Imaging the Circumstellar Environments of Young Binaries in Southern Star-Forming Regions
Chris D. Koresko
A sample of 14 pre-main sequence visual binary systems in southern star-forming regions were imaged at 2 $\mu$m with the Keck I telescope to search for tertiary companions, scattered-light disks, and compact nebulae at linear scales of 5--100 AU. Speckle holography was used to produce images with diffraction-limited resolution and high dynamic range, and photometry in four filters between J (1.25 $\mu$m) and L' (3.8 $\mu$m) was used to provide a diagnostic of the infrared excess. Of the 14 visual binary systems studied, 9 contain components which show evidence for resolved structure. Two of them (WSB~18 and B59-1) have well-resolved tertiary companions separated from the primary stars by $\sim 100$~mas. The remainder are only marginally resolved at the diffraction limit of the telescope. Sz~116, AS~205, Elias~2-22, ESO~H$\alpha$ 282, and B59-2 each have one component which is marginally-resolved along one axis and unresolved on the perpendicular axis, consistent with tertiaries at separations between $\sim 6$ and 40~mas. If all of these systems are hierarchical triples, then fully half of the nominal binaries in the sample are triple. The primaries in Elias~2-49 and WSB~71 are marginally resolved along all position angles, suggesting the presence of dust halos. No disks were unambiguously detected in the sample.
Koresko.tex
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\shorttitle{  Imaging of Southern Young Binaries }
\shortauthors{ Koresko }


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\title{ Imaging the Circumstellar Environments of Young Binaries in Southern
Star-Forming Regions}

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\author{ Chris D. Koresko }
\affil{ Interferometry Science Center, Caltech, Pasadena, CA 91125 }
    
\begin{abstract}

A sample of 14 pre-main sequence visual binary systems in southern
star-forming regions were imaged at 2 $\mu$m with the Keck I telescope to
search for tertiary companions, scattered-light disks, and compact nebulae
at linear scales of 5--100 AU.   Speckle holography was used to produce
images with diffraction-limited resolution and high dynamic range, and
photometry in four filters between J (1.25 $\mu$m) and L' (3.8 $\mu$m) was
used to provide a diagnostic of the infrared excess.  Of the 14 visual
binary systems studied, 9 contain components which show evidence for
resolved structure.  Two of them (WSB~18 and B59-1) have well-resolved
tertiary companions separated from the primary stars by $\sim 100$~mas. 
The remainder are only marginally resolved at the diffraction limit of the
telescope.  Sz~116, AS~205, Elias~2-22, ESO~H$\alpha$ 282, and B59-2 each
have one component which is marginally-resolved along one axis and
unresolved on the perpendicular axis, consistent with tertiaries at
separations between $\sim 6$ and 40~mas.  If all of these systems are
hierarchical triples, then fully half of the nominal binaries in the
sample are triple.  The primaries in Elias~2-49 and WSB~71 are marginally
resolved along all position angles, suggesting the presence of dust
halos.  No disks were unambiguously detected in the sample. 

\end{abstract}

\keywords{ stars: pre-main-sequence --- binaries: visual ---
circumstellar matter --- methods: observational }


\section{ Introduction }

It has recently become clear that the great majority of the young low-mass
stars in the nearest active star-forming regions (SFRs) are found in binary
or multiple systems (Leinert et al. 1993; Ghez et al. 1993; Ghez et al.
1997).  Although there is evidence that the mass in circumstellar material
is smaller in binaries with separations $\simless 100$ AU than in wider
binaries or single stars (Jensen, Mathieu, \& Fuller 1996), a majority of
young binaries emit excess radiation at infrared or submillimeter
wavelengths which is typically presumed to arise in dusty disks.  If the
presence of one or more stellar companions does not severely hamper a disk's
ability to produce a planetary system, then binary and multiple systems
probably contain the majority of the planets in the nearby SFRs. 

At present, only two multiple-star systems are known to contain
well-ordered, edge-on circumstellar disks visible because of their occlusion
of the central star.  The HK~Tauri~B and HV~Tauri~C disks exhibit a
morphology much like that predicted by scattered-light models: A pair of
bright lenslike reflection nebulae trace the upper and lower surfaces of the
disk, while the dense region nearer the midplane is dark (Stapelfeldt et al.
1998; Koresko 1998; Monin \& Bouvier 2000).  There is no central point
source seen in either the visible or near-infrared images.  The
near-infrared color of the HK~Tauri~B star+disk system is bluer than that
of the primary star, presumably because the light we see is scattered by
small particles.  

A handful of visual binary companions to T~Tauri stars are ``infrared
companions" (IRCs), objects unusually faint at visible wavelengths but
bright in the infrared.  The IRCs are mysterious because their spectral
energy distributions show strong reddening and infrared excess reminiscent
of stars at younger evolutionary phases than their primaries ({\it e.g.,}
Koresko, Herbst, \& Leinert 1997).  The structures seen in the two IRCs
which have been resolved in published high-resolution images are not edge-on
disk systems, but a diffuse and irregular nebula (Koresko et al. 1999) and a
close double (Koresko 2000).  Among the present sample, the companion to
Elias~2-22 is known from previous observations (Chelli et al. 1988) to be an
IRC.

Clearly, the circumstellar environments of young binary stars exhibit a
range of phenomena which are accessible to modern imaging techniques.  This
paper describes the first results of an ongoing effort to characterize the
circumstellar environments in a larger sample of young binary systems.  The
sources were chosen from the binaries detected in the visible-light imaging
survey of Reipurth \& Zinnecker (1993; hereafter RZ).  They include all the
binaries, with the exception of Sz~120, with separations between 1\arcsec.0
and 3\arcsec.7 and right ascensions between $16^{\rm h}~06^{\rm m}$ and
$17^{\rm h}~10^{\rm m}$ (equinox 1950). This RA range includes all the RZ
binaries in the Ophiuchus SFR, and several additional groups, for a total of
14 systems.  The range of separations was chosen to avoid having the
seeing-limited Point-Spread Functions (PSFs) overlap or fall off the edges
of the detector.  With one exception, the distances to the targets are given
by RZ as 160~pc.  Their physical separations are larger than 100~AU, so they
are not expected to be strongly depleted of disk material via disk-companion
interactions (Jensen, Mathieu, \& Fuller 1996).  

The sources are a somewhat mixed bag, consisting mainly of T~Tauri stars
but including at least one Herbig~Ae/Be star (Elias~2-49) and a likely
Post-T~Tauri star (WSB~4).  Many of the sources are well-studied, but a
handful lack even published spectral types.  Ten of the 14 systems are
members of RZ's ``systematic survey sample", which for the present purpose
means that they were known to be members of dark clouds $\sim 160$ pc
distant and are included in a list of young stars selected on the basis of
a youth indicator such as near- or far-infrared excess or Ca II HK line
emission.  The remaining four systems are AS~205, both of whose components
are Classical T~Tauri stars (CTTS) (Prato et al. 1997); Elias~2-26,
another CTTS (Martin et al. 1998); Elias~2-49, a Herbig Ae/Be star in
Hillenbrand's Group~I (Hillenbrand 1992); and ESO H$\alpha$~282, about
which little seems to be known beyond the fact that it is an emission-line
source.

The 14 pre-main sequence binary systems were observed using speckle
holography at near-infrared wavelengths $\sim 2~\mu$m to produce
diffraction-limited images with dynamic range of $\sim 2000$ at 0\arcsec.3
from the star, which is much better than would have been possible with
standard speckle imaging.  The improved dynamic range is possible because
the PSF is measured simultaneously with the science observations. 
Near-infrared photometry in the J, H, K, and L' bands was obtained the
following night to measure the infrared excess of each component as a tracer
of warm dust. 

Two new tertiary companions with separations $\sim 0$\arcsec.1 were
detected, and an additional 7 objects were found to be marginally resolved. 
Of these, 5 are extended along only one direction, which is consistent with
the presence of a very close tertiary companion or a narrow extended
structure such as a compact edge-on disk.  The remaining two
marginally-resolved objects were extended in all directions, which would be
consistent with extended nebulosity but not with tertiary companions.  

No new edge-on disks were unambiguously detected.  It is known from
theoretical models ({\it e.g.,} Wood et al. 1998) and from recent
high-resolution images ({\it e.g.,} Burrows et al. 1996) that pre-main
sequence circumstellar disks with fiducial mass distributions are readily
detectable in visible-light or near-infrared images only when they are
viewed along a line of sight close to the disk midplane.  In this
orientation, the direct radiation from the central star is blocked, making
it possible to see the much fainter scattered light which traces the disk
surface. The range of viewing angles over which the disk hides the star, and
therefore the likelihood of detecting it if its orientation is random,
depends on the disk's mass, radius, and flaring, and potentially on the
degree to which the scattering dust has settled toward the midplane.

%QUESTIONS:
%
%  WHAT ARE THE PROPERTIES (AGES...) OF THE CLOUDS CONTAINING THE SAMPLE
%  STARS: OPHIUCHUS, LUPUS III, B40, L162, AND B59?
%
%  CAN THE NUMBER OF DETECTED TERTIARIES BE PREDICTED BY IGNORING THE VISUAL COMPANION
%  AND USING THE COMPANION SURFACE DENSITY RELATION FOUND BY KOHLER ET AL. 1998 AND
%  THE SIMON ET AL. PAPER THEY REFERENCE?
%  
%  WHAT ARE THE CLOUD TEMPERATURES?  IS THERE ANY CORRELATION BETWEEN TEMPERATURE AND
%  THE NUMBER OF TERTIARIES, AS WAS FOUND FOR BINARY COMPANIONS BY BRANDNER ET AL 1996?

\section{ Observations }

\subsection{ Holographic Imaging }

The holography data were taken at the 10~m Keck~1 telescope on 1999~June~1
using the Near-Infrared Camera (NIRC; Matthews \& Soifer 1994) with the
NIRC Image Converter (Matthews et al. 1996) to produce a magnified pixel
spacing of 20 mas, approximately Nyquist-sampling the $\sim 50$ mas
diffraction limit at the $\sim 2~\mu$m observing wavelength.  The field of
the $256\times 256$ pixel detector was 5\arcsec.1.  The optical bandpass
was set using either a standard K photometric filter, or, for stars which
would saturate the detector in the K band, one of the narrower Ks or CH4
filters.  The observations of each visual binary consisted of hundreds of
frames with an integration time of  0.14 sec per frame.  This short
exposure time partially ``froze" the atmospheric seeing, so that the raw
PSF consisted of distinguishable speckles.  The separations of these
visual binaries were large enough compared to the 0\arcsec.5 typical
seeing that only a small fraction of the frames were rejected due to poor
separation of the stars.  The data for each visual binary system were
divided into $\sim 7$ series of frames, with $\sim 140$ frames in each
series.  Series taken on blank sky were used to estimate the background
and bias level.  A journal of the holographic observations is given in
Table \ref{HoloObsJournal}.

Individual frames were calibrated by subtracting mean sky frames, dividing
by flatfield images, and ``fixing" bad pixels.  A model was computed for the
detector ``bleed" signal which extended along the readout direction, as
described in the NIRC manual, and this was subtracted from the calibrated
frame.  For each frame, a subframe centered on each of the stars was
extracted, with one subframe being treated as the science measurement and
the other serving as a measurement of the instantaneous point-spread
function (PSF).  The subframes were either $64\times 64$ or $48\times 48$
pixels in size; the smaller subframes were necessary for doubles whose
separations were less than 64 pixels.  The Fourier power spectra of the
target and PSF subframes, and the cross-spectrum ({\it i.e.,} the Fourier
transform of the cross-correlation) of the two subframes, were computed for
each input frame.  These quantities are meaningful within a roughly
hexagonal region of the $(u,v)$ plane containing spatial frequencies below
the diffraction limit of the hexagonal Keck primary mirror.    

\begin{deluxetable}{lllllll}
\footnotesize
\tablecaption{ Holographic Observations 
\label{HoloObsJournal}}
\tablewidth{0pt}
\tablehead{
\colhead{Object} & \colhead{Alt. Name} & \colhead{Region}  & \colhead{D(pc)} & \colhead{Filter} &
\colhead{Frames} & \colhead{Subframe Size}
} 
\startdata
Sz 116						& HBC 625		 &	Lupus III	& 150	 &   K   &  887     &  64             \\
AS 205						& HBC 254		 &	B 40			& ?		 &   CH4 &  941     &  64             \\
WSB 4 						& 					 &	Oph				& 160	 &   K   &  1014    &  64             \\
WSB 18						& 					 &	Oph				& 160	 &   K   &  868     &  48             \\
WSB 19            & 					 &	Oph				& 160	 &   K   &  870     &  64             \\
WSB 26						& DoAr 23		 &	Oph				& 160	 &   K   &  848     &  48             \\
Elias 2-22			  & DoAr 24E	 &  Oph 			& 160	 &   Ks  &  716     &  64             \\
WSB 35            & 					 &	Oph				& 160	 &   K   &  864     &  64             \\
Elias 2-26				& ROX 15		 &	Oph				& 160	 &   K   &  857     &  64             \\
WSB 71						& 					 &	Oph 			& 160	 &   K   &  813     &  64             \\
Elias 2-49				& HD 150193	 &	Oph				& 160	 &   Ks  &  384     &  48             \\
ESO H$\alpha$ 282 & 					 &	L 162			& 160	 &   K   &  848     &  48             \\
B59-1 						& 					 &	B 59			& 160	 &   K   &  822     &  64             \\
B59-2 						& 					 &	B 59			& 160	 &   K   &  858     &  64             \\
\enddata
\tablecomments{ 
Holographic observations of the 14 pre-main sequence visual binary
systems.  The filters' center wavelengths and half-power bandwidths,
measured in microns, are as follows: K ($2.21 \pm 0.42$), Ks ($2.16 \pm
0.33$), CH4 ($2.27 \pm 0.16$).  Distances are from Reipurth \& Zinnecker 1993.
}
\end{deluxetable}

The cross-spectrum, the target power, and the PSF power were averaged over
the whole series of good frames.  An averaged Fourier power spectrum and
Fourier phase for the target, corrected for the distortions due to the
atmosphere and the telescope, were reconstructed from them as follows:  The
power spectrum was estimated as the ratio of the average target power to the
average PSF power, after removing a noise bias term from each.  The phase
was estimated as the phase of the averaged cross-spectrum.  Uncertainties in
the power spectrum and phase were estimated from the scatter between the
averages computed for the $\sim 7$ series for each visual binary system. The
PSF star in each binary was chosen arbitrarily, and the choice was reversed
if it showed signs of resolved structure.  In no system was there clear
evidence that both stars were resolved, although in principle the technique
would be insensitive to this if the morphologies of the two stars were
sufficiently similar.

The reconstructed Fourier transforms were compared with models of single
stars, double stars, and elliptical Gaussian halos.  These choices clearly
cannot span the range of all possible {\it astrophysical} models, but for
targets which are only marginally resolved, they are sufficient to fit
the features in the data. There is a degree of degeneracy in these model
fits, in the sense that an unresolved star can always be fitted by a
double-star model (with either zero separation or zero brightness ratio) or
by a halo model (with zero axis lengths).  Further, an object which is
marginally resolved along only one axis can be fitted by either a
double-star model or a halo model with a zero semiminor axis, since either
model produces a power spectrum having a single broad stripe, and a nearly
featureless phase, in the measured frequency range.  

The comparisons with models were done separately for each series of
frames.   Only frequencies below that for which the median uncertainty rose
to four times its low-frequency value at were used.  For the double-star and
halo models, the fitting was done 11 times for each series, with the
starting values of the parameters varied randomly, and the fit with the
smallest $\chi^2$ value was chosen to represent that series.  This was
necessary because for some starting parameters the fitting routine became
trapped in local minima in the $\chi^2$ surface.  The final parameter values
for each model were the averages of the results for the series, and the
quoted uncertainties are the standard deviations of the means of those
results.  These statistical uncertainties are likely to be optimistic
because they do not include the effects of systematic biases.  The
estimation of physical parameters is rather delicate and easily affected by
those biases, especially for the marginally-resolved sources. 

For the marginally-resolved sources, the final misfit ($\chi^2$ per degree
of freedom) for the power spectrum was close to unity.  The power spectra
are more useful than the phases for marginally-resolved systems because the
power spectra begin to show strong parameter-dependent structure at lower
spatial frequencies than the phases do.  On the other hand, the position
angles derived from power spectra suffer from an ambiguity of 180 deg ({\it
e.g.,} it is not possible to tell which star in a double is brighter).  For
the fully-resolved sources, which in the present sample are both
primary-tertiary doubles, the fits to the power spectra tend to be poorer. 
This may be because the power spectra, being positive-definite, are
susceptible to biases caused by zero-mean noise, while the phase tends to be
unbiased by it.  The phase fits also produce unambiguous position angles for
resolved doubles.  In summary, the phase data are more useful for resolved
doubles, while the power spectra are more useful for marginally-resolved
objects.


%Because the speckle transfer function of the telescope and atmosphere falls
%rapidly with spatial frequency, the uncertainties in the Fourier transform
%components rise with frequency.  

%THE FOLLOWING BELONGS IN THE DISCUSSION SECTION
%For such ambiguous cases the double-star model is preferred for its
%astrophysical simplicity; the most likely alternative may be a circumstellar
%disk seen tilted just far enough from an edge-on orientation to make the
%front surface dominate the back.  

% *** Comment this out since we never show or refer to the images ***
%An image was made by inverting the power and phase with the use of an
%apodizing function to suppress high-frequency noise.  The apodizing function
%chosen was the product of a Gaussian and a Hanning function which produced a
%final image resolution of 55~mas (FWHM).  The images were then rotated to
%standard orientation.  



\subsection{ Near-Infrared Photometry }

The photometric fluxes of the target visual binary systems were crudely
measured in the J, H, K, and L' filters at the Keck~I telescope on
1999~June~2, by imaging them with NIRC.  For the J, H, and K filters,
subframes of $256\times128$ pixels were used with the Image Converter, while
for the L' filter the Image Converter was removed and the subframe size was
reduced to $32\times 32$ or $64\times 32$ pixels.  These choices were driven
largely by the need to read out the detector fast enough to avoid saturating
it on the brighter stars or, at the longest wavelength, on the blank sky. 
The stars determined the peak signal levels in the J, H, and K filters, so
the exposure times were adjusted between 68~msec and 4~sec, and no coadding
was done.  For the L' filter the frames consisted of 1000 coadds of 5~msec
each.  A total of 10 frames were taken on each target in each filter. 
Periodically, frames were obtained on blank sky to estimate the background
level.  SR3 served as a photometric standard for the J, H, and K filters,
and HD~161793 was used for the L' filter.


The frames were calibrated in the usual way, with sky-subtraction,
flatfielding, and pixel-fixing.  To derive the fluxes in the frames, each
binary was fitted using a model in which the two stars were represented by
circular Gaussians of adjustable size, intensity, and position.  Although
the Gaussians were not very good models for the PSFs, the fact that the PSF
in each image was essentially identical for the two stars permitted the
ratio of their brightnesses and the distance between them to be estimated
accurately using this technique.  The brightness ratios were combined with
total flux measurements to derive the fluxes for the individual stars.  At
J, H, and K, these total fluxes were computed by summing the values of all
pixels within 32 pixels of either star, after subtracting the background
level estimated as the median value of all pixels at least 64 pixels away
from either star.  In the L' filter the total flux was simply the sum of all
pixel-values, after subtraction of a background level.  The photometric
fluxes were calibrated in the usual way by comparison of these numbers with
similar results on standard stars.  No attempt was made to correct for the
differences in airmass, as those corrections were found to be smaller than
the statistical uncertainties.  The final uncertainties in the magnitudes
are approximately 0.2 mag.  

Separations and position angles of the binaries were derived from the model
fits in each filter.  The platescale with IC out of the beam has been
carefully measured by the Keck Observatory staff, and the pixel spacing is
quoted in the instrument manual as 0\arcsec.1500.  By contrast, the
0\arcsec.02 nominal platescale with the IC in is relatively uncertain.  On
the other hand, the measurements of the separation vectors taken with the
smaller pixels are likely to be significantly more precise, in the sense of
having smaller uncertainties if the platescale error is neglected.  To take
advantage of this, the assumed platescale in each of the J, H, and K bands
was adjusted to make the mean separation for all the visual binaries (except
for Sz~116, which was not measured in the L' filter, and Elias~2-49, which
is anomalous as discussed below)  equal to the mean separation in the IC-out
L' images.  The required IC-in pixel spacing of 0\arcsec.0207 was consistent
between the J, H, and K images.  Similarly, the assumed orientation of the
IC-in images was adjusted to make the mean difference between the IC-in and
IC-out position angle measurements vanish.


\section{ Results and Discussion }

The results of the holographic imaging of the target binaries are summarized
in Table \ref{HoloSummary}, and the Fourier power spectra are displayed in
Figure \ref{Power2D}.  The Table presents the values of $\chi^2$ per degree
of freedom ({\it i.e.,} per frequency element in the $(u,v)$ plane) for 
unresolved-star, double-star, and halo models of the target in each of the
14 visual binary systems studied.  For the two fully-resolved doubles, the
fits were made to the Fourier phase, resulting in unambiguous position angle
measurements, while for all other sources the fits were made to the power
spectra.  In the following discussion, a target is considered to be resolved
if either the double-star or the halo model improves the $\chi^2$ value by
more than 0.5 compared to the unresolved-star model.  This criterion
provides a clean separation between the unresolved and extended targets.


\bigskip
\begin{figure*}[htbp]
\epsfxsize=16.5cm
\epsfbox{Koresko.fig1.ps}
\caption[ ]{ Reconstructed visibility amplitudes for the target components
in the 14 visual binaries, displayed with the zero frequency at the center
of each plot, and in the orientation natural to the detector. The measured
amplitudes extend from the zero-frequency point to something close to the
diffraction limit of the Keck primary mirror, whose hexagonal shape is
visible in the data on the brighter stars.  At high frequencies detector
noise produces stripes parallel to the horizontal or vertical axes. The
striped structure consistent with the presence of close tertiary
companions is obvious in Sz~116, WSB~18, and B59-1.  Closer inspection
reveals similar (but broader) central stripes in AS~205,
ESO~H$\alpha$~282, B59-2, and possibly in Elias~2-22.  The amplitudes of
WSB~71 and Elias~2-49 decrease in all directions, which is inconsistent
with a tertiary-star model. }
\label{Power2D}
\end{figure*}


\begin{deluxetable}{llclllc}
\footnotesize
\tablecaption{ Summary of Holographic Imaging Results 
\label{HoloSummary}}
\tablewidth{0pt}
\tablehead{
\colhead{Object} & \colhead{Fit To} & \colhead{Target} & \colhead{$\chi^2_{u}$} & \colhead{$\chi^2_{d}$} & \colhead{$\chi^2_{h}$} & \colhead{Comment}
} 
\startdata
Sz 116            & power & sec & 9.4 & 1.5  & 1.5  &  1   \\  % Marginally-resolved; Consistent with a tertiary or halo.
AS 205            & power & sec & 4.7 & 1.1  & 1.2  &  1   \\  % Secondary is marginally resolved; consistent with a tertiary companion.
WSB 4             & power & pri & 1.1 & 1.1  & 1.1  &  4   \\  % No evidence for a resolved component in this IRC system.
WSB 18            & phase & pri & 96  & 1.2  & 96   &  2   \\  % Primary has a tertiary companion.
WSB 19            & power & sec & 1.6 & 1.6  & 1.6  &  4   \\  % No evidence of resolved structure (but the data are relatively noisy).
WSB 26            & power & pri & 1.1 & 1.1  & 1.1  &  4   \\  % No evidence of resolved structure.
Elias 2-22        & power & sec & 1.9 & 1.1  & 1.1  &  1,5 \\  % The IRC may be marginally resolved; consistent with a tertiary companion.
WSB 35            & power & pri & 1.4 & 1.4  & 1.4  &  4   \\  % No evidence of resolved structure (but affected by noise at high frequency).
Elias 2-26        & power & sec & 1.1 & 1.1  & 1.1  &  4   \\  % No evidence of resolved structure.
WSB 71            & power & pri & 2.1 & 1.5  & 1.3  &  3   \\  % Primary star is marginally resolved; not consistent with a tertiary companion.
Elias 2-49        & power & pri & 3.6 & 2.0  & 1.0  &  3   \\  % Primary is marginally resolved; not consistent with a tertiary companion.
ESO H$\alpha$ 282 & power & sec & 6.6 & 1.5  & 1.5  &  1   \\  % Secondary is marginally resolved; consistent with a tertiary companion.
B59-1             & phase & pri & 47  & 1.2  & 47   &  2   \\  % Primary has tertiary companion.
B59-2             & power & pri & 2.3 & 1.1  & 1.1  &  1   \\  % Primary is marginally resolved; consistent with a tertiary companion.
\enddata
\tablecomments{ 
Fits were made to the Fourier phase data for the two fully-resolved doubles, and to the power spectra for all other targets.
$\chi^2_{u}$, $\chi^2_{s}$, and $\chi^2_{h}$ are the $\chi^2$ per degree of freedom for the unresolved, double, and halo models, respectively.
The comment numbers are:
(1) Marginally-resolved, consistent with a tertiary or a halo.
(2) Fully-resolved double.
(3) Marginally-resolved, consistent with a halo but not a tertiary.
(4) No evidence of resolved structure
(5) Infrared Companion.  This system is also known as DoAr 24E.
}
\end{deluxetable}

The J,H,K,L' photometric measurements for the 14 holographic imaging targets
are given in Table \ref{PhotSummary}.  Measurements of the separation
vectors of their components, which are the averages of those measured at J,
H, and K with the corrected platescale, are included in the table.  The
brightness ratios for the visual binaries are plotted in Figure
\ref{BrightnessRatios}.

\begin{deluxetable}{lrrrrrrrrrrr}
\footnotesize
\tablecaption{ Infrared Photometry and Seeing-Limited Imaging
\label{PhotSummary}}
\tablewidth{0pt}
\tablehead{
\colhead{Object}
 & \colhead{J$_{pri}$} & \colhead{H$_{pri}$} & \colhead{K$_{pri}$} & \colhead{L'$_{pri}$}
 & \colhead{J$_{sec}$} & \colhead{H$_{sec}$} & \colhead{K$_{sec}$} & \colhead{L'$_{sec}$}
 & \colhead{Sep (\arcsec)} & \colhead{PA (deg)}
}

% The Sep and PA numbers in the table are the average of the JHK measurments, rescaled to have the sampe
% average values as the Lp measurements.  The rationale is that the platescale (and probably the orientation)
% of the array are known best in the non-IC field in which the Lp images were taken.
\startdata
    Sz 116				& 10.5  &   9.5  &  9.9  &        &  11.1  & 10.4  &   10.4  &	     &  1.65 & 26.5  \\
    As 205				&  8.3  &   6.9  &  6.2  &   5.4  &   8.8  &  7.5  &    7.0  &   5.9 &  1.34 & 211.3 \\
     WSB 4				& 11.6  &  11.0  & 10.9  &  10.1  &  11.9  & 11.2  &   10.6  &   9.1 &  2.84 & 128.5 \\
    WSB 18				& 11.1  &  10.1  & 10.0  &   8.9  &  11.8  & 10.8  &   10.4  &   9.0 &  1.08 & 80.4  \\
    WSB 19				& 10.8  &   9.7  &  9.6  &   8.4  &  11.5  & 10.5  &   10.4  &   9.3 &  1.53 & 260.7 \\
    WSB 26				& 10.9  &   9.9  &  9.6  &   8.5  &  11.4  & 10.2  &    9.7  &   8.1 &  1.15 & 23.8  \\
Elias 2-22				&  8.7  &   7.5  &  7.1  &   6.1  &  11.6  &  9.1  &    7.7  &   5.6 &  2.05 & 148.6 \\
    WSB 35				& 10.8  &   9.7  &  9.3  &   8.0  &  11.5  & 10.7  &   10.6  &   9.9 &  2.29 & 130.3 \\
Elias 2-26				& 10.5  &   8.8  &  8.4  &   7.2  &  11.9  & 10.2  &    9.6  &   8.3 &  1.44 & 67.0  \\
    WSB 71				& 10.1  &   8.8  &  8.0  &   6.2  &  11.0  & 10.1  &    9.9  &   9.2 &  3.56 & 35.0  \\
Elias 2-49				&  7.1  &   6.4  &  5.9  &   5.2  &   8.5  &  7.8  &    7.8  &   6.8 &  1.09 & 221.7 \\
ESO H$\alpha$ 282 &  9.8  &   8.6  &  7.8  &   6.8  &  11.1  & 10.1  &    9.5  &   8.6 &  1.03 & 251.8 \\
     B59-1				& 11.8  &   9.8  &  9.0  &   7.5  &  13.1  & 11.5  &   11.0  &  10.2 &  3.44 & 109.3 \\
     B59-2				& 10.0  &   9.3  &  9.3  &   8.9  &  11.3  & 10.4  &   10.1  &   8.9 &  3.11 & 207.0 \\
\enddata
% The list below is the output of convert_fits_tables_to_ascii.pro set to use the JHK filters only
%    Sz 116  Sep=  1.594+/-  0.007    PA=  26.2+/-   0.3
%    As 205  Sep=  1.291+/-  0.002    PA= 211.0+/-   0.1
%     WSB 4  Sep=  2.739+/-  0.015    PA= 128.2+/-   0.1
%    WSB 18  Sep=  1.045+/-  0.007    PA=  80.1+/-   0.2
%    WSB 19  Sep=  1.473+/-  0.003    PA= 260.4+/-   0.0
%    WSB 26  Sep=  1.105+/-  0.002    PA=  23.5+/-   0.1
%Elias 2-22  Sep=  1.975+/-  0.006    PA= 148.3+/-   0.2
%    WSB 35  Sep=  2.206+/-  0.001    PA= 130.0+/-   0.1
%Elias 2-26  Sep=  1.384+/-  0.002    PA=  66.7+/-   0.1
%    WSB 71  Sep=  3.434+/-  0.004    PA=  34.7+/-   0.0
%Elias 2-49  Sep=  1.054+/-  0.004    PA= 221.4+/-   0.1
% ESO Ha282  Sep=  0.995+/-  0.001    PA= 251.5+/-   0.1
%     B59-1  Sep=  3.317+/-  0.010    PA= 109.0+/-   0.1
%     B59-2  Sep=  2.995+/-  0.005    PA= 206.7+/-   0.0
\tablecomments{ 
  Near-infrared magnitudes and separation vectors for the holographic target
  binaries, based on corrected JHK measurements as described in the text. 
  The position angles are computed for vectors extending from the brighter
  to the fainter component in the J filter.  They are measured in degrees
  east of North.  The estimated photometric uncertainties are typically 0.2
  mag, and the scatter between the J, H, and K separation vector
  measurements for a given star is typically smaller than 0\arcsec.01 in
  separation and 0.2 deg in position angle.
}
\end{deluxetable}

Out of the 14 visual binary systems (nominally 28 stars) observed, two are
found to be well-resolved doubles, 5 to be marginally resolved along a
single axis (consistent both with doubles with separations less than the
diffraction limit, or with elongated halos), and two were
marginally-resolved along all axes.  The results for the individual resolved
and marginally-resolved sources are discussed below.

%WSB71: DISAGREEMENT BETWEEN NIR AND VISIBLE SEPARATIONS MIGHT BE DUE TO 
%THE NEBULOSITY DETECTED IN THESE HOLOGRAPHIC OBSERVATIONS.

\subsection{ Fully-Resolved \\ Primary-Tertiary Pairs }

Two systems (WSB~18 and B59-1) have well-resolved tertiary companions
separated from their primaries by $\sim 0$\arcsec.1.  Models were fitted to
their Fourier phases in order to derive the brightness ratio $R$, separation
$\rho$, and position angle $\theta$ of the primary-tertiary pairs.  The
results are given in Table \ref{BinaryFits}.  

\begin{deluxetable}{lrrr}
\footnotesize
\tablecaption{ Double-Star Fits 
\label{BinaryFits}}
\tablewidth{0pt}
\tablehead{
\colhead{Object} & \colhead{$\theta$} & \colhead{$R$} & \colhead{Sep} \\
\colhead{} & \colhead{(deg)} & \colhead{}  & \colhead{(mas)}
} 
\startdata
Sz 116		      	 & $143.3  \pm .4$ &   $0.37   \pm .01 $  & $36.9  \pm  .4 $  \\
AS 205						 & $101    \pm 1 $ &   $1      \pm .003$  & $8.5   \pm  .4 $  \\
WSB 18		      	 & $339.55 \pm.05$ &   $0.505  \pm .001$  & $100.4 \pm  .1 $  \\
Elias 2-22      	 & $ 44    \pm 3 $ &   $0.98   \pm .001$  & $5.6   \pm  .2 $  \\
%WSB 71						 & $122    \pm 7 $ &   $0.273  \pm .005$  & $17.4  \pm  .4 $  \\
%WSB 71						 & $19     \pm 4 $ &   $0.61   \pm .04 $  & $19    \pm  1  $  \\
ESO H$\alpha$ 282  & $16.7   \pm .6$ &   $0.78   \pm .02 $  & $12.8  \pm  .1 $  \\
B59-1 		      	 & $218.9  \pm .1$ &   $0.49   \pm .002$  & $106.7 \pm  .1 $  \\
B59-2 		      	 & $113.5  \pm .5$ &   $0.91   \pm .05 $  & $12.2  \pm  .1 $  \\
\enddata
\tablecomments{ 
Double-star fitting parameters for the Fourier phase data on the two objects
with clearly-resolved tertiary companions, and fits to the Fourier power
spectra of the five objects which exhibit marginally-resolved structure
which consistent with the double-star model.  In the latter cases, the $R$
(brightness ratio) and $\rho$ (separation) values must be considered
representative of a class of possible models, and the position angle
($\theta$) values are ambiguous by 180 deg.  The errorbars quoted here are
derived from the standard deviation of the mean derived from the scatter
among fits to $\sim 7$ series of $\sim 140$ frames each.  They do not
include any potential contributions from uncertainties in the platescale or
orientation of the camera, or noise biases. They are very optimistic,
especially for the marginally-resolved objects.
}
\end{deluxetable}


The presence of a tertiary companion to the primary in WSB~18 is
noteworthy in light of the study by Brandner \& Zinnecker (1997), in
which the primary appeared somewhat more luminous than would have been
predicted for a single star with its M2 spectral type and the $5 \times
10^6$ yr age derived by comparing the secondary to the theoretical
pre-main sequence tracks of D'Antona \& Mazzitelli (1994).  It is natural
to suggest that the tertiary companion is responsible for the excess
luminosity, and its detection strengthens the conclusion reached by
Brandner \& Zinnecker (1997) that the components of the binaries in their
sample are coeval.

The projected separations of these primary-tertiary systems correspond to
$\sim 16$~AU at 160~pc.  Assuming that the true separations are not much
larger, that the mass of each close pair is $\sim0.6~{\rm M}_\odot$, and
that the orbits are circular, results in an estimate of $\sim 10^2$ yr for
their orbital periods.  The systems should rotate by $\sim 4$ deg yr$^{-1}$,
making their orbital motion readily detectable from one year to the next.  



\subsection{ Marginally-Resolved Sources }

As discussed above, the nature of the 7 \\ marginally resolved targets is
somewhat ambiguous.  Five of them appear to be extended along only one axis,
making them consistent with both the double-star model (the double having a
separation below the diffraction limit) and the halo model (the minor axis
being much smaller than the major axis).  In these cases, the double-star
model is preferred for its astrophysical simplicity, although it may be
possible to produce an elongated halo of the observed size by a suitable
arrangement of circumstellar material such the inner regions of a nearly
edge-on disk.  The results of fitting the double-star model to these sources
are included in Table \ref{BinaryFits}.


\begin{deluxetable}{lccrrr}
\footnotesize
\tablecaption{ Gaussian Halo Fits to Multiaxial Power Spectra 
\label{GaussianFits}}
\tablewidth{0pt}
\tablehead{
\colhead{Object} & \colhead{comp} & \colhead{Fit to} & \colhead{$\theta$} & \colhead{$a$} & \colhead{$b$} \\
\colhead{} & \colhead{} & \colhead{} & \colhead{(deg)} & \colhead{(mas)} & \colhead{(mas)} 
}
\startdata
WSB 71						& pri & power &  $8   \pm 3$  & $64 \pm 1$ &  $50 \pm 1$  \\
Elias 2-49      	& pri & power &  $116 \pm 3$  & $54 \pm 1$ &  $45 \pm 1$  \\
\enddata
\tablecomments{ 
Gaussian-halo fitting parameters for the Fourier power data on the two objects which are
marginally resolved but not consistent with the double-star model.  The
parameters are the position angle of the major axis and the lengths of the
semimajor and semiminor axes.
}
\end{deluxetable}


The power spectrum of the Sz~116 secondary is consistent with a double whose
separation is just below the diffraction limit.  The best-fit model gives a
projected separation of 40 mas, corresponding to 6 AU at 160 pc, making this
the widest of the potential marginally-resolved doubles in the sample.   
The secondaries in AS~205, Elias~2-22, ESO~H$\alpha$~282, and the primary in
B59-2 also have asymmetrical power spectra consistent with the presence of
tertiary companions with separations ranging from only 6 to 13 mas, well
below the diffraction limit.  If the structures seen in these targets do
reflect tertiary companions, their periods would be only $\sim 1-20$ yr. 
For these marginally-resolved objects, followup holographic observations to
search for significant changes in the position angles of their extended
structures would provide a good test of the tertiary-star interpretation.

%In a marginally-resolved double,  the fitted $R$ and $\rho$ are degenerate
%in the sense that each can be derived from the fit only when the other is
%specified.  Further, the phases are nearly flat in the measured range of
%spatial frequencies, leading to a 180-deg ambiguity in the position angle of
%the putative double.   Alternatively, because the rapidly-varying regions in
%the Fourier phase which would  uniquely identify the structure as a double
%lie beyond the measured frequencies, the power spectrum is also consistent
%with {\it any} model having the appropriate angular size in direction
%perpendicular to the stripe, and a much smaller size in the direction
%parallel to the stripe.  For convenience it was fitted with a ``core-halo"
%model consisting of a two-dimensional, elongated Gaussian halo surrounding a
%central point-source.  The fitting parameters were the ratio $\gamma$ of the
%total flux in the halo to the flux in the point-source, the semimajor axis
%$a$ and semiminor axis $b$, and the position angle of elongation $\theta$. 
%This model could represent scattering in a shell of dust surrounding a star,
%or perhaps the inner regions of a truncated disk seen nearly edge-on.  The
%results of that fit are given in Table \ref{GaussianFits}.


The primary components in Elias~2-49 and WSB~71 are marginally-resolved but
inconsistent with the double-star model because there is no position angle
along which the power spectra do not decrease with frequency.  In these
objects the $\chi^2$ is lower for the halo model than for the double-star
model.  The data are insufficient to determine the morphology of these
sources in any detail, and it may be much more complex than a simple
Gaussian halo.  The marginally-resolved primaries in these systems are much
redder than the unresolved secondaries (see Figure \ref{BrightnessRatios}),
which is not surprising if they are surrounded by dust envelopes, provided
that the overall flux is not dominated by scattering.  These sources merit
followup observations at shorter wavelengths, {\it e.g.,} visible-light
imaging with the {\it Hubble Space Telescope}, to search for extended, low
surface brightness scattered light to which the present holographic
observations are insensitive.

%The case of WSB~71 the primary is especially interesting in light of the
%controversy surrounding the separation of the visual binary. Simon et al.
%(1995) give the separation as 4\arcsec.8 at PA 37 deg.  This separation
%differs by 1\arcsec.3 from the present value,  Zinnecker et al. (1993) give
%3\arcsec.7, which is much more consistent with my 3\arcsec.46.  I'd like to
%try fitting a  line to the plot of separation vs time, but the earlier
%papers don't give the dates of their observations.

%WSB~71 was identified as a pre-main sequence system in an H$\alpha$
%objective prism survey by Dolidze \& Arakelyan (1959), and later by
%Wilking, Schwartz, and Blackwell (WSB; 1987), who found both components to
%be moderately strong H$\alpha$ emitters.  The primary star is much redder
%than the secondary (see Figure \ref{BrightnessRatios}).


\subsection{ Wavelength Dependence of Visual Binary Separation Vectors }

If a star has a close companion with a significantly different infrared
color, or if it is surrounded by an asymmetrical halo of non-gray dust, then
its photocenter will be wavelength-dependent.  A comparison between the
separation vectors measured for a visual binary over a range of wavelengths
can potentially detect this effect.   The RMS scatter in the differences
between the IC-in and IC-out separation values for the binaries, excluding
Sz~116 (which was not measured at L') and Elias~2-49, is 0\arcsec.014.  This
gives an indication of the precision of the individual separation
measurements.  For Elias~2-49 the separation decreases with wavelength from
1\arcsec.09 at J to 0\arcsec.97 at L', a change which is much larger change
than the estimated precision and suggestively similar to the estimated
Gaussian size of the halo surrounding the primary star.  Deeper
high-resolution images of this system at visible or near-infrared
wavelengths would be useful to reveal the structure of the circumstellar
material.  Interestingly, WSB~71, the other system which appears to have a
halo around its primary, has a separation which is not dependent on
wavelength between the J and L' bands.


\subsection{ The Density of Tertiary Companions }

If one assumes that all five marginally-resolved sources which are
compatible with doubles do have physical tertiary companions at the
separations derived from the model fits, then fully half of the nominal
visual binaries in the sample are actually hierarchical triples.  This
perhaps surprising result is discussed below.

The angular surface density $\Sigma(\theta)$ of companions to young stars in
the Taurus, Ophiuchus, and Orion star-forming regions was computed by Simon
(1997) for companion separations $\theta$ as small as 0\arcsec.01.  In the
``binary regime" in which $\theta$ is small enough that $\Sigma$ is
dominated by bound objects, the surface density in all three clusters was
found to be $\Sigma(\theta) \propto \theta^{-b}$, where b is close to 2.0. 
A similar conclusion was reached for an X-ray selected sample of Taurus
stars by Kohler \& Leinert (1998; hereafter KL98) for separations between
0\arcsec.13 and 13\arcsec.  The robustness of this result is impressive in
light of range of physical conditions spanned by the three SFRs.  


\bigskip
\begin{figure*}[htbp]
\epsfxsize=10cm
\epsfbox{Koresko.fig2.ps}
\caption[ ]{ The brightness ratios for the visual binaries in the sample,
estimated by fitting the seeing-limited images with models consisting of a
pair of circular Gaussians, for the four filters.  Sz 116 is not included
on the plot because the L' measurement on it was not successful. }
\label{BrightnessRatios}
\end{figure*}

\bigskip
\begin{figure*}[htbp]
\epsfxsize=10cm
\epsfbox{Koresko.fig3.ps}
\caption[ ]{ The near-infrared colors of the components of the visual
binary systems in the sample are plotted with symbols indicating their
rank ({\it i.e.,} whether they are primary or secondary stars).  Symbols
for the components of each system are joined by a line.  A box surrounding
a symbol indicates that it was marginally or fully resolved by the
holographic observations.  The data reveal no clear correlation between
brightness, color, or structure. }
\label{ColorColor}
\end{figure*}

Suppose that this result applies within the present sample of visual
binaries for tertiaries at separations small compared to the visual binary
separation.  This cannot be strictly true, both because the sample is {\it
statistically} biased by the presence of the visual companions and because
the companions' gravitational influence restricts the range of separations
for which a stable tertiary orbit may exist, but given the large ratio of
the observed outer to inner separations and the steepness of the density
function, it may not be terribly wrong.  The orbital stability restriction
has been estimated by Harrington (1972) and depends on the inclination of
the inner orbit relative to the outer orbit, the eccentricity of the outer
orbit, and the mass ratios of the components, but for an outer-orbit
eccentricity of 0.5 and components of similar mass it requires the major
axis of the outer orbit to be approximately a factor of 10 larger than that
of the inner orbit.  The average projected separation of the visual binaries
in the present sample is 2\arcsec, so according to the Harrington criterion
with the assumed parameters, and neglecting the effects of projection onto
the plane of the sky, the systems on average should permit tertiary
companions with separations no larger than about 200 mas.  Normalizing
$\Sigma$ to produce the number of companions observed by KL98 within the
range of radii to which their observations were sensitive, and integrating
over an annulus extending from the approximate detection limit of 5 mas to
the stability cutoff at 200 mas, leads to a prediction that 11 tertiaries
should be found among the 28 primary and secondary stars in the sample.  
Given the small sample size, the crudeness of the above approximations, and
the possibility that some additional tertiary companions were missed, this
is consistent with the 7 detected here.  

% Leave this for the conclusion.
%The light from an unknown companion will cause the luminosity for a star
%to be overestimated, so that it would be placed incorrectly in the H-R
%diagram.  As noted above, this appears to have been the case for one of
%the present sample (WSB~18).  Given the large fraction of young visual
%binaries which appear to have close tertiaries, a careful search for extra
%companions must be an important part of any effort to use these binaries
%to test theoretical pre-main sequence evolutionary tracks.



% Comment out this section for now since the sample of targets isn't well
% enough understood to make the conclusion as strong as I'd like.

%\subsection{ The Absence of Edge-On Disks }
%
%Perhaps the most striking result of the holographic observations is the
%failure to unambiguously detect any edge-on disks among the sample of 14
%visual binaries.  It is natural to wonder how likely such an outcome is in
%the context of standard models in which the infrared and submillimeter
%excess emission in young low-mass stars arise in axially-symmetric, flared
%disks in hydrostatic equilibrium, composed of material similar to the
%interstellar medium (ISM).  As noted above, a ``normal" PMS disk can be seen
%in visible or near-infrared images if and only if it is viewed along a line
%of sight which suffers enough extinction to hide the star and hot inner
%disk, which would otherwise greatly outshine the faint light scattered by
%the outer disk surface.  The probability that a randomly-oriented disk will
%be detectable then depends on the parameters which determine its apparent
%thickness, {\it i.e.,} the maximum angle between the disk plane and the line
%of sight for which the extinction is greater than some critical value.
%
%The disk model of Wood et al. (1998) is a fairly typical one in that it
%assumes that the disk material has optical properties like those of the
%interstellar medium and treats the radial dependence of the surface density
%and scaleheight as power laws.  In polar coordinates (r,z) the density can
%be expressed as
%
%\begin{equation}
%	\rho(r,z) = \rho_0~\left({r \over R_0}\right)^{-\alpha} ~e^{-{1 \over 2} \left({z \over h(r))}\right)^2}
%\end{equation}
%
%\noindent with
%
%\begin{equation}
%	h(r) = h_0 ~\left({r \over R_0}\right)^{\beta}
%\end{equation}
%
%%\noindent and
%%
%%\begin{equation}
%%	\rho_0 = {{(2\pi)^{-{3 \over 2}}  (\beta - \alpha + 2) M_{disk} h_0^{-1}} \over 
%%	{R_{outer}^2 ({R_{outer} \over R_0})^{\beta - \alpha} - {R_{inner}^2 ({R_{inner} \over R_0})^{\beta - \alpha}}}}
%%\end{equation}
%
%\noindent Here $h_0$ and $\rho_0$ are the scaleheight and mass density in the
%equatorial plane at some fiducial distance $R_0$ from the center of the star.
%
%The extinction along a line of sight from the star to infinity at angle
%$\theta$ above the disk plane is simply the mass opacity multiplied by the
%integral of the mass density in the disk:
%
%%\begin{equation}
%%	\tau(\theta) = \kappa \rho_0 \int_{R_{inner} \over {cos\theta}}^{R_{outer} \over {cos\theta}} 
%%								 ({s~cos\theta \over r_0})^{-\alpha} {\rm exp} [-{1\over 2} ({{s~sin\theta} \over {h_0({{s~cos\theta}\over r_0})^\beta}})^2] ds
%%\end{equation}
%
%\begin{equation}
%	\tau(\theta) = \kappa \rho_0 \int_{R_{inner} \over {cos~\theta}}^{R_{outer} \over {cos~\theta}} 
%								 \left({r \over r_0}\right)^{-\alpha} {\rm exp} \left[-{1\over 2} \left({{z} \over {h_0\left({{r}\over r_0}\right)^\beta}}\right)^2\right] ds
%\end{equation}
%
%\noindent where $r = s$~{\rm cos}~$\theta$ and $z = s$~{\rm sin}~$\theta$.
%
%%\noindent where
%%
%%\begin{eqnarray}
%%	r_{min} & = & {r_{inner} \over {cos(\theta)}} \\
%%	r_{max} & = & {r_{outer} \over {cos(\theta)}} \\
%%	z(r) & = & r~sin(\theta)
%%\end{eqnarray}
%
%
%While the basic parameters for such disks in most of the target systems are
%not well constrained by existing observations, it turns out that the
%detection probabilities are sensitive primarily to the disk scaleheight. 
%This is because of the very rapid falloff of density with height above the
%disk plane.  This point is illustrated in Figure \ref{DiskTauVsScaleheight},
%which plots the optical depth along a line of sight against the angle
%between that line of sight and the disk midplane, and Figure
%\ref{DiskPdetVsScaleheight}, which plots the detection probability for a
%randomly-oriented disk against disk scaleheight for several disk masses. 
%The disk models have parameters typical of those used by Wood et al.
%(1998).  The assumed criterion for detectability is a line-of-sight optical
%depth greater than some critical value at the 2 $\mu$m wavelength of the
%holographic observations.  The detection probability is simply the sine of
%the maximum angle above the disk plane for which this criterion is met.  For
%the purpose of the present calculation, this minimum optical depth is
%assumed to be 5, which is the lower limit on the extinction of the star in
%the HK~Tauri~B edge-on disk system.  It is clear from the plots that the
%detection probability for a disk is not very sensitive to this choice, and
%that it is much more sensitive to the scaleheight than to the mass.
%
%An estimate for the range of detection probabilities for plausible disks in
%the target systems can be derived by choosing sets of disk parameters which
%tend minimize and maximize the apparent thickness, within reasonable limits
%set by theoretical and observational constraints (Wood et al. 1998).  The
%adopted parameter sets are given in Table \ref{DiskParameterSets}.  Despite
%the large range of disk masses, the range of detection probabilities is only
%0.08 for the least-detectable disk to 0.55 for the most-detectable.
%
% 
%
%
%The failure to clearly detect any edge-on disks can be used to constrain
%the properties of the disks around these stars.  Strom et al. (1989) found
%that among their sample of stars in the Taurus-Auriga star-forming
%complex, nearly 60\% of those with ages $< 3\times 10^6$ yr had K-band
%flux excesses which they attributed to the presence of disks, while this
%was true of only 10\% of the stars older than $10^7$ yr.  The
%submillimeter photometric survey of the same star-forming complex by
%Beckwith et al. (1990) detected emission, which they interpreted as coming
%from dusty disks, in 42\% of their targets.  Adopting the latter
%percentage for the southern PMS targets of the present paper, one would
%predict that $\sim 12$ of the nominally 28 stars (14 visual binaries)
%would have disks.  If all of those disks were of the most-detectable type
%described in Table \ref{DiskParameterSets}, and had independent random
%orientations, then the probability that none would produce enough
%extinction to be detected by the holographic observations is $< 10^{-4}$. 
%Even for the least-detectable disks, the probability that all 12 would
%be missed is only 0.35.  Thus it appears likely that these southern young
%stars are deficient in massive disks compared to their cousins in the
%Taurus-Auriga complex, or that their disks are unexpectedly difficult to
%detect using high-resolution imaging in the near-infrared.
%
%One mechanism which could potentially reduce the detectability of disks to
%the imaging observations presented here is the settling of the dust, which
%provides the great majority of the mass opacity in the near-infrared, toward
%the disk midplane.  This would have an effect similar to a reduction in the
%scaleheight.  Dust settling has been proposed by Miyake \& Nakagawa (1995)
%and Chiang et al. (2001) to fit the SEDs of several T~Tauri stars, and by
%D'Alessio et al. (1999) to account for the lack of TTS with SEDs consistent
%with extinction by edge-on disks in the IRAS study of Kenyon et al. (1994). 
%That significant dust settling has occurred in some TTS disks would not be
%surprising in light of the predicted settling timescales ({\it e.g.,} Chiang
%\& Goldreich 1997) which can be expressed as
%
%\begin{equation}
%	t_{settle} \sim 5\times 10^5~{{1~\mu m} \over r_{grain}}~{{M_{disk} \over
%	10^{-2}~M_\odot}}~{\rm yr}
%\end{equation}
%
%For comparison, the typical age of a TTS is $10^6$ yr, and the grain sizes
%in the circumbinary disk associated with UY~Aurigae are estimated to range
%from 0.03 - 0.6 $\mu$m (Close et al. 1998).



% This paragraph is not good enough to publish.
%Given the absence of edge-on disks in the sample, it would be natural to
%suppose that the reddest stars would be the ones most likely to exhibit
%extended structure.  The present dataset provides only weak support for
%this hypothesis.  As parameterized by J-K color, the reddest object is the
%infrared companion in Elias~2-22; conversely, the extended secondary in
%Sz~116 is one of the bluest objects.  The average J-K color of all the
%resolved objects is $1.8 \pm .4$, compared to $1.4 \pm .1$ for the
%unresolved objects.  Leaving out the two objects whose extended structure
%is clearly due to tertiary companions, or considering only the two objects
%whose extended structure is clearly not due to tertiaries, does not
%significantly alter this conclusion.  

\subsection{ Infrared Brightness Ratios }

The brightness ratios for the target visual binaries measured in all four
filters, plotted against the filter wavelengths in
Figure~\ref{BrightnessRatios}, show no clear tendency for the primary star
in a visual binary to be redder than the secondary.  Such a tendency would
be manifested as a generally negative slope in the brightness-ratio curves. 
This contrasts with the result for the sample of Taurus visual binary TTS
observed by Moneti \& Zinnecker (1991), in which the primary stars were
redder than the secondaries.  An examination of the color-color plot in
Figure~\ref{ColorColor} shows no clear correlations between the colors of
the visual binary components, their structure in the holographic
observations, or their relative brightness.
% Note that the MZ comparisons were made using dereddened photometry, but
% I checked their raw numbers and found that their conclusion still holds.


\section{ Conclusions }

The present study is among the first to probe a sample of pre-main sequence
visual binary systems with this combination of high resolution and dynamic
range in the near-infrared.  The results are not very consistent with the
naive expectations that motivated it.  More than half of the systems contain
resolved or marginally-resolved components.  Two tertiary companions were
clearly resolved at $\sim 100$ mas separations from their primaries, and
tertiaries at separations below the diffraction limit are likely to account
for the structure seen in all but two of the marginally-resolved stars,
including the infrared companion (IRC) in Elias~2-22.  Followup observations
to search for orbital motion would be useful.  The presence of such a large
number of tertiary companions would make it necessary to search binary
systems carefully before attempting to use them to test pre-main sequence
tracks, since the light of an undetected tertiary will make a star appear
younger and more luminous that it is.  Two stars showed extended structure
which appears to be due to the presence of dust halos rather than tertiary
companions, and one of these (Elias~2-49) has a separation which decreases
with wavelength.  Strikingly absent is a clear detection of an edge-on
circumstellar disk in any of the binaries in the sample.


%This is not what would have been predicted for a collection of
%randomly-oriented disks with parameters typical of theoretical models, and
%may point to the action of dust settling, although the possibility that the
%disks in these systems simply have small scaleheights or very small masses
%cannot be ruled out.  



% \clearpage
\acknowledgments

It is a pleasure to thank M. Creech-Eakman for her assistance with the
observations, G. Blake, P. Goldreich, and J. Kwan for useful discussions, E.
Gaidos for useful comments on a version of the manuscript, and to an
anonymous referee for an exceptionally useful report.  Data presented herein
were obtained at the W.M. Keck Observatory, which is operated as a
scientific partnership among the California Institute of Technology, the
University of California and the National Aeronautics and Space
Administration.  The Observatory was made possible by the generous financial
support of the W.M. Keck Foundation.  The author wishes to extend special
thanks to those of Hawaiian ancestry on whose sacred mountain he was
privileged to be a guest.  Without their generous hospitality, and the hard
work and dedication of the Keck Observatory staff, the observations
presented herein would not have been possible.  This research was supported
by the National Aeronautics and Space Administration.


% ******************  References ******************

% \clearpage

\begin{thebibliography}{}

%% Steve's big millimeter photometric survey paper
%\bibitem[beckwith(1990)]{beckwith90} Beckwith, S.V.W., Sargent, A.I.,
%Chini, R.S., \& Guesten, R.
%1990, \aj, 99, 924

% * Physical properties of 90AU to 250AU pre-main-sequence binaries.
\bibitem[brandner(1997)]{brandner97} Brandner, W. \& Zinnecker, H.
1997, \aap, 321, 220

% * Hubble Space Telescope Observations of the Disk and Jet of HH 30
\bibitem[burrows(1996)]{burrows96}Burrows, C.J., Stapelfeldt, K.R., Watson,
A.M., Krist, J.E., Ballester, G.E., Clarke, J.T., Crisp, D., Gallagher,
J.S., III, Griffiths, R.E., Hester, Jeff, Hoessel, J.G., Holtzman, J.A.,
Mould, J.R., Scowen, P.A., Trauger, J.T., \& Westphal, J.A.
1996, \apj, 473, 437

% * Detection of the Elias 2-22 (=DoAr 24e) IRC
\bibitem[chelli(1988)]{chelli88} Chelli, A., Cruz-Gonzalez, I., Zinnecker,
H., Carrasco, L., \& Perrier, C. 1988, A\&A, 207, 46

%% Spectral Energy Distributions of T Tauri Stars with Passive Circumstellar Disks
%\bibitem[chiang(1997)]{chiang97} Chiang, E. I. \& Goldreich, P
%1997, \apj, 490, 368

%% Spectral energy distriutions of passive T Tauri and Herbig Ae disks:
%%grain meneralogy, parameter dependences, and comparison with ISL LWS
%%observations
%\bibitem[chiang(2001)]{chiang2001} Chiang, E.I., Joung, M.K., Creech-Eakman, M.J., Qi, C., Kessler, J.E., Blake, G.A., \& van Dishoeck, E.F.
%2001, {\it ApJ,} 547, 1077

% Adaptive Optics Imaging of the Circumbinary Disk around the T Tauri
% Binary UY Aurigae: Estimates of the Binary Mass and Circumbinary Dust
% Grain Size Distribution
\bibitem[close(1998)]{close98}Close, L. M., Dutrey, A., Roddier, F., Guilloteau, S., Roddier, C., Northcott, M., Menard, F., Duvert, G., Graves, J. E., \& Potter, D.
1998, \apj, 499, 883
 
%% Accretion Disks around Young Objects. II. Tests of Well-mixed Models with ISM Dust
%\bibitem[dalessio(1999)]{dalessio99} D'Alessio, P., Calvet, N., Hartmann, L., Lizano, S., \& Cant\'o, J.
%1999, \apj, 527, 893

% * New pre-main-sequence tracks for M less than or equal to 2.5 solar mass as tests of opacities and convection model
\bibitem[dantona(1994)]{dantona94} D'Antona, F. \& Mazzitelli, I.
1994, \apjs, 90, 467

%%The T-association near rho Ophiuchi
%\bibitem[dolidze(1959)]{dolidze59} Dolidze, M. V. \& Arakelian, M. A.
%1959, AZh, 36, 444

% * The multiplicity of T Tauri stars in the star forming regions
%Taurus-Auriga and Ophiuchus-Scorpius: A 2.2 micron speckle imaging survey
\bibitem[ghez(1993)]{ghez93} Ghez, A.M., Neugebauer, G., \& Matthews, K.
1993, \aj, 106, 2005

% * The Multiplicity of Pre-Main-Sequence Stars in Southern Star-forming Regions
\bibitem[ghez(1997)]{ghez97} Ghez, A.M., McCarthy, D.W., Patience, J.L., \& Beck, T.L
1997, \apj, 481, 378

% * Paper giving a criterion for the stability of triple-star orbits
\bibitem[harrington(1972)]{harrington72} Harrington, R.S. 1972, Celest. Mech.
6, 322

% * Hillenbrand HAEBE SED classification paper
\bibitem[hillenbrand(1992)]{hillenbrand92} 
Hillenbrand, L.A., Strom, S.E., Vrba, F.J., \& Keene, J. 1992, \apj, 397, 613

% * The Connection between Submillimeter Continuum Flux and Binary Separation
%in Young Binaries: Evidence of Interaction between Stars and Disks
\bibitem[jensen(1996)]{jensen96}Jensen, E.L.N., Mathieu, R.D., \& Fuller, G.A.
1996, \apj, 458, 312

%% New pre-main-sequence stars in the Taurus-Auriga molecular cloud
%\bibitem[kenyon(1994)]{kenyon94} Kenyon, Scott J.; Gomez, Mercedes; Marzke, Ronald O.; Hartmann, Lee
%1994, \aj, 108, 251

% * Multiplicity of T Tauri stars in Taurus after ROSAT
\bibitem[kohler(1998)]{kohler98} Kohler, R. \& Leinert, Ch.
1998, \aa, 331, 977

% * The Infrared Companions of T Tauri Stars
\bibitem[koresko(1997)]{kor97} Koresko, C.D., Herbst, T.M., \& Leinert, Ch.
1997, \apj, 480, 741

% * A Circumstellar Disk in a Pre-main-sequence Binary Star
\bibitem[koresko(1998)]{kor98} Koresko, C.D. 1998, \apjl, 507, 145

% * Haro 6-10 IRC paper
\bibitem[koresko(1999)]{kor99} Koresko, C.D., Blake, G.A., Brown, M.E.,
Sargent, A.I., \& Koerner, D.W.
1999, \apjl, 525, 49

% * A Third Star in the T Tauri System 
\bibitem[koresko(2000)]{kor2000} Koresko, C.D. 2000, \apjl, 531, 147

% * A systematic approach for young binaries in Taurus
\bibitem[leinert(1993)]{leinert93}Leinert, Ch., Zinnecker, H., Weitzel,
N., Christou, J., Ridgway, S. T., Jameson, R., Haas, M., \& Lenzen, R.
1993, \aap, 278, 129

% * Eduardo Martin paper giving a spectral type of Elias 2-26
\bibitem[martin(1998)]{Martin98} Martin, E.L., Montmerle, T., Gregorio-Hetem, J., Casanova, S.
1998 \mnras, 300, 733

% * NIRC description
\bibitem[matthews(1994)]{matthews94} Matthews, K. \& Soifer, B.T.
1994, Infrared Astronomy with Arrays:  The Next Generation.  I. McLean, Ed.
(Dordrecht:  Kluwer Acadameic Publishers), 239

% * The first diffraction-limited images from the W.M. Keck Telescope.
\bibitem[matthews(1996)]{matthews96} Matthews, K., Ghez, A.M., Weinberger,
A.J., \& Neugebauer, G. 1996, \pasp, 108, 615 

%% Dust particle settling in passive disks around T Tauri stars: Models and IRAS observations
%\bibitem[miyake(1995)]{miyake95} Miyake, K. \& Nakagawa, Y.
%1995, \apj, 441, 361

% * Infrared imaging photometry of binary T Tauri stars
\bibitem[moneti(1991)]{moneti91} Moneti, A.; Zinnecker, H.
1991, \aap, 242, 428

% * Disks in multiple systems: direct imaging of a nearly edge-on
% circumstellar disk in the young triple system HV Tau
\bibitem[monin(2000)]{Monin2000} Monin, J.-L. \& Bouvier, J.
2000, \aap, 356, L75

% * Are Both Stars in a Classic T Tauri Binary Classic T Tauri Stars?
\bibitem[prato97]{Prato97} Prato, L. \& Simon, M. 1997, \apj, 474, 455

% * Visible-light survey for binaries from which my own target list was drawn
\bibitem[reipurth(1993)]{reipurth93} Reipurth, B. \& Zinnecker, H. 1993,
\aap, 278, 81 (RZ)

% * Clustering of young stars in Taurus, Ophiuchus, and the Orion Trapezium
\bibitem[simon(1997)]{simon97} Simon 1997, \apjl, 482, 81

% * An Edge-On Circumstellar Disk in the Young Binary System HK Tauri
\bibitem[stapelfeldt(1998)]{stp98} Stapelfeldt, K.R., Krist, J.E.,
Menard, F., Bouvier, J., Padgett, D.L., \& Burrows, C.J. 1998, 
\apjl, 502, 65

%% Circumstellar material associated with solar-type pre-main-sequence
%% stars - A possible constraint on the timescale for planet building
%\bibitem[strom(1989)]{strom89}Strom, K.M., Strom, S.E., Edwards, S., Cabrit, S., \&  Skrutskie, M.F.
%1989, \aj, 97, 1451

% An H-alpha emission-line survey of the rho Ophiuchi dark cloud complex
\bibitem[wilking(1987)]{wilking87} Wilking, Bruce A.; Schwartz, Richard D.; Blackwell, James H.
1987, \aj, 94, 106

% * Optical and Near-Infrared Model Images of the Circumstellar Environments of Classical T Tauri Stars
Wood, K., Kenyon, S.J., Whitney, B., \& Turnbull, M.
1998, \apj, 497, 404

%Zinnecker's visible-light ESO survey for binaries, presented in a 
%conference (note that these data were published as a paper by Reipurth & 
%Zinnecker).  
%\bibitem[zinnecker(1992)]{zinnecker92} Zinnecker, H., Brandner, W., \&
%Reipurth, B.  1992, IAU Colloq 135, ASP Conference Series, 32, 50


\end{thebibliography}



\clearpage

%\figcaption[fig3.ps]{
%Visibility amplitudes, plotted against the magnitude of the two-dimensional
%spatial frequency, for the targets in the 14 visual binary systems.  The
%plotted values are azimuthal averages over spatial frequencies at the given
%magnitudes.  They have been normalized to unity at the lowest nonzero
%spatial frequency and shifted for clarity.  The identity of the target star
%in each visual binary is noted in parenthesis.  A constant amplitude
%corresponds to an unresolved target.  The curves for the two well-resolved
%primary-tertiary systems (WSB~18 and B59-1) show damped oscillations
%consistent with well-resolved tertiary companions, while the
%marginally-resolved systems (Sz~116, AS~205, WSB~71, Elias~2-49,
%ESO~H$\alpha$~282, B59-2, and possibly Elias~2-22) show gradual downward
%curvatures.  The amplitudes for WSB~19 and WSB~35 curve {\it upward} at high
%frequencies, which taken at face value would point to extended structure in
%the reference stars, but these power spectra are affected by detector noise
%at high frequencies, as can be seen in the two-dimensional plots in Figure
%\ref{Power2D}.
%\label{Power1D}}

%\figcaption[fig3.ps]{
%Optical depth through a standard disk model as a function of the angle
%between the line of sight and the disk plane.  The steepness of this curve
%shows the sensitivity of the apparent thickness of the disk to the
%scaleheight.  The disk model parameters are typical: $M_{disk} =
%10^{-4}~{\rm M}_\odot$, $\alpha = 2$, $\beta = 1.2$, $R_{inner} = 6~{\rm
%R}_\odot$, $R_{outer} = 250~{\rm AU}$, and $H_{100} = 7.92~{\rm AU}$.
%\label{DiskTauVsScaleheight}}

%\figcaption[fig4.ps]{
%Probability that a randomly-oriented circumstellar disk will produce an
%optical depth $> 5$ along a given line of sight.  With the exception of the
%total mass, which is given by the label for each of the four curves, the disk model parameters
%are as for Figure \ref{DiskTauVsScaleheight}.
%\label{DiskPdetVsScaleheight}}


\clearpage

% ****************  TABLES  ****************



%\begin{deluxetable}{lrrrrrrrrrrr}
%\footnotesize
%\tablecaption{ Additional Infrared Photometry and Imaging
%\label{PhotSummaryExtra}}
%\tablewidth{0pt}
%\tablehead{
%\colhead{Object}
% & \colhead{L'$_{pri}$}
% & \colhead{L'$_{sec}$}
% & \colhead{Sep (\arcsec)} & \colhead{PA (deg)}
%} 
%\startdata
%    HO Lup				&   7.6  &  10.3 &  1.54 &  33.3 \\
%     Sz 81				&   8.9  &   9.6 &  1.95 &  17.5 \\
%     S CrA				&   6.0  &   6.5 &  1.31 & 147.5 \\
%    VV CrA				&   5.3  &   5.4 &  2.10 & 222.3 \\
%\enddata
%\tablecomments{ 
%  Near-infrared L'-band magnitudes and separation vectors for four
%  additional pre-main sequence visual binaries.  The values are as
%  described in Table \ref{PhotSummary}, except that primary in each system
%  is defined here as the brighter star in the L' filter.  Note that this
%  causes a reversal of the position angle for the IRC system VV~CrA as
%  compared to the value measured at visible wavelengths by Reipurth \&
%  Zinnecker (1993).  The uncertainties in the separations and position
%  angles may be somewhat larger than those in Table \ref{PhotSummary} due
%  to the smaller number of measurements. 
%}
%\end{deluxetable}





%\begin{deluxetable}{llll}
%\footnotesize
%\tablecaption{ Parameters for Models of Least-Detectable and Most-Detectable Disks 
%\label{DiskParameterSets}}
%\tablewidth{0pt}
%\tablehead{
%\colhead{Parameter} & \colhead{Least} & \colhead{Most} & \colhead{Comment} \\
%\colhead{} & \colhead{Detectable} & \colhead{Detectable} & \colhead{}
%} 
%\startdata
%H$_{100}$    & 7 AU                & 30 AU                &  Disk scaleheight at 100 AU                                    \\
%$M_{disk}$   & $10^{-5}$ $M_\odot$ & $10^{-2}$ $M_\odot$  &  Disk mass                                                     \\
%$R_{inner}$  & 6 $R_\odot$         & 6 $R_\odot$          &  Disk inner radius                                             \\
%$\alpha$     & 2                   & 2                    &  Power-law index for radial dependence of density				 \\
%$\beta$      & 9/8                 & 6/5                  &  Power-law index for radial dependence of scaleheight     \\
%$R_{outer}$  & 250 AU              & 250 AU               &  Disk outer radius                                             \\
%$P_{detect}$ & 0.08                & 0.55                 &  Detection probability (assuming random orientation)           \\
%\enddata
%\end{deluxetable}



\end{document}


% *** Remove this because the 1D projected power spectra are no longer used
%Figure \ref{Power1D} shows the azimuthally-averaged
%amplitudes (square roots of the power spectra), which are useful for
%distinguishing faint structure near or below the diffraction limit.

astro-ph0205131.extracted_bib
\begin{thebibliography}{}

%% Steve's big millimeter photometric survey paper
%\bibitem[beckwith(1990)]{beckwith90} Beckwith, S.V.W., Sargent, A.I.,
%Chini, R.S., \& Guesten, R.
%1990, \aj, 99, 924

% * Physical properties of 90AU to 250AU pre-main-sequence binaries.
\bibitem[brandner(1997)]{brandner97} Brandner, W. \& Zinnecker, H.
1997, \aap, 321, 220

% * Hubble Space Telescope Observations of the Disk and Jet of HH 30
\bibitem[burrows(1996)]{burrows96}Burrows, C.J., Stapelfeldt, K.R., Watson,
A.M., Krist, J.E., Ballester, G.E., Clarke, J.T., Crisp, D., Gallagher,
J.S., III, Griffiths, R.E., Hester, Jeff, Hoessel, J.G., Holtzman, J.A.,
Mould, J.R., Scowen, P.A., Trauger, J.T., \& Westphal, J.A.
1996, \apj, 473, 437

% * Detection of the Elias 2-22 (=DoAr 24e) IRC
\bibitem[chelli(1988)]{chelli88} Chelli, A., Cruz-Gonzalez, I., Zinnecker,
H., Carrasco, L., \& Perrier, C. 1988, A\&A, 207, 46

%% Spectral Energy Distributions of T Tauri Stars with Passive Circumstellar Disks
%\bibitem[chiang(1997)]{chiang97} Chiang, E. I. \& Goldreich, P
%1997, \apj, 490, 368

%% Spectral energy distriutions of passive T Tauri and Herbig Ae disks:
%%grain meneralogy, parameter dependences, and comparison with ISL LWS
%%observations
%\bibitem[chiang(2001)]{chiang2001} Chiang, E.I., Joung, M.K., Creech-Eakman, M.J., Qi, C., Kessler, J.E., Blake, G.A., \& van Dishoeck, E.F.
%2001, {\it ApJ,} 547, 1077

% Adaptive Optics Imaging of the Circumbinary Disk around the T Tauri
% Binary UY Aurigae: Estimates of the Binary Mass and Circumbinary Dust
% Grain Size Distribution
\bibitem[close(1998)]{close98}Close, L. M., Dutrey, A., Roddier, F., Guilloteau, S., Roddier, C., Northcott, M., Menard, F., Duvert, G., Graves, J. E., \& Potter, D.
1998, \apj, 499, 883
 
%% Accretion Disks around Young Objects. II. Tests of Well-mixed Models with ISM Dust
%\bibitem[dalessio(1999)]{dalessio99} D'Alessio, P., Calvet, N., Hartmann, L., Lizano, S., \& Cant\'o, J.
%1999, \apj, 527, 893

% * New pre-main-sequence tracks for M less than or equal to 2.5 solar mass as tests of opacities and convection model
\bibitem[dantona(1994)]{dantona94} D'Antona, F. \& Mazzitelli, I.
1994, \apjs, 90, 467

%%The T-association near rho Ophiuchi
%\bibitem[dolidze(1959)]{dolidze59} Dolidze, M. V. \& Arakelian, M. A.
%1959, AZh, 36, 444

% * The multiplicity of T Tauri stars in the star forming regions
%Taurus-Auriga and Ophiuchus-Scorpius: A 2.2 micron speckle imaging survey
\bibitem[ghez(1993)]{ghez93} Ghez, A.M., Neugebauer, G., \& Matthews, K.
1993, \aj, 106, 2005

% * The Multiplicity of Pre-Main-Sequence Stars in Southern Star-forming Regions
\bibitem[ghez(1997)]{ghez97} Ghez, A.M., McCarthy, D.W., Patience, J.L., \& Beck, T.L
1997, \apj, 481, 378

% * Paper giving a criterion for the stability of triple-star orbits
\bibitem[harrington(1972)]{harrington72} Harrington, R.S. 1972, Celest. Mech.
6, 322

% * Hillenbrand HAEBE SED classification paper
\bibitem[hillenbrand(1992)]{hillenbrand92} 
Hillenbrand, L.A., Strom, S.E., Vrba, F.J., \& Keene, J. 1992, \apj, 397, 613

% * The Connection between Submillimeter Continuum Flux and Binary Separation
%in Young Binaries: Evidence of Interaction between Stars and Disks
\bibitem[jensen(1996)]{jensen96}Jensen, E.L.N., Mathieu, R.D., \& Fuller, G.A.
1996, \apj, 458, 312

%% New pre-main-sequence stars in the Taurus-Auriga molecular cloud
%\bibitem[kenyon(1994)]{kenyon94} Kenyon, Scott J.; Gomez, Mercedes; Marzke, Ronald O.; Hartmann, Lee
%1994, \aj, 108, 251

% * Multiplicity of T Tauri stars in Taurus after ROSAT
\bibitem[kohler(1998)]{kohler98} Kohler, R. \& Leinert, Ch.
1998, \aa, 331, 977

% * The Infrared Companions of T Tauri Stars
\bibitem[koresko(1997)]{kor97} Koresko, C.D., Herbst, T.M., \& Leinert, Ch.
1997, \apj, 480, 741

% * A Circumstellar Disk in a Pre-main-sequence Binary Star
\bibitem[koresko(1998)]{kor98} Koresko, C.D. 1998, \apjl, 507, 145

% * Haro 6-10 IRC paper
\bibitem[koresko(1999)]{kor99} Koresko, C.D., Blake, G.A., Brown, M.E.,
Sargent, A.I., \& Koerner, D.W.
1999, \apjl, 525, 49

% * A Third Star in the T Tauri System 
\bibitem[koresko(2000)]{kor2000} Koresko, C.D. 2000, \apjl, 531, 147

% * A systematic approach for young binaries in Taurus
\bibitem[leinert(1993)]{leinert93}Leinert, Ch., Zinnecker, H., Weitzel,
N., Christou, J., Ridgway, S. T., Jameson, R., Haas, M., \& Lenzen, R.
1993, \aap, 278, 129

% * Eduardo Martin paper giving a spectral type of Elias 2-26
\bibitem[martin(1998)]{Martin98} Martin, E.L., Montmerle, T., Gregorio-Hetem, J., Casanova, S.
1998 \mnras, 300, 733

% * NIRC description
\bibitem[matthews(1994)]{matthews94} Matthews, K. \& Soifer, B.T.
1994, Infrared Astronomy with Arrays:  The Next Generation.  I. McLean, Ed.
(Dordrecht:  Kluwer Acadameic Publishers), 239

% * The first diffraction-limited images from the W.M. Keck Telescope.
\bibitem[matthews(1996)]{matthews96} Matthews, K., Ghez, A.M., Weinberger,
A.J., \& Neugebauer, G. 1996, \pasp, 108, 615 

%% Dust particle settling in passive disks around T Tauri stars: Models and IRAS observations
%\bibitem[miyake(1995)]{miyake95} Miyake, K. \& Nakagawa, Y.
%1995, \apj, 441, 361

% * Infrared imaging photometry of binary T Tauri stars
\bibitem[moneti(1991)]{moneti91} Moneti, A.; Zinnecker, H.
1991, \aap, 242, 428

% * Disks in multiple systems: direct imaging of a nearly edge-on
% circumstellar disk in the young triple system HV Tau
\bibitem[monin(2000)]{Monin2000} Monin, J.-L. \& Bouvier, J.
2000, \aap, 356, L75

% * Are Both Stars in a Classic T Tauri Binary Classic T Tauri Stars?
\bibitem[prato97]{Prato97} Prato, L. \& Simon, M. 1997, \apj, 474, 455

% * Visible-light survey for binaries from which my own target list was drawn
\bibitem[reipurth(1993)]{reipurth93} Reipurth, B. \& Zinnecker, H. 1993,
\aap, 278, 81 (RZ)

% * Clustering of young stars in Taurus, Ophiuchus, and the Orion Trapezium
\bibitem[simon(1997)]{simon97} Simon 1997, \apjl, 482, 81

% * An Edge-On Circumstellar Disk in the Young Binary System HK Tauri
\bibitem[stapelfeldt(1998)]{stp98} Stapelfeldt, K.R., Krist, J.E.,
Menard, F., Bouvier, J., Padgett, D.L., \& Burrows, C.J. 1998, 
\apjl, 502, 65

%% Circumstellar material associated with solar-type pre-main-sequence
%% stars - A possible constraint on the timescale for planet building
%\bibitem[strom(1989)]{strom89}Strom, K.M., Strom, S.E., Edwards, S., Cabrit, S., \&  Skrutskie, M.F.
%1989, \aj, 97, 1451

% An H-alpha emission-line survey of the rho Ophiuchi dark cloud complex
\bibitem[wilking(1987)]{wilking87} Wilking, Bruce A.; Schwartz, Richard D.; Blackwell, James H.
1987, \aj, 94, 106

% * Optical and Near-Infrared Model Images of the Circumstellar Environments of Classical T Tauri Stars
Wood, K., Kenyon, S.J., Whitney, B., \& Turnbull, M.
1998, \apj, 497, 404

%Zinnecker's visible-light ESO survey for binaries, presented in a 
%conference (note that these data were published as a paper by Reipurth & 
%Zinnecker).  
%\bibitem[zinnecker(1992)]{zinnecker92} Zinnecker, H., Brandner, W., \&
%Reipurth, B.  1992, IAU Colloq 135, ASP Conference Series, 32, 50


\end{thebibliography}