A Breakthrough in Astronomical Software Development
Margaret Meixner\altaffilmark{1}
Letizia Stanghellini
% Howard Bond
% and the start of your text. \begin{abstract} % Leave intact I present the results of time-resolved photometry of a selection of central stars of planetary nebulae. The study reveals periodic variability in two of the eight central stars observed, those of NGC 6026 and NGC 6337. The variability matches that expected from a binary system in which a hot primary irradiates a cooler secondary star.
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\title{Two New Close Binary Central Stars of Planetary Nebulae from a Critically Selected Southern Hemisphere Sample}
\titlemark{Two New Close Binary CSPNe}

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\author{Todd Hillwig}
\affil{Department of Physics and Astronomy, Georgia State University}

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\keywords{NGC 6026, NGC 6337, binary stars, photometry}

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\begin{abstract}          % Leave intact
I  present the results of time-resolved photometry of a selection of
central stars of planetary nebulae.  The study reveals periodic variability
in two of the eight central stars observed, those of NGC 6026 and NGC 6337.
The variability matches that expected from a binary system in which a hot
primary irradiates a cooler secondary star.

\end{abstract}

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\section{Introduction}

Soker (1997) explained planetary nebula (PN) structures using orbital
interactions in a binary system, with the companion to the PN progenitor 
being either stellar or substellar (brown dwarf or planet).  Using this 
theory he classified a large number of PNe, based on their morphology, as 
either: single progenitor, close stellar companion with no common 
envelope (CE) phase, close stellar companion with a CE phase, or 
substellar companion with a CE phase.  These predictions provide a good 
basis for testing the binary theory of PN shaping.

It is understood that for close stellar companions undergoing a CE phase, a 
significant fraction should become systems with orbital periods on the 
order of a few days or less.  To 
date, thirteen central stars of planetary nebulae (CSPNe) have been 
identified as close binaries, with orbital periods determined to be $< 16$ 
days (Bond 2000).  Bond (2000) gives the fraction of 
detectable close binaries from a random sample as $\sim 10-15\%$.

Here I present the use of time 
resolved photometry of CSPNe to search for the sinusoidal variations in 
brightness that would be associated with an irradiated hemisphere of a 
stellar companion to the central star.  Eight southern hemisphere CSPNe
classified by Soker (1997) as having a stellar companion that underwent
a CE phase were observed.  The results support the 
classification of NGC 6026 and NGC 6337 as close binary stars.  I 
present orbital periods for these systems.  My detected binary fraction 
is also compared to that discovered previously.

\section {Observations}

The photometric data consist of V-band CCD photometry obtained from 30
April - 4 May, 2002 at the Cerro Tololo Interamerican Observatory 
(CTIO) 0.9m telescope with the T2K Imager.  The observations were 
carried out such that every object was observed each night for at least 
one hour and the five-night spacing was altered to reduce as many aliases 
as possible in the period search algorithm to be utilized.

The exposures were reduced using the DAOPHOT package of IRAF.
The resulting photometry was then analyzed by 
incomplete ensemble photometry (Honeycutt 1992).  The zero points are 
instrumental magnitudes dependent on the instrumental response of the 
CTIO 0.9m system.  

The light curves were analyzed with {\it periodogram}, a 
program which uses the period search technique of Scargle (1982) as 
modified by Horne \& Baliunas (1986).

Six of the eight stars showed no unambiguous variability.  Five of the
six: He 2-141, NGC 3132, NGC 4361, NS 238, and Sp 3, where observed
on at least four nights, with $55\leq t \leq 78$ minutes of consecutive
observations on at least one of those nights.  K 1-1 was observed on
three nights with a maximum duration of 64 minutes.

\section {NGC 6026}

The light curve of NGC 6026 (Figure \ref{6026}) shows obvious
variability and suggests a well-behaved periodic nature over the four
nights on which observations were made.
\begin{figure}[t]
\begin{center}
\epsscale{.50}
\plottwo{6026_lcurve.ps}{6026_phase.ps}
\end{center}
\caption{Light curve for NGC 6026 by night (\textbf{left}) and the
phase-folded light curve of NGC 6026
for the ephemeris in equation \ref{eq2}(\textbf{right}).}\label{6026}
\end{figure}
The periodogram for NGC 6026 showed periodic
variability with a strong one day alias, resulting in a preliminary period
of 0.263 days.  This 
period was taken as a starting point for fitting a sine curve of the form
\begin{equation}
V=V_{mean}+Ksin\Biggl[\frac{2\pi(T-T_o)}{P}+\phi\Biggr]
\end{equation}
where $V$ is the instrumental magnitude, $K$ is the semiamplitude,
and $P$ is the period; either $T_o$ or $\phi$ is a fitted parameter.
For the photometry we fit for $T_o$ plus the remaining parameters and
set $\phi$ = 0.75 as is appropriate for variation dominated by an
irradiation effect.  This produces an ephemeris 
with minimum light at phase zero:
\begin{equation}
T=2452394.945(1)+0.2636(2)E.
\label{eq2}
\end{equation}
Figure \ref{6026} shows the light curve folded on this ephemeris and the
sine curve fit.  The rms variation of the data from the fitted sine curve 
is 0.010 magnitude.

The remaining parameters from the sine fitting are 
$V_{mean}=9.3210(7)$ mag (instrumental) and $K=0.087(1)$ mag.

\section {NGC 6337}

As with NGC 6026, the light curve of NGC 6337 in Figure \ref{6337} shows
apparently well-behaved periodic variability.
\begin{figure}[t]
\begin{center}
\epsscale{.50}
\plottwo{6337_lcurve.ps}{6337_phase.ps}
\end{center}
\caption{Light curve for NGC 6337 by night (\textbf{left}) and the
phase-folded light curve of NGC 6337
for the ephemeris in equation \ref{eq3}(\textbf{right}).}\label{6337}
\end{figure}
The periodogram showed periodic variability with a strong one day alias.
The peak with the most power was located at a period of 0.173 days.
Since NGC 6337 appears as a ring, the nebular background close to the 
central star is at a minimum, allowing more accurate photometry.  This, 
along with the obvious slope to each night of observation allowed all 
but the highest peak to be rejected.  The corresponding period was 
taken as a starting point for fitting a sine curve of the same form 
as Equation 1, for NGC 6026.  The derived photometric ephemeris for 
minimum light in NGC 6337 is then
\begin{equation}
T=2452395.7969(5)+0.1735(3)E.
\label{eq3}
\end{equation}
Figure \ref{6337} shows the light curve folded on this ephemeris and
the sine curve fit.  The rms variation of the data from the fitted
sine curve is 0.016 magnitude.

The remaining parameters from the sine fitting are 
$V_{mean}=12.041(1)$ mag (instrumental) and $K=0.209(3)$ mag.

\section {Comments on Masses, Inclinations, and Binary Fractions}

The short orbital periods allow limits to be placed on several of
the binary system parameters. Taking the masses of both 
CSs to be $\sim 0.6 M_\odot$, typical of CSPNe, Kepler's 2nd law gives
the  binary separation as a function of secondary mass.  The small
deviation from a sine curve, short orbital
periods, and significant reflection effects observed
in the two systems suggest that the companions are cool dwarf stars
which do not fill, or just fill, their Roche lobes.
To find an upper limit for the secondary masses, I will assume that
both secondaries are Main Sequence stars and do not fill their
Roche lobes.  This condition is met for the two systems,
according to their corresponding orbital periods and for
$M_1 = 0.6 M_\odot$, by $M_2(NGC 6026) \leq 0.8 M_\odot$ and
$M_2(NGC 6337) \leq 0.3 M_\odot$.  These correspond to binary
separations of $a(NGC 6026) \leq 1.93 R_\odot$ and
$a(NGC 6337) \leq 1.26 R_\odot$.

A few comments may be made about the binary system inclinations as well.
Corradi et al. (2000) show that the ring-like structure of NGC 6337 is
the narrow waist of a nearly pole-on bipolar PN.  The inclination of 
the PN then must be $\leq 15^\circ$ for the bright inner ring to have no 
apparent ellipticity.
NGC 6026 does not appear ring-shaped, but as a partial ellipse with a 
very faint south-east edge.  If this PN is what Bond (2000) refers to as
a ``wedding ring'' PN, then the inclination must be intermediate and can be
roughly determined from the ellipticity of the observed PN.  The ellipse
measures $\sim32\arcsec \times 42\arcsec$, giving an inclination of
$\sim40^\circ$.  If the binary mechanism for shaping PNe is 
correct, then the close binary CSs must have inclinations equal to
those of the PNe.

Finally, previous studies have found the fraction of detectable close
binary CSPNe to be $\sim10-15\%$ (Bond 2000).  The binary fraction
found here is 25\%, 
much higher than previous studies, though the low number of statistics 
means that the results are reasonably similar.  Since the sample 
observed in this study was not randomly selected, but was based on 
the classifications of Soker (1997), it would be interesting to 
increase the number of CSPNe observed. If the 25\% binary fraction 
persisted, the binary mechanism for PN shaping would be strongly supported, 
specifically the scenario outlined by Soker (1997). 

\acknowledgements
I would like to thank Bill Bagnuolo, Doug Gies, and Bill Nelson for
their generous support and Todd Henry and Alberto Miranda for their 
assistance with the CTIO 0.9m telescope.  This research was funded
by Georgia State University.  

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%			      References
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%           \reference King, I.\ R.\  1975, in Dynamics of Stellar 
%                Systems, ed.\ A.\ Hayli (Dordrecht: Reidel), 99
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\reference Corradi, R.\ L.\ M., Goncalves, D.\ R.\, Villaver, E.\,
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\reference Honeycutt, R.\ K.\ 1992, PASP, 104, 435

\reference Horne, J.\ H.\ \& Baliunas, S.\ L.\ 1986, ApJ, 302, 757 

\reference Scargle, J.\ D.\, 1982, ApJ, 263, 835

\reference Soker, N.\ 1997, ApJS, 112, 487


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