Star Clusters in the Galactic Anticenter Stellar Structure: New Radial Velocities \& Metallicities
Peter M. Frinchaboy\inst{1}
Ricardo R. Mu\~{n}oz\inst{1}
Steven R. Majewski\inst{1}
Eileen D. Friel\inst{2}
Randy L. Phelps\inst{2,3}
William B. Kunkel\inst{4}
The Galactic Anticenter Stellar Structure (GASS) has been identified with excess surface densities of field stars in several large area sky surveys, and with an unusual, string-like grouping of star clusters. Some members of the cluster grouping have radial velocities (RVs) consistent with the observed GASS velocity-longitude trend. We provide new RV measurements of stars in six clusters that have been suggested to be associated with the GASS. We show that the RVs of at least four clusters are consistent with the previously measured RV trend for GASS. We also derive spectroscopic metallicities for four clusters, and provide an improved age-metallicity relation for the clusters apparently associated with GASS.
frinchaboy.tex
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\begin{document}
%
\title*{Star Clusters in the Galactic Anticenter Stellar Structure:
 New Radial Velocities \& Metallicities}
%
%
\toctitle{Star Clusters in the Galactic Anticenter Stellar Structure:
\protect\newline New Radial Velocities \& Metallicities}
% allows explicit linebreak for the table of content
%
%
\titlerunning{Star Clusters in the GASS}
% allows abbreviation of title, if the full title is too long
% to fit in the running head
%
\author{Peter M. Frinchaboy\inst{1}
\and Ricardo R. Mu\~{n}oz\inst{1}
\and Steven R. Majewski\inst{1}
\and \\ Eileen D. Friel\inst{2}
\and Randy L. Phelps\inst{2,3}
\and William B. Kunkel\inst{4}}
%
\authorrunning{Peter Frinchaboy et al.}
% if there are more than two authors,
% please abbreviate author list for running head
%
%
%\institute{Princeton University, Princeton NJ 08544, USA
%\and Universit\'{e} de Paris-Sud,
%     Laboratoire d'Analyse Num\'{e}rique,
%     B\^{a}timent 425,\\
%     F-91405 Orsay Cedex, France}

\institute{University of Virginia, P. O. Box 3818, Charlottesville, VA 22903, USA
\and National Science Foundation, 4201 Wilson Boulevard, Arlington, Virginia 22230 
\and C.~S.~U., Sacramento, 6000 J Street, Sacramento, CA 95819
%\and Department of Physics, U.~C., Davis, 1 Shields Avenue, Davis, CA 95616
\and Las Campanas Observatory, Casilla 601, La Serena, Chile}


\maketitle              % typesets the title of the contribution

\begin{abstract}
The Galactic Anticenter Stellar Structure (GASS) has been identified
with excess surface densities of field stars in several large area sky
surveys, and with an unusual, string-like grouping of star clusters.
Some members of the cluster grouping
have radial velocities (RVs) consistent with the observed GASS velocity-longitude trend.
We provide new RV measurements of stars in six clusters
that have been suggested to be associated with the GASS. We show
that the RVs of at least four clusters are
consistent with the previously measured RV trend for GASS. We also derive
spectroscopic metallicities for four clusters, and provide an
improved age-metallicity relation for the clusters apparently associated with GASS.
\end{abstract}


%\section{Introduction}

We have found that the outer most open clusters found to date seem to 
be strung along the Galactic
anticenter stellar structure (GASS) \cite{fri:pmf04}.  
This structure \cite{fri:newberg}, \cite{fri:yanny}, \cite{fri:ibata}, \cite{fri:majewski},  
\cite{fri:helio}, \cite{fri:crane} was discovered as an excess of stars beyond the apparent 
limit of the Galactic disk.
% and has
%been used to argue for the presence of a distinct, extended stellar
%structure wrapping around the disk at low latitudes and has been referred 
%to as the Moneceross ``Ring'' \cite{fri:newberg}, \cite{fri:yanny}, the Canis Major galaxy
%\cite{fri:ibata}, and GASS \cite{fri:majewski},  \cite{fri:helio}, \cite{fri:crane}.  
%However, because of unfortunate placement behind considerable
%extinction, it has been difficult to get information on the system's
%true shape, orientation, breadth, etc.  Even the location of the
%structure's center (presumably corresponding to a ``nucleus'') remains
%uncertain.
Previous work \cite{fri:crane} has resulted in a number of parameters of the stream including: 
(1) a velocity-longitude trend indicating
a slightly non-circular orbit, (2) a velocity dispersion smaller than
even that of disk stars, (3) a wide metallicity spread from $-1.6 <$ [Fe/H]$ < -0.4$.
%Additionally, we found in \cite{fri:pmf04} that five globular clusters 
%and one open cluster, Tombaugh 2,
%are apparently associated with the stream based on both position
%{\it and} radial velocity (RV). 
%These clusters (Pal 1, NGC 2808, NGC 5286, NGC 2298, XX, XX, XX).


%\section{Radial Velocities}
%
%Text
%\section{Metallicity Analysis}

Spectra for stars in the clusters Berkeley 29, BH 176, and Saurer 1 were collected with the Hydra Spectrograph 
%on the 4-meter telescope at Cerro Tololo Intra-American Observatory. 
and clusters Berkeley 22,  BH 144(ESO 096-SC04), and ESO 093-SC08 were collected 
%UT 2004 August 24--28 
using the RC Spectrograph on 
the Blanco 4-meter telescope. 
The data were reduced using standard IRAF reduction methodology with
%Following overscan correction, trimming, and bias subtraction, the data were
%flatfielded using done lamp exposures that had been spectrally normalized
%to remove the large-scale spectral response.  Spectra were then extracted, 
%sky subtracted, and wavelength-calibrated 
%using comparison lamp spectra taken at the same telescope position 
%as the targets.
RVs determined using {\ttfamily fxcor}. 
%with Heliocentric RV ($V_r$) 
%and Galactocentric RV ($V_{gsr}$) shown in Table~\ref{fri:Tab1}. 
The CaII infrared triplet was measured using the index definitions of 
\cite{fri:az}, transformed to the common system derived in \cite{fri:cole}.
%
%We measured spectroscopic indices for the Ca II infrared triplet. 
%Strong features centered approximately at 8498, 8542, and 8662 were used. 
%The indices are formed by the ratio of the average flux in a bandpass centered on the feature to 
%that in two closely placed regions that reflect the local pseudocontinuum, expressed as a 
%magnitude. 
%The bandpass definitions are summarized in Armandroff \& Zinn (1988). 
%These features are those commonly measured in globular cluster studies 
%The bandpass definitions are summarized in \cite{fri:az}. 
%System offsets were then applied transforming all of the methods to the 
%common system derived in \cite{fri:cole}.  
%
The resulting RVs and metallicities are presented in Table~\ref{fri:Tab1}.  The typical error 
in the metallicities ($\sigma_{[Fe/H]}$) are $\sim$ 0.3 dex.  All clusters measured are 
shown with the GASS longitude-velocity ( $l$ vs. $V_{gsr}$) trend (Fig.~\ref{fri:f1}a) 
and members are used to produce an 
age-metallicity relation %for the GASS cluster system 
(Fig.~\ref{fri:f1}b). 
We find that the clusters Berkeley 29, Saurer 1, ESO 0903-SC08, and possibly BH 144 \& 176, have 
RVs %and/or metallicities 
consistent with being part of the GASS cluster 
system; however, further work is needed to confirm actual membership in GASS. 


\begin{table}
\caption{GASS Cluster Candidate Radial Velocities and Metallicities        }
\begin{center}
\renewcommand{\arraystretch}{0.93}
\setlength\tabcolsep{5pt}
\begin{tabular}{lccccccc}
\hline\noalign{\smallskip}
Cluster & $l$  &  $b$  & \# stars & $V_r$       & $V_{gsr}$
  & [Fe/H] & GASS?\\[-0.5ex]
        &      &       & \scriptsize{Metallicity(RV)}& \scriptsize{(km/s)} &  \scriptsize{(km/s)}
  &        &      \\
\noalign{\smallskip}
\hline
\noalign{\smallskip}
Berkeley 22            & 199.8 &  $-$8.1  &  4(5)  & 106 $\pm$ 9  &  $+$16 & $-$0.97 & N \\
Berkeley 29            & 198.0 &  $+$8.0  &  8(8)  &  26 $\pm$ 6  &  $-$52 & $-$0.62 & Y \\
Berkeley 20            & 203.5 & $-$17.3  &  3(3)  &  78 $\pm$ 6  &  $-$19 & $-$0.68 & (Cal) \\
Saurer 1               & 214.3 &  $-$6.8  &  2(4)  &  98 $\pm$ 9  &  $-$39 & $-$0.49 & Y$^1$ \\
Berkeley 39            & 223.5 & $+$10.1  & 19(19) &  59 $\pm$ 4  & $-$103 & $-$0.27 & (Cal) \\
ESO093$-$SC08          & 293.5 &  $-$4.0  &  0(1)  &  86 $\pm$ 10 & $-$125 &  ...... & Y \\
BH 144  \scriptsize{(ESO096$-$SC04)}   
                       & 305.3 &  $-$3.2  &  2(3)  &  40 $\pm$ 10 & $-$146 & $-$0.51 & ?$^2$ \\
47 Tuc                 & 305.9 & $-$44.9  &  5(.)  &  .......     &  ..... & $-$0.78 & (Cal) \\
BH 176                 & 328.4 &  $-$4.3  &  0(3)  &  13 $\pm$ 5  & $-$100 &  ...... & Y$^2$ \\
\hline 
\end{tabular}
\end{center}
%\renewcommand{\arraystretch}{0.3}
{$^1$\scriptsize 
We find Saurer 1 to be a member due to its $V_{gsr}$; \cite{fri:carraro} did not correct this RV to $V_{gsr}$ before \\[-1ex] 
incorrectly excluding Saurer 1 from the GASS member clusters.  $^2$ It is noted from the spatial \\[-1ex]
distribution of the clusters that GASS should have an elliptical orbit.  If this change is made, \\[-1ex] 
BH 144 and BH 176, along with NGC5286 and NGC 2808, should fit the corrected $l-v_{GSR}$ trend.
 \\[-7ex]} 
%\end{center}
\label{fri:Tab1}
\end{table}

We acknowledge funding by NSF grant AST-0307851, NASA/JPL contract 1228235,
the David and Lucile Packard Foundation, Robert J. Huskey Travel fellowship, 
AAS International Travel Grant, 
and the F.H. Levinson Fund of the Peninsula Community Foundation.
%
\begin{figure}[b]
\begin{center}
\includegraphics[width=.580\textwidth]{frinchaboyF1a.eps} %\label{f1a}
\includegraphics[width=.355\textwidth]{frinchaboyF1b.eps} %\label{f1b}
\end{center}
\caption[]{(a,left) The $l-v_{GSR}$ distribution of objects lying
  within 2.35 kpc of the GASS cluster plane.  
  %Corrections to $v_{gsr}$ assume a solar apex of
  %$(\alpha,\delta)=(18h,30\arcdeg)$ at 20.0 km s$^{-1}$ and an LSR
  %rotation of 220 km s$^{-1}$.  
  The hash marks represent a velocity
  dispersion of 30 km s$^{-1}$ about the $v_{gsr}$ of a circularly
  orbiting object at r$_{GC}=18$ kpc with $v_{circ}=220$ km s$^{-1}$,
  which reasonably matches GASS M giant velocities\cite{fri:crane}. 
  (b,right) AMR of all GASS candidate clusters.  
Circles denote values measured and listed in table, triangles are data
from \cite{fri:pmf04}.}
\label{fri:f1}
%\label{f1b}
%\label{eps1}
\end{figure}
%
\begin{thebibliography}{8.}
\addcontentsline{toc}{section}{References}
%
\bibitem{fri:az} Armandroff, Zinn, AJ, 96, 92 (1988) 

\bibitem{fri:carraro} Carraro, Bresolin, Villanova, Matteucci, Patat, Romaniello,
 (astro-ph/0406679)
  
\bibitem{fri:cole} Cole, Smecker-Hane, Tolstoy, Bosler, Gallagher, MNRAS, 347, 367 (2004)

\bibitem{fri:crane} Crane, Majewski,
  Rocha-Pinto, Frinchaboy, Skrutskie, Law, ApJL, 594, L119 (2003) 
  
\bibitem{fri:pmf04} Frinchaboy,~P., et al., ApJL, 602, L21 (2004)
%Majewski, Reid, Crane, Rocha-Pinto, Phelps, Patterson, Mu\~{n}oz, 

\bibitem{fri:ibata} Ibata, Irwin, Lewis, 
  Ferguson, Tanvir, MNRAS, 340, L21 (2003)
  
\bibitem{fri:majewski} Majewski, Skrutskie, 
  Weinberg, Ostheimer, AJ, 128, 245 (2003)
  
\bibitem{fri:newberg} Newberg,~H., et al.,
  ApJ, 569, 245 (2002)
  
\bibitem{fri:helio} Rocha-Pinto, Majewski, 
   Skrutskie, Crane, ApJL, 594, L115 (2003)
  
\bibitem{fri:yanny} Yanny,~B., et al., ApJ, 588, 824 (2003)

%\bibitem{fri:journ1} W. Frank, A. Seeger: Appl. Phys. A \textbf{3}, 66 (1988)
%
%\bibitem{fri:proc1} W. Greiner, D.N. Poenaru: `Cluster Preformation in
%Closed- and Mid-shell Nuclei'. In:
%\emph{Atomic and Nuclear Clusters, 2nd International Conference at Santorini, Greece,
%June 28--July 2, 1993}, ed. by G.S. Anagnostatos, W. von Oertzen (Springer, Heidelberg 1994)
%pp. 264--266
%
%\bibitem{fri:fhjl1} F. Holzwarth, J. Lenz  et al.: \emph{1readme.
%Further Details on Layout and \LaTeX\, code}. (Springer, Berlin
%Heidelberg 1999)
%\bibitem{fri:proc2} W. Greiner, D.N. Poenaru: `Cluster Preformation in
%Closed- and Mid-shell Nuclei'. In:
%\emph{Atomic and Nuclear Clusters, 2nd International Conference at Santorini, Greece,
%June 28--July 2, 1993}, ed. by G.S. Anagnostatos, W. von Oertzen (Springer, Heidelberg 1994)
%pp. 264--266
%
%\bibitem{fri:fhjl2} F. Holzwarth, J. Lenz  et al.: \emph{1readme.
%Further Details on Layout and \LaTeX\, code}. (Springer, Berlin
%Heidelberg 1999)
%
\end{thebibliography}


%INDEX%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Please check with the editor of your book whether he plans to
% include a "mutual" subject index - if so, please code your entries
% in the standard syntax. For your own purposes you may print your
% "personal" index by using the following commands:
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\end{document}


astro-ph0411127.extracted_bib
\begin{thebibliography}{8.}
\addcontentsline{toc}{section}{References}
%
\bibitem{fri:az} Armandroff, Zinn, AJ, 96, 92 (1988) 

\bibitem{fri:carraro} Carraro, Bresolin, Villanova, Matteucci, Patat, Romaniello,
 (astro-ph/0406679)
  
\bibitem{fri:cole} Cole, Smecker-Hane, Tolstoy, Bosler, Gallagher, MNRAS, 347, 367 (2004)

\bibitem{fri:crane} Crane, Majewski,
  Rocha-Pinto, Frinchaboy, Skrutskie, Law, ApJL, 594, L119 (2003) 
  
\bibitem{fri:pmf04} Frinchaboy,~P., et al., ApJL, 602, L21 (2004)
%Majewski, Reid, Crane, Rocha-Pinto, Phelps, Patterson, Mu\~{n}oz, 

\bibitem{fri:ibata} Ibata, Irwin, Lewis, 
  Ferguson, Tanvir, MNRAS, 340, L21 (2003)
  
\bibitem{fri:majewski} Majewski, Skrutskie, 
  Weinberg, Ostheimer, AJ, 128, 245 (2003)
  
\bibitem{fri:newberg} Newberg,~H., et al.,
  ApJ, 569, 245 (2002)
  
\bibitem{fri:helio} Rocha-Pinto, Majewski, 
   Skrutskie, Crane, ApJL, 594, L115 (2003)
  
\bibitem{fri:yanny} Yanny,~B., et al., ApJ, 588, 824 (2003)

%\bibitem{fri:journ1} W. Frank, A. Seeger: Appl. Phys. A \textbf{3}, 66 (1988)
%
%\bibitem{fri:proc1} W. Greiner, D.N. Poenaru: `Cluster Preformation in
%Closed- and Mid-shell Nuclei'. In:
%\emph{Atomic and Nuclear Clusters, 2nd International Conference at Santorini, Greece,
%June 28--July 2, 1993}, ed. by G.S. Anagnostatos, W. von Oertzen (Springer, Heidelberg 1994)
%pp. 264--266
%
%\bibitem{fri:fhjl1} F. Holzwarth, J. Lenz  et al.: \emph{1readme.
%Further Details on Layout and \LaTeX\, code}. (Springer, Berlin
%Heidelberg 1999)
%\bibitem{fri:proc2} W. Greiner, D.N. Poenaru: `Cluster Preformation in
%Closed- and Mid-shell Nuclei'. In:
%\emph{Atomic and Nuclear Clusters, 2nd International Conference at Santorini, Greece,
%June 28--July 2, 1993}, ed. by G.S. Anagnostatos, W. von Oertzen (Springer, Heidelberg 1994)
%pp. 264--266
%
%\bibitem{fri:fhjl2} F. Holzwarth, J. Lenz  et al.: \emph{1readme.
%Further Details on Layout and \LaTeX\, code}. (Springer, Berlin
%Heidelberg 1999)
%
\end{thebibliography}