Palomar 13: A velocity dispersion inflated by binaries?

Free access article

Issue		A&A Volume 419, Number 2, May IV 2004


Page(s)		533 - 541
Section		Galactic structure, stellar clusters and populations
DOI		http://dx.doi.org/10.1051/0004-6361:20041033

A&A 419, 533-541 (2004)
DOI: 10.1051/0004-6361:20041033

Palomar 13 : A velocity dispersion inflated by binaries?

A. Blecha¹, G. Meylan², P. North³ and F. Royer¹

¹ Observatoire de Genève, Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland
    e-mail: frederic.royer@obs.unige.ch
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
    e-mail: gmeylan@stsci.edu
³ Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne, Observatoire, 1290 Chavannes-des-Bois, Switzerland
    e-mail: Pierre.North@obs.epfl.ch

(Received 14 January 2004 / Accepted 19 February 2004 )

Abstract
Recently, combining radial velocities from Keck/HIRES échelle spectra with published proper motion membership probabilities, Côté et al (2002) observed a sample of 21 stars, probable members of Palomar 13, a globular cluster in the Galactic halo. Their projected velocity dispersion $\sigma_\mathrm{p} = 2.2 \pm 0.4$ km s ^-1 gives a mass-to-light ratio $\mathcal{M}/L_V = 40^{+24}_{-17}$ , about one order of magnitude larger than the usual estimate for globular clusters. We present here radial velocities measured from three different CCD frames of commissioning observations obtained with the new ESO/VLT instrument FLAMES (Fibre Large Array Multi Element Spectrograph). From these data, now publicly available, we measure the homogeneous radial velocities of eight probable members of this globular cluster. A new projected velocity dispersion $\sigma_\mathrm{p} = 0.6$ - $0.9 \pm 0.3$ km s ^-1 implies Palomar 13 mass-to-light ratio $\mathcal{M}/L_V = 3$ - 7, similar to the usual value for globular clusters. We discuss briefly the two most obvious reasons for the previous unusual mass-to-light ratio finding: binaries, now clearly detected, and more homogeneous data from the multi-fibre FLAMES spectrograph.

Key words: Galaxy: globular clusters: individual: Palomar 13 -- techniques: radial velocities -- stars: kinematics -- stars: population II

Offprint request: A. Blecha, andre.blecha@obs.unige.ch

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