Near-infrared proper motions and spectroscopy of infrared excess sources at the Galactic center^⋆

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: eckart@ph1.uni-koeln.de
³ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19, 19001 Santiago, Chile
⁴ Astronomical Institute, Academy of Sciences, Bocni II 1401, 14131 Prague, Czech Republic
⁵ Université de Toulouse, UPS-OMP, IRAP, 31400 Toulouse, France
⁶ CNRS, IRAP, 14 avenue Édouard Belin, 31400 Toulouse, France

Received: 12 July 2012
Accepted: 23 November 2012

Abstract

Context. There are a number of faint compact infrared excess sources in the central stellar cluster of the Milky Way. Their nature and origin is unclear. In addition to several isolated objects of this kind there is a small but dense cluster of comoving sources (IRS13N) located  ~3′′ west of SgrA* just 0.5′′ north of the bright IRS13E cluster of Wolf-Rayet and O-type stars. Based on the analysis of their color and brightness, there are two main possibilities: (1) they may be dust-embedded stars older than a few Myr; or (2) very young, dusty stars with ages younger than 1 Myr.

Aims. We present a first K_s-band identification and proper motions of the IRS13N members, the high-velocity dusty S-cluster object (DSO, also referred to as G2), and other infrared excess sources in the central field. Goal is to constrain the nature of these source.

Methods. The L′- (3.8 μm) K_s- (2.2 μm) and H-band (1.65 μm) observations were carried out using the NACO adaptive optics system at the ESO VLT. Proper motions were obtained by linear fitting of the stellar positions extracted by StarFinder as a function of time, weighted by positional uncertainties, and by Gaussian fitting from high-pass filtered and deconvolved images. We also present results of near-infrared (NIR) H- and K_s-band ESO-SINFONI integral field spectroscopy of the Galactic center cluster ISR13N.

Results. We show that within the uncertainties, the positions and proper motions of the IRS13N sources in K_s- and L′-band are identical. The HK−sL′ colors then indicate that the bright L′-band IRS13N sources are indeed dust-enshrouded stars rather than core-less dust clouds. The proper motions also show that the IRS13N sources are not strongly gravitationally bound to each other. Combined with their NIR colors, this implies that they have been formed recently. For the DSO we obtain proper motions and a K_s-L′-color.

Conclusions. Most of the compact L′-band excess emission sources have a compact H- or K_s-band counterpart and therefore are likely stars with dust shells or disks. Our new results and orbital analysis from our previous work favor the hypothesis that the infrared excess IRS13N members and other dusty sources close to SgrA* are young dusty stars and that star formation at the Galactic center (GC) is a continuously ongoing process. For the DSO the color information indicates that it may be a dust cloud or a dust-embedded star.