Dust embedded sources at the Galactic Center^*

2 to 4 μm imaging and spectroscopy in the central parsec

¹ I Physikalishes Institut, Zülpicher Str. 77, 50937 Köln, Germany e-mail: moultaka@ph1.uni-koeln.de
² Max Planck Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany

Received: 4 December 2003
Accepted: 3 June 2004

Abstract

We present the first L-band spectroscopic observations of a dozen stellar sources in the central 0.5 pc of the GC stellar cluster that are bright in the 2-4 μm wavelength domain. The L-band data were taken with ISAAC at the VLT UT1 (Antu). With the aid of additional K-band spectroscopic data we derive the optical depth spectra of the sources after fitting their continuum emission with a single reddened blackbody continuum. We also derive intrinsic source spectra by correcting the line of sight extinction via the optical depth spectrum of a late type star that is most likely not affected by local dust emission or extinction at the Galactic Center. The good agreement between the two approaches shows that the overall variation of the line-of-sight extinction across the central 0.5 pc is  mag. The extinction-corrected spectra of the hot He-stars resemble pure Rayleigh-Jeans continuum spectra. The intrinsic spectra of all other sources are in agreement with being the result of the continuum emission and absorption features due to the dust in which they are embedded. We interprete both facts as evidence that a significant amount of the absorption takes place within the central parsec of the Galactic Center and is most likely associated with the individual sources there. We find absorption features at m,  m, and m wavelength. Correlations between all three features show that they are very likely to arise in the ISM of the central 0.5 pc. Spatially highly variable hydrogen emission lines seen towards the individual sources give evidence of the complex density and temperature structure of the mini-spiral. The featureless K-band spectra of sources like IRS 21 and IRS 1W are consistent with these sources being massive hot stars embedded in the bow shock created by their motion through the dust and gas of the mini-spiral. The bow shock scenario may be applicable to most of the dust-embedded sources in the central stellar cluster. Spectroscopy of high MIR-excess sources 0.5″ north of the IRS 13 complex is largely consistent with them being YSOs. However, a bow-shock nature of these sources cannot be excluded. The L-band spectrum at the location of SgrA* closely resembles that of a hot O-type star, such as S2, which was very close to Sgr A* at the time of our observations.