L- and M-band imaging observations of the Galactic Center region

In this paper we present near-infrared H-, K-, L- and M-band photometry of the Galactic Center from images obtained at the ESO VLT inMay and August 2002, using the NAOS/CONICA ( H and K) and the ISAAC( L and M) instruments**. The large field of view (70 '' x 70 '') of the ISAAC instrument and the large number of sources identified ( L- M data for a total of 541 sources) allows us to investigate their colors, infrared excesses and the extended dust emission. Our new L- band magnitude calibration reveals an important offset to the traditionally used standard calibrations, which we attribute to the use of the variable star IRS 7 as a flux calibrator. Together with new results on the extinction towards the Galactic Center ( Scoville et al. 2003; Raab 2000), our magnitude calibration results in stellar color properties expected from standard stars and removes any necessity to modify the K- band extinction. The large number of sources for which we have obtained L - M colors allows us to measure the M-band extinction to A(M) = ( 0.056 +/- 0.006) AV, i. e. A(M) approximate to A(L), a considerably higher value than what has so far been assumed. L - M color data has not been investigated previously, due to lack of useful M-band data. We find that this color is a useful diagnostic tool for the preliminary identification of stellar types, since hot and cool stars show a fairly clear L - M color separation, with average L - M colors of 0.46 +/- 0.17 for emission- line stars and - 0.05 +/- 0.27 for cool red giants/AGB stars. This is especially important if visual colors are not available, as in the Galactic Center. For one of the most prominent dust embedded sources, IRS 3, we find extended L- and M-band continuum emission with a characteristic bow-shock shape. An explanation for this unusual appearance is that IRS 3 consists of a massive, hot, young mass-losing star surrounded by an optically thick, extended dust shell, which is pushed northwest by wind from the direction of the IRS 16 cluster and Sgr A*.