2 micron Spectroscopy within 0.''3 of Sgr A*

Donald F. Figer(1), E. E. Becklin(2), Ian S. McLean(2), Andrea M. Gilbert(3), James R. Graham(3), James E. Larkin(2), N. A. Levenson(4), Harry I. Teplitz(5,6), Mavourneen K. Wilcox(2), Mark Morris(2)

(1) Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; figer@stsci.edu
(2) Department of Physics and Astronomy, University of California, Los Angeles, Division of Astronomy, Los Angeles, CA, 90095-1562
(3) Department of Astronomy, University of California, Berkeley, 601 Campbell Hall, Berkeley, CA, 94720-3411
(4) Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218
(5) Laboratory for Astronomy and Solar Physics, Code 681, Goddard Space Flight Center, Greenbelt MD 20771
(6) NOAO Research Associate

Paper: ApJ Letters, April 10th Issue

Abstract:

We present moderate (R 2,700) and high resolution (R 22,000) 2.0-2.4 micron spectroscopy of the central 0.1 square arcseconds of the Galaxy obtained with NIRSPEC, the facility near-infrared spectrometer for the Keck II telescope. The composite spectra do not have any features attributable to the brightest stars in the central cluster, i.e. after background subtraction, W_¹²CO(2-0) < 2 \AA. This stringent limit leads us to conclude that the majority, if not all, of the stars are hotter than typical red giants. Coupled with previously reported photometry, we conclude that the sources are likely OB main sequence stars. In addition, the continuum slope in the composite spectrum is bluer than that of a red giant and is similar to that of the nearby hot star, IRS16NW. It is unlikely that they are late-type giants stripped of their outer envelopes because such sources would be much fainter than those observed. Given their inferred youth ( tau _age < 20 \Myr), we suggest the possibility that the stars have formed within 0.1 pc of the supermassive black hole. We find a newly-identified broad-line component (V_FWHM 1,000 km/sec ) to the 2.2178 micron [\ionFe3] line located within a few arcseconds of Sgr A^*. A similar component is not seen in the Br- gamma emission.

Preprints available from the authors at figer@stsci.edu , or the raw TeX (no figures) if you click here.

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