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Published February 10, 2010 | Published
Journal Article Open

Near-infrared Counterparts to Chandra X-ray Sources Toward the Galactic Center. II. Discovery of Wolf-Rayet Stars and O Supergiants

Abstract

We present new identifications of infrared counterparts to the population of hard X-ray sources near the Galactic center detected by the Chandra X-ray Observatory. We have spectroscopically confirmed 16 new massive stellar counterparts to the X-ray population, including nitrogen-type (WN) and carbon-type (WC) Wolf-Rayet stars, and O supergiants. These discoveries increase the total sample of massive stellar X-ray sources in the Galactic center region to 30 (possibly 31). For the majority of these sources, the X-ray photometry is consistent with thermal emission from plasma having temperatures in the range of kT = 1-8 keV or non-thermal emission having power-law indices in the range of –1 ≲ Γ ≲ 3, and X-ray luminosities in the range of L_X ~ 10^(32)-10^(34) erg s^(–1) (0.5-8.0 keV). Several sources have exhibited X-ray variability of several factors between observations. These X-ray properties are not a ubiquitous feature of single massive stars but are typical of massive binaries, in which the high-energy emission is generated by the collision of supersonic winds, or by accretion onto a compact companion. However, without direct evidence for companions, the possibility of intrinsic hard X-ray generation from single stars cannot be completely ruled out. The spectral energy distributions of these sources exhibit significant infrared excess, attributable to free-free emission from ionized stellar winds, supplemented by hot dust emission in the case of the WC stars. With the exception of one object located near the outer regions of the Quintuplet cluster, most of the new stars appear isolated or in loose associations. Seven hydrogen-rich WN and O stars are concentrated near the Sagittarius B H II region, while other similar stars and more highly evolved hydrogen-poor WN and WC stars lie scattered within ≈50 pc, in projection, of Sagitarrius A West. We discuss various mechanisms capable of generating the observed X-rays and the implications these stars have for massive star formation in the Galaxy's Central Molecular Zone.

Additional Information

© 2010 American Astronomical Society. Print publication: Issue 1 (2010 February 10) received 2009 September 24; accepted for publication 2009 December 8; published 2010 January 21. This research was based upon observations with the Chandra X-ray Observatory. Support was provided by the National Science Foundation (grant AST-0406816). J.M. thanks Fabrice Martins for providing model stellar spectra of the Arches stars for comparison, and comments on the spectral classification of the massive stars presented in this work. We also thank Shogo Nishiyama and Tetsuya Nagata for providing near-infrared photometry from the SIRIUS survey.

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