121 research outputs found
Young, Massive Star Candidates Detected throughout the Nuclear Star Cluster of the Milky Way
Aims. Young, massive stars have been found at projected distances R < 0.5 pc
from supermassive black hole, Sgr A* at the center of our Galay. In recent
years, increasing evidence has been found for the presence of young, massive
stars also at R > 0.5 pc. Our goal in this work is a systematic search for
young, massive star candidates throughout the entire region within R ~ 2.5 pc
of the black hole. Methods. The main criterion for the photometric
identification of young, massive early-type stars is the lack of CO-absorption
in the spectra. We used narrow-band imaging with VLT/ISAAC to search for young,
massive stars within ~2.5 pc of Sgr A*. Results. We have found 63 early-type
star candidates at R < 2.5 pc, with an estimated erroneous identification rate
of only about 20%. Considering their K-band magnitudes and interstellar
extinction, they are candidates for Wolf-Rayet stars, supergiants, or early
O-type stars. Of these, 31 stars are so far unknown young, massive star
candidates, all of which lie at R>0.5pc. The surface number density profile of
the young, massive star candidates can be well fit by a single power-law, with
Gamma = 1.6 +- 0.17 at R < 2.5 pc, which is significantly steeper than that of
the late-type giants that make up the bulk of the observable stars in the NSC.
Intriguingly, this power-law is consistent with the power-law that describes
the surface density of young, massive stars in the same brightness range at R <
0.5 pc. Conclusions. The finding of a significant number of newly identified
early-type star candidates at the Galactic center suggests that young, massive
stars can be found throughout the entire cluster which may require us to modify
existing theories for star formation at the Galactic center. Follow-up studies
are needed to improve the existing data and lay the foundations for a unified
theory of star formation in the Milky Way's NSC.Comment: 20 pages, 11 figures, Accepted for publication in Astronomy and
Astrophysic
Near-Infrared Counterparts to Chandra X-ray Sources toward the Galactic Center. I. Statistics and a Catalog of Candidates
We present a catalog of 5184 candidate infrared counterparts to X-ray sources
detected towards the Galactic center. The X-ray sample contains 9017 point
sources detected in this region by the Chandra X-ray Observatory, including
data from a recent deep survey of the central 2 x 0.8 deg of the Galactic
plane. A total of 6760 of these sources have hard X-ray colors, and the
majority of them lie near the Galactic center, while most of the remaining 2257
soft X-ray sources lie in the foreground. We cross-correlated the X-ray source
positions with the 2MASS and SIRIUS near-infrared catalogs, which collectively
contain stars with a 10-sigma limiting flux of K_s<=15.6 mag. In order to
distinguish absorbed infrared sources near the Galactic center from those in
the foreground, we defined red and blue sources as those which have H-K_s>=0.9
and <=0.9 mag, respectively. We find that 5.8(1.5)% of the hard X-ray sources
have real infrared counterparts, of which 228(99) are red and 166(27) are blue.
The red counterparts are probably comprised of WR/O stars, HMXBs, and
symbiotics near the Galactic center. We also find that 39.4(1.0)% of the soft
X-ray sources have blue infrared counterparts; most of these are probably
coronally active dwarfs in the foreground. There is a noteworthy collection of
~20 red counterparts to hard X-ray sources near the Sagittarius-B H II region,
which are probably massive binaries that have formed within the last several
Myr. For each of the infrared matches to X-ray sources in our catalog we
derived the probability that the association is real, based on the results of
the cross-correlation analysis. The catalog will serve spectroscopic surveys to
identify infrared counterparts to X-ray sources near the Galactic center.Comment: Submitted to ApJ January 16, 2009; accepted July 21, 2009; 30 pages,
6 figure
- …