ApJ, 2007, 659, 1241

Photometric Stellar Variability in the Galactic Center

M. Rafelski, A. M. Ghez, S. D. Hornstein, J. R. Lu, and M. Morris

Light curve of the variable source IRS16SW Light curve of the Wolf-Rayet star IRS29N (top left)

Abstract

We report the results of a diffraction-limited, photometric variability study of the central 5" x 5" of the Galaxy conducted over the past 10 years using speckle imaging techniques on the W. M. Keck I 10 m telescope. Within our limiting magnitude of m_K < 16 mag for images made from a single night of data, we find a minimum of 15 K[2.2 micron]-band variable stars out of 131 monitored stars. The only periodic source in our sample is the previously identified variable IRS 16SW, for which we measure an orbital period of 19.448 +/- 0.002 days. In contrast to recent results, our data on IRS 16SW show an asymmetric phased light curve with a much steeper fall time than rise time, which may be due to tidal deformations caused by the proximity of the stars in their orbits. We also identify a possible wind colliding binary (IRS 29N) based on its photometric variation over a few year timescale, which is likely due to episodic dust production. None of the four luminous blue variable (LBV) candidates in our sample show the characteristic large increase or decrease in luminosity. However, our time baseline is too short to rule them out as LBVs. Among the remaining variable stars, the majority are early-type stars, and three are possibly variable due to line-of-sight extinction variations. For the seven OB stars at the center of our field of view that have well-determined three-dimensional orbits, we see no evidence of flares or dimming of their light, which limits the possibility of a cold, geometrically thin, inactive accretion disk around the supermassive black hole, Sgr A*.

Figure Caption

(a): Phased light curve of IRS 16SW at its period of 9.724 (top) and 19.448 days (bottom). The points are plotted twice for clarity. Both phased diagrams show a rise time that is ~1.6 times longer than the fall time. The similarity of the first half of the data set (1995-2000; diamonds) to the second half (2001-2005; crosses) shows that the asymmetry has been constant over the past 10 yr. This effect could be produced by tidal deformation of two equal-mass stars in an eclipsing binary star system.
(b): Light curves of W-R stars of type WC in our sample. The top left panel is the variable star IRS 29N, which is probably a wind-colliding binary, while the other two are nonvariable stars. The given intrinsic variance is expressed in magnitudes.