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Session 44 - Stellar X-Ray Sources.
Display session, Tuesday, January 14
Metropolitan Ballroom,
The OGLE microlensing survey has serendipitously discovered a over 2000 variable stars in the Baade Window in the direction of the galactic bulge (Udalski et al. 1994 Acta Astr., 44, 317). A surprising result is the apparent large fraction (about 30 percent) of probable spotted, chromospherically active (=CA) stars, also known as RS CVn variables, found in the samples. Like the nearby RS CVn variables, these OGLE variables have low amplitude, periodic light variations with periods of a few days up to several tens of days; they have V-I indices and positions in the color-magnitude diagram that indicate that they are late G- to mid K-giants or subgiants. It is well known that RS CVn stars are strong coronal X-ray sources with L_X = 1-50\times 10^30 ergs cm^-2s^-1. We find that the X-ray emission from these stars, when combined with the expected coronal X-ray emission from other CA variables in the OGLE samples - such as the Algol, W UMa and ellipsoidal binaries - is sufficient to account for most (if not all) of the soft diffuse X-ray background (DXBG) emission observed by ROSAT from the central regions of the Galaxy. The confirmation that these variable stars are CA stars can be made by observing the chief indicators of magnetic chromospheric activity - Ca II H amp; K or H\alpha emission in their spectra.
More sophisicated modelling of selection effects of the CA X-ray sources in the bulge needs to be done, but the preliminary estimates given here indicate that chromospherically active stars (CABs = RS CVn and BY Dra stars, Algols, W UMas) in the galactic bulge may significantly contribute to the soft DXBG observed from the central regions of the Galaxy. Before proceeding, it also would be wise to obtain spectra of some of the OGLE "miscellaneous" variables to search for confirming evidence of chromospheric Ca II H+K or H\alpha emission. We are hope to carry out these initial spectroscopic observations with a multifiber-feed spectrograph during 1997. This work is supported by grants from NSF (AST-9315365), NASA (NAG 5-2494) and the Korea Haksul Foundation.
