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The ChaMP Extended Stellar Survey (ChESS): Photometric and Spectroscopic Properties of Serendipitously Detected Stellar X-Ray Sources
We present 348 X-ray-emitting stars identified from correlating theExtended Chandra Multiwavelength Project (ChaMP), a wide-areaserendipitous survey based on archival X-ray images, with the SloanDigital Sky Survey (SDSS). We use morphological star/galaxy separation,matching to an SDSS quasar catalog, an optical color-magnitude cut, andX-ray data-quality tests to create our catalog, the ChaMP ExtendedStellar Survey (ChESS), from a sample of 2121 matched ChaMP/SDSSsources. Our cuts retain 92% of the spectroscopically confirmed stars inthe original sample while excluding 99.6% of the 684 spectroscopicallyconfirmed extragalactic sources. Fewer than 3% of the sources in ourfinal catalog are previously identified stellar X-ray emitters. For 42catalog members, spectroscopic classifications are available in theliterature. We present new spectral classifications and Hαmeasurements for an additional 79 stars. The catalog is dominated bymain-sequence stars; we estimate the fraction of giants in ChESS is~10%. We identify seven giant stars (including a possible Cepheid and anRR Lyrae star) as ChaMP sources, as well as three cataclysmic variables.We derive distances from ~10 to 2000 pc for the stars in our catalogusing photometric parallax relations appropriate for dwarfs on the mainsequence and calculate their X-ray and bolometric luminosities. Thesestars lie in a unique space in the LX-distance plane, fillingthe gap between the nearby stars identified as counterparts to sourcesin the ROSAT All Sky Survey and the more distant stars detected in deepChandra and XMM-Newton surveys. For 36 newly identified X-ray-emitting Mstars we calculate LHα/Lbol. The quantitiesLHα/Lbol and LX/Lbolare linearly related belowLX/Lbol~3×10-4, whileLHα/Lbol appears to turn over at largerLX/Lbol values. Stars with reliable SDSSphotometry have an ~0.1 mag blue excess in u-g, likely due to increasedchromospheric continuum emission. Photometric metallicity estimatessuggest that the sample is evenly split between the young and old diskpopulations of the Galaxy; the lowest activity sources belong to the olddisk population, a clear signature of the decay of magnetic activitywith age. Future papers will present analyses of source variability andcomparisons of this catalog to models of stellar activity in theGalactic disk.Observations reported here were obtained at the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona.
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