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Register a new userA Chandra X-ray census of the interacting binaries in old open clusters - NGC188
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Added by
Maureen Van Den Berg
Publication date
2018
fields
Physics
and research's language is
English
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We present a new X-ray study of NGC188, one of the oldest open clusters known in our Galaxy (7 Gyr). Our observation with the Chandra X-ray Observatory is aimed at uncovering the population of close interacting binaries in NGC188. We detect 84 sources down to a luminosity of Lx ~ 4e29 erg/s (0.3-7 keV), of which 73 are within the half-mass radius rh. Of the 60 sources inside rh with more than 5 counts, we estimate that ~38 are background sources. We detected 55 new sources, and confirmed 29 sources previously detected by ROSAT and/or XMM-Newton. A total of 13 sources detected are cluster members, and 7 of these are new detections: four active binaries, two blue straggler stars (BSSs), and, surprisingly, an apparently single cluster member on the main sequence (CX33/WOCS5639). One of the BSSs detected (CX84/WOCS5379) is intriguing as its X-ray luminosity cannot be explained by its currently understood configuration as a BSS/white-dwarf binary in an eccentric orbit of ~120 days. Its X-ray detection, combined with reports of short-period optical variability, suggests the presence of a close binary, which would make this BSS system a hierarchical multiple. We also classify one source as a new cataclysmic-variable candidate; it is identified with a known short-period optical variable, whose membership to NGC188 is unknown. We have compared the X-ray emissivity of NGC188 with those of other old Galactic open clusters. Our findings confirm the earlier result that old open clusters have higher X-ray emissivities than other old stellar populations.
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We present the first X-ray study of NGC6791, one of the oldest open clusters known (8 Gyr). Our Chandra observation is aimed at uncovering the population of close interacting binaries down to Lx ~ 1e30 erg/s (0.3-7 keV). We detect 86 sources within 8 arcmin of the cluster center, including 59 inside the half-mass radius. We identify twenty sources with proper-motion cluster members, which are a mix of cataclysmic variables (CVs), active binaries (ABs), and binaries containing sub-subgiants. With follow-up optical spectroscopy we confirm the nature of one CV. We discover one new, X-ray variable candidate CV with Balmer and HeII emission lines in its optical spectrum; this is the first X-ray--selected CV confirmed in an open cluster. The number of CVs per unit mass is consistent with the field, suggesting that the 3-4 CVs observed in NGC6791 are primordial. We compare the X-ray properties of NGC6791 with those of a few old open (NGC6819, M67) and globular clusters (47Tuc, NGC6397). It is puzzling that the number of ABs brighter than 1e30 erg/s normalized by cluster mass is lower in NGC6791 than in M67 by a factor ~3 to 7. CVs, ABs, and sub-subgiants brighter than 1e30 erg/s are under-represented per unit mass in the globular clusters compared to the oldest open clusters, and this accounts for the lower total X-ray luminosity per unit mass of the former. This indicates that the net effect of dynamical encounters may be the destruction of even some of the hardest (i.e. X-ray--emitting) binaries.
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