A CHANDRA X-RAY STUDY OF THE INTERACTING BINARIES IN THE OLD OPEN CLUSTER NGC 6791

Journal Article · Thu Jun 20 04:00:00 EDT 2013 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/770/2/98· OSTI ID:22127093

Van den Berg, Maureen ^[1]; Verbunt, Frank ^[2]; Tagliaferri, Gianpiero; Belloni, Tomaso ^[3]; Bedin, Luigi R. ^[4]; Platais, Imants ^[5]

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1. Astronomical Institute ''Anton Pannekoek'', University of Amsterdam, Science Park 904, 1098 XH Amsterdam (Netherlands)
2. Department of Astrophysics/IMAPP, Radboud University Nijmegen, P.O. Box 9010, 6500 GL Nijmegen (Netherlands)
3. INAF/Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (Italy)
4. INAF/Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, I-35122 Padova (Italy)
5. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 (United States)

We present the first X-ray study of NGC 6791, one of the oldest open clusters known (8 Gyr). Our Chandra observation is aimed at uncovering the population of close interacting binaries down to L{sub X} Almost-Equal-To 1 Multiplication-Sign 10{sup 30} erg s{sup -1} (0.3-7 keV). We detect 86 sources within 8' of the cluster center, including 59 inside the half-mass radius. We identify 20 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 He II emission lines in its optical spectrum; this is the first X-ray-selected CV in an open cluster. The number of CVs per unit mass is consistent with the field, suggesting that the 3-4 CVs observed in NGC 6791 are primordial. We compare the X-ray properties of NGC 6791 with those of a few old open (NGC 6819, M 67) and globular clusters (47 Tuc, NGC 6397). It is puzzling that the number of ABs brighter than 1 Multiplication-Sign 10{sup 30} erg s{sup -1} normalized by cluster mass is lower in NGC 6791 than in M 67 by a factor {approx}3-7. CVs, ABs, and sub-subgiants brighter than 1 Multiplication-Sign 10{sup 30} erg s{sup -1} 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.

• OSTI ID: 22127093
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 770; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
COMPARATIVE EVALUATIONS
KEV RANGE
LUMINOSITY
MASS
NOVAE
PROPER MOTION
SPECTRA
SPECTROSCOPY
X RADIATION