期刊
ASTROPHYSICAL JOURNAL
卷 840, 期 2, 页码 -出版社
IOP PUBLISHING LTD
DOI: 10.3847/1538-4357/aa6af3
关键词
binaries: close; blue stragglers; globular clusters: general; open clusters and associations: general; stars: evolution; stars: variables: general
资金
- HST grant [AR-13910]
- National Science Foundation Astronomy and Astrophysics Postdoctoral Fellowship Award [AST-1302765]
- NASA through Space Telescope Science Institute [AR-13910]
- NASA [NAS5-26555]
- Sigma Xi, the Scientific Research Society
- HST [AR-12845]
- BMBF Verbundforschung [05A14MGA]
- Natural Sciences and Engineering Research Council of Canada
- Natural Sciences and Engineering Research Council of Canada Discovery Grant
- Fonds de recherche du Quebec - Nature et Technologies Nouveaux Chercheurs Grant
- Division Of Astronomical Sciences
- Direct For Mathematical & Physical Scien [1302765] Funding Source: National Science Foundation
Sub-subgiants are stars that are. observed to be redder than normal main-sequence stars and fainter than normal subgiant (and giant) stars in an optical color-magnitude diagram (CMD). The red straggler stars, which lie redward of the red giant branch, may be related and are often grouped together with the sub-subgiants in the literature. These stars defy our standard theory of single-star evolution. and are important tests for binary evolution and stellar collision models. In total, we identify 65 sub-subgiants (SSG) and red stragglers (RS) in 16 open and globular star clusters from the literature; 50 of these, including 43 sub-subgiants, pass our strict membership selection criteria (though the remaining sources may also be cluster members). In addition to their unique location on the CMD, we find that at least 58% (25/43) of sub-subgiants in this sample are X-ray sources with typical 0.5-2.5 keV luminosities of order 10(30)-10(31) erg s(-1). Their X-ray luminosities and optical-to-X-ray flux ratios are similar to those of RS CVn active binaries. At least 65% (28/43) of the sub-subgiants in our sample are variables, 21 of which are known to be radial-velocity binaries. Typical variability periods are less than or similar to 15. days. At least 33% (14/43) of the sub-subgiants are Ha emitters. These observational demographics provide strong evidence that binarity is important for sub-subgiant formation. Finally, we find that the number of sub-subgiants per unit mass increases toward lower-mass clusters, such that the open clusters in our sample have the highest specific frequencies of sub-subgiants.
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