On the origin of sub-subgiant stars. I. Demographics

From National Research Council Canada

DOI	Resolve DOI: https://doi.org/10.3847/1538-4357/aa6af3
Author	Search for: Geller, Aaron M.; Search for: Leiner, Emily M.; Search for: Bellini, Andrea; Search for: Gleisinger, Robert; Search for: Haggard, Daryl; Search for: Kamann, Sebastian; Search for: Leigh, Nathan W. C.; Search for: Mathieu, Robert D.; Search for: Sills, Alison; Search for: Watkins, Laura L.; Search for: Zurek, David
Format	Text, Article
Subject	binaries: close; blue stragglers; globular clusters: general; open clusters and associations: general; stars: evolution; stars: variables: general
Abstract	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 1030–1031 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 lesssim ≲15 days. At least 33% (14/43) of the sub-subgiants are Hα 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.
Publication date	2017-05-08
Publisher	IOP Publishing
In	The Astrophysical Journal 840, no. 2: 66–.
Language	English
Peer reviewed	Yes
NRC publication	This is a non-NRC publication "Non-NRC publications" are publications authored by NRC employees prior to their employment by NRC.
NPARC number	23003237
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Record identifier	5663d047-cec8-42c7-9349-eba9c5fc2fd3
Record created	2018-05-08
Record modified	2020-03-16

Date modified:: 2026-01-18