WIYN Open Cluster Study. LXXI. Spectroscopic Membership and Orbits of NGC 6791 Sub-Subgiants

TL;DR

This study confirms the cluster membership and binary nature of sub-subgiants in NGC 6791, providing orbital data and supporting the idea that SSGs are a distinct class of nonstandard stellar evolution products.

Contribution

First to determine three-dimensional kinematic membership and orbital parameters of SSGs in NGC 6791, highlighting their binary nature and properties.

Findings

Four SSG candidates confirmed as cluster members.

Three SSGs identified as short-period binary systems with known orbits.

Supports SSGs as a new class of nonstandard stellar evolution products.

Abstract

In an optical color-magnitude diagram sub-subgiants (SSGs) lie red of the main sequence and fainter than the base of the red giant branch in a region not easily populated by standard stellar-evolution pathways. In this paper, we present multi-epoch radial velocities for five SSG candidates in the old and metal-rich open cluster NGC 6791 (8 Gyr, [Fe/H] = +0.30). From these data we are able to make three-dimensional kinematic membership determinations and confirm four SSG candidates to be likely cluster members. We also identify three member SSGs as short-period binary systems and present their orbital solutions. These are the first SSGs with known three-dimensional kinematic membership, binary status, and orbital parameters since the two SSGs in M67 studied by Mathieu et al. 2003. We also remark on the other properties of these stars including photometric variability, H$\alpha$ emission, and X-ray luminosity. The membership confirmation of these SSGs in NGC 6791 strengthens the case that SSGs are a new class of nonstandard stellar evolution products, and that a physical mechanism must be found that explains the evolutionary paths of these stars.