Wiyn Open Cluster Study. Lxxi. Spectroscopic Membership and Orbits of NGC 6791 Sub-Subgiants

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, Hα 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.

On the Origin of Sub-subgiant Stars. I. Demographics

“…The authors are currently collecting radial-velocity measurements for the other two SSG candidates through the WIYN Open Cluster Study (WOCS; Mathieu 2000). Though not published in Milliman et al (2016), these WOCS radial velocities indicate that 6371 and 7011 are both short-period SB2 binaries with radial velocities spanning the cluster distribution; we have not yet been able to derive orbital solutions for these stars, so we cannot yet provide conclusive radial-velocity membership probabilities. The five Platais et al (2011) sources are also short-period radialvelocity binaries.…”

“…All of these sources have proper-motion membership probabilities of P P M ≥ 96% (Platais et al 2011). Four of the five Platais et al (2011) candidates (83, 746, 3626, 15561) were confirmed to be radial-velocity members by Milliman et al (2016). The authors are currently collecting radial-velocity measurements for the other two SSG candidates through the WIYN Open Cluster Study (WOCS; Mathieu 2000).…”

“…The five Platais et al (2011) sources are also short-period radialvelocity binaries. Milliman et al (2016) published orbital solutions for 746, 3626 and 15561 (see Table 3). Five of these NGC 6791 sources (four SSGs and one RS) are short-period photometric variables, X-ray sources and Hα emitters (de Marchi et al 2007;Mochejska et al 2002;Kaluzny 2003;Bruntt et al 2003;Mochejska et al 2005;van den Berg et al 2013;Milliman et al 2016).…”

“…Milliman et al (2016) published orbital solutions for 746, 3626 and 15561 (see Table 3). Five of these NGC 6791 sources (four SSGs and one RS) are short-period photometric variables, X-ray sources and Hα emitters (de Marchi et al 2007;Mochejska et al 2002;Kaluzny 2003;Bruntt et al 2003;Mochejska et al 2005;van den Berg et al 2013;Milliman et al 2016). For the variable SSG and RS sources in NGC 6791, photometric variability occurs on periods similar to the radial-velocity orbital period and has been attributed to spot modulation (van den Berg et al 2013).…”

“…We discuss this phenomenon in some detail for specific sources below. Briefly, some stars move around dramatically relative to the isochrone with different filter choices, perhaps due to spot activity and photometric variability (see also, e.g., Milliman et al 2016). Indeed, photometric variability appears to be a defining characteristic of the SSG and RS stars.…”