Sub-Subgiants in the Old Open Cluster M67?

Abstract

We report the discovery of two spectroscopic binaries in the field of the old open cluster M67-S1063 and S1113-whose positions in the color-magnitude diagram place them %1 mag below the subgiant branch. A ROSAT study of M67 independently discovered these stars to be X-ray sources. Both have proper-motion membership probabilities greater than 97%; precise center-of-mass velocities are consistent with the cluster mean radial velocity. S1063 is also projected within one core radius of the cluster center. It is a single-lined binary with a period of 18.396 days and an orbital eccentricity of 0.206. S1113 is a double-lined system with a circular orbit having a period of 2.823094 days. The primary stars of both binaries are subgiants. The secondary of S1113 is likely a 0.9 M main-sequence star, which implies a 1.3 M primary star. We have been unable to explain securely the low apparent luminosities of the primary stars. The colors of S1063 suggest 0.15 mag higher reddening than found for either M67 or through the entire Galaxy in the direction of M67. S1063 could be explained as an extincted M67 subgiant, although the origin of such enhanced extinction is unknown. The photometric properties of S1113 are well modeled by a cluster binary with a 0.9 M mainsequence secondary star. However, the low composite luminosity requires a small (2.0 R ) primary star that would be supersynchronously rotating, in contrast to the short synchronization timescales, the circular orbit, and the periodic photometric variability with the orbital period. Geometric arguments based on a tidally relaxed system suggest a larger (4.0 R ) primary star in a background binary, but such a large star violates the observed flux ratio. Thus, we have not been able to find a compelling solution for the S1113 system. We speculate that S1063 and S1113 may be the products of close stellar encounters involving binaries in the cluster environment and may define alternative stellar evolutionary tracks associated with mass transfer episodes, mergers, and/or dynamical stellar exchanges.

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

“…Finally, the SB2, NGC 2682 S1113, is the only SSG that has a dynamically measured mass ratio. Its mass ratio of 0.7 and the spectroscopic temperature and surface gravity measurements of Mathieu et al (2003) point to a main-sequence companion to the SSG. However, again Mathieu et al (2003) caution that they could not find a self-consistent solution for S1113 that accounts for all of the observations.…”

“…The longest-period binary with an orbital solution, NGC 2682 ID 1063, is also the only eccentric binary (with e = 0.206 ± 0.014). Mathieu et al (2003) note that this eccentricity is typical for normal main-sequence and subgiant binaries in M67 with periods of ∼20 days, but argues against a phase of mass transfer or a large evolved primary, as these would both tend to circularize the orbit (in the absence of any sufficiently recent dynamical encounter, or a tertiary companion, that could have increased the eccentricity).…”

A VIMOS spectroscopy study of photometric variables and straggler candidates inω Centauri

“…At least some candidate red stragglers in ω Cen are X-ray sources, although the cause of their X-ray activity remains obscure (Haggard et al 2010). The two binary red stragglers in M67 studied by Mathieu et al (2003) may belong to the RS CVn class, but their parameters have not been uniquely determined. Mathieu et al (2003) and Mathieu (2008) argue that their evolutionary history might have included mass transfer episodes, mergers, dynamical stellar exchanges, and/or close encounters leading to envelope stripping.…”

“…The two binary red stragglers in M67 studied by Mathieu et al (2003) may belong to the RS CVn class, but their parameters have not been uniquely determined. Mathieu et al (2003) and Mathieu (2008) argue that their evolutionary history might have included mass transfer episodes, mergers, dynamical stellar exchanges, and/or close encounters leading to envelope stripping. In their conclusion, Mathieu et al (2003) point to an intriguing possibility that these systems are the progenitors of blue stragglers.…”

Close binary and other variable stars in the solar-age Galactic open cluster M 67

“…In the CM diagram of the open cluster, the systems S1063 and S1113 lay well below the subgiant branch. Mathieu et al (2003) studied these subgiant members of M 67 spectroscopically and obtained their orbital parameters. S1063 is a single-lined binary with an orbital period of 18.4 days and an eccentricity 0.21.…”