AAS 197, January 2001
Session 41. Star Clusters and Associations
Display, Tuesday, January 9, 2001, 9:30am-7:00pm, Exhibit Hall

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[41.11] Sub-Subgiants in the Old Open Cluster M67

R.D. Mathieu (University of Wisconsin - Madison), D.W. Latham (Harvard-Smithsonian Center for Astrophysics), K.G. Stassun (University of Wisconsin - Madison), G. Torres (Harvard-Smithsonian Center for Astrophysics), M. van den Berg, F. Verbunt (Astronomical Institute, Utrecht)

In the course of a multi-decade survey of the spectroscopic binary population in M67, we have discovered two spectroscopic binaries S1113 and S1063 - whose positions in the color-magnitude diagram place them below the subgiant branch. A comprehensive ROSAT study of stellar X-ray sources in M67 independently discovered these stars to be X-ray sources.

The membership in the cluster of these binaries is as secure as can be achieved at present. Both have proper-motion membership probabilities greater than 97 center-of-mass velocities are precisely consistent with the cluster mean radial velocity. S1063 is also projected within one core radius of the cluster center.

S1113 is a double-lined system with a circular orbit having a period of 2.823094 ± 0.000014 days. The mass ratio is 0.703 ± 0.012; if the primary is an evolved star with unchanged mass, its mass would be 1.3 M\odot and the secondary mass would be 0.9 M\odot (Yale isochrones). The primary is rapidly rotating (vsin i = 53 km/sec), consistent with synchronous rotation at a stellar radius of 4 R\odot. This radius approaches the Roche radius of the primary. All properties of the secondary star are consistent with a main-sequence star of mass 0.9 M\odot.

S1063 is a single-lined binary with a period of 18.396 ± 0.005 days and an orbital eccentricity of 0.206 ± 0.014. If the primary is an evolved star of 1.3 M\odot, then a main-sequence secondary would have a mass between 0.325 M\odot and 0.8 M\odot (the latter based on its non-detection in our spectra). The projected rotational velocity of the primary permits it to be pseudosynchronized with the orbit.

We have obtained light curves of both stars. S1113 is a periodic photometric variable with a period very nearly the same as the orbital period. S1063 is is variable on long timescales, but a clear periodic signature has not been detected as yet.

The origin and evolutionary state of these binaries is as yet unknown.


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