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Session 26 - Stars in the Visible & IR.
Display session, Tuesday, June 10
South Main Hall,

[26.08] Eclipsing Binaries as Astrophysical Laboratories: CM Draconis - Accurate Absolute Physical Properties of Low Mass Stars and an Independent Estimate of the Primordial Helium Abundance

G. P. McCook, E. F. Guinan (Villanova Univ.), D. Saumon (Vanderbilt Univ.), Y. W. Kang (Sejong Univ.)

CM Draconis (Gl 630.1; Vmax = +12.93) is an important eclipsing binary consisting of two dM4.5e stars with an orbital period of 1.2684 days. This binary is a high velocity star (s= 164 km/s) and the brighter member of a common proper motion pair with a cool faint white dwarf companion (LP 101-16). CM Dra and its white dwarf companion were once considered by Zwicky to belong to a class of "pygmy stars", but they turned out to be ordinary old, cool white dwarfs or faint red dwarfs. Lacy (ApJ 218,444L) determined the first orbital and physical properties of CM Dra from the analysis of his light and radial velocity curves. In addition to providing directly measured masses, radii, and luminosities for low mass stars, CM Dra was also recognized by Lacy and later by Paczynski and Sienkiewicz (ApJ 286,332) as an important laboratory for cosmology, as a possible old Pop II object where it may be possible to determine the primordial helium abundance. Recently, Metcalfe et al.(ApJ 456,356) obtained accurate RV measures for CM Dra and recomputed refined elements along with its helium abundance.

Starting in 1995, we have been carrying out intensive RI photoelectric photometry of CM Dra to obtain well defined, accurate light curves so that its fundamental properties can be improved, and at the same time, to search for evidence of planets around the binary from planetary transit eclipses. During 1996 and 1997 well defined light curves were secured and these were combined with the RV measures of Metcalfe et al. (1996) to determine the orbital and physical parameters of the system, including a refined orbital period. A recent version of the Wilson-Devinney program was used to analyze the data. New radii, masses, mean densities, Teff, and luminosities were found as well as a re-determination of the helium abundance (Y). The results of the recent analyses of the light and RV curves will be presented and modelling results discussed. This research is supported by NSF grants AST-9315365 and AST-9528506 which we gratefully acknowledge.

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