The Low-Mass Double-Lined Eclipsing Binary CM Draconis

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Session 85 -- Binaries
Display presentation, Wednesday, 11, 1995, 9:20am - 6:30pm

[85.18] The Low-Mass Double-Lined Eclipsing Binary CM Draconis

Travis S. Metcalfe (University of Arizona), Robert D. Mathieu (University of Wisconsin), David W. Latham, Guillermo Torres (CfA)

CM Draconis is the least massive main-sequence eclipsing double-lined spectroscopic binary currently known. Consequently, this system offers a unique opportunity to test stellar structure models near the bottom of the main sequence. The orbital solution of Lacy (1977) established the masses and radii of the two components with uncertainties of a few percent, but these errors are too large to distinguish between competing models. We present a new double-lined orbital solution based on 233 echelle spectra obtained with the CfA Digital Speedometers over the past ten years. Radial velocities for both components were determined using TODCOR, a two-dimensional correlation technique developed by Zucker and Mazeh (1994). We derive individual masses of $M_{A} = 0.231 \pm 0.002$ and $M_{B} = 0.214 \pm 0.001~M_{\odot}$ and a mass ratio of $q = 0.926 \pm 0.004$. When plotted on a mass-radius diagram using the radii derived by Lacy, the slope defined by the two components of CM Draconis agrees well with the model slopes. The ability to distinguish between models is limited by the uncertainties in the radii and the metal abundance. Following the analysis of Paczy\'nski and Sienkiewicz (1984) we derive bulk helium abundances for the two components of $Y_{A} = 0.31$ and $Y_{B} = 0.30$. The uncertainty is these helium abundances relative to each other is nominally $\pm 0.02$ and depends primarily on the uncertainty in the ratios of the masses and radii. The uncertainty in the absolute helium abundance is considerably larger and depends on the uncertainty in the absolute masses and radii, the parallax, bolometric correction, age, metallicity, and details of the stellar structure models (such as nuclear cross sections). The uncertainty in the absolute helium abundance is nominally $\pm 0.05$, but improved determinations of the radii, parallax, and bolometric correction are especially needed to confirm and improve this uncertainty. CM Draconis appears to be a member of Population II. Thus, it may be able to provide a useful test of the primordial helium abundance.

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