Revised age for CM Draconis and WD 1633+572

Toward a resolution of model-observation radius discrepancies

¹ Department of Physics and AstronomyUppsala University, Box 516, 751 20 Uppsala, Sweden
e-mail: gregory.a.feiden@gmail.com
² Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755, USA

Received: 27 March 2014
Accepted: 16 September 2014

Abstract

We report an age revision for the low-mass detached eclipsing binary CM Draconis and its common proper motion companion, WD 1633+572. An age of 8.5 ± 3.5 Gyr is found by combining an age estimate for the lifetime of WD 1633+572 and an estimate from galactic space motions. The revised age is greater than a factor of two older than previous estimates. Our results provide consistency between the white dwarf age and the system’s galactic kinematics, which reveal the system is a highly probable member of the galactic thick disk. We find the probability that CM Draconis and WD 1633+572 are members of the thick disk is 8500 times greater than the probability that they are members of the thin disk and 170 times greater than the probability they are halo interlopers. If CM Draconis is a member of the thick disk, it is likely enriched in α-elements compared to iron by at least 0.2 dex relative to the Sun. This leads to the possibility that previous studies under-estimate the [Fe/H] value, suggesting the system has a near-solar [Fe/H]. Implications for the long-standing discrepancies between the radii of CM Draconis and predictions from stellar evolution theory are discussed. We conclude that CM Draconis is only inflated by about 2% compared to stellar evolution predictions.