* Astronomy

Blobrana · L

RE: CM Draconis

Blobrana · L

Title: The Metallicity of the CM Draconis System
Authors: Ryan C. Terrien, Scott W. Fleming, Suvrath Mahadevan, Rohit Deshpande, Gregory A. Feiden, Chad F. Bender, Lawrence W. Ramsey

The CM Draconis system comprises two eclipsing mid-M dwarfs of nearly equal mass in a 1.27-day orbit. This well-studied eclipsing binary has often been used for benchmark tests of stellar models, since its components are amongst the lowest mass stars with well-measured masses and radii (~ 1% relative precision). However, as with many other low-mass stars, non-magnetic models have been unable to match the observed radii and effective temperatures for CM Dra at the 5-10% level. To date, the uncertain metallicity of the system has complicated comparison of theoretical isochrones with observations. In this Letter, we use data from the SpeX instrument on the NASA Infrared Telescope Facility (IRTF) to measure the metallicity of the system during primary and secondary eclipses, as well as out of eclipse, based on an empirical metallicity calibration in the H and K near-infrared (NIR) bands. We derive a [Fe/H] = -0.30 ± 0.12 that is consistent across all orbital phases. The determination of [Fe/H] for this system constrains a key dimension of parameter space when attempting to reconcile model isochrone predictions and observations.

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Blobrana · L

Title: Extrasolar planet detection by binary stellar eclipse timing: evidence for a third body around CM Draconis
Authors: H. J. Deeg, B. Ocaña1, V. P. Kozhevnikov, D. Charbonneau, F. T. O'Donovan, and L. R. Doyle

Aims. Our objective is to elucidate the physical process that causes the observed observed-minus-calculated (O-C) behaviour in the M4.5/M4.5 binary CM Dra and to test for any evidence of a third body around the CM Dra system.
Methods. New eclipse minimum timings of CM Dra were obtained between the years 2000 and 2007. The O-C times of the system are fitted against several functions, representing different physical origins of the timing variations.
Results. Using our observational data in conjunction with published timings going back to 1977, a clear non-linearity in O-C times is apparent. An analysis using model-selection statistics gives about equal weight to a parabolic and to a sinusoidal fitting function. Attraction from a third body, either at large distance in a quasi-constant constellation across the years of observations or from a body on a shorter orbit generating periodicities in O-C times is the most likely source of the observed O-C times. The white dwarf GJ 630.1B, a proper motion companion of CM Dra, can however be rejected as the responsible third body. Also, no further evidence of the short-periodic planet candidate described by Deeg et al. (2000, A&A, 358, L5) is found, whereas other mechanisms, such as period changes from stellar winds or Applegate's mechanism can be rejected.
Conclusions. A third body, being either a few-Jupiter-mass object with a period of 18.5 ± 4.5 years or an object in the mass range of 1.5 Jupiter masses to 0.1 solar masses with periods of hundreds to thousands of years is the most likely origin of the observed minimum timing behaviour.

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