The mass and radius of the unseen M dwarf companion in the single-lined eclipsing binary hat-tr-205-013

Thomas G. Beatty, José M. Fernández, David W. Latham, Gáspár Á Bakos, Géza Kovács, Robert W. Noyes, Robert P. Stefanik, Guillermo Torres, Mark E. Everett, Carl W. Hergenrother

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58 Scopus citations


We derive masses and radii for both components in the single-lined eclipsing binary HAT-TR-205-013, which consists of an F7 V primary and a late M dwarf secondary. The system's period is short, P = 2.230736 ± 0.000010 days, with an orbit indistinguishable from circular, e = 0.012 ± 0.021. We demonstrate generally that the surface gravity of the secondary star in a single-lined binary undergoing total eclipses can be derived from characteristics of the light curve and spectroscopic orbit. This constrains the secondary to a unique line in the mass-radius diagram, with M/R2 = constant. For HAT-TR-205-013, we assume the orbit has been tidally circularized and that the primary's rotation has been synchronized and aligned with the orbital axis. Our observed line broadening, Vrot sin irot = 28.9 ± 1.0 km s-1, gives a primary radius of RA = 1.28 ± 0.04 R. Our light-curve analysis leads to the radius of the secondary, RB = 0.167 ± 0.006 R , and the semimajor axis of the orbit, a = 7.54 ± 0.30 M = 0.0351 ± 0.0014 AU. Our single-lined spectroscopic orbit and the semimajor axis then yield the individual masses MB = 0.124 ± 0.010 M and MA = 1.04 ± 0.13 M. Our result for HAT-TR-205-013 B lies above the theoretical mass-radius models from the Lyon group, consistent with results from double-lined eclipsing binaries. The method we describe offers the opportunity to study the very low end of the stellar mass-radius relation.

Original languageEnglish (US)
Pages (from-to)573-582
Number of pages10
JournalAstrophysical Journal
Issue number1 I
StatePublished - Jul 1 2007
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Binaries: eclipsing
  • Binaries: spectroscopic
  • Stars: fundamental parameters
  • Stars: low-mass, brown dwarfs
  • Stars: rotation


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