A Dynamical Mass Measurement for the Pre-Main-Sequence Secondary of the Eclipsing Binary TY CrA

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Session 44 -- Young Stars, T Tauri Stars, Herbig Haro Objects
Display presentation, Wednesday, 1, 1994, 9:20-6:30

[44.08] A Dynamical Mass Measurement for the Pre-Main-Sequence Secondary of the Eclipsing Binary TY CrA

R.D.Mathieu, B.Casey, L.P.Vaz (Wisconsin), J.Andersen (Copenhagen), N.Suntzeff (CTIO), F.Walter (SUNY-Stony Brook)

Using the Danish 50cm telescope at La Silla we have obtained simultaneous uvby light curves of the eclipsing binary TY CrA, located in the Corona Australis star-forming region. We have securely detected the secondary eclipse (2\% depth in $y$). We have also obtained high-resolution (R=15000) echelle spectra in the red. Along with the primary spectrum, absorption lines of the secondary and a previously unknown tertiary component have been found. In particular, both the secondary and tertiary are detected at the Lithium 6708\AA\ line. Based on temperature insensitive lines the tertiary/secondary luminosity ratio at $\sim$6400\AA\ is $\sim$1.5.

When combined with our previous single-lined orbital solution for the primary (Casey, B.W., Mathieu, R.D., Suntzeff, N.B., Lee, C.W., and Cardelli, J.A. 1993, Astron. Journal , 105, 2276) the secondary radial-velocity measurements provide a mass ratio of 0.521$\pm$0.007. Using a modified form of the Wilson-Devinney formalism, our light curve solution gives an inclination angle of 81\deg, masses and radii of (3.2 M$_\odot$, 1.8 R$_\odot$) and (1.7 M$_\odot$, 2.3 R$_\odot$) for the primary and secondary respectively. Based on both spectral classification and uvby colors we adopt a primary effective temperature of 12,000 $\pm$ 500 K. Using Kurucz atmosphere models for both stars in the WD solution, we derive a temperature of 5,000 K for the secondary, thus fully specifying the system. The primary lies on the ZAMS, while the secondary lies at the base of the Hayashi tracks. The secondary provides the first dynamical mass calibration with which to test theoretical calculations of Hayashi tracks. We will evaluate several modern theoretical pre-main sequence evolutionary models with respect to TY CrA.

The $v\sin i$ of the secondary spectrum is 40 km/sec, making the secondary rotation synchronous with the orbital motion. Given that the primary is remarkably subsynchronous (Casey et al. 1993 and new spectra), we conclude that the orbit was tidally circularized by the secondary.

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