The Pre-Main-Sequence Triple TY CrA: Spectroscopic Detection of the Secondary and Tertiary Components
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We present new spectroscopic data for the eclipsing, pre-main-sequence binary TY CrA. Absorption lines from the secondary and from a previously unknown tertiary are detected, including Li I 6708 in both stars which supports the pre-main-sequence status of the system. Radial velocities of the primary and secondary are given, from which we derive minimum masses of M1 sin3 i=3.080.04 M and M2 sin3 i=1.590.02 M, and a mass ratio of q=M2/M1=0.5180.005. Radial-velocity measurements for the tertiary, and variations in the binary center-of-mass velocity, suggest detection of the orbital motion of the tertiary-binary system, but this requires confirmation. Analysis of these motions yields an estimated tertiary mass of 2.40.5 M. We measure stellar luminosity ratios using observations of the Fe I 6400 and Li I 6708 line strengths and application of appropriate bolometric corrections. We find the primary/secondary bolometric luminosity ratio to be between 10 and 33 and the tertiary/secondary bolometric luminosity ratio to be between 0.5 and 3.9, both depending on the secondary and tertiary effective temperatures. Adopting a ZAMS luminosity for the primary, the luminosity ratios indicate the secondary star is in the pre-main-sequence stage, likely near the base of the Hayashi track. The FWHM of primary lines is less than 10 km s-1, which is consistent with earlier v sin i measurements of less than 10 km s-1 and indicates subsynchronous rotation. The projected rotational velocities of the secondary and tertiary are 323 kms-1 and 503 kms-1, respectively. The secondary's rotation is consistent with synchronism for radii calculated from evolutionary models. For radii derived from the observed luminosity, the secondary is slightly subsynchronous, though the uncertainties in this empirical radius determination permit synchronism. We identify a candidate solution for the secondary which is reasonably consistent with the dynamical mass, the spectroscopically determined luminosities, synchronous rotation, and the 1.6 M evolutionary track of Swenson et al. [ApJ, 425, 286 (1994)] at an age of 3 million years.
