Accurate orbital parameters for the bright southern spectroscopic binary ζ Trianguli Australis - an interesting case of a near-circular orbit
Accurate orbital parameters have been obtained for the bright southern single-lined spectroscopic binary star ζ TrA, with the 1-m telescope and Hercules spectrograph at Mt John University Observatory. More than 200 observations have been collected over five months of observation between 2003 March and August. They cover about 12 complete periods (P~ 13 d). High-precision relative radial velocities have been measured by the cross-correlation technique with HRSP, a dedicated reduction software package developed in the Department of Physics and Astronomy, University of Canterbury. An iterative linear least-squares method has been used to improve the initial set of orbital parameters estimated from the geometrical properties of the radial-velocity curve. The final rms error of the least-squares fit is only about 14 m s-1 so the error bars on the orbital parameters are extremely small. As a particularly interesting result, a very low eccentricity of e= 0.01442 +/- 0.00021 has been derived, which proves that the orbit is definitely elliptical and not circular as has been suggested in the literature. The orbital period has also been improved by including some historical observations so that the best value is P= 12.975780 +/- 0.000046 d. The mass of the companion is in the range 0.09-0.45 Msolar, which covers the spectral types between M1V and M7V, on the assumption that the companion is a main-sequence star. In that case the orbital inclination has to be larger than about 14°.