Accurate orbital parameters for the bright southern spectroscopic binary ζ Trianguli Australis – an interesting case of a near-circular orbit

Abstract

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 = 0.014 42 ± 0.000 21 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 = 12.975 780 ± 0.000 046 d. The mass of the companion is in the range 0.09-0.45 M , which covers the spectral types between M1 V and M7 V, on the assumption that the companion is a main-sequence star. In that case the orbital inclination has to be larger than about 14 • .

An Investigation of the Small Eccentricity in the Spectroscopic Binary System ζ TrA

“…We note that the difference between a circular orbit and the observations amounts to as much as 140 m s −1 at some phases, which is essentially the amplitude of the second harmonic in the data. Our conclusion is that this strong and highly significant second harmonic is most probably the result of a small orbital eccentricity as reported by Skuljan et al (2004). We note that the observed third harmonic according to Lucy (2005) has an amplitude of only 5.2 ± 2.0 m s −1 , which is just over twice the error bar of its measurement, and that the predicted third harmonic for an eccentric orbit is only 1.6 m s −1 .…”

“…The model included both tidal distortion and the reflection effect, both of which in principle cause small perturbations of the radial velocity from the true centre of mass value for the primary star. The secondary star was assumed to be between M1V and M7V as there are maximum and minimum limits from the mass function and light ratio (see Skuljan et al 2004). The RV curves generated from the program for three models (F9V+M1V without proximity effects, F9V+M1V with proximity effects and F9V+M7V with proximity effects), using the orbital parameters as derived by Skuljan et al (2004), show no difference in radial velocities between the models and observational data.…”

“…We generated a model of ζ TrA from the 2003 version of the Wilson-Devinney (1971) computer program in order to match the observational data from Skuljan et al (2004). The model included both tidal distortion and the reflection effect, both of which in principle cause small perturbations of the radial velocity from the true centre of mass value for the primary star.…”

“…F9V, P ∼ 13 d) was found by Skuljan et al (2004) to be e = 0.01398±0.00019. Lucy (2005) devised a statistical test of the significance of this result, based on the amplitude and phase of the third harmonic in the Fourier analysis of the radial velocity data, and concluded that the non-zero eccentricity measured does not arise from a slightly eccentric Keplerian orbit, but from proximity effects in the binary.…”

“…In a recent paper, Skuljan et al (2004) derived precise elements of a Keplerian orbit for the F9V star ζ TrA (V = 4.91) in a single-line spectroscopic binary. They used 228 high-precision radial velocities from a high-resolution fibre-fed echelle spectrograph, Hercules.…”