Spectroscopic orbital analysis of the δ Scuti binary, RS Cha – High-resolution spectroscopy reveals a third component

Over the last 40 years variations in the systemic velocity and the observed minus computed time of first conjunction have been observed in the RS Cha binary system. Our goal is to determine the probability for the existence of a third body in this system, and to calculate an orbital solution for this component. A total of 381 high-resolution echelle spectra were obtained at Mount John University Observatory using the 1.0-m McLellan telescope and High Efficiency and Resolution Canterbury University Large Echelle Spectrograph (HERCULES; echelle spectrograph). The spectra were collected during three observing runs occurring over a 15 month period spanning from 2005 November 18 to 2007 February 17, and the data were reduced using the HERCULES reduction software package 2.3. Radial velocities for the 46 echelle orders were generated using Two-Dimensional Correlation, and the velocities from the best 15 orders were selected and used in the calculation of a weighted mean. The weight for each order was determined by generating a preliminary orbital solution for that particular order, using Stern’s method, with the rms of the orbital fit used to calculate the associated weight on the order. Systemic velocities for each of the three observing runs were computed by applying a linear regression to the radial velocities of one star against its companion (i.e. V1 versus V2). The value of the slope and intercept of the regression line are required for calculating the systemic velocity. Analysis of the 381 spectra confirmed the suspected variation of the system velocity during the time-span over which these data were collected. The systemic velocity for each observing run differs significantly (12.13 ± 0.26, 11.41 ± 0.22 and 9.68 ± 0.78 km s −1 ) and combined with four historical (previously published) values they failed the χ 2 test, and imply a 99.9 per cent confidence that a third body exists. Three possible orbital solutions for the third body, with respect to the close binary, were generated using the historical and current systemic velocity values (P = 12.69 ± 0.01 or 24.17 ± 0.01 or 74.45 ± 0.02 d). The orbital solution for the binary was calculated after the effects of the shift in systemic velocity during the course of our data were removed. Values for the period and masses are P =