Long-term Spectroscopic Monitoring of Arcturus

Abstract

Long-term monitoring of the bright K giant star Arcturus (K1.5 III, HR 5340) was conducted using the University of Western Ontario’s Elginfield Observatory. Over 1300 high resolution spectra (λ∕∆λ ≈ 105) were recorded in the λ6250 A spectral region during the 1984 - 2006 observing seasons with 311 simultaneous, precise radial velocity measurements (σ ~ 25 m s' ) obtained from 2002 - 2006. Temperature variations were measured using the well established line-depth ratio, V I λ6251.83 A to Fe I λ6252.56 Â. Velocity spans were obtained from the bisector of the Fe I λ6252.56 Â line. Radial velocities were measured using the telluric absorption technique outlined by Gray & Brown (2006a). Spectroscopic and precise radial velocity measurements reveal variability in Arcturus from days to approximately five years. From velocity span observations, three periods of variability are observed: 1) a semi-regular, 5 ± 1 year magnetic cycle, 2) a 2.0 ± 0.2 year modulation identified as the rotation period, and 3) a 200 ± 5 day period, interpreted as one fourth of the rotation period, indicating the presence of four active longitudes on the surface of Arcturus. Radial velocity measurements exhibit a period of 222 ± 3 days with an amplitude of 150 ± 20 m s', consistent with the observations of Hatzes & Cochrane (1993). This period is coherent over eighteen years and does not correlate with observed bisector or line-depth ratio variations, suggesting it is not associated with stellar variability. A detection of a low mass companion (≥ 4.4 MJupiter), is postulated. Radial velocity observations also demonstrate a scatter of ~ 145 m s' superimposed with the observed 222 day period, resulting from short-term variability on iii the timescale of nights to days. Variations during individual nights range from ~ 50 - 180 m s’1, while variability over consecutive nights ranges from ~ 70 - 160 m s'. Ultimately, this scatter of ~ 145 m s’ limits the accuracy of studies of long-term radial velocity variability of Arcturus. Line-depth ratio measurements indicate no variations in temperature in excess of 10 K over the observed magnetic cycle, rotation period, and short-term variability of Arcturus