Long-lived, long-period radial velocity variations in Aldebaran: A planetary companion and stellar activity^⋆,⋆⋆

¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
e-mail: artie@tls-tautenburg.de
² McDonald Observatory, The University of Texas at Austin, Austin, TX 78712, USA
³ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁴ Korea Astronomy and Space Science Institute, 776, Daedeokdae-Ro, Youseong-Gu, Daejeon 305-348, Korea
⁵ 1234 Hewlett Place, Victoria, BC, V8S 4P7, Canada
⁶ Department of Physics and Astronomy, University of Victoria, Victoria, BC, V8W 3P6, Canada
⁷ Astronomy Department, Box 351580, University of Washington, Seattle, WA 98195-1580, UK
⁸ National Astronomical Research Institute of Thailand, 191 Siriphanich Bldg., Huay Kaew Rd., Suthep, Muang, 50200 Chiang Mai, Thailand
⁹ Crimean Astrophysical Observatory, Nauchny, Crimea, 98409, Ukraine
¹⁰ Physikalisches Institut, Center for Space and Habitability, University of Bern, Sidlerstrasse 5, 3012 Bern, Swithzerland
¹¹ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, 1121 Budapest, Konkoly Thege. Miklós. út 15-17, Hungary
¹² INAF-Osservatorio Astronomico di Padova, vicolo dell’O servatorio 5, 35122 Padova, Italy

Received: 15 December 2014
Accepted: 29 April 2015

Abstract

Aims. We investigate the nature of the long-period radial velocity variations in α Tau first reported over 20 yr ago.

Methods. We analyzed precise stellar radial velocity measurements for α Tau spanning over 30 yr. An examination of the Hα and Ca II λ8662 spectral lines, and Hipparcos photometry was also done to help discern the nature of the long-period radial velocity variations.

Results. Our radial velocity data show that the long-period, low amplitude radial velocity variations are long-lived and coherent. Furthermore, Hα equivalent width measurements and Hipparcos photometry show no significant variations with this period. Another investigation of this star established that there was no variability in the spectral line shapes with the radial velocity period. An orbital solution results in a period of P = 628.96 ± 0.90 d, eccentricity, e = 0.10 ± 0.05, and a radial velocity amplitude, K = 142.1 ± 7.2 m s^-1. Evolutionary tracks yield a stellar mass of 1.13 ± 0.11 M_⊙, which corresponds to a minimum companion mass of 6.47 ± 0.53 M_Jup with an orbital semi-major axis of a = 1.46 ± 0.27 AU. After removing the orbital motion of the companion, an additional period of ≈520 d is found in the radial velocity data, but only in some time spans. A similar period is found in the variations in the equivalent width of Hα and Ca II. Variations at one-third of this period are also found in the spectral line bisector measurements. The ~520 d period is interpreted as the rotation modulation by stellar surface structure. Its presence, however, may not be long-lived, and it only appears in epochs of the radial velocity data separated by ~10 yr. This might be due to an activity cycle.

Conclusions. The data presented here provide further evidence of a planetary companion to α Tau, as well as activity-related radial velocity variations.