Precise radial velocities with BOES

Detection of low-amplitude pulsations in the K-giant α Arietis

¹ Korea Astronomy and Space Science Institute, 61-1 Whaam-Dong Yuseong-Gu, Daejeon 305-348, Korea
² Astrophysical Research Center for the Structure and Evolution of the Cosmos, Sejong University, Seoul 143-747, Korea e-mail: david@arcsec.sejong.ac.kr
³ Astronomical Observatory, Odessa National University, Shevchenko Park, Odessa 65014, Ukraine
⁴ Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu 702-701, Korea
⁵ Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany

Received: 16 March 2005
Accepted: 26 January 2006

Abstract

We present the first results from a high-precision radial velocity study of the K2 III giant star α Ari. Observations were acquired over 6 nights in 2004 using the new high-resolution spectrograph BOES (Bohyunsan Observatory Echelle Spectrograph) of the 1.8-m telescope. A high radial-velocity precision was achieved by using the high-resolution () mode of BOES and an iodine gas absorption cell. The radial velocity measurements made during show coherent, low-amplitude variations with a period of days (or aliases at 0.445 or 0.821 days), and an amplitude of 18.9 m s^-1. Observations of τ Cet over this same interval are constant to within 3 m s^-1. After subtracting the contribution of the 0.57-day period, we find evidence for a second period, days. Observations made on a second run during show that the radial velocity variations are indeed present, but on shorter time-scales and with a lower amplitude. Two probable periods fit the radial velocity measurements from the second run reasonably well: 0.185 days or an alias of 0.256 days. The shorter period coincides with the secondary one found in the earlier measurements. We conclude that, similar to other K-giant pulsating stars, α Ari shows unstable acoustic pulsations or mode switching on time scales of tens of days. The calculated pulsation constants for the dominant 0.571-day period is consistent with third-overtone pulsations, while the secondary periodicity found in both data runs corresponds to a high overtone ().