On the nature of the radial velocity variability of Aldebaran: a search for spectral line bisector variations

The shape of the Tii 6303.8-A spectral line of Aldebaran as measured by the line bisector was investigated using high signal-to-noise ratio, high-resolution data. The goal of this study was to understand the nature of the 643-d period in the radial velocity for this star reported by Hatzes & Cochran. Variations in the line bisector with the radial velocity period would provide strong evidence in support of rotational modulation or stellar pulsations as the cause of the 643-d period. A lack of any bisector variability at this period would support the planet hypothesis. Variations in the line asymmetries are found with a period of 49.93d. These variations are uncorrelated with the 643-d period found previously in the radial velocity measurements. It is demonstrated that this 50-d period is consistent with an m=4 non-radial sectoral g-mode oscillation. The lack of spectral variability with the radial velocity period of 643d may provide strong evidence in support of the hypothesis that this variability stems from the reflex motion of the central star due to a planetary companion having a mass of 11 Jupiter masses. However, this long-period variability may still be the result of a low-order (m=2) pulsation mode as these would cause bisector variations of less than the error measurement.