Periodic Hα variations in GL 581: Further evidence for an activity origin to GL 581d

Abstract

Context. Radial velocity measurements initially showed evidence that the M dwarf GL 581 might host up to six planetary companions. Two of these, GL 581g and GL 581d had orbital distances in the so-called "habitable zone" of the star. The existence of both of these planets have been called into question. Additional radial velocity measurements for GL 581g could not confirm its presence. A study of Hα in GL 581 showed that changes in this activity indicator correlated with radial velocity variations attributed to GL 581d. Thus two planets that were important for studies of habitable planets may be artifacts of stellar activity. Aims. Previous investigations analyzing the same activity data have reached different conclusions regarding the existence of GL 581d. We therefore investigated the Hα variations for GL 581 to assess the nature of the radial velocity variations attributed to the possible planet GL 581d. Methods. We performed a Fourier analysis of the published Hα measurements for GL 581. Fourier components were selectively found and removed in a so-called pre-whitening process thus isolating any variations at the orbital frequency of GL 581d. Results. The frequency analysis yields five significant frequencies, one of which is associated with the 66.7 d orbital period of the presumed planet Gl 581d. The Hα variations at this period show sine-like variations that are 180° out-of-phase with the radial velocity variations of GL 581d. This is seen in the full data set that spans almost 7 years, as well as a subset of the data near the end of the time series that had good temporal sampling over 230 days. Furthermore, no significant temporal variations are found in the ratio of the amplitudes of the Hα index and radial velocity variations. This provides additional evidence that the radial velocity signal attributed to GL 581d is in fact due to stellar activity. Conclusions. The analysis confirms the anti-correlation of the radial velocity of GL 581d with the Hα equivalent width and provides additional strong evidence that the signal of GL 581d is intrinsic to the star.