The Lack of Spectral Variability in 51 Pegasi: Confirmation of the Planet Hypothesis

Abstract

We present-high resolution (R > 200,000), high signal-to-noise spectral data for 51 Pegasi spanning a complete orbital period of the purported planetary companion. A detailed examination of the line bisectors from eight spectral lines as well as the ratio of the line depth of the V I 6251 A line to the Fe I 6253 A line were performed. The changes in the mean bisector velocity span and curvature are about 1 m s-1 and 4 m s-1, respectively. These variations are an order of magnitude less than the variations reported by Gray and are small enough to exclude all nonradial sectoral pulsation modes. The line-depth ratio of V I/Fe I also shows no significant variations, and a least-squares sine fit using the planet period of 4.23 days yields an amplitude of 0.00038 for any line-depth ratio variations. This corresponds to a disk-integrated temperature variation (peak-to-peak) of less than 1.7 K. We conclude that the radial velocity variations are not due to a dynamical stellar atmosphere and that a planetary companion remains as the only viable explanation for the observed radial velocity variations. The lack of bisector variability in our data indicates that the contribution to the observed spectra due to reflected light from the planet is at least 2000 times less than that of the primary light. Assuming a planetary albedo of unity, this places an upper limit of 0.2 R☉ for the radius of the planet.