EXOPLANETS OR DYNAMIC ATMOSPHERES - THE RADIAL-VELOCITY AND LINE-SHAPE VARIATIONS OF 51 PEGASI AND TAU-BOOTIS

The stars 51 Pegasi and tau Bootis show radial velocity variations tha t have been interpreted as resulting from companions with roughly Jovi an mass and orbital periods of a few days. Gray and Gray & Hatzes repo rted that the radial velocity signal of 51 Peg is synchronous with var iations in the shape of the line lambda 6253 Fe I; thus, they argue th at the velocity signal arises not from a companion of planetary mass b ut from dynamic processes in the atmosphere of the star, possibly nonr adial pulsations. Here we seek confirming evidence for line shape or s trength variations in both 51 Peg and tau Boo, using R = 50,000 observ ations taken with the Advanced Fiber Optic Echelle. Because of our rel atively low spectral resolution, we compare our observations with Gray 's line bisector data by fitting observed line profiles to an expansio n in terms of orthogonal (Hermite) functions. To obtain an accurate co mparison, we model the emergent line profiles from rotating and pulsat ing stars, taking the instrumental point-spread function into account. We describe this modeling process in detail. We find no evidence for line profile or strength variations at the radial velocity period in e ither 51 Peg or in tau Boo. For 51 Peg, our upper limit for line shape variations with 4.23 day periodicity is small enough to exclude with 10 sigma confidence the bisector curvature signal reported by Gray & H atzes; the bisector span and relative line depth signals reported by G ray are also not seen, but in this case with marginal (2 sigma) confid ence. We cannot, however, exclude pulsations as the source of 51 Peg's radial velocity variation because our models imply that line shape va riations associated with pulsations should be much smaller than those computed by Gray & Hatzes; these smaller signals are below the detecti on limits both for Gray & Hatzes's data and for our own. tau Boo's lar ge radial velocity amplitude and upsilon sin i make it easier to test for pulsations in this star. Again we find no evidence for periodic li ne shape changes, at a level that rules out pulsations as the source o f the radial velocity variability. We conclude that the planet hypothe sis remains the most likely explanation for the existing data.