Tm. Brown et al., EXOPLANETS OR DYNAMIC ATMOSPHERES - THE RADIAL-VELOCITY AND LINE-SHAPE VARIATIONS OF 51 PEGASI AND TAU-BOOTIS, The Astrophysical journal. Supplement series, 117(2), 1998, pp. 563-585
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.