Testing the planet hypothesis: A search for variability in the spectral-line shapes of 51 Pegasi

A search for variability in the spectral-line shapes of 51 Pegasi is performed by using high signal-to-noise ratio (S/N > 500), high-resolution (0.035 Angstrom) data covering one ''orbital'' period. We find no evidence for variability in the velocity span of the spectral-line bisectors greater than the error of the measurement (sigma approximate to 20 m s(-1)). It is demonstrated that the lack of strong variations in the bisector velocity span can be used to exclude the presence of nonradial sectoral modes with l greater than or equal to 4. The expected change in the bisector velocity span from low-order (l = 1-3) modes is about 10 m s(-1), or one-half the error measurement. Consequently, low-order nonradial pulsations can still account for the observed radial velocity (RV) amplitude and the lack of observed line shape variability. There are also no apparent variations in the equivalent width of a low-excitation V I line measured on three consecutive nights. These measurements place a limit of Delta T = 4 K for any disk-integrated temperature variations of the stellar surface. The projected rotational velocity was also measured using nine spectral lines and assuming a solar-like macroturbulent velocity, yielding a mean value of upsilon sin i = 2.35 +/- 0.1 km s(-1). This, along with the published rotation period of 37 days, yields a minimum stellar radius of 1.7 R.. Although the planet hypothesis is still the most likely explanation for the RV variations, this is still not definite. Precise photometric measurements and spectral observations at higher spectral resolution are needed to exclude with certainty the presence of low-order nonradial pulsations in 51 Pegasi.