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 p erformed by using high signal-to-noise ratio (S/N > 500), high-resolut ion (0.035 Angstrom) data covering one ''orbital'' period. We find no evidence for variability in the velocity span of the spectral-line bis ectors 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 non radial 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, lo w-order nonradial pulsations can still account for the observed radial velocity (RV) amplitude and the lack of observed line shape variabili ty. There are also no apparent variations in the equivalent width of a low-excitation V I line measured on three consecutive nights. These m easurements place a limit of Delta T = 4 K for any disk-integrated tem perature variations of the stellar surface. The projected rotational v elocity was also measured using nine spectral lines and assuming a sol ar-like macroturbulent velocity, yielding a mean value of upsilon sin i = 2.35 +/- 0.1 km s(-1). This, along with the published rotation per iod of 37 days, yields a minimum stellar radius of 1.7 R.. Although th e planet hypothesis is still the most likely explanation for the RV va riations, 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.