A search for variability in the spectral line shapes of tau Bootis: Does this star really have a planet?

An analysis is made of the spectral line shapes of tau Bootis using high-re solution (0.026 Angstrom) and high signal-to-noise (S/N approximate to 400) data in an effort to confirm the planet hypothesis for this star. Changes in the line shape are quantified using spectral line bisectors and line res iduals. We detect no variations in either of these quantities above the lev el of the noise in the data. One spectral line, Fe I lambda 6213, does show a hint of sinusoidal variations in the bisector velocity span when phased to the radial velocity period of 3.3 days, but this is not seen in the bise ctors for two other lines nor in the line residuals. Comparisons of the dat a to the bisector and residual variations expected for nonradial pulsations indicate that we can exclude those sectoral nonradial modes having m>2 and all sectoral modes with k>1, where k is the ratio of the horizontal to ver tical velocities for the pulsations. The lack of line shape variability and the 469 m s(-1) radial velocity amplitude is still consistent with nonradi al sectoral modes m = 1 and possibly with m = 2, but with k approximate to 1, which is at least 3 orders of magnitude less than the predicted value gi ven the 3.3 day period of tau Bootis. Such low values of k can probably be excluded given the lack of photometric variations for this star. Although t he measurements presented here do not prove beyond a doubt that tau Boo has a planetary companion, they do add significantly to the increasing body of evidence in favor of this hypothesis.