Abstract
We report on the discovery of at least 18 pulsation modes in β Pictoris from 697 high-dispersion spectra obtained over a two-week period. These are seen as absorption subfeatures moving across the spectral line profiles, indicating that they are of relatively high spherical harmonic order. They do not resemble the features seen in the Ca H and K lines generally attributed to infalling bodies. We use the correlation function between an unbroadened synthetic spectrum and the observed spectrum to obtain what is essentially a high signal-to-noise ratio representation of the mean line profile. From the correlation intensity as a function of time, we calculated periodograms for every point across the correlation function. In this way, we are able to determine accurate periods for each mode. By phasing the correlation function with the period belonging to a given mode, we can isolate the line profile variations for the mode and deduce the approximate value of the azimuthal order, m. Two of the modes are retrograde, the remainder are prograde and most of them lie in the range 4<=|m|<=10. Comparison with non-adiabatic models shows that the periods are in the range expected for unstable modes in a star with the mass and temperature appropriate to β Pic. Some constraints can be placed on the effective temperature. We also report on photometric observations of β Pic obtained during the same run. From our 14 318 photometric observations combined with the discovery data from Koen (2003), we find two definite pulsation modes with frequencies of 47.4 and 38.1 cycle d^−1. A third frequency at 47.3 cycle d^−1 may also be present. The amplitudes are all smaller than 1.5 mmag.