Asteroseismology of the β Cephei star ν Eridani -: II.: Spectroscopic observations and pulsational frequency analysis

We undertook a multisite spectroscopic campaign for the beta Cephei star nu Eridani. A total of 2294 high-resolution spectra were obtained from telescopes at 11 different observatories around the world. The time base of dedicated multisite observations is 88 d. To this data set we have added 148 older, previously unpublished spectra, such that the overall time-span of the 2442 spectra is 430 d. The analysis of the radial velocity variations derived from the Si III triplet centred on 4560 Angstrom leads to 19 significant frequencies, of which seven correspond to independent pulsation frequencies. Five of these are members of multiplets with an average spacing of 0.018+/-0.002 cd(-1). Our spectroscopic results agree well with those derived from a simultaneous multisite photometric campaign of the star, albeit that we do not recover their low frequency at 0.43218 cd(-1). We find three different candidate frequencies below 1 cd(-1) instead. We also find that the radial velocity amplitude of the main mode has increased by some 30 per cent over the last 15 years, which is consistent with the photometry data. We derive a relative equivalent width variation of 6.5 per cent, which is completely dominated by the main radial mode. The phase difference between the radial velocity and light variations for the main frequency is 97.degrees9+/-1.degrees8, which is clearly deviant from the adiabatic value and confirms the radial nature of the dominant mode. The spectral line broadening leads to an upper limit of 20 km s(-1) for v sin i, which is consistent with the long rotation period derived from the frequency splittings.