A search for pulsations in the HgMn star HD 45975 with CoRoT photometry and ground-based spectroscopy ⋆

The existence of pulsations in HgMn stars is still being debated. To provide the first unambiguous observational detecti on of pulsations in this class of chemically peculiar objects, the bright sta r HD 45975 was monitored for nearly two months by the CoRoT satellite. Independent analyses of the light curve provides evidence of monoperiodic variations with a frequency of 0.7572 d −1 and a peakto-peak amplitude of∼2800 ppm. Multisite, ground-based spectroscopic observations overlapping the CoRoT observations show the star to be a long-period, single-lined binary. Furthermore , with the notable exception of mercury, they reveal the same periodicity as in photometry in the line moments of chemical species exhibiting strong overabundances (e.g., Mn and Y). In contrast, lines of other elements do not show significant variations. As found i n other HgMn stars, the pattern of variability consists in an absorption bump moving redwards across the line profiles. We argue that t he photometric and spectroscopic changes are more consistent with an interpretation in terms of rotational modulation of spots a t the stellar surface. In this framework, the existence of pu lsations producing photometric variations above the∼50 ppm level is unlikely in HD 45975. This provides strong constraints on the excitation/damping of pulsation modes in this HgMn star.