High resolution spectroscopy of HgMn stars: a time of surprises

HUBRIG, S.; COWLEY, C. R.; GONZALEZ, J. F.; CASTELLI, F.

Aveiro, Portugal

Conferencia; Precision Spectroscopy in Astrophysics; 2006

European Southern Observatory - Univ. Lisboa - Univ. Aveiro

The origin of the abundance anomalies observed in late B-type stars with HgMn peculiarity is still poorly understood. Observationally, these stars are characterized by low rotational velocities and weak or non-detectable magnetic fields. The most distinctive features of their atmospheres are an extreme overabundance of Hg (up to 6 dex) and/or Mn (up to 3 dex), a deficiency of He, and isotopic anomalies observed for various elements. The recent discovery of an inhomogeneous distribution of various elements in the atmospheres of ARAur and áAnd challenges our understanding of the nature of HgMn stars. Our high quility spectroscopic observations obtained during the last months led already to the discovery of a number of HgMn stars with spots, proving that the presence of an inhomogeneous distribution on the surface of these stars is a rather common characteristics and not just a rare phenomenon found in the atmospheres of a very few HgMn stars. Typically, inhomogeneous chemical abundance distributions are observed only on the surfaces of magnetic chemically peculiar stars with large-scale organized magnetic fields. In these stars the abundance distribution of certain elements is non-uniform and non-symmetric with respect to the rotation axis. A magnetic field of the order of a few hundred Gauss was already detected in six HgMn stars. The structure of the measured field in HgMn stars must be, however, sufficiently tangled so that it does not produce a strong net observable circular polarization signature.