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Doppler images from dual-site observations of southern rapidly rotating stars - I. Differential rotation on PZ Tel
We present Doppler images of the young, late-type star PZ Tel fromspectroscopic observations at two epochs. The 0.95-d period necessitateddual-site observations in order to obtain full phase coverage. Theresulting maximum entropy reconstructions based on least-squaresdeconvolved profiles derived from ~2800 photospheric absorption linesreveal the presence of starspots at all latitudes. By cross-correlationof constant-latitude strips from the images, we derive a measure ofrotation as a function of latitude. The rate of surface shear is foundto be in close agreement with the solar value. Hα absorptiontransients reveal several prominences to be present at the time ofobservation. We also examine further evidence for the young evolutionarystatus of PZ Tel through radial velocity, and lithium abundance.
| Diagnosis of Stellar Winds and Temperature Structures in Be Stars through the Analysis of Mg II Lines
We compute non-LTE Mg II line profiles for Be stars by considering 12energy-level atoms and supposing that the circumstellar medium can bedescribed by an expanding, spherically symmetric flow in which we assumethe presence of a chromosphere. The line radiative transfer equation wassolved rigorously in spherical coordinates and in the comoving frame.The Mg II line profiles predicted by this model coincide with thoseobserved in some Be stars. The calculation was performed for a range ofeffective temperatures representative of the B spectral type. We havealso analyzed the influence on the line profiles of differenttemperature and velocity distributions in the circumstellar material.The line spectrum variability of a Be star is interpreted as the resultof a variable outward mass flux.
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