HI Infrared Line Formation in Early B-Type Stars

Infrared spectroscopy of early-type stars opens the possibility to study the outer parts of their atmospheres and stellar winds. Hot stars with strong stellar winds show signs of this outflow in UV (e.g. CIV, SiIV, NV) and optical (Hα) P Cygni profiles. Stars with weak winds, such as late O and early B dwarfs, usually have photospheric Hα and weak or absent wind absorption or emission in UV resonance lines of CIV and SiIV. The IR spectra of these stars contain important diagnostic tools of their photospheres and winds, which are becoming accessible with modern IR spectrographs. HI and HeI infrared lines are formed in the outer photosphere and wind where the level populations of quantum levels involved, slightly depart from LTE. Even though these departures are small, they strongly affect the HI infrared line profiles due to the effect of stimulated emission, i.e. hν/kT > 1, and drives the HI infrared lines in emission. Such non-LTE effects make these line transitions very sensitive to the local physical conditions, like density, temperature and mass loss.