Stellar chromospheres

Observations indicating the presence of stellar chromospheres, that is hot envelopes around stars are summarized. Undisputed indicators (called type I) for hot stellar envelopes are spectral lines of highly ionized atoms, Fe ii emission lines and flares in late type stars and the presence of the He i10830 Å line. Whether indicators (called type II) like emission cores in the Ca ii H and K and Mg ii h and k lines or mass loss signify the presence of stellar chromospheres is still somewhat debated, although the discussion points in favour of the usefulness of these indicators. The combined evidence to date shows that all non degenerate type stars have chromospheres except possibly the A stars. There are however theoretical reasons for expecting chromospheres in A stars. Empirical chromosphere models for a rapidly growing sample of stars have recently been constructed on the basis of Ca ii and Mg ii line observations. A discussion of possible heating mechanisms is given and the relative importance of these mechanisms is evaluated. For the low and middle chromosphere the short period acoustic heating mechanism seems to be the dominant process although there are still uncertainties. Both steady state and time dependent theoretical models of stellar chromospheres, based on the short period acoustic heating theory, are discussed, and predictions of these models are compared with results from empirical models. This relatively favourable comparison shows that the explanation of the Wilson-Bappu effect might be at hand.