Properties and nature of Be stars. XVIII. Spectral, light and colour variations of 4 Herculis

An analysis of a rich series of spectroscopic and photometric observations of the Be star 4 Her led to the following conclusions:

4 Her is another example of a long-term Be variable with a type of correlation between the brightness and emission strength, similar to 88 Her (V744 Her) and BU Tau (Pleione). It is argued that the formation of a new Be envelope of 4 Her starts with the creation of a slightly cooler pseudophotosphere at the equatorial regions of the star (seen under some intermediate inclination angle) which only gradually grows into an optically thin extended envelope. Radial-velocity measurements of the centre of the Hα emission and of the photospheric lines confirm the binary nature of the star. The first reliable orbital elements are presented. The 46-d orbit is nearly circular and has a semiamplitude of 5-8 . () kms() -1. An LTE model atmosphere analysis of the photospheric spectrum of the primary leads to T_eff=12500K, log g = 4.0, and v\sin i=300kms^-1. No direct evidence of the low-mass secondary was found and the possibility that the secondary fills its Roche lobe can be safely excluded. The central quasi-emission bumps (CQEB) visible as "doubling" of some shell lines appear during the phase of the formation of a new shell. They are strongest during the light minimum and become fainter as the Hα emission strengthens. An unusual blue-shifted absorption component of the Hα line, never reported before, re-appears strictly periodically in the V peak of the Hα emission at a limited range of velocities and orbital phases. It is argued that the observational facts about 4 Her are probably best reconciled by a model which assumes that the secondary is a hot and rotationally unstable object which looses mass towards the primary via a gas stream. However, some important findings remain unexplained. This research is based on spectra from the \Ond\ and Haute Provence Observatories and on photometry from Hvar, \Ond, Mt. Kobau, Toronto, APT Phoenix-10, and AAVSO observers.