SIMBAD references

This paper investigates so-called shell absorption lines occurring in Be star disks seen edge-on. These lines are counterparts of absorption troughs in P Cyg-type profiles for different geometry and kinematics and offer a probe of a small portion of the circumstellar disk. Contrary to other studies, this investigation concentrates on Fe lines as prototype examples for metallic shell lines, which have an extremely small thermal width (2km/s at 10⁴K). However, it is also demonstrated that incorporation of turbulence provides a powerful tool to generalize the results towards lines from arbitrary ions. As a simple, though not unphysical model, a purely Keplerian, isothermal disk in hydrodynamic equilibrium is adopted. The intention of this paper is to provide simple approximations for easy use. Simple geometrical illustrations of the shell absorption behaviour projected onto the stellar disk are used to illustrate the origin of shell line profiles. It is shown that resulting Fe shell profiles consist of a broad kinematic part and a narrow thermal core, the narrowness of which depends on the intrinsic Doppler width of the line. The outer disk radius R_d may cause, under certain circumstances, cusps at radial velocities ±V_d=R_d^–3/2. These cusps can become the deepest points in the line profile, thus causing a central peak. Finally it is demonstrated that if the disk becomes turbulent, the central peak disappears, and the Fe shell profiles are identical to hydrogenic lines from non-turbulent disks. This study is intended to provide tools for comparison to recent high-resolution, high-S/N measurements of optical shell lines in Be stars. However, it also bears significance for the more general case of central continuum light sources partly occulted by thin disks.