SHELL LINES IN DISKS AROUND BE-STARS .1. SIMPLE APPROXIMATIONS FOR KEPLERIAN DISKS

This paper investigates so-called shell absorption lines occurring in Be stat 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. Contra ry to other studies, this investigation concentrates on Fe lines as pr ototype examples for metallic shell lines, which have an extremely sma ll thermal width (2 km s(-1) at 10(4) K). However, it is also demonstr ated that incorporation of turbulence provides a powerful tool to gene ralize the results towards lines from arbitrary ions. As a simple, tho ugh not unphysical model, a purely Keplerian, isothermal disk in hydro dynamic equilibrium is adopted. The intention of this paper is to prov ide simple approximations for easy use. Simple geometrical illustratio ns of the shell absorption behaviour projected onto the stellar disk a re used to illustrate the origin of shell line profiles. It is shown t hat resulting Fe shell profiles consist of a broad kinematic part and a narrow thermal core, the narrowness of which depends on the intrinsi c Doppler width of the line. The outer disk radius R(d) may cause, und er certain circumstances, cusps at radial velocities +/-V-d = R(d)(-3/ 2). These cusps can become the deepest points in the line profile, thu s causing a central peak. Finally it is demonstrated that if the disk becomes turbulent, the central peak disappears, and the Fe shell profi les are identical to hydrogenic lines from non-turbulent disks. This s tudy is intended to provide tools for comparison to recent high-resolu tion, high-S/N measurements of optical shell lines in Be stars. Howeve r, it also bears significance for the more general case of central con tinuum light sources partly occulted by thin disks.