STELLAR CONTRAILS IN QUASI-STELLAR OBJECTS - THE ORIGIN OF BROAD ABSORPTION-LINES

Active galactic nuclei (AGNs) and quasars often exhibit infrared exces ses at lambda = 2-10 mu m attributable to thermal dust emission. In th is paper we propose that this hot dust is supplied by circumstellar ma ss loss from evolved stars in the nuclear star cluster. The physics of the mass-loss dust, specifically the evaporation temperature, is a cr itical parameter in determining the accretion rate of mass-loss materi al onto the central AGN. For standard interstellar dust grains with an evaporation temperature of 1800 K the dust is destroyed inside a radi us of 1 pc from a central luminosity source of 5 x 10 L.. The mass-los s material inside 1 pc will therefore have a lower radiation pressure efficiency and accrete inward. Outside this critical radius, dust may survive, and the mass loss is accelerated outward owing to the high ra diation pressure efficiency of the dust mixed with the gas. The outflo wing material will consist of discrete trails of debris shed by the in dividual mass-loss stars, and we suggest that these trails produce the broad absorption lines (BALs) seen in 5%-10% of QSOs. The model accou nts naturally for the maximum outflow velocities seen in the BALs (sim ilar to 30,000 km s(-1) and varying as L(1/4)) since this maximum term inal velocity occurs for matter originating at the inner edge of the r adiative equilibrium dust survival zone. Although the radiation pressu re acts on the dust, individual grains will be highly charged (Z simil ar to 10(3)+), and the grains are therefore strongly coupled to the ga s through the ambient magnetic fields. Numerical hydrodynamic calculat ions were done to follow the evolution of mass-loss material. As the o rbiting debris is driven outward by radiation pressure, the trail form s a spiral with initially high pitch angle (similar to 85 degrees). Th e trails are compressed into thin ribbons in the radial direction-init ially by the radiation pressure gradients due to absorption within the trail. After reaching >10(4) km s(-1) radial velocity, the compressio n can be maintained by ram pressure due to an ambient gas of modest de nsity (similar to 10(2) cm(-3)). Each of the stellar contrails will ha ve mean column density similar to 10(19)-10(21) cm(-2), volume density similar to 10(8)-10(9) cm(-3), and thickness 10(11)-10(12) cm along t he line of sight to the AGN-corresponding to parameters deduced from o bservations of the BAL clouds. Assuming minimal expansion perpendicula r to the line of sight at the speed of sound, the width of the trails is 10(15)-10(16) Cm, Or 10(2)-10(3) times the line-of-sight depth. Sin ce the UV-emitting accretion disk probably has a radius of about 2 x 1 0(16) cm, a single trail will only partially cover the continuum, but for the column densities quoted above the observed absorption lines (e .g., C IV) will be optically thick with 1.>10. Since the contrails are nearly radial just after leaving the star when the maximum outward ac celeration occurs, a large range of velocities (similar to 4000 km s(- 1)) will be seen in absorption of the QSO light from each trail, and o nly a few disk-crossing trails are needed to account for the full widt h of broad absorption line troughs.