Bloated stars as AGN broad-line clouds: the emission-line profiles

Abstract

The Bloated Stars Scenario proposes that AGN broad line emission originates in the winds or envelopes of bloated stars (BS). Alexander and Netzer (1994) established that ~ 5e4 BSs with dense, decelerating winds can reproduce the observed emission line spectrum and avoid rapid collisional destruction. Here, we use the observed properties of AGN line profiles to further constrain the model parameters. In the BS model, the origin of the broad profiles is the stellar velocity field in the vicinity of the central black hole. We use a detailed photoionization code and a model of the stellar distribution function to calculate the BS emission line profiles and compare them to a large sample of AGN CIV, CIII] and MgII profiles. We find that the BSs can reproduce the general shape and width of typical AGN profiles as well as the line ratios if (i) The ionizing luminosity to black hole mass ratio is low enough. (ii) The broad line region size is limited by some cutoff mechanism. (iii) The fraction of the BSs in the stellar population falls off roughly as r^-2. (iv) The wind density and/or velocity are correlated with the black hole mass and ionizing luminosity. Under these conditions the strong tidal forces near the black hole play an important role in determining the line emission properties of the BSs. Some discrepancies remain: the calculated BS profiles tend to have weaker wings than the observed ones, and the differences between the profiles of different lines are somewhat smaller than those observed.