Emission lines from tidally disrupted white dwarfs and other evolved stars

Abstract

When a black hole tidally disrupts a star, accretion of the debris will produce a luminous flare and reveal the presence of a dormant black hole. The accretion flare can also photoionize a portion of the post-disruption debris. We present models of the emission line spectrum produced in the debris released when a white dwarf or a horizontal branch star is tidally disrupted by an intermediate-mass black hole, and discuss the possibility of using the emission lines to identify such events and constrain the properties of the black hole. We also compare the white dwarf disruption models with observations of white dwarf tidal disruption candidates in globular clusters associated with NGC 4472 and NGC 1399. The bright [O III] lines observed in each system are consistent with these models, but there are some drawbacks to interpreting these sources as tidally disrupted white dwarfs. On the other hand, models of the emission line spectrum produced when a horizontal branch star is disrupted by a ∼ 100 Mʘ black hole are in good agreement with the source in the NGC 1399 globular cluster. Finally, we describe light curves for the emission lines produced in the debris of a tidally disrupted helium core. The modeled light curves are consistent with the recent observations of Gezari et al. (2012).