Spectroscopic signatures of the tidal disruption of stars by massive black holes

During the tidal disruption of a main-sequence star by a massive black hole (BH) having mass M_BH≲ 10⁷M_⊙, the stellar debris is expected to fall back to the BH at a rate well above the Eddington rate. Some fraction of this gas is predicted to be blown away from the BH, producing an optically bright flare of radiation. We predict the spectra and spectral evolution of tidal disruption events, focusing on the signatures produced by photoionized gas outside the photosphere of this super-Eddington outflow. We show that the spectrum of such an outflow should show absorption lines that are strongly blueshifted relative to the host galaxy, are typically very broad (0.01-0.1c) and are most prominent at ultraviolet wavelengths (e.g. Civ, Lyman α, Ovi) at early times (≲1 month for a ~10⁶M_⊙ BH). There may also be optical absorption lines of hydrogen and Heii if there is a lower velocity component to the outflow (≲0.01c). At later times, the outflow falls out of thermal equilibrium and the continuum emission likely hardens - the absorption lines will then primarily be in the soft X-rays. Supernovae in galactic nuclei are a significant source of confusion in optical surveys for tidal disruption events: we estimate that nuclear Type Ia supernovae are two orders of magnitude more common than tidal disruption events at z~ 0.1 for ground-based surveys. Nuclear Type II supernovae occur at a comparable rate but can be excluded by pre-selecting red galaxies. The contamination from nuclear supernovae can be reduced to a manageable level by using high-resolution follow-up imaging with adaptive optics or the Hubble Space Telescope. Our predictions should help optical transient surveys capitalize on their potential for discovering tidal disruption events.