The Tidal Disruption of Giant Stars and Their Contribution to the Flaring Supermassive Black Hole Population

Abstract

Sun-like stars are thought to be regularly disrupted by supermassive black holes (SMBHs) within galactic nuclei. Yet, as stars evolve off the main sequence their vulnerability to tidal disruption increases drastically as they develop a bifurcated structure consisting of a dense core and a tenuous envelope. Here we present the first hydrodynamic simulations of the tidal disruption of giant stars and show that the core has a substantial influence on the star's ability to survive the encounter. Stars with more massive cores retain large fractions of their envelope mass, even in deep encounters. Accretion flares resulting from the disruption of giant stars should last for tens to hundreds of years. Their characteristic signature in transient searches would not be the t −5/3 decay typically associated with tidal disruption events, but a correlated rise over many orders of magnitude in brightness on months to years timescales. We calculate the relative disruption rates of stars of varying evolutionary stages in typical galactic centers, then use our results to produce Monte Carlo realizations of the expected flaring event populations. We find that the demographics of tidal disruption flares are strongly dependent on both stellar and black hole mass, especially near the limiting SMBH mass scale of ∼ 10 8 M . At this black hole mass, we predict a sharp transition in the SMBH flaring diet beyond which all observable disruptions arise from evolved stars, accompanied by a dramatic cutoff in the overall tidal disruption flaring rate. Black holes less massive than this limiting mass scale will show observable flares from both main sequence and evolved stars, with giants contributing up to 10% of the event rate. The relative fractions of stars disrupted at different evolutionary states can constrain the properties and distributions of stars in galactic nuclei other than our own.

Abundance anomalies in tidal disruption events

“…Evolved stars, however, are only likely to be stripped into the hydrogen burning zone because of the huge jump in density at its base. The stripping of the envelope may also require several pericentric passages rather than occurring a single event (MacLeod et al 2012). Figure 1 shows that low mass stars simply live too long to develop abundance anomalies given the age of the universe.…”

“…To make a rough estimate of the fraction of stars likely to show anomalies, we need a scaling of the disruption rate with stellar mass, a model for the IMF and a star formation history. If we follow Wang & Merritt (2004) or MacLeod et al (2012), the TDE rate scales as R 1/4 * M −1/12 * ≃ M 1/6 * if we roughly scale R * ∝ M * for the MS. Higher mass stars have lower densities and so have higher rates. Stars closer to the main sequence turn off also have lower densities than on the ZAMS, but we make no attempt to include this effect.…”

The X-Ray Through Optical Fluxes and Line Strengths of Tidal Disruption Events

“…Gezari et al (2012) argued that the absence of H lines in PS1-10jh implies the disruption of a He-rich stellar core. The simulations of MacLeod et al (2012), however, show that it is difficult to remove the H envelope of a red giant and disrupt only the He core. Noting that He star disruptions would be exceedingly rare, Guillochon et al (2014) instead argued for the disruption of a normal main-sequence star but with the hydrogen line emission suppressed by photoionization effects (e.g., Korista & Goad 2004).…”

Large decay of X-ray flux in 2XMM J123103.2+110648: evidence for a tidal disruption event

“…Alternatively, it could be a TDE of slow circularization and thus of a relatively long rising phase (Guillochon & Ramirez-Ruiz 2015). Tidal stripping of an evolved star could also result in a relatively slow event (MacLeod et al 2012). However, TDEs of evolved stars should be very rare for a black hole of mass 10 5 M⊙, accounting for only ∼3% of the total TDEs (Kochanek 2016).…”