 Award ID(s):
 2006684
 Publication Date:
 NSFPAR ID:
 10350415
 Journal Name:
 The Astrophysical Journal
 Volume:
 923
 Issue:
 2
 Page Range or eLocationID:
 184
 ISSN:
 0004637X
 Sponsoring Org:
 National Science Foundation
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Abstract Upon entering the tidal sphere of a supermassive black hole, a star is ripped apart by tides and transformed into a stream of debris. The ultimate fate of that debris, and the properties of the bright flare that is produced and observed, depends on a number of parameters, including the energy of the center of mass of the original star. Here we present the results of a set of smoothed particle hydrodynamics simulations in which a 1 M ⊙ , γ = 5/3 polytrope is disrupted by a 10 6 M ⊙ supermassive black hole. Each simulation has a pericenter distance of r p = r t (i.e., β ≡ r t / r p = 1 with r t the tidal radius), and we vary the eccentricity e of the stellar orbit from e = 0.8 up to e = 1.20 and study the nature of the fallback of debris onto the black hole and the longterm fate of the unbound material. For simulations with eccentricities e ≲ 0.98, the fallback curve has a distinct, threepeak structure that is induced by selfgravity. For simulations with eccentricities e ≳ 1.06, the core of the disrupted star reforms following itsmore »

Abstract We present a toy model for the thermal optical/UV/Xray emission from tidal disruption events (TDEs). Motivated by recent hydrodynamical simulations, we assume that the debris streams promptly and rapidly circularize (on the orbital period of the most tightly bound debris), generating a hot quasispherical pressuresupported envelope of radius
R _{v} ∼ 10^{14}cm (photosphere radius ∼10^{15}cm) surrounding the supermassive black hole (SMBH). As the envelope cools radiatively, it undergoes Kelvin–Helmholtz contractionR _{v} ∝t ^{−1}, its temperature risingT _{eff}∝t ^{1/2}while its total luminosity remains roughly constant; the optical luminosity decays as . Despite this similarity to the mass fallback rate $\nu {L}_{\nu}\propto \phantom{\rule{0.50em}{0ex}}{R}_{v}^{2}{T}_{\mathrm{eff}}\propto {t}^{3/2}$ , envelope heating from fallback accretion is subdominant compared to the envelope cooling luminosity except near optical peak (where they are comparable). Envelope contraction can be delayed by energy injection from accretion from the inner envelope onto the SMBH in a regulated manner, leading to a latetime flattening of the optical/Xray light curves, similar to those observed in some TDEs. Eventually, as the envelope contracts to near the circularization radius, the SMBH accretion rate rises to its maximum, in tandem with the decreasing optical luminosity. This coolinginduced (rather than circularizationinduced) delay of up to several hundred days may account for themore » ${\stackrel{\u0307}{M}}_{\mathrm{fb}}\propto {t}^{5/3}$ 
Abstract We develop a Newtonian model of a deep tidal disruption event (TDE), for which the pericenter distance of the star,
r _{p}, is well within the tidal radius of the black hole,r _{t}, i.e., whenβ ≡r _{t}/r _{p}≫ 1. We find that shocks form forβ ≳ 3, but they are weak (with Mach numbers ∼1) for allβ , and that they reach the center of the star prior to the time of maximum adiabatic compression forβ ≳ 10. The maximum density and temperature reached during the TDE follow much shallower relations withβ than the previously predicted and ${\rho}_{\mathrm{max}}\propto {\beta}^{3}$ scalings. Below ${T}_{\mathrm{max}}\propto {\beta}^{2}$β ≃ 10, this shallower dependence occurs because the pressure gradient is dynamically significant before the pressure is comparable to the ram pressure of the freefalling gas, while aboveβ ≃ 10, we find that shocks prematurely halt the compression and yield the scalings and ${\rho}_{\mathrm{max}}\propto {\beta}^{1.62}$ . We find excellent agreement between our results and highresolution simulations. Our results demonstrate that, in the Newtonian limit, the compression experienced by the star is completely independent of the mass of the black hole. We discuss our results in the context of existing (affine) models, polytropic versus nonpolytropic stars, and general relativistic effects, which become important when the pericenter ofmore » ${T}_{\mathrm{max}}\propto {\beta}^{1.12}$ 
ABSTRACT Tidal disruption events (TDEs) occur when a star gets torn apart by the strong tidal forces of a supermassive black hole, which results in the formation of a debris stream that partly falls back towards the compact object. This gas moves along inclined orbital planes that intersect near pericentre, resulting in a socalled ‘nozzle shock’. We perform the first dedicated study of this interaction, making use of a twodimensional simulation that follows the transverse gas evolution inside a given section of stream. This numerical approach circumvents the lack of resolution encountered near pericentre passage in global threedimensional simulations using particlebased methods. As it moves inward, we find that the gas motion is purely ballistic, which near pericentre causes strong vertical compression that squeezes the stream into a thin sheet. Dissipation takes place at the resulting nozzle shock, inducing a rise in pressure that causes the collapsing gas to bounce back, although without imparting significant net expansion. As it recedes to larger distances, this matter continues to expand while remaining thin despite the influence of pressure forces. This gas evolution specifies the strength of the subsequent selfcrossing shock, which we find to be more affected by black hole spin than previouslymore »

Abstract Tidal disruption events with tidal radius r t and pericenter distance r p are characterized by the quantity β = r t / r p , and “deep encounters” have β ≫ 1. It has been assumed that there is a critical β ≡ β c ∼ 1 that differentiates between partial and full disruption: for β < β c a fraction of the star survives the tidal interaction with the black hole, while for β > β c the star is completely destroyed, and hence all deep encounters should be full. Here we show that this assumption is incorrect by providing an example of a β = 16 encounter between a γ = 5/3, solarlike polytrope and a 10 6 M ⊙ black hole—for which previous investigations have found β c ≃ 0.9—that results in the reformation of a stellar core postdisruption that comprises approximately 25% of the original stellar mass. We propose that the core reforms under selfgravity, which remains important because of the compression of the gas both near pericenter, where the compression occurs out of the orbital plane, and substantially after pericenter, where compression is within the plane. We find that the core forms onmore »