Eric R. Coughlin

The repeated tidal stripping of a star by a supermassive black hole, known as a repeating partial tidal disruption event (rpTDE), can give rise to a transient that rebrightens months to years after the first outburst. Among the rpTDE candidates so far observed, some exhibit dimmer peak luminosities during each successive outburst, which is a trend that has not been reproduced from theoretical models when the star survives more than one encounter with the black hole. Here we suggest that this trend can be recovered if the partially disrupted star is initially (i.e., prior to its first mass-stripping event) rapidly rotating, which is expected if the star was placed on its orbit through the Hills breakup of a tidally locked and tight binary. We test this hypothesis with hydrodynamical simulations of high-mass (≥1 M _⊙ ) main-sequence stars repeatedly partially disrupted by a 10 ⁶ M _⊙ black hole, and demonstrate that successively dimmer outbursts are indeed recovered for high (tens of percent of breakup) and prograde (i.e., aligned with the orbital angular momentum) stellar spins. Our results provide strong indirect evidence for the operation of the Hills mechanism in seeding the stars in rpTDEs.

Some high-mass stars likely end their lives in underluminous implosions that leave behind a black hole, known as failed supernovae. However, neutrinos radiated during proto-neutron-star formation generate a weak (Mach ≳ 1) shock wave in the outer layers of the star, which produces a unique transient as it breaks out of the dying star and signals its imminent disappearance. It was recently shown that there are two self-similar solutions that describe the propagation of this weak shock wave, and these solutions simultaneously contain outward-moving ejecta and fallback accretion onto the black hole. Here we show that the larger Mach number solutions are unstable, such that the Mach number of the shock grows with time t and deviates from the self-similar prediction as ∝ t ^α , with α ≲ 0.1, whereas the smaller Mach number solutions are stable. We also show that, above a critical mass loss that is readily achievable in core-collapse supernovae, the shock asymptotically strengthens and approaches the strong limit. Our results imply that it is the mass lost to neutrinos relative to the mass enclosed by the shock wave, as well as the stellar density gradient where the shock forms, that primarily dictate its strength and the amount of material it ejects. These criteria explain why red supergiants, which have relative mass losses well in excess of the critical value at the time of shock formation, more readily eject material and create more luminous explosions compared to more compact progenitors.

When a star passes within the Roche limit of a supermassive black hole (SMBH), it is pulled apart by the BH’s tidal field in a tidal disruption event (TDE). The resulting flare is powered by the circularization and accretion of bound stellar debris, which initially returns to the BH on eccentric orbits in a thin debris stream. The returning fluid elements follow inclined orbits that converge near pericenter, resulting in extreme vertical compression to scales 10 − 4 R ⊙ and the formation of a nozzle shock. Dissipation at the nozzle shock may affect circularization by altering the properties of the debris stream, but its role is the subject of ongoing debate. We develop an idealized model for the debris stream evolution combining 3D smoothed-particle hydrodynamics simulations, the semi-analytic affine model, and 1D finite-volume hydrodynamic simulations. Because our model is computationally cheap, we can unambiguously resolve the nozzle shock, use a realistic equation of state, and follow the debris stream evolution at many different times. Near peak fallback, Hydrogen recombination and molecular Hydrogen formation broaden the stream by a factor ∼ 5 , enhancing dissipation at the nozzle. However, the dissipation is still insufficient to directly circularize the debris by in-plane pressure gradients. Instead, the thicker stream substantially increases the likelihood that the stream self-intersects on the second orbit, despite relativistic nodal precession. The stream properties at self-intersection are sensitive to dissipation at the nozzle and the timing of focal points where the ballistic trajectories of the debris converge. Our results clarify the nozzle shock’s role in circularization in TDEs, providing a foundation for more realistic circularization and emission models.

In a tidal disruption event (TDE), a star is disrupted by the tidal field of a massive black hole, creating a debris stream that returns to the black hole, forms an accretion flow, and powers a luminous flare. Over the last few decades, several numerical studies have concluded that shock-induced dissipation occurs as the stream returns to pericenter (i.e., pre-self-intersection), resulting in efficient circularization of the debris. However, the efficacy of these shocks is the subject of intense debate. We present high-resolution simulations (up to 10 ¹⁰ particles) of the disruption of a solar-like star by a 10 ⁶ M _⊙ black hole with the new, GPU-based, smoothed-particle hydrodynamics (SPH) code sph-exa , including the relativistic apsidal precession of the stellar debris orbits; our simulations run from initial disruption to the moment of stream self-intersection. With ∼10 ⁸ particles—corresponding to the highest-resolution SPH simulations of TDEs in the preexisting literature—we find significant, in-plane spreading of the debris as the stream returns through pericenter, in line with previous works that suggested this is a significant source of dissipation and luminous emission. However, with increasing resolution, this effect is dramatically diminished, and with 10 ¹⁰ particles, there is effectively no change between the incoming and the outgoing stream widths. Our results demonstrate that the paradigm of significant dissipation of kinetic energy during pericenter passage is incorrect, and instead, it is likely that debris circularization is mediated by the originally proposed stream–stream collision scenario.

Extragalactic nuclear transients that exhibit repeating outbursts can be modeled as the repeated dynamical interaction between bound stars and supermassive black holes (SMBHs). A subset of these transients, with recurrence timescales of months‐to‐years, have been explained as accretion flares from the repeated tidal stripping of a star by an SMBH, in a repeating partial tidal disruption event (rpTDE). We outline the scope of the rpTDE model and discuss hydrodynamical simulations and analytical predictions for the stability of stars undergoing repeated mass loss, and the long‐term evolution of these flares as a function of stellar type and orbital parameters. Our findings demonstrate that high‐mass and centrally concentrated stars undergo negligible changes in structure in response to small amounts () of mass loss, and can survive many mass‐stripping encounters with an SMBH. Contrarily, low‐mass and less evolved stars are unstable to mass loss, and would be destroyed within a few orbits. We discuss the implications of these results for constraining the stellar type and orbital parameters of observed sources, such as ASASSN‐14ko, for which flares have been observed, and AT2020vdq, which exhibits a second flare that is brighter than its primary outburst.

A star entering the tidal sphere of a supermassive black hole (SMBH) can be partially stripped of mass, resulting in a partial tidal disruption event (TDE). Here, we develop an analytical model for properties of these events, including the peak fallback rate, M ̇ peak , the time at which the peak occurs, t _peak , and the amount of mass removed from the star, Δ M , for any star and any pericenter distance associated with the stellar orbit about the black hole. We compare the model predictions to 1276 hydrodynamical simulations of partial TDEs of main-sequence stars by a 10 ⁶ M _⊙ SMBH. The model yields t _peak predictions that are in good agreement (to within tens of percent) with the numerical simulations for any stellar mass and age. The agreement for M ̇ peak is weaker due to the influence of self-gravity on the debris stream dynamics, which remains dynamically important for partial TDEs; the agreement for M ̇ peak is, however, to within a factor of ∼2–3 in the majority of cases considered, with larger differences for low-mass stars ( M _⋆ ≲ 0.5 M _⊙ ) on grazing orbits with small mass loss. We show that partial TDE light curves for disruptions caused by ∼10 ⁶ M _⊙ SMBHs can span ∼20–100 days peak timescales, whereas grazing encounters of high-mass stars with high-mass SMBHs can yield longer peak timescales ( t ≳ 1000 days), associated with some observed transients. Our model provides a significant step toward an analytical prescription for TDE light curves and luminosity functions.

Pa 30—the likely remnant of the Galactic Type Iax supernova of 1181 AD—displays an unusual, firework-like morphology, consisting of radial filaments extending from a common center, where a white dwarf (WD) currently drives a very fast wind (speed ≳10 ⁴ km s ⁻¹ ). We propose the filaments arose from the Rayleigh–Taylor-unstable nature of the interface between the circumstellar medium (CSM) and the shocked wind launched by the natal WD; the filaments then elongated intact due to the Kelvin–Helmholtz-stable nature of the large initial density contrast between the wind and CSM, supplemented by the slowly declining wind density profile (relative to homologously expanding ejecta). To support this interpretation, we present two-dimensional hydrodynamical simulations and derive the filament properties, including their speed, density, and temperature, all of which are consistent with observations. We suggest the filaments elongate until the wind and CSM densities become comparable at the contact discontinuity, which occurs within 1–10 yr, and then truncate because the Rayleigh–Taylor instability (RTI) halts. The subsequent Kelvin–Helmholtz instability growth timescale across the current width of the filaments is longer than the age of Pa 30, so they remain intact. The filament-less central region in Pa 30 is therefore more likely a consequence of the finite timescale over which the RTI operates, rather than a wind termination shock. In general, firework-like filaments may form in other systems, provided there is a sufficiently large density contrast between the ejecta and its surroundings.