ALBERT

Description: The potential of tidal disruption of stars to probe otherwise quiescent supermassive black holes cannot be exploited, if their dynamics is not fully understood. So far, the observational appearance of these events has been derived from analytical extrapolations of the debris dynamical properties just after disruption. By means of hydrodynamical simulations, we investigate the subsequent fallback of the stream of debris towards the black hole for stars already bound to the black hole on eccentric orbits. We demonstrate that the debris circularize due to relativistic apsidal precession which causes the stream to self-cross. The circularization time-scale varies between 1 and 10 times the period of the star, being shorter for more eccentric and/or deeper encounters. This self-crossing leads to the formation of shocks that increase the thermal energy of the debris. If this thermal energy is efficiently radiated away, the debris settle in a narrow ring at the circularization radius with shock-induced luminosities of ~10–10 3 L Edd . If instead cooling is impeded, the debris form an extended torus located between the circularization radius and the semi-major axis of the star with heating rates ~1–10 2 L Edd . Extrapolating our results to parabolic orbits, we infer that circularization would occur via the same mechanism in ~1 period of the most bound debris for deeply penetrating encounters to ~10 for grazing ones. We also anticipate the same effect of the cooling efficiency on the structure of the disc with associated luminosities of ~1–10 L Edd and heating rates of ~0.1–1 L Edd . In the latter case of inefficient cooling, we deduce a viscous time-scale generally shorter than the circularization time-scale. This suggests an accretion rate through the disc tracing the fallback rate, if viscosity starts acting promptly.

Description: Most massive galaxies are thought to contain a supermassive black hole in their centre surrounded by a tenuous gas environment, leading to no significant emission. In these quiescent galaxies, tidal disruption events represent a powerful detection method for the central black hole. Following the disruption, the stellar debris evolves into an elongated gas stream, which partly falls back towards the disruption site and accretes on to the black hole producing a luminous flare. Using an analytical treatment, we investigate the interaction between the debris stream and the gas environment of quiescent galaxies. Although we find dynamical effects to be negligible, we demonstrate that Kelvin–Helmholtz instability can lead to the dissolution of the stream into the ambient medium before it reaches the black hole, likely dimming the associated flare. This result is robust against the presence of a typical stellar magnetic field and fast cooling within the stream. Furthermore, we find this effect to be enhanced for disruptions involving more massive black holes and/or giant stars. Consequently, although disruptions of evolved stars have been proposed as a useful probe of black holes with masses 10 8 M , we argue that the associated flares are likely less luminous than expected.

Description: There is mounting observational evidence that most galactic nuclei host both supermassive black holes (SMBHs) and young populations of stars. With an abundance of massive stars, core-collapse supernovae are expected in SMBH spheres of influence. We develop a novel numerical method, based on the Kompaneets approximation, to trace supernova remnant (SNR) evolution in these hostile environments, where radial gas gradients and SMBH tides are present. We trace the adiabatic evolution of the SNR shock until 50 per cent of the remnant is either in the radiative phase or is slowed down below the SMBH Keplerian velocity and is sheared apart. In this way, we obtain shapes and lifetimes of SNRs as a function of the explosion distance from the SMBH, the gas density profile and the SMBH mass. As an application, we focus here exclusively on quiescent SMBHs, because their light may not hamper detections of SNRs and because we can take advantage of the unsurpassed detailed observations of our Galactic Centre. Assuming that properties such as gas and stellar content scale appropriately with the SMBH mass, we study SNR evolution around other quiescent SMBHs. We find that, for SMBH masses over ~10 7 M , tidal disruption of SNRs can occur at less than 10 4 yr, leading to a shortened X-ray emitting adiabatic phase, and to no radiative phase. On the other hand, only modest disruption is expected in our Galactic Centre for SNRs in their X-ray stage. This is in accordance with estimates of the lifetime of the Sgr A East SNR, which leads us to expect one supernova per 10 4 yr in the sphere of influence of Sgr A*.

Description: Supermassive black holes are a key ingredient of galaxy evolution. However, their origin is still highly debated. In one of the leading formation scenarios, a black hole of ~100 M results from the collapse of the inner core of a supermassive star (10 4–5 M ), created by the rapid accumulation (0.1 M  yr –1 ) of pristine gas at the centre of newly formed galaxies at z  ~ 15. The subsequent evolution is still speculative: the remaining gas in the supermassive star can either directly plunge into the nascent black hole or part of it can form a central accretion disc, whose luminosity sustains a surrounding, massive, and nearly hydrostatic envelope (a system called a ‘quasi-star’). To address this point, we consider the effect of rotation on a quasi-star, as angular momentum is inevitably transported towards the galactic nucleus by the accumulating gas. Using a model for the internal redistribution of angular momentum that qualitatively matches results from simulations of rotating convective stellar envelopes, we show that quasi-stars with an envelope mass greater than a few 10 5 M $_{\odot } \times (\rm black\ hole\ mass/100\,\mathrm{M}_{\odot })^{0.82}$ have highly sub-Keplerian gas motion in their core, preventing gas circularization outside the black hole's horizon. Less massive quasi-stars could form but last for only 10 4 yr before the accretion luminosity unbinds the envelope, suppressing the black hole growth. We speculate that this might eventually lead to a dual black hole seed population: (i) massive (〉10 4 M ) seeds formed in the most massive (〉10 8 M ) and rare haloes; (ii) lighter (~10 2 M ) seeds to be found in less massive and therefore more common haloes.

Description: Although the internal structure of white dwarfs is considered to be generally well understood, the source and entity of their viscosity is still very uncertain. We propose here to study white dwarf viscous properties using short-period (〈1 h), detached white dwarf binaries, such as the newly discovered ~12.8 min system (J0651). These binaries are wide enough that mass transfer has not yet started but close enough that the secondary (least massive) component is subject to a measurable tidal deformation. The associated tidal torque transfers orbital energy, which is partially converted into heat by the action of viscosity as the secondary gets spun up. As a consequence, its outer non-degenerate layers expand, and the star puffs up. We self-consistently calculate the fractional change in radius, and the degree of synchronization (ratio of stellar spin to orbital period) as a function of the viscous time. Specializing to the case of J0651, we find that an ~10 per cent discrepancy between the measured radius of the secondary star and predictions of He white dwarf models can be interpreted as tidal inflation if the viscous time-scale is ~4 10 4  yr. Such value is well in the range of various non-microscopic viscosities proposed in the literature like, e.g. tidally induced turbulence, non-linear damping of dynamical tides or internal magnetic stresses with a magnetic field strength ~10–100 G. A 10 per cent tidal inflation is the maximum possible effect in J0651, at its current orbital separation, hence it selects a single value of the viscous time-scale: the latter implies that the system is still far from synchronization. Smaller effects of tidal inflation – well consistent with current uncertainties – would instead correspond to two different viscous time-scales, one longer and one shorter than 4 10 4  yr. In this more general case, the degeneracy can be broken by a joint measurement of the secondary's spin, since the two time-scales imply very different degrees of synchronization. Extrapolating the secondary's expansion into the future, we find that the star will fill its Roche lobe at a separation which is ~1.2–1.5 smaller than the current one. Applying this method to a future sample of systems can allow us to learn whether viscosity changes with mass and/or nuclear composition.

Description: A large number of tidal disruption event (TDE) candidates have been observed recently, often differing in their observational features. Two classes appear to stand out: X-ray and optical TDEs, the latter featuring lower effective temperatures and luminosities. These differences can be explained if the radiation detected from the two categories of events originates from different locations. In practice, this location is set by the evolution of the debris stream around the black hole and by the energy dissipation associated with it. In this paper, we build an analytical model for the stream evolution, whose dynamics is determined by both magnetic stresses and shocks. Without magnetic stresses, the stream always circularizes. The ratio of the circularization time-scale to the initial stream period is t ev / t min  = 8.3( M h /10 6 M ) –5/3 β –3 , where M h is the black hole mass and β is the penetration factor. If magnetic stresses are strong, they can lead to the stream ballistic accretion. The boundary between circularization and ballistic accretion corresponds to a critical magnetic stresses efficiency v A / v c 10 –1 , largely independent of M h and β. However, the main effect of magnetic stresses is to accelerate the stream evolution by strengthening self-crossing shocks. Ballistic accretion therefore necessarily occurs on the stream dynamical time-scale. The shock luminosity associated with energy dissipation is sub-Eddington but decays as t –5/3 only for a slow stream evolution. Finally, we find that the stream thickness rapidly increases if the stream is unable to cool completely efficiently. A likely outcome is its fast evolution into a thick torus, or even an envelope completely surrounding the black hole.

Description: The formation of supermassive black holes is still an outstanding question. In the quasi-star scenario, black hole seeds experience an initial super-Eddington growth, that in less than a million years may leave a 10 4 –10 5 M black hole at the centre of a protogalaxy at z ~ 20–10. Super-Eddington accretion, however, may be accompanied by vigorous mass-loss that can limit the amount of mass that reaches the black hole. In this paper, we critically assess the impact of radiative driven winds, launched from the surface of the massive envelopes from which the black hole accretes. Solving the full wind equations coupled with the hydrostatic structure of the envelope, we find mass outflows with rates between a few tens and 10 4 M  yr –1 , mainly powered by advection luminosity within the outflow. We therefore confirm the claim by Dotan et al. that mass losses can severely affect the black hole seed early growth within a quasi-star. In particular, seeds with mass 〉10 4 M can only form within mass reservoirs 10 7 M , unless they are refilled at huge rates (100 M  yr –1 ). This may imply that only very massive haloes (〉10 9 M ) at those redshifts can harbour massive seeds. Contrary to previous claims, these winds are expected to be relatively bright (10 44 –10 47  erg s –1 ), blue ( T eff ~ 8000 K) objects, that while eluding the Hubble Space Telescope , could be observed by the James Webb Space Telescope.

Description: Appreciable star formation, and, therefore, numerous massive stars, are frequently found near supermassive black holes (SMBHs). As a result, core-collapse supernovae in these regions should also be expected. In this paper, we consider the observational consequences of predicting the fate of supernova remnants (SNRs) in the sphere of influence of quiescent SMBHs. We present these results in the context of ‘autarkic’ nuclei, a model that describes quiescent nuclei as steady-state and self-sufficient environments where the SMBH accretes stellar winds with no appreciable inflow of material from beyond the sphere of influence. These regions have properties such as gas density that scale with the mass of the SMBH. Using predictions of the X-ray lifetimes of SNRs originating in the sphere of influence, we make estimates of the number of core collapse SNRs present at a given time. With the knowledge of lifetimes of SNRs and their association with young stars, we predict a number of core-collapse SNRs that grows from ~1 around Milky Way-like (4.3  x  10 6 M ) SMBHs to ~100 around the highest mass (10 10 M ) SMBHs. The presence of young SNRs will amplify the X-ray emission near quiescent SMBHs, and we show that the total core-collapse SNR emission has the potential to influence soft X-ray searches for very low-luminosity SMBHs. Our SNR lifetime estimates also allow us to predict star formation rates in these regions. Assuming a steady-state replenishment of massive stars, we estimate a star formation rate density of 2  x  10 –4 M yr –1 pc –2 around the Milky Way SMBH, and a similar value around other SMBHs due to a weak dependence on SMBH mass. This value is consistent with currently available observations.