ALBERT

Description: A tidal disruption event occurs when a star wanders close enough to a black hole to be disrupted by its tidal force. The debris of a tidally disrupted star are expected to form an accretion disc around the supermassive black hole. The light curves of these events sometimes show a quasi-periodic modulation of the flux that can be associated with the precession of the accretion disc due to the Lense–Thirring (‘frame-dragging’) effect. Since the initial star orbit is in general inclined with respect to the black hole spin, this misalignment combined with the Lense–Thirring effect leads to a warp in the disc. In this paper, we provide a simple model of the system composed by a thick and narrow accretion disc surrounding a spinning supermassive black hole, with the aim to: (a) compute the expected precession period as a function of the system parameters, (b) discuss the conditions that have to be satisfied in order to have rigid precession, (c) investigate the alignment process, highlighting how different mechanisms play a role leading the disc and the black hole angular momenta into alignment.

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: We explain the axisymmetric gaps seen in recent long-baseline observations of the HL Tau protoplanetary disc with the Atacama Large Millimetre/Submillimetre Array (ALMA) as being due to the different response of gas and dust to embedded planets in protoplanetary discs. We perform global, three-dimensional dusty smoothed particle hydrodynamics calculations of multiple planets embedded in dust/gas discs which successfully reproduce most of the structures seen in the ALMA image. We find a best match to the observations using three embedded planets with masses of 0.2, 0.27 and 0.55 M J in the three main gaps observed by ALMA, though there remain uncertainties in the exact planet masses from the disc model.

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: We present three-dimensional Smoothed Particle Hydrodynamics (SPH) simulations investigating the dependence of the accretion rate on the disc thickness around an equal-mass, circular black hole binary system. We find that for thick/hot discs, with H / R 0.1, the binary torque does not prevent the gas from penetrating the cavity formed in the disc by the binary (in line with previous investigations). The situation drastically changes for thinner discs; in this case the mass accretion rate is suppressed, such that only a fraction (linearly dependent on H / R ) of the available gas is able to flow within the cavity and accrete on to the binary. Extrapolating this result to the cold and thin accretion discs expected around supermassive black hole binary systems implies that this kind of system accretes less material than predicted so far, with consequences not only for the electromagnetic and gravitational waves emissions during the late inspiral phase but also for the recoil speed of the black hole formed after binary coalescence, thus influencing also the evolutionary path both of the binary and of the host galaxy. Our results, being scale-free, are also applicable to equal-mass, circular binaries of stellar mass black holes, such as the progenitor of the recently discovered gravitational wave source GW150914.

Description: In this paper, we discuss the influence of gravitational instabilities in massive protostellar discs on the dynamics of dust grains. Starting from a smoothed particle hydrodynamics simulation, we have computed the evolution of the dust in a quasi-static gas density structure typical of self-gravitating disc. For different grain size distributions, we have investigated the capability of spiral arms to trap particles. We have run 3D radiative transfer simulations in order to construct maps of the expected emission at (sub-)millimetre and near-infrared wavelengths. Finally, we have simulated realistic observations of our disc models at (sub-)millimetre and near-infrared wavelengths as they may appear with the Atacama Large Millimetre/submillimetre Array (ALMA) and the High-Contrast Coronographic Imager for Adaptive Optics (HiCIAO) in order to investigate whether there are observational signatures of the spiral structure. We find that the pressure inhomogeneities induced by gravitational instabilities produce a non-negligible dynamical effect on centimetre-sized particles leading to significant overdensities in spiral arms. We also find that the spiral structure is readily detectable by ALMA over a wide range of (sub-)millimetre wavelengths and by HiCIAO in near-infrared scattered light for non-face-on discs located in the Ophiuchus star-forming region. In addition, we find clear spatial spectral index variations across the disc, revealing that the dust trapping produces a migration of large grains that can be potentially investigated through multiwavelength observations in the (sub-)millimetric. Therefore, the spiral arms observed to date in protoplanetary disc might be interpreted as density waves induced by the development of gravitational instabilities.

Description: We study accretion rates during the gravitational wave-driven merger of a binary supermassive black hole embedded in an accretion disc, formed by gas driven to the centre of the galaxy. We use 3D simulations performed with phantom , a smoothed particle hydrodynamics code. Contrary to previous investigations, we show that there is evidence of a ‘squeezing phenomenon’, caused by the compression of the inner disc gas when the secondary black hole spirals towards the primary. This causes an increase in the accretion rates that always exceed the Eddington rate. We have studied the main features of the phenomenon for a mass ratio q = 10 –3 between the black holes, including the effects of numerical resolution, the secondary accretion radius and the disc thickness. With our disc model with a low aspect ratio, we show that the mass expelled from the orbit of the secondary is negligible (〈5 per cent of the initial disc mass), different to the findings of previous 2D simulations with thicker discs. The increase in the accretion rates in the last stages of the merger leads to an increase in luminosity, making it possible to detect an electromagnetic precursor of the gravitational wave signal emitted by the coalescence.

Description: In this paper, we present simulated Atacama Large Millimeter/sub-millimeter Array (ALMA) observations of self-gravitating circumstellar discs with different properties in size, mass and inclination, located in four of the most extensively studied and surveyed star-forming regions. Starting from a smoothed particle hydrodynamics simulation and representative dust opacities, we have initially constructed maps of the expected emission at sub-mm wavelengths of a large sample of discs with different properties. We have then simulated realistic observations of discs as they may appear with ALMA using the Common Astronomy Software Application ALMA simulator. We find that, with a proper combination of antenna configuration and integration time, the spiral structure characteristic of self-gravitating discs is readily detectable by ALMA over a wide range of wavelengths at distances comparable to TW Hydrae (~50 pc), Taurus-Auriga and Ophiucus (~140 pc) star-forming regions. However, for discs located in Orion complex (~400 pc) only the largest discs in our sample (outer radius of 100 au) show a spatially resolved structure while the smaller ones (outer radius of 25 au) are characterized by a spiral structure that is not conclusively detectable with ALMA.

Description: In this paper, we revisit the issue of estimating the ‘fossil’ disc mass in the circumprimary disc, during the merger of a supermassive black hole binary. As the binary orbital decay speeds up due to the emission of gravitational waves, the gas in the circumprimary disc might be forced to accrete rapidly and could in principle provide a significant electromagnetic counterpart to the gravitational wave emission. Since the luminosity of such flare is proportional to the gaseous mass in the circumprimary disc, estimating such mass accurately is important. Previous investigations of this issue have produced contradictory results, with some authors estimating super-Eddington flares and large disc mass, while others suggesting that the ‘fossil’ disc mass is very low, even less than a Jupiter mass. Here, we perform simple 1D calculations to show that such very low estimates of the disc mass are an artefact of the specific implementation of the tidal torque in 1D models. In particular, for moderate mass ratios of the binary, the usual formula for the torque used in 1D models significantly overestimates the width of the gap induced by the secondary and this artificially leads to a very small leftover circumprimary disc. Using a modified torque, calibrated to reproduce the correct gap width as estimated by 3D models, leads to fossil disc masses of the order of one solar mass. The rapid accretion of the whole circumprimary disc would produce peak luminosities of the order of 1–20 times the Eddington luminosity. Even if a significant fraction of the gas escapes accretion by flowing out the secondary orbit during the merger (an effect not included in our calculations), we would still predict close to Eddington luminosities that might be easily detected.

Description: On 2011 August 11, INTEGRAL discovered the hard X-ray source IGR J17361–4441 near the centre of the globular cluster NGC 6388. Follow-up observations with Chandra showed the position of the transient was inconsistent with the cluster dynamical centre, and thus not related to its possible intermediate mass black hole. The source showed a peculiar hard spectrum ( 0.8) and no evidence of QPOs, pulsations, type-I bursts, or radio emission. Based on its peak luminosity, IGR J17361–4441 was classified as a very faint X-ray transient, and most likely a low-mass X-ray binary. We re-analysed 200 d of Swift /XRT observations, covering the whole outburst of IGR J17361–4441 and find a t –5/3 trend evident in the light curve, and a thermal emission component that does not evolve significantly with time. We investigate whether this source could be a tidal disruption event, and for certain assumptions find an accretion efficiency 3.5 10 –4 ( M Ch / M ) consistent with a massive white dwarf, and a disrupted minor body mass M mb 1.9 10 27 ( M / M Ch ) g in the terrestrial-icy planet regime. These numbers yield an inner disc temperature of the order kT in 0.04 keV, consistent with the blackbody temperature of kT in 0.08 keV estimated by spectral fitting. Although the density of white dwarfs and the number of free-floating planets are uncertain, we estimate the rate of planetary tidal disruptions in NGC 6388 to be in the range 3 10 –6 –3 10 –4 yr –1 . Averaged over the Milky Way globular clusters, the upper limit value corresponds to 0.05 yr –1 , consistent with the observation of a single event by INTEGRAL and Swift .