James Guillochon

A Possible Relativistic Jetted Outburst from a Massive Black Hole Fed by a Tidally Disrupted Star

A recent bright emission observed by the Swift satellite is due to the sudden accretion of a star onto a massive black hole. Gas accretion onto some massive black holes (MBHs) at the centers of galaxies actively powers luminous emission, but most MBHs are considered dormant. Occasionally, a star passing too near an MBH is torn apart by gravitational forces, leading to a bright tidal disruption flare (TDF). Although the high-energy transient Sw 1644+57 initially displayed none of the theoretically anticipated (nor previously observed) TDF characteristics, we show that observations suggest a sudden accretion event onto a central MBH of mass about 106 to 107 solar masses. There is evidence for a mildly relativistic outflow, jet collimation, and a spectrum characterized by synchrotron and inverse Compton processes; this leads to a natural analogy of Sw 1644+57 to a temporary smaller-scale blazar.

HYDRODYNAMICAL SIMULATIONS TO DETERMINE THE FEEDING RATE OF BLACK HOLES BY THE TIDAL DISRUPTION OF STARS: THE IMPORTANCE OF THE IMPACT PARAMETER AND STELLAR STRUCTURE

The disruption of stars by supermassive black holes has been linked to more than a dozen flares in the cores of galaxies out to redshift z ~ 0.4. Modeling these flares properly requires a prediction of the rate of mass return to the black hole after a disruption. Through hydrodynamical simulation, we show that aside from the full disruption of a solar mass star at the exact limit where the star is destroyed, the common assumptions used to estimate , the rate of mass return to the black hole, are largely invalid. While the analytical approximation to tidal disruption predicts that the least-centrally concentrated stars and the deepest encounters should have more quickly-peaked flares, we find that the most-centrally concentrated stars have the quickest-peaking flares, and the trend between the time of peak and the impact parameter for deeply penetrating encounters reverses beyond the critical distance at which the star is completely destroyed. We also show that the most-centrally concentrated stars produced a characteristic drop in shortly after peak when a star is only partially disrupted, with the power law index n being as extreme as –4 in the months immediately following the peak of a flare. Additionally, we find that n asymptotes to – 2.2 for both low- and high-mass stars for approximately half of all stellar disruptions. Both of these results are significantly steeper than the typically assumed n = –5/3. As these precipitous decay rates are only seen for events in which a stellar core survives the disruption, they can be used to determine if an observed tidal disruption flare produced a surviving remnant. We provide fitting formulae for four fundamental quantities of tidal disruption as functions of the stars distance to the black hole at pericenter and its stellar structure: the total mass lost, the time of peak, the accretion rate at peak, and the power-law index shortly after peak. These results should be taken into consideration when flares arising from tidal disruptions are modeled.

PS1-10jh: The Disruption of a Main-sequence Star of Near-solar Composition

When a star comes within a critical distance to a supermassive black hole (SMBH), immense tidal forces disrupt the star, resulting in a stream of debris that falls back onto the SMBH and powers a luminous flare. In this paper, we perform hydrodynamical simulations of the disruption of a main-sequence star by an SMBH to characterize the evolution of the debris stream after a tidal disruption. We demonstrate that this debris stream is confined by self-gravity in the two directions perpendicular to the original direction of the stars travel and as a consequence has a negligible surface area and makes almost no contribution to either the continuum or line emission. We therefore propose that any observed emission lines are not the result of photoionization in this unbound debris, but are produced in the region above and below the forming elliptical accretion disk, analogous to the broad-line region (BLR) in steadily accreting active galactic nuclei. As each line within a BLR is observationally linked to a particular location in the accretion disk, we suggest that the absence of a line indicates that the accretion disk does not yet extend to the distance required to produce that line. This model can be used to understand the spectral properties of the tidal disruption event PS1-10jh, for which He II lines are observed, but the Balmer series and He I are not. Using a maximum likelihood analysis, we show that the disruption of a main-sequence star of near-solar composition can reproduce this event.

Consequences of the Ejection and Disruption of Giant Planets

The discovery of Jupiter-mass planets in close orbits about their parent stars has challenged models of planet formation. Recent observations have shown that a number of these planets have highly inclined, sometimes retrograde orbits about their parent stars, prompting much speculation as to their origin. It is known that migration alone cannot account for the observed population of these misaligned hot Jupiters, which suggests that dynamical processes after the gas disk dissipates play a substantial role in yielding the observed inclination and eccentricity distributions. One particularly promising candidate is planet-planet scattering, which is not very well understood in the nonlinear regime of tides. Through three-dimensional hydrodynamical simulations of multi-orbit encounters, we show that planets that are scattered into an orbit about their parent stars with closest approach distance being less than approximately three times the tidal radius are either destroyed or completely ejected from the system. We find that as few as 9 and as many as 12 of the currently known hot Jupiters have a maximum initial apastron for scattering that lies well within the ice line, implying that these planets must have migrated either before or after the scattering event that brought them to their current positions. If stellar tides are unimportant (Q * 107), disk migration is required to explain the existence of the hot Jupiters present in these systems. Additionally, we find that the disruption and/or ejection of Jupiter-mass planets deposits a Suns worth of angular momentum onto the host star. For systems in which planet-planet scattering is common, we predict that planetary hosts have up to a 35% chance of possessing an obliquity relative to the invariable plane of greater than 90°.

COLLISIONS OF WHITE DWARFS AS A NEW PROGENITOR CHANNEL FOR TYPE Ia SUPERNOVAE

We present the results of a systematic numerical study of an alternative progenitor scenario to produce Type Ia supernova explosions, which is not restricted to the ignition of a CO white dwarf (WD) near the Chandrasekhar mass. In this scenario, a shock-triggered thermonuclear explosion ensues from the collision of two WDs. Consistent modeling of the gas dynamics together with nuclear reactions using both a smoothed particle and a grid-based hydrodynamics code are performed to study the viability of this alternative progenitor channel. We find that shock-triggered ignition and the synthesis of Ni are in fact a natural outcome for moderately massive WD pairs colliding close to head-on. We use a multi-dimensional radiative transfer code to calculate the emergent broadband light curves and spectral time series of these events. The synthetic spectra and light curves compare well to those of normal Type Ia supernovae over a similar B-band decline rate and are broadly consistent with the Phillips relation, although a mild dependence on viewing angle is observed due to the asymmetry of the ejected debris. While event rates within galactic centers and globular clusters are found to be much too low to explain the bulk of the Type Ia supernovae population, they may be frequent enough to make as much as a one percent contribution to the overall rate. Although these rate estimates are still subject to substantial uncertainties, they do suggest that dense stellar systems should provide upcoming supernova surveys with hundreds of such collision-induced thermonuclear explosions per year.

SURFACE DETONATIONS IN DOUBLE DEGENERATE BINARY SYSTEMS TRIGGERED BY ACCRETION STREAM INSTABILITIES

We present three-dimensional simulations on a new mechanism for the detonation of a sub-Chandrasekhar CO white dwarf in a dynamically unstable system where the secondary is either a pure He white dwarf or an He/CO hybrid. For dynamically unstable systems where the accretion stream directly impacts the surface of the primary, the final tens of orbits can have mass accretion rates that range from 10–5 to 10–3 M ☉ s–1, leading to the rapid accumulation of helium on the surface of the primary. After ~10–2 M ☉ of helium has been accreted, the ram pressure of the hot helium torus can deflect the accretion stream such that the stream no longer directly impacts the surface. The velocity difference between the stream and the torus produces shearing which seeds large-scale Kelvin-Helmholtz instabilities along the interface between the two regions. These instabilities eventually grow into dense knots of material that periodically strike the surface of the primary, adiabatically compressing the underlying helium torus. If the temperature of the compressed material is raised above a critical temperature, the timescale for triple-α reactions becomes comparable to the dynamical timescale, leading to the detonation of the primarys helium envelope. This detonation drives shock waves into the primary which tend to concentrate at one or more focal points within the primarys CO core. If a relatively small amount of mass is raised above a critical temperature and density at these focal points, the CO core may itself be detonated.

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

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 stars 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 timescales of months to years. 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 ~108 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.

An Open Catalog for Supernova Data

We present the Open Supernova Catalog, an online collection of observations and metadata for presently 36,000+ supernovae and related candidates. The catalog is freely available on the web (https://sne.space), with its main interface having been designed to be a user-friendly, rapidly-searchable table accessible on desktop and mobile devices. In addition to the primary catalog table containing supernova metadata, an individual page is generated for each supernova which displays its available metadata, light curves, and spectra spanning X-ray to radio frequencies. The data presented in the catalog is automatically rebuilt on a daily basis and is constructed by parsing several dozen sources, including the data presented in the supernova literature and from secondary sources such as other web-based catalogs. Individual supernova data is stored in the hierarchical, human- and machine-readable JSON format, with the entirety of each supernovas data being contained within a single JSON file bearing its name. The setup we present here, which is based upon open source software maintained via git repositories hosted on github, enables anyone to download the entirety of the supernova dataset to their home computer in minutes, and to make contributions of their own data back to the catalog via git. As the supernova dataset continues to grow, especially in the upcoming era of all-sky synoptic telescopes which will increase the total number of events by orders of magnitude, we hope that the catalog we have designed will be a valuable tool for the community to analyze both historical and contemporary supernovae.

THE DYNAMICS, APPEARANCE, AND DEMOGRAPHICS OF RELATIVISTIC JETS TRIGGERED BY TIDAL DISRUPTION OF STARS IN QUIESCENT SUPERMASSIVE BLACK HOLES

We examine the consequences of a model in which relativistic jets can be triggered in quiescent massive black holes when a geometrically thick and hot accretion disk forms as a result of the tidal disruption of a star. To estimate the power, thrust, and lifetime of the jet, we use the mass accretion history onto the black hole as calculated by detailed hydrodynamic simulations of the tidal disruption of stars. We go on to determine the states of the interstellar medium in various types of quiescent galactic nuclei, and describe how this external matter can affect jets propagating through it. We use this information, together with a two-dimensional hydrodynamic model of the structure of the relativistic flow, to study the dynamics of the jet, the propagation of which is regulated by the density stratification of the environment and by its injection history. The breaking of symmetry involved in transitioning from one to two dimensions is crucial and leads to qualitatively new phenomena. At early times, as the jet power increases, the high pressure of the cocoon collimates the jet, increasing its shock velocity as compared to that of spherical models. We show that small velocity gradients, induced near or at the source, steepen into internal shocks and provide a source of free energy for particle acceleration and radiation along the jets channel. The jets terminate at a working surface where they interact strongly with the surrounding medium through a combination of shock waves and instabilities; a continuous flow of relativistic fluid emanating from the nucleus supplies this region with mass, momentum, and energy. Information about the t –5/3 decrease in power supply propagates within the jet at the internal sound speed. As a result, the internal energy at the jet head continues to accumulate until long after the peak feeding rate is reached. An appreciable time delay is thus expected between peaks in the short-wavelength radiation emanating near the jets origin and the long-wavelength emission produced at the head of the jet. Many of the observed properties of the Swift 1644+57/GRB 110328A event can be understood as resulting from accretion onto and jets driven by a 106 M ☉ central mass black hole following the disruption of a sun-like star. With the inclusion of a stochastic contribution to the luminosity due to variations in the feeding rate driven by instabilities near the tidal radius, we find that our model can explain the X-ray light curve without invoking a rarely occurring deep encounter. In conjunction with the number density of black holes in the local universe, we hypothesize that the conditions required to produce the Swift event are not anomalous, but are in fact representative of the jet-driven flare population arising from tidal disruptions.

CIRCUMSTELLAR ABSORPTION IN DOUBLE DETONATION TYPE Ia SUPERNOVAE

Upon formation, degenerate He core white dwarfs are surrounded by a radiative H-rich layer primarily supported by ideal gas pressure. In this Letter, we examine the effect of this H-rich layer on mass transfer in He+C/O double white dwarf binaries that will eventually merge and possibly yield a Type Ia supernova (SN Ia) in the double detonation scenario. Because its thermal profile and equation of state differ from the underlying He core, the H-rich layer is transferred stably onto the C/O white dwarf prior to the He cores tidal disruption. We find that this material is ejected from the binary system and sweeps up the surrounding interstellar medium hundreds to thousands of years before the SN Ia. The close match between the resulting circumstellar medium profiles and values inferred from recent observations of circumstellar absorption in SNe Ia gives further credence to the resurgent double detonation scenario.