Despite their unique astrophysical relevance, the outcome of white dwarf binary mergers has so far only been studied for a very restricted number of systems. Here we present the results of a survey ...with more than 200 simulations systematically scanning the white dwarf binary parameter space. We consider white dwarf masses ranging from 0.2 to 1.2 M⊙ and account for their different chemical compositions. We find excellent agreement with the orbital evolution predicted by mass transfer stability analysis. Much of our effort in this paper is dedicated to determining which binary systems are prone to a thermonuclear explosion just prior to merger or at surface contact. We find that a large fraction of He-accreting binary systems explode: all dynamically unstable systems with accretor masses below 1.1 M⊙ and donor masses above ∼0.4 M⊙ are found to trigger a helium detonation at surface contact. A substantial fraction of these systems could explode at earlier times via detonations induced by instabilities in the accretion stream, as we have demonstrated in our previous work. We do not find definitive evidence for an explosion prior to merger or at surface contact in any of the studied double carbon-oxygen systems. Although we cannot exclude their occurrence if some helium is present, the available parameter space for a successful detonation in a white dwarf binary of pure carbon-oxygen composition is small. We demonstrate that a wide variety of dynamically unstable systems are viable Type Ia candidates. The next decade thus holds enormous promise for the study of these events, in particular with the advent of wide-field synoptic surveys allowing a detailed characterization of their explosive properties.
We present the STARS library, a grid of tidal disruption event (TDE) simulations interpolated to provide the mass fallback rate (dM/dt) to the black hole for a main-sequence star of any stellar mass, ...stellar age, and impact parameter. We use a one-dimensional stellar evolution code to construct stars with accurate stellar structures and chemical abundances, then perform tidal disruption simulations in a three-dimensional adaptive-mesh hydrodynamics code with a Helmholtz equation of state, in unprecedented resolution: from 131 to 524 cells across the diameter of the star. The interpolated library of fallback rates is available on GitHub (github.com/jamielaw-smith/STARS_library) and version 1.0.0 is archived on Zenodo; one can query the library for any stellar mass, stellar age, and impact parameter. We provide new fitting formulae for important disruption quantities ( ) as a function of stellar mass, stellar age, and impact parameter. Each of these quantities varies significantly with stellar mass and stellar age, but we are able to reduce all of our simulations to a single relationship that depends only on stellar structure, characterized by a single parameter , and impact parameter β. We also find that, in general, more centrally concentrated stars have steeper dM/dt rise slopes and shallower decay slopes. For the same ΔM, the dM/dt shape varies significantly with stellar mass, promising the potential determination of stellar properties from the TDE light curve alone. The dM/dt shape depends strongly on stellar structure and to a certain extent stellar mass, meaning that fitting TDEs using this library offers a better opportunity to determine the nature of the disrupted star and the black hole.
astroquery is a collection of tools for requesting data from databases hosted on remote servers with interfaces exposed on the internet, including those with web pages but without formal application ...program interfaces. These tools are built on the Python requests package, which is used to make HTTP requests, and astropy, which provides most of the data parsing functionality. astroquery modules generally attempt to replicate the web page interface provided by a given service as closely as possible, making the transition from browser-based to command-line interaction easy. astroquery has received significant contributions from throughout the astronomical community, including several from telescope archives. astroquery enables the creation of fully reproducible workflows from data acquisition through publication. This paper describes the philosophy, basic structure, and development model of the astroquery package. The complete documentation for astroquery can be found at http://astroquery.readthedocs.io/.
ABSTRACT
The rate of tidal disruption flares (TDFs) per mass of the disrupting black hole encodes information on the present-day mass function (PDMF) of stars in the clusters surrounding super ...massive black holes. We explore how the shape of the TDF rate with black hole mass can constrain the PDMF, with only weak dependence on black hole spin. We show that existing data can marginally constrain the minimum and maximum masses of stars in the cluster, and the high-mass end of the PDMF slope, as well as the overall TDF rate. With $\mathcal {O}(100)$ TDFs expected to be identified with the Zwicky Transient Facility, the overall rate can be highly constrained, but still with only marginal constraints on the PDMF. However, if ${\lesssim } 10 {{\ \rm per\ cent}}$ of the TDFs expected to be found by LSST over a decade ($\mathcal {O}(10^3)$ TDFs) are identified, then precise and accurate estimates can be made for the minimum stellar mass (within a factor of 2) and the average slope of the high-mass PDMF (to within $\mathcal {O}(10{{\ \rm per\ cent}})$) in nuclear star clusters. This technique could be adapted in the future to probe, in addition to the PDMF, the local black hole mass function and possibly the massive black hole binary population.
The late-time light curves of Type Ia supernovae (SNe Ia), observed >900 days after explosion, present the possibility of a new diagnostic for SN Ia progenitor and explosion models. First, however, ...we must discover what physical process (or processes) leads to the slow-down of the light curve relative to a pure 56Co decay, as observed in SNe 2011fe, 2012cg, and 2014J. We present Hubble Space Telescope observations of SN 2015F, taken 600-1040 days past maximum light. Unlike those of the three other SNe Ia, the light curve of SN 2015F remains consistent with being powered solely by the radioactive decay of 56Co. We fit the light curves of these four SNe Ia in a consistent manner and measure possible correlations between the light-curve stretch-a proxy for the intrinsic luminosity of the SN-and the parameters of the physical model used in the fit. We propose a new, late-time Phillips-like correlation between the stretch of the SNe and the shape of their late-time light curves, which we parameterize as the difference between their pseudo-bolometric luminosities at 600 and 900 days: ΔL900 = log(L600/L900). Our analysis is based on only four SNe, so a larger sample is required to test the validity of this correlation. If true, this model-independent correlation provides a new way to test which physical process lies behind the slow-down of SN Ia light curves >900 days after explosion, and, ultimately, fresh constraints on the various SN Ia progenitor and explosion models.
The γ-ray afterglows of tidal disruption events Chen, Xian; Gómez-Vargas, Germán Arturo; Guillochon, James
Monthly Notices of the Royal Astronomical Society,
05/2016, Letnik:
458, Številka:
3
Journal Article
Recenzirano
Odprti dostop
A star wandering too close to a supermassive black hole (SMBH) will be tidally disrupted. Previous studies of such ‘tidal disruption event’ (TDE) mostly focus on the stellar debris that are bound to ...the system, because they give rise to luminous flares. On the other hand, half of the stellar debris in principle are unbound and can stream to a great distance, but so far there is no clear evidence that this ‘unbound debris stream’ (UDS) exists. Motivated by the fact that the circum-nuclear region around SMBHs is usually filled with dense molecular clouds (MCs), here we investigate the observational signatures resulting from the collision between an UDS and an MC, which is likely to happen hundreds of years after a TDE. We focus on γ-ray emission (0.1–105 GeV), which comes from the encounter of shock-accelerated cosmic rays with background protons and, more importantly, is not subject to extinction. We show that because of the high proton density inside an MC, the peak γ-ray luminosity, about 1039 erg s−1, is at least 100 times greater than that in the case without an MC (only with a smooth interstellar medium). The luminosity decays on a time-scale of decades, depending on the distance of the MC, and about a dozen of these ‘TDE afterglows’ could be detected within a distance of about 16 Mpc by the future Cherenkov Telescope Array. Without careful discrimination, these sources potentially could contaminate the searches for starburst galaxies, galactic nuclei containing millisecond pulsars or dark matter annihilation signals.
The [gamma]-ray afterglows of tidal disruption events Chen, Xian; Gómez-Vargas, Germán Arturo; Guillochon, James
Monthly notices of the Royal Astronomical Society,
05/2016, Letnik:
458, Številka:
3
Journal Article
Recenzirano
A star wandering too close to a supermassive black hole (SMBH) will be tidally disrupted. Previous studies of such 'tidal disruption event' (TDE) mostly focus on the stellar debris that are bound to ...the system, because they give rise to luminous flares. On the other hand, half of the stellar debris in principle are unbound and can stream to a great distance, but so far there is no clear evidence that this 'unbound debris stream' (UDS) exists. Motivated by the fact that the circum-nuclear region around SMBHs is usually filled with dense molecular clouds (MCs), here we investigate the observational signatures resulting from the collision between an UDS and an MC, which is likely to happen hundreds of years after a TDE. We focus on γ-ray emission (0.1-10^sup 5^ GeV), which comes from the encounter of shock-accelerated cosmic rays with background protons and, more importantly, is not subject to extinction. We show that because of the high proton density inside an MC, the peak γ-ray luminosity, about 10^sup 39^ erg s^sup -1^, is at least 100 times greater than that in the case without an MC (only with a smooth interstellar medium). The luminosity decays on a time-scale of decades, depending on the distance of the MC, and about a dozen of these 'TDE afterglows' could be detected within a distance of about 16 Mpc by the future Cherenkov Telescope Array. Without careful discrimination, these sources potentially could contaminate the searches for starburst galaxies, galactic nuclei containing millisecond pulsars or dark matter annihilation signals.
We carried out the first multi-wavelength (optical/UV and X-ray) photometric reverberation mapping of a tidal disruption flare (TDF) ASASSN-14li. We find that its X-ray variations are correlated with ...and lag the optical/UV fluctuations by 32 4 days. Based on the direction and the magnitude of the X-ray time lag, we rule out X-ray reprocessing and direct emission from a standard circular thin disk as the dominant source of its optical/UV emission. The lag magnitude also rules out an AGN disk-driven instability as the origin of ASASSN-14li and thus strongly supports the tidal disruption picture for this event and similar objects. We suggest that the majority of the optical/UV emission likely originates from debris stream self-interactions. Perturbations at the self-interaction sites produce optical/UV variability and travel down to the black hole where they modulate the X-rays. The time lag between the optical/UV and the X-rays variations thus correspond to the time taken by these fluctuations to travel from the self-interaction site to close to the black hole. We further discuss these time lags within the context of the three variants of the self-interaction model. High-cadence monitoring observations of future TDFs will be sensitive enough to detect these echoes and would allow us to establish the origin of optical/UV emission in TDFs in general.
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 star's 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 PS 1-10jh, 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.
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