31 research outputs found
Hydrodynamical moving-mesh simulations of the tidal disruption of stars by supermassive black holes
When a star approaches a black hole closely, it may be pulled apart by
gravitational forces in a tidal disruption event (TDE). The flares produced by
TDEs are unique tracers of otherwise quiescent supermassive black holes (SMBHs)
located at the centre of most galaxies. In particular, the appearance of such
flares and the subsequent decay of the light curve are both sensitive to
whether the star is partially or totally destroyed by the tidal field. However,
the physics of the disruption and the fall-back of the debris are still poorly
understood. We are here modelling the hydrodynamical evolution of realistic
stars as they approach a SMBH on parabolic orbits, using for the first time the
moving-mesh code AREPO, which is particularly well adapted to the problem
through its combination of quasi-Lagrangian behaviour, low advection errors,
and high accuracy typical of mesh-based techniques. We examine a suite of
simulations with different impact parameters, allowing us to determine the
critical distance at which the star is totally disrupted, the energy
distribution and the fallback rate of the debris, as well as the hydrodynamical
evolution of the stellar remnant in the case of a partial disruption.
Interestingly, we find that the internal evolution of the remnant's core is
strongly influenced by persistent vortices excited in the tidal interaction.
These should be sites of strong magnetic field amplification, and the
associated mixing may profoundly alter the subsequent evolution of the tidally
pruned star.Comment: 13 pages, 9 figures. Accepted for publication by MNRA
Collisions of red giants in galactic nuclei
In stellar-dense environments, stars can collide with each other. For
collisions close to a supermassive black hole (SMBH), the collisional kinetic
energy can be so large that the colliding stars can be completely destroyed,
potentially releasing an amount of energy comparable to that of a supernova.
These violent events have been examined mostly analytically, with the
non-linear hydrodynamical effects being left largely unstudied. Using the
moving-mesh hydrodynamics code {\small AREPO}, we investigate high-velocity
( km/s) collisions between 1M giants with varying radii,
impact parameters, and initial approaching velocities, and estimate their
observables. Very strong shocks across the collision surface efficiently
convert of the initial kinetic energy into radiation energy. The
outcome is a gas cloud expanding supersonically, homologously, and
quasi-spherically, generating a flare with a peak luminosity erg/s in the extreme UV band ( eV). The luminosity
decreases approximately following a power-law initially, then
after 10 days at which point it would be bright in the
optical band (eV). Subsequent, and possibly even brighter, emission
would be generated due to the accretion of the gas cloud onto the nearby SMBH,
possibly lasting up to multi-year timescales. This inevitable BH-collision
product interaction can contribute to the growth of BHs at all mass scales, in
particular, seed BHs at high redshifts. Furthermore, the proximity of the
events to the central BH makes them a potential tool for probing the existence
of dormant BHs, even very massive ones which cannot be probed by tidal
disruption events.Comment: 16 pages, 14 figures, 2 tables, submitted to MNRAS, comments welcome,
movies here:
https://www.youtube.com/playlist?list=PLxLK3qI02cQd9lyIo6DIqm1tQnx_-G3U
Optimizing Distributed Tensor Contractions using Node-Aware Processor Grids
We propose an algorithm that aims at minimizing the inter-node communication
volume for distributed and memory-efficient tensor contraction schemes on
modern multi-core compute nodes. The key idea is to define processor grids that
optimize intra-/inter-node communication volume in the employed contraction
algorithms. We present an implementation of the proposed node-aware
communication algorithm into the Cyclops Tensor Framework (CTF). We demonstrate
that this implementation achieves a significantly improved performance for
matrix-matrix-multiplication and tensor-contractions on up to several hundreds
modern compute nodes compared to conventional implementations without using
node-aware processor grids. Our implementation shows good performance when
compared with existing state-of-the-art parallel matrix multiplication
libraries (COSMA and ScaLAPACK). In addition to the discussion of the
performance for matrix-matrix-multiplication, we also investigate the
performance of our node-aware communication algorithm for tensor contractions
as they occur in quantum chemical coupled-cluster methods. To this end we
employ a modified version of CTF in combination with a coupled-cluster code
(Cc4s). Our findings show that the node-aware communication algorithm is also
able to improve the performance of coupled-cluster theory calculations for
real-world problems running on tens to hundreds of compute nodes.Comment: 15 pages, 4 figure
The origin of the cosmic gamma-ray background in the MeV range
There has been much debate about the origin of the diffuse --ray
background in the MeV range. At lower energies, AGNs and Seyfert galaxies can
explain the background, but not above 0.3 MeV. Beyond 10 MeV
blazars appear to account for the flux observed. That leaves an unexplained gap
for which different candidates have been proposed, including annihilations of
WIMPS. One candidate are Type Ia supernovae (SNe Ia). Early studies concluded
that they were able to account for the --ray background in the gap,
while later work attributed a significantly lower contribution to them.
All those estimates were based on SN Ia explosion models which did not
reflect the full 3D hydrodynamics of SNe Ia explosions. In addition, new
measurements obtained since 2010 have provided new, direct estimates of high-z
SNe Ia rates beyond 2. We take into account these new advances to see
the predicted contribution to the gamma--ray background.
We use here a wide variety of explosion models and a plethora of new
measurements of SNe Ia rates. SNe Ia still fall short of the observed
background. Only for a fit, which would imply 150\% systematic error in
detecting SNe Ia events, do the theoretical predictions approach the observed
fluxes. This fit is, however, at odds at the highest redshifts with recent SN
Ia rates estimates. Other astrophysical sources such as FSRQs do match the
observed flux levels in the MeV regime, while SNe Ia make up to 30--50\% of the
observed flux.Comment: 40 pages, 13 Figures, accepted to be published in Ap
Type Ia supernovae from exploding oxygen-neon white dwarfs
The progenitor problem of Type Ia supernovae (SNe Ia) is still unsolved. Most
of these events are thought to be explosions of carbon-oxygen (CO) white dwarfs
(WDs), but for many of the explosion scenarios, particularly those involving
the externally triggered detonation of a sub-Chandrasekhar mass WD (sub-M Ch
WD), there is also a possibility of having an oxygen-neon (ONe) WD as
progenitor. We simulate detonations of ONe WDs and calculate synthetic
observables from these models. The results are compared with detonations in CO
WDs of similar mass and observational data of SNe Ia. We perform hydrodynamic
explosion simulations of detonations in initially hydrostatic ONe WDs for a
range of masses below the Chandrasekhar mass (M Ch), followed by detailed
nucleosynthetic postprocessing with a 384-isotope nuclear reaction network. The
results are used to calculate synthetic spectra and light curves, which are
then compared with observations of SNe Ia. We also perform binary evolution
calculations to determine the number of SNe Ia involving ONe WDs relative to
the number of other promising progenitor channels. The ejecta structures of our
simulated detonations in sub-M Ch ONe WDs are similar to those from CO WDs.
There are, however, small systematic deviations in the mass fractions and the
ejecta velocities. These lead to spectral features that are systematically less
blueshifted. Nevertheless, the synthetic observables of our ONe WD explosions
are similar to those obtained from CO models. Our binary evolution calculations
show that a significant fraction (3-10%) of potential progenitor systems should
contain an ONe WD. The comparison of our ONe models with our CO models of
comparable mass (1.2 Msun) shows that the less blueshifted spectral features
fit the observations better, although they are too bright for normal SNe Ia.Comment: 6 pages, 5 figure
Do electron-capture supernovae make neutron stars? First multidimensional hydrodynamic simulations of the oxygen deflagration
Context. In the classical picture, electron-capture supernovae and the accretion-induced collapse of oxygen-neon white dwarfs undergo an oxygen deflagration phase before gravitational collapse produces a neutron star. These types of core collapse events are postulated to explain several astronomical phenomena. In this work, the oxygen deflagration phase is simulated for the first time using multidimensional hydrodynamics.
Aims. By simulating the oxygen deflagration with multidimensional hydrodynamics and a level-set-based flame approach, new insights can be gained into the explosive deaths of 8−10 M⊙ stars and oxygen-neon white dwarfs that accrete material from a binary companion star. The main aim is to determine whether these events are thermonuclear or core-collapse supernova explosions, and hence whether neutron stars are formed by such phenomena.
Methods. The oxygen deflagration is simulated in oxygen-neon cores with three different central ignition densities. The intermediate density case is perhaps the most realistic, being based on recent nuclear physics calculations and 1D stellar models. The 3D hydrodynamic simulations presented in this work begin from a centrally confined flame structure using a level-set-based flame approach and are performed in 2563 and 5123 numerical resolutions.
Results. In the simulations with intermediate and low ignition density, the cores do not appear to collapse into neutron stars. Instead, almost a solar mass of material becomes unbound from the cores, leaving bound remnants. These simulations represent the case in which semiconvective mixing during the electron-capture phase preceding the deflagration is inefficient. The masses of the bound remnants double when Coulomb corrections are included in the equation of state, however they still do not exceed the effective Chandrasekhar mass and, hence, would not collapse into neutron stars. The simulations with the highest ignition density (log 10ρc = 10.3), representing the case where semiconvective mixing is very efficient, show clear signs that the core will collapse into a neutron star