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Register a new userRelativistic Tidal Disruption and Nuclear Ignition of White Dwarf Stars by Intermediate Mass Black Holes
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Patrick Christopher Fragile
Publication date
2018
fields
Physics
and research's language is
English
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We present results from general relativistic calculations of the tidal disruption of white dwarf stars from near encounters with intermediate mass black holes. We follow the evolution of 0.2 and $0.6 M_odot$ stars on parabolic trajectories that approach $10^3$ - $10^4 M_odot$ black holes as close as a few Schwarzschild radii at periapsis, paying particular attention to the effect tidal disruption has on thermonuclear reactions and the synthesis of intermediate to heavy ion elements. These encounters create diverse thermonuclear environments characteristic of Type I supernovae and capable of producing both intermediate and heavy mass elements in arbitrary ratios, depending on the strength (or proximity) of the interaction. Nuclear ignition is triggered in all of our calculations, even at weak tidal strengths $beta sim 2.6$ and large periapsis radius $R_P sim 28$ Schwarzschild radii. A strong inverse correlation exists between the mass ratio of calcium to iron group elements and tidal strength, with $beta lesssim 5$ producing predominately calcium-rich debris. At these moderate to weak interactions, nucleosynthesis is not especially efficient, limiting the total mass and outflows of calcium group elements to $< 15$% of available nuclear fuel. Iron group elements however continue to be produced in greater quantity and ratio with increasing tidal strength, peaking at $sim 60$% mass conversion efficiency in our closest encounter cases. These events generate short bursts of gravitational waves with characteristic frequencies 0.1-0.7 Hz and strain amplitudes $0.5times10^{-22}$ - $3.5times10^{-22}$ at 10 Mpc source distance.
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We present results from general relativistic calculations of nuclear ignition in white dwarf stars triggered by near encounters with rotating intermediate mass black holes with different spin and alignment parameters. These encounters create thermonuclear environments characteristic of Type Ia supernovae capable of producing both calcium and iron group elements in arbitrary ratios, depending primarily on the proximity of the interaction which acts as a strong moderator of nucleosynthesis. We explore the effects of black hole spin and spin-orbital alignment on burn product synthesis to determine whether they might also be capable of moderating reactive flows. When normalized to equivalent impact penetration, accounting for frame dragging corrections, the influence of spin is weak, no more than 25% as measured by nuclear energy release and mass of burn products, even for near maximally rotating black holes. Stars on prograde trajectories approach closer to the black hole and produce significantly more unbound debris and iron group elements than is possible by encounters with nonrotating black holes or by retrograde orbits, at more than 50% mass conversion efficiency. The debris contains several radioisotopes, most notably Ni56, made in amounts that produce sub-luminous (but still observable) light curves compared to branch-normal SNe Ia.
We present a numerical investigation of the tidal disruption of white dwarfs by moderately massive black holes, with particular reference to the centers of dwarf galaxies and globular clusters. Special attention is given to the fate of white dwarfs of all masses that approach the black hole close enough to be disrupted and severely compressed to such extent that explosive nuclear burning can be triggered. Consistent modeling of the gas dynamics together with the nuclear reactions allows for a realistic determination of the explosive energy release. In the most favorable cases, the nuclear energy release may be comparable to that of typical type Ia supernovae. Although the explosion will increase the mass fraction escaping on hyperbolic orbits, a good fraction of the debris remains to be swallowed by the hole, causing a bright soft X-ray flare lasting for about a year. Such transient signatures, if detected, would be a compelling testimony for the presence of a moderately mass black hole (below $10^5 M_odot$).
We present the first simulations of the tidal disruption of stars with realistic structures and compositions by massive black holes (BHs). We build stars in the stellar evolution code MESA and simulate their disruption in the 3D adaptive-mesh hydrodynamics code FLASH, using an extended Helmholtz equation of state and tracking 49 elements. We study the disruption of a 1$M_odot$ star and 3$M_odot$ star at zero-age main sequence (ZAMS), middle-age, and terminal-age main sequence (TAMS). The maximum BH mass for tidal disruption increases by a factor of $sim$2 from stellar radius changes due to MS evolution; this is equivalent to varying BH spin from 0 to 0.75. The shape of the mass fallback rate curves is different from the results for polytropes of Guillochon & Ramirez-Ruiz (2013). The peak timescale $t_{rm peak}$ increases with stellar age, while the peak fallback rate $dot M_{rm peak}$ decreases with age, and these effects diminish with increasing impact parameter $beta$. For a $beta=1$ disruption of a 1$M_odot$ star by a $10^6 M_odot$ BH, from ZAMS to TAMS, $t_{rm peak}$ increases from 30 to 54 days, while $dot M_{rm peak}$ decreases from 0.66 to 0.14 $M_odot$/yr. Compositional anomalies in nitrogen, helium, and carbon can occur before the peak timescale for disruptions of MS stars, which is in contrast to predictions from the frozen-in model. More massive stars can show stronger anomalies at earlier times, meaning that compositional constraints can be key in determining the mass of the disrupted star. The abundance anomalies predicted by these simulations provide a natural explanation for the spectral features and varying line strengths observed in tidal disruption events.
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 remnants 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.
The tidal disruption of stars by (super-)massive black holes in galactic nuclei has been discussed in theoretical terms for about 30 years but only in the past decade have we been able to detect such events in substantial numbers. Thus, we are now starting to carry out observational tests of models for the disruption. We are also formulating expectations for the inspiral and disruption of white dwarfs by intermediate-mass black holes with masses $< 10^5;{rm M}_odot$. Such events are very rich with information and open a new window to intermediate-mass black holes, thought to live in dwarf galaxies and star clusters. They can inform us of the demographics of intermediate-mass black holes, stellar populations and dynamics in their immediate vicinity, and the physics of accretion of hydrogen-deficient material. The combination of upcoming transient surveys using ground-based, electromagnetic observatories and low-frequency gravitational wave observations is ideal for exploiting tidal disruptions of white dwarfs. The detection rate of gravitational wave signals, optimistically, may reach a few dozen per year in a volume up to $zapprox 0.1$. Gravitational wave observations are particularly useful because they yield the masses of the objects involved and allow determination of the spin of the black hole, affording tests of physical models for black hole formation and growth. They also give us advance warning of the electromagnetic flares by weeks or more. The right computing infrastructure for modern models for the disruption process and event rates will allow us to make the most of the upcoming observing facilities.
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