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Register a new userTidal Disruption of Inclined or Eccentric Binaries by Massive Black Holes
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Binary stars that are on close orbits around massive black holes (MBH) such as Sgr A* in the center of the Milky Way are liable to undergo tidal disruption and eject a hypervelocity star. We study the interaction between such a MBH and circular binaries for general binary orientations and penetration depths (i.e. binaries penetrate into the tidal radius around the BH). We show that for very deep penetrators, all binaries are disrupted when the binary rotation axis is roughly oriented toward the BH or it is in the opposite direction. The surviving chance becomes significant when the angle between the binary rotation axis and the BH direction is between pi /4 and 3 pi /4. The surviving chance is as high as $sim$ 20$%$ when the binary rotation axis is perpendicular to the BH direction. The angular dependence is opposite for very shallow penetrators where coplanar prograde orbits have the lowest surviving chance (or equivalently most vulnerable). We provide numerical fits to the disruption probability and energy gain at the the BH encounter as a function of the penetration depth. The latter can be simply rescaled in terms of binary masses, their initial separation and the binary-to-BH mass ratio to evaluate the ejection velocity of a binary members in various systems. We also investigate the disruption of coplanar, eccentric binaries by a MBH. It is shown that for highly eccentric binaries retrograde orbits have a significantly increased disruption probability and ejection velocities compared to the circular binaries.
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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.
Supermassive black hole binaries (SMBHBs) are products of galaxy mergers, and are important in testing Lambda cold dark matter cosmology and locating gravitational-wave-radiation sources. A unique electromagnetic signature of SMBHBs in galactic nuclei is essential in identifying the binaries in observations from the IR band through optical to X-ray. Recently, the flares in optical, UV, and X-ray caused by supermassive black holes (SMBHs) tidally disrupting nearby stars have been successfully used to observationally probe single SMBHs in normal galaxies. In this Letter, we investigate the accretion of the gaseous debris of a tidally disrupted star by a SMBHB. Using both stability analysis of three-body systems and numerical scattering experiments, we show that the accretion of stellar debris gas, which initially decays with time $propto t^{-5/3}$, would stop at a time $T_{rm tr} simeq eta T_{rm b}$. Here, $eta sim0.25$ and $T_{rm b}$ is the orbital period of the SMBHB. After a period of interruption, the accretion recurs discretely at time $T_{rm r} simeq xi T_b$, where $xi sim 1$. Both $eta$ and $xi$ sensitively depend on the orbital parameters of the tidally disrupted star at the tidal radius and the orbit eccentricity of SMBHB. The interrupted accretion of the stellar debris gas gives rise to an interrupted tidal flare, which could be used to identify SMBHBs in non-active galaxies in the upcoming transient surveys.
Galaxy mergers produce supermassive black hole binaries, which emit gravitational waves prior to their coalescence. We perform three-dimensional hydrodynamic simulations to study the tidal disruption of stars by such a binary in the final centuries of its life. We find that the gas stream of the stellar debris moves chaotically in the binary potential and forms accretion disks around both black holes. The accretion light curve is modulated over the binary orbital period owing to relativistic beaming. This periodic signal allows to detect the decay of the binary orbit due to gravitational wave emission by observing two tidal disruption events that are separated by more than a decade.
After the Tidal Disruption Event (TDE) of a star around a SuperMassive Black Hole (SMBH), if the stellar debris stream rapidly circularizes and forms a compact disk, the TDE emission is expected to peak in the soft X-ray or far Ultra-Violet (UV). The fact that many TDE candidates are observed to peak in the near UV and optical has challenged conventional TDE emission models. By idealizing a disk as a nested sequence of elliptical orbits which communicate adiabatically via pressure forces, and are heated by energy dissipated during the circularization of the nearly parabolic debris streams, we investigate the dynamics and thermal emission of highly eccentric TDE disks, including the effect of General-Relativistic apsidal precession from the SMBH. We calculate the properties of uniformly precessing, apsidally aligned, and highly eccentric TDE disks, and find highly eccentric disk solutions exist for realistic TDE properties (SMBH and stellar mass, periapsis distance, etc.). Taking into account compressional heating (cooling) near periapsis (apoapsis), we find our idealized eccentric disk model can produce emission consistent with the X-ray and UV/Optical luminosities of many optically bright TDE candidates. Our work attempts to quantify the thermal emission expected from the shock-heating model for TDE emission, and finds stream-stream collisions are a promising way to power optically bright TDEs.
We argue that the `changing look AGN recently reported by LaMassa et al. could be a luminous flare produced by the tidal disruption of a super-solar mass star passing just a few gravitational radii outside the event horizon of a $sim 10^8 M_{odot}$ nuclear black hole. This flare occurred in a massive, star forming galaxy at redshift $z=0.312$, robustly characterized thanks to repeated late-time photometric and spectroscopic observations. By taking difference-photometry of the well sampled multi-year SDSS Stripe-82 light-curve, we are able to probe the evolution of the nuclear spectrum over the course of the outburst. The tidal disruption event (TDE) interpretation is consistent with the very rapid rise and the decay time of the flare, which displays an evolution consistent with the well-known $t^{-5/3}$ behaviour (with a clear superimposed re-brightening flare). Our analysis places constraints on the physical properties of the TDE, such as the putative disrupted stars mass and orbital parameters, as well as the size and temperature of the emitting material. The properties of the broad and narrow emission lines observed in two epochs of SDSS spectra provide further constraints on the circum-nuclear structure, and could be indicative that the system hosted a moderate-luminosity AGN as recently as a few $10^4$ years ago, and is likely undergoing residual accretion as late as ten years after peak, as seen from the broad H$alpha$ emission line. We discuss the complex interplay between tidal disruption events and gas accretion episodes in galactic nuclei, highlighting the implications for future TDE searches and for estimates of their intrinsic rates.
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