skip to main content

Title: On the Role of Dynamical Cooling in the Dynamics of Circumbinary Disks

Hydrodynamical interactions between binaries and circumbinary disks (CBDs) play an important role in a variety of astrophysical systems, from young stellar binaries to supermassive black hole binaries. Previous simulations of CBDs have mostly employed locally isothermal equations of state. We carry out 2D viscous hydrodynamic simulations of CBDs around equal-mass, circular binaries, treating the gas thermodynamics by thermal relaxation toward equilibrium temperature (the constant-βcooling ansatz, whereβis the cooling time in units of the local Keplerian time). As an initial study, we use the grid-based codeAthena++on a polar grid, covering an extended disk outside the binary co-orbital region. We find that with a longer cooling time, the accretion variability is gradually suppressed, and the morphology of the CBD becomes more symmetric. The disk also shows evidence of hysteresis behavior depending on the initial conditions. Gas cooling also affects the rate of angular momentum transfer between the binary and the CBD, where given our adopted disk thickness and viscosity (H/r∼ 0.1 andα∼ 0.1), the binary orbit expands while undergoing accretion for mostβvalues between 0 and 4.0 except over a narrow range of intermediateβvalues. The validity of using a polar grid excising the central domain is also discussed.

more » « less
Author(s) / Creator(s):
; ;
Publisher / Repository:
DOI PREFIX: 10.3847
Date Published:
Journal Name:
The Astrophysical Journal
Medium: X Size: Article No. 175
["Article No. 175"]
Sponsoring Org:
National Science Foundation
More Like this

    Hydrodynamical interaction in circumbinary discs (CBDs) plays a crucial role in various astrophysical systems, ranging from young stellar binaries to supermassive black hole binaries in galactic centres. Most previous simulations of binary-disc systems have adopted locally isothermal equation of state. In this study, we use the grid-based code Athena++ to conduct a suite of two-dimensional viscous hydrodynamical simulations of circumbinary accretion on a Cartesian grid, resolving the central cavity of the binary. The gas thermodynamics is treated by thermal relaxation towards an equilibrium temperature (based on the constant − β cooling ansatz, where β is the cooling time in units of the local Keplerian time). Focusing on equal mass, circular binaries in CBDs with (equilibrium) disc aspect ratio H/R = 0.1, we find that the cooling of the disc gas significantly influences the binary orbital evolution, accretion variability, and CBD morphology, and the effect depends sensitively on the disc viscosity prescriptions. When adopting a constant kinematic viscosity, a finite cooling time (β ≳ 0.1) leads to a binary inspiral as opposed to an outspiral and the CBD cavity becomes more symmetric. When adopting a dynamically varying α-viscosity, binary inspiral only occurs within a narrow range of cooling time (corresponding to β around 0.5).

    more » « less
  2. Abstract

    Double white dwarf (WD) binaries are increasingly being discovered at short orbital periods where strong tidal effects and significant tidal heating signatures may occur. We assume that the tidal potential of the companion excites outgoing gravity waves within the WD primary, the dissipation of which leads to an increase in the WD’s surface temperature. We compute the excitation and dissipation of the waves in cooling WD models in evolvingMESAbinary simulations. Tidal heating is self-consistently computed and added to the models at every time step. As a binary inspirals to orbital periods less than ∼20 minutes, the WD’s behavior changes from cooling to heating, with temperature enhancements that can exceed 10,000 K compared with nontidally heated models. We compare a grid of tidally heated WD models to observed short-period systems with hot WD primaries. While tidal heating affects theirTeff, it is likely not the dominant luminosity. Instead, these WDs are probably intrinsically young and hot, implying that the binaries formed at short orbital periods. The binaries are consistent with undergoing common envelope evolution with a somewhat low efficiencyαCE. We delineate the parameter space where the traveling wave assumption is most valid, noting that it breaks down for WDs that cool sufficiently, where standing waves may instead be formed.

    more » « less
  3. Abstract

    We present fully relativistic predictions for the electromagnetic emission produced by accretion disks surrounding spinning and nonspinning supermassive binary black holes on the verge of merging. We use the codeBothrosto post-process data from 3D general relativistic magnetohydrodynamic simulations via ray-tracing calculations. These simulations model the dynamics of a circumbinary disk and the mini-disks that form around two equal-mass black holes orbiting each other at an initial separation of 20 gravitational radii, and evolve the system for more than 10 orbits in the inspiral regime. We model the emission as the sum of thermal blackbody radiation emitted by an optically thick accretion disk and a power-law spectrum extending to hard X-rays emitted by a hot optically thin corona. We generate time-dependent spectra, images, and light curves at various frequencies to investigate intrinsic periodic signals in the emission, as well as the effects of the black hole spin. We find that prograde black hole spin makes mini-disks brighter since the smaller innermost stable circular orbit angular momentum demands more dissipation before matter plunges to the horizon. However, compared to mini-disks in larger separation binaries with spinning black holes, our mini-disks are less luminous: unlike those systems, their mass accretion rate is lower than in the circumbinary disk, and they radiate with lower efficiency because their inflow times are shorter. Compared to a single black hole system matched in mass and accretion rate, these binaries have spectra noticeably weaker and softer in the UV. Finally, we discuss the implications of our findings for the potential observability of these systems.

    more » « less

    The upcoming Laser Interferometer Space Antenna (LISA) is expected to detect gravitational waves (GWs) from massive black hole binaries (MBHB). Finding the electromagnetic (EM) counterparts for these GW events will be crucial for understanding how and where MBHBs merge, measuring their redshifts, constraining the Hubble constant and the graviton mass, and for other novel science applications. However, due to poor GW sky localization, multiwavelength, time-dependent EM models are needed to identify the right host galaxy. We studied merging MBHBs embedded in a circumbinary disc (CBD) using high-resolution two-dimensional simulations, with a Γ-law equation of state, incorporating viscous heating, shock heating, and radiative cooling. We simulate the binary from large separation until after merger, allowing us to model the decoupling of the binary from the CBD. We compute the EM signatures and identify distinct features before, during, and after the merger. Our main result is a multiband EM signature: we find that the MBHB produces strong thermal X-ray emission until 1–2 d prior to the merger. However, as the binary decouples from the CBD, the X-ray-bright minidiscs rapidly shrink in size, become disrupted, and the accretion rate drops precipitously. As a result, the thermal X-ray luminosity drops by orders of magnitude, and the source remains X-ray dark for several days, regardless of any post-merger effects such as GW recoil or mass-loss. Looking for the abrupt spectral change where the thermal X-ray disappears is a tell-tale EM signature of LISA mergers that does not require extensive pre-merger monitoring.

    more » « less

    We present the to-date largest parameter space exploration of binaries in circumbinary discs (CBDs), deriving orbital evolution prescriptions for eccentric, unequal mass binaries from our suite of hydrodynamic simulations. In all cases, binary eccentricities evolve towards steady state values that increase with mass ratio, and saturate at an equilibrium eccentricity eb,eq ∼ 0.5 in the large mass ratio regime, in line with resonant theory. For binaries accreting at their combined Eddington limit, a steady state eccentricity can be achieved within a few megayears. Once at their steady state eccentricities, binaries with qb ≳ 0.3 evolve towards coalescence, while lower mass ratio systems expand due to CBD torques. We discuss implications for population studies of massive black hole binaries, protostars in binary systems, and post-common envelope binaries observed by ground-based gravitational wave detectors.

    more » « less