53 research outputs found

    Protoplanetary Disks as (Possibly) Viscous Disks

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    Protoplanetary disks are believed to evolve on Myr timescales in a diffusive (viscous) manner as a result of angular momentum transport driven by internal stresses. Here we use a sample of 26 protoplanetary disks resolved by ALMA with measured (dust-based) masses and stellar accretion rates to derive the dimensionless α\alpha-viscosity values for individual objects, with the goal of constraining the angular momentum transport mechanism. We find that the inferred values of α\alpha do not cluster around a single value, but instead have a broad distribution extending from 10−410^{-4} to 0.040.04. Moreover, they correlate with neither the global disk parameters (mass, size, surface density) nor the stellar characteristics (mass, luminosity, radius). However, we do find a strong linear correlation between α\alpha and the central mass accretion rate M˙\dot M. This correlation is unlikely to result from the direct physical effect of M˙\dot M on disk viscosity on global scales. Instead, we suggest that it is caused by the decoupling of stellar M˙\dot M from the global disk characteristics in one of the following ways. (1) The behavior (and range) of α\alpha is controlled by a yet unidentified parameter (e.g. ionization fraction, magnetic field strength, or geometry), ultimately driving the variation of M˙\dot M. (2) The central M˙\dot M is decoupled from the global viscous mass accretion rate as a result of an instability or mass accumulation (or loss) in the inner disk. (3) Perhaps the most intriguing possibility is that angular momentum in protoplanetary disks is transported non-viscously, e.g. via magnetohydrodynamic winds or spiral density waves

    Spin Evolution and Cometary Interpretation of the Interstellar Minor Object 1I/2017 'Oumuamua

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    Observations of the first interstellar minor object 1I/2017 'Oumuamua did not reveal direct signs of outgassing that would have been natural if it had volatile-rich composition. However, a recent measurement by Micheli et al (2018) of a substantial non-gravitational acceleration affecting the orbit of this object has been interpreted as resulting from its cometary activity, which must be rather vigorous. Here we critically re-assess this interpretation by exploring the implications of measured non-gravitational acceleration for the 'Oumuamua's rotational state. We show that outgassing torques should drive rapid evolution of 'Oumuamua's spin (on a timescale of a few days), assuming torque asymmetry typical for the Solar System comets. However, given the highly elongated shape of the object, its torque asymmetry is likely higher, implying even faster evolution. This would have resulted in rapid rotational fission of 'Oumuamua during its journey through the Solar System and is clearly incompatible with the relative stability of its rotational state inferred from photometric variability. Based on these arguments, as well as the lack of direct signs of outgassing, we conclude that the classification of 'Oumuamua as a comet (invoked to explain its claimed anomalous acceleration) is questionable

    Envelopes of embedded super-Earths - I. Two-dimensional simulations

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    Measurements of exoplanetary masses and radii have revealed a population of massive super-Earths --- planets sufficiently large that, according to one dimensional models, they should have turned into gas giants. To better understand the origin of these objects, we carry out hydrodynamical simulations of planetary cores embedded in a nascent protoplanetary disk. In this first paper of a series, to gain intuition as well as to develop useful diagnostics, we focus on two-dimensional simulations of the flow around protoplanetary cores. We use the pluto code to study isothermal and adiabatic envelopes around cores of sub- to super-thermal masses, fully resolving the envelope properties down to the core surface. Owing to the conservation of vortensity, envelopes acquire a substantial degree of rotational support when the core mass increases beyond the thermal mass, suggesting a limited applicability of one-dimensional models for describing the envelope structure. The finite size of the core (relatively large for super-Earths) also controls the amount of rotational support in the entire envelope. Steady non-axisymmetric shocks develop in the supersonic envelopes of high-mass cores, triggering mass accretion and turbulent mixing in their interiors. We also examine the influence of the gas self-gravity on the envelope structure. Although it only weakly alters the properties of the envelopes, the gas gravity has significant effect on the properties of the density waves triggered by the core in the protoplanetary disk

    Envelopes of embedded super-Earths – II. Three-dimensional isothermal simulations

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    Massive planetary cores embedded in protoplanetary discs are believed to accrete extended atmospheres, providing a pathway to forming gas giants and gas-rich super-Earths. The properties of these atmospheres strongly depend on the nature of the coupling between the atmosphere and the surrounding disc. We examine the formation of gaseous envelopes around massive planetary cores via three-dimensional inviscid and isothermal hydrodynamic simulations. We focus the changes in the envelope properties as the core mass varies from low (sub-thermal) to high (super-thermal) values, a regime relevant to close-in super-Earths. We show that global envelope properties such as the amount of rotational support or turbulent mixing are mostly sensitive to the ratio of the Bondi radius of the core to its physical size. High-mass cores are fed by supersonic inflows arriving along the polar axis and shocking on the densest parts of the envelope, driving turbulence and mass accretion. Gas flows out of the core's Hill sphere in the equatorial plane, describing a global mass circulation through the envelope. The shell of shocked gas atop the core surface delimits regions of slow (inside) and fast (outside) material recycling by gas from the surrounding disc. While recycling hinders the runaway growth towards gas giants, the inner regions of protoplanetary atmospheres, more immune to mixing, may remain bound to the planet

    Compact Object Binary Mergers Driven by Cluster Tides: A New Channel for LIGO/Virgo Gravitational-wave Events

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    The detections of gravitational waves produced in mergers of binary black holes (BH) and neutron stars (NS) by LIGO/Virgo have stimulated interest in the origin of the progenitor binaries. Dense stellar systems - globular and nuclear star clusters - are natural sites of compact object binary formation and evolution towards merger. Here we explore a new channel for the production of binary mergers in clusters, in which the tidal field of the cluster secularly drives the binary to high eccentricity (even in the absence of a central massive black hole) until gravitational wave emission becomes important. We employ the recently developed secular theory of cluster tide-driven binary evolution to compute present day merger rates for BH-BH, NS-BH and NS-NS binaries, varying cluster potential and central concentration of the binary population (but ignoring cluster evolution and stellar flybys for now). Unlike other mechanisms, this new dynamical channel can produce a significant number of mergers out to cluster-centric distances of several pc. For NS-NS binaries we find merger rates in the range 0.01−0.070.01-0.07 Gpc−3^{-3} yr−1^{-1} from globular clusters and 0.1−0.20.1-0.2 Gpc−3^{-3} yr−1^{-1} from cusped nuclear clusters. For NS-BH and BH-BH binaries we find small merger rates from globular clusters, but a rate of 0.1−0.20.1 - 0.2 Gpc−3^{-3} yr−1^{-1} from cusped nuclear clusters, contributing to the observed LIGO/Virgo rate at the level of several per cent. Therefore, cluster tide-driven mergers constitute a new channel that can be further explored with current and future gravitational wave detectors

    Secular Evolution Driven by Massive Eccentric Disks/Rings: An Apsidally Aligned Case

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    Massive eccentric disks (gaseous or particulate) orbiting a dominant central mass appear in many astrophysical systems, including planetary rings, protoplanetary and accretion disks in binaries, and nuclear stellar disks around supermassive black holes in galactic centers. We present an analytical framework for treating the nearly Keplerian secular dynamics of test particles driven by the gravity of an eccentric, apsidally aligned, zero-thickness disk with arbitrary surface density and eccentricity profiles. We derive a disturbing function describing the secular evolution of coplanar objects, which is explicitly related (via one-dimensional, convergent integrals) to the disk surface density and eccentricity profiles without using any ad hoc softening of the potential. Our analytical framework is verified via direct orbit integrations, which show it to be accurate in the low-eccentricity limit for a variety of disk models (for disk eccentricity < 0.1-0.2). We find that free precession in the potential of a disk with a smooth surface density distribution can naturally change from prograde to retrograde within the disk. Sharp disk features - edges and gaps - are the locations where this tendency is naturally enhanced, while the precession becomes very fast. Radii where free precession changes sign are the locations where substantial (formally singular) growth of the forced eccentricity of the orbiting objects occurs. Based on our results, we formulate a self-consistent analytical framework for computing an eccentricity profile for an aligned, eccentric disk (with a prescribed surface density profile) capable of precessing as a solid body under its own self-gravity

    Secular dynamics of binaries in stellar clusters - II. Dynamical evolution

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    Dense stellar clusters are natural sites for the origin and evolution of exotic objects such as relativistic binaries (potential gravitational wave sources), blue stragglers, etc. We investigate the secular dynamics of a binary system driven by the global tidal field of an axisymmetric stellar cluster in which the binary orbits. In a companion paper (Hamilton & Rafikov 2019a) we developed a general Hamiltonian framework describing such systems. The effective (doubly-averaged) Hamiltonian derived there encapsulates all information about the tidal potential experienced by the binary in its orbit around the cluster in a single parameter Γ\Gamma. Here we provide a thorough exploration of the phase-space of the corresponding secular problem as Γ\Gamma is varied. We find that for Γ>1/5\Gamma > 1/5 the phase-space structure and the evolution of binary orbital element are qualitatively similar to the Lidov-Kozai problem. However, this is only one of four possible regimes, because the dynamics are qualitatively changed by bifurcations at Γ=1/5,0,−1/5\Gamma = 1/5,0,-1/5. We show how the dynamics are altered in each regime and calculate characteristics such as secular evolution timescale, maximum possible eccentricity, etc. We verify the predictions of our doubly-averaged formalism numerically and find it to be very accurate when its underlying assumptions are fulfilled, typically meaning that the secular timescale should exceed the period of the binary around the cluster by ≳10−102\gtrsim 10-10^2 (depending on the cluster potential and binary orbit). Our results may be relevant for understanding the nature of a variety of exotic systems harboured by stellar clusters
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