53 research outputs found
Protoplanetary Disks as (Possibly) Viscous Disks
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 -viscosity values for individual objects, with the goal
of constraining the angular momentum transport mechanism. We find that the
inferred values of do not cluster around a single value, but instead
have a broad distribution extending from to . 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 and the central mass accretion
rate . This correlation is unlikely to result from the direct physical
effect of on disk viscosity on global scales. Instead, we suggest that
it is caused by the decoupling of stellar from the global disk
characteristics in one of the following ways. (1) The behavior (and range) of
is controlled by a yet unidentified parameter (e.g. ionization
fraction, magnetic field strength, or geometry), ultimately driving the
variation of . (2) The central 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
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
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
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
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 Gpc yr from
globular clusters and Gpc yr from cusped nuclear
clusters. For NS-BH and BH-BH binaries we find small merger rates from globular
clusters, but a rate of Gpc yr 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
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
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On the Planetary Interpretation of Multiple Gaps and Rings in Protoplanetary Disks Seen by ALMA
It has been recently suggested that the multiple concentric rings and gaps
discovered by ALMA in many protoplanetary disks may be produced by a single
planet, as a result of the complex propagation and dissipation of the multiple
spiral density waves it excites in the disk. Numerical efforts to verify this
idea have largely utilized the so-called locally isothermal approximation with
a prescribed disk temperature profile. However, in protoplanetary disks this
approximation does not provide an accurate description of the density wave
dynamics on scales of tens of au. Moreover, we show that locally isothermal
simulations tend to overestimate the contrast of ring and gap features, as well
as misrepresent their positions, when compared to simulations in which the
energy equation is evolved explicitly. This outcome is caused by the
non-conservation of the angular momentum flux of linear perturbations in
locally isothermal disks. We demonstrate this effect using simulations of
locally isothermal and adiabatic disks (with essentially identical temperature
profiles) and show how the dust distributions, probed by mm wavelength
observations, differ between the two cases. Locally isothermal simulations may
thus underestimate the masses of planets responsible for the formation of
multiple gaps and rings on scales of tens of au observed by ALMA. We suggest
that caution should be exercised in using the locally isothermal simulations to
explore planet-disk interaction, as well as in other studies of wave-like
phenomena in astrophysical disks
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1I/2017 'Oumuamua-like Interstellar Asteroids as Possible Messengers from Dead Stars
Discovery of the first interstellar asteroid (ISA) - 1I/2017 'Oumuamua -
raised a number of questions regarding its origin. Many of them relate to its
lack of cometary activity, suggesting refractory composition of 'Oumuamua. Here
we explore the possibility that 'Oumuamua-like ISAs are produced in tidal
disruption events (TDEs) of refractory planetoids (asteroids, dwarf planets,
etc.) by the white dwarfs (WDs). This idea is supported by existing
spectroscopic observations of metal-polluted WDs, hinting at predominantly
volatile-poor composition of accreted material. We show that such TDEs sourced
by realistic planetary systems (including a population of >1000 km planetoids
and massive perturbers - Neptune-to-Saturn mass planets) can eject to
interstellar space up to 30% of planetary mass involved in them. Collisional
fragmentation, caused by convergent vertical motion of the disrupted
planetoid's debris inside the Roche sphere of the WD, channels most of the
original mass into 0.1-1 km fragments, similar to 'Oumuamua. Such size spectrum
of ISAs (very different from the top-heavy distributions expected in other
scenarios) implies that planetary TDEs can account for a significant fraction
(up to ~30% under optimistic assumptions) of the ISAs. This figure is based on
existing observations of WD metal pollution and accounts for observational
biases by using realistic models of circum-WD planetary systems. ISAs should
exhibit kinematic characteristics similar to old, dynamically hot Galactic
populations; we interpret 'Oumuamua's slow Galactic motion as a statistical
fluctuation. ISA ejection in individual planetary TDEs is highly anisotropic,
resulting in large fluctuations of their space density. We also show that other
ISA production mechanisms involving stellar remnants - direct ejection by
massive planets around WDs and SN explosions - have difficulty explaining
'Oumuamua-like ISAs
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Fast and Slow Precession of Gaseous Debris Disks around Planet-accreting White Dwarfs
Spectroscopic observations of some metal-rich white dwarfs (WDs), believed to
be polluted by planetary material, reveal the presence of compact gaseous
metallic disks orbiting them. The observed variability of asymmetric,
double-peaked emission line profiles in about half of such systems could be
interpreted as the signature of precession of an eccentric gaseous debris disk.
The variability timescales --- from decades down to yr (recently inferred
for the debris disk around HE 1349--2305) --- are in rough agreement with the
rate of general relativistic (GR) precession in the test particle limit.
However, it has not been demonstrated that this mechanism can drive such a
fast, coherent precession of a radially extended (out to ) gaseous
disk mediated by internal stresses (pressure). Here we use the linear theory of
eccentricity evolution in hydrodynamic disks to determine several key
properties of eccentric modes in gaseous debris disks around WDs. We find a
critical dependence of both the precession period and radial eccentricity
distribution of the modes on the inner disk radius, . For small
inner radii, , the modes are
GR-driven, with periods of yr. For , the modes are pressure-dominated, with periods of yr. Correspondence between the variability periods and inferred inner radii
of the observed disks is in general agreement with this trend. In particular,
the short period of HE 1349--2305 is consistent with its small .
Circum-WD debris disks may thus serve as natural laboratories for studying the
evolution of eccentric gaseous disks
Secular dynamics of binaries in stellar clusters - II. Dynamical evolution
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 . Here we provide a thorough
exploration of the phase-space of the corresponding secular problem as
is varied. We find that for 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 . 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 (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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