11 research outputs found
The origin of the negative torque density in disk-satellite interaction
Tidal interaction between a gaseous disk and a massive orbiting perturber is
known to result in angular momentum exchange between them. Understanding
astrophysical manifestations of this coupling such as gap opening by planets in
protoplanetary disks or clearing of gas by binary supermassive black holes
(SMBHs) embedded in accretion disks requires knowledge of the spatial
distribution of the torque exerted on the disk by a perturber. Recent
hydrodynamical simulations by Dong et al (2011) have shown evidence for the
tidal torque density produced in a uniform disk to change sign at the radial
separation of scale heights from the perturber's orbit, in clear
conflict with the previous studies. To clarify this issue we carry out a linear
calculation of the disk-satellite interaction putting special emphasis on
understanding the behavior of the perturbed fluid variables in physical space.
Using analytical as well as numerical methods we confirm the reality of the
negative torque density phenomenon and trace its origin to the overlap of
Lindblad resonances in the vicinity of the perturber's orbit - an effect not
accounted for in previous studies. These results suggest that calculations of
the gap and cavity opening in disks by planets and binary SMBHs should rely on
more realistic torque density prescriptions than the ones used at present.Comment: 18 pages, 6 figures, accepted to Ap
Disk-satellite interaction in disks with density gaps
Gravitational coupling between a gaseous disk and an orbiting perturber leads
to angular momentum exchange between them which can result in gap opening by
planets in protoplanetary disks and clearing of gas by binary supermassive
black holes (SMBHs) embedded in accretion disks. Understanding the co-evolution
of the disk and the orbit of the perturber in these circumstances requires
knowledge of the spatial distribution of the torque exerted by the latter on a
highly nonuniform disk. Here we explore disk-satellite interaction in disks
with gaps in linear approximation both in Fourier and in physical space,
explicitly incorporating the disk non-uniformity in the fluid equations.
Density gradients strongly displace the positions of Lindblad resonances in the
disk (which often occur at multiple locations), and the waveforms of modes
excited close to the gap edge get modified compared to the uniform disk case.
The spatial distribution of the excitation torque density is found to be quite
different from the existing prescriptions: most of the torque is exerted in a
rather narrow region near the gap edge where Lindblad resonances accumulate,
followed by an exponential fall-off with the distance from the perturber.
Despite these differences, for a given gap profile the full integrated torque
exerted on the disk agrees with the conventional uniform disk theory prediction
at the level of ~10%. The nonlinearity of the density wave excited by the
perturber is shown to decrease as the wave travels out of the gap, slowing down
its nonlinear evolution and damping. Our results suggest that gap opening in
protoplanetary disks and gas clearing around SMBH binaries can be more
efficient than the existing theories predict. They pave the way for
self-consistent calculations of the gap structure and the orbital evolution of
the perturber using accurate prescription for the torque density behavior.Comment: corrected typos in reference
The dynamics of inner dead-zone boundaries in protoplanetary disks
In protoplanetary disks, the inner radial boundary between the MRI turbulent
(`active') and MRI quiescent (`dead') zones plays an important role in models
of the disk evolution and in some planet formation scenarios. In reality, this
boundary is not well-defined: thermal heating from the star in a passive disk
yields a transition radius close to the star (<0.1 au), whereas if the disk is
already MRI active, it can self-consistently maintain the requisite
temperatures out to a transition radius of roughly 1 au. Moreover, the
interface may not be static; it may be highly fluctuating or else unstable. In
this paper, we study a reduced model of the dynamics of the active/dead zone
interface that mimics several important aspects of a real disk system. We find
that MRI-transition fronts propagate inward (a `dead front' suppressing the
MRI) if they are initially at the larger transition radius, or propagate
outward (an `active front' igniting the MRI) if starting from the smaller
transition radius. In both cases, the front stalls at a well-defined
intermediate radius, where it remains in a quasi-static equilibrium. We propose
that it is this new, intermediate stalling radius that functions as the true
boundary between the active and dead zones in protoplanetary disks. These
dynamics are likely implicated in observations of variable accretion, such as
FU Ori outbursts, as well as in those planet formation theories that require
the accumulation of solid material at the dead/active interface.Comment: 16 pages, 10 figures; MNRAS accepted; v3 final correction
Tracing the power-law component in the energy spectrum of black hole candidates as a function of the QPO frequency
We investigated the relation between the centroid frequency of the
quasi-periodic oscillation observed in the power density spectra of a sample of
galactic black-hole candidates with the power-law photon index obtained from
spectral fits. Our aim is to avoid inner accretion disk radius determination
directly from spectral fits, given the uncertainties of the absolute values
obtained in that way, but to base our analysis on the likely association of QPO
frequency to a characteristic radius. We used archival RXTE data of GRS
1915+105 and published parameters for GRO 1655-40, XTE J1550-564, XTE J1748-288
and 4U 1630-47. While for low values of the QPO frequency, the two parameters
are clearly correlated for each source, there is evidence for a turnoff in the
correlation above a characteristic frequency, different for different sources.
We discuss the possible nature of this turnoff.Comment: 11 pages, 10 figures. Accepted for publication on Astronomy &
Astrophysic
Resolved Images of Large Cavities in Protoplanetary Transition Disks
Circumstellar disks are thought to experience a rapid "transition" phase in
their evolution that can have a considerable impact on the formation and early
development of planetary systems. We present new and archival high angular
resolution (0.3" = 40-75 AU) Submillimeter Array (SMA) observations of the 880
micron dust continuum emission from 12 such transition disks in nearby
star-forming regions. In each case, we directly resolve a dust-depleted disk
cavity around the central star. Using radiative transfer calculations, we
interpret these dust disk structures in a homogeneous, parametric model
framework by reproducing their SMA visibilities and SEDs. The cavities in these
disks are large (R_cav = 15-73 AU) and substantially depleted of small
(~um-sized) dust grains, although their mass contents are still uncertain. The
structures of the remnant material at larger radii are comparable to normal
disks. We demonstrate that these large cavities are common among the
millimeter-bright disk population, comprising at least 20% of the disks in the
bright half of the millimeter luminosity (disk mass) distribution. Utilizing
these results, we assess some of the physical mechanisms proposed to account
for transition disk structures. As has been shown before, photoevaporation
models do not produce the large cavity sizes, accretion rates, and disk masses
representative of this sample. It would be difficult to achieve a sufficient
decrease of the dust optical depths in these cavities by particle growth alone:
substantial growth (to meter sizes or beyond) must occur in large (tens of AU)
regions of low turbulence without also producing an abundance of small
particles. Given those challenges, we suggest instead that the observations are
most commensurate with dynamical clearing due to tidal interactions with
low-mass companions --young brown dwarfs or giant planets on long-period
orbits.Comment: ApJ, in pres
Protoplanetary Disk Structures in Ophiuchus
We present a high angular resolution (0.3" = 40 AU) SMA survey of the 870
micron thermal continuum emission from 9 of the brightest, and therefore most
massive, circumstellar disks in the ~1 Myr-old Ophiuchus star-forming region.
Using 2-D radiative transfer calculations, we simultaneously fit the observed
continuum visibilities and broadband spectral energy distribution for each disk
with a parametric structure model. Compared to previous millimeter studies,
this survey includes significant upgrades in modeling, data quality, and
angular resolution that provide improved constraints on key structure
parameters, particularly those that characterize the spatial distribution of
mass in the disks. In the context of a surface density profile motivated by
similarity solutions for viscous accretion disks, the best-fit models for the
sample disks have characteristic radii R_c = 20-200 AU, high disk masses M_d =
0.005-0.14 M_sun, and a narrow range of radial surface density gradients around
a median = 0.9. These density structures are used in conjunction with
accretion rate estimates from the literature to help characterize the viscous
evolution of the disk material. Using the standard prescription for disk
viscosities, those combined constraints indicate that = 0.0005-0.08.
Three of the sample disks show large (R = 20-40 AU) central cavities in their
continuum emission morphologies, marking extensive zones where dust has been
physically removed and/or has significantly diminished opacities. Based on the
current requirements of planet formation models, these emission cavities and
the structure constraints for the sample as a whole suggest that these young
disks may eventually produce planetary systems, and have perhaps already
started. (abridged)Comment: ApJ in press: 51 pages, 13 figure