313 research outputs found
Orbital Stability of Multi-Planet Systems: Behavior at High Masses
In the coming years, high contrast imaging surveys are expected to reveal the
characteristics of the population of wide-orbit, massive, exoplanets. To date,
a handful of wide planetary mass companions are known, but only one such
multi-planet system has been discovered: HR8799. For low mass planetary
systems, multi-planet interactions play an important role in setting system
architecture. In this paper, we explore the stability of these high mass,
multi-planet systems. While empirical relationships exist that predict how
system stability scales with planet spacing at low masses, we show that
extrapolating to super-Jupiter masses can lead to up to an order of magnitude
overestimate of stability for massive, tightly packed systems. We show that at
both low and high planet masses, overlapping mean motion resonances trigger
chaotic orbital evolution, which leads to system instability. We attribute some
of the difference in behavior as a function of mass to the increasing
importance of second order resonances at high planet-star mass ratios. We use
our tailored high mass planet results to estimate the maximum number of planets
that might reside in double component debris disk systems, whose gaps may
indicate the presence of massive bodies.Comment: Accepted to Ap
Planet-disc interaction on a freely moving mesh
General-purpose, moving-mesh schemes for hydrodynamics have opened the
possibility of combining the accuracy of grid-based numerical methods with the
flexibility and automatic resolution adaptivity of particle-based methods. Due
to their supersonic nature, Keplerian accretion discs are in principle a very
attractive system for applying such freely moving mesh techniques. However, the
high degree of symmetry of simple accretion disc models can be difficult to
capture accurately by these methods, due to the generation of geometric grid
noise and associated numerical diffusion, which is absent in polar grids. To
explore these and other issues, in this work we study the idealized problem of
two-dimensional planet-disc interaction with the moving-mesh code AREPO. We
explore the hydrodynamic evolution of discs with planets through a series of
numerical experiments that vary the planet mass, the disc viscosity and the
mesh resolution, and compare the resulting surface density, vortensity field
and tidal torque with results from the literature. We find that the performance
of the moving-mesh code in this problem is in accordance with published
results, showing good consistency with grid codes written in polar coordinates.
We also conclude that grid noise and mesh distortions do not introduce
excessive numerical diffusion. Finally, we show how the moving-mesh approach
can naturally increase resolution in regions of high densityaround planets and
planetary wakes, while retaining the background flow at low resolution. This
provides an alternative to the difficult task of implementing adaptive mesh
refinement in conventional polar-coordinate codes.Comment: 21 pages, 15 figures, 2 tables. Updated to match version published by
MNRA
Correcting for Activity Effects on the Temperatures, Radii, and Estimated Masses of Low-Mass Stars and Brown Dwarfs
We present empirical relations for determining the amount by which the
effective temperatures and radii---and therefore the estimated masses---of
low-mass stars and brown dwarfs are altered due to chromospheric activity.
Accurate estimates of stellar radii are especially important in the context of
searches for transiting exoplanets, which rely upon the assumed stellar
radius/density to infer the planet radius/density. Our relations are based on a
large set of well studied low-mass stars in the field and on a set of benchmark
low-mass eclipsing binaries. The relations link the amount by which an active
object's temperature is suppressed, and its radius inflated, to the strength of
its Halpha emission. These relations are found to approximately preserve
bolometric luminosity. We apply these relations to the peculiar brown-dwarf
eclipsing binary 2M0535-05, in which the active, higher-mass brown dwarf has a
cooler temperature than its inactive, lower-mass companion. The relations
correctly reproduce the observed temperatures and radii of 2M0535-05 after
accounting for the Halpha emission; 2M0535-05 would be in precise agreement
with theoretical isochrones were it inactive. The relations that we present are
applicable to brown dwarfs and low-mass stars with masses below 0.8 Msun and
for which the activity, as measured by Halpha, is in the range -4.6 < log
Lha/Lbol < -3.3. We expect these relations to be most useful for correcting
radius and mass estimates of low-mass stars and brown dwarfs over their active
lifetimes (few Gyr). We also discuss the implications of this work for
determinations of young cluster IMFs.Comment: To appear in Cool Stars 17 proceeding
Dynamics of small grains in transitional discs
Transitional discs have central regions characterised by significant
depletion of both dust and gas compared to younger, optically-thick discs.
However, gas and dust are not depleted by equal amounts: gas surface densities
are typically reduced by factors of , but small dust grains are
sometimes depleted by far larger factors, to the point of being undetectable.
While this extreme dust depletion is often attributed to planet formation, in
this paper we show that another physical mechanism is possible: expulsion of
grains from the disc by radiation pressure. We explore this mechanism using 2D
simulations of dust dynamics, simultaneously solving the equation of radiative
transfer with the evolution equations for dust diffusion and advection under
the combined effects of stellar radiation and hydrodynamic interaction with a
turbulent, accreting background gas disc. We show that, in transition discs
that are depleted in both gas and dust fraction by factors of
compared to minimum mass Solar nebular values, and where the ratio of accretion
rate to stellar luminosity is low (
yr ), radiative clearing of any remaining
m and larger grains is both rapid and inevitable. The process is
size-dependent, with smaller grains removed fastest and larger ones persisting
for longer times. Our proposed mechanism thus naturally explains the extreme
depletion of small grains commonly-found in transition discs. We further
suggest that the dependence of this mechanism on grain size and optical
properties may explain some of the unusual grain properties recently discovered
in a number of transition discs. The simulation code we develop is freely
available.Comment: 20 pages, 15 figures; MNRAS in press; compared to the previous
version, this one has added simulations exploring the impact of adding
laminar accretion flows to the model; movies of simulation results available
from http://www.mso.anu.edu.au/~krumholz/movies.htm
Protoplanetary Disks in Ophiuchus as Seen From ALMA
We present a high angular resolution (), high
sensitivity ( mJy) survey of the 870 m continuum emission
from the circumstellar material around 49 pre-main sequence stars in the
Ophiuchus molecular cloud. Because most millimeter instruments have resided in
the northern hemisphere, this represents the largest high-resolution,
millimeter-wave survey of the circumstellar disk content of this cloud. Our
survey of 49 systems comprises 63 stars; we detect disks associated with 29
single sources, 11 binaries, 3 triple systems and 4 transition disks. We
present flux and radius distributions for these systems; in particular, this is
the first presentation of a reasonably complete probability distribution of
disk radii at millimeter-wavelengths. We also compare the flux distribution of
these protoplanetary disks with that of the disk population of the
Taurus-Auriga molecular cloud. We find that disks in binaries are both
significantly smaller and have much less flux than their counterparts around
isolated stars. We compute truncation calculations on our binary sources and
find that these disks are too small to have been affected by tidal truncation
and posit some explanations for this. Lastly, our survey found 3 candidate
gapped disks, one of which is a newly identified transition disk with no
signature of a dip in infrared excess in extant observations.Comment: 26 pages, 16 figures. Accepted for publication in Ap
ALMA Measurements of Circumstellar Material in the GQ Lup System
We present ALMA observations of the GQ Lup system, a young Sun-like star with
a substellar mass companion in a wide-separation orbit. These observations of
870 m continuum and CO J=3-2 line emission with beam size
( AU) resolve the disk of dust and gas surrounding the primary star, GQ
Lup A, and provide deep limits on any circumplanetary disk surrounding the
companion, GQ Lup b. The circumprimary dust disk is compact with a FWHM of
AU, while the gas has a larger extent with a characteristic radius of
AU. By forward-modeling the velocity field of the circumprimary
disk based on the CO emission, we constrain the mass of GQ Lup A to be , where is a known distance, and
determine that we view the disk at an inclination angle of
and a position angle of . The
upper limit on the 870 m flux density of any circumplanetary
disk associated with GQ Lup b of mJy implies an upper limit on the dust
disk mass of for standard assumptions about optically thin
emission. We discuss proposed mechanisms for the formation of wide-separation
substellar companions given the non-detection of circumplanetary disks around
GQ Lup b and other similar systems.Comment: 11 pages, 4 figure
The VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey of Perseus Protostars. VI. Characterizing the Formation Mechanism for Close Multiple Systems
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations
of multiple protostar systems in the Perseus molecular cloud previously
detected by the Karl G. Jansky Very Large Array (VLA). We observed 17 close
(600~AU separation) multiple systems at 1.3~mm in continuum and five
molecular lines (i.e., \twco, \cateo, \thco, HCO, SO) to characterize the
circum-multiple environments in which these systems are forming. We detect at
least one component in the continuum for the 17 multiple systems. In three
systems, one companion is not detected, and for two systems the companions are
unresolved at our observed resolution. We also detect circum-multiple dust
emission toward 8 out of 9 Class 0 multiples. Circum-multiple dust emission is
not detected toward any of the 8 Class I multiples. Twelve systems are detected
in the dense gas tracers toward their disks/inner envelopes. For these 12
systems, we use the dense gas observations to characterize their formation
mechanism. The velocity gradients in the circum-multiple gas are clearly
orthogonal to the outflow directions in 8 out of the 12 systems, consistent
with disk fragmentation. Moreover, only two systems with separations 200~AU
are \textit{inconsistent} with disk fragmentation, in addition to the two
widest systems (500~AU). Our results suggest that disk fragmentation via
gravitational instability is an important formation mechanism for close
multiple systems, but further statistics are needed to better determine the
relative fraction formed via this method.Comment: 48 Pages, 26 Figures, 7 Tables, Accepted by Ap
A Model-Independent Mass and Moderate Eccentricity for β Pic b
We use a cross-calibration of Hipparcos and Gaia DR2 astrometry for
Pic to measure the mass of the giant planet Pic b in a comprehensive joint orbit analysis that includes published relative
astrometry and radial velocities. Our mass uncertainty is somewhat higher than
previous work because our astrometry from the Hipparcos-Gaia Catalog of
Accelerations accounts for the error inflation and systematic terms that are
required to bring the two data sets onto a common astrometric reference frame,
and because we fit freely for the host-star mass .
This first model-independent mass for a directly imaged planet is inconsistent
with cold-start models given the age of the Pic moving group
Myr) but consistent with hot- and warm-start models, concordant with past work.
We find a higher eccentricity for Pic b compared to
previous orbital fits. If confirmed by future observations, this eccentricity
may help explain inner edge, scale height, and brightness asymmetry of
Pic's disk. It could also potentially signal that Pic b has migrated
inward to its current location, acquiring its eccentricity from interaction
with the 3:1 outer Lindblad resonance in the disk.Comment: ApJ Letters, accepte
Stellar orbit evolution in close circumstellar disc encounters
The formation and early evolution of circumstellar discs often occurs within
dense, newborn stellar clusters. For the first time, we apply the moving-mesh
code AREPO, to circumstellar discs in 3-D, focusing on disc-disc interactions
that result from stellar fly-bys. Although a small fraction of stars are
expected to undergo close approaches, the outcomes of the most violent
encounters might leave an imprint on the discs and host stars that will
influence both their orbits and their ability to form planets. We first
construct well-behaved 3-D models of self-gravitating discs, and then create a
suite of numerical experiments of parabolic encounters, exploring the effects
of pericenter separation r_p, disc orientation and disc-star mass ratio
(M_d/M_*) on the orbital evolution of the host stars. Close encounters (2r_p<~
disc radius) can truncate discs on very short time scales. If discs are
massive, close encounters facilitate enough orbital angular momentum extraction
to induce stellar capture. We find that for realistic primordial disc masses
M_d<~0.1M_*, non-colliding encounters induce minor orbital changes, which is
consistent with analytic calculations of encounters in the linear regime. The
same disc masses produce entirely different results for grazing/colliding
encounters. In the latter case, rapidly cooling discs lose orbital energy by
radiating away the energy excess of the shock-heated gas, thus causing capture
of the host stars into a bound orbit. In rare cases, a tight binary with a
circumbinary disc forms as a result of this encounter.Comment: 20 pages, 14 figures, 1 table. Published by MNRA
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