11,046 research outputs found
Coplanar Circumbinary Debris Disks
We present resolved Herschel images of circumbinary debris disks in the alpha
CrB (HD139006) and beta Tri (HD13161) systems. We find that both disks are
consistent with being aligned with the binary orbital planes. Though secular
perturbations from the binary can align the disk, in both cases the alignment
time at the distances at which the disk is resolved is greater than the stellar
age, so we conclude that the coplanarity was primordial. Neither disk can be
modelled as a narrow ring, requiring extended radial distributions. To satisfy
both the Herschel and mid-IR images of the alpha CrB disk, we construct a model
that extends from 1-300AU, whose radial profile is broadly consistent with a
picture where planetesimal collisions are excited by secular perturbations from
the binary. However, this model is also consistent with stirring by other
mechanisms, such as the formation of Pluto-sized objects. The beta Tri disk
model extends from 50-400AU. A model with depleted (rather than empty) inner
regions also reproduces the observations and is consistent with binary and
other stirring mechanisms. As part of the modelling process, we find that the
Herschel PACS beam varies by as much as 10% at 70um and a few % at 100um. The
70um variation can therefore hinder image interpretation, particularly for
poorly resolved objects. The number of systems in which circumbinary debris
disk orientations have been compared with the binary plane is now four. More
systems are needed, but a picture in which disks around very close binaries
(alpha CrB, beta Tri, and HD 98800, with periods of a few weeks to a year) are
aligned, and disks around wider binaries (99 Her, with a 50 yr period) are
misaligned, may be emerging. This picture is qualitatively consistent with the
expectation that the protoplanetary disks from which the debris emerged are
more likely to be aligned if their binaries have shorter periods.Comment: accepted to MNRA
Predicting the frequencies of diverse exo-planetary systems
Extrasolar planetary systems range from hot Jupiters out to icy comet belts
more distant than Pluto. We explain this diversity in a model where the mass of
solids in the primordial circumstellar disk dictates the outcome. The star
retains measures of the initial heavy-element (metal) abundance that can be
used to map solid masses onto outcomes, and the frequencies of all classes are
correctly predicted. The differing dependences on metallicity for forming
massive planets and low-mass cometary bodies are also explained. By
extrapolation, around two-thirds of stars have enough solids to form Earth-like
planets, and a high rate is supported by the first detections of low-mass
exo-planets.Comment: 5 pages, 2 figures; accepted by MNRA
Searching for a dusty cometary belt around TRAPPIST-1 with ALMA
Low-mass stars might offer today the best opportunities to detect and characterize planetary systems, especially those harbouring close-in low-mass temperate planets. Among those stars, TRAPPIST-1 is exceptional since it has seven Earth-sized planets, of which three could sustain liquid water on their surfaces. Here we present new and deep ALMA observations of TRAPPIST-1 to look for an exo-Kuiper belt which can provide clues about the formation and architecture of this system. Our observations at 0.88 mm did not detect dust emission, but can place an upper limit of 23 µJy if the belt is smaller than 4 au, and 0.15 mJy if resolved and 100 au in radius. These limits correspond to low dust masses of ̃10-5 to 10-2 M⊕, which are expected after 8 Gyr of collisional evolution unless the system was born with a >20 M⊕ belt of 100 km-sized planetesimals beyond 40 au or suffered a dynamical instability. This 20 M⊕ mass upper limit is comparable to the combined mass in TRAPPIST-1 planets, thus it is possible that most of the available solid mass in this system was used to form the known planets. A similar analysis of the ALMA data on Proxima Cen leads us to conclude that a belt born with a mass ≳1 M⊕ in 100 km-sized planetesimals could explain its putative outer belt at 30 au. We recommend that future characterizations of debris discs around low-mass stars should focus on nearby and young systems if possible
ALMA and Herschel Observations of the Prototype Dusty and Polluted White Dwarf G29-38
ALMA Cycle 0 and Herschel PACS observations are reported for the prototype,
nearest, and brightest example of a dusty and polluted white dwarf, G29-38.
These long wavelength programs attempted to detect an outlying, parent
population of bodies at 1-100 AU, from which originates the disrupted
planetesimal debris that is observed within 0.01 AU and which exhibits L_IR/L =
0.039. No associated emission sources were detected in any of the data down to
L_IR/L ~ 1e-4, generally ruling out cold dust masses greater than 1e24 - 1e25 g
for reasonable grain sizes and properties in orbital regions corresponding to
evolved versions of both asteroid and Kuiper belt analogs. Overall, these null
detections are consistent with models of long-term collisional evolution in
planetesimal disks, and the source regions for the disrupted parent bodies at
stars like G29-38 may only be salient in exceptional circumstances, such as a
recent instability. A larger sample of polluted white dwarfs, targeted with the
full ALMA array, has the potential to unambiguously identify the parent
source(s) of their planetary debris.Comment: 8 pages, 5 figures and 1 table. Accepted to MNRA
Steady-state evolution of debris disks around A stars
In this paper a simple analytical model for the steady-state evolution of
debris disks due to collisions is confronted with Spitzer observations of main
sequence A stars. All stars are assumed to have planetesimal belts with a
distribution of initial masses and radii. In the model disk mass is constant
until the largest planetesimals reach collisional equilibrium whereupon the
mass falls off oc 1/t. We find that the detection statistics and trends seen at
both 24 and 70um can be fitted well by the model. While there is no need to
invoke stochastic evolution or delayed stirring to explain the statistics, a
moderate rate of stochastic events is not ruled out. Potentially anomalous
systems are identified by a high dust luminosity compared with the maximum
permissible in the model (HD3003, HD38678, HD115892, HD172555). Their
planetesimals may have unusual properties (high strength or low eccentricity)
or this dust could be transient. While transient phenomena are also favored for
a few systems in the literature, the overall success of our model, which
assumes planetesimals in all belts have the same strength, eccentricity and
maximum size, suggests a large degree of uniformity in the outcome of planet
formation. The distribution of planetesimal belt radii, once corrected for
detection bias, follows N(r) oc r^{-0.8+-0.3} for 3-120AU. Since the inner edge
is often attributed to an unseen planet, this provides a unique constraint on
the planetary systems of A stars. It is also shown that P-R drag may sculpt the
inner edges of A star disks close to the Spitzer detection threshold (HD2262,
HD19356, HD106591, HD115892). This model can be readily applied to the
interpretation of future surveys, and predictions are made for the upcoming
SCUBA-2 survey, including that >17% of A stars should be detectable at 850um.Comment: Accepted by Ap
Asymmetric Heating of the HR 4796A Dust Ring Due to Pericenter Glow
We have obtained new resolved images of the well-studied HR 4796A dust ring
at 18 and 25 microns with the 8-meter Gemini telescopes. These images confirm
the previously observed spatial extent seen in mid-IR, near-IR, and optical
images of the source. We detect brightness and temperature asymmetries such
that dust on the NE side is both brighter and warmer than dust in the SW. We
show that models of so-called pericenter glow account for these asymmetries,
thus both confirming and extending our previous analyses. In this scenario, the
center of the dust ring is offset from the star due to gravitational
perturbations of a body with an eccentric orbit that has induced a forced
eccentricity on the dust particle orbits. Models with 2-micron silicate dust
particles and a forced eccentricity of 0.06 simultaneously fit the observations
at both wavelengths. We also show that parameters used to characterize the
thermal-emission properties of the disk can also account for the disk asymmetry
observed in shorter-wavelength scattered-light images.Comment: accepted for publication in A&A; 7 pages, 4 figure
Combined visible and near-infrared OPA for wavelength scaling experiments in strong-field physics
We report the operation of an optical parametric amplifier (OPA) capable of
producing gigawatt peak-power laser pulses with tunable wavelength in either
the visible or near-infrared spectrum. The OPA has two distinct operation modes
(i) generation of >350 uJ, sub 100 fs pulses, tunable between 1250 - 1550 nm;
(ii) generation of >190 uJ, sub 150 fs pulses tunable between 490 - 530 nm. We
have recorded high-order harmonic spectra over a wide range of driving
wavelengths. This flexible source of femtosecond pulses presents a useful tool
for exploring the wavelength-dependence of strong-field phenomena, in both the
multi-photon and tunnel ionization regimes.Comment: 14 pages, 9 figures, This paper was published in Proceedings of SPIE
10088, Nonlinear Frequency Generation and Conversion: Materials and Devices
XVI, doi 10.1117/12.225077
Debris discs around nearby Solar analogues
An unbiased search for debris discs around nearby Sun-like stars is reported.
Thirteen G-dwarfs at 12-15 parsecs distance were searched at 850 \umum
wavelength, and a disc is confirmed around HD 30495. The estimated dust mass is
0.008 M with a net limit \la 0.0025 M for the average
disc of the other stars. The results suggest there is not a large missed
population of substantial cold discs around Sun-like stars -- HD 30495 is a
bright rather than unusually cool disc, and may belong to a few hundred Myr-old
population of greater dust luminosity. The far-infared and millimetre survey
data for Sun-like stars are well fitted by either steady state or stirred
models, provided that typical comet belts are comparable in size to that in the
Solar System.Comment: published in MNRA
Multi-Epoch Observations of HD69830: High Resolution Spectroscopy and Limits to Variability
The main-sequence solar-type star HD69830 has an unusually large amount of
dusty debris orbiting close to three planets found via the radial velocity
technique. In order to explore the dynamical interaction between the dust and
planets, we have performed multi-epoch photometry and spectroscopy of the
system over several orbits of the outer dust. We find no evidence for changes
in either the dust amount or its composition, with upper limits of 5-7% (1
per spectral element) on the variability of the {\it dust spectrum}
over 1 year, 3.3% (1 ) on the broad-band disk emission over 4 years,
and 33% (1 ) on the broad-band disk emission over 24 years. Detailed
modeling of the spectrum of the emitting dust indicates that the dust is
located outside of the orbits of the three planets and has a composition
similar to main-belt, C-type asteroids asteroids in our solar system.
Additionally, we find no evidence for a wide variety of gas species associated
with the dust. Our new higher SNR spectra do not confirm our previously claimed
detection of HO ice leading to a firm conclusion that the debris can be
associated with the break-up of one or more C-type asteroids formed in the dry,
inner regions of the protoplanetary disk of the HD69830 system. The modeling of
the spectral energy distribution and high spatial resolution observations in
the mid-infrared are consistent with a 1 AU location for the emitting
material
Collisional modelling of the AU Microscopii debris disc
The spatially resolved AU Mic debris disc is among the most famous and
best-studied debris discs. We aim at a comprehensive understanding of the dust
production and the dynamics of the disc objects with in depth collisional
modelling including stellar radiative and corpuscular forces. Our models are
compared to a suite of observational data for thermal and scattered light
emission, ranging from the ALMA radial surface brightness profile at 1.3mm to
polarisation measurements in the visible. Most of the data can be reproduced
with a planetesimal belt having an outer edge at around 40au and subsequent
inward transport of dust by stellar winds. A low dynamical excitation of the
planetesimals with eccentricities up to 0.03 is preferred. The radial width of
the planetesimal belt cannot be constrained tightly. Belts that are 5au and
17au wide, as well as a broad 44au-wide belt are consistent with observations.
All models show surface density profiles increasing with distance from the star
as inferred from observations. The best model is achieved by assuming a stellar
mass loss rate that exceeds the solar one by a factor of 50. While the SED and
the shape of the ALMA profile are well reproduced, the models deviate from the
scattered light data more strongly. The observations show a bluer disc colour
and a lower degree of polarisation for projected distances <40au than predicted
by the models. The problem may be mitigated by irregularly-shaped dust grains
which have scattering properties different from the Mie spheres used. From
tests with a handful of selected dust materials, we derive a preference for
mixtures of silicate, carbon, and ice of moderate porosity. We address the
origin of the unresolved central excess emission detected by ALMA and show that
it cannot stem from an additional inner belt alone. Instead, it should derive,
at least partly, from the chromosphere of the central star.Comment: Astronomy and Astrophysics (accepted for publication), 18 pages, 11
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