422 research outputs found
The local dust foregrounds in the microwave sky: I. Thermal emission spectra
Analyses of the cosmic microwave background (CMB) radiation maps made by the
Wilkinson Microwave Anisotropy Probe (WMAP) have revealed anomalies not
predicted by the standard inflationary cosmology. In particular, the power of
the quadrupole moment of the CMB fluctuations is remarkably low, and the
quadrupole and octopole moments are aligned mutually and with the geometry of
the Solar system. It has been suggested in the literature that microwave sky
pollution by an unidentified dust cloud in the vicinity of the Solar system may
be the cause for these anomalies. In this paper, we simulate the thermal
emission by clouds of spherical homogeneous particles of several materials.
Spectral constraints from the WMAP multi-wavelength data and earlier infrared
observations on the hypothetical dust cloud are used to determine the dust
cloud's physical characteristics. In order for its emissivity to demonstrate a
flat, CMB-like wavelength dependence over the WMAP wavelengths (3 through 14
mm), and to be invisible in the infrared light, its particles must be
macroscopic. Silicate spheres from several millimetres in size and carbonaceous
particles an order of magnitude smaller will suffice. According to our
estimates of the abundance of such particles in the Zodiacal cloud and
trans-neptunian belt, yielding the optical depths of the order of 1E-7 for each
cloud, the Solar-system dust can well contribute 10 microKelvin (within an
order of magnitude) in the microwaves. This is not only intriguingly close to
the magnitude of the anomalies (about 30 microKelvin), but also alarmingly
above the presently believed magnitude of systematic biases of the WMAP results
(below 5 microKelvin) and, to an even greater degree, of the future missions
with higher sensitivities, e.g. PLANCK.Comment: 33 pages, 9 figures, 1 table. The Astrophysical Journal, 2009,
accepte
Collisions and drag in debris discs with eccentric parent belts
Context: High-resolution images of circumstellar debris discs reveal
off-centred rings that indicate past or ongoing perturbation, possibly caused
by secular gravitational interaction with unseen stellar or substellar
companions. The purely dynamical aspects of this departure from radial symmetry
are well understood. However, the observed dust is subject to additional forces
and effects, most notably collisions and drag. Aims: To complement the studies
of dynamics, we therefore aim to understand how new asymmetries are created by
the addition of collisional evolution and drag forces, and existing ones
strengthened or overridden. Methods: We augmented our existing numerical code
"Analysis of Collisional Evolution" (ACE) by an azimuthal dimension, the
longitude of periapse. A set of fiducial discs with global eccentricities
ranging from 0 to 0.4 is evolved over giga-year timescales. Size distribution
and spatial variation of dust are analysed and interpreted. The basic impact of
belt eccentricity on spectral energy distributions (SEDs) and images is
discussed.
Results: We find features imposed on characteristic timescales. First,
radiation pressure defines size cutoffs that differ between periapse and
apoapse, resulting in an asymmetric halo. The differences in size distribution
make the observable asymmetry of the halo depend on wavelength. Second,
collisional equilibrium prefers smaller grains on the apastron side of the
parent belt, reducing the effect of pericentre glow and the overall asymmetry.
Third, Poynting-Robertson drag fills the region interior to an eccentric belt
such that the apastron side is more tenuous. Interpretation and prediction of
the appearance in scattered light is problematic when spatial and size
distribution are coupled.Comment: Accepted for publication in A&A, 14 pages, 16 figure
Collisional modelling of the debris disc around HIP 17439
We present an analysis of the debris disc around the nearby K2 V star HIP
17439. In the context of the Herschel DUNES key programme the disc was observed
and spatially resolved in the far-IR with the Herschel PACS and SPIRE
instruments. In a first model, Ertel et al. (2014) assumed the size and radial
distribution of the circumstellar dust to be independent power laws. There, by
exploring a very broad range of possible model parameters several scenarios
capable of explaining the observations were suggested. In this paper, we
perform a follow-up in-depth collisional modelling of these scenarios trying to
further distinguish between them. In our models we consider collisions, direct
radiation pressure, and drag forces, i.e. the actual physical processes
operating in debris discs. We find that all scenarios discussed in Ertel et al.
are physically sensible and can reproduce the observed SED along with the PACS
surface brightness profiles reasonably well. In one model, the dust is produced
beyond 120au in a narrow planetesimal belt and is transported inwards by
Poynting-Robertson and stellar wind drag. A good agreement with the observed
radial profiles would require stellar winds by about an order of magnitude
stronger than the solar value, which is not supported, although not ruled out,
by observations. Another model consists of two spatially separated planetesimal
belts, a warm inner and a cold outer one. This scenario would probably imply
the presence of planets clearing the gap between the two components. Finally,
we show qualitatively that the observations can be explained by assuming the
dust is produced in a single, but broad planetesimal disc with a surface
density of solids rising outwards, as expected for an extended disc that
experiences a natural inside-out collisional depletion. Prospects of
discriminating between the competing scenarios by future observations are
discussed.Comment: Astronomy and Astrophysics (accepted for publication). 11 pages, 8
figure
The cold origin of the warm dust around epsilon Eridani
Context: The K2V star eps Eri hosts one known inner planet, an outer Kuiper
belt analog, and an inner disk of warm dust. Spitzer/IRS measurements indicate
that the warm dust is present at distances as close as a few AU from the star.
Its origin is puzzling, since an "asteroid belt" that could produce this dust
would be unstable because of the known inner planet. Aims: Here we test the
hypothesis that the observed warm dust is generated by collisions in the outer
belt and is transported inward by Poynting-Robertson (P-R) drag and strong
stellar winds. Methods: We simulated a steady-state distribution of dust
particles outside 10AU with a collisional code and in the inner region (r<10AU)
with single-particle numerical integrations. By assuming homogeneous spherical
dust grains composed of water ice and silicate, we calculated the thermal
emission of the dust and compared it with observations. We investigated two
different orbital configurations for the inner planet inferred from RV
measurements, one with a highly eccentric orbit of e=0.7 and another one with a
moderate one of e=0.25. We also produced a simulation without a planet.
Results: Our models can reproduce the shape and magnitude of the observed SED
from mid-IR to sub-mm wavelengths, as well as the Spitzer/MIPS radial
brightness profiles. The best-fit dust composition includes both ice and
silicates. The results are similar for the two possible planetary orbits and
without a planet. Conclusions: The observed warm dust in the system can indeed
stem from the outer belt and be transported inward by P-R and stellar wind
drag. The inner planet has little effect on the distribution of dust, so that
the planetary orbit could not be constrained. Reasonable agreement between the
model and observations can only be achieved by relaxing the assumption of
purely silicate dust and assuming a mixture of silicate and ice in comparable
amounts.Comment: 9 pages, 9 figures, abstract abridge
First L band detection of hot exozodiacal dust with VLTI/MATISSE
For the first time, we observed the emission of hot exozodiacal dust in L band. We used the new instrument MATISSE at the Very Large Telescope Interferometer to detect the hot dust around Îș Tuc with a significance of 3Ï to 6Ï at wavelengths between 3.37 and 3.85ÎŒm and a dust-to-star flux ratio of 5 to 7 per centâ . We modelled the spectral energy distribution based on the new L band data alone and in combination with H band data published previously. In all cases we find 0.58ÎŒm grains of amorphous carbon to fit the Îș Tuc observations the best, however, also nanometre or micrometre grains and other carbons or silicates reproduce the observations well. Since the H band data revealed a temporal variability, while our Lband data were taken at a different epoch, we combine them in different ways. Depending on the approach, the best fits are obtained for a narrow dust ring at a stellar distance in the 0.1â029âau range and thus with a temperature between 940 and 1430Kâ . Within the 1Ï uncertainty dust location and temperature are confined to 0.032â1.18au and 600â2000Kâ
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
figure
Constraints on the structure of hot exozodiacal dust belts
Recent interferometric surveys of nearby main-sequence stars show a faint but significant near-infrared excess in roughly two dozen systems, i.e. around 10â30 per cent of stars surveyed. This excess is attributed to dust located in the immediate vicinity of the star, the origin of which is highly debated. We used previously published interferometric observations to constrain the properties and distribution of this hot dust. Considering both scattered radiation and thermal re-emission, we modelled the observed excess in nine of these systems. We find that grains have to be sufficiently absorbing to be consistent with the observed excess, while dielectric grains with pure silicate compositions fail to reproduce the observations. The dust should be located within âŒ0.01â1âau from the star depending on its luminosity. Furthermore, we find a significant trend for the disc radius to increase with the stellar luminosity. The dust grains are determined to be below 0.2--0.5ÎŒm, but above 0.02--0.15ÎŒm in radius. The dust masses amount to (0.2â3.5) Ă 10â»âč Mâ. The near-infrared excess is probably dominated by thermal re-emission, though a contribution of scattered light up to 35ââperâcent cannot be completely excluded. The polarization degree predicted by our models is always below 5ââperâcent, and for grains smaller than âŒ0.2ÎŒm even below 1ââperâcent. We also modelled the observed near-infrared excess of another 10 systems with poorer data in the mid-infrared. The basic results for these systems appear qualitatively similar, yet the constraints on the dust location and the grain sizes are weaker
Sub-millimeter images of a dusty Kuiper belt around eta Corvi
We present sub-millimeter and mid-infrared images of the circumstellar disk
around the nearby F2V star eta Corvi. The disk is resolved at 850um with a size
of ~100AU. At 450um the emission is found to be extended at all position
angles, with significant elongation along a position angle of 130+-10deg; at
the highest resolution (9.3") this emission is resolved into two peaks which
are to within the uncertainties offset symmetrically from the star at 100AU
projected separation. Modeling the appearance of emission from a narrow ring in
the sub-mm images shows the observed structure cannot be caused by an edge-on
or face-on axisymmetric ring; the observations are consistent with a ring of
radius 150+-20AU seen at 45+-25deg inclination. More face-on orientations are
possible if the dust distribution includes two clumps similar to Vega; we show
how such a clumpy structure could arise from the migration over 25Myr of a
Neptune mass planet from 80-105AU. The inner 100AU of the system appears
relatively empty of sub-mm emitting dust, indicating that this region may have
been cleared by the formation of planets, but the disk emission spectrum shows
that IRAS detected an additional hot component with a characteristic
temperature of 370+-60K (implying a distance of 1-2AU). At 11.9um we found the
emission to be unresolved with no background sources which could be
contaminating the fluxes measured by IRAS. The age of this star is estimated to
be ~1Gyr. It is very unusual for such an old main sequence star to exhibit
significant mid-IR emission. The proximity of this source makes it a perfect
candidate for further study from optical to mm wavelengths to determine the
distribution of its dust.Comment: 22 pages, 4 figures. Scheduled for publication in ApJ 10 February
2005 issu
Transience of hot dust around sun-like stars
There is currently debate over whether the dust content of planetary systems
is stochastically regenerated or originates in planetesimal belts evolving in
steady state. In this paper a simple model for the steady state evolution of
debris disks due to collisions is developed and confronted with the properties
of the emerging population of 7 sun-like stars that have hot dust <10AU. The
model shows there is a maximum possible disk mass at a given age, since more
massive primordial disks process their mass faster. The corresponding maximum
dust luminosity is f_max=0.00016r^(7/3)/t_age. The majority (4/7) of the hot
disks exceed this limit by >1000 and so cannot be the products of massive
asteroid belts, rather the following systems must be undergoing transient
events characterized by an unusually high dust content near the star: eta
Corvi, HD69830, HD72905 and BD+20307. It is also shown that the hot dust cannot
originate in a recent collision in an asteroid belt, since there is also a
maximum rate at which collisions of sufficient magnitude to reproduce a given
dust luminosity can occur. Further it is shown that the planetesimal belt
feeding the dust in these systems must be located further from the star than
the dust, typically at >2AU. Other notable properties of the 4 hot dust systems
are: two also have a planetesimal belt at >10AU (eta Corvi and HD72905); one
has 3 Neptune mass planets at <1AU (HD69830); all exhibit strong silicate
features in the mid-IR. We consider the most likely origin for the dust in
these systems to be a dynamical instability which scattered planetesimals
inwards from a more distant planetesimal belt in an event akin to the Late
Heavy Bombardment in our own system, the dust being released from such
planetesimals in collisions and possibly also sublimation.Comment: 16 pages, accepted by ApJ, removed HD128400 as hot dust candidat
Collisional Velocities and Rates in Resonant Planetesimal Belts
We consider a belt of small bodies around a star, captured in one of the
external or 1:1 mean-motion resonances with a massive perturber. The objects in
the belt collide with each other. Combining methods of celestial mechanics and
statistical physics, we calculate mean collisional velocities and collisional
rates, averaged over the belt. The results are compared to collisional
velocities and rates in a similar, but non-resonant belt, as predicted by the
particle-in-a-box method. It is found that the effect of the resonant lock on
the velocities is rather small, while on the rates more substantial. The
collisional rates between objects in an external resonance are by about a
factor of two higher than those in a similar belt of objects not locked in a
resonance. For Trojans under the same conditions, the collisional rates may be
enhanced by up to an order of magnitude. Our results imply, in particular,
shorter collisional lifetimes of resonant Kuiper belt objects in the solar
system and higher efficiency of dust production by resonant planetesimals in
debris disks around other stars.Comment: 31 pages, 11 figures (some of them heavily compressed to fit into
arxiv-maximum filesize), accepted for publication at "Celestial Mechanics and
Dynamical Astronomy
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