4,685 research outputs found
Collisional Processes in Extrasolar Planetsimal Disks - Dust Clumps in Fomalhaut's Debris Disk
This paper presents a model for the outcome of collisions between
planetesimals in a debris disk and assesses the impact of collisional processes
on the structure and size distribution of the disk. The model is presented by
its application to Fomalhaut's collisionally replenished dust disk; a recent
450 micron image of this disk shows a clump embedded within it with a flux ~5
per cent of the total. The following conclusions are drawn: (i) SED modelling
is consistent with Fomalhaut's disk having a collisional cascade size
distribution extending from bodies 0.2 m in diameter down to 7 micron-sized
dust. (ii) Collisional lifetime arguments imply that the cascade starts with
planetesimals 1.5-4 km in diameter. Any larger bodies must be predominantly
primordial. (iii) Constraints on the timescale for the ignition of the cascade
are consistent with these primordial planetesimals having a distribution that
extends up to 1000km, resulting in a disk mass of 5-10 times the minimum mass
solar nebula. (iv) The debris disk is expected to be intrinsically clumpy,
since planetesimal collisions result in dust clumps. The intrinsic clumpiness
of Fomalhaut's disk is below current detection limits, but could be detectable
by future observatories such as the ALMA, and could provide the only way of
determining the primordial planetesimal population. (v) The observed clump
could have originated in a collision between two runaway planetesimals, both
larger than 1400 km diameter. It is unlikely that we should witness such an
event unless both the formation of these runaways and the ignition of the
collisional cascade occurred within the last ~10 Myr. (vi) Another explanation
for Fomalhaut's clump is that ~5 per cent of the planetesimals in the ring are
trapped in 1:2 resonance with a planet orbiting at 80 AU.Comment: 21 pages, 13 figures, accepted by MNRA
Time variation of fundamental couplings and dynamical dark energy
Scalar field dynamics may give rise to a nonzero cosmological variation of
fundamental constants. Within different scenarios based on the unification of
gauge couplings, the various claimed observations and bounds may be combined in
order to trace or restrict the time history of the couplings and masses. If the
scalar field is responsible for a dynamical dark energy or quintessence,
cosmological information becomes available for its time evolution. Combining
this information with the time variation of couplings, one can determine the
interaction strength between the scalar and atoms, which may be observed by
tests of the Weak Equivalence Principle. We compute bounds on the present rate
of coupling variation from experiments testing the differential accelerations
for bodies with equal mass and different composition and compare the
sensitivity of various methods. In particular, we discuss two specific models
of scalar evolution: crossover quintessence and growing neutrino models.Comment: 26 pages, 2 figures; minor typos & added references, to be published
in JCA
Investigation of the fundamental constants stability based on the reactor Oklo burn-up analysis
The burn-up for SC56-1472 sample of the natural Oklo reactor zone 3 was
calculated using the modern Monte Carlo codes. We reconstructed the neutron
spectrum in the core by means of the isotope ratios: Sm/Sm and
Lu/Lu. These ratios unambiguously determine the spectrum index
and core temperature. The effective neutron absorption cross section of
Sm calculated using this spectrum was compared with experimental one.
The disagreement between these two values allows to limit a possible shift of
the low laying resonance of Sm even more . Then, these limits were
converted to the limits for the change of the fine structure constant .
We found that for the rate of change the inequality is fulfilled, which is of the next
higher order than our previous limit.Comment: 16 pages, 7 figure
Gastro-oesophageal reflux disease - Impact of guidelines on GP management
Copyright © 2008 Royal Australian College of General Practitioners Copyright to Australian Family Physician. Reproduced with permission. Permission to reproduce must be sought from the publisher, The Royal Australian College of General Practitioners.Background: This program examined the impact of clinical guidelines and a 3 year self audit process on general practitioners’ diagnosis and management of gastro-oesophageal reflux disease. Methods: Nine hundred and sixty-six Australian GPs participated in a retrospective five step clinical audit reporting on data for 28 622 patients. Results: General practitioners demonstrated significant improvements in their diagnosis and management of gastro-oesophageal reflux disease across the audit period, including: - a significant 3% decrease in use of endoscopy - improved GP assessment and identification of risk factors and exacerbants - significant increases in GP recommendations for patient weight loss and dietary change (7 and 10% respectively) - a significant 4% reduction in patient use of medications that may exacerbate reflux symptoms. Discussion: The findings provide a snapshot of current diagnostic and management practices in Australian general practice, and highlight the benefits of clinical audit as a tool for eliciting evidence based, guideline driven practice change.Catherine N Kirby, Leon Piterman, Mark R Nelson and John Den
Herschel GASPS spectral observations of T Tauri stars in Taurus: unraveling far-infrared line emission from jets and discs
At early stages of stellar evolution young stars show powerful jets and/or
outflows that interact with protoplanetary discs and their surroundings.
Despite the scarce knowledge about the interaction of jets and/or outflows with
discs, spectroscopic studies based on Herschel and ISO data suggests that gas
shocked by jets and/or outflows can be traced by far-IR (FIR) emission in
certain sources. We want to provide a consistent catalogue of selected atomic
([OI] and [CII]) and molecular (CO, OH, and HO) line fluxes observed in
the FIR, separate and characterize the contribution from the jet and the disc
to the observed line emission, and place the observations in an evolutionary
picture. The atomic and molecular FIR (60-190 ) line emission of
protoplanetary discs around 76 T Tauri stars located in Taurus are analysed.
The observations were carried out within the Herschel key programme Gas in
Protoplanetary Systems (GASPS). The spectra were obtained with the
Photodetector Array Camera and Spectrometer (PACS). The sample is first divided
in outflow and non-outflow sources according to literature tabulations. With
the aid of archival stellar/disc and jet/outflow tracers and model predictions
(PDRs and shocks), correlations are explored to constrain the physical
mechanisms behind the observed line emission. The much higher detection rate of
emission lines in outflow sources and the compatibility of line ratios with
shock model predictions supports the idea of a dominant contribution from the
jet/outflow to the line emission, in particular at earlier stages of the
stellar evolution as the brightness of FIR lines depends in large part on the
specific evolutionary stage. [Abridged Abstract]Comment: 37 pages, 27 figures, accepted for publication in A&
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
Jacoby Creek Site Inventory Assessment and Management Recommendations for Parcel 502-052-022
Jacoby Creek is a 16.6 mi2 watershed that discharges into Humboldt Bay in Humboldt County, California. Legacy use of this watershed consisted of unmanaged logging and diking to create pastureland for agriculture until the 1970’s. Currently, 187 acres of the Jacoby Creek watershed is owned by the Jacoby Creek Land Trust (JCLT). JCLT focuses on creating a space for recreation and conservation within the Jacoby Creek watershed.
The parcel (parcel 502-052-022) was assessed to identify social trails, invasive species, tree density, and upland understory vegetation to provide restoration recommendations. Non-native invasive species, such as Hedera helix (English ivy) and Rubus armeniacus (Himalayan Blackberry), without proper management have the potential to grow and outcompete native flora. A system of social trails were found, most of which lead to less accessible routes. It is recommended that JCLT discourage the use of social trails by covering trail entrances and with online notices and other forms of outreach. A tree density assessment was conducted on the previously logged, second growth forest, showing it is of high density. A variable density thinning treatment is recommended to allow for healthier trees similar to old growth redwood forest conditions
The nature of mid-infrared excesses from hot dust around Sun-like stars
(ABRIDGED) Studies of debris disks have shown that most systems are analogous
to the Edgeworth-Kuiper Belt. However a rare subset of sun-like stars possess
dust which lies in the terrestrial planet region. In this study we aim to
determine how many sources with apparent mid-IR excess are truly hosts of warm
dust, and investigate where the dust must lie. We observed using mid-IR imaging
with TIMMI2, VISIR and MICHELLE a sample of FGK main sequence stars reported to
have hot dust. A new modelling approach was developed to determine the
constraints that can be set on the radial extent of excess emission. We confirm
the presence of warm dust around 3 of the candidates (eta Corvi, HD145263 and
HD202406), and present constraints on the emitting dust regions. Of 2
alternative models for the eta Corvi excess emission, we find that a model with
1 hot dust component at <3 AU (combined with the known submm dust population)
fits the data better at the 2.6sigma level than an alternative model with 2
populations of dust in the mid-IR. We identify several systems which have a
companion (HD65277 and HD79873) or background object (HD53246, HD123356 and
HD128400) responsible for their mid-infrared excess, and for 3 other systems we
were able to rule out a point-like source near the star at the level of excess
observed in lower resolution observations (HD12039, HD69830 and HD191089). Hot
dust sources are either young and possibly primordial or transitional, or have
relatively small radius steady-state planetesimal belts, or they are old and
luminous with transient emission. High resolution imaging can be used to
constrain the location of the disk and help to discriminate between different
models of disk emission. For some small disks, interferometry is needed to
resolve the disk location.Comment: Accepted for publication in Astronomy & Astrophysic
Gravitational Stirring in Planetary Debris Disks
We describe gravitational stirring models of planetary debris disks using a
new multi-annulus planetesimal evolution code. The current code includes
gravitational stirring and dynamical friction; future studies will include
coagulation, fragmentation, Poynting-Robertson drag, and other physical
processes. We use the results of our calculations to investigate the physical
conditions required for small bodies in a planetesimal disk to reach the
shattering velocity and begin a collisional cascade. Our results demonstrate
that disks composed primarily of bodies with a single size will not undergo a
collisional cascade which produces small dust grains at 30-150 AU on timescales
of 1 Gyr or smaller. Disks with a size distribution of bodies reach conditions
necessary for a collisional cascade in 10 Myr to 1 Gyr if the disk is at least
as massive as a minimum mass solar nebula and if the disk contains objects with
radii of 500 km or larger. The estimated 500 Myr survival time for these disks
is close to the median age of roughly 400 Myr derived for nearby stars with
dusty disks.Comment: 23 pages of text + 16 Figures; to appear in the Astronomical Journal,
January 200
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