10,931 research outputs found
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
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
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
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
[Job] Locked and [Un]loaded: The Effect of the Affordable Care Act Dependency Mandate on Reenlistment in the U.S. Army
One concern with employer-based health insurance is job lock or the inability for employees to leave their current employment for better opportunities for fear of losing benefits. We use the implementation of the Affordable Care Act’s dependency mandate as a natural experiment. Data from the United States Army overcome some limitations in previous studies including the ability to examine workers with fixed contract expiration dates, uniform pay, and health coverage. We find that the ACA decreased reenlistment rates by 3.13 percent for enlisted soldiers aged 23–25. We also find that younger veterans who leave the army are more likely to attend college. These findings show that the ACA reduced job lock and increased college-going
[Job] Locked and [Un]loaded: The Effect of the Affordable Care Act Dependency Mandate on Reenlistment in the U.S. Army
One concern with employer-based health insurance is job lock or the inability for employees to leave their current employment for better opportunities for fear of losing benefits. We use the implementation of the Affordable Care Act’s dependency mandate as a natural experiment. Data from the United States Army overcome some limitations in previous studies including the ability to examine workers with fixed contract expiration dates, uniform pay, and health coverage. We find that the ACA decreased reenlistment rates by 3.13 percent for enlisted soldiers aged 23–25. We also find that younger veterans who leave the army are more likely to attend college. These findings show that the ACA reduced job lock and increased college-going
[Job] Locked and [Un]loaded: The Effect of the Affordable Care Act Dependency Mandate on Reenlistment in the U.S. Army
Hydrodynamic View of Wave-Packet Interference: Quantum Caves
Wave-packet interference is investigated within the complex quantum
Hamilton-Jacobi formalism using a hydrodynamic description. Quantum
interference leads to the formation of the topological structure of quantum
caves in space-time Argand plots. These caves consist of the vortical and
stagnation tubes originating from the isosurfaces of the amplitude of the wave
function and its first derivative. Complex quantum trajectories display
counterclockwise helical wrapping around the stagnation tubes and hyperbolic
deflection near the vortical tubes. The string of alternating stagnation and
vortical tubes is sufficient to generate divergent trajectories. Moreover, the
average wrapping time for trajectories and the rotational rate of the nodal
line in the complex plane can be used to define the lifetime for interference
features.Comment: 4 pages, 3 figures (major revisions with respect to the previous
version have been carried out
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