21,428 research outputs found
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
Ordinary differential equations which linearize on differentiation
In this short note we discuss ordinary differential equations which linearize
upon one (or more) differentiations. Although the subject is fairly elementary,
equations of this type arise naturally in the context of integrable systems.Comment: 9 page
Differentiation Of Murine Erythroleukemic Cells During Exposure To Microwave Radiation
Cultures of murine erythroleukemic cells undergoing erythroid differentiation in response to induction by hexamethylene bisacetamide (HMBA) were exposed to 1180-MHz microwave (MW) radiation for 48 h while maintained at 37.4°C by variable-temperature air flow. Exposures at 1180 MHz were at 5.5, 11, and 22 mW/cm2 with a normalized specific absorption rate of 3.32 W/kg per mW/cm2. HMBA-induced control cells were incubated in a 37.4°C water bath. Mean cell doubling time was 16.5 h in both the irradiated cultures and the control cultures. About 65% of the cells of irradiated cultures and control cultures were benzidine-positive differentiated cells. Both the irradiated cultures and the control cultures contained approximately 58 μg of hemoglobin/mg total cytoplasmic protein. The absence of any change in these parameters suggests that MW radiation at 1180 MHz and similar frequencies exerts no effect on proliferation and differentiation of mammalian cells in the absence of hyperthermia
The polarization of the planet-hosting WASP-18 system
We report observations of the linear polarization of the WASP-18 system,
which harbors a very massive ( approx 10 M_J) planet orbiting very close to its
star with an orbital period of 0.94 days. We find the WASP-18 system is
polarized at about 200 parts-per-million (ppm), likely from the interstellar
medium predominantly, with no strong evidence for phase dependent modulation
from reflected light from the planet. We set an upper limit of 40 ppm (99%
confidence level) on the amplitude of a reflected polarized light planetary
signal. We compare the results with models for a number of processes that may
produce polarized light in a planetary system to determine if we can rule out
any phenomena with this limit. Models of reflected light from thick clouds can
approach or exceed this limit, but such clouds are unlikely at the high
temperature of the WASP-18b atmosphere. Additionally, we model the expected
polarization resulting from the transit of the planet across the star and find
this has an amplitude of about 1.6 ppm, which is well below our detection
limits. We also model the polarization due to the tidal distortion of the star
by the massive planet and find this is also too small to be measured currently.Comment: 23 pages, 10 Figures, 6 Tables, Accepted to A
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
The Sloan Lens ACS Survey. VIII. The relation between environment and internal structure of early-type galaxies
We study the relation between the internal structure of early-type galaxies
and their environment using 70 strong gravitational lenses from the Sloan ACS
Lens Survey. The Sloan database is used to determine two measures of
overdensity of galaxies around each lens: the projected number density of
galaxies inside the tenth nearest neighbor (\Sigma_{10}) and within a cone of
radius one h^{-1} Mpc (D_1). Our main results are: 1) The average overdensity
is somewhat larger than unity, consistent with lenses preferring overdense
environments as expected for massive early-type galaxies (12/70 lenses are in
known groups/clusters). 2) The distribution of overdensities is
indistinguishable from that of "twin" non-lens galaxies selected from SDSS to
have the same redshift and stellar velocity dispersion \sigma_*. Thus, within
our errors, lens galaxies are an unbiased population, and the SLACS results can
be generalized to the overall population of early-type galaxies. 3) Typical
contributions from external mass distribution are no more than a few per cent,
reaching 10-20% (~0.05-0.10 external convergence) only in the most extreme
overdensities. 4) No significant correlation between overdensity and slope of
the mass density profile of the lens is found. 5) Satellite galaxies (those
with a more luminous companion) have marginally steeper mass density profiles
than central galaxies (those without). This result suggests that tidal
stripping may affect the mass structure of early-type galaxies down to kpc
scales probed by strong lensing, when they fall into larger structures
[ABRIDGED].Comment: ApJ, in press; minor changes with respect to v
Deployer Performance Results for the TSS-1 Mission
Performance of the Tethered Satellite System (TSS) Deployer during the STS-46 mission (July and August 1992) is analyzed in terms of hardware operation at the component and system level. Although only a limited deployment of the satellite was achieved (256 meters vs 20 kilometers planned), the mission served to verify the basic capability of the Deployer to release, control and retrieve a tethered satellite. - Deployer operational flexibility that was demonstrated during the flight is also addressed. Martin Marietta was the prime contractor for the development of the Deployer, under management of the NASA George C. Marshall Space Flight Center (MSFC). The satellite was provided by Alenia, Torino, Italy under contract to the Agencia Spaziale Italiana (ASI). Proper operation of the avionics components and the majority of mechanisms was observed during the flight. System operations driven by control laws for the deployment and retrieval of the satellite were also successful for the limited deployment distance. Anomalies included separation problems for one of the two umbilical connectors between the Deployer and satellite, tether jamming (at initial Satellite fly-away and at a deployment distance of 224 meters), and a mechanical interference which prevented tether deployment beyond 256 meters. The Deployer was used in several off-nominal conditions to respond to these anomalies, which ultimately enabled a successful satellite retrieval and preservation of hardware integrity for a future re-flight. The paper begins with an introduction defining the significance of the TSS-1 mission. The body of the paper is divided into four major sections: (1) Description of Deployer System and Components, (2) Deployer Components/Systems Demonstrating Successful Operation, (3) Hardware Anomalies and Operational Responses, and (4) Design Modifications for the TSS-1R Re-flight Mission. Conclusions from the TSS-1 mission, including lessons learned are presented at the end of the manuscript
Linearly polarized X-ray flares following short gamma-ray bursts
Soft X-ray flares were detected to follow the short-duration gamma-ray burst
GRB 050724. The temporal properties of the flares suggest that they are likely
due to the late time activity of the central engine. We argue that if short
GRBs are generated through compact star mergers, as is supported by the recent
observations, the jet powering the late X-ray flares must be launched via
magnetic processes rather than via neutrino-antineutrino annihilations. As a
result, the X-ray flares following short GRBs are expected to be linearly
polarized. The argument may also apply to the X-ray flares following long GRBs.
Future observations with the upcoming X-ray polarimeters will test this
prediction.Comment: 4 pages (no figure), accepted for publication in ApJL, typos
correcte
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