336 research outputs found
Infrared Emission by Dust Around lambda Bootis Stars: Debris Disks or Thermally Emitting Nebulae?
We present a model that describes stellar infrared excesses due to heating of
the interstellar (IS) dust by a hot star passing through a diffuse IS cloud.
This model is applied to six lambda Bootis stars with infrared excesses.
Plausible values for the IS medium (ISM) density and relative velocity between
the cloud and the star yield fits to the excess emission. This result is
consistent with the diffusion/accretion hypothesis that lambda Bootis stars (A-
to F-type stars with large underabundances of Fe-peak elements) owe their
characteristics to interactions with the ISM. This proposal invokes radiation
pressure from the star to repel the IS dust and excavate a paraboloidal dust
cavity in the IS cloud, while the metal-poor gas is accreted onto the stellar
photosphere. However, the measurements of the infrared excesses can also be fit
by planetary debris disk models. A more detailed consideration of the
conditions to produce lambda Bootis characteristics indicates that the majority
of infrared-excess stars within the Local Bubble probably have debris disks.
Nevertheless, more distant stars may often have excesses due to heating of
interstellar material such as in our model.Comment: 10 pages, 5 figures, 4 tables, accepted by ApJ, emulateap
Characterization of the optical properties of the buried contact of the JWST MIRI Si:As infrared blocked impurity band detectors
The Mid-Infrared Instrument MIRI on-board the James Webb Space Telescope uses
three Si:As impurity band conduction detector arrays. MIRI medium resolution
spectroscopic measurements (R3500-1500) in the 5~ to 28~
wavelength range show a 10-30\% modulation of the spectral baseline; coherent
reflections of infrared light within the Si:As detector arrays result in
fringing. We quantify the shape and impact of fringes on spectra of optical
sources observed with MIRI during ground testing and develop an optical model
to simulate the observed modulation. We use our optical model in conjunction
with the MIRI spectroscopic data to show that the properties of the buried
contact inside the MIRI Si:As detector have a significant effect on the
fringing behavior.Comment: 11 pages, 7 figures, SPIE Astronomical Telescopes + Instrumentation
2020, submitted to SPI
Observability of the General Relativistic Precession of Periastra in Exoplanets
The general relativistic precession rate of periastra in close-in exoplanets
can be orders of magnitude larger than the magnitude of the same effect for
Mercury. The realization that some of the close-in exoplanets have significant
eccentricities raises the possibility that this precession might be detectable.
We explore in this work the observability of the periastra precession using
radial velocity and transit light curve observations. Our analysis is
independent of the source of precession, which can also have significant
contributions due to additional planets and tidal deformations. We find that
precession of the periastra of the magnitude expected from general relativity
can be detectable in timescales of <~ 10 years with current observational
capabilities by measuring the change in the primary transit duration or in the
time difference between primary and secondary transits. Radial velocity curves
alone would be able to detect this precession for super-massive, close-in
exoplanets orbiting inactive stars if they have ~100 datapoints at each of two
epochs separated by ~20 years. We show that the contribution to the precession
by tidal deformations may dominate the total precession in cases where the
relativistic precession is detectable. Studies of transit durations with Kepler
might need to take into account effects arising from the general relativistic
and tidal induced precession of periastra for systems containing close-in,
eccentric exoplanets. Such studies may be able to detect additional planets
with masses comparable to that of Earth by detecting secular variations in the
transit duration induced by the changing longitude of periastron.Comment: 13 pages, 5 figures. Accepted for publication in Ap
Acetyl-l-carnitine normalizes the impaired long-term potentiation and spine density in a rat model of global ischemia
Aim: The aim of this study was to enhance the knowledge regarding actors and intentions in the development process of a mobile phone platform for self-management of hypertension. Methods: Our research approach was a 14-month longitudinal “real-time ethnography” method of description and analysis. Data were collected through focus groups with patients and providers, patient interviews, and design meetings with researchers and experts. The analysis was informed by the concepts of actors and inscriptions in actor-network theory (ANT). Results: Our study showed that laypersons, scientific actors, as well as technology itself, might influence development processes of support for self-management of hypertension. The intentions were inscribed into the technology design as well as the models of learning and treatment. Conclusions: The study highlighted important aspects of how actors and intentions feature in the development of the mobile phone platform to support self-management of hypertension. The study indicated the multifacetedness of the participating actors, including the prominent role of technology. The concrete results of such processes included questions in the self-report system, learning and treatment models
Breathing Spots in a Reaction-Diffusion System
A quasi-2-dimensional stationary spot in a disk-shaped chemical reactor is
observed to bifurcate to an oscillating spot when a control parameter is
increased beyond a critical value. Further increase of the control parameter
leads to the collapse and disappearance of the spot. Analysis of a bistable
activator-inhibitor model indicates that the observed behavior is a consequence
of interaction of the front with the boundary near a parity breaking front
bifurcation.Comment: 4 pages RevTeX, see also http://chaos.ph.utexas.edu/ and
http://t7.lanl.gov/People/Aric
Age and nutritional state influence the effects of cholecystokinin on energy balance
AbstractCholecystokinin (CCK) is anorexic, irrespective whether it is applied intraperitoneally (IP) or intracerebroventricularly (ICV) in male Wistar rats. The metabolic effects depend on the route of administration: by the IP route it elicits hypothermia (presumably by type-1 receptors, CCK1R-s), while ICV administration is followed by fever-like hypermetabolism and hyperthermia via activation of CCK2R-s, which latter response seems to be most important in the postprandial (compensatory) hypermetabolism. The efficacy of the IP injected CCK varies with age: it causes strong anorexia in young adult 4 and 6-months old and again in old rats (aged 18–24months), but the middle-aged (12-month old) ones seem to be resistant to this effect. Such pattern of effects may contribute to the explanation of age-related obesity observed in middle-aged animals as well as to the aging anorexia and loss of body weight in old ones. Diet-induced obesity accelerates the appearance of CCK-resistance as well as the return of high sensitivity to CCK in further aging, while chronic calorie-restriction prevents the development of resistance, as if the speed of the age-related regulatory changes was altered by the nutritional state. The effects of ICV applied CCK also change with age: the characteristic anorexic and hypermetabolic/hyperthermic effects can be observed in young adult rats, but the effects gradually and monotonically decline with age and disappear by the old age of 24months. These disparate age-related patterns of CCK efficacy upon peripheral or central administration routes may indicate that although both peripheral and central CCKR-s exert anorexic effects, they may have dissimilar roles in the regulation of overall energy balance
Probing the stability of gravastars by dropping dust shells onto them
As a preparation for the dynamical investigations, this paper begins with a
short review of the three-layer gravastar model with distinguished attention to
the structure of the pertinent parameter space of gravastars in equilibrium.
Then the radial stability of these types of gravastars is studied by
determining their response for the totally inelastic collision of their surface
layer with a dust shell. It is assumed that the dominant energy condition holds
and the speed of sound does not exceed that of the light in the matter of the
surface layer. While in the analytic setup the equation of state is kept to be
generic, in the numerical investigations three functionally distinct classes of
equations of states are applied. In the corresponding particular cases the
maximal mass of the dust shell that may fall onto a gravastar without
converting it into a black hole is determined. For those configurations which
remain stable the excursion of their radius is assigned. It is found that even
the most compact gravastars cannot get beyond the lower limit of the size of
conventional stars, provided that the dominant energy condition holds in both
cases. It is also shown---independent of any assumption concerning the matter
interbridging the internal de Sitter and the external Schwarzschild
regions---that the better is a gravastar in mimicking a black hole the easier
is to get the system formed by a dust shell and the gravastar beyond the event
horizon of the composite system. In addition, a generic description of the
totally inelastic collision of spherical shells in spherically symmetric
spacetimes is also provided in the appendix.Comment: 29 pages, 10 figure
The quantum efficiency and diffractive image artifacts of Si:As IBC mid-IR detector arrays at 5 10 m: Implications for the JWST/MIRI detectors
Arsenic doped back illuminated blocked impurity band (BIBIB) silicon
detectors have advanced near and mid-IR astronomy for over thirty years; they
have high quantum efficiency (QE), especially at wavelengths longer than 10
m, and a large spectral range. Their radiation hardness is also an asset
for space based instruments. Three examples of Si:As BIBIB arrays are used in
the Mid-InfraRed Instrument (MIRI) of the James Webb Space Telescope (JWST),
observing between 5 and 28 m. In this paper, we analyze the parameters
leading to high quantum efficiency (up to 60\%) for the MIRI devices
between 5 and 10 m. We also model the cross-shaped artifact that was first
noticed in the 5.7 and 7.8 m Spitzer/IRAC images and has since also been
imaged at shorter wavelength (m) laboratory tests of the MIRI
detectors. The artifact is a result of internal reflective diffraction off the
pixel-defining metallic contacts to the readout detector circuit. The low
absorption in the arrays at the shorter wavelengths enables photons diffracted
to wide angles to cross the detectors and substrates multiple times. This is
related to similar behavior in other back illuminated solid-state detectors
with poor absorption, such as conventional CCDs operating near 1 m. We
investigate the properties of the artifact and its dependence on the detector
architecture with a quantum-electrodynamic (QED) model of the probabilities of
various photon paths. Knowledge of the artifact properties will be especially
important for observations with the MIRI LRS and MRS spectroscopic modes.Comment: 17 pages, 15 figures, accepted for publication in PAS
HST and Spitzer Observations of the HD 207129 Debris Ring
A debris ring around the star HD 207129 (G0V; d = 16.0 pc) has been imaged in
scattered visible light with the ACS coronagraph on the Hubble Space Telescope
and in thermal emission using MIPS on the Spitzer Space Telescope at 70 microns
(resolved) and 160 microns (unresolved). Spitzer IRS (7-35 microns) and MIPS
(55-90 microns) spectrographs measured disk emission at >28 microns. In the HST
image the disk appears as a ~30 AU wide ring with a mean radius of ~163 AU and
is inclined by 60 degrees from pole-on. At 70 microns it appears partially
resolved and is elongated in the same direction and with nearly the same size
as seen with HST in scattered light. At 0.6 microns the ring shows no
significant brightness asymmetry, implying little or no forward scattering by
its constituent dust. With a mean surface brightness of V=23.7 mag per square
arcsec, it is the faintest disk imaged to date in scattered light.Comment: 28 pages, 8 figure
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