5,791 research outputs found
Importance of Biologically Active Aurora-like Ultraviolet Emission: Stochastic Irradiation of Earth and Mars by Flares and Explosions
(Abridged) We show that sizeable fractions of incident ionizing radiation
from stochastic astrophysical sources can be redistributed to biologically and
chemically important UV wavelengths, a significant fraction of which can reach
the surface. This redistribution is mediated by secondary electrons, resulting
from Compton scattering and X-ray photoabsorption, with energies low enough to
excite atmospheric molecules and atoms, resulting in a rich aurora-like
spectrum. We calculate the fraction of energy redistributed into biologically
and chemically important wavelength regions for spectra characteristic of
stellar flares and supernovae using a Monte-Carlo transport code written for
this problem and then estimate the fraction of this energy that is transmitted
from the atmospheric altitudes of redistribution to the surface for a few
illustrative cases. Redistributed fractions are found to be of order 1%, even
in the presence of an ozone shield. This result implies that planetary
organisms will be subject to mutationally significant, if intermittent,
fluences of UV-B and harder radiation even in the presence of a narrow-band UV
shield like ozone. We also calculate the surficial transmitted fraction of
ionizing radiation and redistributed ultraviolet radiation for two illustrative
evolving Mars atmospheres whose initial surface pressures were 1 bar. Our
results suggest that coding organisms on planets orbiting low-mass stars (and
on the early Earth) may evolve very differently than on contemporary Earth,
with diversity and evolutionary rate controlled by a stochastically varying
mutation rate and frequent hypermutation episodes.Comment: 21 pages, 2 figures, accepted for publication in Origins of Life and
Evolution of the Biospher
Transport of Ionizing Radiation in Terrestrial-like Exoplanet Atmospheres
(Abridged) The propagation of ionizing radiation through model atmospheres of
terrestrial-like exoplanets is studied for a large range of column densities
and incident photon energies using a Monte Carlo code we have developed to
treat Compton scattering and photoabsorption. Incident spectra from parent star
flares, supernovae, and gamma-ray bursts are modeled and compared to energetic
particles in importance. We find that terrestrial-like exoplanets with
atmospheres thinner than about 100 g cm^-2 transmit and reprocess a significant
fraction of incident gamma-rays, producing a characteristic, flat surficial
spectrum. Thick atmospheres (>~ 100 g cm^-2) efficiently block even gamma-rays,
but nearly all incident energy is redistributed into diffuse UV and visible
aurora-like emission, increasing the effective atmospheric transmission by many
orders of magnitude. Depending on the presence of molecular UV absorbers and
atmospheric thickness, up to 10% of the incident energy can reach the surface
as UV reemission. For the Earth, between 2 x 10^-3 and 4 x 10^-2 of the
incident flux reaches the ground in the biologically effective 200--320 nm
range, depending on O_2/O_3 shielding. Finally, we suggest that transient
atmospheric ionization layers can be frequently created at low altitudes. We
conclude that these events can produce frequent fluctuations in atmospheric
ionization levels and surficial UV fluxes on terrestrial-like planets.Comment: 59 pages, 15 figures; in press in Icarus; minor edits, no results
change
Polar phonons and intrinsic dielectric response of the ferromagnetic insulating spinel CdCrS from first principles
We have studied the dielectric properties of the ferromagnetic spinel
CdCrS from first principles. Zone-center phonons and Born effective
charges were calculated by frozen-phonon and Berry phase techniques within
LSDA+U. We find that all infrared-active phonons are quite stable within the
cubic space group. The calculated static dielectric constant agrees well with
previous measurements. These results suggest that the recently observed
anomalous dielectric behavior in CdCrS is not due to the softening of a
polar mode. We suggest further experiments to clarify this point
Recommended from our members
"Am iz kwiin" (I'm his queen): Combining interpretative phenomenological analysis with a feminist approach to work with gems in a resource-constrained setting
This article focuses on working with gems using a feminist approach to interpretative phenomenological analysis (IPA) in a resource-constrained setting. The research explores the experiences of maternal disclosure of HIV to children of HIV positive mothers in Kingston, Jamaica. A feminist approach helps recognise power imbalances within research relationships and the women’s lived experiences. We present three “gems” which illuminate women’s lived experiences and explore how popularised representations of women’s sexuality and mothering influence disclosure discourses. We use emotion work as a conceptual resource to structure the women’s narratives and challenge existing policy discourses, which arguably represent disclosure within a binary, rationalist, decision-making framework. This article adds to global literature on maternal HIV disclosure and problematises policy discourses by bringing into relief the emotion work women engage in when deciding if and how to communicate their HIV status to their children. It adds to the body of research using IPA, particularly in resource-constrained settings where IPA has thus far had little application
Recommended from our members
Amazonian Hydrothermal Alteration Comparing Nakhlite Secondary Mineralogy to Water Rock Reaction Experiments
We report on results from experiments with Mars analog materials under diagenetic conditions. The mineralogical results of our experiments suggest that an important type of fluid alteration in the Amazonian may be short duration (e.g. less than 1 year) events from near neutral, dilute brines, that were able to exchange CO2 either directly, or via ice reservoirs, with the atmosphere
Acute supplementation of N-acetylcysteine does not affect muscle blood flow and oxygenation characteristics during handgrip exercise
Citation: Smith, J. R., Broxterman, R. M., Ade, C. J., Evans, K. K., Kurti, S. P., Hammer, S. M., . . . Harms, C. A. (2016). Acute supplementation of N-acetylcysteine does not affect muscle blood flow and oxygenation characteristics during handgrip exercise. Physiological Reports, 4(7), 1-10. doi:10.14814/phy2.12748N-acetylcysteine (NAC; antioxidant and thiol donor) supplementation has improved exercise performance and delayed fatigue, but the underlying mechanisms are unknown. One possibility is NAC supplementation increases limb blood flow during severe-intensity exercise. The purpose was to determine if NAC supplementation affected exercising arm blood flow and muscle oxygenation characteristics. We hypothesized that NAC would lead to higher limb blood flow and lower muscle deoxygenation characteristics during severe-intensity exercise. Eight healthy nonendurance trained men (21.8 ± 1.2 years) were recruited and completed two constant power handgrip exercise tests at 80% peak power until exhaustion. Subjects orally consumed either placebo (PLA) or NAC (70 mg/kg) 60 min prior to handgrip exercise. Immediately prior to exercise, venous blood samples were collected for determination of plasma redox balance. Brachial artery blood flow (BABF) was measured via Doppler ultrasound and flexor digitorum superficialis oxygenation characteristics were measured via near-infrared spectroscopy. Following NAC supplementaiton, plasma cysteine (NAC: 47.2 ± 20.3 ?mol/L vs. PLA: 9.6 ± 1.2 ?mol/L; P = 0.001) and total cysteine (NAC: 156.2 ± 33.9 ?mol/L vs. PLA: 132.2 ± 16.3 ?mol/L; P = 0.048) increased. Time to exhaustion was not significantly different (P = 0.55) between NAC (473.0 ± 62.1 sec) and PLA (438.7 ± 58.1 sec). Resting BABF was not different (P = 0.79) with NAC (99.3 ± 31.1 mL/min) and PLA (108.3 ± 46.0 mL/min). BABF was not different (P = 0.42) during exercise or at end-exercise (NAC: 413 ± 109 mL/min; PLA: 445 ± 147 mL/min). Deoxy-[hemoglobin+myoglobin] and total-[hemoglobin+myoglobin] were not significantly different (P = 0.73 and P = 0.54, respectively) at rest or during exercise between conditions. We conclude that acute NAC supplementation does not alter oxygen delivery during exercise in men. © 2016 Published by the American Physiological Society and The Physiological Society
SN2008am: A Super-Luminous Type IIn Supernova
We present observations and interpretation of the Type IIn supernova SN
2008am discovered by the ROTSE Supernova Verification Project (RSVP). SN 2008am
peaked at approximately -22.3 mag at a redshift of z=0.2338, giving it a peak
luminosity of 3 x 10^{44}erg/s and making it one of the most luminous
supernovae ever observed. The total radiated energy is ~ 2 x 10^{51} erg.
Photometric observations in the ultraviolet, optical and infrared bands
(J,H,Ks) constrain the SED evolution. We obtained six optical spectra of the
supernova, five on the early decline from maximum light and a sixth nearly a
year later plus a very late-time spectrum (~2 yr) of the host galaxy. The
spectra of SN 2008am show strong Balmer-line and He I lambda 5876A emission
with intermediate widths (~25A) in the first ~40 days after optical maximum. We
examine a variety of models for the line wings and conclude that multiple
scattering is most likely, implying that our spectra contain no specific
information on the bulk flow velocity. We examine a variety of models for the
ROTSE light curve subject to the rise time and the nature of the spectra,
including radioactive decay, shocks in optically-thick and optically-thin
circumstellar media (CSM) and a magnetar. The most successful model is one for
which the CSM is optically-thick and in which diffusion of forward
shock-deposited luminosity gives rise to the observed light curve. Diffusion of
the shock-deposited energy from the forward shock is found to be important to
account for the rising part of the light curve. Although there are differences
in detail, SN 2008am appears to be closely related to other super-luminous Type
IIn supernovae, SN 2006gy, SN 2006tf and perhaps SN 2008iy, that may represent
the deaths of very massive LBV-type progenitors and for which the luminosity is
powered by the interaction of the ejecta with a dense circumstellar medium.Comment: 58 pages, 14 figure
- …