13,350 research outputs found
White Dwarf Cosmochronology in the Solar Neighborhood
The study of the stellar formation history in the solar neighborhood is a
powerful technique to recover information about the early stages and evolution
of the Milky Way. We present a new method which consists of directly probing
the formation history from the nearby stellar remnants. We rely on the volume
complete sample of white dwarfs within 20 pc, where accurate cooling ages and
masses have been determined. The well characterized initial-final mass relation
is employed in order to recover the initial masses (1 < M/Msun < 8) and total
ages for the local degenerate sample. We correct for moderate biases that are
necessary to transform our results to a global stellar formation rate, which
can be compared to similar studies based on the properties of main-sequence
stars in the solar neighborhood. Our method provides precise formation rates
for all ages except in very recent times, and the results suggest an enhanced
formation rate for the solar neighborhood in the last 5 Gyr compared to the
range 5 < Age (Gyr) < 10. Furthermore, the observed total age of ~10 Gyr for
the oldest white dwarfs in the local sample is consistent with the early
seminal studies that have determined the age of the Galactic disk from stellar
remnants. The main shortcoming of our study is the small size of the local
white dwarf sample. However, the presented technique can be applied to larger
samples in the future.Comment: 25 pages, 10 figures, accepted for publication in the Astrophysical
Journa
Solar modulation and interplanetary gradients of the galactic electrons flux, 1977 - 1984
The flux of electrons with energy from approx. 10 to 180 MeV measured with the electron telescope on the Voyager 1 and 2 spacecraft in the heliocentric radial range 1 - 22 AU between 1977 and 1984 is reported. Jovian electrons were clearly observable between 1978 and 1983 (radial range 2 - 12 AU) at energies below approx. 50 MeV. Above approx. 50 MeV the electron intensity exhibited temporal variations generally related to the 11 year modulation of protons 75 MeV. The overall magnitude of the electron intensity changes between the maximum intensity observed in 1977 and the minimum intensity in 1981 was a factor approx. 2, also comparable to that observed for 75 MeV protons. By early 1985 the electron intensity had apparently recovered to the level observed in 1977 whereas the proton intensity was still about 20% lower. A detailed interpretation of these electron variations in all energy channels depends on an accurate subtraction of background induced by energetic protons of a few 100 MeV. This subtraction is facilitated by calibration results at several energies
Simple statistical models predict C-to-U edited sites in plant mitochondrial RNA
BACKGROUND: RNA editing is the process whereby an RNA sequence is modified from the sequence of the corresponding DNA template. In the mitochondria of land plants, some cytidines are converted to uridines before translation. Despite substantial study, the molecular biological mechanism by which C-to-U RNA editing proceeds remains relatively obscure, although several experimental studies have implicated a role for cis-recognition. A highly non-random distribution of nucleotides is observed in the immediate vicinity of edited sites (within 20 nucleotides 5' and 3'), but no precise consensus motif has been identified. RESULTS: Data for analysis were derived from the the complete mitochondrial genomes of Arabidopsis thaliana, Brassica napus, and Oryza sativa; additionally, a combined data set of observations across all three genomes was generated. We selected datasets based on the 20 nucleotides 5' and the 20 nucleotides 3' of edited sites and an equivalently sized and appropriately constructed null-set of non-edited sites. We used tree-based statistical methods and random forests to generate models of C-to-U RNA editing based on the nucleotides surrounding the edited/non-edited sites and on the estimated folding energies of those regions. Tree-based statistical methods based on primary sequence data surrounding edited/non-edited sites and estimates of free energy of folding yield models with optimistic re-substitution-based estimates of ~0.71 accuracy, ~0.64 sensitivity, and ~0.88 specificity. Random forest analysis yielded better models and more exact performance estimates with ~0.74 accuracy, ~0.72 sensitivity, and ~0.81 specificity for the combined observations. CONCLUSIONS: Simple models do moderately well in predicting which cytidines will be edited to uridines, and provide the first quantitative predictive models for RNA edited sites in plant mitochondria. Our analysis shows that the identity of the nucleotide -1 to the edited C and the estimated free energy of folding for a 41 nt region surrounding the edited C are the most important variables that distinguish most edited from non-edited sites. However, the results suggest that primary sequence data and simple free energy of folding calculations alone are insufficient to make highly accurate predictions
The Energy Spectrum of Jovian Electrons in Interplanetary Space
The energy spectrum of electrons with energies approx 10 to approx 180 MeV measured with the electron telescope on the Voyager 1 and 2 spacecraft in interplanetary space from 1978 to 1983 is studied. The kinetic energy of electrons is determined by double dE/dx measurements from the first two detectors (D sub 1, D sub 2) of a stack of eight solid state detectors and by the range of particle penetration into the remaining six detectors (D sub 3 to D sub 8) which are interleaved with tungsten absorbers. From 1978 to 1983 (radial range approximately 2 to a pproximately 12 AU) electrons of Jovian origin were clearly observable for electrons stopping in D(sub 3(E approximately greater than 4 MeV)) and in D(sub 4 (E approximately greater than 8 MeV)). For electrons stopping in D(sub 5(E approximately greather than 12 MeV)), the jovian flux dominated the galactic electron flux for a period of approximately one year near the encounter with Jupiter. Jovian electrons were also observed in D(sub 6(E approximately greater than 21 MeV)) but not in D(sub 7(E approximately greater than 28 MeV)). A detailed interpretation of the electron variations in all energy channels depends on an accurate subtraction of background induced by energetic protons of a few 100 MeV. This substraction is facilitated by laboratory calibration results at several energies. Further results on the differential energy spectrum of Jovian electrons and limits on the maximum detected energies will be reported
The Field White Dwarf Mass Distribution
We revisit the properties and astrophysical implications of the field white
dwarf mass distribution in preparation of Gaia applications. Our study is based
on the two samples with the best established completeness and most precise
atmospheric parameters, the volume-complete survey within 20 pc and the Sloan
Digital Sky Survey (SDSS) magnitude-limited sample. We explore the modelling of
the observed mass distributions with Monte Carlo simulations, but find that it
is difficult to constrain independently the initial mass function (IMF), the
initial-to-final-mass relation (IFMR), the stellar formation history (SFH), the
variation of the Galactic disk vertical scale height as a function of stellar
age, and binary evolution. Each of these input ingredients has a moderate
effect on the predicted mass distributions, and we must also take into account
biases owing to unidentified faint objects (20 pc sample), as well as unknown
masses for magnetic white dwarfs and spectroscopic calibration issues (SDSS
sample). Nevertheless, we find that fixed standard assumptions for the above
parameters result in predicted mean masses that are in good qualitative
agreement with the observed values. It suggests that derived masses for both
studied samples are consistent with our current knowledge of stellar and
Galactic evolution. Our simulations overpredict by 40-50% the number of massive
white dwarfs (M > 0.75 Msun) for both surveys, although we can not exclude a
Salpeter IMF when we account for all biases. Furthermore, we find no evidence
of a population of double white dwarf mergers in the observed mass
distributions.Comment: 15 pages, 16 figures, accepted for publication in MNRA
Morphological stability of electromigration-driven vacancy islands
The electromigration-induced shape evolution of two-dimensional vacancy
islands on a crystal surface is studied using a continuum approach. We consider
the regime where mass transport is restricted to terrace diffusion in the
interior of the island. In the limit of fast attachment/detachment kinetics a
circle translating at constant velocity is a stationary solution of the
problem. In contrast to earlier work [O. Pierre-Louis and T.L. Einstein, Phys.
Rev. B 62, 13697 (2000)] we show that the circular solution remains linearly
stable for arbitrarily large driving forces. The numerical solution of the full
nonlinear problem nevertheless reveals a fingering instability at the trailing
end of the island, which develops from finite amplitude perturbations and
eventually leads to pinch-off. Relaxing the condition of instantaneous
attachment/detachment kinetics, we obtain non-circular elongated stationary
shapes in an analytic approximation which compares favorably to the full
numerical solution.Comment: 12 page
GRB 071028B, a burst behind large amounts of dust in an unabsorbed galaxy
We report on the discovery and properties of the fading afterglow and
underlying host galaxy of GRB 071028B, thereby facilitating a detailed
comparison between these two. Observations were performed with the Gamma-ray
Burst Optical and Near-infrared Detector at the 2.2 m telescope on the La Silla
Paranal Observatory in Chile. We conducted five observations from 1.9 d to
227.2 d after the trigger and obtained deep images in the g'r'i'z' and JHKs
bands. Based on accurate seven-channel photometry covering the optical to
near-infrared wavelength range, we derive a photometric redshift of z = 0.94
+0.05 -0.10 for the unabsorbed host galaxy of GRB 071028B. In contrast, we show
that the afterglow with an intrinsic extinction of AV(SB) = (0.70 +/- 0.11) mag
is moderately absorbed and requires a relatively flat extinction curve.
According to the reported Swift/BAT observations, the energetics yield an
isotropic energy release of E(gamma,iso.,rest) = (1.4 +2.4 -0.7) x 10^51 erg.Comment: 8 pages, 5 figures, accepted for publication in A&
Large periodic time variations of termination shock particles between ~0.5-20 mev and 6-14 mev electrons measured by the crs experiment on Voyager 2 as it crossed into the heliosheath in 2007: An example of freshly accelerated cosmic rays?
We have examined features in the structure of the heliosheath using the fine scale time variations of termination shock particles (TSP) between ~0.5 - 20 MeV and electrons between 2.5-14 MeV measured by the CRS instrument as the V2 spacecraft crossed the heliospheric termination shock in 2007. The very disturbed heliosheath at V2 is particularly noteworthy for strong periodic intensity variations of the TSP just after V2 crossed the termination shock (2007.66) reaching a maximum between 2007.75 and 2008.0. A series of 42/21 day periodicities was observed at V2 along with spectral changes of low energy TSP and the acceleration of 6-14 MeV electrons. Evidence is presented for the acceleration of TSP and electrons at the times of the 42/21 day periodicities just after V2 crossed the HTS. Spectra for TSP between 2-20 MeV and electrons between 2.5-14 MeV are derived for three time periods including the time of the HTS crossing. The energy spectra of TSP and electrons at these times of intensity peaks are very similar above ~3 MeV, with exponents of a power law spectrum between -3.0 and -3.6. The ratio of TSP intensities to electron intensities at the same energy is ~500. The electron intensity peaks and minima are generally out of phase with those of nuclei by ~1/2 of a 42 day cycle. These charge dependent intensity differences and the large periodic intensity changes could provide new clues as to a possible acceleration mechanism
Direct glass bonded high specific power silicon solar cells for space applications
A lightweight, radiation hard, high performance, ultra-thin silicon solar cell is described that incorporates light trapping and a cover glass as an integral part of the device. The manufacturing feasibility of high specific power, radiation insensitive, thin silicon solar cells was demonstrated experimentally and with a model. Ultra-thin, light trapping structures were fabricated and the light trapping demonstrated experimentally. The design uses a micro-machined, grooved back surface to increase the optical path length by a factor of 20. This silicon solar cell will be highly tolerant to radiation because the base width is less than 25 microns making it insensitive to reduction in minority carrier lifetime. Since the silicon is bonded without silicone adhesives, this solar cell will also be insensitive to UV degradation. These solar cells are designed as a form, fit, and function replacement for existing state of the art silicon solar cells with the effect of simultaneously increasing specific power, power/area, and power supply life. Using a 3-mil thick cover glass and a 0.3 g/sq cm supporting Al honeycomb, a specific power for the solar cell plus cover glass and honeycomb of 80.2 W/Kg is projected. The development of this technology can result in a revolutionary improvement in high survivability silicon solar cell products for space with the potential to displace all existing solar cell technologies for single junction space applications
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