7,871 research outputs found
Measuring ^{12}C(&alpha,&gamma)^{16}O from White Dwarf Asteroseismology
During helium burning in the core of a red giant, the relative rates of the
3&alpha and ^{12}C(&alpha,&gamma)^{16}O reactions largely determine the final
ratio of carbon to oxygen in the resulting white dwarf star. The uncertainty in
the 3&alpha reaction at stellar energies due to the extrapolation from
high-energy laboratory measurements is relatively small, but this is not the
case for the ^{12}C(&alpha,&gamma)^{16}O reaction. Recent advances in the
analysis of asteroseismological data on pulsating white dwarf stars now make it
possible to obtain precise measurements of the central ratio of carbon to
oxygen, providing a more direct way to measure the ^{12}C(&alpha,&gamma)^{16}O
reaction rate at stellar energies. We assess the systematic uncertainties of
this approach and quantify small shifts in the measured central oxygen
abundance originating from the observations and from model settings that are
kept fixed during the optimization. Using new calculations of white dwarf
internal chemical profiles, we find a rate for the ^{12}C(&alpha,&gamma)^{16}O
reaction that is significantly higher than most published values. The accuracy
of this method may improve as we modify some of the details of our description
of white dwarf interiors that were not accessible through previous
model-fitting methods.Comment: 8 pages, 4 figures, 3 tables, uses emulateapj5.sty, Accepted for
publication in the Astrophysical Journa
Selection of high-z supernovae candidates
Deep, ground based, optical wide-field supernova searches are capable of
detecting a large number of supernovae over a broad redshift range up to z~1.5.
While it is practically unfeasible to obtain spectroscopic redshifts of all the
supernova candidates right after the discovery, we show that the magnitudes and
colors of the host galaxies, as well as the supernovae, can be used to select
high-z supernova candidates, for subsequent spectroscopic and photometric
follow-up.
Using Monte-Carlo simulations we construct criteria for selecting galaxies in
well-defined redshift bands. For example, with a selection criteria using B-R
and R-I colors we are able to pick out potential host galaxies for which z>0.85
with 80% confidence level and with a selection efficiency of 64-86%. The method
was successfully tested using real observations from the HDF.
Similarly, we show that that the magnitude and colors of the supernova
discovery data can be used to constrain the redshift. With a set of cuts based
on V-R and R-I in a search to m_I~25, supernovae at z~1 can be selected in a
redshift interval sigma_z <0.15.Comment: 33 pages, 13 figures, accepted for publication in PASP (March 2002
issue
The core/envelope symmetry in pulsating stars
We demonstrate that there is an inherent symmetry in the way high-overtone
stellar pulsations sample the core and the envelope, which can potentially lead
to an ambiguity in the asteroseismologically derived locations of internal
structures. We provide an intuitive example of the source of this symmetry by
analogy with a vibrating string. For the stellar case, we focus on the white
dwarf stars, establishing the practical consequences for high-order white dwarf
pulsations both analytically and numerically. In addition, we verify the effect
empirically by cross-fitting two different structural models, and we discuss
the consequences that this approximate symmetry may have for past and present
asteroseismological fits of the pulsating DBV, GD 358. Finally, we show how the
signatures of composition transition zones that are brought about by physically
distinct processes may be used to help alleviate this potential ambiguity in
our asteroseismological interpretation of the pulsation frequencies observed in
white dwarf stars.Comment: Accepted for publication in MNRAS; 8 pages, 8 figure
Galaxy number counts- IV. surveying the Herschel deep field in the near-infrared
(abridged) We present results from two new near-infrared imaging surveys. One
covers 47.2 arcmin^2 to K(3\sigma)<20 whilst a second, deeper survey covers a
sub-area of 1.8 arcmin^2 to K(3\sigma)<22.75. Over the entire area we have
extremely deep UBRI photometry. Our K- counts are consistent with the
predictions of non-evolving models with 0 < q0 <0.5. The K-selected (B-K)
galaxy colour distributions move sharply bluewards fainter than K~20 and at at
brighter magnitudes (K<20) our observed colour distributions indicate a
deficiency of red, early-type galaxies at z~1 in comparison with passively
evolving models. This implies either a pure luminosity evolution (PLE) model
with a low level of continuing star-formation following an an initial burst, or
dynamical merging. At fainter magnitudes, the continuing bluewards trend
observed in (B-K) can be explained purely in terms of passively evolving PLE
models. Our observed numbers of (I-K)>4 galaxies at K<20 exhibit the same
deficiency, suggesting that at least part of the larger deficit observed in
(B-K) at K<20 may be due to star-formation rather than dynamical merging.
Finally, as we and others have noted, the number-redshift distribution at
18<K<19 of recent, deep K- selected redshift surveys is well fitted by
non-evolving models; passively evolving models with a Salpeter or Scalo initial
mass functions overpredict the numbers of galaxies with z>1. Dynamical merging
is one possible solution to reduce the numbers of these galaxies but a
dwarf-dominated IMF for early-type galaxies could offer an alternative
explanation; we show that this model reproduces both the optical-infrared
colour distributions and the K- band galaxy counts.Comment: 15 pages, 9 figures, revised version, requires astrobib.sty,
mn-abs.sty, submitted to MNRA
Evolutionary Timescale of the DAV G117-B15A: The Most Stable Optical Clock Known
We observe G117-B15A, the most precise optical clock known, to measure the
rate of change of the main pulsation period of this blue-edge DAV white dwarf.
Even though the obtained value is only within 1 sigma, Pdot = (2.3 +/- 1.4) x
10^{-15} s/s, it is already constraining the evolutionary timescale of this
cooling white dwarf star.Comment: Accepted for publication in ApJ
The effect of 3-He diffusion on the pulsational spectra of DBV models
We consider envelopes of DB white dwarfs which are not composed of pure 4-He,
but rather a mixture of 3-He and 4-He. Given this assumption, the same
diffusive processes which produce a relatively pure H layer overlying a He
layer in the DA's should work to produce a 3-He layer overlying a 4-He layer in
the DB's. We examine the relevant timescales for diffusion in these objects,
and compare them to the relevant evolutionary timescales in the context of the
DBV white dwarfs. We then explore the consequences which 3-He separation has on
the pulsational spectra of DBV models. Since GD 358 is the best-studied member
of this class of variables, we examine fits to its observed pulsation spectrum.
We find that the inclusion of a 3-He layer results in a modest improvement in a
direct fit to the periods, while a fit to the period spacings is significantly
improved.Comment: 4 pages, 4 figure
Coordination between proteasome impairment and caspase activation leading to TAU pathology: neuroprotection by cAMP
Neurofibrillary tangles (NFTs) are hallmarks of Alzheimer’s disease (AD). The main component of NFTs is TAU, a highly soluble microtubule-associated protein. However, when TAU is cleaved at Asp421 by caspases it becomes prone to aggregation leading to NFTs. What triggers caspase activation resulting in TAU cleavage remains unclear. We investigated in rat cortical neurons a potential coordination between proteasome impairment and caspase activation. We demonstrate that upon proteasome inhibition, the early accumulation of detergent-soluble ubiquitinated (SUb) proteins paves the way to caspase activation and TAU pathology. This occurs with two drugs that inhibit the proteasome by different means: the product of inflammation prostaglandin J2 (PGJ2) and epoxomicin. Our results pinpoint a critical early event, that is, the buildup of SUb proteins that contributes to caspase activation, TAU cleavage, TAU/Ub-protein aggregation and neuronal death. Furthermore, to our knowledge, we are the first to demonstrate that elevating cAMP in neurons with dibutyryl-cAMP (db-cAMP) or the lipophilic peptide PACAP27 prevents/ diminishes caspase activation, TAU cleavage and neuronal death induced by PGJ2, as long as these PGJ2-induced changes are moderate. db-cAMP also stimulated proteasomes, and mitigated proteasome inhibition induced by PGJ2. We propose that targeting cAMP/PKA to boost proteasome activity in a sustainable manner could offer an effective approach to avoid early accumulation of SUb proteins and later caspase activation, and TAU cleavage, possibly preventing/delaying AD neurodegeneration
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