1,048 research outputs found
Effects of dark matter annihilation on the first stars
We study the evolution of the first stars in the universe (Population III)
from the early pre-Main Sequence until the end of helium burning in the
presence of WIMP dark matter annihilation inside the stellar structure. The two
different mechanisms that can provide this energy source are the contemporary
contraction of baryons and dark matter, and the capture of WIMPs by scattering
off the gas with subsequent accumulation inside the star. We find that the
first mechanism can generate an equilibrium phase, previously known as a "dark
star", which is transient and present in the very early stages of pre-MS
evolution. The mechanism of scattering and capture acts later, and can support
the star virtually forever, depending on environmental characteristic of the
dark matter halo and on the specific WIMP model.Comment: Proceedings of the IAU Symposium 255, "Low-Metallicity Star
Formation: From the First Stars to Dwarf Galaxies"; L.K. Hunt, S. Madden and
R. Schneider ed
Life Products of Stars
We attempt to document complete energetic transactions of stars in their
life. We calculate photon and neutrino energies that are produced from stars in
their each phase of evolution from 1 to 8 M_sun, using the state-of-the-art
stellar evolution code, tracing the evolution continuously from pre-main
sequence gravitational contraction to white dwarfs. We also catalogue
gravitational and thermal energies and helium, and heavier elements that are
stored in stars and those ejected into interstellar space in each evolutionary
phase.Comment: 26 pages, including 8 figures and 3 tables. Submitted to ApJ
Evolution and Yields of Extremely Metal Poor Intermediate Mass Stars
Intermediate mass stellar evolution tracks from the main sequence to the tip
of the AGB for five initial masses (2 to 6Msun) and metallicity Z=0.0001 have
been computed. The detailed 1D structure and evolution models include
exponential overshooting, mass loss and a detailed nucleosynthesis network with
updated nuclear reaction rates. The network includes a two-particle heavy
neutron sink for approximating neutron density in the He-shell flash. It is
shown how the neutron-capture nucleosynthesis is important in models of very
low metallicity for the formation of light neutron-heavy species, like sodium
or the heavy neon and magnesium isotopes. The models have high resolution, as
required for modeling the third dredge-up. All sequences have been followed
from the pre-main sequence to the end of the AGB when all envelope mass is
lost. Detailed structural and chemical model properties as well as yields are
presented. This set of stellar models is based on standard assumptions and
updated input physics. It can be confronted with observations of
extremely-metal poor stars and may be used to assess the role of AGB stars in
the origin of abundance anomalies of some Globular Cluster members of
correspondingly low metallicity.Comment: 40 pages, 11 figures, to appear in ApJS, including 5 electronic
table
The Ocular Albinism type 1 (OA1) G-Protein Coupled Receptor functions with MART-1 at early stages of melanogenesis to control melanosome identity and composition
OA1 (GPR143; GPCR, G-protein-coupled receptor), the protein product of the ocular albinism type 1 gene, encodes a pigment-cell-specific GPCR that localizes intracellularly to melanosomes. OA1 mutations result in ocular albinism due to alterations in melanosome formation, suggesting that OA1 is a key player in the biogenesis of melanosomes. To address the function of OA1 in melanosome biogenesis, we have used siRNA inactivation and combined morphological and biochemical methods to investigate melanosome ultrastructure, melanosomal protein localization and expression in human pigmented melanocytic cells. OA1 loss of function leads to decreased pigmentation and causes formation of enlarged aberrant premelanosomes harboring disorganized fibrillar structures and displaying proteins of mature melanosomes and lysosomes at their membrane. Moreover, we show that OA1 interacts biochemically with the premelanosomal protein MART-1. Inactivation of MART-1 by siRNA leads to a decreased stability of OA1 and is accompanied by similar defects in premelanosome biogenesis and composition. These data show for the first time that melanosome composition and identity are regulated at early stages by OA1 and that MART-1 likely acts as an escort protein for this GPCR
The s-Process in Rotating Asymptotic Giant Branch Stars
(abridged) We model the nucleosynthesis during the thermal pulse phase of a
rotating, solar metallicity AGB star of 3M_sun. Rotationally induced mixing
during the thermal pulses produces a layer (~2E-5M_sun) on top of the CO-core
where large amounts of protons and C12 co-exist. We follow the abundance
evolution in this layer, in particular that of the neutron source C13 and of
the neutron poison N14. In our AGB model mixing persists during the entire
interpulse phase due to the steep angular velocity gradient at the
core-envelope interface. We follow the neutron production during the interpulse
phase, and find a resulting maximum neutron exposure of tau_max =0.04 mbarn^-1,
which is too small to produce any significant s-process. In parametric models,
we then investigate the combined effects of diffusive overshooting from the
convective envelope and rotationally induced mixing. Models with overshoot and
weaker interpulse mixing - as perhaps expected from more slowly rotating stars
- yield larger neutron exposures. We conclude that the incorporation of
rotationally induce mixing processes has important consequences for the
production of heavy elements in AGB stars. Through a distribution of initial
rotation rates it may lead to a natural spread in the neutron exposures
obtained in AGB stars of a given mass - as appears to be required by
observations. Our results suggest that both processes, diffusive overshoot and
rotational mixing, may be required to obtain a consistent description of the
s-process in AGB stars which fulfils all observational constraints. Finally, we
find that mixing due to rotation within our current framework does increase the
production of N15 in the partial mixing zone, however still falling short of
what seems required by observations.Comment: 50 pages, 13 figures, ApJ in press, tentatively scheduled for v593 n2
August 20, 200
The Contribution of TP-AGB and RHeB Stars to the Near-IR Luminosity of Local Galaxies: Implications for Stellar Mass Measurements of High Redshift Galaxies
Using high spatial resolution HST WFC3 and ACS imaging of resolved stellar
populations, we constrain the contribution of thermally-pulsing asymptotic
giant branch (TP-AGB) stars and red helium burning (RHeB) stars to the 1.6 um
near-infrared (NIR) luminosities of 23 nearby galaxies. The TP-AGB phase
contributes as much as 17% of the integrated F160W flux, even when the red
giant branch is well populated. The RHeB population contribution can match or
even exceed the TP-AGB contribution, providing as much as 21% of the integrated
F160W light. The NIR mass-to-light (M/L) ratio should therefore be expected to
vary significantly due to fluctuations in the star formation rate over
timescales from 25 Myr to several Gyr. We compare our observational results to
predictions based on optically derived star formation histories and stellar
population synthesis (SPS) models, including models based on the Padova
isochrones (used in popular SPS programs). The SPS models generally reproduce
the expected numbers of TP-AGB stars in the sample. The same SPS models,
however, give a larger discrepancy in the F160W flux contribution from the
TP-AGB stars, over-predicting the flux by a weighted mean factor of 2.3 +/-0.8.
This larger offset is driven by the prediction of modest numbers of high
luminosity TP-AGB stars at young (<300 Myrs) ages. The best-fit SPS models
simultaneously tend to under-predict the numbers and fluxes of stars on the
RHeB sequence, typically by a factor of 2.0+/-0.6 for galaxies with significant
numbers of RHeBs. Coincidentally, over-prediction of the TP-AGB and
under-prediction of the RHeBs result in a NIR M/L ratio largely unchanged for a
rapid star formation rate. However, the NIR-to-optical flux ratio of galaxies
could be significantly smaller than AGB-rich models would predict, an outcome
that has been observed in some intermediate redshift post-starburst galaxies.
(Abridged)Comment: 28 Pages, 12 Figures, 5 Tables, Accepted for Publication in the
Astrophysical Journa
A Dramatic Decrease in Carbon Star Formation in M31
We analyze resolved stellar near-infrared photometry of 21 HST fields in M31
to constrain the impact of metallicity on the formation of carbon stars.
Observations of nearby galaxies show that the carbon stars are increasingly
rare at higher metallicity. Models indicate that carbon star formation
efficiency drops due to the decrease in dredge-up efficiency in metal-rich
thermally-pulsing Asymptotic Giant Branch (TP-AGB) stars, coupled to a higher
initial abundance of oxygen. However, while models predict a metallicity
ceiling above which carbon stars cannot form, previous observations have not
yet pinpointed this limit. Our new observations reliably separate carbon stars
from M-type TP-AGB stars across 2.6-13.7 kpc of M31's metal-rich disk using HST
WFC3/IR medium-band filters. We find that the ratio of C to M stars (C/M)
decreases more rapidly than extrapolations of observations in more metal-poor
galaxies, resulting in a C/M that is too low by more than a factor of 10 in the
innermost fields and indicating a dramatic decline in C star formation
efficiency at metallicities higher than [M/H] -0.1 dex. The
metallicity ceiling remains undetected, but must occur at metallicities higher
than what is measured in M31's inner disk ([M/H] +0.06 dex).Comment: 16 pages, 13 Figures; text clarifications in response to the referee.
Results are unchanged; accepted for publication in Ap
On the typical timescale for the chemical enrichment from SNeIa in Galaxies
We calculate the type Ia supernova rate for different star formation
histories in galaxies by adopting the most popular and recent progenitor
models. We show that the timescale for the maximum in the type Ia supernova
rate, which corresponds also to time of the maximum enrichment, is not unique
but is a strong function of the adopted stellar lifetimes, initial mass
function and star formation rate. This timescale varies from Myr
for an instantaneous starburst to 0.3 Gyr for a typical elliptical
galaxy to Gyr for a disk of a spiral Galaxy like the Milky Way.
We also show that the typical timescale of 1 Gyr, often quoted as the typical
timescale for the type Ia supernovae, is just the time at which, in the solar
neighbourhood, the Fe production from supernovae Ia starts to become important
and not the time at which SNe Ia start to explode. As a cosequence of this, a
change in slope in the [O/Fe] ratio is expected in correspondance of this
timescale. We conclude that the suggested lack of supernovae Ia at low
metallicities produces results at variance with the observed [O/Fe] vs. [Fe/H]
relation in the solar region. We also compute the supernova Ia rates for
different galaxies as a function of redshift and predict an extended maximum
between redshift and for elliptical galaxies, and two
maxima, one at and the other at , for spiral galaxies,
under the assumption that galaxies start forming stars at and
, .Comment: 25 pages, 6 figures, accepted for pubblication from Ap
MASYS. The AKARI spectroscopic survey of Symbiotic Stars in the Magellanic Clouds
MASYS is the AKARI spectroscopic survey of Symbiotic Stars in the Magellanic
Clouds, and one of the European Open Time Observing Programmes approved for the
AKARI (Post-Helium) Phase-3. It is providing the first ever near-IR spectra of
extragalactic symbiotic stars. The observations are scheduled to be completed
in July 2009.Comment: 4 pages, 1 table. To appear in the Proceedings of the Conference
"AKARI, a light to illuminate the misty Universe", Fukutake Hall, The
University of Tokyo, Japan, 16-19 February 200
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