40 research outputs found
Updated stellar yields from Asymptotic Giant Branch models
An updated grid of stellar yields for low to intermediate-mass
thermally-pulsing Asymptotic Giant Branch (AGB) stars are presented. The models
cover a range in metallicity Z = 0.02, 0.008, 0.004, and 0.0001, and masses
between 1Msun to 6Msun. New intermediate-mass Z = 0.0001 AGB models are also
presented, along with a finer mass grid than used in previous studies. The
yields are computed using an updated reaction rate network that includes the
latest NeNa and MgAl proton capture rates, with the main result that between ~6
to 30 times less Na is produced by intermediate-mass models with hot bottom
burning. In low-mass AGB models we investigate the effect on the production of
light elements of including some partial mixing of protons into the intershell
region during the deepest extent of each third dredge-up episode. The protons
are captured by the abundant 12C to form a 13C pocket. The 13C pocket increases
the yields of 19F, 23Na, the neutron-rich Mg and Si isotopes, 60Fe, and 31P.
The increase in 31P is by factors of ~4 to 20, depending on the metallicity.
Any structural changes caused by the addition of the 13C pocket into the
He-intershell are ignored. However, the models considered are of low mass and
any such feedback is likely to be small. Further study is required to test the
accuracy of the yields from the partial-mixing models. For each mass and
metallicity, the yields are presented in a tabular form suitable for use in
galactic chemical evolution studies or for comparison to the composition of
planetary nebulae.Comment: Accepted for publication in MNRAS; 15 page
Asymptotic Giant Branch stars at low metallicity: the challenging interplay between mass loss and molecular opacities
We investigate the main physical properties of low-metallicity Asymptotic
Giant Branch stars, with the aim of quantifying the uncertainties that
presently affect the predicted chemical yields of these stars, associated to
mass loss and description of molecular opacities. We find that above a
threshold mass, M ~ 3.5Msun for Z=0.001, the results are little dependent on
the opacity treatment, as long as hot-bottom burning prevents the surface C/O
ratio from exceeding unity; the yields of these massive AGB stars are expected
to be mostly determined by the efficiency of convection, with a relatively mild
dependence on the mass-loss description. A much higher degree of uncertainty is
associated to the yields of less massive models, which critically depend on the
adopted molecular opacities. An interval of masses exists, say 2.0-3.0Msun,
(the exact range depends on mass loss), in which HBB may be even extinguished
following the cooling produced by the opacity of C-bearing molecules. The
yields of these stars are the most uncertain, the variation range being the
largest (up to ~ 2dex) for the nitrogen and sodium yields. For very low-mass
models, not experiencing hot-bottom burning (M< 1.5Msun),the description of
mass loss and the treatment of the convective boundaries are crucial for the
occurrence of the third dredge-up, with sizable consequences on the CNO yields.Comment: 13 pages, 8 figures. Accepted for publication on MNRA
Abundance patterns of multiple populations in Globular Clusters: a chemical evolution model based on yields from AGB ejecta
A large number of spectroscopic studies have provided evidence of the
presence of multiple populations in globular clusters by revealing patterns in
the stellar chemical abundances. This paper is aimed at studying the origin of
these abundance patterns. We explore a model in which second generation (SG)
stars form out of a mix of pristine gas and ejecta of the first generation of
asymptotic giant branch stars. We first study the constraints imposed by the
spectroscopic data of SG stars in globular clusters on the chemical properties
of the asymptotic and super asymptotic giant branch ejecta. With a simple
one-zone chemical model, we then explore the formation of the SG population
abundance patterns focussing our attention on the Na-O, Al-Mg anticorrelations
and on the helium distribution function. We carry out a survey of models and
explore the dependence of the final SG chemical properties on the key
parameters affecting the gas dynamics and the SG formation process. Finally, we
use our chemical evolution framework to build specific models for NGC 2808 and
M4, two Galactic globular clusters which show different patterns in the Na-O
and Mg-Al anticorrelation and have different helium distributions. We find that
the amount of pristine gas involved in the formation of SG stars is a key
parameter to fit the observed O-Na and Mg-Al patterns. The helium distribution
function for these models is in general good agreement with the observed one.
Our models, by shedding light on the role of different parameters and their
interplay in determining the final SG chemical properties, illustrate the basic
ingredients, constraints and problems encountered in this self-enrichment
scenario which must be addressed by more sophisticated chemical and
hydrodynamic simulations.Comment: 19 pages, 10 figures, MNRAS accepte
Thermohaline instability and rotation-induced mixing. I - Low- and intermediate-mass solar metallicity stars up to the end of the AGB
(abridged) Numerous spectroscopic observations provide compelling evidence
for non-canonical processes that modify the surface abundances of low- and
intermediate-mass stars beyond the predictions of standard stellar theory. We
study the effects of thermohaline instability and rotation-induced mixing in
the 1-4 Msun range at solar metallicity. We present evolutionary models by
considering both thermohaline and rotation-induced mixing in stellar interior.
We discuss the effects of these processes on the chemical properties of stars
from the zero age main sequence up to the end of the second dredge-up on the
early-AGB for intermediate-mass stars and up to the AGB tip for low-mass stars.
Model predictions are compared to observational data for
lithium,12C/13C,[N/C],[Na/Fe],16O/17O, and 16O/18O in Galactic open clusters
and in field stars with well-defined evolutionary status,as well as in
planetary nebulae. Thermohaline mixing simultaneously accounts for the observed
behaviour of 12C/13C,[N/C], and lithium in low-mass stars that are more
luminous than the RGB bump, and its efficiency is increasing with decreasing
initial stellar mass. On the TP-AGB,thermohaline mixing leads to lithium
production, although the 7Li yields remain negative. Although the 3He stellar
yields are much reduced thanks to this process, we find that solar-metallicity,
low-mass stars remain net 3He producers. Rotation-induced mixing is found to
change the stellar structure so that in the mass range between \sim 1.5 and 2.2
Msun the thermohaline instability occurs earlier on the red giant branch than
in non-rotating models. Finally rotation accounts for the observed star-to-star
abundance variations at a given evolutionary status, and is necessary to
explain the features of CN-processed material in intermediate-mass stars.Comment: 18 pages, 22 figures, accepted for publication in A&
Gene Expression Profiling Reveals New Aspects of PIK3CA Mutation in ERalpha-Positive Breast Cancer: Major Implication of the Wnt Signaling Pathway
BACKGROUND: The PI3K/AKT pathway plays a pivotal role in breast cancer development and maintenance. PIK3CA, encoding the PI3K catalytic subunit, is the oncogene exhibiting a high frequency of gain-of-function mutations leading to PI3K/AKT pathway activation in breast cancer. PIK3CA mutations have been observed in 30% to 40% of ERα-positive breast tumors. However the physiopathological role of PIK3CA mutations in breast tumorigenesis remains largely unclear. METHODOLOGY/PRINCIPAL FINDINGS: To identify relevant downstream target genes and signaling activated by aberrant PI3K/AKT pathway in breast tumors, we first analyzed gene expression with a pangenomic oligonucleotide microarray in a series of 43 ERα-positive tumors with and without PIK3CA mutations. Genes of interest were then investigated in 249 ERα-positive breast tumors by real-time quantitative RT-PCR. A robust collection of 19 genes was found to be differently expressed in PIK3CA-mutated tumors. PIK3CA mutations were associated with over-expression of several genes involved in the Wnt signaling pathway (WNT5A, TCF7L2, MSX2, TNFRSF11B), regulation of gene transcription (SEC14L2, MSX2, TFAP2B, NRIP3) and metal ion binding (CYP4Z1, CYP4Z2P, SLC40A1, LTF, LIMCH1). CONCLUSION/SIGNIFICANCE: This new gene set should help to understand the behavior of PIK3CA-mutated cancers and detailed knowledge of Wnt signaling activation could lead to novel therapeutic strategies
Sustaining a Corpus for Spoken Turkish Discourse Accessibility and Corpus Management Issues
This paper addresses the issues of the long-term availability of language resources and the financing of resource maintenance in the context of the web-based corpus management system employed in the Spoken Turkish Corpus (STC), which operates with EXMARaLDA. Section 2 overviews the capacities of the corpus management system with respect to its software infrastructure, online presentation, metadata management, and interoperability. Section 3 describes the plan foreseen in STC for sustaining the resource, and dwells on the ethical issues surrounding the conflicting demands of free resources for non-commercial research and resource maintenance