7,507 research outputs found
Dynamic structure factor for 3He in two-dimensions
Recent neutron scattering experiments on 3He films have observed a zero-sound
mode, its dispersion relation and its merging with -and possibly emerging from-
the particle-hole continuum. Here we address the study of the excitations in
the system via quantum Monte Carlo methods: we suggest a practical scheme to
calculate imaginary time correlation functions for moderate-size fermionic
systems. Combined with an efficient method for analytic continuation, this
scheme affords an extremely convincing description of the experimental
findings.Comment: 5 pages, 5 figure
Lithium-7 surface abundance in pre-MS stars. Testing theory against clusters and binary systems
The disagreement between theoretical predictions and observations for surface
lithium abundance in stars is a long-standing problem, which indicates that the
adopted physical treatment is still lacking in some points. However, thanks to
the recent improvements in both models and observations, it is interesting to
analyse the situation to evaluate present uncertainties. We present a
consistent and quantitative analysis of the theoretical uncertainties affecting
surface lithium abundance in the current generation of models. By means of an
up-to-date and well tested evolutionary code, FRANEC, theoretical errors on
surface 7Li abundance predictions, during the pre-main sequence (pre-MS) and
main sequence (MS) phases, are discussed in detail. Then, the predicted surface
7Li abundance was tested against observational data for five open clusters,
namely Ic 2602, \alpha Per, Blanco1, Pleiades, and Ngc 2516, and for four
detached double-lined eclipsing binary systems. Stellar models for the
aforementioned clusters were computed by adopting suitable chemical
composition, age, and mixing length parameter for MS stars determined from the
analysis of the colour-magnitude diagram of each cluster. We restricted our
analysis to young clusters, to avoid additional uncertainty sources such as
diffusion and/or radiative levitation efficiency. We confirm the disagreement,
within present uncertainties, between theoretical predictions and 7Li
observations for standard models. However, we notice that a satisfactory
agreement with observations for 7Li abundance in both young open clusters and
binary systems can be achieved if a lower convection efficiency is adopted
during the pre-MS phase with respect to the MS one.Comment: 10 pages, 5 figures. Accepted for publication in A&
Effect of planet ingestion on low-mass stars evolution: the case of 2MASS J08095427--4721419 star in the Gamma Velorum cluster
We analysed the effects of planet ingestion on the characteristics of a
pre-MS star similar to the Gamma Velorum cluster member 2MASS
J08095427--4721419 (#52). We discussed the effects of changing the age at
which the accretion episode occurs, the mass of the ingested planet and its
chemical composition. We showed that the mass of the ingested planet required
to explain the current [Fe/H]^#52 increases by decreasing the age and/or
by decreasing the Iron content of the accreted matter.
We compared the predictions of a simplified accretion method -- where only
the variation of the surface chemical composition is considered -- with that of
a full accretion model that properly accounts for the modification of the
stellar structure. We showed that the two approaches result in different
convective envelope extension which can vary up to 10 percent. We discussed the
impact of the planet ingestion on a stellar model in the colour-magnitude
diagram, showing that a maximum shift of about 0.06 dex in the colour and 0.07
dex in magnitude are expected and that such variations persist even much later
the accretion episode. We also analysed the systematic bias in the stellar mass
and age inferred by using a grid of standard non accreting models to recover
the characteristics of an accreting star. We found that standard non accreting
models can safely be adopted for mass estimate, as the bias is <= 6 percent,
while much more caution should be used for age estimate where the differences
can reach about 60 percent.Comment: Accepted for publication in MNRAS. 13 pages, 3 tables, 9 figure
Astrophysical implications of the proton-proton cross section updates
The p(p,e^+ \nu_e)^2H reaction rate is an essential ingredient for
theoretical computations of stellar models. In the past several values of the
corresponding S-factor have been made available by different authors. Prompted
by a recent evaluation of S(E), we analysed the effect of the adoption of
different proton-proton reaction rates on stellar models, focusing, in
particular, on the age of mid and old stellar clusters (1-12 Gyr) and on
standard solar model predictions. By comparing different widely adopted p(p,e^+
\nu_e)^2H reaction rates, we found a maximum difference in the temperature
regimes typical of main sequence hydrogen-burning stars (5x10^6 - 3x10^7 K) of
about 3%. Such a variation translates into a change of cluster age
determination lower than 1%. A slightly larger effect is observed in the
predicted solar neutrino fluxes with a maximum difference, in the worst case,
of about 8%. Finally we also notice that the uncertainty evaluation of the
present proton-proton rate is at the level of few \permil, thus the p(p,e^+
\nu_e)^2H reaction rate does not constitute anymore a significant uncertainty
source in stellar models.Comment: accepte
Structure, rotational dynamics, and superfluidity of small OCS-doped He clusters
The structural and dynamical properties of OCS molecules solvated in Helium
clusters are studied using reptation quantum Monte Carlo, for cluster sizes
n=3-20 He atoms. Computer simulations allow us to establish a relation between
the rotational spectrum of the solvated molecule and the structure of the He
solvent, and of both with the onset of superfluidity. Our results agree with a
recent spectroscopic study of this system, and provide a more complex and
detailed microscopic picture of this system than inferred from experiments.Comment: 4 pages. TeX (requires revtex4) + 3 ps figures (1 color
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