856 research outputs found
Burst Neutrinos from Nitrogen Flash
Neutrinos give a novel probe to explore deep interior of astrophysical
objects, which otherwise is not accessible with optical observations; among
notable examples are solar and supernova neutrinos. We show that there is a new
class of strong neutrino emission from helium burning, N + alpha --> 18F gamma
followed by beta decay 18F --> 18O + e+ + nu_e, that gives a maximum neutrino
luminosity of 10^8 times the solar bolometric luminosity at the helium-core
flash of a 1 M_sun star, whereas the flash is not observable by optical means.
This means that the neutrino flux, of average energy of 0.382 MeV, will be 10%
the solar CNO neutrino flux on Earth if the star is located at 10pc.Comment: Accepted to ApJ Letters. Replaced version with extended discussion.
Some additional references adde
Bayesian analysis of ages, masses, and distances to cool stars with non-LTE spectroscopic parameters
For studies of Galactic evolution, the accurate characterization of stars in
terms of their evolutionary stage and population membership is of fundamental
importance. A standard approach relies on extracting this information from
stellar evolution models but requires the effective temperature, surface
gravity, and metallicity of a star obtained by independent means. In previous
work, we determined accurate effective temperatures and non-LTE logg and [Fe/H]
(NLTE-Opt) for a large sample of metal-poor stars, -3<[Fe/H]<-0.5, selected
from the RAVE survey. As a continuation of that work, we derive here their
masses, ages, and distances using a Bayesian scheme and GARSTEC stellar tracks.
For comparison, we also use stellar parameters determined from the widely-used
1D LTE excitation-ionization balance of Fe (LTE-Fe). We find that the latter
leads to systematically underestimated stellar ages, by 10-30%, but
overestimated masses and distances. Metal-poor giants suffer from the largest
fractional distance biases of 70%. Furthermore, we compare our results with
those released by the RAVE collaboration for the stars in common (DR3, Zwitter
et al. 2010, Seibert et al. 2011). This reveals -400 to +400 K offsets in
effective temperature, -0.5 to 1.0 dex offsets in surface gravity, and 10 to
70% in distances. The systematic trends strongly resemble the correlation we
find between the NLTE-Opt and LTE-Fe parameters, indicating that the RAVE DR3
data may be affected by the physical limitations of the 1D LTE synthetic
spectra. Our results bear on any study, where spectrophotometric distances
underlie stellar kinematics. In particular, they shed new light on the debated
controversy about the Galactic halo origin raised by the SDSS/SEGUE
observations.Comment: 13 pages and 15 figures. Accepted for publication in MNRA
A Quantitative Analysis of the Solar Composition Problem
Abstract We perform a quantitative analysis of the solar composition problem by using a statistical approach that allows us to combine the information provided by helioseismic and solar neutrino data in an effective way. We show that the opacity profile of the Sun is well constrained by the solar observational properties. In the context of a two parameter analysis in which elements are grouped as volatiles (i.e. C, N, O and Ne) and refractories (i.e. Mg, Si, S, Fe), the optimal surface composition is found by increasing the abundance of volatiles by (45 ± 4) % and that of refractories by (19 ± 3) % with respect to the values provided by Asplund et al., 2009. As an additional result of our analysis, we show that the best fit to the observational data is obtained with values of input parameters of the standard solar models (radiative opacities, gravitational settling rate, the astrophysical factors S 34 and S 17) that differ at the ∼ 1σ level from those presently adopted
The more the merrier: grid based modelling of Kepler dwarfs with 5-dimensional stellar grids
We present preliminary results of our grid based modelling (GBM) of the
dwarf/subgiant sample of stars observed with Kepler including global
asteroseismic parameters. GBM analysis in this work is based on a large grid of
stellar models that is characterized by five independent parameters: model mass
and age, initial metallicity (\zini), initial helium (\yini), and mixing
length parameter (). Using this grid relaxes assumptions used in
all previous GBM work where the initial composition is determined by a single
parameter and that is fixed to a solar-calibrated value. The new
grid allows us to study, for example, the impact of different galactic chemical
enrichment models on the determination of stellar parameters such as mass
radius and age. Also, it allows to include new results from stellar atmosphere
models on in the GBM analysis in a simple manner. Alternatively,
it can be tested if global asteroseismology is a useful tool to constraint our
ignorance on quantities such as \yini and . Initial findings
show that mass determination is robust with respect to freedom in the latter
quantities, with a 4.4\% maximum deviation for extreme assumptions regarding
prior information on \yini-\zini relations and . On the other
hand, tests carried out so far seem to indicate that global seismology does not
have much power to constrain \yini-\zini relations of values
without resourcing to additional information.Comment: To appear in the Proceedings of the joint TASC2/KASC9 workshop -
SPACEINN & HELAS8 conference. 4 page
Solar Neutrinos
The study of solar neutrinos has given since ever a fundamental contribution
both to astroparticle and to elementary particle physics, offering an ideal
test of solar models and offering at the same time relevant indications on the
fundamental interactions among particles. After reviewing the striking results
of the last two decades, which were determinant to solve the long standing
solar neutrino puzzle and refine the Standard Solar Model, we focus our
attention on the more recent results in this field and on the experiments
presently running or planned for the near future. The main focus at the moment
is to improve the knowledge of the mass and mixing pattern and especially to
study in detail the lowest energy part of the spectrum, which represents most
of solar neutrino spectrum but is still a partially unexplored realm. We
discuss this research project and the way in which present and future
experiments could contribute to make the theoretical framemork more complete
and stable, understanding the origin of some "anomalies" that seem to emerge
from the data and contributing to answer some present questions, like the exact
mechanism of the vacuum to matter transition and the solution of the so called
solar metallicity problem.Comment: 51 pages, to be published in Special Issue on Neutrino Physics,
Advances in High Energy Physics Hindawi Publishing Corporation 201
Bayesian mass and age estimates for transiting exoplanet host stars
The mean density of a star transited by a planet, brown dwarf or low mass
star can be accurately measured from its light curve. This measurement can be
combined with other observations to estimate its mass and age by comparison
with stellar models. Our aim is to calculate the posterior probability
distributions for the mass and age of a star given its density, effective
temperature, metallicity and luminosity. We computed a large grid of stellar
models that densely sample the appropriate mass and metallicity range. The
posterior probability distributions are calculated using a Markov-chain
Monte-Carlo method. The method has been validated by comparison to the results
of other stellar models and by applying the method to stars in eclipsing binary
systems with accurately measured masses and radii. We have explored the
sensitivity of our results to the assumed values of the mixing-length
parameter, , and initial helium mass fraction, Y. For a star
with a mass of 0.9 solar masses and an age of 4 Gyr our method recovers the
mass of the star with a precision of 2% and the age to within 25% based on the
density, effective temperature and metallicity predicted by a range of
different stellar models. The masses of stars in eclipsing binaries are
recovered to within the calculated uncertainties (typically 5%) in about 90% of
cases. There is a tendency for the masses to be underestimated by about 0.1
solar masses for some stars with rotation periods Pd. Our method
makes it straightforward to determine accurately the joint posterior
probability distribution for the mass and age of a star eclipsed by a planet or
other dark body based on its observed properties and a state-of-the art set of
stellar models.Comment: Accepted for publication in A&A. 9 pages, 4 figures. Source code for
the software described is available from
http://sourceforge.net/projects/bagemas
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