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

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 ($\alpha_{mlt}$). Using this grid relaxes assumptions used in all previous GBM work where the initial composition is determined by a single parameter and that $\alpha_{mlt}$ 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 $\alpha_{mlt}$ 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 $\alpha_{mlt}$. 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 $\alpha_{mlt}$. 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 $\alpha_{mlt}$ values without resourcing to additional information.Comment: To appear in the Proceedings of the joint TASC2/KASC9 workshop - SPACEINN & HELAS8 conference. 4 page

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

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, $\alpha_{\rm MLT}$, 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 P$_{\rm rot}< 7$d. 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