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Cerdanyola del Vallès, Spain

Serenelli A.,Institute Ciencias del Espacio ICE CSIC IEEC
European Physical Journal A | Year: 2016

Standard solar models (SSMs) provide a reference framework across a number of research fields: solar and stellar models, solar neutrinos, particle physics the most conspicuous among them. The accuracy of the physical description of the global properties of the Sun that SSMs provide has been challenged in the last decade by a number of developments in stellar spectroscopic techniques. Over the same period of time, solar neutrino experiments, and Borexino in particular, have measured the four solar neutrino fluxes from the pp-chains that are associated with 99% of the nuclear energy generated in the Sun. Borexino has also set the most stringent limit on CNO energy generation, only ∼ 40% larger than predicted by SSMs. More recently, and for the first time, radiative opacity experiments have been performed at conditions that closely resemble those at the base of the solar convective envelope. In this article, we review these developments and discuss the current status of SSMs, including its intrinsic limitations. © 2016, SIF, Springer-Verlag Berlin Heidelberg. Source

Silva Aguirre V.,University of Aarhus | Davies G.R.,University of Aarhus | Davies G.R.,University of Birmingham | Basu S.,Yale University | And 32 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2015

We present a study of 33 Kepler planet-candidate host stars for which asteroseismic observations have sufficiently high signal-to-noise ratio to allow extraction of individual pulsation frequencies. We implement a new Bayesian scheme that is flexible in its input to process individual oscillation frequencies, combinations of them, and average asteroseismic parameters, and derive robust fundamental properties for these targets. Applying this scheme to grids of evolutionary models yields stellar properties with median statistical uncertainties of 1.2 per cent (radius), 1.7 per cent (density), 3.3 per cent (mass), 4.4 per cent (distance), and 14 per cent (age), making this the exoplanet host-star sample with the most precise and uniformly determined fundamental parameters to date. We assess the systematics from changes in the solar abundances and mixing-length parameter, showing that they are smaller than the statistical errors.We also determine the stellar propertieswith three other fitting algorithms and explore the systematics arising from using different evolution and pulsation codes, resulting in 1 per cent in density and radius, and 2 per cent and 7 per cent in mass and age, respectively. We confirm previous findings of the initial helium abundance being a source of systematics comparable to our statistical uncertainties, and discuss future prospects for constraining this parameter by combining asteroseismology and data from space missions. Finally, we compare our derived properties with those obtained using the global average asteroseismic observables along with effective temperature and metallicity, finding excellent level of agreement. Owing to selection effects, our results show that the majority of the high signal-to-noise ratio asteroseismic Kepler host stars are older than the Sun. © 2015 The Authors. Source

Chaplin W.J.,University of Birmingham | Chaplin W.J.,University of Aarhus | Lund M.N.,University of Birmingham | Lund M.N.,University of Aarhus | And 62 more authors.
Publications of the Astronomical Society of the Pacific | Year: 2015

We present the first detections by the NASA K2 mission of oscillations in solar-type stars, using short-cadence data collected during K2 Campaign 1 (C1). We understand the asteroseismic detection thresholds for C1-like levels of photometric performance, and we can detect oscillations in subgiants having dominant oscillation frequencies around 1000 μHz. Changes to the operation of the fine-guidance sensors are expected to give significant improvements in the high-frequency performance from C3 onwards. A reduction in the excess high-frequency noise by a factor of 2.5 in amplitude would bring main-sequence stars with dominant oscillation frequencies as high as ≃2500 μHz into play as potential asteroseismic targets for K2. © 2015, The Astronomical Society of the Pacific. All rights reserved. Source

Casagrande L.,The Australian National University | Casagrande L.,University of California at Santa Barbara | Silva Aguirre V.,University of California at Santa Barbara | Silva Aguirre V.,University of Aarhus | And 17 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2016

The existence of a vertical age gradient in the Milky Way disc has been indirectly known for long. Here, wemeasure it directly for the first time with seismic ages, using red giants observed by Kepler. We use Strömgren photometry to gauge the selection function of asteroseismic targets, and derive colour and magnitude limits where giants with measured oscillations are representative of the underlying population in the field. Limits in the 2MASS system are also derived. We lay out a method to assess and correct for target selection effects independent of Galaxy models. We find that low-mass, i.e. old red giants dominate at increasing Galactic heights, whereas closer to theGalactic plane they exhibit a wide range of ages and metallicities. Parametrizing this as a vertical gradient returns approximately 4 Gyr kpc-1 for the disc we probe, although with a large dispersion of ages at all heights. The ages of stars show a smooth distribution over the last ≃10 Gyr, consistent with a mostly quiescent evolution for the Milky Way disc since a redshift of about 2. We also find a flat age-metallicity relation for disc stars. Finally, we show how to use secondary clump stars to estimate the present-day intrinsic metallicity spread, and suggest using their number count as a new proxy for tracing the ageing of the disc. This work highlights the power of asteroseismology for Galactic studies; however, we also emphasize the need for better constraints on stellar mass-loss, which is a major source of systematic age uncertainties in red giant stars. © 2015 The Authors. Source

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