Calibrating Convective properties of Solar-like Stars in the Kepler Field of View

Stellar models generally use simple parametrizations to treat convection. The most widely used parametrization is the so-called "Mixing Length Theory" where the convective eddy sizes are described using a single number, \alpha, the mixing-length parameter. This is a free parameter, and the general practice is to calibrate \alpha using the known properties of the Sun and apply that to all stars. Using data from NASA's Kepler mission we show that using the solar-calibrated \alpha is not always appropriate, and that in many cases it would lead to estimates of initial helium abundances that are lower than the primordial helium abundance. Kepler data allow us to calibrate \alpha for many other stars and we show that for the sample of stars we have studied, the mixing-length parameter is generally lower than the solar value. We studied the correlation between \alpha and stellar properties, and we find that \alpha increases with metallicity. We therefore conclude that results obtained by fitting stellar models or by using population-synthesis models constructed with solar values of \alpha are likely to have large systematic errors. Our results also confirm theoretical expectations that the mixing-length parameter should vary with stellar properties.Comment: 16 pages, 4 figures, accepted for publication in ApJ

Asteroseismology of the open clusters NGC 6791, NGC 6811, and NGC 6819 from nineteen months of Kepler photometry

We studied solar-like oscillations in 115 red giants in the three open clusters NGC 6791, NGC 6811, and NGC 6819, based on photometric data covering more than 19 months with NASA's Kepler space telescope. We present the asteroseismic diagrams of the asymptotic parameters \delta\nu_02, \delta\nu_01 and \epsilon, which show clear correlation with fundamental stellar parameters such as mass and radius. When the stellar populations from the clusters are compared, we see evidence for a difference in mass of the red giant branch stars, and possibly a difference in structure of the red clump stars, from our measurements of the small separations \delta\nu_02 and \delta\nu_01. Ensemble \'{e}chelle diagrams and upper limits to the linewidths of l = 0 modes as a function of \Delta\nu of the clusters NGC 6791 and NGC 6819 are also shown, together with the correlation between the l = 0 ridge width and the T_eff of the stars. Lastly, we distinguish between red giant branch and red clump stars through the measurement of the period spacing of mixed dipole modes in 53 stars among all the three clusters to verify the stellar classification from the color-magnitude diagram. These seismic results also allow us to identify a number of special cases, including evolved blue stragglers and binaries, as well as stars in late He-core burning phases, which can be potentially interesting targets for detailed theoretical modeling.Comment: 30 pages, 15 figures, 1 table, accepted to Ap

TESS asteroseismology of the known red-giant host stars HD 212771 and HD 203949

International audienc

Infância, exclusão social e educação como utopia realizável

A infância, como construção social, tem sofrido, no decurso da 2ª modernidade, processos de reinstitucionalização que, em larga medida, põem em causa as representações e imagens das crianças, dominantes nos últimos 200 anos. A análise da (re)construção das identidades sociais e das subjectividades infantis constitui, desse modo, uma tarefa teórica da mais exigente actualidade. O que, entretanto, aqui se assinala é que este processo de reinstitucionalização da infância, apesar da construção de consensos globais sobre os direitos das crianças, tem vindo a aumentar os factores e as condições de exclusão das gerações mais jovens face aos direitos sociais e da cidadania. Neste artigo inventariam-se alguns dos principais indicadores de exclusão, considerando diversos espaços estruturais, e assinalam-se alguns dos pontos de ruptura por onde pode passar a construção de uma educação escolar centrada na afirmação activa dos direitos das crianças.Along the second modernity, childhood, as a social construction, has undergone reinstatement processes that question the representations and images of children that have prevailed over last two hundred years. An analysis of the (re) construction of the social identities and childhood subjectivity thus constitute a theoretical task of the most contemporary exigence. Nevertheless, even though a global consensus about children’s rights has been constructed, the increase of factors and conditions that exclude the youngest generations from social rights and citizenship, induced by this reinstatement process of childhood, is here emphasized. This paper lists some of the main indicators of exclusion, considering different structural spaces, and pinpoints some turning points through which the construction of a school education centered on the active affirmation of the children’s rights could be attained.CIFPEC/CIEC - Centro de Investigação em Estudos da Criança, UM (UI 644 e 317 da FCT

Revisiting the Red Giant Branch Hosts KOI-3886 and ι Draconis.:Detailed Asteroseismic Modeling and Consolidated Stellar Parameters

Asteroseismology is playing an increasingly important role in the characterization of red giant host stars and their planetary systems. Here, we conduct detailed asteroseismic modeling of the evolved red giant branch (RGB) hosts KOI-3886 and ι Draconis, making use of end-of-mission Kepler (KOI-3886) and multisector TESS (ι Draconis) time-series photometry. We also model the benchmark star KIC 8410637, a member of an eclipsing binary, thus providing a direct test to the seismic determination. We test the impact of adopting different sets of observed modes as seismic constraints. Inclusion of .,"= 1 and 2 modes improves the precision of the stellar parameters, albeit marginally, compared to adopting radial modes alone, with 1.9%-3.0% (radius), 5%-9% (mass), and 19%-25% (age) reached when using all p-dominated modes as constraints. Given the very small spacing of adjacent dipole mixed modes in evolved RGB stars, the sparse set of observed g-dominated modes is not able to provide extra constraints, further leading to highly multimodal posteriors. Access to multiyear time-series photometry does not improve matters, with detailed modeling of evolved RGB stars based on (lower-resolution) TESS data sets attaining a precision commensurate with that based on end-of-mission Kepler data. Furthermore, we test the impact of varying the atmospheric boundary condition in our stellar models. We find the mass and radius estimates to be insensitive to the description of the near-surface layers, at the expense of substantially changing both the near-surface structure of the best-fitting models and the values of associated parameters like the initial helium abundance, Y i . Attempts to measure Y i from seismic modeling of red giants may thus be systematically dependent on the choice of atmospheric physics

TESS Asteroseismology of α\alpha Mensae: Benchmark Ages for a G7 Dwarf and its M-dwarf Companion

Asteroseismology of bright stars has become increasingly important as a method to determine fundamental properties (in particular ages) of stars. The Kepler Space Telescope initiated a revolution by detecting oscillations in more than 500 main-sequence and subgiant stars. However, most Kepler stars are faint, and therefore have limited constraints from independent methods such as long-baseline interferometry. Here, we present the discovery of solar-like oscillations in $\alpha$ Men A, a naked-eye (V=5.1) G7 dwarf in TESS's Southern Continuous Viewing Zone. Using a combination of astrometry, spectroscopy, and asteroseismology, we precisely characterize the solar analog alpha Men A (Teff = 5569 +/- 62 K, R = 0.960 +/- 0.016 Rsun, M = 0.964 +/- 0.045 Msun). To characterize the fully convective M dwarf companion, we derive empirical relations to estimate mass, radius, and temperature given the absolute Gaia magnitude and metallicity, yielding M = 0.169 +/- 0.006, R = 0.19 +/- 0.01 and Teff = 3054 +/- 44 K. Our asteroseismic age of 6.2 +/- 1.4 (stat) +/- 0.6 (sys) Gyr for the primary places $\alpha$ Men B within a small population of M dwarfs with precisely measured ages. We combined multiple ground-based spectroscopy surveys to reveal an activity cycle of 13.1 +/- 1.1 years, a period similar to that observed in the Sun. We used different gyrochronology models with the asteroseismic age to estimate a rotation period of ~30 days for the primary. Alpha Men A is now the closest (d=10pc) solar analog with a precise asteroseismic age from space-based photometry, making it a prime target for next-generation direct imaging missions searching for true Earth analogs.Comment: Accepted to The Astrophysical Journal; 15 pages, 10 figure

The Second APOKASC Catalog: The Empirical Approach

We present a catalog of stellar properties for a large sample of 6676 evolved stars with APOGEE spectroscopic parameters and \textit{Kepler} asteroseismic data analyzed using five independent techniques. Our data includes evolutionary state, surface gravity, mean density, mass, radius, age, and the spectroscopic and asteroseismic measurements used to derive them. We employ a new empirical approach for combining asteroseismic measurements from different methods, calibrating the inferred stellar parameters, and estimating uncertainties. With high statistical significance, we find that asteroseismic parameters inferred from the different pipelines have systematic offsets that are not removed by accounting for differences in their solar reference values. We include theoretically motivated corrections to the large frequency spacing ($\Delta \nu$) scaling relation, and we calibrate the zero point of the frequency of maximum power ($\nu_{\rm max}$) relation to be consistent with masses and radii for members of star clusters. For most targets, the parameters returned by different pipelines are in much better agreement than would be expected from the pipeline-predicted random errors, but 22\% of them had at least one method not return a result and a much larger measurement dispersion. This supports the usage of multiple analysis techniques for asteroseismic stellar population studies. The measured dispersion in mass estimates for fundamental calibrators is consistent with our error model, which yields median random and systematic mass uncertainties for RGB stars of order 4\%. Median random and systematic mass uncertainties are at the 9\% and 8\% level respectively for RC stars.Comment: 29 pages, 26 figures. Submitted ApJSupp. Comments welcome. For access to the main data table (Table 5) use https://www.dropbox.com/s/k33td8ukefwy5tv/APOKASC2_Table5.txt?dl=0; for access to the individual pipeline values (Table 6) use https://www.dropbox.com/s/vl9s2p3obftrv8m/APOKASC2_Table6.txt?dl=

Seismic analysis of four solar-like stars observed during more than eight months by Kepler

Having started science operations in May 2009, the Kepler photometer has been able to provide exquisite data of solar-like stars. Five out of the 42 stars observed continuously during the survey phase show evidence of oscillations, even though they are rather faint (magnitudes from 10.5 to 12). In this paper, we present an overview of the results of the seismic analysis of 4 of these stars observed during more than eight months.Comment: 5 pages, 1 figure. To appear in the ASP proceedings of "The 61st Fujihara seminar: Progress in solar/stellar physics with helio- and asteroseismology", 13th-17th March 2011, Hakone, Japa