593 research outputs found

    Quiet Sun Magnetic Field Measurements Based on Lines with Hyperfine Structure

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    The Zeeman pattern of MnI lines is sensitive to hyperfine structure (HFS) and, they respond to hG magnetic field strengths differently from the lines used in solar magnetometry. This peculiarity has been employed to measure magnetic field strengths in quiet Sun regions. However, the methods applied so far assume the magnetic field to be constant in the resolution element. The assumption is clearly insufficient to describe the complex quiet Sun magnetic fields, biasing the results of the measurements. We present the first syntheses of MnI lines in realistic quiet Sun model atmospheres. The syntheses show how the MnI lines weaken with increasing field strength. In particular, kG magnetic concentrations produce NnI 5538 circular polarization signals (Stokes V) which can be up to two orders of magnitude smaller than the weak magnetic field approximation prediction. Consequently, (1) the polarization emerging from an atmosphere having weak and strong fields is biased towards the weak fields, and (2) HFS features characteristic of weak fields show up even when the magnetic flux and energy are dominated by kG fields. For the HFS feature of MnI 5538 to disappear the filling factor of kG fields has to be larger than the filling factor of sub-kG fields. Stokes V depends on magnetic field inclination according to the simple consine law. Atmospheres with unresolved velocities produce asymmetric line profiles, which cannot be reproduced by simple one-component model atmospheres. The uncertainty of the HFS constants do not limit the use of MnI lines for magnetometry.Comment: Accepted for publication in ApJ. 10 pages, 14 figure

    The effect of diffusion on the Red Giant luminosity function 'bump'

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    This paper investigates the effect of microscopic diffusion of helium and heavy elements on the location of the Red Giant Branch Luminosity Function Bump in Population II stellar models. To this aim updated evolutionary models taking into account diffusion from the Main Sequence until the Zero Age Horizontal Branch have been computed. The observational luminosity difference between the RGB bump and the ZAHB, as collected for a sample of galactic globular clusters, has been compared with the corresponding theoretical values obtained by adopting both canonical and diffusive models. We find that the effect of diffusion, even if slightly improving the agreement between observations and theory, is negligible with respect to the observational uncertainties. In any case the theoretical predictions in models with and without diffusion appear in agreement with the observational results within the estimated errors. Thus canonical models can be still safely adopted, at least until much more accurate observational data will be available.Comment: TeX, 6 pages, uses mnrass.sty (included), 2 postscript figures, in publication on MNRA

    Helioseismology can test the Maxwell-Boltzmann distribution

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    Nuclear reactions in stars occur between nuclei in the high-energy tail of the energy distribution and are sensitive to possible deviations from the standard equilibrium thermal-energy distribution. We are able to derive strong constraints on such deviations by using the detailed helioseismic information of the solar structure. If a small deviation is parameterized with a factor exp{-delta*(E/kT)^2}, we find that delta should lie between -0.005 and +0.002. However, even values of delta as small as 0.003 would still give important effects on the neutrino fluxes.Comment: 10 pages in ReVTeX + 1 postscript figure. Submitted to Phys. Lett.

    Cumulative physical uncertainty in modern stellar models. II. The dependence on the chemical composition

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    We extend our work on the effects of the uncertainties on the main input physics for the evolution of low-mass stars. We analyse the dependence of the cumulative physical uncertainty affecting stellar tracks on the chemical composition. We calculated more than 6000 stellar tracks and isochrones, with metallicity ranging from Z = 0.0001 to 0.02, by changing the following physical inputs within their current range of uncertainty: 1H(p,nu e+)2H, 14N(p,gamma)15O and triple-alpha reaction rates, radiative and conductive opacities, neutrino energy losses, and microscopic diffusion velocities. The analysis was performed using a latin hypercube sampling design. We examine in a statistical way the dependence on the variation of the physical inputs of the turn-off (TO) luminosity, the central hydrogen exhaustion time (t_H), the luminosity and the helium core mass at the red-giant branch (RGB) tip, and the zero age horizontal branch (ZAHB) luminosity in the RR Lyrae region. For the stellar tracks, an increase from Z = 0.0001 to Z = 0.02 produces a cumulative physical uncertainty in TO luminosity from 0.028 dex to 0.017 dex, while the global uncertainty on t_H increases from 0.42 Gyr to 1.08 Gyr. For the RGB tip, the cumulative uncertainty on the luminosity is almost constant at 0.03 dex, whereas the one the helium core mass decreases from 0.0055 M_sun to 0.0035 M_sun. The dependence of the ZAHB luminosity error is not monotonic with Z, and it varies from a minimum of 0.036 dex at Z = 0.0005 to a maximum of 0.047 dex at Z = 0.0001. Regarding stellar isochrones of 12 Gyr, the cumulative physical uncertainty on the predicted TO luminosity and mass increases respectively from 0.012 dex to 0.014 dex and from 0.0136 M_sun to 0.0186 M_sun. Consequently, for ages typical of galactic globular clusters, the uncertainty on the age inferred from the TO luminosity increases from 325 Myr to 415 Myr.Comment: Accepted for publication in A&

    On the age of Galactic bulge microlensed dwarf and subgiant stars

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    Recent results by Bensby and collaborators on the ages of microlensed stars in the Galactic bulge have challenged the picture of an exclusively old stellar population. However, these age estimates have not been independently confirmed. In this paper we verify these results by means of a grid-based method and quantify the systematic biases that might be induced by some assumptions adopted to compute stellar models. We explore the impact of increasing the initial helium abundance, neglecting the element microscopic diffusion, and changing the mixing-length calibration in theoretical stellar track computations. We adopt the SCEPtER pipeline with a novel stellar model grid for metallicities [Fe/H] from -2.00 to 0.55 dex, and masses in the range [0.60; 1.60] Msun from the ZAMS to the helium flash at the red giant branch tip. We show for the considered evolutionary phases that our technique provides unbiased age estimates. Our age results are in good agreement with Bensby and collaborators findings and show 16 stars younger than 5 Gyr and 28 younger than 9 Gyr over a sample of 58. The effect of a helium enhancement as large as Delta Y/Delta Z = 5 is quite modest, resulting in a mean age increase of metal rich stars of 0.6 Gyr. Even simultaneously adopting a high helium content and the upper values of age estimates, there is evidence of 4 stars younger than 5 Gyr and 15 younger than 9 Gyr. For stars younger than 5 Gyr, the use of stellar models computed by neglecting microscopic diffusion or by assuming a super-solar mixing-length value leads to a mean increase in the age estimates of about 0.4 Gyr and 0.5 Gyr respectively. Even considering the upper values for the age estimates, there are four stars estimated younger than 5 Gyr is in both cases. Thus, the assessment of a sizeable fraction of young stars among the microlensed sample in the Galactic bulge appears robust.Comment: Accepted for publication in A&A. Abstract shortene

    Mixing-length estimates from binary systems. A theoretical investigation on the estimation errors

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    We performed a theoretical investigation on the mixing-length parameter recovery from an eclipsing double-lined binary system. We focused on a syntetic system composed by a primary of mass M = 0.95 Msun and a secondary of M = 0.85 Msun. Monte Carlo simulations were conducted at three metallicities, and three evolutionary stages of the primary. For each configuration artificial data were sampled assuming an increasing difference between the mixing-length of the two stars. The mixing length values were reconstructed using three alternative set-ups. A first method, which assumes full independence between the two stars, showed a great difficulty to constrain the mixing-length values: the recovered values were nearly unconstrained with a standard deviation of 0.40. The second technique imposes the constraint of common age and initial chemical composition for the two stars in the fit. We found that αml,1\alpha_{ml,1} values match the ones recovered under the previous configuration, but αml,2\alpha_{ml,2} values are peaked around unbiased estimates. This occurs because the primary star provides a much more tight age constraint in the joint fit than the secondary. Within this second scenario we also explored, for systems sharing a common αml\alpha_{ml}, the difference in the mixing-length values of the two stars only due to random fluctuations owing to the observational errors. The posterior distribution of these differences was peaked around zero, with a large standard deviation of 0.3 (15\% of the solar-scaled value). The third technique also imposes the constraint of a common mixing-length value for the two stars, and served as a test for identification of wrong fitting assumptions. In this case the common mixing-length is mainly dictated by the value of αml,2\alpha_{ml,2}. [...] For Δαml>0.4\Delta \alpha_{ml} > 0.4 less than half of the systems can be recovered and only 20% at Δαml=1.0\Delta \alpha_{ml} = 1.0.Comment: Abstract abridge

    Evolution of the habitable zone of low-mass stars. Detailed stellar models and analytical relationships for different masses and chemical compositions

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    We study the temporal evolution of the habitable zone (HZ) of low-mass stars - only due to stellar evolution - and evaluate the related uncertainties. These uncertainties are then compared with those due to the adoption of different climate models. We computed stellar evolutionary tracks from the pre-main sequence phase to the helium flash at the red-giant branch tip for stars with masses in the range [0.70 - 1.10] Msun, metallicity Z in the range [0.005 - 0.04], and various initial helium contents. We evaluated several characteristics of the HZ, such as the distance from the host star at which the habitability is longest, the duration of this habitability, the width of the zone for which the habitability lasts one half of the maximum, and the boundaries of the continuously habitable zone (CHZ) for which the habitability lasts at least 4 Gyr. We developed analytical models, accurate to the percent level or lower, which allowed to obtain these characteristics in dependence on the mass and the chemical composition of the host star. The metallicity of the host star plays a relevant role in determining the HZ. The initial helium content accounts for a variation of the CHZ boundaries as large as 30% and 10% in the inner and outer border. The computed analytical models allow the first systematic study of the variability of the CHZ boundaries that is caused by the uncertainty in the estimated values of mass and metallicity of the host star. An uncertainty range of about 30% in the inner boundary and 15% in the outer one were found. We also verified that these uncertainties are larger than that due to relying on recently revised climatic models, which leads to a CHZ boundaries shift within 5% with respect to those of our reference scenario. We made an on-line tool available that provides both HZ characteristics and interpolated stellar tracks.Comment: Accepted for publication in A&A, abstract abridge

    Cumulative physical uncertainty in modern stellar models I. The case of low-mass stars

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    Using our updated stellar evolutionary code, we quantitatively evaluate the effects of the uncertainties in the main physical inputs on the evolutionary characteristics of low mass stars from the main sequence to the zero age horizontal branch (ZAHB). We calculated more than 3000 stellar tracks and isochrones, with updated solar mixture, by changing the following physical inputs within their current range of uncertainty: 1H(p,nu e+)2H, 14N(p,gamma)15O, and triple-alpha reaction rates, radiative and conductive opacities, neutrino energy losses, and microscopic diffusion velocities. We performed a systematic variation on a fixed grid, in a way to obtain a full crossing of the perturbed input values. The effect of the variations of the chosen physical inputs on relevant stellar evolutionary features, such as the turn-off luminosity, the central hydrogen exhaustion time, the red-giant branch (RGB) tip luminosity, the helium core mass, and the ZAHB luminosity in the RR Lyrae region are statistically analyzed. For a 0.9 Msun model, the cumulative uncertainty on the turn-off, the RGB tip, and the ZAHB luminosities accounts for ±\pm 0.02 dex, ±\pm 0.03 dex, and ±\pm 0.045 dex respectively, while the central hydrogen exhaustion time varies of about ±\pm 0.7 Gyr. The most relevant effect is due to the radiative opacities uncertainty; for the later evolutionary stages the second most important effect is due to the triple-alpha reaction rate uncertainty. For an isochrone of 12 Gyr, we find that the isochrone turn-off log luminosity varies of ±\pm 0.013 dex, the mass at the isochrone turn-off varies of ±\pm 0.015 Msun, and the difference between ZAHB and turn-off log-luminosity varies of ±\pm 0.05 dex. The effect of the physical uncertainty affecting the age inferred from turn-off luminosity and from the vertical method are of ±\pm 0.375 Gyr and ±\pm 1.25 Gyr respectively.Comment: Accepteted for pubblication in A&A. The abstract is shortened to fill in the arxiv abstract fiel

    Uncertainties in grid-based estimates of stellar mass and radius. SCEPtER: Stellar CharactEristics Pisa Estimation gRid

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    Some aspects of the systematic and statistical errors affecting grid-based estimation of stellar masses and radii have still not been investigated well. We study the impact on mass and radius determination of the uncertainty in the input physics, in the mixing-length value, in the initial helium abundance, and in the microscopic diffusion efficiency adopted in stellar model computations. We consider stars with mass in the range [0.8 - 1.1] Msun and evolutionary stages from the zero-age main sequence to the central hydrogen exhaustion. Stellar parameters were recovered by a maximum-likelihood technique, comparing the observations constraints to a grid of stellar models. Synthetic grids with perturbed input were adopted to estimate the systematic errors arising from the current uncertainty in model computations. We found that the statistical error components, owing to the current typical uncertainty in the observations, are nearly constant in all cases at about 4.5% and 2.2% on mass and radius determination, respectively. The systematic bias on mass and radius determination due to a variation of ±\pm 1 in Delta Y/Delta Z is ±\pm 2.3% and ±\pm 1.1%; the one due to a change of ±\pm 0.24 in the value of the mixing-length is ±\pm 2.1% and ±\pm 1.0%; the one due to a variation of ±\pm 5% in the radiative opacity is ∓\mp 1.0% and ∓\mp 0.45%. An important bias source is to neglect microscopic diffusion, which accounts for errors of about 3.7% and 1.5% on mass and radius. The cumulative effects of the considered uncertainty sources can produce biased estimates of stellar characteristics. Comparison of the results of our technique with other grid techniques shows that the systematic biases induced by the differences in the estimation grids are generally greater than the statistical errors involved.Comment: Accepted for publication in A&A. Abstract shortene
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