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  • 1
    Electronic Resource
    Electronic Resource
    Primordial Helium and ΔY/ΔZ from H II Regions and from Fine Structure in the Main Sequence Based on Hipparcos Parallaxes (1998)
    Springer
    Space science reviews 84 (1998), S. 115-126 
    Details
    ISSN: 1572-9672
    Keywords: galaxies: abundances ; ISM: abundances ; stars: abundances ; stars: evolution
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The primordial helium abundance YP is important for cosmology and the ratio ΔY/ΔZ of the changes relative to primordial abundances constrains models of stellar evolution. While the most accurate estimates of YP come from emission lines in extragalactic H II regions, they involve an extrapolation to zero metallicity which itself is closely tied up with the slope ΔY/ΔZ. Recently certain systematic effects have come to light in this exercise which make it useful to have an independent estimate of ΔY/ΔZ from fine structure in the main sequence of nearby stars. We derive such an estimate from Hipparcos data for stars with Z ≤ Z⊙ and find values between 2 and 3, which are consistent with stellar models, but still have a large uncertainty.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005098630276
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    Paper (German National Licenses)
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  • 2
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    Modelling the Milky Way – I. Method and first results fitting the thick disc and halo with DES-Y3 data (2020)
    Oxford University Press
    Details
    Publication Date: 2020-07-09
    Description: We present a technique to fit the stellar components of the Galaxy by comparing Hess Diagrams (HDs) generated from trilegal models to real data. We apply this technique, which we call mwfitting, to photometric data from the first 3 yr of the Dark Energy Survey (DES). After removing regions containing known resolved stellar systems such as globular clusters, dwarf galaxies, nearby galaxies, the Large Magellanic Cloud, and the Sagittarius Stream, our main sample spans a total area of ∼2300 deg2. We further explore a smaller subset (∼1300 deg2) that excludes all regions with known stellar streams and stellar overdensities. Validation tests on synthetic data possessing similar properties to the DES data show that the method is able to recover input parameters with a precision better than 3 per cent. We fit the DES data with an exponential thick disc model and an oblate double power-law halo model. We find that the best-fitting thick disc model has radial and vertical scale heights of 2.67 ± 0.09 kpc and 925 ± 40 pc, respectively. The stellar halo is fit with a broken power-law density profile with an oblateness of 0.75 ± 0.01, an inner index of 1.82 ± 0.08, an outer index of 4.14 ± 0.05, and a break at 18.52 ± 0.27 kpc from the Galactic centre. Several previously discovered stellar overdensities are recovered in the residual stellar density map, showing the reliability of mwfitting in determining the Galactic components. Simulations made with the best-fitting parameters are a promising way to predict Milky Way star counts for surveys such as the LSST and Euclid.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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  • 3
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    Uncertainties on near-core mixing in red-clump stars: effects on the period spacing and on the luminosity of the AGB bump (2015)
    Oxford University Press
    Details
    Publication Date: 2015-09-27
    Description: Low-mass stars in the He-core-burning (HeCB) phase play a major role in stellar, galactic, and extragalactic astrophysics. The ability to predict accurately the properties of these stars, however, depends on our understanding of convection, which remains one of the key open questions in stellar modelling. We argue that the combination of the luminosity of the AGB bump (AGBb) and the period spacing of gravity modes ( 1 ) during the HeCB phase provides us with a decisive test to discriminate between competing models of these stars. We use the Modules for Experiments in Stellar Astrophysics (MESA), a Bag of Stellar Tracks and Isochrones (BaSTI), and PAdova & TRieste Stellar Evolution Code (PARSEC) stellar evolution codes to model a typical giant star observed by Kepler . We explore how various near-core-mixing scenarios affect the predictions of the above-mentioned constraints, and we find that 1 depends strongly on the prescription adopted. Moreover we show that the detailed behaviour of 1 shows the signature of sharp variations in the Brunt–Väisälä frequency, which could potentially give additional information about near-core features. We find evidence for the AGBb among Kepler targets, and a first comparison with observations shows that, even if standard models are able to reproduce the luminosity distribution, no standard model can account for satisfactorily the period spacing of HeCB stars. Our analysis allows us to outline a candidate model to describe simultaneously the two observed distributions: a model with a moderate overshooting region characterized by an adiabatic thermal stratification. This prescription will be tested in the future on cluster stars, to limit possible observational biases.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    Connecting the evolution of thermally pulsing asymptotic giant branch stars to the chemistry in their circumstellar envelopes - I. Hydrogen cyanide (2015)
    Oxford University Press
    Details
    Publication Date: 2015-12-13
    Description: We investigate the formation of hydrogen cyanide (HCN) in the inner circumstellar envelopes of thermally pulsing asymptotic giant branch (TP-AGB) stars. A dynamic model for periodically shocked atmospheres, which includes an extended chemo-kinetic network, is for the first time coupled to detailed evolutionary tracks for the TP-AGB phase computed with the colibri code. We carried out a calibration of the main shock parameters (the shock formation radius r s,0 and the effective adiabatic index $\gamma _{\rm ad}^{\rm eff}$ ) using the circumstellar HCN abundances recently measured for a populous sample of pulsating TP-AGB stars. Our models recover the range of the observed HCN concentrations as a function of the mass-loss rates, and successfully reproduce the systematic increase of HCN moving along the M-S-C chemical sequence of TP-AGB stars, which traces the increase of the surface C/O ratio. The chemical calibration brings along two important implications for the physical properties of the pulsation-induced shocks: (i) the first shock should emerge very close to the photosphere ( r s,0 ~= 1 R ), and (ii) shocks are expected to have a dominant isothermal character $(\gamma _{\rm ad}^{\rm eff}\simeq 1)$ in the denser region close to the star (within ~3–4 R ), implying that radiative processes should be quite efficient. Our analysis also suggests that the HCN concentrations in the inner circumstellar envelopes are critically affected by the H–H 2 chemistry during the post-shock relaxation stages. Given the notable sensitiveness of the results to stellar parameters, this paper shows that such chemo-dynamic analyses may indeed provide a significant contribution to the broader goal of attaining a comprehensive calibration of the TP-AGB evolutionary phase.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    On the interpretation of sub-giant branch morphologies of intermediate-age star clusters with extended main sequence turnoffs (2015)
    Oxford University Press
    Details
    Publication Date: 2015-04-30
    Description: High-quality photometry of many star clusters in the Magellanic Clouds with ages of 1–2 Gyr revealed main sequence turnoffs (MSTOs) that are significantly wider than can be accounted for by a simple stellar population (SSP). Such extended MSTOs (eMSTOs) are often interpreted in terms of an age spread of several 10 8  yr, challenging the traditional view of star clusters as being formed in a single star formation episode. Li et al. and Bastian & Niederhofer recently investigated the sub-giant branches (SGBs) of NGC 1651, NGC 1806, and NGC 1846, three star clusters in the Large Magellanic Cloud (LMC) that exhibit an eMSTO. They argued that the SGB of these star clusters can be explained only by an SSP. We study these and two other similar star clusters in the LMC, using extensive simulations of SSPs including unresolved binaries. We find that the shapes of the cross-SGB profiles of all star clusters in our sample are in fact consistent with their cross-MSTO profiles when the latter are interpreted as age distributions. Conversely, SGB morphologies of star clusters with eMSTOs are found to be inconsistent with those of simulated SSPs. Finally, we create parsec isochrones from tracks featuring a grid of convective overshoot levels and a very fine grid of stellar masses. A comparison of the observed photometry with these isochrones shows that the morphology of the red clump (RC) of such star clusters is also consistent with that implied by their MSTO in the age spread scenario. We conclude that the SGB and RC morphologies of star clusters featuring eMSTOs are consistent with the scenario in which the eMSTOs are caused by a distribution of stellar ages.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    PARSEC evolutionary tracks of massive stars up to 350 M{odot} at metallicities 0.0001 (2015)
    Oxford University Press
    Details
    Publication Date: 2015-07-15
    Description: We complement the parsec data base of stellar evolutionary tracks with new models of massive stars, from the pre-main-sequence phase to the central carbon ignition. We consider a broad range of metallicities, 0.0001 ≤  Z  ≤ 0.04 and initial masses up to M ini = 350 M . The main difference with respect to our previous models of massive stars is the adoption of a recent formalizm accounting for the mass-loss enhancement when the ratio of the stellar to the Eddington luminosity, e , approaches unity. With this new formalizm, the models are able to reproduce the Humphreys–Davidson limit observed in the Galactic and Large Magellanic Cloud colour–magnitude diagrams, without an ad hoc mass-loss enhancement. We also follow the predictions of recent wind models indicating that the metallicity dependence of the mass-loss rates becomes shallower when e approaches unity. We thus find that the more massive stars may suffer from substantial mass-loss even at low metallicity. We also predict that the Humphreys–Davidson limit should become brighter at decreasing metallicity. We supplement the evolutionary tracks with new tables of theoretical bolometric corrections, useful to compare tracks and isochrones with the observations. For this purpose, we homogenize existing stellar atmosphere libraries of hot and cool stars (Potsdam Wolf–Rayet, atlas9 and phoenix ) and we add, where needed, new atmosphere models computed with wm-basic . The mass, age and metallicity grids are fully adequate to perform detailed investigations of the properties of very young stellar systems, both in local and distant galaxies. The new tracks supersede the previous old padova models of massive stars.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    Synthetic photometry for M and K giants and stellar evolution: hydrostatic dust-free model atmospheres and chemical abundances (2016)
    Oxford University Press
    Details
    Publication Date: 2016-02-27
    Description: Based on a grid of hydrostatic spherical comarcs models for cool stars, we have calculated observable properties of these objects, which will be mainly used in combination with stellar evolution tracks and population synthesis tools. The high-resolution opacity sampling and low-resolution convolved spectra as well as bolometric corrections for a large number of filter systems are made electronically available. We exploit those data to study the effect of mass, C/O ratio and nitrogen abundance on the photometry of K and M giants. Depending on effective temperature, surface gravity and the chosen wavelength ranges, variations of the investigated parameters cause very weak to moderate and, in the case of C/O values close to 1, even strong shifts of the colours. For the usage with stellar evolution calculations, they will be treated as correction factors applied to the results of an interpolation in the main quantities. When we compare the synthetic photometry to observed relations and to data from the Galactic bulge, we find in general a good agreement. Deviations appear for the coolest giants showing pulsations, mass-loss and dust shells, which cannot be described by hydrostatic models.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    Envelope overshooting in low-metallicity intermediate- and high-mass stars: a test with the Sagittarius dwarf irregular galaxy (2015)
    Oxford University Press
    Details
    Publication Date: 2015-12-01
    Description: We check the performance of the Padova TRieste Stellar Evolution Code ( parsec ) tracks in reproducing the blue loops of intermediate age and young stellar populations at very low metallicity. We compute new evolutionary parsec tracks of intermediate- and high-mass stars from 2 to 350 M with enhanced envelope overshooting (EO), EO = 2 H P and 4 H P , for very low metallicity, Z  = 0.0005. The input physics, including the mass-loss rate, has been described in parsec , version V1.2. By comparing the synthetic colour–magnitude diagrams (CMDs) obtained from the different sets of models with EO = 0.7 H P (the standard parsec tracks), 2 H P and 4 H P , with deep observations of the Sagittarius dwarf irregular galaxy (SagDIG), we find that the overshooting scale EO = 2 H P best reproduces the observed loops. This result is consistent with that obtained by Tang et al. for Z in the range 0.001–0.004. We also discuss the dependence of the blue loop extension on the adopted instability criterion. Contrary to what has been stated in the literature, we find that the Schwarzschild criterion, instead of the Ledoux criterion, favours the development of blue loops. Other factors that could affect the CMD comparisons, such as differential internal extinction or the presence of binary systems, are found to have negligible effects on the results. Thus, we confirm that, in the presence of core overshooting during the H-burning phase, a large EO is needed to reproduce the main features of the central He-burning phase of intermediate- and high-mass stars.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    Probing interstellar extinction near the 30 Doradus nebula with red giant stars (2014)
    Oxford University Press
    Details
    Publication Date: 2014-01-16
    Description: We have studied the interstellar extinction in a field of ~ 3 arcmin x 3 arcmin located about 6 arcmin southwest of 30 Doradus in the Large Magellanic Cloud (LMC). Hubble Space Telescope observations in the U , B , V , I and Hα bands reveal patchy extinction in this field. The colour–magnitude diagram (CMD) shows an elongated stellar sequence, almost parallel to the main sequence (MS), which is in reality made up of stars of the red giant clump (RC) spread across the CMD by the uneven levels of extinction in this region. Since these objects are all at the same distance from us and share very similar physical properties, we can derive quantitatively both the extinction law in the range 3000–8000 Å and the absolute extinction towards about 100 objects, setting statistically significant constraints on the dust grains properties in this area. We find an extinction curve considerably flatter than the standard Galactic one and than those obtained before for the LMC. The derived value of R V  = 5.6 ± 0.3 implies that in this region larger grains dominate. Upper MS stars span a narrower range of E ( B – V ) values than RC objects, at variance with what has been found elsewhere in the LMC.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    Improving PARSEC models for very low mass stars (2014)
    Oxford University Press
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    Publication Date: 2014-09-17
    Description: Many stellar models present difficulties in reproducing basic observational relations of very low mass stars (VLMS), including the mass–radius relation and the optical colour–magnitudes of cool dwarfs. Here, we improve parsec (PAdova-TRieste Stellar Evolution Code) models on these points. We implement the T– relations from phoenix BT-Settl model atmospheres as the outer boundary conditions in the parsec code, finding that this change alone reduces the discrepancy in the mass–radius relation from 8 to 5 per cent. We compare the models with multiband photometry of clusters Praesepe and M67, showing that the use of T– relations clearly improves the description of the optical colours and magnitudes. But anyway, using both Kurucz and phoenix model spectra, model colours are still systematically fainter and bluer than the observations. We then apply a shift to the above T– relations, increasing from 0 at T eff = 4730 K to ~14 per cent at T eff = 3160 K, to reproduce the observed mass–radius relation of dwarf stars. Taking this experiment as a calibration of the T – relations, we can reproduce the optical and near-infrared colour–magnitude diagrams of low-mass stars in the old metal-poor globular clusters NGC 6397 and 47 Tuc, and in the intermediate-age and young solar-metallicity open clusters M67 and Praesepe. Thus, we extend parsec models using this calibration, providing VLMS models more suitable for the lower main-sequence stars over a wide range of metallicities and wavelengths. Both sets of models are available on parsec webpage.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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