arXiv subjects

Achim Weiss

Publications and source records attributed to Achim Weiss.

At least 55 records · Page 3Linked to original sources

arXiv2011

Helium burning in moderate-mass stars

The evolution of low- and intermediate mass stars at the onset and during core helium burning is reviewed. Particular emphasis is laid on structural differences, which may allow to identify a star's nature and evolutionary phase in spite of the fact that it is found in a region of the Hertzsprung-Russell-Diagram objects from both mass ranges may populate. Seismic diagnostics which are sensitive to the temperature and density profile at the border of the helium core and outside of it may be the most promising tool.

astro-ph.SR↗

arXiv2011

The age of the Milky Way halo stars from the Sloan Digital Sky Survey

We determined the age of the stellar content of the Galactic halo by considering main-sequence turn-off stars. From the large number of halo stars provided by Sloan Digital Sky Survey, we could accurately detect the turn-off as a function of metallicity of the youngest dominant population, which was done by looking at the hottest (bluest) stars of a population. Using the turn-off temperature of a population of a given metallicity, we looked for the isochrones with that turn-off temperature and metallicity and found no age gradient as a function of metallicity. This would mean that this dominating population of the Galactic halo formed rapidly, probably during the collapse of the proto-Galactic gas. Moreover, we could find a significant number of stars with hotter temperatures than the turn-off, which might be blue horizontal branch (BHB) stars, blue stragglers, or main sequence stars that are younger than the dominant population and were probably formed in external galaxies and accreted later on to our Milky Way. Motivated by the current debate about the efficiency of gravitational settling (atomic diffusion) in the interior of old solar-type stars, we used isochrones with and without settling to determine the ages. When ignoring diffusion in the isochrones we obtained ages of 14-16 Gyr. This result is a strong argument against inhibited diffusion in old halo field stars, since it results in a serious conflict with the age of the Universe of 13.7 Gyr. The age obtained including diffusion in the isochrones was 10-12 Gyr, which agrees with the absolute age of the old globular clusters in the inner halo.

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arXiv2010

Stellar atmosphere parameters with MAx, a MAssive compression of x^2 for spectral fitting

MAx is a new tool to estimate parameters from stellar spectra. It is based on the maximum likelihood method, with the likelihood compressed in a way that the information stored in the spectral fluxes is conserved. The compressed data are given by the size of the number of parameters, rather than by the number of flux points. The optimum speed-up reached by the compression is the ratio of the data set to the number of parameters. The method has been tested on a sample of low-resolution spectra from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey for the estimate of metallicity, effective temperature and surface gravity, with accuracies of 0.24 dex, 130K and 0.5 dex, respectively. Our stellar parameters and those recovered by the SEGUE Stellar Parameter Pipeline agree reasonably well. A small sample of high-resolution VLT-UVES spectra were also used to test the method and the results have been compared to a more classical approach. The speed and multi-resolution capability of MAx combined with its performance compared with other methods indicates that it will be a useful tool for the analysis of upcoming spectral surveys.

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arXiv2010

On using the CMD morphology of M67 to test solar abundances

The open cluster M67 has solar metallicity and an age of about 4Gyr. The turn-off mass is close to the minimum mass for which solar metallicity stars develop a convective core during main sequence evolution as a result of the development of hydrogen burning through the CNO-cycle. The morphology of the color-magnitude-diagram (CMD) of M67 around the turn-off shows a clear hook-like feature, direct sign that stars close to the turn-off have convective cores. VandenBerg et al. investigated the possibility of using the morphology of the M67 turn-off to put constraints on the solar metallicity, particularly CNO elements, for which solar abundances have been revised downwards by more than 30% over the last few years. Here, we extend their work filling in the gaps in their analysis. To this aim, we compute isochrones appropriate for M67 using new (low metallicity) and old (high metallicity) solar abundances and study whether the characteristic turn-off in the CMD of M67 can be reproduced or not. We also study the importance of other constitutive physics on determining the presence of such a hook, particularly element diffusion, overshooting and nuclear reaction rates. We find that using the new solar abundance determinations, with low CNO abundances, makes it more difficult to reproduce the characteristic CMD of M67. This result is in agreement with results by VandenBerg et al. However, changes in the constitutive physics of the models, particularly overshooting, can influence and alter this result to the extent that isochrones constructed with models using low CNO solar abundances can also reproduce the turn-off morphology in M67. We conclude that only if all factors affecting the turn-off morphology are completely under control (and this is not the case), M67 could be used to put constraints on solar abundances.

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arXiv2009

Detection of solar-like oscillations from Kepler photometry of the open cluster NGC 6819

Asteroseismology of stars in clusters has been a long-sought goal because the assumption of a common age, distance and initial chemical composition allows strong tests of the theory of stellar evolution. We report results from the first 34 days of science data from the Kepler Mission for the open cluster NGC 6819 -- one of four clusters in the field of view. We obtain the first clear detections of solar-like oscillations in the cluster red giants and are able to measure the large frequency separation and the frequency of maximum oscillation power. We find that the asteroseismic parameters allow us to test cluster-membership of the stars, and even with the limited seismic data in hand, we can already identify four possible non-members despite their having a better than 80% membership probability from radial velocity measurements. We are also able to determine the oscillation amplitudes for stars that span about two orders of magnitude in luminosity and find good agreement with the prediction that oscillation amplitudes scale as the luminosity to the power of 0.7. These early results demonstrate the unique potential of asteroseismology of the stellar clusters observed by Kepler.

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arXiv2009

New Asymptotic Giant Branch models for a range of metallicities

We present a new grid of stellar model calculations for stars on the Asymptotic Giant Branch between 1.0 and 6.0 M_sun. Our grid consists of 5 chemical mixtures between Z=0.0005 and Z=0.04, with both solar-like and $α$-element enhanced metal ratios. We treat consistently the carbon-enhancement of the stellar envelopes by using opacity tables with varying C/O-ratio and by employing theoretical mass loss rates for carbon stars. The low temperature opacities have been calculated specifically for this project. For oxygen stars we use an empirical mass loss formalism. The third dredge-up is naturally obtained by including convective overshooting. Our models reach effective temperatures in agreement with earlier synthetic models, which included approximative carbon-enriched molecular opacities and show good agreement with empirically determined carbon-star lifetimes. A fraction of the models could be followed into the post-AGB phase, for which we provide models in a mass range supplementing previous post-AGB calculations. Our grid constitutes the most extensive set of AGB-models, calculated with the latest physical input data and treating carbon-enhancement due to the third dredge-up most consistently.

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arXiv2008

Semi-empirical white dwarf initial-final mass relationships: a thorough analysis of systematic uncertainties due to stellar evolution models

Using the most recent results about white dwarfs in 10 open clusters, we revisit semi-empirical estimates of the initial-final mass relation in star clusters, with emphasis on the use of stellar evolution models. We discuss the influence of these models on each step of the derivation. One intention of our work is to use consistent sets of calculations both for the isochrones and the white dwarf cooling tracks. The second one is to derive the range of systematic errors arising from stellar evolution theory. This is achieved by using different sources for the stellar models and by varying physical assumptions and input data. We find that systematic errors, including the determination of the cluster age, are dominating the initial mass values, while observational uncertainties influence the final mass primarily. After having determined the systematic errors, the initial-final mass relation allows us finally to draw conclusions about the physics of the stellar models, in particular about convective overshooting.

astro-ph↗

arXiv2008

CoRoT/ESTA-TASK 1 and TASK 3 comparison of the internal structure and seismic properties of representative stellar models: Comparisons between the ASTEC, CESAM, CLES, GARSTEC and STAROX codes

We compare stellar models produced by different stellar evolution codes for the CoRoT/ESTA project, comparing their global quantities, their physical structure, and their oscillation properties. We discuss the differences between models and identify the underlying reasons for these differences. The stellar models are representative of potential CoRoT targets. Overall we find very good agreement between the five different codes, but with some significant deviations. We find noticeable discrepancies (though still at the per cent level) that result from the handling of the equation of state, of the opacities and of the convective boundaries. The results of our work will be helpful in interpreting future asteroseismology results from CoRoT.

astro-ph↗

arXiv2006

On the primordial scenario for abundance variations within globular clusters. The isochrone test

Self-enrichment processes occurring in the early stages of a globular cluster lifetime are generally invoked to explain the observed CNONaMgAl abundance anticorrelations within individual Galactic globulars.We have tested, with fully consistent stellar evolution calculations, if theoretical isochrones for stars born with the observed abundance anticorrelations satisfy the observational evidence that objects with different degrees of these anomalies lie on essentially identical sequences in the Color-Magnitude-Diagram (CMD). To this purpose, we have computed for the first time low-mass stellar models and isochrones with an initial metal mixture that includes the extreme values of the observed abundance anticorrelations, and varying initial He mass fractions. Comparisons with 'normal' alpha-enhanced isochrones and suitable Monte Carlo simulations that include photometric errors show that a significant broadening of the CMD sequences occurs only if the helium enhancement is extremely large (in this study, when Y=0.35) in the stars showing anomalous abundances. Stellar luminosity functions up to the Red Giant Branch tip are also very weakly affected, apart from - depending on the He content of the polluting material - the Red Giant Branch bump region. We also study the distribution of stars along the Zero Age Horizontal Branch, and derive general constraints on the relative location of objects with and without abundance anomalies along the observed horizontal branches of globular clusters.

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arXiv2003

The age of the oldest Open Clusters

We determine ages of 71 old Open Clusters by a two-step method: we use main-squence fitting to 10 selected clusters, in order to obtain their distances, and derive their ages from comparison with our own isochrones used before for Globular Clusters. We then calibrate the morphological age indicator delta(V), which can be obtained for all remaining clusters, in terms of age and metallicity. Particular care is taken to ensure consistency in the whole procedure. The resulting Open Cluster ages connect well to our previous Globular Cluster results. From the Open Cluster sample, as well as from the combined sample, questions regarding the formation process of Galactic components are addressed. The age of the oldest open clusters (NGC6791 and Be17) is of the order of 10 Gyr. We determine a delay by 2.0+-1.5 Gyr between the start of the halo and thin disk formation, whereas thin and thick disk started to form approximately at the same time. We do not find any significant age-metallicity relationship for the open cluster sample. The cumulative age distribution of the whole open cluster sample shows a moderately significant (~2sigma level) departure from the predictions for an exponentially declining dissolution rate with timescale of 2.5 Gyr. The cumulative age distribution does not show any trend with galactocentric distance, but the clusters with larger height to the Galactic plane have an excess of objects between 2-4 and 6 Gyr with respect to their counterpart closer to the plane of the Galaxy.

astro-ph↗

arXiv2003

Spectroscopic study of blue compact galaxies III. Empirical population synthesis

This is the third paper of a series dedicated to the study of the star formation rates, star formation histories, metallicities and dust contents of a sample of blue compact galaxies (BCGs). We constrain the stellar contents of 73 blue compact galaxies by analyzing their continuum spectra and the equivalent widths of strong stellar absorption features using a technique of empirical population synthesis based on a library of observed star-cluster spectra. Our results indicate that blue compact galaxies are typically age-composite stellar systems; in addition to young stars, intermediate-age and old stars contribute significantly to the 5870 A continuum emission of most galaxies in our sample. The stellar populations of blue compact galaxies also span a variety of metallicities. The ongoing episodes of star formation started typically less than a billion years ago. Some galaxies may be undergoing their first global episode of star formation, while for most galaxies in our sample, older stars are found to contribute up to half the optical emission. Our results suggest that BCGs are primarily old galaxies with discontinuous star formation histories. The absorption equivalent widths measured in the synthetic spectra range from typically 1.5 A for Halpha, to 2 -- 5 A for Hbeta, Hgamma, and Hdelta. The implied accurate measurements of emission-line intensities will be used in a later study to constrain the star formation rates and gas-phase chemical element abundances of blue compact galaxies.

astro-ph↗

arXiv2001

A contribution of 26Al to the O-Al anticorrelation in globular cluster red giants

We modify our combined ("deep mixing" plus "primordial") scenario explaining the star-to-star abundance variations in globular cluster red giants in such a way that it confirms with new experimental data: (i) the new and better constrained (NACRE) thermonuclear reaction rates and (ii) a discovery of the O-Na anticorrelation in stars below the main-sequence turn-off in the cluster NGC6752. For the latter we propose that some main-sequence dwarfs in globular clusters accrete material lost by red giant primary components of close binaries during a common envelope event. As a consequence of the new reaction rates, we are drawn to the conclusion that the anomalies in [Al/Fe] in globular cluster red giants are in fact manifestations of 26Al (instead of the stable isotope 27Al) abundance variations produced by deep mixing.

astro-ph↗

arXiv2001

Atomic diffusion in metal-poor stars: II. Predictions for the Spite plateau

We have computed a grid of up-to-date stellar evolutionary models including atomic diffusion, in order to study the evolution with time of the surface Li abundance in low-mass metal-poor stars. We discuss in detail the dependence of the surface Li evolution on the initial metallicity and stellar mass, and compare the abundances obtained from our models with the available Li measurements in Pop II stars. While it is widely accepted that the existence of the Spite Li-plateau for these stars is a strong evidence that diffusion is inhibited, we show that, when taking into account observational errors, uncertainties in the Li abundance determinations, in the effective temperature scale, and in particular the size of the observed samples of stars, the Spite plateau and the Li abundances in subgiant branch stars can be reproduced also by models including fully efficient diffusion, provided that the most metal-poor field halo objects are between 13.5 and 14 Gyr old. We provide the value of the minimum number of plateau stars to observe, for discriminating between efficient or inhibited diffusion. From our models with diffusion we derive that the average Li abundance along the Spite plateau is about a factor of 2 lower than the primordial one. As a consequence, the derived primordial Li abundance is consistent with a high helium and low deuterium Big Bang Nucleosynthesis; this implies a high cosmological baryon density as inferred from the analyses of the cosmic microwave background.

astro-ph↗

arXiv2000

Evolutionary tracks and isochrones for alpha-enhanced stars

We present four large sets of evolutionary tracks for stars with initial chemical compositions [Y=0.250, Z=0.008], [Y=0.273, Z=0.019], [Y=0.320, Z=0.040] and [Y=0.390, Z=0.070] and enhancement of alpha elements with respect to the solar pattern. The major improvement with respect to previous similar calculations is that we use consistent opacities - i.e. computed with the same chemical composition as adopted in the stellar models - over the whole relevant range of temperatures. For the same initial chemical compositions [Y,Z] and otherwise identical input physics we present also new evolutionary sequences with solar-scaled mixtures of abundances. Based on these stellar models we calculate the corresponding sets of isochrones both in the Johnson-Cousins UBVRIJHK and HST/WFPC2 photometric systems. Furthermore, we derive integrated magnitudes, colours and mass-to-light ratios for ideal single stellar populations with total mass equal to 1 Msun. Finally, the major changes in the tracks, isochrones, and integrated magnitudes and colours passing from solar-scaled to alpha-enhanced mixtures are briefly outlined. Retrieval of the complete data set is possible via http://pleiadi.pd.astro.it .

astro-ph↗

arXiv2000

Age-luminosity relations for low-mass metal-poor stars

We present a grid of evolutionary calculations for metal-poor low-mass stars for a variety of initial helium and metal abundances. The intention is mainly to provide a database for deriving directly stellar ages of halo and globular cluster stars for which basic stellar parameters are known, but the tracks can also be used for isochrone or luminosity function construction, since they extend to the tip of the red giant branch. Fitting formulae for age-luminosity relations are provided as well. The uncertainties of the evolutionary ages due to inherent shortcomings in the models and due to the unclear effectiveness of diffusion are discussed. A first application to field single stars is presented.

astro-ph↗

arXiv2000

Evolution and surface abundances of red giants experiencing deep mixing

We have calculated the evolution of low metallicity red giant stars under the assumption of deep mixing between the convective envelope and the hydrogen burning shell. We find that the extent of the observed abundance anomalies, and in particular the universal O-Na anticorrelation, can be totally explained by mixing which does not lead to significant helium enrichment of the envelope. On the other hand, models with extremely deep mixing and strong helium enrichment predict anomalies of sodium and oxygen, which are much larger than the observed ones. This latter result depends solely on the nucleosynthesis inside the hydrogen burning shell, but not on the details of the mixing descriptions. These, however, influence the evolution of surface abundances with brightness, which we compare with the limited observational material available. Our models allow, nevertheless, to infer details on the depth and speed of the mixing process in several clusters. Models with strong helium enrichment evolve to high luminosities and show an increased mass loss. However, under peculiar assumptions, red giants reach very high luminosities even without extreme helium mixing. Due to the consequently increased mass loss, such models could be candidates for blue horizontal branch stars, and, at the same time, would be consistent with the observed abundance anomalies.

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arXiv1999

Colour transformations for isochrones in the VI-plane

Reliable temperature-colour transformations are a necessary ingredient for isochrones to be compared with observed colour-magnitude-diagrams of globular clusters. We show that both theoretical and empirical published transformations to (V-I) to a large extend exhibit significant differences between them. Based on these comparisons we argue for particular transformations for dwarfs and giants to be preferred. We then show that our selected combination of transformations results in fits of V-(V-I)-CMDs of a quality which is comparable to that of our earlier V-(B-V) isochrones for a wide range of cluster metallicities. The cluster parameters, such as reddening, are consistent with those derived in (B-V). Therefore, at least in the case of the fit with our own isochrones - based on the particular distance scale provided by our own horizontal branch models, and on the treatment of convection by the mixing-length theory having l/H_p calibrated on our solar model - the chosen transformations appear to lead to self-consistent (V-I) isochrones. Our isochrones are now well tested and self-consistent for B, V and I photometric data.

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arXiv1998

Helioseismological constraint on solar axion emission

Helioseismological sound-speed profiles severely constrain possible deviations from standard solar models, allowing us to derive new limits on anomalous solar energy losses by the Primakoff emission of axions. For an axion-photon coupling $g_{aγ} < 5 x 10^(-10) GeV^(-1)$ the solar model is almost indistinguishable from the standard case, while $g_{aγ} > 10 x 10^(-10) GeV^(-1)$ is probably excluded, corresponding to an axion luminosity of about $0.20 L_(sun)$. This constraint on $g_{aγ}$ is much weaker than the well-known globular-cluster limit, but about a factor of 3 more restrictive than previous solar limits. Our result is primarily of interest to the large number of current or proposed search experiments for solar axions because our limit defines the maximum g_{aγ}$ for which it is self-consistent to use a standard solar model to calculate the axion luminosity.

hep-ph↗