arXiv subjects

Achim Weiss

Publications and source records attributed to Achim Weiss.

59 records · Page 4Linked to original sources

arXiv1998

Metal-rich globular clusters in the galactic disk: new age determinations and the relation to halo clusters

New age determinations of the galactic disk globular clusters 47 Tuc, M71 and NGC6352 have been performed with our up-to-date alpha-enhanced stellar models. We find that all three clusters are about 9.2 Gyr old and therefore coeval with the oldest disk white dwarfs. Several arguments are presented which indicate that the initial helium content of the stars populating these clusters is close to the solar one. We also revisit a total of 28 halo clusters, for which we use an updated [Fe/H] scale. This new metallicity scale leads on average to an age reduction of around 0.8 Gyr relative to our previous results. We compare the predicted cluster distances, which result from our dating method, with the most recent distances based on HIPPARCOS parallaxes of local subdwarfs. We further demonstrate that for the most metal-rich clusters scaled-solar isochrones no longer can be used to replace alpha-enhanced ones at the same total metallicity. The implications of the presented age determinations are discussed in the context of the formation history of the Galaxy.

astro-ph↗

arXiv1997

The age of the most nearby star

We address the question how accurately stellar ages can be determined by stellar evolution theory. We select the star with the best observational material available - our Sun. We determine the solar age by fitting solar evolution models to a number of observational quantities including several obtained from helioseismology, such as photospheric helium abundance or p-mode frequencies. Different cases with respect to the number of free parameters and that of the observables to be fitted are investigated. Age is one of the free parameters determined by the procedure. We find that the neglect of hydrogen-helium-diffusion leads to ages deviating by up to 100% from the true, meteoritic solar age. Our best models including diffusion yield ages by about 10% too high. The implication for general stellar age determination is that a higher accuracy than that can not be expected, even with the most up-to-date models. Our results also confirm that diffusion as treated presently in solar models is slightly too effective.

astro-ph↗

arXiv1997

Chronology of the halo globular cluster system formation

Using up-to-date stellar models and isochrones we determine the age of 25 galactic halo clusters. The clusters are distributed into four groups according to metallicity. We measure the absolute age of a reference cluster in each group, and then find the relative ages of the other clusters relative to this one. This combination yields the most reliable results. We find that the oldest cluster group on average is 11.8+-0.9 Gyr or 12.3+-0.3 Gyr old, depending on whether we include Arp2 and Rup106. The average age of all clusters is about 10.5 Gyr. Questions concerning a common age for all clusters and a relation between metallicity and age are addressed. The groups of lower metallicity appear to be coeval, but our results indicate that globally the sample has an age spread without a simple age-metallicity relation.

astro-ph↗

arXiv1995

3He in stars of low and intermediate mass

We follow the evolution of 3He in stars of mass 0.8 -- 10 M (solar units) for Pop. I and II compositions from the main sequence until advanced stages on the AGB. Under standard assumptions we confirm earlier results of more restricted investigations that low-mass stars up to 5 M are net producers of 3He. We show that the inclusion of additional mixing due to diffusion beneath the convective envelope simultaneously leads to observed carbon isotope anomalies observed in globular cluster Red Giants and to a strong reduction of 3He, such that stars exhibiting such anomalies will have destroyed 3He contrary to the standard picture.

astro-ph↗

arXiv1994

Arc Statistics with Realistic Cluster Potentials. II. Influence of Cluster Asymmetry and Substructure

We construct a sample of numerical models for clusters of galaxies and employ these to investigate their capability of imaging background sources into long arcs. Emphasis is laid on the statistics of these arcs. We study cross sections for arc length and length-to-width ratio and optical depths for these arc properties, we examine the distribution of arc widths and curvature radii among long arcs, and we compare these results to predictions based on simplified (radially symmetric) cluster models. We find that the capability of the numerically modeled clusters to produce long arcs is larger by about two orders of magnitude than that of spherically symmetric cluster models with the same observable parameters (core radii and velocity dispersions), and that they are similarly efficient as singular isothermal spheres with the same velocity-dispersion distribution. The influence of source ellipticity is also investigated; we find that the optical depth for arcs with a length-to-width ratio $\ga10$ is significantly larger for elliptical than for circular sources. Given these results, we conclude that spherically symmetric lens models for galaxy clusters, adapted to the observable parameters of these clusters, grossly underestimate the frequency of long arcs. We attribute this difference between numerically constructed and simplified analytical lens models to the abundance and the extent of intrinsic asymmetry and to substructure in galaxy clusters.

astro-ph↗