Primordial nucleosynthesis with massive #tau# neutrinos

A massive long-lived #tau# neutrino in the MeV regime modifies the primordial light-element abundances predicted by big-bang nucleosynthesis (BBN) calculations. This effect has been used to derive limits on m_#nu#__#tau#_. Because recently the observational situation has become somewhat confusing and, in part, intrinsically inconsistent, we reconsider the BBN limits on m_#nu#__#tau#_. To this end we use our newly developed BBN code to calculate the primordial abundances as a function of m_#nu#__#tau#_ and of the cosmic baryon density #eta#. We derive concordance regions in the #eta#-m_#nu#__#tau#_-plane for several sets of alleged primordial abundances. In some cases a concordance region exists only for a nonvanishing m_#nu#__#tau#_. At the present time BBN does not provide clear evidence for or against a #tau# neutrino mass. (orig.)28 refs.Available from TIB Hannover: RR 4697(999) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Episodic lithium production by extra-mixing in red giants

SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Deep diffusive mixing in globular-cluster red giants

Additional mixing between the hydrogen burning shell and the base of convective envelope in stars on their first ascent of the red giant branch is traditionally considered as a possible cause of the star-to-star abundance variations in globular clusters. In the present paper we model this deep mixing by diffusion. Nuclear kinetics equations coupled with diffusion terms are solved to follow a time evolution of stellar surface abundances for isotopes taking part in reactions of the pp-chains, CNO-, NeNa- and MgAl-cycles. Our task is to reproduce the ''global anticorrelation'' of [O/Fe] versus [Na/Fe] found recently in globular cluster and halo giants by Kraft et al. (1993) and the correlation of [C/Fe] with M_V observed in M 92 (Langer et al. 1986). It is shown that both correlations can be accounted for by deep diffusive mixing with parameters for mixing depth and rate adjusted individually for each correlation. The deep mixing in red giants with solar metallicity is demonstrated to allow the interpretation of anomalous "1"7O/"1"6O and "1"8O/"1"6O isotope ratios in S and C asymptotic giant branch stars. In the conclusion we propose an observational test for the discussed deep mixing. (orig.)Available from TIB Hannover: RR 4697(868) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Homogeneous age dating of 55 galactic globular clusters Glues to the galaxcy formation mechanisms

SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

"3He in stars of low and intermediate mass

We follow the evolution of "3He in stars of mass 0.8... 10M_sun 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 5M_sun 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. (orig.)Available from TIB Hannover: RR 4697(905) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Could intermediate-mass AGB stars produce star-to-star abundance variations in globular-cluster red giants?

We performed detailed nucleosynthesis calculations in hydrogen and helium burning shells as well as for hot bottom burning at the base of convective envelope in a low-metallicity (Z=10"-"4) intermediate-mass star during thermal-pulsing AGB evolution. Based on complete stellar models, up-to-date simple analytical expressions were used to describe the model star and its evolution. Our study concentrated on surface abundances of light elements, such as C, N, O, Na and Al, and their isotopes in order to test a hypothesis of Cottrell and Da Costa (1981) frequently invoked to explain star-to-star abundance variations in globular-cluster red giants. It is shown that this hypothesis of primordial contamination of intracluster matter by nuclear products from early generation AGB stars fails to reproduce the observed O depletion and Al enhancement. We propose an alternative mechanism which combines some primordial composition anomalies with atmospheric abundance effects produced by deep mixing in globular-cluster red giants. We also suggest observational tests for the verification of our model. (orig.)53 refs.Available from TIB Hannover: RR 4697(941) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Chronology of the halo globular cluster system formation

Using updated stellar models and isochrones we determine the age of 25 galactic halo clusters. The clusters are distributed into four groups of metallicity. We use a combination of two methods for obtaining either the absolute age of a template cluster within each group or relative ages for all other clusters of the same group 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 cluster groups of lower metallicity appear to be almost coeval, but our results indicate that globally the sample has an age spread without a simple age-metallicity relation. (orig.)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

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 obsefvational 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. (orig.)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

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

SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

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 NGC 6352 have been performed with our up-to-date #alpha#-enhanced stellar models. We find that all three clusters are about 9.5 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 sub-dwarfs. 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. (orig.)63 refs.SIGLEAvailable from TIB Hannover: RR 4697(1079) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman