Using the most recent results about white dwarfs (WDs) in ten open clusters, we revisit semiempirical estimates of the initial-final mass relation (IFMR) 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 WD 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.
Red Giant Branch Stars: The Theoretical Framework Salaris, Maurizio; Cassisi, Santi; Weiss, Achim
Publications of the Astronomical Society of the Pacific,
04/2002, Letnik:
114, Številka:
794
Journal Article
Recenzirano
Odprti dostop
Theoretical predictions of red giant branch stars’ effective temperatures, colors, luminosities, and surface chemical abundances are a necessary tool for the astrophysical interpretation of the ...visible–near‐infrared integrated light from unresolved stellar populations, the color‐magnitude diagrams of resolved stellar clusters and galaxies, and spectroscopic determinations of red giant chemical abundances. On the other hand, the comparison with empirical constraints provides a stringent test for the accuracy of present generations of red giant models.
We review the current status of red giant star modeling, discussing in detail the still‐existing uncertainties affecting the model input physics (e.g., electron conduction opacity, treatment of the superadiabatic convection) and the adequacy of the physical assumptions built into the model computations.
We compare theory with several observational features of the red giant branch in Galactic globular clusters, such as the luminosity function “bump,” the luminosity of the red giant branch tip, and the envelope chemical abundance patterns, to show the level of agreement between current stellar models and empirical data concerning the stellar luminosities, star counts, and surface chemical abundances.
We describe the Garching Stellar Evolution Code. General features, treatment of the microphysics, details of the numerical solution, handling and particularities are discussed. The standard solar ...model serves as the most basic benchmark to test the accurateness of the code and is presented, too.
ABSTRACT
Thermal bombs are a widely used method to artificially trigger explosions of core-collapse supernovae (CCSNe) to determine their nucleosynthesis or ejecta and remnant properties. Recently, ...their use in spherically symmetric (1D) hydrodynamic simulations led to the result that 56,57Ni and 44Ti are massively underproduced compared to observational estimates for Supernova 1987A, if the explosions are slow, i.e. if the explosion mechanism of CCSNe releases the explosion energy on long time-scales. It was concluded that rapid explosions are required to match observed abundances, i.e. the explosion mechanism must provide the CCSN energy nearly instantaneously on time-scales of some ten to order 100 ms. This result, if valid, would disfavour the neutrino-heating mechanism, which releases the CCSN energy on time-scales of seconds. Here, we demonstrate by 1D hydrodynamic simulations and nucleosynthetic post-processing that these conclusions are a consequence of disregarding the initial collapse of the stellar core in the thermal-bomb modelling before the bomb releases the explosion energy. We demonstrate that the anticorrelation of 56Ni yield and energy-injection time-scale vanishes when the initial collapse is included and that it can even be reversed, i.e. more 56Ni is made by slower explosions, when the collapse proceeds to small radii similar to those where neutrino heating takes place in CCSNe. We also show that the 56Ni production in thermal-bomb explosions is sensitive to the chosen mass cut and that a fixed mass layer or fixed volume for the energy deposition cause only secondary differences. Moreover, we propose a most appropriate setup for thermal bombs.
Auch Sterne altern Weiss, Achim
Physik in unserer Zeit,
03/2020, Letnik:
51, Številka:
2
Journal Article
Zusammenfassung Sterne teilen uns ihr Alter nicht direkt mit. Da sie sich aber verändern, kann daraus ihr Alter bestimmt werden. Die meisten Bestimmungsmethoden sind modellabhängig. Daher ist es ...wichtig, die Physik der Sterne möglichst genau zu modellieren. Immerhin sind die Methoden aber gut genug, um daraus die Geschichte der Milchstraße, von externen Galaxien und auch des Universums zu rekonstruieren 12.
Auch Sterne altern Weiss, Achim
Physik in unserer Zeit,
March 2020, Letnik:
51, Številka:
2
Journal Article
Zusammenfassung
Sterne teilen uns ihr Alter nicht direkt mit. Da sie sich aber verändern, kann daraus ihr Alter bestimmt werden. Die meisten Bestimmungsmethoden sind modellabhängig. Daher ist es ...wichtig, die Physik der Sterne möglichst genau zu modellieren. Immerhin sind die Methoden aber gut genug, um daraus die Geschichte der Milchstraße, von externen Galaxien und auch des Universums zu rekonstruieren 12.
Das Alter des Universums und der Sterne zu ermitteln, gehört seit jeher zu den zentralen Aufgaben der Astrophysik. Mittlerweile gibt es hierfür eine Vielzahl von Methoden. Einige liefern erstaunlich genaue Werte, andere sind unsicherer. Was also wissen wir heute und mit welcher Genauigkeit über das Alter von Sternen?
Auch Sterne altern Weiss, Achim
Physik in unserer Zeit,
03/2020, Letnik:
51, Številka:
2
Journal Article
ZusammenfassungSterne teilen uns ihr Alter nicht direkt mit. Da sie sich aber verändern, kann daraus ihr Alter bestimmt werden. Die meisten Bestimmungsmethoden sind modellabhängig. Daher ist es ...wichtig, die Physik der Sterne möglichst genau zu modellieren. Immerhin sind die Methoden aber gut genug, um daraus die Geschichte der Milchstraße, von externen Galaxien und auch des Universums zu rekonstruieren 12.
Improving 1D Stellar Models with 3D Atmospheres Mosumgaard, Jakob Rørsted; Silva Aguirre, Víctor; Weiss, Achim ...
EPJ Web of Conferences,
01/2017, Letnik:
160
Journal Article, Conference Proceeding
Recenzirano
Odprti dostop
Stellar evolution codes play a major role in present-day astrophysics, yet they share common issues. In this work we seek to remedy some of those by the use of results from realistic and highly ...detailed 3D hydrodynamical simulations of stellar atmospheres. We have implemented a new temperature stratification extracted directly from the 3D simulations into the Garching Stellar Evolution Code to replace the simplified atmosphere normally used. Secondly, we have implemented the use of a variable mixing-length parameter, which changes as a function of the stellar surface gravity and temperature – also derived from the 3D simulations. Furthermore, to make our models consistent, we have calculated new opacity tables to match the atmospheric simulations. Here, we present the modified code and initial results on stellar evolution using it.
We calculated synthetic spectra for typical chemical element mixtures (i.e., a standard α-enhanced distribution, and distributions displaying CN and ONa anticorrelations) found in the various ...subpopulations harboured by individual Galactic globular clusters. From the spectra we determined bolometric corrections to the standard Johnson-Cousins and Strömgren filters and finally predicted colours. These bolometric corrections and colour-transformations, coupled to our theoretical isochrones with the appropriate chemical composition, provided us with a complete and self-consistent set of theoretical predictions for the effect of abundance variations on the observed cluster colour–magnitude diagrams. CNO abundance variations affect mainly wavelengths shorter than ~400 nm owing to the rise of molecular absorption bands in cooler atmospheres. As a consequence, colour and magnitude changes are largest in the blue filters, independently of using broad or intermediate bandpasses. Colour–magnitude diagrams involving uvy and UB filters (and their various possible colour combinations) are therefore best suited to infer photometrically the presence of multiple stellar generations in individual clusters. They are particularly sensitive to variations in the N abundance, with the largest variations affecting the red giant branch (RGB) and lower main sequence (MS). BVI diagrams are expected to display multiple sequences only if the different populations are characterized by variations of the C+N+O sum and/or helium abundance that lead to changes in luminosity and effective temperature, but leave the flux distribution above 400 nm practically unaffected. A variation of just the helium abundance up to the level we investigate here exclusively affects the interior structure of stars, and is largely irrelevant for the atmospheric structure and the resulting flux distribution in the whole wavelength range spanned by our analysis.
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