Simple Chemical Evolution Models Research Articles

The age-metallicity relationship for the clusters of the Large Magellanic Cloud

Moderate dispersion spectrophotometric scans with the intensified Reticon array constructed by S. Shectman have been obtained using the du Pont telescope of the Las Campanas Observatory for 38 stars expected to be members of 15 clusters in the Large Magellanic Cloud. Ages for these clusters are deduced from a transformation of their classification in the scheme of Searle, Wilkinson, and Bagnuolo. Abundances are derived from the scans using a crude analysis applied to computer-generated pseudoequivalent widths calibrated by identical observations of 42 stars in six galactic globular clusters and by several galactic supergiants. A strong age-metallicity relationship is found and the chemical history of the LMC is discussed. Unlike the solar neighborhood or galactic halo, simple models of chemical evolution are adequate to fit our derived age-abundance correlation.

Chemical evolution with inhibited star formation rate

In this paper a simple model of chemical evolution is proposed, according to which active phases of star formation occured, each followed by a quiescent phase where the formation of stars withm≲1m⊙ was totally inhibited. The contraction of the gaseous component of the system was not stopped during quiescent phases, according to observed distributions of chemical and dynamical parameters in objects of different populations. Building up a detailed model we assume that: (i) star formation is inhibited during a given active phase, at a rate which is connected to the number of supernovae born at that time; (ii) total mass is conserved in the unit comoving volume; (iii) instantaneous recycling approximation holds. According to these hypothesis, theoretical metal abundance distributions as function of a single parameter\(\bar \mu _f \) may be derived and then compared with empirical distributions related to different kinds of populations. It is found that theoretical curves systematically overestimate the objects of lower metallicity and underestimate the objects of higher metallicity in respect to observations, as a possible consequence of the last two assumptions reported above. If we leave the detailed explanation of this effect to more refined models and taking other observational data into account, the chemical evolution of both Galactic spheroid component and solar neighbourhood may be derived and the results are to be considered as essentially qualitative. If three active phases of star formation occurred during the history of the Galactic spheroid component, the yield of metals,\(\hat p\), had to increase passing from an active phase to the next even if\(\hat p\,\alpha \,Z\) (metal content) and the disk mass fraction,RD, had not to exceed a value of about 0.4 when\(\hat p\, \approx 0.01\) is assumed as a characteristic value for the disk component and the main part of this last is thought to have formed at the end of the second active phase. If a single active (and no quiescent) phase occurred during the history of the solar neighbourhood, the yield did not increase appreciably up to to-day and no more than half (if any) of the initial mass could escape from the system in form of galactic wind. A disk mass fractionRD≲0.2 is further derived, if a birth-rate stellar function smoothly dependent on the gas mass fraction and a ratio of mean past to present star formation rate of the order of unity are to be reproduced, in agreement with recent observations.

Where is Population III

view Abstract Citations (144) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Where is population III ? Bond, H. E. Abstract A simple one-zone model of chemical evolution in the galactic halo predicts that the halo should contain appreciable numbers of 'Population III' stars (roughly defined as stars having less than 1/1000 solar metal content, or Fe/H less than -3). However, unbiased samples of halo globular clusters, dwarf spheroidal systems, and high-velocity subdwarfs all show a statistically significant lack of objects with Population III metallicity. Even extensive special searches for field stars of ultralow metallicity made by the writer and others have failed to reveal more than a very small number of objects that may have Fe/H less than -3. A chemical evolution model in which the primordial gas stars with a small but nonzero metallicity near Fe/H = -2.6 is shown to predict a metallicity distribution function in the galactic halo that is in good agreement with the observations. Since it seems unlikely that the galaxy literally formed from gas with nonzero metallicity, the most probable explanation seems to be that low-mass stars cannot easily form from gas of extremely low metallicity. Thus the stellar mass function of Population III was such that very few long-lived stars with a mass less than approximately 0.8 solar mass were formed. If Population III produced any globular clusters, they are now 'extinct'; that is, they contain only nonluminous remnants of massive stars. It has been known for some years that the galactic disk lacks stars with Fe/H less than approximately -1, and this situation has been referred to as the 'G-dwarf problem'. This paper shows that a 'G-dwarf problem' also exists in the galactic halo. Publication: The Astrophysical Journal Pub Date: September 1981 DOI: 10.1086/159186 Bibcode: 1981ApJ...248..606B Keywords: Chemical Evolution; Galactic Evolution; Globular Clusters; Metallic Stars; Star Distribution; Abundance; Distribution Functions; G Stars; Solar System; Stellar Evolution; Stellar Mass; Astrophysics full text sources ADS | data products SIMBAD (2)

Abundances and Chemical Evolution of the Galactic Center

AbstractFrom observations of the galactic center using various techniques radioastronomy, millimeter waves (molecules) – infrared and gamma rays, the interstellar matter of this region* appears to have been strongly processed into stars : the gas density is much lower than in the solar neighbourhood. From CO measurements one knows that there are many molecular clouds such as SgrB2 where stars are forming now. From IR measurements, there are some indication that low mass stars are relatively more numerous in such regions than in the external regions of the galaxy. Finally the heavy element abundances show three important features (i) the possibility of strong enhancements in elements such as N and in a less extent 0 and Ne (the so called abundance gradients), (ii) Some specific enhancements of isotopes such43C,44N and also47O relative to42C,45N and43O (iii) Deuterium seems to have a lower abundance than in other parts of the galaxy such as the solar neighbourhood. Simple models of chemical evolution have been designed to account for such features and are rewiewed here.