arXiv subjects
Publications and source records attributed to B. E. J. Pagel.
Changing the form of the star cluster mass function (CMF) can effectively change the upper end of the stellar initial mass function. The yields of from supernovae are very sensitive to the mass of the progenitor star. We show that by changing the parameters of the CMF, it is possible to change the yields of oxygen and magnesium by a factor of $\sim 1.5$ and of metals in general by a factor of $\sim 1.8$.
Highlights of the meeting include new insights into r and s-processes and an explosion of interesting abundance data on stars in the Galactic halo and in dwarf spheroidal galaxies. I include in this summary a few suggestions on the chemical evolution of the Thick and Thin disks, and on the present and past distribution of baryons and metals in the universe.
CNO abundances in galaxies bear on issues of galactic evolution as well as stellar evolution and nucleosynthesis. Knowledge about them in dwarf and spiral galaxies depends mainly on emission lines from H II regions, with information from stars and supernova remnants available for galaxies in the Local Group. Oxygen abundances can be related to both local and global properties of the parent galaxies such as gas fraction, surface brightness, galaxy luminosity, rotation speed and morphological type, and in H II regions they range from 1/30th solar (using the new calibration) to slightly above solar. C/O is a more or less smoothly increasing function of O/H, flat at low metallicities and rising above 12+log(O/H) = 8.0 with a 45 degree slope. N/O behaves quite similarly in the traditional `primary followed by secondary' style, but with a substantial scatter at least around O-abundance 8.0. C/N ratios show no clear trend with metallicity.
An attempt is made to guess the overall cosmic abundance of `metals' and the contribution made by the energy released in their production to the total intensity of extragalactic background light (EBL). With a comparable or somewhat larger amount coming from white dwarfs, and a probably quite modest contribution from AGNs, one can fairly easily account for the lower end of the range of existing estimates for the total EBL intensity (50to 60 nwt m$^{-2}$ sterad$^{-1}$), but it seems more difficult should some higher estimates (90 to 100 in the same units) prove to be correct.
Thin and Thick disk metallicities overlap, but alpha(Fe) relations differ; there is also a hiatus in time before the first thin-disk stars, with a minimum metallicity [Fe/H] = -0.6. Thus there are two `G-dwarf' problems. We fit metallicity distribution functions (MDFs) for the two disks derived by Wyse & Gilmore (1995) with analytical models (for O and Mg). The Thick disk fits a simple inflow model with effective yield and final [Mg/H] about the same as for the Thin Disk, excluding continuous mass loss from the former as a source of prompt initial enrichment (PIE) for the latter. The Thin disk has a narrow MDF fitted (rather poorly) with a closed model having PIE from some other source, or more elegantly with a slow infall model similar to the `two-inflow' scenario of Matteucci et al., but with a PIE and a severe break in the age-metallicity relation. Implications for radio-active cosmochronology are investigated.
Chemical evolution of galaxies brings together ideas on stellar evolution and nucleosynthesis with theories of galaxy formation, star formation and galaxy evolution, with all their associated uncertainties. In a new perspective brought about by the Hubble Deep Field and follow-up investigations of global star formation rates, diffuse background etc., it has become necessary to consider the chemical composition of dark baryonic matter as well as that of visible matter in galaxies.
We try to understand the s- and r-process elements vs Ti/Fe plots derived by Jehin et al. (1999) for mildly metal-poor stars within the framework of the analytical semi-empirical models for these elements by Pagel & Tautvaisiene (1995, 1997). Jehin et al. distinguished two Pop II subgroups: IIa with alpha/Fe and s-elements/Fe increasing together, which they attribute to pure SNII activity, and IIb with constant alpha/Fe and a range in s/Fe which they attribute to a prolonged accretion phase in parent globular clusters. However, their sample consists mainly of thick-disk stars with only 4 clear halo members, of which two are `anomalous' in the sense defined by Nissen & Schuster (1997). Only the remaining two halo stars (and one in Nissen & Schuster's sample) depart significantly from Y/Ti (or s/alpha) ratios predicted by our model.
Initial conditions are set by Big bang nucleosynthesis from which we know that 90 per cent of baryons are dark and have essentially unknown chemical composition. In our own Galaxy, there are many clues from individual stars in different populations whereas in elliptical galaxies the data largely come from integrated spectra, but these raise problems enough like the Mg/Fe and G-dwarf problems. Irregular and blue compact galaxies display the primary--secondary transition in N/O; this in turn may be relevant to element ratios observed in damped Lyman-alpha systems at high red-shift, which offer rather little evidence for for pure SNII synthesis such as is found in the Galactic halo stars. A recent estimate of past star formation rates as a function of red-shift is presented and the appropriateness of the conventional conversion factor of 42 from SFR to metal production is discussed. For any reasonable value of this conversion factor, it is clear that most of the metals existing at z=2.5 have yet to be detected.
We have extended our analytical chemical evolution modelling ideas for the Galaxy to the Magellanic Clouds. Unlike previous authors, we assume neither a steepened IMF nor selective galactic winds, since among the alpha-particle elements only oxygen shows a large deficit relative to iron and a similar deficit is also found in Galactic supergiants. Thus we assume yields and time delays identical to those we previously assumed for the solar neighbourhood. We include inflow and non-selective galactic winds and consider both smooth and bursting star formation rates, the latter giving a better fit to the age-metallicity relations. We predict essentially solar abundance ratios for primary elements and these seem to fit most of the data within their substantial scatter. Our LMC model also gives a remarkably good fit to the anomalous Galactic halo stars discovered by Nissen & Schuster (1997). Our models predict current ratios of SNIa to core-collapse supernova rates enhanced by 50 per cent and 25 per cent respectively relative to the solar neighbourhood, in fair agreement with ratios found by Cappellaro et al. (1993) for Sdm-Im relative to Sbc galaxies, but these ratios are sensitive to detailed assumptions about the bursts. We consider that the high SNIa/SNII ratios found in some late-type galaxies are related to their star formation history rather than to any differences in the IMF.
The slope Delta Y/Delta Z is a quantity of interest in relation to stellar evolution, the initial mass function and the determination of the primordial helium abundance. In this paper we estimate Delta Y/Delta Z from fine structure in the Main Sequence of nearby stars from Hipparcos data for stars with Z \leq Zsun and find a value of about 3, which is consistent with what has been found in extragalactic H II regions and with stellar models for suitable upper limits to the initial masses of supernovae according to the IMF slope adopted.
We try to explain the spatial variation of primordial deuterium suggested by some observations by varying leptonic chemical potentials. The variation of the latter may take place in some scenarios of leptogenesis. The model predicts a large mass fraction of $^4 He$ (35-60%) and $^7 Li$ (up to $10^{-9}$) in deuterium-rich regions. Because of lepton family symmetry, the angular variations of cosmic microwave background radiation can be sufficiently small although still observable in future measurements.