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Reference: Richard S. Ellis, (1974). Stellar abundances and nucleosynthesis. DPhil. University of Oxford.Citable link to this page:

 

Stellar abundances and nucleosynthesis

Abstract: Stellar abundances are essential parameters for many branchesof astronomy. They are also particularly important as tests fortheories of nucleosynthesis. Despite the encouraging results fromband photometry, detailed spectroscopic analyses are still important infurthering knowledge of physical processes occurring in a variety ofstellar objects. This thesis presents the first of a series ofabsolute abundances for the late-type giant Arcturus. The broaderaim of the investigation is to make Arcturus a reliable standardstar, and also it is hoped that equilibrium assumptions in its atmospherecan be scrutinised.An essential prerequisite to a model atmosphere is the continuumflux curve. Certain discrepancies near the flux maximum promptedthe construction of a spectrum scanner which was used to obtain fluxesof Arcturus at the Wise Observatory, Israel. All known flux resultshave been collated for Arcturus, and the continuum flux curve ispresented for the region 3800 Å to 13 microns.Ways of measuring the effective temperature are discussed. Theonly adequate method for Arcturus and other cool stars involves comparingthe relative energy distribution with model predictions. Asall present models fail to account for the flux below 5000 Å thismethod is not as accurate as it could be. A value of 4450 ± 50┬░K isadopted for Arcturus; most of this error is due to the unrealisticnature of the models.Using this temperature a grid of line-blanketed models withdifferent gravities has been constructed. The suggested gravity-indicatorrequires a precise value for the microturbulence, and forthis preliminary analysis a model with log g = 1.7 was selected.This value, reported by several investigators, is later shown to befairly realistic.Uncertain oscillator strengths continue to plague abundancework in astronomy, though the Oxford furnace is now producingprecision values for several elements. Using this furnace a series ofneutral titanium lines of direct astrophysical interest have beenmeasured, and they are accurate to at least ±0.03 dex(7%) on arelative scale. Considerable effort is still needed in establishingaccurate absolute scales however; these results have been made absoluteby referring to theoretical and observational lifetimes.The internal consistency of the Ti I values has been verifiedby studying the solar abundance of titanium. Rf restricting theanalysis to good quality weak lines, uncertainties in damping andmicroturbulence have been avoided. A log abundance of 4.81 ± 0.08is reported (on the log H = 12.00 scale).For the Arcturus analysis, a few iron lines have also beenincluded. In this star the abundances are very sensitive to the valueassumed for the microturbulence. The empirical nature of this parameteris emphasised. A value of 2.1 ± 0.2 km sec−1 was found bydemanding an unique abundance for all lines. Using this velocitygave absolute abundances of:- log n(Ti) = 4.42 ± 0.15 log n(Fe) = 7.37 ± 0.10 With suitable solar abundances these imply [Ti/H] = −0.39 ± 0.07,[Fe/H] = −0.33 ± 0.10.Theories accounting for the origin of iron-peak elements arediscussed. Explosive Silicon burning cannot account for all theobserved isotopes, and the equilibrium process has been re-introducedto explain some anomalies. These theories are examined in relationto supernova models and Galactic evolution, and also with regard tothe broader purpose and prospects for measurements of stellarabundances.

Type of Award:DPhil Level of Award:Doctoral Awarding Institution: University of Oxford Notes:This thesis was digitised thanks to the generosity of Dr Leonard Polonsky

Bibliographic Details

Issue Date: 1974Identifiers

Urn: uuid:c930d309-ce96-429d-8cbb-a914c931a8cf

Source identifier: 601870681 Item Description

Type: Thesis;

Language: eng Tiny URL: td:601870681

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Author: Richard S. Ellis - institutionUniversity of Oxford oxfordCollegeWolfson College - - - - Bibliographic Details Issue Date: 1974 -

Source: https://ora.ox.ac.uk/objects/uuid:c930d309-ce96-429d-8cbb-a914c931a8cf



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