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On the Galactic Chemical Evolution of Sulphur

Matrozis, Elvijs LU (2013) In Lund Observatory Examensarbeten ASTM31 20131
Department of Astronomy and Theoretical Physics
Lund Observatory
Abstract
Sulphur is one of the more interesting chemical elements from a scientific perspective. As a volatile element, sulphur could potentially be used as a tracer of galactic properties, such as star-formation rate and initial mass function, in both the Milky Way and beyond. But first its origin and production has to be understood. Knowledge of Galactic chemical evolution of sulphur is therefore vital. Previous results from spectroscopic analyses of metal-poor stars in our Galaxy have lead to incompatible conclusions: 1) the chemical evolution of sulphur qualitatively follows that of other alpha elements (the standard scenario); 2) large amounts of sulphur were somehow produced in the early Universe, and since then the sulphur production rate... (More)
Sulphur is one of the more interesting chemical elements from a scientific perspective. As a volatile element, sulphur could potentially be used as a tracer of galactic properties, such as star-formation rate and initial mass function, in both the Milky Way and beyond. But first its origin and production has to be understood. Knowledge of Galactic chemical evolution of sulphur is therefore vital. Previous results from spectroscopic analyses of metal-poor stars in our Galaxy have lead to incompatible conclusions: 1) the chemical evolution of sulphur qualitatively follows that of other alpha elements (the standard scenario); 2) large amounts of sulphur were somehow produced in the early Universe, and since then the sulphur production rate has dropped substantially; 3) a combination of the two, indicating a heterogeneous chemical evolution of the Milky Way. In this project we employ the technique of spectrum synthesis to derive sulphur abundances in the atmospheres of a sample of 39 giant and dwarf stars from low (Fe/H ∼ −2.5) to solar metallicities, based on calculated one-dimensional, local thermodynamic equilibriumMARCS models. This study aims to improve on previous studies by: 1) using a homogeneously determined set of stellar parameters to reduce the random errors in the derived abundances; 2) using the forbidden sulphur line at 1082 nm which has only recently become available for analysis and is more robust against some of the assumptions made in the models.

We find that 22 of our 39 stars have Fe/H < −1. The sulphur abundances of
these stars show general agreement with the standard scenario and with the predictions of contemporary models of Galactic chemical evolution. The continued rise in sulphur abundances towards lower metallicity, as found in some studies, is not confirmed, however, more observations of stars with metallicities between −1.5 and −1.0 are needed to reach more definite conclusions.

We conclude that relatively moderate errors in stellar parameters can lead to large errors in sulphur abundances, underscoring the need for accurate stellar parameters. We further find that the forbidden sulphur line can be detected in giant stars with Fe/H > ∼ −2.3 and is found to give lower abundance than other, more model-dependent diagnostics. However, when these model-dependencies are corrected for, other diagnostics give systematically lower abundances than the forbidden sulphur line, suggesting that either these corrections are overestimated or other corrections, likely due to our
assumption of one dimension, can not be neglected. (Less)
Abstract (Swedish)
Alla naturligt förekommande grundämnen, med få undantag, har skapats i stjärnors inre. Några av dem har skapats långsamt och kontinuerligt och stora mängder har ansamlats under årmiljonerna. Andra har skapats under några få sekunder i några av de mest våldsamma explosionerna i universum, nämligen de s.k. supernovorna. Genom att studera dessa processers avtryck, kan vi fördjupa vår kunskap om, t.ex., hur galaxer bildas och utvecklas. Stjärnor är utmärkta laboratorier för sådana studier. Det ljus som stjärnor avger beror på dess temperatur, massa, storlek, och, viktigast, på de grundämnen som finns på stjärnans yta. Stjärnljus tillåter oss alltså att sluta oss till hur stora dessa grundämnens förekomster var i Vintergatan vid den tidpunkt då... (More)
Alla naturligt förekommande grundämnen, med få undantag, har skapats i stjärnors inre. Några av dem har skapats långsamt och kontinuerligt och stora mängder har ansamlats under årmiljonerna. Andra har skapats under några få sekunder i några av de mest våldsamma explosionerna i universum, nämligen de s.k. supernovorna. Genom att studera dessa processers avtryck, kan vi fördjupa vår kunskap om, t.ex., hur galaxer bildas och utvecklas. Stjärnor är utmärkta laboratorier för sådana studier. Det ljus som stjärnor avger beror på dess temperatur, massa, storlek, och, viktigast, på de grundämnen som finns på stjärnans yta. Stjärnljus tillåter oss alltså att sluta oss till hur stora dessa grundämnens förekomster var i Vintergatan vid den tidpunkt då stjärnan bildades. Genom att studera ljuset från stjärnor med olikaåldrar, kan vi ta ögonblicksbilder av vår galax vid dessa olika tidpunkter och därmed förstå hur ymnigheterna av de olika grundämnena har ändrats över tid och vad detta betyder.

I denna avhandling har jag studerat grundämnet svavels kosmiska historia i detalj genom att studera spektra av omkring 40 stjärnor. Svavel är ett av de tio mest förekommande grundämnena i universum. Tidigare studier har kommit fram till olika slutsatser och innebörd för Vintergatans utveckling i stort. Olika studier besvarar t.ex. följande frågor på olika sätt: bildades det relativt sett fler tunga stjärnor när vår galax var yngre? Är dagens supernovor annorlunda jämfört med tidigare generationer av supernovor? Liknar svavels historia några andra grundämnens, eller är den unik?

Resultaten som redovisas i denna avhandling stödjer iden att svavels historia speglar den av kisel och kalcium, vilket innebär att svavel skapades i explosionsstadiet av supernovor av typ II. Dessa är resultatet av normala massiva stjärnor, då deras bränsle tagit slut och då de alltså inte kan hålla emot gravitationen längre. Våra resultat stöds av aktuella teoretiska modeller of galaktisk kemisk utveckling. (Less)
Please use this url to cite or link to this publication:
author
Matrozis, Elvijs LU
supervisor
organization
course
ASTM31 20131
year
type
H2 - Master's Degree (Two Years)
subject
publication/series
Lund Observatory Examensarbeten
report number
2013-EXA74
language
English
id
3806540
date added to LUP
2013-06-04 14:06:13
date last changed
2013-06-05 13:28:18
@misc{3806540,
  abstract     = {Sulphur is one of the more interesting chemical elements from a scientific perspective. As a volatile element, sulphur could potentially be used as a tracer of galactic properties, such as star-formation rate and initial mass function, in both the Milky Way and beyond. But first its origin and production has to be understood. Knowledge of Galactic chemical evolution of sulphur is therefore vital. Previous results from spectroscopic analyses of metal-poor stars in our Galaxy have lead to incompatible conclusions: 1) the chemical evolution of sulphur qualitatively follows that of other alpha elements (the standard scenario); 2) large amounts of sulphur were somehow produced in the early Universe, and since then the sulphur production rate has dropped substantially; 3) a combination of the two, indicating a heterogeneous chemical evolution of the Milky Way. In this project we employ the technique of spectrum synthesis to derive sulphur abundances in the atmospheres of a sample of 39 giant and dwarf stars from low (Fe/H ∼ −2.5) to solar metallicities, based on calculated one-dimensional, local thermodynamic equilibriumMARCS models. This study aims to improve on previous studies by: 1) using a homogeneously determined set of stellar parameters to reduce the random errors in the derived abundances; 2) using the forbidden sulphur line at 1082 nm which has only recently become available for analysis and is more robust against some of the assumptions made in the models.

We find that 22 of our 39 stars have Fe/H < −1. The sulphur abundances of
these stars show general agreement with the standard scenario and with the predictions of contemporary models of Galactic chemical evolution. The continued rise in sulphur abundances towards lower metallicity, as found in some studies, is not confirmed, however, more observations of stars with metallicities between −1.5 and −1.0 are needed to reach more definite conclusions.

We conclude that relatively moderate errors in stellar parameters can lead to large errors in sulphur abundances, underscoring the need for accurate stellar parameters. We further find that the forbidden sulphur line can be detected in giant stars with Fe/H > ∼ −2.3 and is found to give lower abundance than other, more model-dependent diagnostics. However, when these model-dependencies are corrected for, other diagnostics give systematically lower abundances than the forbidden sulphur line, suggesting that either these corrections are overestimated or other corrections, likely due to our
assumption of one dimension, can not be neglected.},
  author       = {Matrozis, Elvijs},
  language     = {eng},
  note         = {Student Paper},
  series       = {Lund Observatory Examensarbeten},
  title        = {On the Galactic Chemical Evolution of Sulphur},
  year         = {2013},
}