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The origin and chemical evolution of iron-peak and neutron-capture elements in the Milky Way disk

Battistini, Chiara LU (2015)
Abstract (Swedish)
Popular Abstract in Swedish

I Big Bang skapades 3 grundämnen: H, He, och spår av Li. Alla andra grundämnen har bildats i stjärnor. Lågmassiva stjärnor som vår sol kan genom kärnreaktioner skapa grundämnen upp till och med syre i det periodiska systemet, medan mer massiva stjärnor kan skapa grundämnen ända upp till järn. Tyngre grundämnen än järn produceras in slutstadierna av stjärnornas liv, till exempel i supernavaexplosioner, då de sprider massor med tyngre grundämnen i det interstellära mediet, vilket i sin tur kommer att berika den kemiska sammansättningen av nästkommande generationer av stjärnor.

Även om vi har en någorlunda god teoretisk förståelse om hur stjärnor utvecklas och om grundämnesnukleosyntesen,... (More)
Popular Abstract in Swedish

I Big Bang skapades 3 grundämnen: H, He, och spår av Li. Alla andra grundämnen har bildats i stjärnor. Lågmassiva stjärnor som vår sol kan genom kärnreaktioner skapa grundämnen upp till och med syre i det periodiska systemet, medan mer massiva stjärnor kan skapa grundämnen ända upp till järn. Tyngre grundämnen än järn produceras in slutstadierna av stjärnornas liv, till exempel i supernavaexplosioner, då de sprider massor med tyngre grundämnen i det interstellära mediet, vilket i sin tur kommer att berika den kemiska sammansättningen av nästkommande generationer av stjärnor.

Även om vi har en någorlunda god teoretisk förståelse om hur stjärnor utvecklas och om grundämnesnukleosyntesen, som i stort klarar av att relativt bra matcha observationer, så fattas flera bitar i pusslet om grundämnenas kosmiska ursprung. Exempelvis så verkar det om vissa grundämnen som ligger nära Fe i det periodiska systemet (järngruppsämnen) har ett annat ursprung än Fe. Relativt Fe, så uppvisar Cr och Ni platta halttrender, vilket tyder på samma ursprung, medan Sc och Co påminner mer om syre, som inte är ett järngrup- psämne. Samtidigt så kan inte kemiska utvecklingsmodeller producera de observerade mängderna av flera av dessa ämnen. För de tyngre grundämnena som skapas genom neu- troninfångning, så finns det goda indikationer på att de så kallade s-processämnena (som bara behöver en låg neutronflux) bildats i låg- eller mellanmassiva stjärnor som befinner sig på den asymptotiska jättegrenen. Exempelvis så finns det observerationer av vissa instabila isotoper av s-processämnen i atmosfärerna hos jättestjärnor. Då halveringstiden är kort så kan dessa isotoper inte komma från tidigare generationer av stjärnor. Dock så är den producerade mängden s-processämnen väldigt osäker eftersom man inte känner till vilka nukleosynteskanaler som är mest aktiva i dessa faser av stjärnutvecklingen. Faktum är att jättestjärnor genomgår flera episoder där de inre lagrena blandas vilket kan påverka antagandena för stjärnutvecklingsmodeller. Å andra sidan så har källorna för r-processämnena (de som skapas då neutronfluxen är hög) sedan länge antagits komma från massiva stjärnor då de i sina slutstadier exploderar som supernovor eftersom neutronflödet då är väldigt stort. Dock så kan inte teoretiska modeller av nukleosyntesen i dessa supernovor förklara de höga ymnigheterna av r-processämnen som observerats i metallfattiga stjärnor, som borde vara sparsamt berikade av tidigare stjärngenerationer.

Detta avhandlingsarbete ämnar att öka vår föståelse för det kosmiska ursprunget för de udda järngruppsämnena Sc, V, Mn, och Co, samt de tyngre ämnena Sr, Zr, La, Ce, Nd, Sm, och Eu, som bildats genom neutroninfångning. Speciellt oklart för dessa grundämnen är från vilka källor de kommer och på vilka tidsskalor de berikat det interstellära mediet. Vi har använt oss högupplösta spektra för ett stort urval av relativt närbelägna sollika dvärgstjärnor, vilket tillåter oss att studera deras detaljerade kemiska sammansättning. Dvärgstjärnor är synnerligen lämpade för denna typ av studier då deras atmosfärer fungerar som tidskapslar, intakta med den kemiska sammansättning hos den interstellära gas de skapades ur för flera miljarder år sedan.

Våra resultat pekar på att Sc, V, och Co skapas när massiva stjärnor exploderar. För Co ser vi vidare indikationer på att större mängder skapades i Vintergatans tidiga stadier, från väldigt massiva stjärnor (mer än 20 gånger solens massa). Mn verkar ha ett mer komplext ursprung, dels först från explosioner av massiva stjärnor, och senare även från termonukleära explosioner av kompakta objekt. De tyngre neutroninfångningsämnena härkommer vanligtvis från en blandning av s- och r-processer som är aktiva på olika tidsskalor eftersom de kommer från stjärnor med olika massor. Vårt arbete verkar bekräfta de teoretiska beräkningarna för hur mycket och varifrån de kommer. Resultaten indikerar också att Sr och Zr kommer från samma källor, liksom La och Ce, samt Nd och Sm. Vidare undersökningar av ymnigheterna av dessa ämnen för stjärnor av olika åldrar tycks visa att de huvudsakliga produktionskällorna har ändrats för ungefär 8 miljarder år sedan.



Popular Abstract in English

In the Big Bang only three elements were created: H, He, and traces of Li. All the other elements that we see today come from the stars. Low-mass stars like our Sun can fuse elements up to O, while more massive stars can fuse elements up to Fe. Heavier elements are produced in the final stages, when the stars die in supernovae explosions and disperse all the elements they have produced into the interstellar medium, to be used by the next generations of stars that then will have enriched chemical compositions.

Even if we have a good theoretical understanding on the different phases of stellar evolution and nucleosynthesis of chemical elements that are able to match observations (e.g. HR diagram, nuclear reaction rates or the presence of specific elements in the stellar atmospheres), pieces in the puzzle of the cosmic origin of the elements are missing. The elements close to Fe in the periodic table (iron-peak elements) seem to not share the same production path as Fe. For example Cr and Ni present flat abundance trends when compared to Fe, indicating a common origin, while Sc and Co are more similar to O and the α-elements (e.g. Mg, Si, Ca, Ti), that are not iron-peak elements. At the same time chemical evolution models of how the elements are formed cannot always reconcile the amount of material produced in nucleosynthesis with observations. For neutron-capture elements instead there are good evidences that the production sites of s-process elements (produced when a low flux of neutrons is available) are the asymptotic giant branch phase of intermediate- and low-mass stars. For example, unstable isotopes of some neutron-capture elements, are observed in the atmospheres of giant stars and since their half-life times are short compared to the giant phase lifetime, they cannot have formed in a previous generation of stars. Furthermore in the case of s-process elements, the amount of elements that is produced is very uncertain because the exact nucleosynthesis in this stellar phase is not completely understood. In fact giant stars experience several episodes of mixing in their interior layers that alter the assumptions that are used to construct stellar evolution model. On the other hand, the production site of r-process elements (that form when an intense neutron flux is available) has been for long time considered to be the final explosion of massive stars because that environment provides a high flux of neutrons. Unfortunately models of supernovae explosions cannot match the high abundances of the r-process elements that are observed in extremely metal-poor stars that are supposed to have formed when the gas was not very enriched from previous generations of stars.

The aim of this thesis is to try to improve our understanding of the odd-Z iron-peak elements Sc, V, Mn, and Co and also neutron-capture elements Sr, Zr, La, Ce, Nd, Sm, and Eu. These elements have still open issues like formation sites and timescale of formation and enrichment of the interstellar medium. To do this we have used high-resolution spectra for a large sample of nearby solar-type dwarf stars, enabling us to do a detailed analysis of their chemical compositions. The study of dwarf stars is incredibly valuable because they act as time capsules, retaining chemical information of the interstellar gas cloud they formed from.

Our results indicate that Sc, V, and Co are produced in the final explosions of massive stars. In the case of Co the indication is that more Co was produced in the early stages of the Milky Way from very massive stars (more than 20 times the mass of the Sun). Mn, however, presents a more complex history, being first produced in the explosions of massive stars and then in later stages of the Galactic evolution from the thermonuclear explosions of compact objects. Neutron-capture elements are usually produced in a mixture of s- and r-processes that are active at different timescales. Our work seems to confirm the most recent theoretical calculations on how much and where these elements are produced. Our results indicate that Sr and Zr have the same production sites, and similarly for La and Ce, and for Nd and Sm. Investigation of the abundances at different stellar ages seems to indicate a change in production sites in the Galaxy for Sr, Zr, Nd, Sm, and Eu around 8 Gyr ago, coinciding with an apparent transition in the history of the Galactic disk. (Less)
Abstract
All the elements heavier than Li are created during stellar evolution. Even if our knowledge of this process is good, many questions still remain. For instance, the odd-Z iron-peak elements Sc, V, Mn, and Co together with neutron-capture elements Sr, Zr, La, Ce, Nd, Sm, and Eu have still unclear production sites because theoretical models and observational evidences are not in agreement. Having a clear picture of how, when, and where the different elements formed is important because elements with known origins can be used to study the chemical evolution of stellar populations and improve our knowledge on how our Galaxy and its stellar population formed and evolved.

The aim of this thesis is to try to improve the understanding of... (More)
All the elements heavier than Li are created during stellar evolution. Even if our knowledge of this process is good, many questions still remain. For instance, the odd-Z iron-peak elements Sc, V, Mn, and Co together with neutron-capture elements Sr, Zr, La, Ce, Nd, Sm, and Eu have still unclear production sites because theoretical models and observational evidences are not in agreement. Having a clear picture of how, when, and where the different elements formed is important because elements with known origins can be used to study the chemical evolution of stellar populations and improve our knowledge on how our Galaxy and its stellar population formed and evolved.

The aim of this thesis is to try to improve the understanding of the production and evolution of these elements. The analysis was performed using a large sample of high-resolution spectra of dwarf stars in the solar neighbourhood, to probe the different populations of the Milky Way, with particular interest to the Galactic thin and thick disks.

Our results indicate that Sc, V, and Co are produced in Type II supernovae. In particular Co abundances seem to point to high production of Co in very massive stars in the early stage of the Galaxy. As soon as Type Ia supernovae start to enrich the interstellar medium with Fe, the abundance trends decrease to reach solar values. On the other hand Mn is produced in core collapse supernovae in the early stages of the Galaxy, but appears later to be produced in higher quantities in Type Ia supernovae. Neutron-capture elements are produced in a mix of two different processes, depending on the intensity of the neutron flux that happen in different sites and at different timescales in the Galactic evolution. Our results are generally in agreement with the recent theoretical calculation on how much and where these elements are produced. The same production sites can be inferred for Sr and Zr, for La and Ce, and for Nd and Sm. Finally, the different abundance trends at different stellar ages seems to indicate a change in the production sites between the transition from thick disk to thin disk for Sr, Zr, Nd, Sm, and Eu. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Randich, Sofia, INAF - Osservatorio astrofisico di Arcetri, Firenze, Italy
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Stars: abundances, Stars: solar-type, Galaxy: disk, Galaxy: evolution, Galaxy: solar neighbourhood
pages
85 pages
publisher
Department of Astronomy and Theoretical Physics, Lund University
defense location
Lundmark lecture hall (Lundmarksalen), Deparment of Astronomy and Theoretical Physics
defense date
2015-06-05 09:00
ISBN
978-91-7623-293-4
language
English
LU publication?
yes
id
b4cb1ed8-2d99-4916-a3dc-b9afc3db7031 (old id 5337273)
date added to LUP
2015-05-13 15:16:36
date last changed
2016-09-19 08:45:11
@misc{b4cb1ed8-2d99-4916-a3dc-b9afc3db7031,
  abstract     = {All the elements heavier than Li are created during stellar evolution. Even if our knowledge of this process is good, many questions still remain. For instance, the odd-Z iron-peak elements Sc, V, Mn, and Co together with neutron-capture elements Sr, Zr, La, Ce, Nd, Sm, and Eu have still unclear production sites because theoretical models and observational evidences are not in agreement. Having a clear picture of how, when, and where the different elements formed is important because elements with known origins can be used to study the chemical evolution of stellar populations and improve our knowledge on how our Galaxy and its stellar population formed and evolved.<br/><br>
The aim of this thesis is to try to improve the understanding of the production and evolution of these elements. The analysis was performed using a large sample of high-resolution spectra of dwarf stars in the solar neighbourhood, to probe the different populations of the Milky Way, with particular interest to the Galactic thin and thick disks.<br/><br>
Our results indicate that Sc, V, and Co are produced in Type II supernovae. In particular Co abundances seem to point to high production of Co in very massive stars in the early stage of the Galaxy. As soon as Type Ia supernovae start to enrich the interstellar medium with Fe, the abundance trends decrease to reach solar values. On the other hand Mn is produced in core collapse supernovae in the early stages of the Galaxy, but appears later to be produced in higher quantities in Type Ia supernovae. Neutron-capture elements are produced in a mix of two different processes, depending on the intensity of the neutron flux that happen in different sites and at different timescales in the Galactic evolution. Our results are generally in agreement with the recent theoretical calculation on how much and where these elements are produced. The same production sites can be inferred for Sr and Zr, for La and Ce, and for Nd and Sm. Finally, the different abundance trends at different stellar ages seems to indicate a change in the production sites between the transition from thick disk to thin disk for Sr, Zr, Nd, Sm, and Eu.},
  author       = {Battistini, Chiara},
  isbn         = {978-91-7623-293-4},
  keyword      = {Stars: abundances,Stars: solar-type,Galaxy: disk,Galaxy: evolution,Galaxy: solar neighbourhood},
  language     = {eng},
  pages        = {85},
  publisher    = {ARRAY(0x82def68)},
  title        = {The origin and chemical evolution of iron-peak and neutron-capture elements in the Milky Way disk},
  year         = {2015},
}