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Nucleosynthesis in accretion disks around black holes

Frankel, Neige LU (2017) In Lund Observatory Examensarbeten ASTM31 20171
Department of Astronomy and Theoretical Physics
Lund Observatory
Abstract
Nucleosynthesis is the mechanism which produces new elements in nuclear reactions. Nuclear reaction rates are highly temperature dependant, and nuclear reactions take place in very hot environments. Current theories predict that the light elements such as hydrogen and helium were produced during the Big Bang. On the other hand, the core of stars produce heavier elements through nuclear fusion. These new elements are then released to the interstellar medium through stellar winds, enriching the gas which will form later generations of stars. Black hole accretion disks also can contain material of high temperatures generated by high accretion rates, allowing nuclear fusion to take place. Nucleosynthesis products can be expelled in winds... (More)
Nucleosynthesis is the mechanism which produces new elements in nuclear reactions. Nuclear reaction rates are highly temperature dependant, and nuclear reactions take place in very hot environments. Current theories predict that the light elements such as hydrogen and helium were produced during the Big Bang. On the other hand, the core of stars produce heavier elements through nuclear fusion. These new elements are then released to the interstellar medium through stellar winds, enriching the gas which will form later generations of stars. Black hole accretion disks also can contain material of high temperatures generated by high accretion rates, allowing nuclear fusion to take place. Nucleosynthesis products can be expelled in winds driven by a super-Eddington accretion, and enrich the interstellar medium.
I wrote a computer program 1 integrating a nuclear burning network in a black hole accretion disk for various ranges of black hole mass and accretion rates. I found that the accretion rate needed for nucleosynthesis to take place increases with the black hole mass. The highest temperatures are located in the inner disk, and the black hole event horizon increases linearly with its mass, preventing the disk from attaining high temperatures. For a stellar mass black hole, highly super-Eddington accretion rates allow nuclear burning and powerful winds. Such accretion rates can be supplied by unstable mass transfer during the disruption of a white dwarf. The alpha chain reactions, involving captures of helium nuclei, structure the disk composition radially, with isotope abundances dominating at specific radii. Assuming a given fraction of the disk material is expelled in winds due to Super-Eddington accretion, and knowing the rate at which such events happen in the Galaxy allowed me to compute upper limits of the contribution of accretion disks to the interstellar medium enrichment. Comparing this production to combined stellar yields from stars, I find that black hole–white dwarf accretion disks produce at most 10 −4 times the amount of the same elements that stars produce. This result shows that such a small contribution can be neglected to the overall content of the Galaxy. But the nucleosynthesis involved in general may perhaps play a role in observing these systems, for example a light curve emitted by radioactive elements produced in these short-lived black hole accretion disks. (Less)
Popular Abstract (French)
La nucléosynthèse dans les disques d'accrétion autour de trous noirs.

"Nous sommes faits de poussières d'étoiles", comme le décrit Hubert Reeves, reprenant, dans son magnifique ouvrage sur la synthèse des éléments dans l'Univers, l'idée de Carl Sagan. Cela signifie que les éléments qui nous compensent ajourd'hui, carbone, oxygène, calcium, etc., ont été fabriqués à partir de l'hydrogène et de l'hélium hérités du Big Bang, dans les denses coeurs brûlants des étoiles. Mais est-ce là la fin de l'histoire ? La synthèse des éléments nécessite des conditions extrêmes de températures et densités. Ces conditions sont atteignables dans les disques formés par le le gas qui se fait avaler par les trous noirs, objets si denses que même la lumière... (More)
La nucléosynthèse dans les disques d'accrétion autour de trous noirs.

"Nous sommes faits de poussières d'étoiles", comme le décrit Hubert Reeves, reprenant, dans son magnifique ouvrage sur la synthèse des éléments dans l'Univers, l'idée de Carl Sagan. Cela signifie que les éléments qui nous compensent ajourd'hui, carbone, oxygène, calcium, etc., ont été fabriqués à partir de l'hydrogène et de l'hélium hérités du Big Bang, dans les denses coeurs brûlants des étoiles. Mais est-ce là la fin de l'histoire ? La synthèse des éléments nécessite des conditions extrêmes de températures et densités. Ces conditions sont atteignables dans les disques formés par le le gas qui se fait avaler par les trous noirs, objets si denses que même la lumière ne peut s'en échapper. Du gas orbitant un trou noir chauffe par friction, permettant à des réactions nucléaires d'avoir lieu. Si une partie des éléments produits peu s'échapper, à l'aide de vents par exemple, alors peut-être que les disques d'accrétion autour des trous noirs jouent un rôle dans la synthèse des éléments nous composant. Seulement, les températures ne sont suffisantes que dans de rares conditions qui mettent en jeu la destruction d'objets compacts, tels que les naines blanches et les étoiles à neutrons, qui sont respectivement les restes de vies d'étoiles légères, et massives. Le travail décrit dans ce mémoire se concentre sur les réactions nucléaires dans les disques d'accrétion provenant de ma destructions d'une naine blanche par un trou noir. Comme ces événements sont très rares, leur contribution à la synthèse des éléments qui nous cmposent aujourd'hui est négligeable comparée à tout ce que les nombreuses étoiles de notre galaxie produisent. En revanche, le cas des étoiles à neutrons n'a pas dit son dernier mot et est toujours un sujet de recherche actuel. Quoi qu'il en soit, les cas les plus prometteurs de synthèse des éléments dans les disques d'accrétion autour des trous noirs mettent en jeu des étoiles, donc il est bien vrai, nous sommes des poussières d'étoiles. (Less)
Popular Abstract (Swedish)
Nukleosynthes i ackretionsskrivor kring svarta hål.

Hur bildas atomerna som bygger upp oss människor - till exempel atomer av kol, syre eller kalcium? Den idag gâllande teorin är att lättare atomer såsom väte, helium och litium skapades vid "Big Bang", medan en supernova från en döende stjärna producerade och frigjorde bland annat kol, syre, kalcium och järn. Det som stjärnnor och Big Bang har gemensamt i sin förmåga att fungera som kärnreaktorer är att de utmärks av väldigt hög värme och en hög densitet. Ett svart hål som muycket snabbt fångar gaser från en omgivande ansamlingsskiva, som vi kallar ackretionsskiva, skapar också mycket hög värme och hög densitet. Detta tillstånd har också alla fôrutsättningar att fungera som en... (More)
Nukleosynthes i ackretionsskrivor kring svarta hål.

Hur bildas atomerna som bygger upp oss människor - till exempel atomer av kol, syre eller kalcium? Den idag gâllande teorin är att lättare atomer såsom väte, helium och litium skapades vid "Big Bang", medan en supernova från en döende stjärna producerade och frigjorde bland annat kol, syre, kalcium och järn. Det som stjärnnor och Big Bang har gemensamt i sin förmåga att fungera som kärnreaktorer är att de utmärks av väldigt hög värme och en hög densitet. Ett svart hål som muycket snabbt fångar gaser från en omgivande ansamlingsskiva, som vi kallar ackretionsskiva, skapar också mycket hög värme och hög densitet. Detta tillstånd har också alla fôrutsättningar att fungera som en kärnreactor och att skapa nya tyngre grundåmnen.
I mitt examenarsbete har jag undersökt kärnreaktioner i sådana ackretionsskivor kring svarta hål. Temperaturen och densiteten blir tillråckligt hög endast om infångningen av gaser sker mycket hastigt. Detta kan inträffa om ansamlingsskivan kommer från en vit dvärg, vilken ökar i storlek när den kastat ut massa. Jag har skrivit ett program som beräknar hur mycket av nua tyngre atomer som skapas och frigörs. Jämfört med stjärnor är produktionen av nya grundämnen betydligt mindre förekommande. Eftersom en vit dvärg utgör slutprodukten från en död stjärna, kan vi alltid säga att vi människor har erhållit våra byggstenar från stjärnstoft. (Less)
Please use this url to cite or link to this publication:
author
Frankel, Neige LU
supervisor
organization
course
ASTM31 20171
year
type
H2 - Master's Degree (Two Years)
subject
keywords
accretion disk, Nucleosynthesis, black hole, high energy, white dwarf tidal disruption
publication/series
Lund Observatory Examensarbeten
report number
2017-EXA114
language
English
id
8912003
date added to LUP
2017-06-09 17:22:27
date last changed
2017-06-09 17:22:27
@misc{8912003,
  abstract     = {Nucleosynthesis is the mechanism which produces new elements in nuclear reactions. Nuclear reaction rates are highly temperature dependant, and nuclear reactions take place in very hot environments. Current theories predict that the light elements such as hydrogen and helium were produced during the Big Bang. On the other hand, the core of stars produce heavier elements through nuclear fusion. These new elements are then released to the interstellar medium through stellar winds, enriching the gas which will form later generations of stars. Black hole accretion disks also can contain material of high temperatures generated by high accretion rates, allowing nuclear fusion to take place. Nucleosynthesis products can be expelled in winds driven by a super-Eddington accretion, and enrich the interstellar medium. 
I wrote a computer program 1 integrating a nuclear burning network in a black hole accretion disk for various ranges of black hole mass and accretion rates. I found that the accretion rate needed for nucleosynthesis to take place increases with the black hole mass. The highest temperatures are located in the inner disk, and the black hole event horizon increases linearly with its mass, preventing the disk from attaining high temperatures. For a stellar mass black hole, highly super-Eddington accretion rates allow nuclear burning and powerful winds. Such accretion rates can be supplied by unstable mass transfer during the disruption of a white dwarf. The alpha chain reactions, involving captures of helium nuclei, structure the disk composition radially, with isotope abundances dominating at specific radii. Assuming a given fraction of the disk material is expelled in winds due to Super-Eddington accretion, and knowing the rate at which such events happen in the Galaxy allowed me to compute upper limits of the contribution of accretion disks to the interstellar medium enrichment. Comparing this production to combined stellar yields from stars, I find that black hole–white dwarf accretion disks produce at most 10 −4 times the amount of the same elements that stars produce. This result shows that such a small contribution can be neglected to the overall content of the Galaxy. But the nucleosynthesis involved in general may perhaps play a role in observing these systems, for example a light curve emitted by radioactive elements produced in these short-lived black hole accretion disks.},
  author       = {Frankel, Neige},
  keyword      = {accretion disk,Nucleosynthesis,black hole,high energy,white dwarf tidal disruption},
  language     = {eng},
  note         = {Student Paper},
  series       = {Lund Observatory Examensarbeten},
  title        = {Nucleosynthesis in accretion disks around black holes},
  year         = {2017},
}