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The astronomical consequences of primordial black holes

Lönnblad Ohlin, Loke LU (2019) In Lund Observatory Examensarbeten ASTM31 20191
Lund Observatory
Abstract
In the radiation or early matter dominated eras of the Universe, there is a theoretical possibility for the formation of black holes (BHs). These primordial black holes (PBHs) have been thought of as a possible candidate for dark matter (DM), and an attractive one at that as it would explain DM using only baryonic physics. Constraints from theory and observations such as microlensing has nearly excluded the existence of large amounts of PBHs in most ranges of BH masses. However, there are still three mass ranges that remain relatively unconstrained and could therefore make up a large fraction of DM. These include asteroid mass (∼ 10[sup]13[\sup] − 10[sup]14[\sup] kg), sub-lunar (∼ 10[sup]17[\sup] − 10[sup]19[\sup] kg) and stellar mass... (More)
In the radiation or early matter dominated eras of the Universe, there is a theoretical possibility for the formation of black holes (BHs). These primordial black holes (PBHs) have been thought of as a possible candidate for dark matter (DM), and an attractive one at that as it would explain DM using only baryonic physics. Constraints from theory and observations such as microlensing has nearly excluded the existence of large amounts of PBHs in most ranges of BH masses. However, there are still three mass ranges that remain relatively unconstrained and could therefore make up a large fraction of DM. These include asteroid mass (∼ 10[sup]13[\sup] − 10[sup]14[\sup] kg), sub-lunar (∼ 10[sup]17[\sup] − 10[sup]19[\sup] kg) and stellar mass (10[sup]31[\sup] − 10[sup]32[\sup] kg) PBHs. This project focuses on what impact a large population of PBHs in the lower mass ranges would have on stellar populations, and if any further constraints on PBH masses can be inferred from this.

Focusing on three environments; the galactic centre, solar neighbourhood and dwarf galaxies, we consider the collision, capture and settling of small PBHs inside of stars such as main sequence star, White Dwarfs (WDs) and Neutron Stars (NSs). As the PBHs that we consider are so low mass, if they make up a significant fraction of the DM their number density would be high, and therefore collisions between them and stars would be frequent. The PBHs are small, so during the collision the PBHs simply fly through the star. However due to energy dissipation into the medium of the stars, the PBH can become bound, and over multiple subsequent orbits lose more energy until they have become completely trapped inside of the star. Once this happens, the star is consumed by the PBH. This process of disrupting stars cannot be too efficient (as otherwise stars could not exists), which previous research has used to put constraints on the abundance of low mass PBHs based on the survival of star.

We model this energy dissipation, integrating the trajectories of the PBHs through a target star. We find that the energy dissipation is small, and therefore PBHs end up on too wide orbits. Due to the wide orbits, and long orbital periods, the PBHs are therefore susceptible to being scattered by intruding stars. As the energy dissipation is weaker for lower mass PBHs, and therefore their subsequent orbits become larger, this puts a lower limit on the mass range for PBHs were they can settle inside of stars with this process. This is important, as in the denser environments, were collisions and capture is the most frequent, the probability for an intruding star to eject the PBH is higher. As such, the mass range PBHs can have in order to settle is often already constrained by microlensing constraints. Previous research on the capture of PBHs in stars have not taken this into account, and therefore their results may be significantly changed.

In addition to attempting to capture onto single targets, we also consider capturing in binary stars and planetary systems. As the orbits of the target bodies have comparable velocities as that expected of the incoming PBHs, the three body interaction can exchange energy and angular momentum between the PBH and the binary. Preforming n-body simulations, treating the PBHs as test particles, we find that this exchange more often than not lead to the PBHs gaining energy, and are subsequently ejected from the binary. Furthermore as the energy change due to the three body interaction is higher than that of the energy dissipation into the medium of the star, this is the dominant process. Therefore we suggests that the settling of PBHs into stars in binary systems is improbable, invalidating previous research which has determined merger rates of systems containing a star consumed by a PBH, without considering the three body interaction.

Finally we estimate the rate at which the consumption of a white dwarf or neutron star would be observed. Using a model for a milky way equivalent galaxy and a NS population synthesis for the galactic distribution of NSs, we determine the cosmological consumption rates of WDs and NSs. Assuming that the consumption of a NS or WD is either similar to Type 1a supernovae or gamma ray bursts, we determine the observable rate of such an event. We find that, due to the aforementioned scattering by intruding stars during the settling of a PBH inside of the target, the observational rate is only significant for mass ranges already constrained by microlensing, and we are unable to derive further constraints on the abundance of PBHs. (Less)
Popular Abstract (Swedish)
Inom modern astronomi är mörk materia en av dem viktigaste byggstenarna i universum. Även om vi inte kan se mörk materia, tror vi att uppemot 85 \% av all materia består av det. Faktum är att vi behöver denna mängd för att förstå hur stjärnor rör sig runt Galaxen, och hur galaxer formas. Trots dess betydelse för vår nuvarande förståelse inom astronomi har vi inte kunnat identifiera vad mörk materia skulle kunna bestå av. De mest populära teorierna tror att det skulle kunna vara små partiklar, men dessa har ännu inte hittats, ens i våra mest avancerade detektorer.

En annan teori för vad mörk materia skulle kunna vara är vad som kallas uråldriga svarta hål (primordial black holes på engelska). På tidigt sjuttiotal kom Steven Hawking på... (More)
Inom modern astronomi är mörk materia en av dem viktigaste byggstenarna i universum. Även om vi inte kan se mörk materia, tror vi att uppemot 85 \% av all materia består av det. Faktum är att vi behöver denna mängd för att förstå hur stjärnor rör sig runt Galaxen, och hur galaxer formas. Trots dess betydelse för vår nuvarande förståelse inom astronomi har vi inte kunnat identifiera vad mörk materia skulle kunna bestå av. De mest populära teorierna tror att det skulle kunna vara små partiklar, men dessa har ännu inte hittats, ens i våra mest avancerade detektorer.

En annan teori för vad mörk materia skulle kunna vara är vad som kallas uråldriga svarta hål (primordial black holes på engelska). På tidigt sjuttiotal kom Steven Hawking på att i dem första sekunderna av universum skulle svarta hål kunna bildas. Vi vet inte om detta faktiskt har skett, men om vi skulle kunna hitta ett sådant svart hål skulle det ge oss insikt i det tidiga universum. Dessutom, om tillräckligt många av dessa svarta hål bildades skulle de förklara all mörk materia i universum.

Dessa uråldriga svarta hål skulle vara unika från de vi redan vet existerar, eftersom de nästan skulle kunna ha vilken massa som helst, från några hundratusendelar gram, till miljoner gånger solens massa. Detta skiljer uråldriga svarta hål från de svarta hålen vi vet existerar, och därmed skulle vi kunna urskilja dem i observationer. Även om vi inte kan utesluta dem helt, med noggranna observationer och undersökningar har vi kunnat säga att svarta hål med specifika massor inte finns i tillräckligt stora mängder för att förklara mörk materia. Men, det är fortfarande så att all mörk materia skulle kunna bestå av små svarta hål, där våra observationer inte kan se dem. Dessa svarta hål skulle antingen kunna ha massor liknande asteroider, eller c.a.~tusen till hundra tusen gånger mindre än månen. Eftersom svarta hål är kompakta skulle de minsta av dessa inte vara mycket större än en atom, medan de största skulle vara lika stora som våglängden för synligt ljus.

I detta projektet har jag undersökt vad som skulle ske ifall all mörk materia bestod av dessa små uråldriga svarta hål. Specifikt har jag teoretiskt försökt se vad som händer om de skulle kollidera med stjärnor. Eftersom det skulle finnas extremt många av dessa svarta hål, så hade dessa kollisioner vara vanliga. På grund av att svarta hålen är väldigt små, åker de för det mesta bara igenom stjärnan, men genom diverse fysikaliska processer skulle svarta hålen saktats ner av stjärnans täta innehål. Om svarta hålen saktas ner tillräckligt kan de bli gravitationellt bundna till stjärnan, och om de åker igenom stjärnan flertal gånger fastna inuti stjärnan. Om detta sker skulle stjärnan tillslut sväljas upp av svarta hålet.

Jag har använt mig av tidigare studier och förbättrat deras teoretiska modeller, samt skapat datorsimuleringar för att se hur ofta ett uråldrigt svart hål fastnar inuti stjärnor och liknande objekt, så som vita dvärgar och neutronstjärnor. Jag har visat att denna processen är mycket mindre effektiv än tidigare trott, bland annat genom att ta hänsyn till att ett svart hål som blir bunden till en stjärna först hamnar långt bort från stjärnan innan de åker igenom igen. Därmed finns det en stor sannolikhet att en annan stjärna kommer tillräckligt nära för att via gravitationell kraft dra bort svarta hålet. Jag har även visat att det är näst intill omöjligt att svarta hålen fastnar i en stjärna som är en medlem i ett binärt stjärnsystem, eftersom gravitationskraften från de två stjärnorna är för våldsam, och det lilla svarta hålet kastas ut.

Jag har använt mig av mina resultat för beräkna hur ofta vi skulle kunna se stjärnor bli uppätna av uråldriga svart hål. Fastän detta sällan sker, händer det tillräckligt ofta för neutronstjärnor och vita dvärgar för att vi skulle kunna se det om vi observerar flertal galaxer. Dock gäller detta endast för svarta hål med massor som vi redan kan utesluta genom tidigare observationer. (Less)
Please use this url to cite or link to this publication:
author
Lönnblad Ohlin, Loke LU
supervisor
organization
course
ASTM31 20191
year
type
H2 - Master's Degree (Two Years)
subject
publication/series
Lund Observatory Examensarbeten
report number
2019-EXA150
language
English
id
8979720
date added to LUP
2019-06-10 12:19:39
date last changed
2019-06-10 12:19:39
@misc{8979720,
  abstract     = {In the radiation or early matter dominated eras of the Universe, there is a theoretical possibility for the formation of black holes (BHs). These primordial black holes (PBHs) have been thought of as a possible candidate for dark matter (DM), and an attractive one at that as it would explain DM using only baryonic physics. Constraints from theory and observations such as microlensing has nearly excluded the existence of large amounts of PBHs in most ranges of BH masses. However, there are still three mass ranges that remain relatively unconstrained and could therefore make up a large fraction of DM. These include asteroid mass (∼ 10[sup]13[\sup] − 10[sup]14[\sup] kg), sub-lunar (∼ 10[sup]17[\sup] − 10[sup]19[\sup] kg) and stellar mass (10[sup]31[\sup] − 10[sup]32[\sup] kg) PBHs. This project focuses on what impact a large population of PBHs in the lower mass ranges would have on stellar populations, and if any further constraints on PBH masses can be inferred from this.

Focusing on three environments; the galactic centre, solar neighbourhood and dwarf galaxies, we consider the collision, capture and settling of small PBHs inside of stars such as main sequence star, White Dwarfs (WDs) and Neutron Stars (NSs). As the PBHs that we consider are so low mass, if they make up a significant fraction of the DM their number density would be high, and therefore collisions between them and stars would be frequent. The PBHs are small, so during the collision the PBHs simply fly through the star. However due to energy dissipation into the medium of the stars, the PBH can become bound, and over multiple subsequent orbits lose more energy until they have become completely trapped inside of the star. Once this happens, the star is consumed by the PBH. This process of disrupting stars cannot be too efficient (as otherwise stars could not exists), which previous research has used to put constraints on the abundance of low mass PBHs based on the survival of star. 

We model this energy dissipation, integrating the trajectories of the PBHs through a target star. We find that the energy dissipation is small, and therefore PBHs end up on too wide orbits. Due to the wide orbits, and long orbital periods, the PBHs are therefore susceptible to being scattered by intruding stars. As the energy dissipation is weaker for lower mass PBHs, and therefore their subsequent orbits become larger, this puts a lower limit on the mass range for PBHs were they can settle inside of stars with this process. This is important, as in the denser environments, were collisions and capture is the most frequent, the probability for an intruding star to eject the PBH is higher. As such, the mass range PBHs can have in order to settle is often already constrained by microlensing constraints. Previous research on the capture of PBHs in stars have not taken this into account, and therefore their results may be significantly changed.

In addition to attempting to capture onto single targets, we also consider capturing in binary stars and planetary systems. As the orbits of the target bodies have comparable velocities as that expected of the incoming PBHs, the three body interaction can exchange energy and angular momentum between the PBH and the binary. Preforming n-body simulations, treating the PBHs as test particles, we find that this exchange more often than not lead to the PBHs gaining energy, and are subsequently ejected from the binary. Furthermore as the energy change due to the three body interaction is higher than that of the energy dissipation into the medium of the star, this is the dominant process. Therefore we suggests that the settling of PBHs into stars in binary systems is improbable, invalidating previous research which has determined merger rates of systems containing a star consumed by a PBH, without considering the three body interaction.

Finally we estimate the rate at which the consumption of a white dwarf or neutron star would be observed. Using a model for a milky way equivalent galaxy and a NS population synthesis for the galactic distribution of NSs, we determine the cosmological consumption rates of WDs and NSs. Assuming that the consumption of a NS or WD is either similar to Type 1a supernovae or gamma ray bursts, we determine the observable rate of such an event. We find that, due to the aforementioned scattering by intruding stars during the settling of a PBH inside of the target, the observational rate is only significant for mass ranges already constrained by microlensing, and we are unable to derive further constraints on the abundance of PBHs.},
  author       = {Lönnblad Ohlin, Loke},
  language     = {eng},
  note         = {Student Paper},
  series       = {Lund Observatory Examensarbeten},
  title        = {The astronomical consequences of primordial black holes},
  year         = {2019},
}