Making Hypervelocity Stars
(2020) In Lund Observatory Examensarbeten ASTK02 20201Lund Observatory - Undergoing reorganization
- Abstract
- In this thesis, I investigate if the binaries that are being tidally disrupted by means of the Hills mechanism, producing hypervelocity stars, could have been transported to the galactic centre by two-body scattering. To survive a scattering event, the binary must be hard, meaning that the binding energy of the binary is higher than the kinetic energy of the collider. If the binary is soft, the binary is disrupted at the scattering event and cannot contribute to the production of hypervelocity stars. From 43 observed hypervelocity stars, I calculate a distribution of the binary separation for each binary prior to disruption. Assuming that the mass of the collider is 1M*, the fraction of binaries considered hard in the distribution is... (More)
- In this thesis, I investigate if the binaries that are being tidally disrupted by means of the Hills mechanism, producing hypervelocity stars, could have been transported to the galactic centre by two-body scattering. To survive a scattering event, the binary must be hard, meaning that the binding energy of the binary is higher than the kinetic energy of the collider. If the binary is soft, the binary is disrupted at the scattering event and cannot contribute to the production of hypervelocity stars. From 43 observed hypervelocity stars, I calculate a distribution of the binary separation for each binary prior to disruption. Assuming that the mass of the collider is 1M*, the fraction of binaries considered hard in the distribution is calculated for three binary mass ratios, 2:1, 1:1, 1:2, between the ejected and captured star and distances from the galactic centre, 0.1, 0.3, 1 pc. No binaries within 0.1 pc could be considered hard since the limiting binary separation at that distance from the galactic centre is less than the physical size of both binary components. For binaries with a mass ratio of 1:2, scattered 1 pc from the galactic centre, 10^−2 − 10^−1 of all binaries would be considered hard. Equivalently, to sustain the expected disruption rate of 10−5 yr^−1, 100-1000 binaries needs to two-body scatter every 1 Myr. The observations of B-stars on highly eccentric orbits around Sgr A*, so called S-stars, indicate that hypervelocity stars might be produced by higher mass binaries, corresponding to a mass ratio of 1:4 for the observed hyper velocity stars. This increase in mass increases the fraction of hard binaries by magnitude of 10. This is still too inefficient to solely sustain the expected disruption rate without depleting the galactic centre of stars within 1 pc. Although, the possibility of a 1:4 mass ratio S-star binary surviving a scattering event is large enough to not rule out as possible origin of individual hypervelocity stars. (Less)
- Popular Abstract (Swedish)
- Hypervelocity stars, på svenska hyperhastighetsstjärnor, är stjärnor som observerats med så hög hastighet att de kan lämna vår galax, Vintergatan. Den första hyperhastighetsstjärnan upptäcktes av R. Brown m. fl. (2005), och sedan dess har många fler upptäckts. J. G.Hills presenterade (1988) en teori om att hyperhastighetsstjärnor härstammar från binära stjärnor (två stjärnor som kretsar varandra) som interagerar med det supermassiva svarta hålet i Vintergatans centrum. Ett fenomen kallat Hills Mekanismen, se Figur 3.1.
När den binära stjärnan kommer för nära det svarta hålet, slits den isär av det enorma gravitationsfältet. En av stjärnorna fångas i omloppsbana runt det svarta hålet medans den andra stjärnan slungas iväg ut i rymden.... (More) - Hypervelocity stars, på svenska hyperhastighetsstjärnor, är stjärnor som observerats med så hög hastighet att de kan lämna vår galax, Vintergatan. Den första hyperhastighetsstjärnan upptäcktes av R. Brown m. fl. (2005), och sedan dess har många fler upptäckts. J. G.Hills presenterade (1988) en teori om att hyperhastighetsstjärnor härstammar från binära stjärnor (två stjärnor som kretsar varandra) som interagerar med det supermassiva svarta hålet i Vintergatans centrum. Ett fenomen kallat Hills Mekanismen, se Figur 3.1.
När den binära stjärnan kommer för nära det svarta hålet, slits den isär av det enorma gravitationsfältet. En av stjärnorna fångas i omloppsbana runt det svarta hålet medans den andra stjärnan slungas iväg ut i rymden. Föreställ dig två tennisbollar sammankopplade med en tråd. Om du snurrar tennisbollarna så fort du kan och plötsligt klipper av tråden, då flyger tennisbollen du inte höll i handen iväg med hög hastighet.
För att en sådan splittring ska kunna ske, måste den binära stjärnan komma väldigt nära det svarta hålet, betydligt närmare än vad som tidigare observerats. Detta innebär att det måste existera någon process som styr de binära stjärnorna mot galaxens centrum, t.ex. kollisioner eller andra massiva objekt som kan ändra på den binära stjärnans riktning. Målet med min avhandling är att undersöka olika transportmetoder. Varje transportmetod kommer fodra specifika egenskaper hos den binära stjärnan, t.ex. massa eller hastighet. Från de observerade hyperhastighetsstjärnorna kan egenskaperna hos den tillhörande binära stjärnan beräknas med hjälp av teorin föreslagen av J. G. Hills. Egenskaperna av den beräknade binära stjärnan kan sedan jämföras med de begärda egenskaperna av varje transportmetod.
Galaxens centrum är ett av de svåraste områdena att observera p.g.a. allt ljus från den stora stjärnpopulationen och även stora mängder damm. Försök urskilja en tändtändsticka framför en strålkastare, i en sandstorm. När J. G. Hills föreslog sin teori om hyperhastighetsstjärnor för andra astronomer var det för att bevisa existensen av ett supermassivt svart hål i galaxens centrum. Idag är existensen av Vintergatans svarta hål Sgr A* bekräftad, men hyperhastighetsstjärnor kan fortfarande vara till stor hjälp för oss genom att få en bättre uppfattning om klimatet för himlakroppar i galaxens centrum. Alla framsteg inom dynamiken av galaxens centrum kan vara viktiga för åtskilliga inriktningar inom astronomi. (Less) - Popular Abstract
- Hypervelocity Stars are stars that have been observed to move at such high speeds that
they might exit the galaxy. The first hypervelocity star was observed by R. Brown et al.
(2005) and since then, many more have been discovered through different sky surveys. In
1988, J. G. Hills proposed the idea of hypervelocity stars being created from binary stars
(two stars orbiting each other) that interacts with the super massive black hole in the
centre of the Milky Way, a phenomenon called the Hills Mechanism.
When the binary star gets too close to the black hole, the enormous gravitational force
from the black role rips the two stars apart, capturing one of the stars and sending the
other one flying into space. Imagine spinning two... (More) - Hypervelocity Stars are stars that have been observed to move at such high speeds that
they might exit the galaxy. The first hypervelocity star was observed by R. Brown et al.
(2005) and since then, many more have been discovered through different sky surveys. In
1988, J. G. Hills proposed the idea of hypervelocity stars being created from binary stars
(two stars orbiting each other) that interacts with the super massive black hole in the
centre of the Milky Way, a phenomenon called the Hills Mechanism.
When the binary star gets too close to the black hole, the enormous gravitational force
from the black role rips the two stars apart, capturing one of the stars and sending the
other one flying into space. Imagine spinning two tennis balls connected by a string by
holding one of the tennis balls and moving your arm. Then, when you cannot spin the
balls any faster, you cut the string connecting the balls. The tennis ball not held by your
hand then flies off with high speed.
For this sort of star ejection to occur, the binary star needs to get really close to the
black hole, much closer than where binaries are usually found. This means that there must
be some process that feeds these binaries towards the galactic centre, e.g. collisions further
out from the galactic centre or some other massive object that could deflect the binaries.
The objective of my thesis is to investigate the different possible transportation methods.
Each transportation method will infer some specific range of properties, e.g. size, mass and
or velocity of the binary. From the observed hypervelocity stars, the original binaries can
be calculated using the theory proposed by J. G. Hills. The properties of the calculated
binaries can then be compared to the inferred properties of each transportation method.
The galactic centre is one of the most difficult places to observe in our galaxy since it
is very bright but also full of space dust obscuring the view. Try distinguishing a match lit
in front of some headlights, in a sandstorm. When J. G. Hills suggested that astronomers
should be on the lookout for hypervelocity stars, it was to prove the existence of a super
massive black hole in the centre of the Milky Way. Today, hypervelocity stars can still be
of great use to help us understand the environment for celestial bodies neighbouring the
galactic centre. Any form of discoveries regarding the dynamics of the galactic centre will
be beneficial to many fields of astronomy. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/student-papers/record/9016282
- author
- Forsberg, Frans LU
- supervisor
- organization
- alternative title
- Skapandet av Hyperhastighetsstjärnor
- course
- ASTK02 20201
- year
- 2020
- type
- M2 - Bachelor Degree
- subject
- keywords
- Stellar Dynamics, Binary Stars, Galactic Centre, Binary Relaxation
- publication/series
- Lund Observatory Examensarbeten
- report number
- 2020-EXA163
- language
- English
- id
- 9016282
- date added to LUP
- 2020-06-26 14:36:18
- date last changed
- 2022-07-05 15:25:16
@misc{9016282,
abstract = {{In this thesis, I investigate if the binaries that are being tidally disrupted by means of the Hills mechanism, producing hypervelocity stars, could have been transported to the galactic centre by two-body scattering. To survive a scattering event, the binary must be hard, meaning that the binding energy of the binary is higher than the kinetic energy of the collider. If the binary is soft, the binary is disrupted at the scattering event and cannot contribute to the production of hypervelocity stars. From 43 observed hypervelocity stars, I calculate a distribution of the binary separation for each binary prior to disruption. Assuming that the mass of the collider is 1M*, the fraction of binaries considered hard in the distribution is calculated for three binary mass ratios, 2:1, 1:1, 1:2, between the ejected and captured star and distances from the galactic centre, 0.1, 0.3, 1 pc. No binaries within 0.1 pc could be considered hard since the limiting binary separation at that distance from the galactic centre is less than the physical size of both binary components. For binaries with a mass ratio of 1:2, scattered 1 pc from the galactic centre, 10^−2 − 10^−1 of all binaries would be considered hard. Equivalently, to sustain the expected disruption rate of 10−5 yr^−1, 100-1000 binaries needs to two-body scatter every 1 Myr. The observations of B-stars on highly eccentric orbits around Sgr A*, so called S-stars, indicate that hypervelocity stars might be produced by higher mass binaries, corresponding to a mass ratio of 1:4 for the observed hyper velocity stars. This increase in mass increases the fraction of hard binaries by magnitude of 10. This is still too inefficient to solely sustain the expected disruption rate without depleting the galactic centre of stars within 1 pc. Although, the possibility of a 1:4 mass ratio S-star binary surviving a scattering event is large enough to not rule out as possible origin of individual hypervelocity stars.}},
author = {{Forsberg, Frans}},
language = {{eng}},
note = {{Student Paper}},
series = {{Lund Observatory Examensarbeten}},
title = {{Making Hypervelocity Stars}},
year = {{2020}},
}