Advanced

The thermal structure of exoplanet atmospheres

Agnew, Matthew LU (2015) In Lund Observatory Examensarbeten ASTM31 20151
Lund Observatory
Department of Astronomy and Theoretical Physics
Abstract
Context. The discovery of hot Jupiters, giant gas exoplanets on tight orbits close to their host star, has proven instrumental in the study of exoplanet atmospheres through transit spectroscopy. Just as a temperature increase with height manifests in Earth’s atmosphere due to ozone absorbing ultraviolet radiation, transit spectra of some hot Jupiters has yielded evidence that suggests the presence of a similar temperature inversion in their stratosphere. A possible mechanism driving such an inversion is the presence of an analogous molecule to ozone. One potential candidate is titanium oxide (TiO) since it is one of the first molecules to form at high temperatures. However, due to the intersection of the vapour pressure curve of TiO and... (More)
Context. The discovery of hot Jupiters, giant gas exoplanets on tight orbits close to their host star, has proven instrumental in the study of exoplanet atmospheres through transit spectroscopy. Just as a temperature increase with height manifests in Earth’s atmosphere due to ozone absorbing ultraviolet radiation, transit spectra of some hot Jupiters has yielded evidence that suggests the presence of a similar temperature inversion in their stratosphere. A possible mechanism driving such an inversion is the presence of an analogous molecule to ozone. One potential candidate is titanium oxide (TiO) since it is one of the first molecules to form at high temperatures. However, due to the intersection of the vapour pressure curve of TiO and the pressure–temperature curve of some hot Jupiters, further investigation is required to determine if TiO can occur at suciently high abundances, or if it condenses out such as due to the presence of a cold-trap.

Aims. The goal of this project is to determine the pressure, density and temperature structure of an exoplanet in thermal and hydrostatic equilibrium and to identify if a temperature inversion will manifest.

Methods. A model is developed using the properties of exoplanet HD 209458b and a program is written to numerically and dynamically compute the thermal structure of a hot Jupiter atmosphere by solving the radiative transfer equations. The model takes into account depletion of TiO from destruction at high temperatures and condensation below the vapour pressure curve.

Results. The results show that a weak thermal inversion occurs on HD 209458b at TiO mixing fractions greater than solar Ti abundances and when a sucient fraction of the visible waveband interacts with the TiO. The proportion of incident visible radiation that must interact with the TiO in order to drive an inversion decreases as TiO abundance increases.

Conclusions. For TiO abundances above solar Ti levels, the model shows a weak thermal inversion can manifest in the upper atmosphere of HD 209458b for pressures below ~ 10^4 bar. This falls within the allowable pressure range as existing literature constrains inversions to pressures below ~ 10^3 bar on HD 209458b. However, inversions at such low pressures are not currently observable. (Less)
Popular Abstract (Swedish)
Heta Jupitrar är gasjättar som Jupiter vars omloppsbana för dem närmre sin värdstjärna än Merkurius bana kring Solen. Sen deras upptäckt har de visat sig vara väldigt viktiga i vår förståelse för exoplanet-atmosfärer. På Jorden minskar temperaturen av atmosfären med höjd över marken, förutom i ett område där temperaturen plötsligt går upp igen då ozonet som fångar UV-ljuset befinner sig på den höjden. Detta brukar kallas en temperaturinversion. Observationer har visat att heta Jupitrar har liknande mekanismer i sina atmosfärer. Eftersom det är just temperaturinversionen som gör att Jorden kan beh ̊alla vatten så kan liknande mekanismer på andra planeter vara kritiska för deras habilitet.

Ett potentiellt sätt att ha en... (More)
Heta Jupitrar är gasjättar som Jupiter vars omloppsbana för dem närmre sin värdstjärna än Merkurius bana kring Solen. Sen deras upptäckt har de visat sig vara väldigt viktiga i vår förståelse för exoplanet-atmosfärer. På Jorden minskar temperaturen av atmosfären med höjd över marken, förutom i ett område där temperaturen plötsligt går upp igen då ozonet som fångar UV-ljuset befinner sig på den höjden. Detta brukar kallas en temperaturinversion. Observationer har visat att heta Jupitrar har liknande mekanismer i sina atmosfärer. Eftersom det är just temperaturinversionen som gör att Jorden kan beh ̊alla vatten så kan liknande mekanismer på andra planeter vara kritiska för deras habilitet.

Ett potentiellt sätt att ha en temperaturinversion i en exoplanetatmosfär är närvaron av en molekyl analog till ozon. Titaniumoxid är en av de första molekyler som formas vid hög temperatur och är därför en kandidat för mekanismen i de väldigt varma atmosfärerna hos heta Jupitrar. Problemet är att Titaniumoxid kan potentiellt kondensera till moln och regna ner på marken precis som vatten gör på Jorden och därför bli kvar i den undre atmosfären. Detta skulle kunna innebära att tillräckligt höga koncentrationer av Titaniumoxid aldrig kan n ̊as i den övre atmosfären för att en temperaturinversion ska inträffa.

Målet med detta projektet är att bestämma tryck-, densitets- och temperatursstrukturen hos en exoplanet och identifiera ifall en temperaturinversion kommer att inträffa. För detta syfte har en modell utvecklats som använder sig av egenskaperna hos en välobserverad het Jupiter (HD 209458b) för att dynamiskt och numeriskt beräkna strukturen i en het Jupiter-atmosfär. (Less)
Please use this url to cite or link to this publication:
author
Agnew, Matthew LU
supervisor
organization
course
ASTM31 20151
year
type
H2 - Master's Degree (Two Years)
subject
keywords
exoplanet, atmospheres, cold trap, hot jupiter, thermal inversion, thermal structure
publication/series
Lund Observatory Examensarbeten
report number
2015-EXA99
language
English
id
5470981
date added to LUP
2015-06-12 16:03:36
date last changed
2015-06-12 16:03:36
@misc{5470981,
  abstract     = {Context. The discovery of hot Jupiters, giant gas exoplanets on tight orbits close to their host star, has proven instrumental in the study of exoplanet atmospheres through transit spectroscopy. Just as a temperature increase with height manifests in Earth’s atmosphere due to ozone absorbing ultraviolet radiation, transit spectra of some hot Jupiters has yielded evidence that suggests the presence of a similar temperature inversion in their stratosphere. A possible mechanism driving such an inversion is the presence of an analogous molecule to ozone. One potential candidate is titanium oxide (TiO) since it is one of the first molecules to form at high temperatures. However, due to the intersection of the vapour pressure curve of TiO and the pressure–temperature curve of some hot Jupiters, further investigation is required to determine if TiO can occur at suciently high abundances, or if it condenses out such as due to the presence of a cold-trap.

Aims. The goal of this project is to determine the pressure, density and temperature structure of an exoplanet in thermal and hydrostatic equilibrium and to identify if a temperature inversion will manifest.

Methods. A model is developed using the properties of exoplanet HD 209458b and a program is written to numerically and dynamically compute the thermal structure of a hot Jupiter atmosphere by solving the radiative transfer equations. The model takes into account depletion of TiO from destruction at high temperatures and condensation below the vapour pressure curve.

Results. The results show that a weak thermal inversion occurs on HD 209458b at TiO mixing fractions greater than solar Ti abundances and when a sucient fraction of the visible waveband interacts with the TiO. The proportion of incident visible radiation that must interact with the TiO in order to drive an inversion decreases as TiO abundance increases.

Conclusions. For TiO abundances above solar Ti levels, the model shows a weak thermal inversion can manifest in the upper atmosphere of HD 209458b for pressures below ~ 10^4 bar. This falls within the allowable pressure range as existing literature constrains inversions to pressures below ~ 10^3 bar on HD 209458b. However, inversions at such low pressures are not currently observable.},
  author       = {Agnew, Matthew},
  keyword      = {exoplanet,atmospheres,cold trap,hot jupiter,thermal inversion,thermal structure},
  language     = {eng},
  note         = {Student Paper},
  series       = {Lund Observatory Examensarbeten},
  title        = {The thermal structure of exoplanet atmospheres},
  year         = {2015},
}