Using Exoplanets to Determine Photospheric Properties of Fast-Rotating A-type Stars

Abstract

Exoplanets and their host stars are intrinsically related as they form from the same accretion disc, and are therefore expected to have similar chemical composition. Thus, knowing the composition of exoplanet host stars is important for developing planetary formation and migration theories. There has been a recent uptick in studies of ultra-hot Jupiter atmospheres as their orbital configurations provide an ideal test-bed for transmission spectroscopy, enabling detections of many chemical species. However, this class of exoplanets tend to orbit fast-rotating A-type stars with severely rotation broadened spectra.

Line blending in rotation broadened spectra hinders the accurate measurement of photospheric parameters and chemical abundances in the stellar photosphere. We bypass this problem of rotation broadened spectra by introducing a new method of isolating the non-broadened stellar spectrum using the effect of a transiting exoplanet. During an exoplanet transit, the planet obscures part of the light emitted from the star. The spectrum of this light is characteristic of the stellar disc at the planet's location and is less affected by rotation broadening. This allows us to resolve weaker absorption lines and hence more precisely measure chemical abundances in fast-rotating exoplanet host stars. This will ultimately allow ultra-hot Jupiter systems to be better characterised in terms of their evolutionary mechanisms.

With the obscured stellar spectrum from behind the transiting exoplanet, $F_{\text{obsc}}$, at every time-series observation, we reconstruct the disc-integrated non-broadened stellar spectrum, which we term the isolated spectrum, $F_{\star,N}$. This method is tested with a proof-of-concept using synthetically generated spectra and then applied to real observations from HARPS. We focus on the WASP-189 system which consists of the ultra-hot Jupiter WASP-189 b transiting across its fast-rotating A-type host star. We then apply standard spectral fitting procedures to the isolated spectrum, $F_{\star,N}$, to obtain best-fit global photospheric parameters ($T_{\text{eff}}$, $\log g$ and [Fe/H]) and individual chemical abundances. We have proven that we can obtain photospheric parameters with significantly improved accuracy compared with the broadened stellar spectrum. This method is not limited to ultra-hot Jupiter systems, but can be also be applied to other transiting systems in which abundance determinations via spectral synthesis are imprecise due to severe line blending.

Popular Abstract (Swedish)

Människan har drömt om liv bortanför jorden, även innan upptäckten av den första exoplaneten runt en solliknande stjärna år 1995. Idag, med över 5600 bekräftade exoplaneter, är möjligheterna om liv på en annan planet bortom vår fantasi. Men innan vi ens försöker förstå oss på utomjordiskt liv, så måste vi förstå vad dessa planeter består av. Planeter och deras världsstjärnor är sammankopplade då de bildades från samma material. Att veta egenskaperna hos stjärnan är därmed det första steget till att veta planetens egenskaper.

En het Jupiter är en typ av exoplanet som är fokus för många atmosfäriska studier. Dessa gasjättar har en liknande storlek och massa som Jupiter, men är belägna närmare deras världsstjärna och blir på så sätt också mycket mer bestrålade av sin världsstjärna. På grund av detta når deras dags sida temperaturer på åtminstone 1600 K. WASP-189 b är ett exempel på en ultra-het Jupiter som befinner sig 20 gånger så nära sin stjärna som jorden gör solen. Denna planet, med en massa nästan två gånger Jupiters, har en dags sida med temperaturer på över 3000 K. Världsstjärnan WASP-189 är en snabbt roterande stjärna av spektraltyp A, som visar extrem rotationsbreddning i dess spektrallinjer på grund av Dopplereffekten. Detta orsakar absorptionslinjer i dess spektrum att blandas samman, vilket gör det svårt att urskilja individuella linjer och därmed att definiera deras linjeprofiler för att bestämma kemiska sammansättningar i stjärnans fotosfär. Ifall vi vet den kemiska kompositionen av stjärnan, så hjälper detta oss oerhört i framtida studier av exoplaneters atmosfärer.

Med denna avhandling föreslår vi en lösning till ett rotations-breddat spektrum med en ny metod som vi har utvecklat. När vi observerar spektrumet av en stjärna med en redan känd exoplanet, så kommer planeten ibland att röra sig framför stjärnan, och på så sätt dölja en del av dess observerade flux och därmed också en del av spektrumet. Genom att isolera det dolda spektrumet, som är representativt för det lokala spektrumet hos stjärn-skivan, kan vi rekonstruera hela stjärnans spektrum utan rotationsbreddningens försämrande effekter. Vi kan nu passa spektral modeller till detta nya, isolerade spektrum och mer precist mäta de kemiska sammansättningarna hos världsstjärnan. Detta kommer inte bara underlätta framtida studier om heta Jupiter-system, utan också studier relaterade till stjärnor av spektraltyp A.

Popular Abstract

Humankind has dreamed of life beyond our Earth, even before the discovery of the first exoplanet around a Sun-like star in 1995. Today, with over 5600 confirmed exoplanets, the possibilities of life on another planet are beyond our imagination. But before we even begin to fathom extraterrestrial life, we must understand what these planets are composed of. Planets and their host stars are intrinsically related as they are formed from the same materials. Hence, knowing the properties of the host star is the first step to knowing the planet.

Hot Jupiters are a class of exoplanet that are the focus of many atmospheric studies. These are gas giants with a similar size and mass to Jupiter, but are instead located closer to their host star such that they are much more irradiated by their host star, making their dayside temperatures at least 1600 K. WASP-189 b is an example of an ultra-hot Jupiter located 20 times closer to its star than Earth is to the Sun. This planet, with a mass almost twice that of Jupiter, has a dayside temperature of over 3000 K. The host star WASP-189 is a fast-rotating A-type star, which exhibits extreme rotation broadening in its spectral lines due to the Doppler effect. This causes absorption lines in the spectrum to blend together, making it difficult to resolve individual lines and hence fit their line profiles to determine chemical abundances of elements that are present in the stellar photosphere. If we know the chemical composition of the host star, this will help us immensely in future exoplanet atmosphere studies.

With this thesis, we propose a solution to the rotation broadened spectra with a new method we have developed. When we observe the spectrum of a star with a known transiting planet, the planet will sometimes pass in front of the star, obscuring some of the observed flux, and hence the spectrum. We isolate this obscured spectrum, which is representative of the local spectrum on the stellar disc, and use it to reconstruct the entire stellar spectrum without the debilitating effects of rotation broadening. We can now fit spectral models to this new, isolated spectrum and measure more precisely the chemical abundances of the host star. Not only will this help with future studies on hot Jupiter systems, but also studies involving A-type stars.

• author: Lam, Madeline
• supervisor: Jens Hoeijmakers
• organization: Astrophysics; Department of Physics
• course: ASTM32 20241
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: planets and satellites: gaseous planets, stars: abundances, stars: individual: WASP-189, stars: planetary systems, techniques: spectroscopic
• report number: 2024-EXA231
• language: English
• id: 9166247
• date added to LUP: 2024-11-05 08:28:53
• date last changed: 2024-11-05 08:28:53