Hyperfine structure measurements in scandium for IR spectroscopy

Abstract

Stellar abundances are important for understanding the chemical evolution of galaxies and provide us with constraints on stellar evolution and supernova nucleosynthesis. To determine stellar abundances accurately, atomic data is crucial. This data is incomplete for many elements, especially in the infrared region. In particular, hyperfine structure (hfs) data in the near-infrared region is lacking for neutral scandium. Incorrect scandium abundances are obtained when hfs is neglected in stellar abundance analysis, which has led to contradicting theories of the synthesis process of scandium. In this project, hfs constants for neutral scandium are derived by means of laboratory experiments. A spectrum was produced using a hollow cathode discharge lamp and the spectrum was recorded using a Fourier Transform Spectrometer. A model has been developed to produce synthetic spectra of hfs multiplets. Using the nonlinear least squares method, the model was fitted to observed transitions in order to find the best fit hfs constants. The hfs constants have been determined for 95 levels in neutral scandium. For 52 of these levels, the hfs constants were determined for the first time. An agreement with literature values was found. The behaviour of the developed program was probed given an offset in initial values, and various signal to noise ratios and widths of the structure. Additionally, oscillator strengths have been derived for 590 hfs transitions, using the Höln-Kronig intensity rule. The hfs data derived in this project will allow for more accurate analysis of scandium lines in stellar spectra. The transition in scandium from $3d4s(^3D)4p$ $^4D_{{7}/{2}}$ to $3d^2(^3F)4s$ $^4F_{7/2}$ in the spectrum of the star Alpha Boötis has been investigated in the light of the new hfs data and the new hfs constants were able to reproduce the observed line profile.

Popular Abstract (Swedish)

Allting är uppbyggt av atomer, inte bara träd, djur eller bläcket i dessa bokstäver, utan även astronomiska objekt så som stjärnor och planeter. Stjärnor är i princip extremt varma och kompakta moln av atomer, som skickar ut ljus i alla riktningar. Astronomer är intresserade av vilken sammansättning av atomer som bygger upp stjärnor eftersom detta hjälper dem att förstå hur stjärnor och galaxer bildas och utvecklas. Information om vilka ämnen en stjärna består av kan fås från ljuset de skickar ut. Ljuset som skickas ut från en stjärna innehåller olika färger, eller våglängder. För att förstå varför det är så behöver vi förstå vilka processer som sker i en stjärna. Den inre delen av en stjärna skickar ut kontinuerligt ljus i alla våglängder. Atomer i den yttre delen av stjärnan absorberar en del av det kontinuerliga ljuset. På grund av detta kommer intensiteten för vissa våglängder att minska, detta kallas absorptionslinjer. Varje grundämne har ett unikt set av absorptionslinjer. Det emitterade ljuset från en stjärna är karakteriserat av de atomer som finns stjärnan. Med spektroskopi studerar man spektrumet av en stjärna genom att mäta intensiteten som funktion av våglängd. Den kemiska sammansättningen av en stjärna kan således bestämmas genom att analysera spektrumet. Man kan även studera förhållande som råder i stjärnan genom att analysera formen av absorptionslinjerna. Till exempel kan man bestämma mängden av ett grundämne genom att mäta djupet på en absorptionslinje, eftersom mängden ljus som absorberas är ett mått på mängden atomer av grundämnet i stjärnan. Man kan även studera parametrar så som temperatur och gravitation med hjälp av absorptionslinjer. För att bestämma dessa stjärnparametrar måste de unika spektrallinjerna för de olika grundämnena vara kända från laboratoriemätningar. En del grundämnen har inneboende egenskaper som påverkar formen av spektrallinjerna och därför försvårar stjärnanalysen. Ett sådant ämne är scandium, som uppvisar breda spektrallinjer på grund av en inneboende effekt som kallas hyperfinstruktur. För att analysera scandiumlinjer i en stjärna är det nödvändigt att veta hur hyperfinstrukturen påverkar formen för varje linje. I detta arbete har hyperfinstrukturen mätts i laboratoriespektra för ett antal scandiumlinjer. En teoretiskmodell har utvecklades för att anpassa den experimentella datan och beskriva hyperfinstrukturen. Tack vare hyperfinstrukturerna som bestämts i detta arbete kan man få en bättre förståelse av ljuset som stjärnor skickar ut och vad som sker i det enorma universum som omger vår lilla blå planet.

Popular Abstract

Everything is made up out of atoms. Not only trees, animals or the ink in these letters, but also astronomical objects such as stars and planets. Stars are basically extremely hot and dense clouds of atoms, emitting light in all directions. Astronomers are interested in which atoms build up stars, as this helps us understand how stars and galaxies evolve. Information about the composition of a star can be found from the light emitted by the star. The light emitted by a star consists out of many colours, or so-called wavelengths. To understand this, we should consider the processes that occur inside a star. The inner part of a star shines with a constant intensity at all wavelengths. Atoms in the outer part of a star absorb parts of the light that was emitted by the inner part. However, atoms only absorb light with specific wavelengths characteristic for the atom. For this reason, the intensity shows dips at certain wavelengths which are called absorption lines. Each element has its own unique set of absorption lines. In this way, the light emitted by a star is characterized by the atoms that are present in the star. Astronomers measure the light from stars using a method called spectroscopy. In spectroscopy, the spectrum of a star is recorded by measuring the intensity of the light at various wavelengths. By identifying the absorption lines in the recorded spectrum, one can infer which atoms are present in the star. Moreover, from the shape of an absorption line, the conditions in a star can be probed. For instance, from the depth of an absorption line one can determine the amount of a certain element that is present in a star. This is because the amount of light that is absorbed is related to the number of atoms. The shape of an absorption line can also tell us about the temperature and gravity in a star. Though, before all these parameters can be derived, the unique sets of absorption lines for different elements needs to be known. Luckily, the set of absorption lines characterizing an element can be measured in the lab. For some elements however, intrinsic processes influence the shape of the absorption lines, which complicates probing these stellar conditions. The element scandium for instance, shows very broad absorption lines. This is because the shape of scandium absorption lines in stellar spectra is not only affected by the stellar temperature, gravity and scandium abundance, but also by an intrinsic process called hyperfine structure splitting. In order to analyse scandium absorption lines in stellar spectra, it is necessary to know the influence that hyperfine structure splitting has on the shape of the absorption line. This can be described using so-called hyperfine structure constants. In this project, these constants have been measured for several scandium lines by means of laboratory experiments. This was done by recording a spectrum of pure scandium in the lab. A model was built to reproduce the shape of the recorded scandium lines using the hyperfine structure constants. The model was fit to the observed spectrum, which returned the hyperfine structure constants for several scandium lines. Using the constants determined in this thesis, we are better able to read the light from the stars and determine what is going on in the vast universe surrounding our small blue rock.

• author: van Deelen, Felix
• supervisor: Hampus Nilsson; Henrik Hartman
• organization: Lund Observatory - Undergoing reorganization; Department of Astronomy and Theoretical Physics - Undergoing reorganization
• course: ASTM31 20171
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Astrophysics, Atomic Data, Hyperfine Structure, Stellar Spectroscopy
• publication/series: Lund Observatory Examensarbeten
• report number: 2017-EXA113
• language: English
• id: 8912010
• date added to LUP: 2017-06-09 16:49:53
• date last changed: 2017-06-09 16:49:53