Using the resolution profile of the HERMES spectrograph to model spectral lines

Nonkulovski, Kristijan (author), Kos, Janez (mentor)

The main objective of this master thesis is to characterize the resolution profile of the HERMES spectrograph used in the GALAH project. Approximately 100 spectra from different stars were used in the whole analysis that were taken as part of the regular GALAH observations. First, we evaluated the line spread function from which we get the resolution profile in two of the four wavelength bands of the spectrograph, red channel 6481-6739 \AA and infra red channel 7590-7890 \AA. We find that the modeled resolution profile and the measured FWHM are in a very good agreement in both of the wavelength bands. Second, we use three methods for modeling the spectral lines. We start by synthesizing spectra with very high resolution R=300000 and then apply the different methods. With the first one, we fix the resolution of the synthetic spectra using the modeled resolution profile. In the second method, we degrade the synthetic spectra to the lowest common resolution R=22000, and in the third, we assume the nominal resolution of the spectrograph R=28000 for both the observed and the synthetic spectra. The synthetic spectra were generated with the help of the iSpec tool and best fit value was calculated with the \chi^2 goodness of fit test. For both of the methods, we calculate the relative uncertainty of the extracted parameters: effective temperature (T_eff), surface gravity (log g), metallicity [M/H], rotational velocity (vsini) and abundances of sodium, calcium, and silicon. Effective temperature and surface gravity show almost no change in the relative uncertainty between the first two methods, metallicity has a lower relative uncertainty when using the lowest common resolution, and rotational velocity is poorly determined with all of the methods. For all of the abundances, we observe better precision when using the modeled resolution profile. The accuracy of the chemical abundances was also calculated by comparing spectra that have similar element abundances in our star cluster. We found that elements (silicon and metallicity) that are determined by modeling lines that are close to the edges of the wavelength range show are more affected by the changing resolution across the wavelength band. Elements in the central part of the wavelength range have the same accuracy for the first two methods. We also conclude that when assuming the nominal resolution of the spectrograph we get the worst precision and accuracy for all of the parameters.

• Language: English
• Keywords: spectrograph: HERMES, resolution profile, spectral analysis, stars: abundances, stars: fundamental parameters
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2023
• PID: 20.500.12556/RUL-152306
• COBISS.SI-ID: 172412931
• Publication date in RUL: 18.11.2023

Secondary language

• Language: Slovenian
• Title: Uporaba resolucijskega profila spektrografa HERMES za modeliranje sprektralnih črt

Abstract

Glavni cilj te magistrske naloge je karakterizacija spektrografa HERMES, ki ga uporablja projekt GALAH preko analize njegovega resolucijskega profila. Uporabili smo približno 100 različnih spektrov različnih zvezd, ki jih dobimo iz tekočih GALAH opazovanjih. Naprej smo preučevali funkcijo, ki nam poda instrumentalno razširitev črt in posledično resolucijski profila po celotnem območju valovnih dolžin. Prilagajali smo črte zemljske atmosfere v dveh od vseh štiri območja valovne dolžine spektrografa (rdeči 6481 ⠒ 6739 Å and infra rdeči 7590 ⠒ 7890 Å kanal) iz česa smo dobili celotno širino na polovični višini. Ugotovili smo, da se modelirani resolucijski profil in izmerjena celotna širina na polovični višini ujemata zelo dobro. V drugem delu te magistrske naloge smo uporabili tri različne metode za modeliranje spektralnih črt. Modeliranje črt je bilo narejeno preko sintetizacije spektrov z visoko resolucijo R = 300000 s pomočjo programa iSpec. S prvo metodo sintetični spekter popravimo za modelirani resolucijski profil tako, da naredimo konvolucijo med obema. V drugi metodi degradiramo resolucijo opazovanih in sintetičnih spektrov na vrednost R = 22000. Pri zadnji metodi predpostavimo nominalno resolucijo spektrografa R = 28000 za resolucijo opazovanih spektrov in posledično degradiramo resolucijo sintetičnih spektrov na to vrednost. Najbolj prilegajoč sintetični spekter smo izbrali preko χ2 testa. Za vse metode izračunamo relativno negotovost v pridobljenih parametrih: efektivna temperatura zvezde (Teff), gravitacija na površju (log g), kovinskost [M/H], rotacijska hitrost (vsini) in zastopanost natrija, kalcija in silicija. Efektivna temperatura in gravitacija na površju zvezd ne kažeta velike spremembe če primerjamo prvi dve metodi, kovinskost ima manjšo relativno negotovost, ko degradiramo resolucijo na R = 22000. Rotacijska hitrost je pri vseh metodah zelo slabo določena. Točnost omenjenih parametrov je tudi bila izražunana tako, da smo predpostavili homogenost v zastopanosti elementov znotraj opazovane kopice. Dobili smo, da zastopanosti elementov, ki ju dobimo preko črte, ki se nahajajo na robovih čutijo večji vpliv iz spremenljive resolucije. Zastopanost silicija in kovinskost, ki ju dobimo iz modeliranih črt v sredini opazovanega območja valovne dolžine kaže enako točnost pri prvih dveh metodah. Glede na izmerjeno točnost in relativna negotovost zastopanosti elementov lahko zaključimo, da ne smemo predpostaviti no- minalno resolucijo spektrografa ko merimo zastopanost kemičnih elementov.

• Keywords: spektrograf: HERMES, resolucijski profil, analiza spektrov, zvezde: zastopanost elementov, zvezde: osnovni parametri