An Infrared Stellar Thermometer: Exploiting IGRINS Spectra

Sammanfattning: Using the infrared (IR) spectral range when measuring stellar spectra is beneficial compared to using the optical range mainly when observing distant stars through the dust dense interstellar regions in the Galactic mid-plane. Methods to determine stellar parameters from IR spectra are, however, not fully developed yet because technological difficulties in creating reliable IR detectors have slowed the advance of such methods. The effective temperature of a star is considered to be one of the most fundamental parameters of a star because it is used in stellar classifications and also because it is directly related to the star's energy output, which furthermore affects the formation of spectral lines. Therefore, it is important for astronomers to be able to determine accurate stellar temperatures. This thesis aims to investigate whether OH and CO molecular lines from IR spectra in the H-band ($15,000$ - $18,000 \,$Å) can be used to determine the effective temperature of stars. These lines' sensitivity to the stellar surface gravity, oxygen abundance, and for the CO lines, the carbon abundance were explored as well. There already exist many methods to determine the effective temperature of stars, but the majority of these use optical spectra instead of IR. In this thesis, spectral synthesis is used to determine the temperatures of 34 K-giants. Their spectra were observed with the near-IR, high-resolution spectrometer, IGRINS mounted on the $4.3\,$m Lowell Discovery Telescope in Flagstaff, Arizona. The stellar temperatures that are determined in this thesis are benchmarked against optically determined temperatures from Jönsson et al. (in prep). Other stellar parameters that were assumed in the spectral synthesis, such as the surface gravity, the oxygen abundance and the carbon abundance, were also taken from Jönsson et al. (in prep). The results found in this thesis show that OH lines provide accurate temperatures within $\abs{100}$ K and that they are insensitive to the surface gravity ($1 \,$K/$0.2 \,$dex) but very sensitive to the oxygen abundance ($120 \,$K/$0.2 \,$dex). It was further found that the CO lines were sensitive to the surface gravity ($100 \,$K/$0.2 \,$dex) and the carbon abundance ($320\,$K/$0.2\,$dex for cooler stars and $220\,$K/$0.2\,$dex for hotter stars), but less sensitive to the oxygen abundance ($20 \,$K/$0.2 \,$dex). A combination of the CO and OH lines showed that the sensitivities decreased compared to the most sensitive linelist ($-13 \,$K/$0.2 \,$dex for the surface gravity and $100 \,$K/$0.2 \,$dex for the oxygen abundance). The combination also acquired properties from both linelists such as their sensitivities and estimated temperatures that lie in between the temperatures determined from the separate linelists.