A spectroscopic census of titanium lines in the near-infrared

Abstract

Context. The adverse effects of interstellar extinction makes the optical range unsuitable for spectroscopic analysis of stellar populations in dust-obscured regions of the Milky Way. With new instruments available, high-quality spectroscopy in the near-infrared (near-IR or NIR) range is now feasible. However, this region of the electromagnetic spectrum is relatively unexplored for the purpose of investigating stellar spectra, when compared to the optical. Hence the evaluation of elements with high amounts of spectral lines in the H- and K-bands of the near-infrared is highly relevant. Titanium is a promising candidate as it possesses many lines in the near-IR.

Aims. The objective of this project is to provide a list of recommended titanium spectral lines suitable for determining abundances and stellar parameters. Additionally, the aim is to investigate the abundance trends of titanium when compared to metallicity. The hope is that it may prove useful in future research of the chemical evolution of our galaxy as well as general spectroscopy.

Methods. High-resolution (R∼ 45000) NIR spectra of 37 K- and 45 M-type giants were obtained from the Immersion GRating INfrared Spectrograph (IGRINS) mounted on the South Gemini Telescope. The software Spectroscopy Made Easy (SME) was utilised to evaluate the analytical quality of spectral lines by calculating chemical abundances from synthetic spectra fitted to observed data.

Results. For the cooler M-type giants, a total of 33 Ti-1 and 2 Ti-2 spectral lines were initially concluded to likely produce good results. Upon further investigation, respectively 19 and 2 of these proved to be suitable for spectroscopic analysis. For the warmer K-type giants, a total of 22 Ti-1 and 2 Ti-2 spectral lines were initially concluded to likely produce good results. Upon further investigation, respectively 13 and 2 of these proved to be suitable for spectroscopic analysis.

Conclusions. Titanium offers a viable option in near-IR spectroscopy of K- and M-type giants, for the purpose of studying their chemical evolution and determining stellar parameters through spectral analysis. Discrepancies in calculated abundance between the H- and K-bands limited the number of useful Ti lines in the latter. The issue is seemingly related to the non-local thermodynamic equilibrium (NLTE) models used by SME, and is not unique to titanium. Nonetheless, a sufficient amount of titanium spectral lines are available in the near-IR for both K- and M-type giant stars.

Popular Abstract

In the 17th century, Sir Isaac Newton used a prism to split white sunlight into a rainbow of colours, the so called visible spectrum. Over a century later, Joseph von Fraunhofer would come to invent the spectroscope. Much like the prism of Newton, this instrument could display the colour spectrum of light, but now in much greater detail. With his new invention, he would come to discover dark lines in the visible spectrum of our Sun. Fraunhofer also turned his spectroscope to the brightest star in the night sky, Sirius. He saw that its spectrum was different from that of our Sun, inadvertently laying the foundations for stellar spectroscopy. These spectral lines were eventually named Fraunhofer lines after him. Over three decades after his passing, Gustav Kirchhoff and Robert Bunsen realised that the Fraunhofer lines observed were in fact created by atoms in the outermost layers of the Sun absorbing outgoing light. Light consists of electromagnetic radiation of different wavelengths. For those wavelengths visible to our eyes, they correspond to different colours. Atoms and molecules can absorb and emit light of specific wavelengths, depending on the chemical. This means that the dark absorption lines in a star’s spectrum become a sort of fingerprint for that star, showing what chemicals are present and in what abundances. The abundance of an element is the relative amount of that chemical present in a star. Metallicity is the total measure of all elements heavier than hydrogen present. Stars that formed together tend to have a similar fingerprint of abundances and metallicity. By using spectroscopy to measure these quantities, one can infer which stars formed together and which are outliers. This enables conclusions about the movement of stars and groups of stars, which can help researchers explain the structure of our galaxy. The field of stellar spectroscopy has progressed far since the time of Newton and Fraunhofer. Modern technology has enabled more detailed analysis of a wide range of different objects. However, there are still many challenges to be overcome. One such issue faced by modern day astronomers is the so called interstellar extinction of light. Space is far from empty, there are giant clouds of gas and dust scattered around the space between stars. These can absorb and scatter light, making it difficult to properly observe a star located on the other side. This limits what stars are available for study, denying access to dust-obscured stellar populations of high interest. The near-infrared, or near-IR for short, is less prone to interstellar extinction, as interstellar gas and dust is mostly transparent to near-IR light. When it comes to spectroscopy however, the near-infrared region is less explored than the optical. This means that we lack expertise on what approaches are best suited for measuring chemical abundances and calculating stellar parameters. To this end, it is valuable to investigate what elements are appropriate for near-IR spectroscopy. Titanium is a fairly abundant element with a high amount of spectral lines in the NIR, which allows us to be picky and only use the better lines. This yields greater statistical accuracy when measuring the abundance, since we have a higher number of high-quality data points. It is therefore a promising candidate for use in near-IR spectroscopy, and needs to be evaluated.

• author: Otréus, Axel
• supervisor: Nils Ryde; Govind Nandakumar
• organization: Department of Physics; Astrophysics
• course: FYSK04 20251
• year: 2025
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• report number: 2025-EXA252
• other publication id: 2025-EXA252
• language: English
• id: 9210944
• date added to LUP: 2025-08-29 07:57:40
• date last changed: 2025-08-29 07:57:40