Temperature sensitivity of OH-lines in the H-band of K-M giants

Abstract

To properly understand regions of space behind interstellar dust, like the Galactic bulge, infrared (IR) observations are needed, as it passes through the dust with minimal absorption. In the past few decades, the development of IR telescopes has allowed for a much deeper understanding of our Galaxy’s stellar populations and history. To properly use the data which these IR telescopes produce however, techniques for finding primary parameters of the observed stars are needed however. One molecule which has been found to be reliably temperature sensitive in the past has been OH, which fills the H-band (15000˚A - 18000˚A) with hundreds of absorption lines. It has not been known which of these work the best, as previously existing methods only checked subsets of OH-lines, as well as checking over smaller star samples. It is the goal of this thesis to find a list of high quality OH-lines which all are sensitive to changes in stellar parameters, so that future researchers may use this list as a basis for finding the parameters of stars. These would include the effective temperature, surface gravity and metallicity. Using 78000 K-M giant spectra acquired from APOGEE DR17, 68 OH-lines have been found to be temperature sensitive out of a total 186 OH lines. This was found from relating line depth and stellar surface temperature, which showed a linear relation for these 68 OH-lines. These 68 lines also lacked any major blends with other atoms and molecules. In addition, a subset of 20 lines of these 68 were shown to be significantly less scattered than the rest.

Popular Abstract

As we look out into our Galaxy with today’s telescopes, we notice many types of stars. Some are recognisable with comparable mass, chemical composition and size to our sun. Some are a little stranger with different masses, radii and elemental make-up. Another variable one might even see with ones own eye is color. Warmer stars have a bluer tint and colder stars are redder. This is in fact one of the most fundamental parameters of any star, surface temperature. Knowing the surface temperature of a star can lead a researcher do deduce its age and chemical composition, and with enough stars, this allows them to puzzle together the story of our Galaxy. My project strives to more accurately figure out this surface temperature, allowing future scientists to tell us of the Galaxy’s tale, billions of years in the making. An area of our Galaxy that we still lack a lot of insights on is the galactic bulge, also known as the centre of our Galaxy. The reason for this lack of knowledge is cosmic dust. In the galactic disk, which we are located in, clouds of dust have gathered over billions of years. These clouds absorb and scatter optical light, making many of our previous and current telescopes unable to gather data in any region like the bulge. Luckily, infrared light in the near infrared (NIR) does pass through them with ease making cooler red stars optical behind the clouds for NIR telescopes. These are for example stars like the sun which have grown old, increasing their radii and luminosity considerably and cooling off, resulting in them becoming red giants. So what can these red giants tell us? Through spectroscopy, the process of splitting light into its constituent colors, known as wavelengths, one can see which wavelengths of light is radiated by these stars. From this spectrum of wavelengths, emission peaks and absorption troughs can be spotted. These are related to elements in the atmosphere of the observed star. For example, a star with a lot of iron would have deep absorption lines where iron atoms are known to absorb light. This holds true for all elements and molecules. By then relating the absorption strength with other intrinsic properties of a star’s surface, like surface gravity and surface temperature, one can figure out he chemical make-up of the given star. By then knowing the elements in many stars of the bulge, we get closer to a full understanding of the Milky Way’s history. Not all wavelengths that any given atom or molecule absorbs are created equal how-ever. The absorption lines of the molecule OH for example are generally known to decrease in strength gradually when going from 3000 stars, down to non-existence in stars hotter than 4000. This is due to the molecule breaking down into its constituents the hotter its environment. And though this is generally true, this does not hold for all lines. Some lines are very weak even in the coldest of stars, some lines break the relation totally by being both deep and shallow in similarly cold stars, and some lines do hold to the relation but blend too much with neighboring lines to be useful. Therefore, my goal with this thesis is to figure out which specific lines of the molecule OH are most sensitive and least scattered to temperature. Only a handful of all possible OH-lines in the near infrared have been discovered to be temperature sensitive, but by analysing the line depth of all viable lines, many more lines may be used to calibrate the surface temperature of bulge star temperatures.

• author: Lindqvist, Kasper
• supervisor: Nils Ryde; Govind Nandakumar
• organization: Astrophysics; Department of Physics
• course: ASTK03 20232
• year: 2024
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• report number: 2024-EXA219
• language: English
• id: 9173980
• date added to LUP: 2024-09-09 10:33:33
• date last changed: 2024-09-09 10:33:33