Infrared dwarfs: Surface gravity sensitivity in H-band spectra

Abstract

The age of the center of our galaxy is poorly known. Determining the ages of individual stars in the galactic center usually requires isochrone fitting on a Hertzsprung-Russel diagram, which in turn requires knowledge of the star’s fundamental parameters, such as surface gravity, which is often determined with spectroscopic methods; and metallicity, which affects the shape of the isochrone. This can only be done for dwarf and subgiant stars, as in their respective regions of the diagram the isochrones are well separated and distinct. Such analysis has been performed for dwarf stars in the outer regions of the galactic bulge, but not for dwarf stars in the inner bulge due to the high amounts of extinction. However, infrared wavelengths are less extincted. To this end, this project investigates infrared wavelengths of synthetic stellar spectra for spectral lines sensitive to changes of 0.25, 0.5, 0.75, and 1.0 dex in log g. Synthetic spectra are produced in PySME. Changes in log g are investigated by dividing two synthetic spectra varying by a certain dex in log g while sharing all other input parameters to create the so-called response curve, where regions that vary between both spectra have values greater or less than 1. Peaks in this response curve thus indicate spectral lines sensitive to changes in surface gravity. Response peaks with a strength greater than 2% are analyzed qualitatively as a large sample, as well as individually in representative cases. A small analysis with synthetic Gaussian noise is performed to qualitatively determine at what signal-to-noise ratios (S/R) the synthetic spectrum or the response curve become unrecognizable to inform future observations of these spectral lines in bulge stars. It is concluded that large numbers of sensitive peaks exist in the chosen wavelength range, most are considered weak (of response ≈ 2%). Of the identified peaks, C I, Si I, Mg I, and H I transitions account for the majority of sensitive spectral lines. These are suggested as avenues for future theoretical work in synthetic models of dwarf stars.

Popular Abstract

Astronomy has often been called humanity’s oldest science: ever since we could recognize ourselves as such, humans have been gazing at the stars and wondering. Our access to technology has only expanded our horizons of observation, which uncover new questions about the universe and our place within it. Even though we can now observe galaxies billions of light years away, some of the most enigmatic stars are a little closer to home. Aside from the stars themselves, our galaxy is composed in large part of free-floating gas and dust, molded by gravity, radiation pressure, and supernova shockwaves into all sorts of beautiful shapes. However, this dust also absorbs and scatters starlight, causing stars behind concentrations of this dust to become significantly dimmed in a phenomenon known as extinction. Depending on where a star is located in the galaxy relative to us, it could be subject to varying amounts of extinction, and those lying in the galactic plane (where most of this dust is located) will be the most heavily extincted. The stars of the galactic center, known as the bulge, have long remained mysterious due to the intense extinction their light suffers before it reaches our telescopes. Some of these stars can be made up to 100 times dimmer—for small dwarf stars, this can make them virtually unobservable. Thus, if we are to understand this population of stars, novel observation methods must be employed. To this end, this project focuses on infrared light, which is much less subject to extinction than shorter wavelengths.

Scientists can learn a great deal about a star by analyzing the imprint certain elements leave on the star’s light. Such a graph is known as a spectrum. It is a fingerprint of sorts, showing the composition of the star (by which lines appear at what wavelength), as well as many of the star’s properties (by the shape and strength of specific spectral lines). Two of these properties, the effective temperature and surface gravity of the star, have well-known effects on the spectral lines of the visible spectrum. However, in the case of surface gravity, knowing exactly which spectral lines are affected is necessary, and this is not well known for the infrared region. In order to make observation of the bulge dwarfs with infrared telescopes at all feasible, these spectral lines must first be identified, and adequate candidates for study selected from this sample. This, in short, is the aim of this project. Learning more about the dwarf stars of the galactic center has numerous implications for our understanding of both our galaxy itself and how galaxies form generally. Perhaps most of these stars are incredibly similar in age and composition, suggesting a common origin in space and a rapid phase of star formation. Perhaps they have a great range of ages and all sorts of masses and temperatures, which would point to a continuous seeding of the galactic center with stars from diverse locations. Perhaps we will uncover a situation we have yet to conceive, and unearth even more questions about the history of our galaxy. Whatever the case, these stars present a tremendous opportunity to expand our knowledge horizon, and with it enrich both science and humanity.

• author: Rocha Novella, Santiago
• supervisor: Thomas Bensby
• organization: Department of Physics; Astrophysics
• course: FYSK04 20241
• year: 2024
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: dwarf stars, infrared, extinction, galaxy, Milky Way, galactic bulge, bulge, spectroscopy, synthetic spectra, fundamental parameters, stellar age
• report number: 2024-EXA237
• language: English
• id: 9171938
• date added to LUP: 2024-08-19 10:47:10
• date last changed: 2024-08-19 10:47:10