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The extended atmospheres of red supergiants

Gonzalez I Tora, G (2023) The extended atmospheres of red supergiants. Doctoral thesis, Liverpool John Moores University.

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Red supergiants (RSGs) are evolved massive stars in a stage preceding core-collapse supernova. Investigating evolved-phases of these stars is key to understanding the cosmic matter cycle of our Universe, since they enrich the cosmos with newly formed elements.
The work shown in this thesis has two main focuses: one of these aims is to probe the temperature dependence of RSGs on metallicity. In context, the temperatures of RSGs are thought to be a manifestation of the Hayashi limit, which fixes the minimum effective temperature a star can have while still maintaining hydrostatic equilibrium. The Hayashi limit is expected to depend on metallicity (Z) in such a way that lower-Z RSGs are warmer. This prediction could be tested by studying the average effective temperatures of RSGs in galaxies with different metallicities.

We investigate the Z-dependence of the Hayashi limit by analysing RSGs in the low-Z galaxy Wolf-Lundmark-Mellote (WLM), and compare with the RSGs in the higher-Z environments of the Small Magellanic Cloud (SMC) and Large Magellanic Cloud (LMC). We find a correlation between metallicity and the observed effective temperatures. We also compare the observations with evolutionary models, and find that while the trend is similar, there is a shift between observations and model predictions. This mismatch between observations and models could be due to wrong assumptions in the description of convection for massive stars.

Secondly, the work in this thesis succeeds on modelling the spatially extended atmospheres of RSGs, where the mass loss is initiated, by plugging in the effect of a stellar wind in a stellar structure model. The physical processes that extend the atmospheres of RSGs are still not fully understood, and remain one of the key questions in stellar astrophysics. In this thesis, we propose a new 1D method to study the extended atmospheres of these cold stars, by adding the effect of a semi-empirical stellar wind based on observations of Betelgeuse. We then can compute the intensities, spectral energy distributions and visibilities matching the observations for the different instruments in the Very Large Telescope Interferometer (VLTI).

Specifically, the work first discusses the robustness of our results when comparing with the atmospheric structure of HD 56879 and V602 Car of published VLTI/AMBER data, comprising the wavelength range of $1.8<\lambda<2.5\,\mu$m. Then, we also compare our extended model to newly acquired data of AH Sco, KW Sgr, V602 Car, CK Car and V460 Car with the instruments VLTI/GRAVITY and MATISSE, that comprise a broader wavelength range ($1.8<\lambda<13\,\mu$m). This later work represents the most complete spectro-interferometric study up to date of RSGs. We find that our model can accurately match these observations for the first time, showing the enormous potential of this methodology to reproduce extended atmospheres of RSGs and to constrain temperature and density stratifications as well as mass-loss rates.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: astronomy; stellar astrophysics; astrophysics; stars; massive stars; mass-loss; stellar winds; red supergiants; interferometry; atomspheres; spectroscopy
Subjects: Q Science > QB Astronomy
Divisions: Astrophysics Research Institute
SWORD Depositor: A Symplectic
Date Deposited: 18 Sep 2023 14:50
Last Modified: 18 Sep 2023 14:51
DOI or Identification number: 10.24377/LJMU.t.00021374
Supervisors: Davies, B
URI: https://researchonline.ljmu.ac.uk/id/eprint/21374

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