Understanding the life of evolved stars using modern astronomical tools and techniques : case study of pulsating GW Vir, hot subdwarf B stars and their progenitors
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2022-08
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Universidad de Valparaíso
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Instituto de Fisica y Astronomia
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With the advance of high precision and high duty cycle photometric monitoring from the Transiting Exoplanet Survey Satellite (TESS) mission, unprecedented asteroseismic measurements and tools have become available for pulsating hot subdwarf B (sdB), white dwarf (WD) and pre-white dwarf stars. In this thesis, we present a detailed asteroseismic and spectroscopic analysis of long-period pulsating sdB and GW Vir WD stars observed with TESS in order to compare the observations with model predictions based on stellar evolution computations coupled with adiabatic pulsation computations.
A small percentage of the sdB population (about 10%) was discovered to be pulsating, allowing us to provide observational constraints for stellar models. Pulsating long-period sdB stars constitute a well-established class of variable stars that exhibit brightness variations with periods of up to a few hours and have amplitudes smaller than 0.1 percent of their mean brightness. The oscillation frequencies are associated with low degree (l < 3) medium to high-order (10 < n < 60) gravity(g)- modes, allowing us to investigate the deep interior of these stars. SdB asteroseismology has undergone substantial progress, thanks to the availability of space missions such as Kepler/K2 and TESS. We applied standard seismic tools for mode identification, including asymptotic period spacings and rotational frequency multiplets. For the dipole (l = 1) and quadrupole (l = 2) modes, we looked for a constant period spacing based on the results of the Kolmogorov-Smirnov and Inverse Variance tests. We computed stellar evolution models using the LPCODE stellar evolution code and computed dipole g-mode frequencies with the adiabatic nonradial pulsation code LP- PUL. Derived observational mean period spacings were then compared to the mean period spacings from detailed stellar evolution computations coupled with the adiabatic pulsation computations of g-modes. We find that the mean period spacings obtained for models with small convective cores, as predicted by a pure Schwarzschild criterion, are incompatible with the observations. The models with a standard, mod- est convective boundary mixing at the boundary of the convective core are in better agreement with the observed mean period spacings and are therefore more realistic.
In the second and third part of the thesis, we aimed at searching for the hydrogen(H)-deficient pulsating pre-white dwarf stars called GW Vir stars that exhibit atmospheres rich in carbon, oxygen and helium. We processed and analyzed the high-precision TESS photometric light curves of the four target stars, and derived their oscillation frequencies. For each of these TESS targets, we obtained low- resolution spectra and fitted model atmospheres in order to derive their fundamental atmospheric parameters. We performed an asteroseismological analysis of these stars on the basis of GW Vir white dwarf evolutionary models that take into account the complete evolution of the progenitor stars. We searched for patterns of uniform period spacings in order to constrain the stellar mass of the stars, and employed a detailed model by best-matching all the observed frequencies with those computed from models. Using the high-quality data collected by the TESS space mission and follow-up spectroscopy, we discovered and characterized four new GW Vir stars.
In the final part, we focused on the volume limited sample of low mass red giant stars, the progenitor systems of wide orbit hot sdB + main sequence (MS) binaries that are the product of a stable roche-lobe-overflow mechanism. With a homogeneously created list from Gaia, we aim at performing a spectroscopic survey to find binary systems that include low-mass red giants near the tip of the Red Giant Branch, which are predicted to be the direct progenitors of hot subdwarf B stars. We obtained high- resolution spectra for 88 stars out of which 38 stars were observed in two epochs in order to determine the binary fraction. Combining these measurements with DR2 ra- dial velocity and astrometric excess noise from early Data Release 3 (eDR3), we found
41 binary candidates. We presented 33 low-mass red giant stars that are in binary systems with orbital period between 100 and 900 days from both the ground base surveys and Gaia Data Release 3 (DR3). Using high-quality astrometric measurements provided by the Gaia mission coupled with high-resolution spectroscopy from the ground, we provided a powerful method to search for low-mass red giant stars in binary systems.
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ESTRELLAS GIGANTES ROJAS, BINARIAS ECLIPSANTES, ESPECTROSCOPIA ASTRONOMICA