Examinando por Autor "Uzundag, Murat"

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  • Miniatura
    Ítem
    About the existence of warm H-rich pulsating white dwarfs
    (European Southern Observatory (ESO), 2020) Althaus, Leandro G.; Córsico, Alejandro H.; Uzundag, Murat; Vučković, Maja; Baran, Andrzej S.; Bell, Keaton J.; Camisassa, María E.; Calcaferro, Leila M.; De Gerónimo, Francisco C.; Oliveira Kepler, Souza; Silvotti, Roberto
    Context. The possible existence of warm (Teff ∼ 19 000 K) pulsating DA white dwarf (WD) stars, hotter than ZZ Ceti stars, was predicted in theoretical studies more than 30 yr ago. These studies reported the occurrence of g-mode pulsational instabilities due to the κ mechanism acting in the partial ionization zone of He below the H envelope in models of DA WDs with very thin H envelopes (MH/M⋆ ≲ 10−10). However, to date, no pulsating warm DA WD has been discovered, despite the varied theoretical and observational evidence suggesting that a fraction of WDs should be formed with a range of very low H content. Aims. We re-examine the pulsational predictions for such WDs on the basis of new full evolutionary sequences. We analyze all the warm DAs observed by the TESS satellite up to Sector 9 in order to search for the possible pulsational signal. Methods. We computed WD evolutionary sequences of masses 0.58 and 0.80 M⊙ with H content in the range −14.5 ≲ log(MH/M⋆)≲ − 10, appropriate for the study of pulsational instability of warm DA WDs. Initial models were extracted from progenitors that were evolved through very late thermal pulses on the early cooling branch. We use LPCODE stellar code into which we have incorporated a new full-implicit treatment of time-dependent element diffusion to precisely model the H–He transition zone in evolving WD models with very low H content. The nonadiabatic pulsations of our warm DA WD models were computed in the effective temperature range of 30 000 − 10 000 K, focusing on ℓ = 1 g modes with periods in the range 50 − 1500 s. Results. We find that traces of H surviving the very late thermal pulse float to the surface, eventually forming thin, growing pure H envelopes and rather extended H–He transition zones. We find that such extended transition zones inhibit the excitation of g modes due to partial ionization of He below the H envelope. Only in the cases where the H–He transition is assumed much more abrupt than predicted by diffusion do models exhibit pulsational instability. In this case, instabilities are found only in WD models with H envelopes in the range of −14.5 ≲ log(MH/M⋆)≲ − 10 and at effective temperatures higher than those typical for ZZ Ceti stars, in agreement with previous studies. None of the 36 warm DAs observed so far by TESS satellite are found to pulsate. Conclusions. Our study suggests that the nondetection of pulsating warm DAs, if WDs with very thin H envelopes do exist, could be attributed to the presence of a smooth and extended H–He transition zone. This could be considered as indirect proof that element diffusion indeed operates in the interior of WDs.
  • Miniatura
    Ítem
    Asteroseismic analysis of variable hot subdwarf stars observed with TESS
    (European Southern Observatory (ESO), 2021) Uzundag, Murat; Vuckovic, Maja; Németh, Péter; Bertolami, M. Miller; Silvotti, Roberto; Baran, Andrzej S.; Telting, John H.; Reed, Mike; Shoaf, K. A.; Østensen, Roy H.; Sahoo, Sumanta K.
    Context. We present photometric and spectroscopic analyses of gravity (g-mode) long-period pulsating hot subdwarf B (sdB) stars, also called V1093 Her stars, observed by the TESS space telescope in both 120 s short-cadence and 20 s ultra-short-cadence mode during the survey observation and the extended mission of the southern ecliptic hemisphere. Aims. We performed a detailed asteroseismic and spectroscopic analysis of five pulsating sdB stars observed with TESS in order to compare the observations with model predictions based on our stellar evolution computations coupled with adiabatic pulsation computations. Methods. We processed and analyzed TESS observations of long-period pulsating hot subdwarf B stars. We used standard pre-whitening techniques on the datasets to extract the pulsation periods from the TESS light curves. We applied standard seismic tools for mode identification, including asymptotic period spacings and rotational frequency multiplets. Based on the values obtained from Kolmogorov-Smirnov and Inverse Variance tests, we searched for a constant period spacing for dipole (l = 1) and quadrupole (l = 2) modes. We calculated the mean period spacing for l = 1 and l = 2 modes and estimated the errors by means of a statistical resampling analysis. For all stars, atmospheric parameters were derived by fitting synthetic spectra to the newly obtained low-resolution spectra. We computed stellar evolution models using the LPCODE stellar evolution code, and computed l = 1 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. Results. We detect 73 frequencies, most of which are identified as dipole and quadrupole g-modes with periods spanning from ∼3000 s to ∼14 500 s. The derived mean period spacing of dipole modes is concentrated in a narrow region ranging from 251 s to 256 s, while the mean period spacing for quadrupole modes spans from 145 s to 154 s. The atmospheric parameters derived from spectroscopic data are typical of long-period pulsating sdB stars with an effective temperature ranging from 23 700 K to 27 600 K and surface gravity spanning from 5.3 dex to 5.5 dex. In agreement with the expectations from theoretical arguments and previous asteroseismological works, 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. We find that models with a standard, modest 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. Conclusions. Using high-quality space-based photometry collected by the TESS mission coupled with low-resolution spectroscopy from the ground, we provide a global comparison of the observations with model predictions by means of a robust indicator such as the mean period spacing. All five objects that we analyze in this work show remarkable homogeneity in both seismic and spectroscopic properties.
  • Miniatura
    Ítem
    Looking into the cradle of the grave: J22564-5910, a young post-merger hot subdwarf?
    (European Southern Observatory (ESO), 2021) Vos, Joris; Pelisoli, Ingrid; Budaj, Jan; Reindl, Nicole; Schaffenroth, Veronika; Bobrick, Alexey; Geier, Stephan; Hermes, Jj; Nemeth, Peter; Østensen, Roy; Reding, Joshua S.; Uzundag, Murat; Vučković, Maja
    Context. We present the discovery of J22564–5910, a new type of hot subdwarf (sdB) which shows evidence of gas present in the system and it has shallow, multi-peaked hydrogen and helium lines which vary in shape over time. All observational evidence points towards J22564–5910 being observed very shortly after the merger phase that formed it. Aims. Using high-resolution, high signal-to-noise spectroscopy, combined with multi-band photometry, Gaia astrometry, and TESS light curves, we aim to interpret these unusual spectral features. Methods. The photometry, spectra, and light curves were all analysed, and their results were combined in order to support our interpretation of the observations: the likely presence of a magnetic field combined with gas features around the sdB. Based on the triple-peaked H lines, the magnetic field strength was estimated and, by using the SHELLSPEC code, qualitative models of gas configurations were fitted to the observations. Results. All observations can either be explained by a magnetic field of ∼650 kG, which enables the formation of a centrifugal magnetosphere, or a non-magnetic hot subdwarf surrounded by a circumstellar gas disc or torus. Both scenarios are not mutually exclusive and both can be explained by a recent merger. Conclusions. J22564–5910 is the first object of its kind. It is a rapidly spinning sdB with gas still present in the system. It is the first post-merger star observed this early after the merger event, and as such it is very valuable system to test merger theories. If the magnetic field can be confirmed, it is not only the first magnetic sdB, but it hosts the strongest magnetic field ever found in a pre-white dwarf object. Thus, it could represent the long sought-after immediate ancestor of strongly magnetic white dwarfs.
  • Miniatura
    Ítem
    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
    (Universidad de Valparaíso, 2022-08) Uzundag, Murat; Vuckovic, Maja (Supervisora)
    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.