ORCID Profile
0000-0003-0842-2374
Current Organisation
KU Leuven
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Publisher: EDP Sciences
Date: 05-2020
DOI: 10.1051/0004-6361/202037452
Abstract: Context. Eclipsing, spectroscopic double-lined binary star systems are excellent laboratories for calibrating theories of stellar interior structure and evolution. Their precise and accurate masses and radii measured from binary dynamics offer model-independent constraints and challenge current theories of stellar evolution. Aims. We aim to investigate the mass discrepancy in binary stars. This is the significant difference between stellar components’ masses measured from binary dynamics and those inferred from models of stellar evolution via positions of the components in the T eff − log g Kiel diagram. We study the effect of near-core mixing on the mass of the convective core of the stars and interpret the results in the context of the mass discrepancy. Methods. We fitted stellar isochrones computed from a grid of MESA stellar evolution models to a homogeneous s le of eleven high-mass binary systems. Two scenarios are considered where in idual stellar components of a binary system are treated independent of each other and where they are forced to have the same age and initial chemical composition. We also study the effect of the microturbulent velocity and turbulent pressure on the atmosphere model structure and stellar spectral lines, and its link with the mass discrepancy. Results. We find that the mass discrepancy is present in our s le and that it is anti-correlated with the surface gravity of the star. No correlations are found with other fundamental and atmospheric parameters, including the stellar mass. The mass discrepancy can be partially accounted for by increasing the amount of near-core mixing in stellar evolution models. We also find that ignoring the microturbulent velocity and turbulent pressure in stellar atmosphere models of hot evolved stars results in the overestimation of their effective temperature by up to 8%. Together with enhanced near-core mixing, this can almost entirely account for the ∼30% mass discrepancy found for the evolved primary component of V380 Cyg. Conclusions. We find a strong link between the mass discrepancy and the convective core mass. The mass discrepancy can be solved by considering the combined effect of extra near-core boundary mixing and the consistent treatment in the spectrum analysis of hot evolved stars. Our binary modelling results in convective core masses between 17 and 35% of the stellar mass, which is in excellent agreement with the results from gravity-mode asteroseismology of single stars. This implies larger helium core masses near the end of the main sequence than have been anticipated so far.
Publisher: EDP Sciences
Date: 06-2021
DOI: 10.1051/0004-6361/202140466
Abstract: Context. Asteroseismic modelling of the internal structure of main-sequence stars born with a convective core has so far been based on homogeneous analyses of space photometric Kepler light curves of four years in duration, to which most often incomplete inhomogeneously-deduced spectroscopic information was added to break degeneracies. Aims. Our goal is twofold: (1) to compose an optimal s le of gravity-mode pulsators observed by the Kepler space telescope for joint asteroseismic and spectroscopic stellar modelling, and (2) to provide spectroscopic parameters for its members, deduced in a homogeneous way. Methods. We assembled HERMES high-resolution optical spectroscopy at the 1.2 m Mercator telescope for 111 dwarfs, whose Kepler light curves allowed for the determination of their near-core rotation rates. Our spectroscopic information offers additional observational input to also model the envelope layers of these non-radially pulsating dwarfs. Results. We determined stellar parameters and surface abundances from atmospheric analysis with spectrum normalisation based on a new machine-learning tool. Our results suggest a systematic overestimation of metallicity ([M/H]) in the literature for the studied F-type dwarfs, presumably due to normalisation limitations caused by the dense line spectrum of these rotating stars. CNO surface abundances were found to be uncorrelated with the rotation properties of the F-type stars. For the B-type stars, we find a hint of deep mixing from C and O abundance ratios N abundance uncertainties are too great to reveal a correlation of N with the rotation of the stars. Conclusions. Our spectroscopic stellar parameters and abundance determinations allow for the future joint spectroscopic, astrometric ( Gaia ), and asteroseismic modelling of this legacy s le of gravity-mode pulsators, with the aim of improving our understanding of transport processes in the core-hydrogen burning phase of stellar evolution.
Publisher: American Astronomical Society
Date: 08-02-2019
Publisher: EDP Sciences
Date: 30-10-2023
Publisher: EDP Sciences
Date: 08-2023
DOI: 10.1051/0004-6361/202346108
Abstract: Context. Modern stellar structure and evolution theory suffers from a lack of observational calibration for the interior physics of intermediate- and high-mass stars. This leads to discrepancies between theoretical predictions and observed phenomena that are mostly related to angular momentum and element transport. Analyses of large s les of massive stars connecting state-of-the-art spectroscopy to asteroseismology may provide clues as to how to improve our understanding of their interior structure. Aims. We aim to deliver a s le of O- and B-type stars at metallicity regimes of the Milky Way and the Large Magellanic Cloud (LMC) galaxies with accurate atmospheric parameters from high-resolution spectroscopy, along with a detailed investigation of line-profile broadening, both for the benefit of future asteroseismic studies. Methods. After describing the general aims of our two Large Programs, we develop a dedicated methodology to fit spectral lines and deduce accurate global stellar parameters from high-resolution multi-epoch UVES and FEROS spectroscopy. We use the best available atmosphere models for three regimes covered by our global s le, given its breadth in terms of mass, effective temperature, and evolutionary stage. Results. Aside from accurate atmospheric parameters and locations in the Hertzsprung-Russell diagram, we deliver detailed analyses of macroturbulent line broadening, including estimations of the radial and tangential components. We find that these two components are difficult to disentangle from spectra with signal-to-noise ratios of below 250. Conclusions. Future asteroseismic modelling of the deep interior physics of the most promising stars in our s le will provide much needed information regarding OB stars, including those of low metallicity in the LMC.
Publisher: EDP Sciences
Date: 02-2023
DOI: 10.1051/0004-6361/202244808
Abstract: Context. Unambiguous ex les of the influence of tides on self-excited, free stellar pulsations have recently been observationally detected in space-based photometric data. Aims. We aim to investigate U Gru and contextualise it within the growing class of tidally influenced pulsators. An initial analysis of U Gru revealed frequencies spaced by the orbital frequency that are difficult to explain by currently proposed tidal mechanisms. Methods. We reinvestigate the TESS photometry for U Gru alongside new UVES spectroscopy. We analyse the UVES spectroscopy with least-squares deconvolution and spectral disentangling techniques, and perform an atmospheric analysis. We remove the binary signature from the light curve using an effective model in order to investigate the pulsation signal in the residuals. We track the litudes and phases of the residual pulsations as a function of the orbital period to reveal their tidal influence. Results. We establish that U Gru is likely a hierarchical triple system. We identify a single p -mode oscillation that exhibits litude and phase variation over the binary orbit. We propose a toy model to demonstrate that the series of frequencies separated by the orbital frequency can be reproduced by eclipse mapping. We find no evidence of modulation to the other independent oscillation modes. Conclusions. We demonstrate that U Gru hosts at least one tidally perturbed pulsation. Additionally, we argue that eclipse mapping of the dominant, tidally perturbed mode can produce the series of frequencies separated by the observed orbital frequency. Our simulations show that the effects of eclipse mapping are mode dependent, and are not expected to produce an observable signature for all pulsation modes in an eclipsing binary.
Publisher: EDP Sciences
Date: 09-2022
DOI: 10.1051/0004-6361/202243839
Abstract: Context . Spectroscopic data are necessary to break degeneracies in the asteroseismic modelling of the interior structure in high- and intermediate-mass stars. With the TESS mission, the number of bright intermediate-mass B-type stars with long photometric light curves that are suitable for detailed asteroseismic studies has increased substantially compared to the pre-TESS era. Aims . We derive precise photospheric stellar parameters for a s le of 166 B-type stars with TESS light curves through a homogeneous spectroscopic analysis. The variability types of these s le stars are also classified based on all currently available TESS sectors, and they are ultimately prioritised according to their astrophysical potential. Methods . We obtained high-resolution spectra for all 166 targets with the FEROS spectrograph in the context of a large program. The spectra were reduced with the CERES pipeline, which we adapted to improve the quality of the reduced spectra. These spectra were subsequently analysed with ZETA-P AYNE , a machine-learning-based spectrum analysis algorithm, to infer precise stellar labels for all stars in the s le. Furthermore, the least-squares deconvolution (LSD) method was employed to investigate spectral line profile variability (LPV) and isolate binary systems from presumably single stars. Results . The LSD profile analysis identified 26 spectroscopic double-lined binaries the remainder of the s le are 42 supergiants in the Large Magellanic Cloud galaxy and 98 Galactic stars, both with and without apparent LPV. For the Galactic single stars and single-lined spectroscopic binaries, we determine their five main surface parameters: effective temperature ( T eff ), surface gravity (log g ), global metallicity ([M/H]), projected rotational velocity ( v sin i ), and microturbulent velocity ( ξ ) with average formal precisions of 70 K, 0.03 dex, 0.07 dex, 8 km s −1 , and 0.7 km s −1 , respectively. The average internal uncertainties we find for FEROS spectra with our spectrum analysis method are 430 K( T eff ), 0.12 dex (log g ), 0.13 dex ([M/H]), 12kms −1 ( v sin i ), and 2 kms −1 ( ξ ). Conclusions . We find spectroscopic evidence that 8 of the 98 galactic single or SB1 variables are fast-rotating gravity-mode pulsators occurring in between the slowly pulsating B (SPB) stars and δ Scuti instability strips. The g -mode frequencies of these pulsators are shifted to relatively high frequency values due to their rotation, and their apparently too low T eff relative to the SPB instability region can in most cases be explained by the gravity darkening effect. We also discover 13 new HgMn stars in the Galactic s le of which only one is found in a spectroscopic binary, resulting in a biased and therefore unreliable low binary rate of only 8%.
Publisher: American Astronomical Society
Date: 26-05-2020
Publisher: American Astronomical Society
Date: 21-10-2021
Abstract: The NASA Transiting Exoplanet Survey Satellite (TESS) is observing tens of millions of stars with time spans ranging from ∼27 days to about 1 yr of continuous observations. This vast amount of data contains a wealth of information for variability, exoplanet, and stellar astrophysics studies but requires a number of processing steps before it can be fully utilized. In order to efficiently process all the TESS data and make it available to the wider scientific community, the TESS Data for Asteroseismology working group, as part of the TESS Asteroseismic Science Consortium, has created an automated open-source processing pipeline to produce light curves corrected for systematics from the short- and long-cadence raw photometry data and to classify these according to stellar variability type. We will process all stars down to a TESS magnitude of 15. This paper is the next in a series detailing how the pipeline works. Here, we present our methodology for the automatic variability classification of TESS photometry using an ensemble of supervised learners that are combined into a metaclassifier. We successfully validate our method using a carefully constructed labeled s le of Kepler Q9 light curves with a 27.4 days time span mimicking single-sector TESS observations, on which we obtain an overall accuracy of 94.9%. We demonstrate that our methodology can successfully classify stars outside of our labeled s le by applying it to all ∼167,000 stars observed in Q9 of the Kepler space mission.
No related grants have been discovered for Andrew Tkachenko.