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Accurate ages of stars
EC | FP7 | SP2 | ERC
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  • Measuring amplitudes of harmonics and combination frequencies in variable stars

    Bellinger, Earl P.; Wysocki, Daniel; Kanbur, Shashi M. (2015)
    Projects: EC | STELLARAGES (338251)
    Discoveries of RR Lyrae and Cepheid variable stars with multiple modes of pulsation have increased tremendously in recent years. The Fourier spectra of these stars can be quite complicated due to the large number of combination frequencies that can exist between their modes. As a result, light-curve fits to these stars often suffer from undesirable ringing effects that arise from noisy observations and poor phase coverage. These non-physical overfitting artifacts also occur when fitting the h...

    Surface effects on the red giant branch

    Ball, W. H.; Themeßl, N.; Hekker, S. (2018)
    Projects: EC | STELLARAGES (338251)
    Individual mode frequencies have been detected in thousands of individual solar-like oscillators on the red giant branch (RGB). Fitting stellar models to these mode frequencies, however, is more difficult than in main-sequence stars. This is partly because of the uncertain magnitude of the surface effect: the systematic difference between observed and modelled frequencies caused by poor modelling of the near-surface layers. We aim to study the magnitude of the surface effect in RGB stars. Sur...

    Automated determination of g-mode period spacing of red-giant stars

    Datta, Abhisek; Mazumdar, Anwesh; Gupta, Umang; Hekker, Saskia (2014)
    Projects: EC | STELLARAGES (338251)
    The Kepler satellite has provided photometric timeseries data of unprecedented length, duty cycle and precision. To fully analyse these data for the tens of thousands of stars observed by Kepler, automated methods are a prerequisite. Here we present an automated procedure to determine the period spacing of gravity modes in red-giant stars ascending the red-giant branch. The gravity modes reside in a cavity in the deep interior of the stars and provide information on the conditions in the stel...

    Models of red giants in the CoRoT asteroseismology fields combining asteroseismic and spectroscopic constraints

    Lagarde, N.; Miglio, A.; Eggenberger, P.; Morel, T.; Montalbán, J.; Mosser, B.; Rodrigues, T. S.; Girardi, L.; Rainer, M.; Poretti, E.; Barban, C.; Hekker, S.; Kallinger, T.; Valentini, M.; Carrier, F.;... (2015)
    Projects: EC | STELLARAGES (338251)
    Context. The availability of asteroseismic constraints for a large sample of red giant stars from the CoRoT and Kepler missions paves the way for various statistical studies of the seismic properties of stellar populations. Aims: We use a detailed spectroscopic study of 19 CoRoT red giant stars to compare theoretical stellar evolution models to observations of the open cluster NGC 6633 and field stars. Methods: In order to explore the effects of rotation-induced mixing and thermohaline inst...

    A search for binary candidates among the fundamental mode RR Lyrae stars observed by Kepler

    Guggenberger, Elisabeth; Steixner, Jakob (2014)
    Projects: EC | STELLARAGES (338251)
    Although roughly half of all stars are considered to be part of binary or multiple systems, there are only two confirmed cases of RR Lyrae pulsators with companions. One of them is TU Uma (Wade et al 1999) - a classical RR Lyrae star in a very eccentric orbit - and the other is OGLE-BLG-RRLYR-02792 (Pietrzynski et al 2012). Considering the wealth of well-studied RR Lyrae stars, this number is astoundingly low. Having more RR Lyrae stars in binary systems at hand would be extremely valuable to...

    Asteroseismology of red giants & galactic archaeology

    Hekker, Saskia (2017)
    Projects: EC | STELLARAGES (338251)
    Red-giant stars are low- to intermediate-mass ($M \lesssim 10$~M$_{\odot}$) stars that have exhausted hydrogen in the core. These extended, cool and hence red stars are key targets for stellar evolution studies as well as galactic studies for several reasons: a) many stars go through a red-giant phase; b) red giants are intrinsically bright; c) large stellar internal structure changes as well as changes in surface chemical abundances take place over relatively short time; d) red-giant stars e...

    Grid-based seismic modelling at high and low signal-to-noise ratios HD 181420 and HD 175272

    Hekker, S.; Ball, W. H. (2014)
    Projects: EC | STELLARAGES (338251)
    Context. Recently, the CoRoT target HD 175272 (F5V), which shows a weak signal of solar-like oscillations, was modelled by a differential asteroseismic analysis (Ozel et al. 2013, A&A, 558, A79) relative to a seismically similar star, HD 181420 (F2V), for which there is a clear signature of solar-like oscillations. The results provided by Ozel et al. indicate the possibility of HD 175272 having subsolar mass, while being of the order of 1000 K hotter than the Sun. This seems unphysical – stan...


    Wu, T.; Li, Y.; Hekker, S. (2014)
    Projects: EC | STELLARAGES (338251)
    Stellar mass $M$, radius $R$, and gravity $g$ are important basic parameters in stellar physics. Accurate values for these parameters can be obtained from the gravitational interaction between stars in multiple systems or from asteroseismology. Stars in a cluster are thought to be formed coevally from the same interstellar cloud of gas and dust. The cluster members are therefore expected to have some properties in common. These common properties strengthen our ability to constrain stellar mod...

    The discovery of a planetary candidate around the evolved low-mass Kepler giant star HD 175370

    Hrudková, M.; Hatzes, A.; Karjalainen, R.; Lehmann, H.; Hekker, S.; Hartmann, M.; Tkachenko, A.; Prins, S.; van Winckel, H.; de Nutte, R.; Dumortier, L.; Frémat, Y.; Hensberge, H.; Jorissen, A.; Lampens, P.;... (2016)
    Projects: EC | STELLARAGES (338251)
    We report on the discovery of a planetary companion candidate with a minimum mass M sin i = 4.6 ± 1.0 M[SUB]Jupiter[/SUB] orbiting the K2 III giant star HD 175370 (KIC 007940959). This star was a target in our programme to search for planets around a sample of 95 giant stars observed with Kepler. This detection was made possible using precise stellar radial velocity measurements of HD 175370 taken over five years and four months using the coudé echelle spectrograph of the 2-m Alfred Jensch Te...

    Young [$\alpha$/Fe]-enhanced stars discovered by CoRoT and APOGEE: What is their origin?

    Chiappini, C.; Anders, F.; Rodrigues, T. S.; Miglio, A.; Montalban, J.; Mosser, B.; Girardi, L.; Valentini, M.; Noels, A.; Morel, T.; Minchev, I.; Steinmetz, M.; Santiago, B. X.; Schultheis, M.; Martig, M.;... (2015)
    Projects: EC | STELLARAGES (338251)
    We report the discovery of a group of apparently young CoRoT red-giant stars exhibiting enhanced [α/Fe] abundance ratios (as determined from APOGEE spectra) with respect to solar values. Their existence is not explained bystandard chemical evolution models of the Milky Way, and shows that the chemical-enrichment history of the Galactic disc is more complex. We find similar stars in previously published samples for which isochrone-ages could be reliably obtained, although in smaller relative n...
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