A. Tkachenko - Academia.edu (original) (raw)

Papers by A. Tkachenko

Astronomische Nachrichten, 2012

We used high-quality Kepler photometry and recent spectroscopic data to investigate the Kepler ta... more We used high-quality Kepler photometry and recent spectroscopic data to investigate the Kepler target KIC 005988140 considering three possible scenario's: (1) binarity, (2) co-existence of δ Scuti-and γ Doradus-type oscillations, and (3) rotational modulation with spots over the stellar surface. Using spectrum synthesis, we derived improved atmospheric parameters from high-resolution, high signal-to-noise spectra. The star is of spectral type A7.5 IV-III with a metallicity slightly lower than that of the Sun. The Fourier analyses of both the photometric and the spectroscopic data reveal two dominant frequencies -with an integer ratio equal to two -in the low frequency regime. Nine more frequencies located in the typical δ Scuti range were also detected. The mean light and radial velocity curves which are not exactly in antiphase, show a similar double-wave shape. We therefore suggest on a preliminary basis that the physical cause of the observed light and radial velocity variations is the simultaneous occurrence of (δ Sct-type) pressure modes and stellar rotation coupled to inhomogeneities on the surface of this late A-type star.

Astronomy & Astrophysics, 2014

More than 40 years of ground-based photometric observations of the δ Sct star 4 CVn revealed 18 i... more More than 40 years of ground-based photometric observations of the δ Sct star 4 CVn revealed 18 independent oscillation frequencies, including radial as well as non-radial p-modes of low spherical degree ≤ 2. From 2008 to 2011, more than 2000 spectra were obtained at the 2.1-m Otto-Struve telescope at the McDonald Observatory. We present the analysis of the line-profile variations, based on the Fourier-parameter fit method, detected in the absorption lines of 4 CVn, which carry clear signatures of the pulsations. From a non-sinusoidal, periodic variation of the radial velocities, we discovered that 4 CVn is an eccentric binary system, with an orbital period P orb = 124.44 ± 0.03 d and an eccentricity e = 0.311 ± 0.003. We firmly detect 20 oscillation frequencies, 9 of which are previously unseen in photometric data, and attempt mode identification for the two dominant modes, f 1 = 7.3764 d −1 and f 2 = 5.8496 d −1 , and determine the prograde or retrograde nature of 7 of the modes. The projected rotational velocity of the star, v eq sin i 106.7 km s −1 , translates to a rotation rate of v eq /v crit ≥ 33%. This relatively high rotation rate hampers unique mode identification, since higher-order effects of rotation are not included in the current methodology. We conclude that, in order to achieve unambiguous mode identification for 4 CVn, a complete description of rotation and the use of blended lines have to be included in mode-identification techniques.

Astronomy & Astrophysics, 2014

Context. Massive stars are important for the chemical enrichment of the universe. Since internal ... more Context. Massive stars are important for the chemical enrichment of the universe. Since internal mixing processes influence their life, it is of high importance to place constraints on the corresponding physical parameters, such as core overshooting and the internal rotation profile, to calibrate their stellar structure and evolution models. Although asteroseismology was shown to be able to deliver the most precise constraints so far, the number of detailed seismic studies delivering quantitative results is limited. Aims. Our goal is to extend this limited sample with an in-depth case study and provide a well constrained set of asteroseismic parameters, contributing to the ongoing mapping efforts of the instability strips of the β Cep and slowly pulsating B (SPB) stars. Methods. We derived fundamental parameters from high-resolution spectra using spectral synthesis techniques. We used custom masks to obtain optimal light curves from the original pixel level data from the Kepler satellite. We used standard time-series analysis tools to construct a set of significant pulsation modes which provide the basis for the seismic analysis carried out afterwards. Results. We find that KIC 10526294 is a cool SPB star, one of the slowest rotators ever found. Despite this fact, the length of Kepler observations is sufficient to resolve narrow rotationally split multiplets for each of its nineteen quasi-equally spaced dipole modes. The number of detected consecutive (in radial order) dipole modes in this series is higher than ever before. The observed amount of splitting shows an increasing trend towards longer periods, which -largely independent of the seismically calibrated stellar modelspoints towards a non-rigid internal rotation profile. From the average splitting we deduce a rotation period of ∼ 188 days. From seismic modelling we find that the star is young with a central hydrogen mass fraction X c > 0.64; it has a core overshooting α ov ≤ 0.15.

ABSTRACT Fourier parameters (frequencies (fj), amplitudes (Aj& (thetaj)) of the selected ... more ABSTRACT Fourier parameters (frequencies (fj), amplitudes (Aj& (thetaj)) of the selected peaks having a signal-to-noise ratio (S/N) above 4 for KIC 4931738 and KIC 6352430 when computed in a 1d-1 window after prewhitening. The displayed S/N values are calculated in a window of 1d-1 or 3d-1 centred on the given frequency, or from the full periodogram (from 0d-1 to 24.47d-1). A complete list of all significant frequencies is available upon request. (2 data files).

PLATO 2.0 has recently been selected for ESA's M3 launch opportunity (2022/24). Providing accurat... more PLATO 2.0 has recently been selected for ESA's M3 launch opportunity (2022/24). Providing accurate key planet parameters (radius, mass, density and age) in statistical numbers, it addresses fundamental questions such as: How do planetary systems form and evolve? Are there other systems with planets like ours, including potentially habitable planets? The PLATO 2.0 instrument consists of 34 small aperture telescopes (32 with 25 s readout cadence and 2 with 2.5 s cadence) providing a wide field-of-view (2232 deg 2 ) and a large photometric magnitude range (4-16 mag). It focuses on bright (4-11 mag) stars in wide fields to detect and characterize planets down to Earth-size by photometric transits, whose masses can then be determined by ground-based radial-velocity follow-up measurements. Asteroseismology will be performed for these bright stars to obtain highly accurate stellar parameters, including masses and ages. The combination of bright targets and asteroseismology results in high accuracy for the bulk planet parameters: 2 %, 4-10 % and 10 % for planet radii, masses and ages, respectively. The planned baseline observing strategy includes two long pointings (2-3 years) to detect and bulk characterize planets reaching into the habitable zone (HZ) of solar-like stars and an additional step-and-stare phase to cover in total about 50 % of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect and characterize hundreds of small planets, and thousands of planets in the Neptune to gas giant regime out to the HZ. It will therefore provide the first large-scale catalogue of bulk characterized planets with accurate radii, masses, mean densities and ages. This catalogue will include terrestrial planets at intermediate orbital distances, where surface temperatures are moderate. Coverage of this parameter range with statistical numbers of bulk characterized planets is unique to PLATO 2.0. The PLATO 2.0 catalogue allows us to e.g.:complete our knowledge of planet diversity for low-mass objects, -correlate the planet mean density-orbital distance distribution with predictions from planet formation theories,-constrain the influence of planet migration and scattering on the architecture of multiple systems, and -specify how planet and system parameters change with host star characteristics, such as type, metallicity and age. The catalogue will allow us to study planets and planetary systems at different evolutionary phases. It will further provide a census for small, low-mass planets. This will serve to identify objects which retained their primordial hydrogen atmosphere and in general the typical characteristics of planets in such a low-mass, low-density range. Planets detected by PLATO 2.0 will orbit bright stars and many of them will be targets for future atmosphere spectroscopy exploring their atmospheres. Furthermore, the mission has the potential to detect exomoons, planetary rings, binary and Trojan planets. The planetary science possible with PLATO 2.0 is complemented by its impact on stellar and galactic science via asteroseismology as well as light curves of all kinds of variable stars, together with observations of stellar clusters of different ages. This will allow us to improve stellar models and study stellar activity. A large number of well-known ages from red giant stars will probe the structure and evolution of our Galaxy. Asteroseismic ages of bright stars for different phases of stellar evolution allow calibrating stellar age-rotation relationships. Together with the results of ESA's Gaia mission, the results of PLATO 2.0 will provide a huge legacy to planetary, stellar and galactic science. Exp Astron (2014) 38:249-330 251

Monthly Notices of the Royal Astronomical Society: Letters, 2010

The system AR Aur is a young late B-type double-lined eclipsing binary with a primary star of HgM... more The system AR Aur is a young late B-type double-lined eclipsing binary with a primary star of HgMn peculiarity. We applied the Doppler imaging method to reconstruct the distribution of Fe and Y over the surface of the primary using spectroscopic time series obtained in 2005 and from 2008 October to 2009 February. The results show a remarkable evolution of the element distribution and overabundances. Measurements of the magnetic field with the moment technique using several elements reveal the presence of a longitudinal magnetic field of the order of a few hundred gauss in both stellar components and a quadratic field of the order of 8 kG on the surface of the primary star.

Monthly Notices of the Royal Astronomical Society: Letters, 2012

We report the discovery of low-amplitude gravity-mode oscillations in the massive binary star V38... more We report the discovery of low-amplitude gravity-mode oscillations in the massive binary star V380 Cyg, from 180 d of Kepler custom-aperture space photometry and 5 months of high-resolution high signal-to-noise spectroscopy. The new data are of unprecedented quality and allowed to improve the orbital and fundamental parameters for this binary. The orbital solution was subtracted from the photometric data and led to the detection of periodic intrinsic variability with frequencies of which some are multiples of the orbital frequency and others are not. Spectral disentangling allowed the detection of line-profile variability in the primary. With our discovery of intrinsic variability interpreted as gravity mode oscillations, V380 Cyg becomes an important laboratory for future seismic tuning of the near-core physics in massive B-type stars.

Monthly Notices of the Royal Astronomical Society, 2014

Science (New York, N.Y.), Jan 8, 2011

Hierarchical triple systems comprise a close binary and a more distant component. They are import... more Hierarchical triple systems comprise a close binary and a more distant component. They are important for testing theories of star formation and of stellar evolution in the presence of nearby companions. We obtained 218 days of Kepler photometry of HD 181068 (magnitude of 7.1), supplemented by ground-based spectroscopy and interferometry, which show it to be a hierarchical triple with two types of mutual eclipses. The primary is a red giant that is in a 45-day orbit with a pair of red dwarfs in a close 0.9-day orbit. The red giant shows evidence for tidally induced oscillations that are driven by the orbital motion of the close pair. HD 181068 is an ideal target for studies of dynamical evolution and testing tidal friction theories in hierarchical triple systems.

Astronomy & Astrophysics, 2014

Context. The unparalleled photometric data obtained by NASA's Kepler space telescope led to an im... more Context. The unparalleled photometric data obtained by NASA's Kepler space telescope led to an improved understanding of red giant stars and binary stars. Seismology allows us to constrain the properties of red giants. In addition to eclipsing binaries, eccentric non-eclipsing binaries, exhibiting ellipsoidal modulations, have been detected with Kepler. Aims. We aim to study the properties of eccentric binary systems containing a red giant star and derive the parameters of the primary giant component. Methods. We apply asteroseismic techniques to determine masses and radii of the primary component of each system. For a selected target, light and radial velocity curve modelling techniques are applied to extract the parameters of the system and its primary component. Stellar evolution and its effects on the evolution of the binary system are studied from theoretical models. Results. The paper presents the asteroseismic analysis of 18 pulsating red giants in eccentric binary systems, for which masses and radii were constrained. The orbital periods of these systems range from 20 to 440 days. The results of our ongoing radial velocity monitoring program with the HERMES spectrograph reveal an eccentricity range of e = 0.2 to 0.76. As a case study we present a detailed analysis of KIC 5006817, whose rich oscillation spectrum allows for a detailed seismic analysis. From seismology we constrain the rotational period of the envelope to be at least 165 d, which is roughly twice the orbital period. The stellar core rotates 13 times faster than the surface. From the spectrum and radial velocities we expect that the Doppler beaming signal should have a maximum amplitude of 300 ppm in the light curve. Fixing the mass and radius to the asteroseismically determined values, from our binary modelling we find a value of the gravity darkening exponent that is significantly larger than expected. Through binary modelling, we determine the mass of the secondary component to be 0.29±0.03 M ⊙ . Conclusions. For KIC 5006817 we exclude pseudo-synchronous rotation of the red giant with the orbit. The comparison of the results from seismology and modelling of the light curve shows a possible alignment of the rotational and orbital axis at the 2σ level. Red giant eccentric systems could be progenitors of cataclysmic variables and hot subdwarf B stars.

Astronomy & Astrophysics, 2013

Aims. We present the first binary modelling results for the pulsating eclipsing binary KIC 112856... more Aims. We present the first binary modelling results for the pulsating eclipsing binary KIC 11285625, discovered by the Kepler mission. An automated method to disentangle the pulsation spectrum and the orbital variability in high quality light curves, was developed and applied. The goal was to obtain accurate orbital and component properties, in combination with essential information derived from spectroscopy. Methods. A binary model for KIC 11285625 was obtained, using a combined analysis of high-quality space-based Kepler light curves and ground-based high-resolution HERMES echelle spectra. The binary model was used to separate the pulsation characteristics from the orbital variability in the Kepler light curve in an iterative way. We used an automated procedure to perform this task, based on the JKTEBOP binary modelling code, and adapted codes for frequency analysis and prewhitening of periodic signals. Using a disentangling technique applied to the composite HERMES spectra, we obtained a higher signal-to-noise mean component spectrum for both the primary and the secondary. A model grid search method for fitting synthetic spectra was used for fundamental parameter determination for both components. Results. Accurate orbital and component properties of KIC 11285625 were derived, and we have obtained the pulsation spectrum of the γ Dor pulsator in the system. Detailed analysis of the pulsation spectrum revealed amplitude modulation on a time scale of a hundred days, and strong indications of frequency splittings at both the orbital frequency, and the rotational frequency derived from spectroscopy.

Astronomy & Astrophysics, 2012

Context. The predicted orbital period histogram of a subdwarf B (sdB) population is bimodal with ... more Context. The predicted orbital period histogram of a subdwarf B (sdB) population is bimodal with a peak at short ( < 10 days) and long ( > 250 days) periods. Observationally, however, there are many short-period sdB systems known, but only very few long-period sdB binaries are identified. As these predictions are based on poorly understood binary interaction processes, it is of prime importance to confront the predictions to well constrained observational data. We therefore initiated a monitoring program to find and characterize long-period sdB stars. Aims. In this contribution we aim to determine the absolute dimensions of the long-period binary system PG 1104+243 consisting of an sdB and a main-sequence (MS) component, and determine its evolution history. Methods. High-resolution spectroscopy time-series were obtained with HERMES at the Mercator telescope at La Palma, and analyzed to determine the radial velocities of both the sdB and MS components. Photometry from the literature was used to construct the spectral energy distribution (SED) of the binary. Atmosphere models were used to fit this SED and determine the surface gravity and temperature of both components. The gravitational redshift provided an independent confirmation of the surface gravity of the sdB component.

Astronomy & Astrophysics, 2013

Context. The NASA exoplanet search mission Kepler is currently providing a wealth of light curves... more Context. The NASA exoplanet search mission Kepler is currently providing a wealth of light curves of ultra-high quality from space. Aims. We used high-quality Kepler photometry and spectroscopic data to investigate the Kepler target and binary candidate KIC 5988140. We aim to interpret the observed variations of KIC 5988140 considering three possible scenarios: binarity, co-existence of δ Sct-and γ Dor-type oscillations, and rotational modulation caused by an asymmetric surface intensity distribution. Methods. We used the spectrum synthesis method to derive the fundamental parameters T eff , log g, [M/H], and v sin i from the newly obtained high-resolution, high S/N spectra. Frequency analyses of both the photometric and the spectroscopic data were performed. Results. The star has a spectral type of A7.5 IV-III and a metallicity slightly lower than that of the Sun. Both Fourier analyses reveal the same two dominant frequencies F 1 =2F 2 =0.688 and F 2 =0.344 d −1 . We also detected in the photometry the signal of nine more, significant frequencies located in the typical range of δ Sct pulsation. The light and radial velocity curves follow a similar, stable double-wave pattern which are not exactly in anti-phase but show a relative phase shift of about 0.1 period between the moment of minimum velocity and that of maximum light. Conclusions. Such findings are incompatible with the star being a binary system. We next show that, for all possible (limit) configurations of a spotted surface, the predicted light-to-velocity amplitude ratio is almost two orders larger than the observed value, which pleads against rotational modulation. The same argument also invalidates the explanation in terms of pulsations of type γ Dor (i.e. hybrid pulsations). We confirm the occurrence of various independent δ Sct-type pressure modes in the Kepler light curve. With respect to the low-frequency content, however, we argue that the physical cause of the remaining light and radial velocity variations of this late A-type star remains unexplained by any of the presently considered scenarios.

Astronomy & Astrophysics, 2013

Context. OB stars are important in the chemistry and evolution of the Universe, but the sample of... more Context. OB stars are important in the chemistry and evolution of the Universe, but the sample of targets well understood from an asteroseismological point of view is still too limited to provide feedback on the current evolutionary models. Our study extends this sample with two spectroscopic binary systems. Aims. Our goal is to provide orbital solutions, fundamental parameters and abundances from disentangled high-resolution high signalto-noise spectra, as well as to analyse and interpret the variations in the Kepler light curve of these carefully selected targets. This way we continue our efforts to map the instability strips of β Cep and slowly pulsating B stars using the combination of high-resolution ground-based spectroscopy and uninterrupted space-based photometry. Methods. We fit Keplerian orbits to radial velocities measured from selected absorption lines of high-resolution spectroscopy using synthetic composite spectra to obtain orbital solutions. We use revised masks to obtain optimal light curves from the original pixeldata from the Kepler satellite, which provided better long term stability compared to the pipeline processed light curves. We use various time-series analysis tools to explore and describe the nature of variations present in the light curve. Results. We find two eccentric double-lined spectroscopic binary systems containing a total of three main sequence B-type stars (and one F-type component) of which at least one in each system exhibits light variations. The light curve analysis (combined with spectroscopy) of the system of two B stars points towards the presence of tidally excited g modes in the primary component. We interpret the variations seen in the second system as classical g mode pulsations driven by the κ mechanism in the B type primary, and explain the unexpected power in the p mode region as a result of nonlinear resonant mode excitation.

Space Telescopes and Instrumentation 2008: Ultraviolet to Gamma Ray, 2008

Spatially resolved X-ray spectroscopy with high spectral resolution allows the study of astrophys... more Spatially resolved X-ray spectroscopy with high spectral resolution allows the study of astrophysical processes in extended sources with unprecedented sensitivity. This includes the measurement of abundances, temperatures, densities, ionisation stages as well as turbulence and velocity structures in these sources. An X-ray calorimeter is planned for the Russian mission Spektr Röntgen-Gamma (SRG), to be launched in 2011. During the first half year (pointed phase) it will study the dynamics and composition of of the hot gas in massive clusters of galaxies and in supernova remnants (SNR). During the survey phase it will produce the first all sky maps of line-rich spectra of the interstellar medium (ISM). Spectral analysis will be feasible for typically every 5° x 5° region on the sky. Considering the very short time-scale for the development of this instrument it consists of a combination of well developed systems. For the optics an extra eROSITA mirror, also part of the Spektr-RG payload, will be used. The detector will be based on spare parts of the detector flown on Suzaku combined with a rebuild of the electronics and the cooler will be based on the design for the Japanese mission NeXT. In this paper we will present the science and give an overview of the instrument.

Optics for EUV, X-Ray, and Gamma-Ray Astronomy VI, 2013

Astronomische Nachrichten, 2012

We used high-quality Kepler photometry and recent spectroscopic data to investigate the Kepler ta... more We used high-quality Kepler photometry and recent spectroscopic data to investigate the Kepler target KIC 005988140 considering three possible scenario's: (1) binarity, (2) co-existence of δ Scuti-and γ Doradus-type oscillations, and (3) rotational modulation with spots over the stellar surface. Using spectrum synthesis, we derived improved atmospheric parameters from high-resolution, high signal-to-noise spectra. The star is of spectral type A7.5 IV-III with a metallicity slightly lower than that of the Sun. The Fourier analyses of both the photometric and the spectroscopic data reveal two dominant frequencies -with an integer ratio equal to two -in the low frequency regime. Nine more frequencies located in the typical δ Scuti range were also detected. The mean light and radial velocity curves which are not exactly in antiphase, show a similar double-wave shape. We therefore suggest on a preliminary basis that the physical cause of the observed light and radial velocity variations is the simultaneous occurrence of (δ Sct-type) pressure modes and stellar rotation coupled to inhomogeneities on the surface of this late A-type star.

Astronomy & Astrophysics, 2014

More than 40 years of ground-based photometric observations of the δ Sct star 4 CVn revealed 18 i... more More than 40 years of ground-based photometric observations of the δ Sct star 4 CVn revealed 18 independent oscillation frequencies, including radial as well as non-radial p-modes of low spherical degree ≤ 2. From 2008 to 2011, more than 2000 spectra were obtained at the 2.1-m Otto-Struve telescope at the McDonald Observatory. We present the analysis of the line-profile variations, based on the Fourier-parameter fit method, detected in the absorption lines of 4 CVn, which carry clear signatures of the pulsations. From a non-sinusoidal, periodic variation of the radial velocities, we discovered that 4 CVn is an eccentric binary system, with an orbital period P orb = 124.44 ± 0.03 d and an eccentricity e = 0.311 ± 0.003. We firmly detect 20 oscillation frequencies, 9 of which are previously unseen in photometric data, and attempt mode identification for the two dominant modes, f 1 = 7.3764 d −1 and f 2 = 5.8496 d −1 , and determine the prograde or retrograde nature of 7 of the modes. The projected rotational velocity of the star, v eq sin i 106.7 km s −1 , translates to a rotation rate of v eq /v crit ≥ 33%. This relatively high rotation rate hampers unique mode identification, since higher-order effects of rotation are not included in the current methodology. We conclude that, in order to achieve unambiguous mode identification for 4 CVn, a complete description of rotation and the use of blended lines have to be included in mode-identification techniques.

Astronomy & Astrophysics, 2014

Context. Massive stars are important for the chemical enrichment of the universe. Since internal ... more Context. Massive stars are important for the chemical enrichment of the universe. Since internal mixing processes influence their life, it is of high importance to place constraints on the corresponding physical parameters, such as core overshooting and the internal rotation profile, to calibrate their stellar structure and evolution models. Although asteroseismology was shown to be able to deliver the most precise constraints so far, the number of detailed seismic studies delivering quantitative results is limited. Aims. Our goal is to extend this limited sample with an in-depth case study and provide a well constrained set of asteroseismic parameters, contributing to the ongoing mapping efforts of the instability strips of the β Cep and slowly pulsating B (SPB) stars. Methods. We derived fundamental parameters from high-resolution spectra using spectral synthesis techniques. We used custom masks to obtain optimal light curves from the original pixel level data from the Kepler satellite. We used standard time-series analysis tools to construct a set of significant pulsation modes which provide the basis for the seismic analysis carried out afterwards. Results. We find that KIC 10526294 is a cool SPB star, one of the slowest rotators ever found. Despite this fact, the length of Kepler observations is sufficient to resolve narrow rotationally split multiplets for each of its nineteen quasi-equally spaced dipole modes. The number of detected consecutive (in radial order) dipole modes in this series is higher than ever before. The observed amount of splitting shows an increasing trend towards longer periods, which -largely independent of the seismically calibrated stellar modelspoints towards a non-rigid internal rotation profile. From the average splitting we deduce a rotation period of ∼ 188 days. From seismic modelling we find that the star is young with a central hydrogen mass fraction X c > 0.64; it has a core overshooting α ov ≤ 0.15.

ABSTRACT Fourier parameters (frequencies (fj), amplitudes (Aj&amp; (thetaj)) of the selected ... more ABSTRACT Fourier parameters (frequencies (fj), amplitudes (Aj&amp; (thetaj)) of the selected peaks having a signal-to-noise ratio (S/N) above 4 for KIC 4931738 and KIC 6352430 when computed in a 1d-1 window after prewhitening. The displayed S/N values are calculated in a window of 1d-1 or 3d-1 centred on the given frequency, or from the full periodogram (from 0d-1 to 24.47d-1). A complete list of all significant frequencies is available upon request. (2 data files).

PLATO 2.0 has recently been selected for ESA's M3 launch opportunity (2022/24). Providing accurat... more PLATO 2.0 has recently been selected for ESA's M3 launch opportunity (2022/24). Providing accurate key planet parameters (radius, mass, density and age) in statistical numbers, it addresses fundamental questions such as: How do planetary systems form and evolve? Are there other systems with planets like ours, including potentially habitable planets? The PLATO 2.0 instrument consists of 34 small aperture telescopes (32 with 25 s readout cadence and 2 with 2.5 s cadence) providing a wide field-of-view (2232 deg 2 ) and a large photometric magnitude range (4-16 mag). It focuses on bright (4-11 mag) stars in wide fields to detect and characterize planets down to Earth-size by photometric transits, whose masses can then be determined by ground-based radial-velocity follow-up measurements. Asteroseismology will be performed for these bright stars to obtain highly accurate stellar parameters, including masses and ages. The combination of bright targets and asteroseismology results in high accuracy for the bulk planet parameters: 2 %, 4-10 % and 10 % for planet radii, masses and ages, respectively. The planned baseline observing strategy includes two long pointings (2-3 years) to detect and bulk characterize planets reaching into the habitable zone (HZ) of solar-like stars and an additional step-and-stare phase to cover in total about 50 % of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect and characterize hundreds of small planets, and thousands of planets in the Neptune to gas giant regime out to the HZ. It will therefore provide the first large-scale catalogue of bulk characterized planets with accurate radii, masses, mean densities and ages. This catalogue will include terrestrial planets at intermediate orbital distances, where surface temperatures are moderate. Coverage of this parameter range with statistical numbers of bulk characterized planets is unique to PLATO 2.0. The PLATO 2.0 catalogue allows us to e.g.:complete our knowledge of planet diversity for low-mass objects, -correlate the planet mean density-orbital distance distribution with predictions from planet formation theories,-constrain the influence of planet migration and scattering on the architecture of multiple systems, and -specify how planet and system parameters change with host star characteristics, such as type, metallicity and age. The catalogue will allow us to study planets and planetary systems at different evolutionary phases. It will further provide a census for small, low-mass planets. This will serve to identify objects which retained their primordial hydrogen atmosphere and in general the typical characteristics of planets in such a low-mass, low-density range. Planets detected by PLATO 2.0 will orbit bright stars and many of them will be targets for future atmosphere spectroscopy exploring their atmospheres. Furthermore, the mission has the potential to detect exomoons, planetary rings, binary and Trojan planets. The planetary science possible with PLATO 2.0 is complemented by its impact on stellar and galactic science via asteroseismology as well as light curves of all kinds of variable stars, together with observations of stellar clusters of different ages. This will allow us to improve stellar models and study stellar activity. A large number of well-known ages from red giant stars will probe the structure and evolution of our Galaxy. Asteroseismic ages of bright stars for different phases of stellar evolution allow calibrating stellar age-rotation relationships. Together with the results of ESA's Gaia mission, the results of PLATO 2.0 will provide a huge legacy to planetary, stellar and galactic science. Exp Astron (2014) 38:249-330 251

Monthly Notices of the Royal Astronomical Society: Letters, 2010

The system AR Aur is a young late B-type double-lined eclipsing binary with a primary star of HgM... more The system AR Aur is a young late B-type double-lined eclipsing binary with a primary star of HgMn peculiarity. We applied the Doppler imaging method to reconstruct the distribution of Fe and Y over the surface of the primary using spectroscopic time series obtained in 2005 and from 2008 October to 2009 February. The results show a remarkable evolution of the element distribution and overabundances. Measurements of the magnetic field with the moment technique using several elements reveal the presence of a longitudinal magnetic field of the order of a few hundred gauss in both stellar components and a quadratic field of the order of 8 kG on the surface of the primary star.

Monthly Notices of the Royal Astronomical Society: Letters, 2012

We report the discovery of low-amplitude gravity-mode oscillations in the massive binary star V38... more We report the discovery of low-amplitude gravity-mode oscillations in the massive binary star V380 Cyg, from 180 d of Kepler custom-aperture space photometry and 5 months of high-resolution high signal-to-noise spectroscopy. The new data are of unprecedented quality and allowed to improve the orbital and fundamental parameters for this binary. The orbital solution was subtracted from the photometric data and led to the detection of periodic intrinsic variability with frequencies of which some are multiples of the orbital frequency and others are not. Spectral disentangling allowed the detection of line-profile variability in the primary. With our discovery of intrinsic variability interpreted as gravity mode oscillations, V380 Cyg becomes an important laboratory for future seismic tuning of the near-core physics in massive B-type stars.

Monthly Notices of the Royal Astronomical Society, 2014

Science (New York, N.Y.), Jan 8, 2011

Hierarchical triple systems comprise a close binary and a more distant component. They are import... more Hierarchical triple systems comprise a close binary and a more distant component. They are important for testing theories of star formation and of stellar evolution in the presence of nearby companions. We obtained 218 days of Kepler photometry of HD 181068 (magnitude of 7.1), supplemented by ground-based spectroscopy and interferometry, which show it to be a hierarchical triple with two types of mutual eclipses. The primary is a red giant that is in a 45-day orbit with a pair of red dwarfs in a close 0.9-day orbit. The red giant shows evidence for tidally induced oscillations that are driven by the orbital motion of the close pair. HD 181068 is an ideal target for studies of dynamical evolution and testing tidal friction theories in hierarchical triple systems.

Astronomy & Astrophysics, 2014

Context. The unparalleled photometric data obtained by NASA's Kepler space telescope led to an im... more Context. The unparalleled photometric data obtained by NASA's Kepler space telescope led to an improved understanding of red giant stars and binary stars. Seismology allows us to constrain the properties of red giants. In addition to eclipsing binaries, eccentric non-eclipsing binaries, exhibiting ellipsoidal modulations, have been detected with Kepler. Aims. We aim to study the properties of eccentric binary systems containing a red giant star and derive the parameters of the primary giant component. Methods. We apply asteroseismic techniques to determine masses and radii of the primary component of each system. For a selected target, light and radial velocity curve modelling techniques are applied to extract the parameters of the system and its primary component. Stellar evolution and its effects on the evolution of the binary system are studied from theoretical models. Results. The paper presents the asteroseismic analysis of 18 pulsating red giants in eccentric binary systems, for which masses and radii were constrained. The orbital periods of these systems range from 20 to 440 days. The results of our ongoing radial velocity monitoring program with the HERMES spectrograph reveal an eccentricity range of e = 0.2 to 0.76. As a case study we present a detailed analysis of KIC 5006817, whose rich oscillation spectrum allows for a detailed seismic analysis. From seismology we constrain the rotational period of the envelope to be at least 165 d, which is roughly twice the orbital period. The stellar core rotates 13 times faster than the surface. From the spectrum and radial velocities we expect that the Doppler beaming signal should have a maximum amplitude of 300 ppm in the light curve. Fixing the mass and radius to the asteroseismically determined values, from our binary modelling we find a value of the gravity darkening exponent that is significantly larger than expected. Through binary modelling, we determine the mass of the secondary component to be 0.29±0.03 M ⊙ . Conclusions. For KIC 5006817 we exclude pseudo-synchronous rotation of the red giant with the orbit. The comparison of the results from seismology and modelling of the light curve shows a possible alignment of the rotational and orbital axis at the 2σ level. Red giant eccentric systems could be progenitors of cataclysmic variables and hot subdwarf B stars.

Astronomy & Astrophysics, 2013

Aims. We present the first binary modelling results for the pulsating eclipsing binary KIC 112856... more Aims. We present the first binary modelling results for the pulsating eclipsing binary KIC 11285625, discovered by the Kepler mission. An automated method to disentangle the pulsation spectrum and the orbital variability in high quality light curves, was developed and applied. The goal was to obtain accurate orbital and component properties, in combination with essential information derived from spectroscopy. Methods. A binary model for KIC 11285625 was obtained, using a combined analysis of high-quality space-based Kepler light curves and ground-based high-resolution HERMES echelle spectra. The binary model was used to separate the pulsation characteristics from the orbital variability in the Kepler light curve in an iterative way. We used an automated procedure to perform this task, based on the JKTEBOP binary modelling code, and adapted codes for frequency analysis and prewhitening of periodic signals. Using a disentangling technique applied to the composite HERMES spectra, we obtained a higher signal-to-noise mean component spectrum for both the primary and the secondary. A model grid search method for fitting synthetic spectra was used for fundamental parameter determination for both components. Results. Accurate orbital and component properties of KIC 11285625 were derived, and we have obtained the pulsation spectrum of the γ Dor pulsator in the system. Detailed analysis of the pulsation spectrum revealed amplitude modulation on a time scale of a hundred days, and strong indications of frequency splittings at both the orbital frequency, and the rotational frequency derived from spectroscopy.

Astronomy & Astrophysics, 2012

Context. The predicted orbital period histogram of a subdwarf B (sdB) population is bimodal with ... more Context. The predicted orbital period histogram of a subdwarf B (sdB) population is bimodal with a peak at short ( < 10 days) and long ( > 250 days) periods. Observationally, however, there are many short-period sdB systems known, but only very few long-period sdB binaries are identified. As these predictions are based on poorly understood binary interaction processes, it is of prime importance to confront the predictions to well constrained observational data. We therefore initiated a monitoring program to find and characterize long-period sdB stars. Aims. In this contribution we aim to determine the absolute dimensions of the long-period binary system PG 1104+243 consisting of an sdB and a main-sequence (MS) component, and determine its evolution history. Methods. High-resolution spectroscopy time-series were obtained with HERMES at the Mercator telescope at La Palma, and analyzed to determine the radial velocities of both the sdB and MS components. Photometry from the literature was used to construct the spectral energy distribution (SED) of the binary. Atmosphere models were used to fit this SED and determine the surface gravity and temperature of both components. The gravitational redshift provided an independent confirmation of the surface gravity of the sdB component.

Astronomy & Astrophysics, 2013

Context. The NASA exoplanet search mission Kepler is currently providing a wealth of light curves... more Context. The NASA exoplanet search mission Kepler is currently providing a wealth of light curves of ultra-high quality from space. Aims. We used high-quality Kepler photometry and spectroscopic data to investigate the Kepler target and binary candidate KIC 5988140. We aim to interpret the observed variations of KIC 5988140 considering three possible scenarios: binarity, co-existence of δ Sct-and γ Dor-type oscillations, and rotational modulation caused by an asymmetric surface intensity distribution. Methods. We used the spectrum synthesis method to derive the fundamental parameters T eff , log g, [M/H], and v sin i from the newly obtained high-resolution, high S/N spectra. Frequency analyses of both the photometric and the spectroscopic data were performed. Results. The star has a spectral type of A7.5 IV-III and a metallicity slightly lower than that of the Sun. Both Fourier analyses reveal the same two dominant frequencies F 1 =2F 2 =0.688 and F 2 =0.344 d −1 . We also detected in the photometry the signal of nine more, significant frequencies located in the typical range of δ Sct pulsation. The light and radial velocity curves follow a similar, stable double-wave pattern which are not exactly in anti-phase but show a relative phase shift of about 0.1 period between the moment of minimum velocity and that of maximum light. Conclusions. Such findings are incompatible with the star being a binary system. We next show that, for all possible (limit) configurations of a spotted surface, the predicted light-to-velocity amplitude ratio is almost two orders larger than the observed value, which pleads against rotational modulation. The same argument also invalidates the explanation in terms of pulsations of type γ Dor (i.e. hybrid pulsations). We confirm the occurrence of various independent δ Sct-type pressure modes in the Kepler light curve. With respect to the low-frequency content, however, we argue that the physical cause of the remaining light and radial velocity variations of this late A-type star remains unexplained by any of the presently considered scenarios.

Astronomy & Astrophysics, 2013

Context. OB stars are important in the chemistry and evolution of the Universe, but the sample of... more Context. OB stars are important in the chemistry and evolution of the Universe, but the sample of targets well understood from an asteroseismological point of view is still too limited to provide feedback on the current evolutionary models. Our study extends this sample with two spectroscopic binary systems. Aims. Our goal is to provide orbital solutions, fundamental parameters and abundances from disentangled high-resolution high signalto-noise spectra, as well as to analyse and interpret the variations in the Kepler light curve of these carefully selected targets. This way we continue our efforts to map the instability strips of β Cep and slowly pulsating B stars using the combination of high-resolution ground-based spectroscopy and uninterrupted space-based photometry. Methods. We fit Keplerian orbits to radial velocities measured from selected absorption lines of high-resolution spectroscopy using synthetic composite spectra to obtain orbital solutions. We use revised masks to obtain optimal light curves from the original pixeldata from the Kepler satellite, which provided better long term stability compared to the pipeline processed light curves. We use various time-series analysis tools to explore and describe the nature of variations present in the light curve. Results. We find two eccentric double-lined spectroscopic binary systems containing a total of three main sequence B-type stars (and one F-type component) of which at least one in each system exhibits light variations. The light curve analysis (combined with spectroscopy) of the system of two B stars points towards the presence of tidally excited g modes in the primary component. We interpret the variations seen in the second system as classical g mode pulsations driven by the κ mechanism in the B type primary, and explain the unexpected power in the p mode region as a result of nonlinear resonant mode excitation.

Space Telescopes and Instrumentation 2008: Ultraviolet to Gamma Ray, 2008

Spatially resolved X-ray spectroscopy with high spectral resolution allows the study of astrophys... more Spatially resolved X-ray spectroscopy with high spectral resolution allows the study of astrophysical processes in extended sources with unprecedented sensitivity. This includes the measurement of abundances, temperatures, densities, ionisation stages as well as turbulence and velocity structures in these sources. An X-ray calorimeter is planned for the Russian mission Spektr Röntgen-Gamma (SRG), to be launched in 2011. During the first half year (pointed phase) it will study the dynamics and composition of of the hot gas in massive clusters of galaxies and in supernova remnants (SNR). During the survey phase it will produce the first all sky maps of line-rich spectra of the interstellar medium (ISM). Spectral analysis will be feasible for typically every 5° x 5° region on the sky. Considering the very short time-scale for the development of this instrument it consists of a combination of well developed systems. For the optics an extra eROSITA mirror, also part of the Spektr-RG payload, will be used. The detector will be based on spare parts of the detector flown on Suzaku combined with a rebuild of the electronics and the cooler will be based on the design for the Japanese mission NeXT. In this paper we will present the science and give an overview of the instrument.

Optics for EUV, X-Ray, and Gamma-Ray Astronomy VI, 2013