Category Archives: Publications

Asteroseismology of 19 low-luminosity red giant stars from Kepler

Authors. F. Perez Hernandez, R. A. Garcia, E. Corsaro, S. A. Triana, J. De Ridder

JournalAstronomy & Astrophysics

Abstract. Frequencies of acoustic and mixed modes in red giant stars are now determined with high precision thanks to the long continuous observations provided by the NASA Kepler mission. Here we consider the eigenfrequencies of nineteen low-luminosity red giant stars selected by Corsaro et al. (2015) for a detailed peak-bagging analysis. Our objective is to obtain stellar parameters by using individual mode frequencies and spectroscopic information. We use a forward modelling technique based on a minimization procedure combining the frequencies of the p modes, the period spacing of the dipolar modes, and the spectroscopic data. Consistent results between the forward modelling technique and values derived from the seismic scaling relations are found but the errors derived using the former technique are lower. The average error for log g is 0.002 dex, compared to 0.011 dex from the frequency of maximum power and 0.10 dex from the spectroscopic analysis. Relative errors in the masses and radii are on average 2 and 0.5 per cent respectively, compared to 3 and 2 per cent derived from the scaling relations. No reliable determination of the initial helium abundances and the mixing length parameters could be made. Finally, for our grid of models with a given input physics, we found that low-mass stars require higher values of the overshooting parameter.

Links. A&ANASA ADS, arXiv

The shrinking Sun: a systematic error in local correlation tracking of solar granulation

Authors. B. Löptien, A. C. Birch, T. L. Duvall Jr., L. Gizon, J. Schou

JournalAstronomy & Astrophysics

Abstract. Context. Local correlation tracking of granulation (LCT) is an important method for measuring horizontal flows in the photosphere. This method exhibits a systematic error that looks like a flow converging towards disk center, also known as the shrinking-Sun effect.
Aims. We aim at studying the nature of the shrinking-Sun effect for continuum intensity data and at deriving a simple model that can explain its origin.
Methods. We derived LCT flow maps by running the local correlation tracking code FLCT on tracked and remapped continuum intensity maps provided by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory. We also computed flow maps from synthetic continuum images generated from STAGGER code simulations of solar surface convection. We investigated the origin of the shrinking-Sun effect by generating an average granule from synthetic data from the simulations.
Results. The LCT flow maps derived from HMI and from the simulations exhibit a shrinking-Sun effect of comparable magnitude. The origin of this effect is related to the apparent asymmetry of granulation originating from radiative transfer effects when observing with a viewing angle inclined from vertical. This causes, in combination with the expansion of the granules, an apparent motion towards disk center.

Links. A&ANASA ADS, arXiv

Sensitivity Kernels for Flows in Time-Distance Helioseismology: Extension to Spherical Geometry

Authors. Vincent G. A. Böning, Markus Roth, Wolfgang Zima, Aaron C. Birch, Laurent Gizon

Journal. The Astrophysical Journal

Abstract. We extend an existing Born approximation method for calculating the linear sensitivity of helioseismic travel times to flows from Cartesian to spherical geometry. This development is necessary for using the Born approximation for inferring large-scale flows in the deep solar interior. In a first sanity check, we compare two fmode kernels from our spherical method and from an existing Cartesian method. The horizontal and total integrals agree to within 0.3 %. As a second consistency test, we consider a uniformly rotating Sun and a travel distance of 42 degrees. The analytical travel-time difference agrees with the forward-modelled travel-time difference to within 2 %. In addition, we evaluate the impact of different choices of filter functions on the kernels for a meridional travel distance of 42 degrees. For all filters, the sensitivity is found to be distributed over a large fraction of the convection zone. We show that the kernels depend on the filter function employed in the data analysis process. If modes of higher harmonic degree (90l170) are permitted, a noisy pattern of a spatial scale corresponding to l260 appears near the surface. When mainly low-degree modes are used (l70), the sensitivity is concentrated in the deepest regions and it visually resembles a ray-path-like structure. Among the different low-degree filters used, we find the kernel for phase-speed filtered measurements to be best localized in depth.

Links. ApJNASA ADS, arXiv

Sub-Inertial Gravity Modes in the B8V Star KIC 7760680 Reveal Moderate Core Overshooting and Low Vertical Diffusive Mixing

Authors. Ehsan Moravveji, Richard H. D. Townsend, Conny Aerts, Stephane Mathis

Journal. The Astrophysical Journal

Abstract. KIC 7760680 is so far the richest slowly pulsating B star, by exhibiting 36 consecutive dipole (=1) gravity (g-) modes. The monotonically decreasing period spacing of the series, in addition to the local dips in the pattern confirm that KIC 7760680 is a moderate rotator, with clear mode trapping in chemically inhomogeneous layers. We employ the traditional approximation of rotation to incorporate rotational effects on g-mode frequencies. Our detailed forward asteroseismic modelling of this g-mode series reveals that KIC 7760680 is a moderately rotating B star with mass 3.25 M. By simultaneously matching the slope of the period spacing, and the number of modes in the observed frequency range, we deduce that the equatorial rotation frequency of KIC 7760680 is 0.4805 day1, which is 26\% of its Roche break up frequency. The relative deviation of the model frequencies and those observed is less than one percent. We succeed to tightly constrain the exponentially-decaying convective core overshooting parameter to fov0.024±0.001. This means that convective core overshooting can coexist with moderate rotation. Moreover, models with exponentially-decaying overshoot from the core outperform those with the classical step-function overshoot. The best value for extra diffusive mixing in the radiatively stable envelope is confined to logDext0.75±0.25 (with Dext in cm2 sec1), which is notably smaller than theoretical predictions.

Links. ApJNASA ADS, arXiv

SpaceInn hare-and-hounds exercise: Estimation of stellar properties using space-based asteroseismic data

Authors. D. R. Reese, W. J. Chaplin, G. R. Davies, A. Miglio, H. M. Antia, W. H. Ball, S. Basu, G. Buldgen, J. Christensen-Dalsgaard, H. R. Coelho, S. Hekker, G. Houdek, Y. Lebreton, A. Mazumdar, T. S. Metcalfe, V. Silva Aguirre, D. Stello, K. Verma

Journal. Astronomy & Astrophysics

Abstract. Context: Detailed oscillation spectra comprising individual frequencies for numerous solar-type stars and red giants are or will become available. These data can lead to a precise characterisation of stars.
Aims: Our goal is to test and compare different methods for obtaining stellar properties from oscillation frequencies and spectroscopic constraints, in order to evaluate their accuracy and the reliability of the error bars.
Methods: In the context of the SpaceInn network, we carried out a hare-and-hounds exercise in which one group produced “observed” oscillation spectra for 10 artificial solar-type stars, and various groups characterised these stars using either forward modelling or acoustic glitch signatures.
Results: Results based on the forward modelling approach were accurate to 1.5 % (radius), 3.9 % (mass), 23 % (age), 1.5 % (surface gravity), and 1.8 % (mean density). For the two 1 Msun stellar targets, the accuracy on the age is better than 10 % thereby satisfying PLATO 2.0 requirements. The average accuracies for the acoustic radii of the base of the convection zone, the He II ionisation, and the Gamma_1 peak were 17 %, 2.4 %, and 1.9 %, respectively. Glitch fitting analysis seemed to be affected by aliasing problems for some of the targets.
Conclusions: Forward modelling is the most accurate approach, but needs to be complemented by model-independent results from, e.g., glitch analysis. Furthermore, global optimisation algorithms provide more robust error bars.

Links. A&A, NASA ADS, arXiv

Chromatic line-profile tomography to reveal exoplanetary atmospheres: application to HD 189733b

Authors. Francesco Borsa, Monica Rainer, Ennio Poretti

Journal. Astronomy & Astrophysics

Abstract. Transmission spectroscopy can be used to constrain the properties of exoplanetary atmospheres. During a transit, the light blocked from the atmosphere of the planet leaves an imprint in the light coming from the star. This has been shown for many exoplanets with different techniques, with both photometry and spectroscopy. We aim at testing chromatic line-profile tomography as a new tool to investigate exoplanetary atmospheres. The signal imprinted on the cross-correlation function (CCF) by a planet transiting its star is dependent on the planet-to-star radius ratio. We want to verify if the precision reachable on the CCF obtained from a subset of the spectral orders of the HARPS spectrograph is enough to discriminate the radius of a planet at different wavelengths. Methods. We analyze HARPS archival data of three transits of HD 189733b. We divide the HARPS spectral range in 7 broadbands, calculating for each band the ratio between the area of the out-of-transit CCF and the area of the signal imprinted by the planet on it during the full part of the transit. We take into account the effect of the limb darkening using the theoretical coefficients of a linear law. Averaging the results of three different transits allows us to obtain a good quality broadband transmission spectrum of HD 189733b, with a precision greater than that of the chromatic RM effect. Results. We proved chromatic line-profile tomography to be an interesting way to reveal broadband transmission spectra of exoplanets: our analysis of the atmosphere of HD 189733b is in agreement with other ground- and space-based observations. The independent analysis of different transits puts in evidence the probability that stellar activity plays a role in the extracted transmission spectrum. Care has thus to be taken when claiming for Rayleigh scattering in the atmosphere of exoplanets orbiting active stars using only one transit.

Links. A&ANASA ADS, arXiv

HD 41641: A classical δ Sct-type pulsator with chemical signatures of an Ap star

Authors. A. Escorza, K. Zwintz, A. Tkachenko, T. Van Reeth, T. Ryabchikova, C. Neiner, E. Poretti, M. Rainer, E. Michel, A. Baglin, C. Aerts

Journal. Astronomy & Astrophysics

Abstract. Context. Among the known groups of pulsating stars, δ Sct stars are one of the least understood. Theoretical models do not predict the oscillation frequencies that observations reveal. Complete asteroseismic studies are necessary to improve these models and better understand the internal structure of these targets.
Aims: We study the δ Sct star HD 41641 with the ultimate goal of understanding its oscillation pattern.
Methods: The target was simultaneously observed by the CoRoT space telescope and the HARPS high-resolution spectrograph. The photometric data set was analyzed with the software package PERIOD04, while FAMIAS was used to analyze the line profile variations. The method of spectrum synthesis was used for spectroscopically determining the fundamental atmospheric parameters and individual chemical abundances.
Results: A total of 90 different frequencies was identified and analyzed. An unambiguous identification of the azimuthal order of the surface geometry could only be provided for the dominant p-mode, which was found to be a nonradial prograde mode with m = +1. Using Teff and log g, we estimated the mass, radius, and evolutionary stage of HD 41641. We find HD 41641 to be a moderately rotating, slightly evolved δ Sct star with subsolar overall atmospheric metal content and unexpected chemical peculiarities.
Conclusions: HD 41641 is a pure δ Sct pulsator with p-mode frequencies in the range from 10 d-1 to 20 d-1. This pulsating star presents chemical signatures of an Ap star and rotational modulation due to surface inhomogeneities, which we consider indirect evidence of the presence of a magnetic field.The CoRoT space mission was developed and operated by the French space agency CNES, with participation of ESA’s RSSD and Science Programmes, Austria, Belgium, Brazil, Germany, and Spain.This work uses ground-based spectroscopic observations made with the HARPS instrument at the 3.6 m-ESO telescope (La Silla, Chile) under the Large Program 185.D-0056.

Links. A&ANASA ADS, arXiv

Large size and slow rotation of the trans-Neptunian object (225088) 2007 OR10 discovered from Herschel and K2 observations

Authors. Pál, András; Kiss, Csaba; Müller, Thomas G.; Molnár, László; Szabó, Róbert; Szabó, Gyula M.; Sárneczky, Krisztián; Kiss, László L.

Journal. The Astronomical Journal

Abstract. We present the first comprehensive thermal and rotational analysis of the second most distant trans-Neptunian object (225088) 2007 OR10. We combined optical light curves provided by the Kepler space telescope — K2 extended mission and thermal infrared data provided by the Herschel Space Observatory. We found that (225088) 2007 OR10 is likely to be larger and darker than derived by earlier studies: we obtained a diameter of d=1535^{+75}_{-225} km which places (225088) 2007 OR10 in the biggest top three trans-Neptunian objects. The corresponding visual geometric albedo is p_V=0.089^{+0.031}_{-0.009}. The light curve analysis revealed a slow rotation rate of P_rot=44.81+/-0.37 h, superseded by a very few objects only. The most likely light-curve solution is double-peaked with a slight asymmetry, however, we cannot safely rule out the possibility of having a rotation period of P_rot=22.40+/-0.18 h which corresponds to a single-peaked solution. Due to the size and slow rotation, the shape of the object should be a MacLaurin ellipsoid, so the light variation should be caused by surface inhomogeneities. Its newly derived larger diameter also implies larger surface gravity and a more likely retention of volatiles — CH_4, CO and N_2 — on the surface.

Links. AJNASA ADS, arXiv

Magnetic activity cycles in solar-like stars: The cross-correlation technique of p-mode frequency shifts

Authors. C. Regulo, R. A. Garcia, J. Ballot

Journal. Astronomy & Astrophysics

Abstract. Aims. We aim studying the use of cross-correlation techniques to infer the frequency shifts induced by changing magnetic fields in the p-mode frequencies and provide precise estimation of the error bars. Methods. This technique and the calculation of the associated errors is first tested and validated on the Sun where the p-mode magnetic behaviour is very well known. These validation tests are performed on 6000-day time series of Sun-as-a-star observations delivered by the SoHO spacecraft. Errors of the frequency shifts are quantified through Monte Carlo simulations. The same methodology is then applied to three solar-like oscillating stars: HD 49933, observed by CoRoT, as well as KIC 3733735 and KIC 7940546 observed by Kepler. Results. We first demonstrate the reliability of the error bars computed with the Monte Carlo simulations using the Sun. From the three analyzed stars we confirm the presence of a magnetic activity cycle with this methodology in HD 49933 and we unveil seismic signature of on going magnetic variations in KIC 3733735. Finally, the third star, KIC 7940546, seems to be in a quiet regime.

Links. A&ANASA ADS, arXiv

Spectroscopic survey of γ Doradus stars – I. Comprehensive atmospheric parameters and abundance analysis of γ Doradus stars

Authors. F. Kahraman-Alicavus, E. Niemczura, P. De Cat, E. Soydugan, Z. Kolaczkowski, J. Ostrowski, J. H. Telting, K. Uytterhoeven, E. Poretti, M. Rainer, J. C. Suarez, L. Mantegazza, P. Kilmartin, K. R. Pollard

Journal. Monthly Notices of the Royal Astronomical Society

Abstract. We present a spectroscopic survey of known and candidate γ\,Doradus stars. The high-resolution, high signal-to-noise spectra of 52 objects were collected by five different spectrographs. The spectral classification, atmospheric parameters (\teff, logg, ξ), vsini and chemical composition of the stars were derived. The stellar spectral and luminosity classes were found between G0-A7 and IV-V, respectively. The initial values for \teff\ and \logg\ were determined from the photometric indices and spectral energy distribution. Those parameters were improved by the analysis of hydrogen lines. The final values of \teff, \logg\ and ξ were derived from the iron lines analysis. The \teff\ values were found between 6000\,K and 7900\,K, while \logg\,values range from 3.8 to 4.5\,dex. Chemical abundances and vsini values were derived by the spectrum synthesis method. The vsini values were found between 5 and 240\,km\,s1. The chemical abundance pattern of γ\,Doradus stars were compared with the pattern of non-pulsating stars. It turned out that there is no significant difference in abundance patterns between these two groups. Additionally, the relations between the atmospheric parameters and the pulsation quantities were checked. A strong correlation between the vsini and the pulsation periods of γ\,Doradus variables was obtained. The accurate positions of the analysed stars in the H-R diagram have been shown. Most of our objects are located inside or close to the blue edge of the theoretical instability strip of γ\,Doradus.