Publications (25)29.9 Total impact
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Article: Estimating the p-mode frequencies of the solar twin 18 Sco
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ABSTRACT: Solar twins have been a focus of attention for more than a decade, because their structure is extremely close to that of the Sun. Today, thanks to high-precision spectrometers, it is possible to use asteroseismology to probe their interiors. Our goal is to use time series obtained from the HARPS spectrometer to extract the oscillation frequencies of 18 Sco, the brightest solar twin. We used the tools of spectral analysis to estimate these quantities. We estimate 52 frequencies using an MCMC algorithm. After examination of their probability densities and comparison with results from direct MAP optimization, we obtain a minimal set of 21 reliable modes. The identification of each pulsation mode is straightforwardly accomplished by comparing to the well-established solar pulsation modes. We also derived some basic seismic indicators using these values. These results offer a good basis to start a detailed seismic analysis of 18 Sco using stellar models.Astronomy and Astrophysics 09/2012; 544:A106. · 4.59 Impact Factor -
Article: Seismic analysis of four solar-like stars observed during more than eight months by Kepler
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ABSTRACT: Having started science operations in May 2009, the Kepler photometer has been able to provide exquisite data of solar-like stars. Five out of the 42 stars observed continuously during the survey phase show evidence of oscillations, even though they are rather faint (magnitudes from 10.5 to 12). In this paper, we present an overview of the results of the seismic analysis of 4 of these stars observed during more than eight months.10/2011; -
Article: Asteroseismology from multi-month Kepler photometry: the evolved Sun-like stars KIC 10273246 and KIC 10920273
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ABSTRACT: Context. The evolved main-sequence Sun-like stars KIC 10273246 (F-type) and KIC 10920273 (G-type) were observed with the NASA Kepler satellite for approximately ten months with a duty cycle in excess of 90%. Such continuous and long observations are unprecedented for solar-type stars other than the Sun. Aims. We aimed mainly at extracting estimates of p-mode frequencies – as well as of other individual mode parameters – from the power spectra of the light curves of both stars, thus providing scope for a full seismic characterization. Methods. The light curves were corrected for instrumental effects in a manner independent of the Kepler Science Pipeline. Estimation of individual mode parameters was based both on the maximization of the likelihood of a model describing the power spectrum and on a classic prewhitening method. Finally, we employed a procedure for selecting frequency lists to be used in stellar modeling. Results. A total of 30 and 21 modes of degree l = 0, 1, 2 – spanning at least eight radial orders – have been identified for KIC 10273246 and KIC 10920273, respectively. Two avoided crossings (l = 1 ridge) have been identified for KIC 10273246, whereas one avoided crossing plus another likely one have been identified for KIC 10920273. Good agreement is found between observed and predicted mode amplitudes for the F-type star KIC 10273246, based on a revised scaling relation. Estimates are given of the rotational periods, the parameters describing stellar granulation and the global asteroseismic parameters ∆ν and ν max .C. Régulo. 09/2011; 81113131415(5). -
Article: Asteroseismology from multi-month Kepler photometry: the evolvedSun-like stars KIC 10273246 and KIC 10920273
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ABSTRACT: The evolved main-sequence Sun-like stars KIC 10273246 (F-type) and KIC 10920273 (G-type) were observed with the NASA Kepler satellite for approximately ten months with a duty cycle in excess of 90%. Such continuous and long observations are unprecedented for solar-type stars other than the Sun. We aimed mainly at extracting estimates of p-mode frequencies - as well as of other individual mode parameters - from the power spectra of the light curves of both stars, thus providing scope for a full seismic characterization. The light curves were corrected for instrumental effects in a manner independent of the Kepler Science Pipeline. Estimation of individual mode parameters was based both on the maximization of the likelihood of a model describing the power spectrum and on a classic prewhitening method. Finally, we employed a procedure for selecting frequency lists to be used in stellar modeling. A total of 30 and 21 modes of degree l=0,1,2 - spanning at least eight radial orders - have been identified for KIC 10273246 and KIC 10920273, respectively. Two avoided crossings (l=1 ridge) have been identified for KIC 10273246, whereas one avoided crossing plus another likely one have been identified for KIC 10920273. Good agreement is found between observed and predicted mode amplitudes for the F-type star KIC 10273246, based on a revised scaling relation. Estimates are given of the rotational periods, the parameters describing stellar granulation and the global asteroseismic parameters $\Delta\nu$ and $\nu_{\rm{max}}$.Astronomy and Astrophysics 08/2011; 534:A6 (14pp). · 4.59 Impact Factor -
Article: Accurate p-mode measurements of the G0V metal-rich CoRoT target HD 52265
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ABSTRACT: The star HD 52265 is a G0V metal-rich exoplanet-host star observed in the seismology field of the CoRoT space telescope from November 2008 to March 2009. The satellite collected 117 days of high-precision photometric data on this star, showing that it presents solar-like oscillations. HD 52265 was also observed in spectroscopy with the Narval spectrograph at the same epoch. We characterise HD 52265 using both spectroscopic and seismic data. The fundamental stellar parameters of HD 52265 were derived with the semi-automatic software VWA, and the projected rotational velocity was estimated by fitting synthetic profiles to isolated lines in the observed spectrum. The parameters of the observed p modes were determined with a maximum-likelihood estimation. We performed a global fit of the oscillation spectrum, over about ten radial orders, for degrees l=0 to 2. We also derived the properties of the granulation, and analysed a signature of the rotation induced by the photospheric magnetic activity. Precise determinations of fundamental parameters have been obtained: Teff = 6100 +- 60 K, log g = 4.35 +- 0.09, [M/H] = 0.19 +- 0.05, as well as vsini = 3.6 +0.3 -1.0 km/s. We have measured a mean rotation period P_rot = 12.3 +- 0.15 days, and find a signature of differential rotation. The frequencies of 31 modes are reported in the range 1500-2550 micro-Hz. The large separation exhibits a clear modulation around the mean value = 98.3 +- 0.1 micro-Hz. Mode widths vary with frequency along an S-shape with a clear local maximum around 1800 micro-Hz. We deduce lifetimes ranging between 0.5 and 3 days for these modes. Finally, we find a maximal bolometric amplitude of about 3.96 +- 0.24 ppm for radial modes.05/2011; -
Article: The radius and mass of the close solar twin 18 Scorpii derived from asteroseismology and interferometry
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ABSTRACT: The growing interest in solar twins is motivated by the possibility of comparing them directly to the Sun. To carry on this kind of analysis, we need to know their physical characteristics with precision. Our first objective is to use asteroseismology and interfer-ometry on the brightest of them: 18 Sco. We observed the star during 12 nights with HARPS for seismology and used the PAVO beam-combiner at CHARA for interferometry. An average large frequency separation 134.4 ± 0.3 μHz and angular and linear radiuses of 0.6759 ± 0.0062 mas and 1.010 ± 0.009 R were estimated. We used these values to derive the mass of the star, 1.02 ± 0.03 M .Astronomy and Astrophysics 01/2011; 526:L4. · 4.59 Impact Factor -
Article: Probing the parameter space of HD 49933: a comparison between global and local methods
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ABSTRACT: We present two independent methods for studying the global stellar parameter space (mass M, age, initial chemical composition X_0, Z_0) of HD 49933 with seismic data. Using a local minimization and an MCMC algorithm, we obtain consistent results for the determination of the stellar properties: M = 1.1 - 1.2 M_solar, Age ~ 3.0 Gyr, Z_0 ~ 0.008. A description of the error ellipses can be defined using Singular Value Decomposition techniques, and this is validated by comparing the errors with those from the MCMC method. Comment: to be published in JPCS11/2010; -
Article: A Precise Asteroseismic Age and Radius for the Evolved Sun-like Star KIC 11026764
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ABSTRACT: The primary science goal of the Kepler Mission is to provide a census of exoplanets in the solar neighborhood, including the identification and characterization of habitable Earth-like planets. The asteroseismic capabilities of the mission are being used to determine precise radii and ages for the target stars from their solar-like oscillations. Chaplin et al. published observations of three bright G-type stars, which were monitored during the first 33.5 days of science operations. One of these stars, the subgiant KIC 11026764, exhibits a characteristic pattern of oscillation frequencies suggesting that it has evolved significantly. We have derived asteroseismic estimates of the properties of KIC 11026764 from Kepler photometry combined with ground-based spectroscopic data. We present the results of detailed modeling for this star, employing a variety of independent codes and analyses that attempt to match the asteroseismic and spectroscopic constraints simultaneously. We determine both the radius and the age of KIC 11026764 with a precision near 1%, and an accuracy near 2% for the radius and 15% for the age. Continued observations of this star promise to reveal additional oscillation frequencies that will further improve the determination of its fundamental properties.The Astrophysical Journal 10/2010; 723(2):1583. · 6.02 Impact Factor -
Article: A multi-site campaign to measure solar-like oscillations in Procyon. II. Mode frequencies
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ABSTRACT: We have analyzed data from a multi-site campaign to observe oscillations in the F5 star Procyon. The data consist of high-precision velocities that we obtained over more than three weeks with eleven telescopes. A new method for adjusting the data weights allows us to suppress the sidelobes in the power spectrum. Stacking the power spectrum in a so-called echelle diagram reveals two clear ridges that we identify with even and odd values of the angular degree (l=0 and 2, and l=1 and 3, respectively). We interpret a strong, narrow peak at 446 muHz that lies close to the l=1 ridge as a mode with mixed character. We show that the frequencies of the ridge centroids and their separations are useful diagnostics for asteroseismology. In particular, variations in the large separation appear to indicate a glitch in the sound-speed profile at an acoustic depth of about 1000 s. We list frequencies for 55 modes extracted from the data spanning 20 radial orders, a range comparable to the best solar data, which will provide valuable constraints for theoretical models. A preliminary comparison with published models shows that the offset between observed and calculated frequencies for the radial modes is very different for Procyon than for the Sun and other cool stars. We find the mean lifetime of the modes in Procyon to be 1.29 +0.55/-0.49 days, which is significantly shorter than the 2-4 days seen in the Sun.02/2010; -
Article: Accurate fundamental parameters for 23 bright solar-type stars
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ABSTRACT: We combine results from interferometry, asteroseismology and spectroscopy to determine accurate fundamental parameters of 23 bright solar-type stars, from spectral type F5 to K2 and luminosity classes III to V. For some stars we can use direct techniques to determine the mass, radius, luminosity and effective temperature, and we compare with indirect methods that rely on photometric calibrations or spectroscopic analyses. We use the asteroseismic information available in the literature to infer an indirect mass with an accuracy of 4-15 percent. From indirect methods we determine luminosity and radius to 3 percent. For Teff we find a slight offset of -40+-20 K between the spectroscopic method and the direct method, meaning the spectroscopic temperatures are too high. From the spectroscopic analysis we determine the detailed chemical composition for 13 elements, including Li, C and O. We find no significant offset between the spectroscopic surface gravity and the value from combining asteroseismology with radius estimates. From the spectroscopy we also determine vsini and we present a new calibration of macro- and microturbulence. From the comparison between the results from the direct and spectroscopic methods we claim that we can determine Teff, log g, and [Fe/H] with absolute accuracies of 80 K, 0.08 dex, and 0.07 dex. The indirect methods are important to obtain reliable estimates of the fundamental parameters of relatively faint stars when interferometry cannot be used. Our study is the first to compare direct and indirect methods for a large sample of stars, and we conclude that indirect methods are valid, although slight corrections may be needed. Comment: Accepted by MNRAS. Abstract abridged02/2010; -
Article: The asteroseismic potential of Kepler: first results for solar-type stars
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ABSTRACT: We present preliminary asteroseismic results from Kepler on three G-type stars. The observations, made at one-minute cadence during the first 33.5d of science operations, reveal high signal-to-noise solar-like oscillation spectra in all three stars: About 20 modes of oscillation may be clearly distinguished in each star. We discuss the appearance of the oscillation spectra, use the frequencies and frequency separations to provide first results on the radii, masses and ages of the stars, and comment in the light of these results on prospects for inference on other solar-type stars that Kepler will observe. Comment: 16 pages, 3 figures, 1 table; now accepted for publication in ApJ LettersThe Astrophysical Journal Letters 01/2010; 713:L169. · 5.53 Impact Factor -
Article: Lithium abundance as a boundary condition for age and mass determination of solar twin stars
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ABSTRACT: We explore the non-standard mixing history of five solar twins to determine as precisely as possible their mass and age. For this, we computed a grid of evolutionary models with non-standard mixing at given metallicities with the Toulouse-Geneva code for a range of stellar masses. We choose the evolutionary model that best fit the low lithium abundances observed in the solar twins. Our best model for each solar twin provides a mass and age solution constrained by their Li content and Teff determination. Li depletion due to the additional mixing in solar-twins is strongly mass dependent. An accurate lithium abundance measurement connected with non-standard models provides a more precise information about the age and mass better than that determined only by classical methods.Proceedings of the International Astronomical Union 07/2009; 5:422 - 423. -
Article: The exoplanet-host star iota Horologii: an evaporated member of the primordial Hyades cluster
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ABSTRACT: We show that the exoplanet-host star iota Horologii, alias HD17051, which belongs to the so-called Hyades stream, was formed within the primordial Hyades stellar cluster and has evaporated towards its present location, 40 pc away. This result has been obtained unambiguously by studying the acoustic oscillations of this star, using the HARPS spectrometer in La Silla Observatory (ESO, Chili). Besides the fact that $\iota$ Hor belongs to the Hyades stream, we give evidence that it has the same metallicity, helium abundance, and age as the other stars of the Hyades cluster. They were formed together, at the same time, in the same primordial cloud. This result has strong implications for theories of stellar formation. It also indicates that the observed overmetallicity of this exoplanet-host star, about twice that of the Sun, is original and not caused by planet accretion during the formation of the planetary system. Comment: to be published in A&A letters03/2008; -
Article: Oscillations in Procyon A: First results from a multi-site campaign
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ABSTRACT: Procyon A is a bright F5IV star in a binary system. Although the distance, mass and angular diameter of this star are all known with high precision, the exact evolutionary state is still unclear. Evolutionary tracks with different ages and different mass fractions of hydrogen in the core pass, within the errors, through the observed position of Procyon A in the Hertzsprung-Russell diagram. For more than 15 years several different groups have studied the solar-like oscillations in Procyon A to determine its evolutionary state. Although several studies independently detected power excess in the periodogram, there is no agreement on the actual oscillation frequencies yet. This is probably due to either insufficient high-quality data (i.e., aliasing) or due to intrinsic properties of the star (i.e., short mode lifetimes). Now a spectroscopic multi-site campaign using 10 telescopes world-wide (minimizing aliasing effects) with a total time span of nearly 4 weeks (increase the frequency resolution) is performed to identify frequencies in this star and finally determine its properties and evolutionary state.11/2007; -
Article: Asteroseismology of α Centauri A. Evidence of rotational splitting
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ABSTRACT: Context: Asteroseismology provides a unique tool for studying stellar interiors. Recently p modes have been detected on the bright solar-like star α Cen A thanks to high-precision radial-velocity measurements. However, a better characterisation of these p modes is clearly needed to constrain theoretical models. Aims: We observed α Cen A during five nights using the HARPS spectrograph in order to improve our knowledge of the seismic properties of this star. Methods: We performed high-precision radial-velocity sequences and computed the acoustic spectrum of α Cen A. Results: We identify 34 p modes with angular degree l=0-3 in the frequency range 1.8-2.9 mHz and amplitude range 13-48 cm s-1, in agreement with previous seismic studies. We find an enhancement of the frequency scatter with the angular degree l that indicates, considering the high inclination axis of α Cen A, rotational splitting and explains the low values of previously suggested mode lifetimes. We also derive new values for the small separations that take the effect of rotational splitting into account. Conclusions: Our seismic study of α Cen A leads to a list of now well-identified p-mode frequencies and shows the importance of taking the rotation into account in order to properly characterise the p modes even in quite short campaigns. Based on observations collected with the HARPS spectrograph at the 3.6-m telescope (La Silla Observatory, ESO, Chile: program 075D-0800A).Astronomy and Astrophysics 06/2007; 470:295-302. · 4.59 Impact Factor -
Article: Asteroseismology of alpha Cen A. Evidence of rotational splitting
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ABSTRACT: Asteroseismology provides a unique tool for studying stellar interiors. Recently p modes have been detected on the bright solar-like star alpha Cen A thanks to high-precision radial-velocity measurements. However a better characterisation of these p modes is clearly needed to constrain theoretical models. We observed alpha Cen A during five nights using the HARPS spectrograph in order to improve our knowledge of the seismic properties of this star. We performed high-precision radial-velocity sequences and computed the acoustic spectrum of alpha Cen A. We identify 34 p modes with angular degree l=0-3 in the frequency range 1.8-2.9 mHz and amplitude range 13-48 cm/s, in agreement with previous seismic studies. We find an enhancement of the frequency scatter with the angular degree l that indicates, considering the high inclination axis of alpha Cen A, rotational splitting and explains the low values of previously suggested mode lifetimes. We also derive new values for the small separations that take the effect of rotational splitting into account . Our seismic study of alpha Cen A leads to a list of now well identified p-mode frequencies and shows the importance of taking the rotation into account in order to properly characterise the p modes even in quite short campaigns. Comment: 10 pages, 9 figures. To be published in A&A06/2007; -
Article: Asteroseismology of α Cen A
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ABSTRACT: Asteroseismology, which consists in measuring properties of p-mode oscillations, provides a unique tool for studying stellar interiors . Recently p modes have been detected on the bright solar-like star α Cen A thanks to high- precision radial- velocity measurements. However a better characterization of p modes is clearly needed to constrain theoretical models. We observed α Cen A during five nights using the high precision spectrograph HARPS in order to improve our knowledge of the seismic properties of this star and to investigate its mode lifetimes and rotational splitting. We obtained high- precision radial- velocity sequences and computed the acoustic spectrum of α Cen A . We estimate the scatter in the frequencies for each angular degree ( ℓ) and analyse it to constrain the rotational splitting and mode lifetimes. We identify 34 p modes with angular degree ℓ = 0 - 3 in the frequency range 1.8 - 2 .9 mHz and amplitude range 13 - 4 8 cms, in agreement with the previous seismic studies. We find a clear enhancement of the frequency scatter with the angular degree ( ℓ) , which indicates rotational splitting and explains the low value of mode lifetimes previously suggested by Kjeldsen et al. (2005). Combining all of the existing seismic measurements, we determine a mode lifetime of days and a rotational rate. We also derive new values for the small separations that take into account the effect of rotational splitting. Our seismic study of α Cen A leads to a list of now well- identified p-mode frequencies and shows the importance of taking into account the rotation in order to properly characterize the p modes even for quite short campaigns.07/2006; 17:26. -
Article: Asteroseismology of the planet-hosting star mu Arae. II. Seismic analysis
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ABSTRACT: As most exoplanets host stars, HD 160691 (alias mu Ara) presents a metallicity excess in its spectrum compared to stars without detected planets. This excess may be primordial, in which case the star would be completely overmetallic, or it may be due to accretion in the early phases of planetary formation, in which case it would be overmetallic only in its outer layers. As discussed in a previous paper (Bazot and Vauclair 2004), seismology can help choosing between the two scenarios. This star was observed during eight nights with the spectrograph HARPS at La Silla Observatory. Forty three p-modes have been identified (Bouchy et al. 2005). In the present paper, we discuss the modelisation of this star. We computed stellar models iterated to present the same observable parameters (luminosity, effective temperature, outer chemical composition) while the internal structure was different according to the two extreme assumptions : original overmetallicity or accretion. We show that in any case the seismic constraints lead to models in complete agreement with the external parameters deduced from spectroscopy and from the Hipparcos parallax (L and Teff). We discuss the tests which may lead to a choice between the two typical scenarios. We show that the ``small separation'' seem to give a better fit for the accretion case than for the overmetallic case, but in spite of the very good data the uncertainties are still too large to conclude. We discuss the observations which would be needed to go further and solve this question.05/2005; -
Article: The HARPS survey for southern extra-solar planets II. A 14 Earth-masses exoplanet around mu Arae
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ABSTRACT: In this letter we present the discovery of a very light planetary companion to the star mu Ara (HD160691). The planet orbits its host once every 9.5days, and induces a sinusoidal radial velocity signal with a semi-amplitude of 4.1 m/s, the smallest Doppler amplitude detected so far. These values imply a mass of m2 sini = 14 earth-masses. This detection represents the discovery of a planet with a mass slightly smaller than that of Uranus, the smallest ``ice giant" in our Solar System. Whether this planet can be considered an ice giant or a super-earth planet is discussed in the context of the core-accretion and migration models. Comment: Replaced on 2004-09-10 by final version, accepted for publication in A&A Letters08/2004; -
Article: Asteroseismology of exoplanets hosts stars : tests of internal metallicity
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ABSTRACT: Exoplanets host stars present a clear metallicity excess compared to stars without detected planets, with an average overabundance of 0.2 dex. This excess may be primordial, in which case the stars should be overmetallic down to their center, or it may be due to accretion in the early phases of planetary formation, in which case the stars would be overmetallic only in their outer layers. In the present paper, we show the differences in the internal structure of stars, according to the chosen scenario. Namely two stars with the same observable parameters (luminosity, effective temperature, outer chemical composition) are completely different in their interiors according to their past histories, which we reconstitute through the computations of their evolutionary tracks. It may happen that stars with an initial overmetallicity have a convective core while the stars which suffered accretion do not. We claim that asteroseismic studies of these exoplanets host stars can give clues about their internal structures and metallicities, which may help in understanding planetary formation. Comment: 24 pages, 18 figures, accepted by A&A07/2004;
Top co-authors
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Institutions
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2009–2010
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University of Porto
Porto, Distrito do Porto, Portugal
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2006
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Aarhus University
- Department of Physics and Astronomy
Aars, Region North Jutland, Denmark
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