arXiv subjects

H. Bruntt

Publications and source records attributed to H. Bruntt.

At least 55 records · Page 3Linked to original sources

arXiv2011

Cepheid investigations using the Kepler space telescope

We report results of initial work done on selected candidate Cepheids to be observed with the Kepler space telescope. Prior to the launch 40 candidates were selected from previous surveys and databases. The analysis of the first 322 days of Kepler photometry, and recent ground-based follow-up multicolour photometry and spectroscopy allowed us to confirm that one of these stars, V1154 Cyg (KIC 7548061), is indeed a 4.9-d Cepheid. Using the phase lag method we show that this star pulsates in the fundamental mode. New radial velocity data are consistent with previous measurements, suggesting that a long-period binary component is unlikely. No evidence is seen in the ultra-precise, nearly uninterrupted Kepler photometry for nonradial or stochastically excited modes at the micromagnitude level. The other candidates are not Cepheids but an interesting mix of possible spotted stars, eclipsing systems and flare stars.

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arXiv2011

Transiting exoplanets from the CoRoT space mission: XIII. CoRoT-14b: an unusually dense very hot Jupiter

In this paper, the CoRoT Exoplanet Science Team announces its 14th discovery. Herein, we discuss the observations and analyses that allowed us to derive the parameters of this system: a hot Jupiter with a mass of $7.6 \pm 0.6$ Jupiter masses orbiting a solar-type star (F9V) with a period of only 1.5 d, less than 5 stellar radii from its parent star. It is unusual for such a massive planet to have such a small orbit: only one other known exoplanet with a higher mass orbits with a shorter period.

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arXiv2010

Asteroseismic modelling of the solar-type subgiant star $β$ Hydri

Comparing models and data of pulsating stars is a powerful way to understand the stellar structure better.βHyi is an evolved solar-type pulsator with mixed modes in its frequency spectrum, making it very interesting for asteroseismic studies.The goal of this work is to search for the best model of the solar-type star βHyi, based on up-to-date non-seismic and seismic data.We present a revised list of frequencies for 33 modes, which we produced by analysing the power spectrum of the published observations again using a new weighting scheme that minimises the daily sidelobes.We ran several grids of evolutionary models with different input parameters and different physics, using the stellar evolutionary code ASTEC.For the models that are inside the observed error box of βHyi, we computed their frequencies with the pulsation code ADIPLS.We used two approaches to find the model that oscillates with the frequencies that are closest to the observed frequencies of βHyi:(i)we assume that the best model is the one that reproduces the star's interior based on the radial oscillation frequencies alone, to which we have applied the correction for the near-surface effects;(ii)we assume that the best model is the one that produces the lowest value of the chi-square (\chi2), i.e. that minimises the difference between the observed frequencies of all available modes and the model predictions, after all model frequencies are corrected for near-surface effects.We show that after applying this correction to the frequencies of the best models, we can reproduce the observed modes well, including the mixed modes.The model that gives the lowest value of the \chi2 is a post-main-sequence model with a mass of 1.04 MSun and a metallicity slightly lower than that of the Sun.Our results underscore the importance of having individual frequencies to constrain the properties of the stellar model.

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arXiv2010

Oscillations and magnetic fields in the G8 star EK Eridani

Asteroseismology can provide information that is otherwise not easily accessible, like the stellar mass and the evolutionary stage. Strong magnetic fields are usually accompanied by rapid rotation, which makes asteroseismology difficult due to spectral line broadening. We have found what may turn out to be the Rosetta Stone of the stars: A slowly rotating solar-like star with a strong magnetic field. We have recently detected solar-like oscillations in this active sub-giant, but with amplitudes much lower than expected. We suggest that the large-scale magnetic field alters the pulsations, which become magneto-acoustic in nature. Here we present our results and discuss possible implications and how this may open up a new frontier in the studies of magnetic fields and stellar evolution.

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arXiv2010

Atmospheric parameters of red giants in the Kepler field

Accurate fundamental parameters of stars are mandatory for the asteroseismic investigation of the Kepler mission to succeed. We will determine the atmospheric parameters for a sample of 6 well-studied bright K giants to confirm that our method produces reliable results. We then apply the same method to 14 K giants that are targets for the Kepler mission. We have used high-resolution, high signal-to-noise spectra from the FIES spectrograph on the Nordic Optical Telescope. We used the iterative spectral synthesis method VWA to derive the fundamental parameters from carefully selected high-quality iron lines and pressure-sensitive Calcium lines. We find good agreement with parameters from the literature for the 6 bright giants. We compared the spectroscopic values with parameters based on photometric indices in the Kepler Input Catalogue (KIC). We identify serious problems with the KIC values for [Fe/H] and find a large RMS scatter of 0.5 dex. The log g values in KIC agree reasonably well with the spectroscopic values with a scatter of 0.25 dex, when excluding two low-metallicity giants. The Teffs from VWA and KIC agree well with a scatter of about 85 K. We also find good agreement with log g and Teff derived from asteroseismic analyses for seven Kepler giant targets. We have determined accurate fundamental parameters of 14 giants using spectroscopic data. The large discrepancies between photometric and spectroscopic values of [Fe/H] emphasize the need for further detailed spectroscopic follow-up of the Kepler targets. This is mandatory to be able to produce reliable constraints for detailed asteroseismic analyses and for the interpretation of possible exo-planet candidates found around giant stars.

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arXiv2010

HD 46375: seismic and spectropolarimetric analysis of a young Sun hosting a Saturn-like planet

HD 46375 is known to host a Saturn-like exoplanet orbiting at 0.04 AU from its host star. Stellar light reflected by the planet was tentatively identified in the 34-day CoRoT run acquired in October-November 2008. We constrain the properties of the magnetic field of HD 46375 based on spectropolarimetric observations with the NARVAL spectrograph at the Pic du Midi observatory. In addition, we use a high-resolution NARVAL flux spectrum to contrain the atmospheric parameters. With these constraints, we perform an asteroseismic analysis and modelling of HD 46375 using the frequencies extracted from the CoRoT light curve. We used Zeeman Doppler imaging to reconstruct the magnetic map of the stellar surface. In the spectroscopic analysis we fitted isolated lines using 1D LTE atmosphere models. This analysis was used to constrain the effective temperature, surface gravity, and chemical composition of the star. To extract information about the p-mode oscillations, we used a technique based on the envelope autocorrelation function (EACF). From the Zeeman Doppler imaging observations, we observe a magnetic field of ~5 gauss. From the spectral analysis, HD 46375 is inferred to be an unevolved K0 type star with high metallicity [Fe/H]=+0.39. Owing to the relative faintness of the star (m_hip=8.05), the signal-to-noise ratio is too low to identify individual modes. However, we measure the p-mode excess power and large separation Delta nu_0=153.0 +/- 0.7 muHz. We are able do constrain the fundamental parameters of the star thanks to spectrometric and seismic analyses. We conclude that HD 46375 is similar to a young version of Alpha-CenB. This work is of special interest because of its combination of exoplanetary science and asteroseismology, which are the subjects of the current Kepler mission and the proposed PLATO mission.

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arXiv2010

Possible detection of phase changes from the non-transiting planet HD 46375b by CoRoT

The present work deals with the detection of phase changes in an exoplanetary system. HD 46375 is a solar analog known to host a non-transiting Saturn-mass exoplanet with a 3.0236 day period. It was observed by the CoRoT satellite for 34 days during the fall of 2008. We attempt to identify at optical wavelengths, the changing phases of the planet as it orbits its star. We then try to improve the star model by means of a seismic analysis of the same light curve and the use of ground-based spectropolarimetric observations. The data analysis relies on the Fourier spectrum and the folding of the time series. We find evidence of a sinusoidal signal compatible in terms of both amplitude and phase with light reflected by the planet. Its relative amplitude is Delta Fp/F* = [13.0, 26.8] ppm, implying an albedo A=[0.16, 0.33] or a dayside visible brightness temperature Tb ~ [1880,2030] K by assuming a radius R=1.1 R_Jup and an inclination i=45 deg. Its orbital phase differs from that of the radial-velocity signal by at most 2 sigma_RV. However, the tiny planetary signal is strongly blended by another signal, which we attribute to a telluric signal with a 1 day period. We show that this signal is suppressed, but not eliminated, when using the time series for HD 46179 from the same CoRoT run as a reference. This detection of reflected light from a non-transiting planet should be confirmable with a longer CoRoT observation of the same field. In any case, it demonstrates that non-transiting planets can be characterized using ultra-precise photometric lightcurves with present-day observations by CoRoT and Kepler. The combined detection of solar-type oscillations on the same targets (Gaulme et al. 2010a) highlights the overlap between exoplanetary science and asteroseismology and shows the high potential of a mission such as Plato.

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arXiv2010

Asteroseismology of red giants from the first four months of Kepler data: Fundamental parameters

Clear power excess in a frequency range typical for solar-type oscillations in red giants has been detected in more than 1000 stars, which have been observed during the first 138 days of the science operation of the NASA Kepler satellite. This sample includes stars in a wide mass and radius range with spectral types G and K, extending in luminosity from the bottom of the giant branch up to high-luminous red giants. The high-precision asteroseismic observations with Kepler provide a perfect source for testing stellar structure and evolutionary models, as well as investigating the stellar population in our Galaxy. We fit a global model to the observed frequency spectra, which allows us to accurately estimate the granulation background signal and the global oscillation parameters, such as the frequency of maximum oscillation power. We find regular patterns of radial and non-radial oscillation modes and use a new technique to automatically identify the mode degree and the characteristic frequency separations between consecutive modes of the same spherical degree. In most cases, we can also measure the small separation. The seismic parameters are used to estimate stellar masses and radii and to place the stars in an H-R diagram by using an extensive grid of stellar models that covers a wide parameter range. Using Bayesian techniques throughout our analysis allows us to determine reliable uncertainties for all parameters. We provide accurate seismic parameters and their uncertainties for a large sample of red giants and determine their asteroseismic fundamental parameters. We investigate the influence of the stars' metallicities on their positions in the H-R diagram. We study the red-giant populations in the red clump and bump and compare them to a synthetic population and find a mass and metallicity gradient in the red clump and clear evidence of a secondary-clump population.

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arXiv2010

A precise asteroseismic age and radius for the evolved Sun-like star KIC 11026764

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. (2010) 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.

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arXiv2010

Age and helium content of the open cluster NGC 6791 from multiple eclipsing binary members. I. Measurements, methods, and first results

Earlier measurements of the masses and radii of the detached eclipsing binary V20 in the open cluster NGC 6791 were accurate enough to demonstrate that there are significant differences between current stellar models. Here we improve on those results and add measurements of two additional detached eclipsing binaries, the cluster members V18 and V80. The enlarged sample sets much tighter constraints on the properties of stellar models than has hitherto been possible, thereby improving both the accuracy and precision of the cluster age. We employed (i) high-resolution UVES spectroscopy of V18, V20 and V80 to determine their spectroscopic effective temperatures, [Fe/H] values, and spectroscopic orbital elements, and (ii) time-series photometry from the Nordic Optical Telescope to obtain the photometric elements. The masses and radii of the V18 and V20 components are found to high accuracy, with errors on the masses in the range 0.27-0.36% and errors on the radii in the range 0.61-0.92%. V80 is found to be magnetically active, and more observations are needed to determine its parameters accurately. The metallicity of NGC 6791 is measured from disentangled spectra of the binaries and a few single stars to be [Fe/H]= +0.29 \pm 0.03 (random) \pm 0.07 (systematic). The cluster reddening and apparent distance modulus are found to be E(B - V) = 0.160 \pm 0.025 and (m - M)V = 13.51 \pm 0.06 . A first model comparison shows that we can constrain the helium content of the NGC 6791 stars, and thus reach a more accurate age than previously possible. It may be possible to constrain additional parameters, in particular the C, N, and O abundances. This will be investigated in paper II.

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arXiv2010

Atmospheric parameters and pulsational properties for a sample of $δ$\,Sct, $γ$\,Dor, and hybrid {\it Kepler} targets

We report spectroscopic observations for 19 $δ$\,Sct candidates observed by the {\it Kepler} satellite both in long and short cadence mode. For all these stars, by using spectral synthesis, we derive the effective temperature, the surface gravity and the projected rotational velocity. An equivalent spectral type classification has been also performed for all stars in the sample. These determinations are fundamental for modelling the frequency spectra that will be extracted from the {\it Kepler} data for asteroseismic inference. For all the 19 stars, we present also periodograms obtained from {\it Kepler} data. We find that all stars show peaks in both low- ($γ$\,Dor; g mode) and high-frequency ($δ$\,Sct; p mode) regions. Using the amplitudes and considering 5\,c/d as a boundary frequency, we classified 3 stars as pure $γ$\,Dor, 4 as $γ$\,Dor\,-\,$δ$\ hybrid, Sct, 5 as $δ$\,Sct\,-\,$γ$\,Dor hybrid, and 6 as pure $δ$\,Sct. The only exception is the star KIC\,05296877 which we suggest could be a binary.

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arXiv2010

Transiting exoplanets from the CoRoT space mission XIV. CoRoT-11b: a transiting massive "hot-Jupiter" in a prograde orbit around a rapidly rotating F-type star

The CoRoT exoplanet science team announces the discovery of CoRoT-11b, a fairly massive hot-Jupiter transiting a V=12.9 mag F6 dwarf star (M*=1.27 +/- 0.05 Msun, R*=1.37 +/- 0.03 Rsun, Teff=6440 +/- 120 K), with an orbital period of P=2.994329 +/- 0.000011 days and semi-major axis a=0.0436 +/- 0.005 AU. The detection of part of the radial velocity anomaly caused by the Rossiter-McLaughlin effect shows that the transit-like events detected by CoRoT are caused by a planet-sized transiting object in a prograde orbit. The relatively high projected rotational velocity of the star (vsini=40+/-5 km/s) places CoRoT-11 among the most rapidly rotating planet host stars discovered so far. With a planetary mass of mp=2.33+/-0.34 Mjup and radius rp=1.43+/-0.03 Rjup, the resulting mean density of CoRoT-11b (rho=0.99+/-0.15 g/cm^3) can be explained with a model for an inflated hydrogen-planet with a solar composition and a high level of energy dissipation in its interior.

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arXiv2010

Transiting exoplanets from the CoRoT space mission. XI. CoRoT-8b: a hot and dense sub-Saturn around a K1 dwarf

We report the discovery of CoRoT-8b, a dense small Saturn-class exoplanet that orbits a K1 dwarf in 6.2 days, and we derive its orbital parameters, mass, and radius. We analyzed two complementary data sets: the photometric transit curve of CoRoT-8b as measured by CoRoT and the radial velocity curve of CoRoT-8 as measured by the HARPS spectrometer. We find that CoRoT-8b is on a circular orbit with a semi-major axis of 0.063 +/- 0.001 AU. It has a radius of 0.57 +/- 0.02 RJ, a mass of 0.22 +/- 0.03 MJ, and therefore a mean density 1.6 +/- 0.1 g/cm^3. With 67 % of the size of Saturn and 72 % of its mass, CoRoT-8b has a density comparable to that of Neptune (1.76 g/cm^3). We estimate its content in heavy elements to be 47-63 Earth masses, and the mass of its hydrogen-helium envelope to be 7-23 Earth masses. At 0.063 AU, the thermal loss of hydrogen of CoRoT-8b should be no more than about 0.1 % over an assumed integrated lifetime of 3~Ga.

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arXiv2010

Transiting exoplanets from the CoRoT space mission. XIII. CoRoT-13b: a dense hot Jupiter in transit around a star with solar metallicity and super-solar lithium content

We announce the discovery of the transiting planet CoRoT-13b. Ground based follow-up in CFHT and IAC80 confirmed CoRoT's observations. The mass of the planet was measured with the HARPS spectrograph and the properties of the host star were obtained analyzing HIRES spectra from the Keck telescope. It is a hot Jupiter-like planet with an orbital period of 4.04 days, 1.3 Jupiter masses, 0.9 Jupiter radii, and a density of 2.34 g cm-3. It orbits a G0V star with Teff=5945K, M*=1.09 Msun, R*=1.01 Rsun, solar metallicity, a lithium content of +1.45 dex, and an estimated age between 0.12 and 3.15 Gyr. The lithium abundance of the star is consistent with its effective temperature, activity level, and age range derived from the stellar analysis. The density of the planet is extreme for its mass. It implies the existence of an amount of heavy elements with a mass between about 140 and 300 Mearth.

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arXiv2010

Transiting exoplanets from the CoRoT space mission. XII. CoRoT-12b: a short-period low-density planet transiting a solar analog star

We report the discovery by the CoRoT satellite of a new transiting giant planet in a 2.83 days orbit about a V=15.5 solar analog star (M_* = 1.08 +- 0.08 M_sun, R_* = 1.1 +- 0.1 R_sun, T_eff = 5675 +- 80 K). This new planet, CoRoT-12b, has a mass of 0.92 +- 0.07 M_Jup and a radius of 1.44 +- 0.13 R_Jup. Its low density can be explained by standard models for irradiated planets.

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arXiv2010

Solar-like oscillations in cluster stars

We present a brief overview of the history of attempts to obtain a clear detection of solar-like oscillations in cluster stars, and discuss the results on the first clear detection, which was made by the Kepler Asteroseismic Science Consortium (KASC) Working Group 2.

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arXiv2010

CoRoT-10b: a giant planet in a 13.24 day eccentric orbit

The space telescope CoRoT searches for transiting extrasolar planets by continuously monitoring the optical flux of thousands of stars in several fields of view. We report the discovery of CoRoT-10b, a giant planet on a highly eccentric orbit (e=0.53 +/- 0.04) revolving in 13.24 days around a faint (V=15.22) metal-rich K1V star. We use CoRoT photometry, radial velocity observations taken with the HARPS spectrograph, and UVES spectra of the parent star to derive the orbital, stellar and planetary parameters. We derive a radius of the planet of 0.97 +/- 0.07 R_Jup and a mass of 2.75 +/- 0.16 M_Jup. The bulk density, rho_pl=3.70 +/- 0.83 g/cm^3, is ~2.8 that of Jupiter. The core of CoRoT-10b could contain up to 240 M_Earth of heavy elements. Moving along its eccentric orbit, the planet experiences a 10.6-fold variation in insolation. Owing to the long circularisation time, tau_circ > 7 Gyr, a resonant perturber is not required to excite and maintain the high eccentricity of CoRoT-10b.

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arXiv2010

Improved stellar parameters of CoRoT-7

Accurate parameters of the host stars of exoplanets are important for the interpretation of the new planet systems that continue to emerge. The CoRoT satellite recently discovered a transiting rocky planet with a density similar to the inner planets in our solar system, a so-called Super Earth. This planet is orbiting a relatively faint G9V star called CoRoT-7, and we wish to refine its physical properties, which are important for the interpretation of the properties of the planet system. We used spectra from HARPS@ESO-3.6m and UVES@VLT-8.2m. From the analysis of Fe-1 and Fe-2 lines we determine Teff, log g and microturbulence. We use the Balmer lines to constrain Teff and pressure sensitive Mg-1b and Ca lines to constrain log g. From the analysis we find Teff=5250+-60K, log g = 4.47+-0.05, [M/H]=+0.12+-0.06, and vsini = 1.1 km/s. We compared the L/M ratio with isochrones to constrain the evolutionary status. Using the age estimate of 1.2-2.3 Gyr based on stellar activity, we determine the mass and radius 0.91+-0.03 Msun and 0.82+-0.04 Rsun. With these updated constraints we fitted the CoRoT transit light curve for CoRoT-7b. We revise the planet radius to be slightly smaller, R = 1.58+-0.10 Rearth, and the density becomes higher, rho = 7.2+-1.8 g/cm3. The host star CoRoT-7 is a slowly rotating, metal rich, unevolved type G9V star. The star is relatively faint (V=11.7) and its fundamental parameters can only be determined through indirect methods. Our methods rely on detailed spectral analyses that depend on the adopted model atmospheres. From the analysis of spectra of stars with well-known parameters with similar parameters to CoRoT-7 (the Sun and alpha Cen B) we demonstrate that our methods are robust within the claimed uncertainties. Therefore our methods can be reliably used in subsequent analyses of similar exoplanet host stars.

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