A. Hatzes

Basic physical parameters of a selected sample of evolved stars

We present the detailed spectroscopic analysis of 72 evolved stars, which were previously studied for accurate radial velocity variations. Using one Hyades giant and another well studied star as the reference abundance, we determine the [Fe/H] for the whole sample. These metallicities, together with the Teff values and the absolute V-band magnitude derived from Hipparcos parallaxes, are used to estimate basic stellar parameters (ages, masses, radii, (B−V)0 and log g) using theoretical isochrones and a Bayesian estimation method. The (B−V)0 values so estimated turn out to be in excellent agreement (to within ∼0.05 mag) with the observed (B−V), confirming the reliability of the Teff−(B−V)0 relation used in the isochrones. On the other hand, the estimated log g values are typically 0.2 dex lower than those derived from spectroscopy; this effect has a negligible impact on [Fe/H] determinations. The estimated diameters θ have been compared with limb darkening-corrected ones measured with independent methods, finding an agreement better than 0.3 mas within the 1 <θ< 10 mas interval (or, alternatively, finding mean differences of just 6%). We derive the age-metallicity relation for the solar neighborhood; for the first time to our knowledge, such a relation has been derived from observations of field giants rather than from open clusters and field dwarfs and subdwarfs. The age-metallicity relation is characterized by close-to-solar metallicities for stars younger than ∼4 Gyr, and by a large [Fe/H] spread with a trend towards lower metallicities for higher ages. In disagreement with other studies, we find that the [Fe/H] dispersion of young stars (less than 1 Gyr) is comparable to the observational errors, indicating that stars in the solar neighbourhood are formed from interstellar matter of quite homogeneous chemical composition. The three giants of our sample which have been proposed to host planets are not metal rich; this result is at odds with those for main sequence stars. However, two of these stars have masses much larger than a solar mass so we may be sampling a different stellar population from most radial velocity searches for extrasolar planets. We also confirm the previous indication that the radial velocity variability tends to increase along the RGB, and in particular with the stellar radius.

The CoRoT-7 planetary system: two orbiting super-Earths

We report on an intensive observational campaign carried out with HARPS at the 3.6 m telescope at La Silla on the star CoRoT-7. Additional simultaneous photometric measurements carried out with the Euler Swiss telescope have demonstrated that the observed radial velocity variations are dominated by rotational modulation from cool spots on the stellar surface. Several approaches were used to extract the radial velocity signal of the planet(s) from the stellar activity signal. First, a simple pre-whitening procedure was employed to find and subsequently remove periodic signals from the complex frequency structure of the radial velocity data. The dominant frequency in the power spectrum was found at 23 days, which corresponds to the rotation period of CoRoT-7. The 0.8535 day period of CoRoT-7b planetary candidate was detected with an amplitude of 3.3 m s −1 . Most other frequencies, some with amplitudes larger than the CoRoT-7b signal, are most likely associated with activity. A second approach used harmonic decomposition of the rotational period and up to the first three harmonics to filter out the activity signal from radial velocity variations caused by orbiting planets. After correcting the radial velocity data for activity, two periodic signals are detected: the CoRoT-7b transit period and a second one with a period of 3.69 days and an amplitude of 4 m s −1 . This second signal was also found in the pre-whitening analysis. We attribute the second signal to a second, more remote planet CoRoT-7c . The orbital solution of both planets is compatible with circular orbits. The mass of CoRoT-7b is 4.8 ± 0. 8( M⊕) and that of CoRoT-7c is 8.4 ± 0. 9( M⊕), assuming both planets are on coplanar orbits. We also investigated the false positive scenario of a blend by a faint stellar binary, and this may be rejected by the stability of the bisector on a nightly scale. According to their masses both planets belong to the super-Earth planet category. The average density of CoRoT-7b is ρ = 5.6 ± 1. 3gc m −3 , similar to the Earth. The CoRoT-7 planetary system provides us with the first insight into the physical nature of short period super-Earth planets recently detected by radial velocity surveys. These planets may be denser than Neptune and therefore likely made of rocks like the Earth, or a mix of water ice and rocks.

Transiting exoplanets from the CoRoT space mission - XXIV. CoRoT-25b and CoRoT-26b: two low-density giant planets

We report the discovery of two transiting exoplanets, CoRoT-25b and CoRoT-26b, both of low density, one of which is in the Saturn mass-regime. For each star, ground-based complementary observations through optical photometry and radial velocity measurements secured the planetary nature of the transiting body and allowed us to fully characterize them. For CoRoT-25b we found a planetary mass of 0.27 similar to 0.04 M-Jup, a radius of 1.08(-0.10)(+0.3) R-Jup and hence a mean density of 0.15(-0.06)(+ 0.15) g cm(-3). The planet orbits an F9 mainsequence star in a 4.86-day period, that has a V magnitude of 15.0, solar metallicity, and an age of 4.5(-2.0) (+1.8)-Gyr. CoRoT-26b orbits a slightly evolved G5 star of 9.06 +/- 1.5-Gyr age in a 4.20-day period that has solar metallicity and a V magnitude of 15.8. With a mass of 0.52 +/- 0.05 MJup, a radius of 1.26(-0.07)(+0.13) R-Jup, and a mean density of 0.28(-0.07)(+0.09) g cm(-3), it belongs to the low-mass hot-Jupiter population. Planetary evolution models allowed us to estimate a core mass of a few tens of Earth mass for the two planets with heavy-element mass fractions of 0.52(-0.15)(+0.08) and 0.26(-0.08)(+0.05), respectively, assuming that a small fraction of the incoming flux is dissipated at the center of the planet. In addition, these models indicate that CoRoT-26b is anomalously large compared with what standard models could account for, indicating that dissipation from stellar heating could cause this size.

Solar-like Oscillations in Low-luminosity Red Giants: First Results from Kepler

We have measured solar-like oscillations in red giants using time-series photometry from the first 34 days of science operations of the Kepler Mission. The light curves, obtained with 30 minute sampling, reveal clear oscillations in a large sample of G and K giants, extending in luminosity from the red clump down to the bottom of the giant branch. We confirm a strong correlation between the large separation of the oscillations (Δν) and the frequency of maximum power (νmax). We focus on a sample of 50 low-luminosity stars (νmax > 100 μHz, L <~ 30 L sun) having high signal-to-noise ratios and showing the unambiguous signature of solar-like oscillations. These are H-shell-burning stars, whose oscillations should be valuable for testing models of stellar evolution and for constraining the star formation rate in the local disk. We use a new technique to compare stars on a single echelle diagram by scaling their frequencies and find well-defined ridges corresponding to radial and non-radial oscillations, including clear evidence for modes with angular degree l = 3. Measuring the small separation between l = 0 and l = 2 allows us to plot the so-called C-D diagram of δν02 versus Δν. The small separation δν01 of l = 1 from the midpoint of adjacent l = 0 modes is negative, contrary to the Sun and solar-type stars. The ridge for l = 1 is notably broadened, which we attribute to mixed modes, confirming theoretical predictions for low-luminosity giants. Overall, the results demonstrate the tremendous potential of Kepler data for asteroseismology of red giants.

Transiting exoplanets from the CoRoT space mission II. CoRoT-Exo-2b: A transiting planet around an active G star

Context. The CoRoT mission, a pioneer in exoplanet searches from space, has completed its first 150 days of continuous observations of ∼12 000 stars in the galactic plane. An analysis of the raw data identifies the most promising candidates and triggers the ground-based follow-up. Aims. We report on the discovery of the transiting planet CoRoT-Exo-2b, with a period of 1.743 days, and characterize its main parameters. Methods. We filter the CoRoT raw light curve of cosmic impacts, orbital residuals, and low frequency signals from the star. The folded light curve of 78 transits is fitted to a model to obtain the main parameters. Radial velocity data obtained with the SOPHIE, CORALIE and HARPS spectrographs are combined to characterize the system. The 2.5 min binned phase-folded light curve is affected by the effect of sucessive occultations of stellar active regions by the planet, and the dispersion in the out of transit part reaches a level of 1.09 × 10 −4 in flux units. Results. We derive a radius for the planet of 1.465 ± 0.029 RJup and a mass of 3.31 ± 0.16 MJup, corresponding to a density of 1.31 ± 0.04 g/cm 3 . The large radius of CoRoT-Exo-2b cannot be explained by current models of evolution of irradiated planets.

Transiting exoplanets from the CoRoT space mission I - CoRoT-Exo-1b: a low-density short-period planet around a G0V star

Context. The pioneer space mission for photometric planet searches, CoRoT, steadily monitors about 12,000 stars in each of its fields of view; it is able to detect transit candidates early in the processing of the data and before the end of a run. Aims. We report the detection of the first planet discovered by CoRoT and characterizing it with the help of follow-up observations. Methods. Raw data were filtered from outliers and residuals at the orbital period of the satellite. The orbital parameters and the radius of the planet were estimated by best fitting the phase folded light curve with 34 successive transits. Doppler measurements with the SOPHIE spectrograph permitted us to secure the detection and to estimate the planet mass. Results. The accuracy of the data is very high with a dispersion in the 2.17 min binned phase-folded light curve that does not exceed 3.10-4 in flux unit. The planet orbits a mildly metal-poor G0V star of magnitude V=13.6 in 1.5 days. The estimated mass and radius of the star are 0.95+-0.15Msun and 1.11+-0.05Rsun. We find the planet has a radius of 1.49+-0.08Rjup, a mass of 1.03+-0.12Mjup, and a particularly low mean density of 0.38 +-0.05g cm-3.

Improved stellar parameters of CoRoT-7 A star hosting two super Earths

Context. Accurate parameters of the host stars of exoplanets are needed for interpreting the new planetary systems. 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. The mass was determined using ground-based follow-up spectroscopy, which also revealed a second, non-transiting super Earth. Aims. These planets are orbiting a relatively faint (m(V) = 11.7) G9V star called CoRoT-7. We wish to refine the determination of the physical properties of the host star, which are important for the interpretation of the properties of the planetary system. Methods. We used high-quality spectra from HARPS at the ESO3.6 m and UVES at the VLT 8.2 m. We used various methods to analyse the spectra using 1D LTE atmospheric models. From the analysis of Fe I and Fe II lines we determined the effective temperature, surface gravity and microturbulence. We used the Balmer lines to constrain the effective temperature and pressure-sensitive Mg 1b and Ca lines to constrain the surface gravity. We analysed both single spectra and co-add spectra to identify systematic errors. We determine the projected rotational velocity and macroturbulence by fitting the line shapes of isolated lines. We finally determined an approximate absolute magnitude from the Wilson-Bappu effect. Results. From the analysis of the three best spectra, we find T(eff) = 5250 +/- 60 K, log g = 4.47 +/- 0.05, [M/H] = +0.12 +/- 0.06, and v sin i = 1.1(0.5)(+1.0) km s(-1). The chemical composition of 20 analysed elements is consistent with uniform scaling by the metallicity +0.12 dex. 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. 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 M(circle dot) and 0.82 +/- 0.04 R(circle plus). With these updated constraints we fitted the CoRoT transit light curve for CoRoT-7b. The revised planet radius is slightly smaller, R = 1.58 +/- 0.10 R(circle plus), and the density becomes slightly higher, rho = 7.2 +/- 1.8 g cm(-3).

A giant planet around the massive giant star HD 13189

Most extrasolar planet discoveries using radial velocity measurements have been for solar-like G-stars. In order to understand better the role stellar mass for the formation of planets we must learn more about the frequency of planetary companions around both high- and low-mass stars. Radial velocity searches for planets around high mass main-sequence stars are difficult due to the paucity of lines and often rapid rotation of these early-type stars. On the other hand, evolved stars that have moved off the main sequence offer us the possibility of searching for planets around higher mass stars by means of precise radial velocity measurements. Here we present radial velocity measurements for the star HD 13189 using measurements taken at the Thuringer Landessternwarte Tautenburg, the Harlan J. Smith Telescope at McDonald Observatory, and the Hobby-Eberly Telescope. We classify the spectral type of this star as K2 with luminosity class II. The radial velocity measurements show long-period variations with a period of 472 days and an amplitude of 173 m s −1 . The Ca II S-index is consistent with an inactive star and this shows no variations with the radial velocity period. We also investigated possible changes in the line shapes by measuring spectral line bisectors. These show no variations with the radial velocity period. We interpret the 472-day period as being caused by a sub-stellar companion. Based on the estimated absolute magnitude and a comparison to evolutionary tracks we estimate the mass of the progenitor star between 2 and 7 Mwhich results in a projected mass of the companion of m sin i = 8-20 MJ. HD 13189 may be the most massive star known to possess an extrasolar planet. This suggests that the formation of giant planets can also occur around early-type stars. HD 13189 also shows significant short term radial velocity variability on time scales of days that is most likely due to stellar oscillations. This behavior is typical for K giant stars.

Characteristics of solar-like oscillations in red giants observed in the CoRoT ⋆ exoplanet field

Context. Observations during the first long run (∼150 days) in the exo-planet field of CoRoT increase the number of G-K giant stars for which solar-like oscillations are observed by a factor of 100. This opens the possibility to study the characteristics of their oscillations in a statistical sense. Aims. We aim to understand the statistical distribution of the frequencies of maximum oscillation power (νmax) in red giants and to search for a possible correlation between νmax and the large separation (Δν). Methods. Red giants with detectable solar-like oscillations are identified using both semi-automatic and manual procedures. For these stars, we determine νmax as the centre of a Gaussian fit to the oscillation power excess. For the determination of Δν ,w e use the autocorrelation of the Fourier spectra, the comb response function and the power spectrum of the power spectrum. Results. The resulting νmax distribution shows a pronounced peak between 20−40 μHz. For about half of the stars we obtain Δν with at least two methods. The correlation between νmax and Δν follows the same scaling relation as inferred for solar-like stars. Conclusions. The shape of the νmax distribution can partly be explained by granulation at low frequencies and by white noise at high frequencies, but the population density of the observed stars turns out to be also an important factor. From the fact that the correlation between Δν and νmax for red giants follows the same scaling relation as obtained for sun-like stars, we conclude that the sound travel time over the pressure scale height of the atmosphere scales with the sound travel time through the whole star irrespective of evolution. The fraction of stars for which we determine Δν does not correlate with νmax in the investigated frequency range, which confirms theoretical predictions.

The Extrasolar Planet ϵ Eridani b: Orbit and Mass*

Hubble Space Telescope (HST) observations of the nearby (3.22 pc) K2 V star Eridani have been combined with ground-based astrometric and radial velocity data to determine the mass of its known companion. We model the astrometric and radial velocity measurements simultaneously to obtain the parallax, proper motion, perturbation period, perturbation inclination, and perturbation size. Because of the long period of the companion, Eri b, we extend our astrometric coverage to a total of 14.94 yr (including the 3 yr span of the HST data) by including lower precision ground-based astrometry from the Allegheny Multichannel Astrometric Photometer. Radial velocities now span 1980.8-2006.3. We obtain a perturbation period, P = 6.85 ± 0.03 yr, semimajor axis α = 1.88 ± 0.20 mas, and inclination i = 301 ± 38. This inclination is consistent with a previously measured dust disk inclination, suggesting coplanarity. Assuming a primary mass M* = 0.83 M⊙, we obtain a companion mass M = 1.55MJ ± 0.24MJ. Given the relatively young age of Eri (~800 Myr), this accurate exoplanet mass and orbit can usefully inform future direct-imaging attempts. We predict the next periastron at 2007.3 with a total separation ρ = 03 at position angle P.A. = -27°. Orbit orientation and geometry dictate that Eri b will appear brightest in reflected light very nearly at periastron. Radial velocities spanning over 25 yr indicate an acceleration consistent with a Jupiter-mass object with a period in excess of 50 yr, possibly responsible for one feature of the dust morphology, the inner cavity.