G. Catanzaro

Spectroscopic survey of Kepler stars. I. HERMES/Mercator observations of A- and F-type stars

The Kepler space mission provided near-continuous and high-precision photometry of about 207 000 stars, which can be used for asteroseismology. However, for successful seismic modeling it is equally important to have accurate stellar physical parameters. Therefore, supplementary ground-based data are needed. We report the results of the analysis of high-resolution spectroscopic data of A- and F-type stars from the Kepler field, which were obtained with the HERMES spectrograph on the Mercator telescope. We determined spectral types, atmospheric parameters and chemical abundances for a sample of 117 stars. Hydrogen Balmer, Fe I, and Fe II lines were used to derive effective temperatures, surface gravities, and microturbulent velocities. We determined chemical abundances and projected rotational velocities using a spectrum synthesis technique. The atmospheric parameters obtained were compared with those from the Kepler Input Catalogue (KIC), confirming that the KIC effective temperatures are underestimated for A stars. Effective temperatures calculated by spectral energy distribution fitting are in good agreement with those determined from the spectral line analysis. The analysed sample comprises stars with approximately solar chemical abundances, as well as chemically peculiar stars of the Am, Ap, and λ Boo types. The distribution of the projected rotational velocity, vsini, is typical for A and F stars and ranges from 8 to about 280 km s −1 , with a mean of 134 km s −1 .

The excitation of solar-like oscillations in a δ Sct star by efficient envelope convection

Delta Scuti (δ Sct) stars are opacity-driven pulsators with masses of 1.5–2.5M⊙, their pulsations resulting from the varying ionization of helium. In less massive stars such as the Sun, convection transports mass and energy through the outer 30 per cent of the star and excites a rich spectrum of resonant acoustic modes. Based on the solar example, with no firm theoretical basis, models predict that the convective envelope in δ Sct stars extends only about 1 per cent of the radius, but with sufficient energy to excite solar-like oscillations. This was not observed before the Kepler mission, so the presence of a convective envelope in the models has been questioned. Here we report the detection of solar-like oscillations in the δ Sct star HD 187547, implying that surface convection operates efficiently in stars about twice as massive as the Sun, as the ad hoc models predicted.

The Dwarf project: Eclipsing binaries – precise clocks to discover exoplanets

We present a new observational campaign, Dwarf, aimed at detection of circumbinary extrasolar planets using the timing of the minima of low-mass eclipsing binaries. The observations will be performed within an extensive network of relatively small to medium-size telescopes with apertures of similar to 20-200 cm. The starting sample of the objects to be monitored contains (i) low-mass eclipsing binaries with M and K components, (ii) short-period binaries with a sdB or sdO component, and (iii) post-common-envelope systems containing a WD, which enable to determine minima with high precision. Since the amplitude of the timing signal increases with the orbital period of an invisible third component, the timescale of the project is long, at least 5-10 years. The paper gives simple formulas to estimate the suitability of individual eclipsing binaries for the circumbinary planet detection. Intrinsic variability of the binaries (photospheric spots, flares, pulsation etc.) limiting the accuracy of the minima timing is also discussed. The manuscript also describes the best observing strategy and methods to detect cyclic timing variability in the minima times indicating the presence of circumbinary planets. First test observations of the selected targets are presented ((c) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Atmospheric parameters and pulsational properties for a sample of δ Sct, γ Dor and hybrid Kepler targets

We report spectroscopic observations for 19 δ Sct candidates observed by the 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 Kepler data for asteroseismic inference. For all the 19 stars, we also present periodograms obtained from 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 cycles d −1 as a boundary frequency, we classified three stars as pure γ Dor, four as γ Dor–δ hybrid Sct, five as δ Sct–γ Dor hybrid and six as pure δ Sct. The only exception is the star KIC 05296877, which we suggest could be a binary.

Balmer lines as Teff and log g indicators for non-solar composition atmospheres An application to the extremely helium-weak star HR6000

Although the importance of a correct abundance assumption in the determination of effective temperature and surface gravity of a star has been demonstrated in the literature, this determination is often still extremely simplified, neglecting the effects of non-solar chemical abundances. In this paper we show how the modeling of the profiles of Hδ and Hγ, commonly used as Teff and log g indicators, is affected when the chemical composition is far from the standard one. As a target for our study we selected the chemically peculiar star HR 6000. Comparing the observed and synthetic profiles of Hδ and Hγ we obtained Teff = 12 950 K and log g = 4.05; the atmospheric model has been computed with a metal opacity scale evaluated for (M/H) = −0.5 and He/H = 0. A number of FeII lines have been used to infer the rotational velocity (ve sini = 0k m s −1 )a nd the heliocentric radial velocity (RV = 0.67 km s −1 ). By requiring that the abundance of iron is independent of the 96 measured equivalent widths, we determined the microturubulence velocity (ξ = 0k m s −1 ). The abundance pattern coming from our study is similar to the one inferred from UV lines by Castelli et al. (1985), with the exception of O, Al, Si, Sc and Ni. Possible causes for these descrepancies are discussed. With respect to the Sun, we found the iron peak elements to be normal or overabundant and the light elements, with the exception of Na and P, to be extremely underabundant. We find that HR 6000 is one of the most He-underabundant among the chemically peculiar stars.

LAMOST OBSERVATIONS IN THE KEPLER FIELD. I. DATABASE OF LOW-RESOLUTION SPECTRA*

The nearly continuous light curves with micromagnitude precision provided by the space mission Kepler are revolutionising our view of pulsating stars. They have revealed a vast sea of low-amplitude pulsation modes that were undetectable from Earth. The long time base of Kepler light curves allows an accurate determination of frequencies and amplitudes of pulsation modes needed for in-depth asteroseismic modeling. However, for an asteroseismic study to be successful, the first estimates of stellar parameters need to be known and they can not be derived from the Kepler photometry itself. The Kepler Input Catalog (KIC) provides values for the effective temperature, the surface gravity and the metallicity, but not always with a sufficient accuracy. Moreover, information on the chemical composition and rotation rate is lacking. We are collecting low-resolution spectra for objects in the Kepler field of view with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, Xinglong observatory, China). All of the requested fields have now been observed at least once. In this paper we describe those observations and provide a database of use to the whole astronomical community.

Activity indicators and stellar parameters of the Kepler targets - An application of the ROTFIT pipeline to LAMOST-Kepler stellar spectra

Aims. A comprehensive and homogeneous determination of stellar parameters for the stars observed by the Kepler space telescope is necessary for statistical studies of their properties. As a result of the large number of stars monitored by Kepler , the largest and more complete databases of stellar parameters published to date are multiband photometric surveys. The LAMOST- Kepler survey, whose spectra are analyzed in the present paper, was the first large spectroscopic project, which started in 2011 and aimed at filling that gap. In this work we present the results of our analysis, which is focused on selecting spectra with emission lines and chromospherically active stars by means of the spectral subtraction of inactive templates. The spectroscopic determination of the atmospheric parameters for a large number of stars is a by-product of our analysis. Methods. We have used a purposely developed version of the code ROTFIT for the determination of the stellar parameters by exploiting a wide and homogeneous collection of real star spectra, namely the Indo US library. We provide a catalog with the atmospheric parameters ( T eff , log  g , and [Fe/H]), radial velocity (RV), and an estimate of the projected rotation velocity ( v sin i ). For cool stars ( T eff ≤ 6000 K), we also calculated the H α and Ca ii-IRT fluxes, which are important proxies of chromospheric activity. Results. We have derived the RV and atmospheric parameters for 61 753 spectra of 51 385 stars. The average uncertainties, which we estimate from the stars observed more than once, are about 12 km s -1 , 1.3%, 0.05 dex, and 0.06 dex for RV, T eff , log  g , and [Fe/H], respectively, although they are larger for the spectra with a very low signal-to-noise ratio. Literature data for a few hundred stars (mainly from high-resolution spectroscopy) were used to peform quality control of our results. The final accuracy of the RV is about 14 km s -1 . The accuracy of the T eff , log  g , and [Fe/H] measurements is about 3.5%, 0.3 dex, and 0.2 dex, respectively. However, while the T eff values are in very good agreement with the literature, we noted some issues with the determination of [Fe/H] of metal poor stars and the tendency, for log  g , to cluster around the values typical for main-sequence and red giant stars. We propose correction relations based on these comparisons and we show that this does not have a significant effect on the determination of the chromospheric fluxes. The RV distribution is asymmetric and shows an excess of stars with negative RVs that are larger at low metallicities. Despite the rather low LAMOST resolution, we were able to identify interesting and peculiar objects, such as stars with variable RV, ultrafast rotators, and emission-line objects. Based on the H α and Ca ii-IRT fluxes, we found 442 chromospherically active stars, one of which is a likely accreting object. The availability of precise rotation periods from the Kepler photometry allowed us to study the dependency of these chromospheric fluxes on the rotation rate for a very large sample of field stars.

Orbital solutions for SB2 systems with a HgMn component

From a new set of spectroscopic observations we determined orbital parameters of six SB2 systems with one or both components being HgMn stars. We slightly refined the orbital periods for HD 32964, HD 173524, HD 174933 and HD 216494. Our results for HD 358 are in agreement with the previous literature studies. Regarding HD 33647, our orbital period is shorter than previous determinations. HD 173524 is a triple system. From the variations of the γ-velocity deduced from our and literature data, we refined the orbital period and we estimated the eccentricity of the third companion, for which we get e ≈ 0.13 and Porb = 36 ± 3 years. HD 191110 and HD 216494 seem to be the only synchronous SB2 within our sample. Following the hypothesis of rotational axes perpendicular to the orbital plane, we estimated the angle of the orbital plane to the line of sight. We identified in our spectra the signature of the Hβ of the third component of HD 216494 and we highlight a slight variation of the γ-velocity due to the presence of this component.

Spots on Am stars

We investigate the light variations of 15 Am stars using four years of high-precision photometry from the Kepler spacecraft and an additional 14 Am stars from the K2 Campaign 0 field. We find that most of the Am stars in the Kepler field have light curves characteristic of rotational modulation due to star spots. Of the 29 Am stars observed, 12 are {\delta} Scuti variables and one is a {\gamma} Doradus star. One star is an eclipsing binary and another was found to be a binary from time-delay measurements. Two Am stars show evidence for flares which are unlikely to be due to a cool companion. The fact that 10 out of 29 Am stars are rotational variables and that some may even flare strongly suggests that Am stars possess significant magnetic fields. This is contrary to the current understanding that the enhanced metallicity in these stars is due to diffusion in the absence of a magnetic field. The fact that so many stars are {\delta} Scuti variables is also at odds with the prediction of diffusion theory. We suggest that a viable alternative is that the metal enhancement could arise from accretion.

Asteroseismology of solar-type stars with Kepler: III. Ground-based data

We report on the ground-based follow-up program of spectroscopic and photometric observations of solar-like asteroseismic targets for the Kepler space mission. These stars constitute a large group of more than a thousand objects which are the subject of an intensive study by the Kepler Asteroseismic Science Consortium Working Group 1 (KASC WG-1). In the current work we will discuss the methods we use to determine the fundamental stellar atmospheric parameters using high-quality stellar spectra. These provide essential constraints for the asteroseismic modelling and make it possible to verify the parameters in the Kepler Input Catalogue (KIC) (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)