B. Montesinos

EXPORT: Spectral classification and projected rotational velocities of Vega-type and pre-main sequence stars

In this paper we present the rst comprehensive results extracted from the spectroscopic campaigns car- ried out by the EXPORT (EXoPlanetary Observational Research Team) consortium. During 1998{1999, EXPORT carried out an intensive observational eort in the framework of the origin and evolution of protoplanetary sys- tems in order to obtain clues on the evolutionary path from the early stages of the pre-main sequence to stars with planets already formed. The spectral types of 70 stars, and the projected rotational velocities, v sini ,o f 45 stars, mainly Vega-type and pre-main sequence, have been determined from intermediate- and high-resolution spectroscopy, respectively. The rst part of the work is of fundamental importance in order to accurately place the stars in the HR diagram and determine the evolutionary sequences; the second part provides information on the kinematics and dynamics of the stars and the evolution of their angular momentum. The advantage of using the same observational conguration and methodology for all the stars is the homogeneity of the set of pa- rameters obtained. Results from previous work are revised, leading in some cases to completely new determinations of spectral types and projected rotational velocities; for some stars no previous studies were available.

DUst around NEarby Stars. The survey observational results

Context. Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar system counterparts are the asteroid and Edgeworth-Kuiper belts. Aims. The DUNES survey aims at detecting extra-solar analogues to the Edgeworth-Kuiper belt around solar-type stars, putting in this way the solar system into context. The survey allows us to address some questions related to the prevalence and properties of planetesimal systems. Methods. We used Herschel/PACS to observe a sample of nearby FGK stars. Data at 100 and 160 mu m were obtained, complemented in some cases with observations at 70 mu m, and at 250, 350 and 500 mu m using SPIRE. The observing strategy was to integrate as deep as possible at 100 mu m to detect the stellar photosphere. Results. Debris discs have been detected at a fractional luminosity level down to several times that of the Edgeworth-Kuiper belt. The incidence rate of discs around the DUNES stars is increased from a rate of similar to 12.1% +/- 5% before Herschel to similar to 20.2% +/- 2%. A significant fraction (similar to 52%) of the discs are resolved, which represents an enormous step ahead from the previously known resolved discs. Some stars are associated with faint far-IR excesses attributed to a new class of cold discs. Although it cannot be excluded that these excesses are produced by coincidental alignment of background galaxies, statistical arguments suggest that at least some of them are true debris discs. Some discs display peculiar SEDs with spectral indexes in the 70-160 mu m range steeper than the Rayleigh-Jeans one. An analysis of the debris disc parameters suggests that a decrease might exist of the mean black body radius from the F-type to the K-type stars. In addition, a weak trend is suggested for a correlation of disc sizes and an anticorrelation of disc temperatures with the stellar age.

On the simultaneous optical and near-infrared variability of pre-main sequence stars

For a complete understanding of the physical processes causing the photometric variability of pre-main sequence systems, simultaneous optical and near-IR observations are required to disentangle the emission from the stars and that from their associated circumstellar disks. Data of this sort are extremely rare and little systematic work has been reported to date. The work presented in this paper is a systematic attempt in this direction. It presents an analysis of the simultaneous optical and near-IR photometric variability of 18 Herbig Ae/Be and T Tauri stars which were observed in October 98 by the EXPORT collaboration. The time dierence between the UBVRI and JHK measurements is less than 1 hour in50% of the data and the largest dierence is around 2 hours in only10% of the data. Twelve stars appear to show a correlation between the optical and near-IR variability trends, which suggests a common physical origin such as spots and/or variable extinction. The optical and near-IR variability is uncorrelated in the rest of the objects, which suggests it originates in distinctly dierent regions. In general, the optical variability qualitatively follows the predictions of starspots or variable extinction. As far as the near-IR is concerned, the simultaneity of the observations demonstrates that for most objects the flux is largely produced by their circumstellar disks and, consequently, in many cases the near-IR fluctuations must be attributed to structural variations of such disks producing variations of their thermal emission and/or scattered light. The observed near-IR changes of up to around 1 mag on timescales of 1{2 days provide interesting challenges for understanding the mechanisms generating such remarkable variabilities, an issue insuciently investigated until now but one which deserves further theoretical and modeling eorts.

Parameters of Herbig Ae/Be and Vega-type stars ⋆

Context. This work presents the characterization of 27 young early-type stars, most of them in the age range 1‐10 Myr, and three ‐suspected‐ hot companions of post-T Tauri stars belonging to the Lindroos binary sample. Most of these objects show IR excesses in their spectral energy distributions, which are indicati ve of the presence of disks. The work is relevant in the fields o f stellar physics, physics of disks and formation of planetary systems. Aims. The aim of the work is the determination of the effective temperature, gravity, metallicity, mass, luminosi ty and age of these stars. An accurate modelling of their disks needs, as a previ ous step, the knowledge of most of these parameters, since they will determine the energy input received by the disk and hence, its geometry and global properties. Methods. Spectral energy distributions and mid-resolution spectra were used to estimate Teff, the effective temperature. The comparison of the profiles of the Balmer lines with synthetic pro files provides the value of the stellar gravity, g∗. High-resolution optical observations and synthetic spectra are used to estimate the metallicity, [M/H]. Once Teff, g∗ and [M/H] are known for each star, evolutionary tracks and isochrones provide estimations of the mass, luminosity, age and distances (or upper limits in some cases). The method is original in the sense that it is distance-indep endent, i.e. the estimation of the stellar parameters does n ot require, as it happens in other works, the knowledge of the distance to the object. Results. Stellar parameters (effective temperature, gravity, metallicity, mass, luminosi ty, age and distances ‐or upper limits) are obtained for the sample of stars mentioned above. A detailed discussion on some individual objects, in particular VV Ser, RR Tau, 49 Cet and the three suspected hot companions of post-T Tauris, is presented. Conclusions. These results, apart from their intrinsic interest, would b e extremely valuable to proceed a step further and attempt to model the disks surrounding the stars. The paper also shows the diffi culty posed by the morphology and behaviour of the system star+disk in the computation of the stellar parameters.

The Evershed effect in sunspots as a siphon flow along a magnetic flux tube

The Evershed effect—a wavelength shift and profile asymmetry in the spectral lines observed from the outer regions of sunspots (the penumbra)—has been interpreted as a radial outflow of gas from the sunspot, but the dynamics of the flow have not been fully understood. Although the Evershed effect seems to stop abruptly at the outer edge of the penumbra, the outflow itself must continue, though tracing its path has proved difficult. Theoretical, and observational studies have suggested that much of the continuing flow may follow magnetic field lines that go below the visible surface of the Sun at or just beyond the edge of the penumbra, and recent observations have now confirmed this picture. Here we show, using theoretical calculations based on a more realistic model, that the flow acts like a siphon which is driven along a magnetic flux tube by the pressure drop between the endpoints of the tube.

Accretion rates and accretion tracers of Herbig Ae/Be stars

Context. The scarcity of accretion rate estimates and accretion tracers available for Herbig Ae/Be (HAeBe) stars contrasts with the extensive studies for lower mass objects. Aims. This work aims to derive accretion rates from the UV Balmer excess for a sample of 38 HAeBe stars. We look for possible empirical correlations with the strength of the Hα ,( Oi)6300, and Brγ emission lines. Methods. Shock modelling within the context of magnetospheric accretion (MA) was applied to each star. We obtained the accretion rates from the excess in the Balmer discontinuity, derived from mean values of multi-epoch Johnsons UBphotometry. The accretion rates were related to both mean Hα luminosities, Hα 10% widths, and (O i)6300 luminosities from simultaneous spectra, and to Brγ luminosities from the literature. Results. The typical -median- mass accretion rate is 2 × 10 −7 Myr −1 in our sample, 36% of the stars showing values ≤10 −7 Myr −1 , 35% between 10 −7 and 10 −6 , and 29% > 10 −6 Myr −1 . The model fails to reproduce the large Balmer excesses shown by the four hottest stars (T∗ > 12 000 K). When accretion is related to the stellar masses and luminosities (1 ≤ M∗/M� ≤ 6; 2 ≤ L∗/L� ≤ 10 3 ), we derive u Macc ∝ M 5 and Lacc ∝ L 1.2 ∗ , with scatter. Empirical calibrations relating the accretion and the Hα ,( Oi)6300, and Brγ luminosities are provided. The slopes in our expressions are slightly shallower than those for lower mass stars, but the difference is within the uncertainties, except for the (O i)6300 line. The Hα 10% width is uncorrelated with u Macc, unlike for the lower mass regime. The mean Hα width shows higher values as the projected rotational velocities of HAe stars increase, which agrees with MA. The accretion rate variations in the sample are typically lower than 0.5 dex on timescales of days to months. Our data suggest that the changes in the Balmer excess are uncorrelated to the simultaneous changes of the line luminosities. Conclusions. The Balmer excesses and Hα line widths of HAe stars can be interpreted within the context of MA, which is not the case for several HBes. The steep trend relating u Macc and M∗ can be explained from the mass-age distribution characterizing HAeBe stars. The line luminosities used for low-mass objects are also valid to estimate typical accretion rates for the intermediate-mass regime under similar empirical expressions. However, we suggest that several of these calibrations are driven by the stellar luminosity.

Chromospheric activity and rotation of FGK stars in the solar vicinity - An estimation of the radial velocity jitter

Context. Chromospheric activity produces both photometric and spectroscopic variations that can be mistaken as planets. Large spots crossing the stellar disc can produce planet-like periodic variations in the light curve of a star. These spots clearly affect the spectral line profiles, and their perturbations alter the line centroids creating a radial velocity jitter that might “contaminate” the variations induced by a planet. Precise chromospheric activity measurements are needed to estimate the activity-induced noise that should be expected for a given star. Aims. We obtain precise chromospheric activity measurements and projected rotational velocities for nearby (d ≤ 25 pc) cool (spectral types F to K) stars, to estimate their expected activity-related jitter. As a complementary objective, we attempt to obtain relationships between fluxes in different activity indicator lines, that permit a transformation of traditional activity indicators, i.e., Ca II H & K lines, to others that hold noteworthy advantages. Methods. We used high resolution (~50 000) echelle optical spectra. Standard data reduction was performed using the IRAF ECHELLE package. To determine the chromospheric emission of the stars in the sample, we used the spectral subtraction technique. We measured the equivalent widths of the chromospheric emission lines in the subtracted spectrum and transformed them into fluxes by applying empirical equivalent width and flux relationships. Rotational velocities were determined using the cross-correlation technique. To infer activity-related radial velocity (RV) jitter, we used empirical relationships between this jitter and the R’_HK index. Results. We measured chromospheric activity, as given by different indicators throughout the optical spectra, and projected rotational velocities for 371 nearby cool stars. We have built empirical relationships among the most important chromospheric emission lines. Finally, we used the measured chromospheric activity to estimate the expected RV jitter for the active stars in the sample.

EXPORT: Optical photometry and polarimetry of Vega-type and pre-main sequence stars ?

This paper presents optical UBVRI broadband photo-polarimetry of the EXPORT sample obtained at the 2.5 m Nordic Optical Telescope. The database consists of multi-epoch photo-polarimetry of 68 pre-main-sequence and main-sequence stars. An investigation of the polarization variability indicates that 22 objects are variable at the 3 sigma level in our data. All these objects are pre-main sequence stars, consisting of both T Tauri and Herbig Ae/Be objects while the main sequence, Vega type and post-T Tauri type objects are not variable. The polarization properties of the variable sources are mostly indicative of the UXOR-type behaviour; the objects show highest polarization when the brightness is at minimum. We add seven new objects to the class of UXOR variables (BH Cep, VX Cas, DK Tau, HK Ori, LkH alpha 234, KK Oph and RY Ori). The main reason for their discovery is the fact that our data-set is the largest in its kind, indicating that many more young UXOR-type pre-main sequence stars remain to be discovered. The set of Vega-like systems has been investigated for the presence of intrinsic polarization. As they lack variability, this was done using indirect methods, and apart from the known case of BD+31 degrees 643, the following stars were found to be strong candidates to exhibit polarization due to the presence of circumstellar disks: 51 Oph, BD+31 degrees 643C, HD 58647 and HD 233517.

A spectroscopy study of nearby late-type stars, possible members of stellar kinematic groups

Context. Nearby late-type stars are excellent targets for seeking young objects in stellar associations and moving groups. The origin of these structures is still misunderstood, and lists of moving group members often change with time and also from author to author. Most members of these groups have been identified by means of kinematic criteria, leading to an important contamination of previous lists by old field stars. Aims. We attempt to identify unambiguous moving group members among a sample of nearby-late type stars by studying their kinematics, lithium abundance, chromospheric activity, and other age-related properties. Methods. High-resolution echelle spectra (R ∼ 57 000) of a sample of nearby late-type stars are used to derive accurate radial velocities that are combined with the precise Hipparcos parallaxes and proper motions to compute galactic-spatial velocity components. Stars are classified as possible members of the classical moving groups according to their kinematics. The spectra are also used to study several age-related properties for young late-type stars, i.e., the equivalent width of the lithium Li i 6707.8 A line or the R � index. Additional information like X-ray fluxes from the ROSAT All-Sky Survey or the presence of debris discs is also taken into account. The different age estimators are compared and the moving group membership of the kinematically selected candidates are discussed. Results. From a total list of 405 nearby stars, 102 have been classified as moving group candidates according to their kinematics. i.e., only ∼25.2% of the sample. The number reduces when age estimates are considered, and only 26 moving group candidates (25.5% of the 102 candidates) have ages in agreement with the star having the same age as an MG member.

Kepler-91b: a planet at the end of its life Planet and giant host star properties via light-curve variations ,

Context. The evolution of planetary systems is intimately linked to the evolution of their host star. Our understanding of the whole planetary evolution process is based on the large planet diversity observed so far. To date, only few tens of planets have been discovered orbiting stars ascending the Red Giant Branch. Although several theories have been proposed, the question of how planets die remains open due to the small number statistics, making clear the need of enlarging the sample of planets around post-main sequence stars. Aims. In this work we study the giant star Kepler-91 (KIC 8219268) in order to determine the nature of a transiting companion. This system was detected by the Kepler Space Telescope, which identified small dims in its light cur ve with a period of 6.246580±0.000082 days. However, its planetary confirmation is needed due to th e large pixel size of the Kepler camera which can hide other stellar configurations able to mimic planet-like transit events. Methods. We analyse Kepler photometry to: 1) re-calculate transit parameters, 2) stud y the light-curve modulations, and 3) to perform an asteroseismic analysis (accurate stellar parameter det ermination) by identifying solar-like oscillations on the periodogram. We also used a high-resolution and high signal-to-noise ratio spec trum obtained with the Calar Alto Fiber-fed ´ Echelle spectrograph (CAFE) to measure stellar properties. Additionally, false-positiv e scenarios were rejected by obtaining high-resolution images with the AstraLux lucky-imaging camera on the 2.2 m telescope at the Calar Alto Observatory. Results. We confirm the planetary nature of the object transiting the s tar Kepler-91 by deriving a mass of Mp = 0.88 +0.17 −0.33 MJup and a planetary radius of Rp = 1.384 +0.011 −0.054 RJup. Asteroseismic analysis produces a stellar radius of R⋆ = 6.30± 0.16 R⊙ and a mass of M⋆ = 1.31± 0.10 M⊙. We find that its eccentric orbit ( e = 0.066 +0.013 −0.017 ) is just 1.32 +0.07 −0.22 R⋆ away from the stellar atmosphere at the pericenter. We also detected three small dims in the phase-folded light-curve. The combination of two of them agrees with the theoretical characteristics expected for secondary eclip se. Conclusions. Kepler-91b could be the previous stage of the planet engulfment, recently detected for BD+48 740. Our estimations show that Kepler-91b will be swallowed by its host star in less than 55 Myr. Among the confirmed planets around giant stars, this is the planetary-mass body closest to its host star. At pericen ter passage, the star subtends an angle of 48 ◦ , covering around 10% of the sky as seen from the planet. The planetary atmosphere seems to be inflated probably due to the high stellar irradiation.