Binary stars hosting exoplanets are a unique laboratory where chemical tagging can be performed to measure with high accuracy the elemental abundances of both stellar components, with the aim to investigate the formation of planets and their subseque
nt evolution. Here, we present a high-precision differential abundance analysis of the XO-2 wide stellar binary based on high resolution HARPS-N@TNG spectra. Both components are very similar K-dwarfs and host planets. Since they formed presumably within the same molecular cloud, we expect they should possess the same initial elemental abundances. We investigate if the presence of planets can cause some chemical imprints in the stellar atmospheric abundances. We measure abundances of 25 elements for both stars with a range of condensation temperature $T_{rm C}=40-1741$ K, achieving typical precisions of $sim 0.07$ dex. The North component shows abundances in all elements higher by $+0.067 pm 0.032$ dex on average, with a mean difference of +0.078 dex for elements with $T_{rm C} > 800$ K. The significance of the XO-2N abundance difference relative to XO-2S is at the $2sigma$ level for almost all elements. We discuss the possibility that this result could be interpreted as the signature of the ingestion of material by XO-2N or depletion in XO-2S due to locking of heavy elements by the planetary companions. We estimate a mass of several tens of $M_{oplus}$ in heavy elements. The difference in abundances between XO-2N and XO-2S shows a positive correlation with the condensation temperatures of the elements, with a slope of $(4.7 pm 0.9) times 10^{-5}$ dex K$^{-1}$, which could mean that both components have not formed terrestrial planets, but that first experienced the accretion of rocky core interior to the subsequent giant planets.
We present the results of FLAMES/UVES and FLAMES/GIRAFFE spectroscopic observations of 23 low-mass stars in the L1615/L1616 cometary cloud, complemented with FORS2 and VIMOS spectroscopy of 31 additional stars in the same cloud. L1615/L1616 is a come
tary cloud where the star formation was triggered by the impact of the massive stars in the Orion OB association. From the measurements of the lithium abundance and radial velocity, we confirm the membership of our sample to the cloud. We use the equivalent widths of the H$alpha$, H$beta$, and the HeI $lambda$5876, $lambda$6678, $lambda$7065 AA$ $emission lines to calculate the accretion luminosities, $L_{rm acc}$, and the mass accretion rates, $dot M_{rm acc}$. We find in L1615/L1616 a fraction of accreting objects ($sim 30%$), which is consistent with the typical fraction of accretors in T associations of similar age ($sim 3$ Myr). The mass accretion rate for these stars shows a trend with the mass of the central object similar to that found for other star-forming regions, with a spread at a given mass which depends on the evolutionary model used to derive the stellar mass. Moreover, the behavior of the $2MASS/WISE$ colors with $dot M_{rm acc}$ indicates that strong accretors with $log dot M_{rm acc} gt -8.5$ dex show large excesses in the $JHK{rm s}$ bands, as in previous studies. We also conclude that the accretion properties of the L1615/L1616 members are similar to those of young stellar objects in T associations, like Lupus.
Radial velocities, elemental abundances, and accretion properties of members of star-forming regions (SFRs) are important for understanding star and planet formation. While infrared observations reveal the evolutionary status of the disk, optical spe
ctroscopy is fundamental to acquire information on the properties of the central star and on the accretion characteristics. 2MASS archive data and the Spitzer c2d survey of the Chamaeleon II dark cloud have provided disk properties of a large number of young stars. We complement these data with spectroscopy with the aim of providing physical stellar parameters and accretion properties. We use FLAMES/UVES+GIRAFFE observations of 40 members of Cha II to measure radial velocities through cross-correlation technique, Li abundances by means of curves of growth, and for a suitable star elemental abundances of Fe, Al, Si, Ca, Ti, and Ni using the code MOOG. From the equivalent widths of the Halpha, Hbeta, and the HeI-5876, 6678, 7065 Angstrom emission lines, we estimate the mass accretion rates, dMacc/dt, for all the objects. We derive a radial velocity distribution for the Cha II stars (<Vrad>=11.4+-2.0 km/s). We find dMacc/dt prop. to Mstar^1.3 and to Age^(-0.82) in the 0.1-1.0 Msun mass regime, and a mean dMacc/dt for Cha II of ~7*10^(-10) Msun/yr. We also establish a relationship between the HeI-7065 Angstrom line emission and the accretion luminosity. The radial velocity distributions of stars and gas in Cha II are consistent. The spread in dMacc/dt at a given stellar mass is about one order of magnitude and can not be ascribed entirely to short timescale variability. Analyzing the relation between dMacc/dt and the colors in Spitzer and 2MASS bands, we find indications that the inner disk changes from optically thick to optically thin at dMacc/dt~10^(-10) Msun/yr. Finally, the disk fraction is consistent with the age of Cha II.
We present stellar parameters and abundances of 11 elements (Li, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, and Zn) of 13 F6-K2 main-sequence stars in the young groups AB Doradus, Carina Near, and Ursa Major. The exoplanet-host star iota Horologii is also a
nalysed. The three young associations have lithium abundance consistent with their age. All other elements show solar abundances. The three groups are characterised by a small scatter in all abundances, with mean [Fe/H] values of 0.10 (sigma=0.03), 0.08 (sigma=0.05), and 0.01 (sigma=0.03) dex for AB Doradus, Carina Near, and Ursa Major, respectively. The distribution of elemental abundances appears congruent with the chemical pattern of the Galactic thin disc in the solar vicinity, as found for other young groups. This means that the metallicity distribution of nearby young stars, targets of direct-imaging planet-search surveys, is different from that of old, field solar-type stars, i.e. the typical targets of radial velocity surveys. The young planet-host star iota Horologii shows a lithium abundance lower than that found for the young association members. It is found to have a slightly super-solar iron abundance ([Fe/H]=0.16+-0.09), while all [X/Fe] ratios are similar to the solar values. Its elemental abundances are close to those of the Hyades cluster derived from the literature, which seems to reinforce the idea of a possible common origin with the primordial cluster.
We present a study of the large-scale spatial distribution of 6482 RASS X-ray sources in approximately 5000 deg^2 in the direction of Orion. We examine the astrophysical properties of a sub-sample of ~100 optical counterparts, using optical spectrosc
opy. This sub-sample is used to investigate the space density of the RASS young star candidates by comparing X-ray number counts with Galactic model predictions. We characterize the observed sub-sample in terms of spectral type, lithium content, radial and rotational velocities, as well as iron abundance. A population synthesis model is then applied to analyze the stellar content of the RASS in the studied area. We find that stars associated with the Orion star-forming region do show a high lithium content. A population of late-type stars with lithium equivalent widths larger than Pleiades stars of the same spectral type (hence younger than ~70-100 Myr) is found widely spread over the studied area. Two new young stellar aggregates, namely X-ray Clump 0534+22 (age~2-10 Myr) and X-ray Clump 0430-08 (age~2-20 Myr), are also identified. The spectroscopic follow-up and comparison with Galactic model predictions reveal that the X-ray selected stellar population in the direction of Orion is characterized by three distinct components, namely the clustered, the young dispersed, and the widespread field populations. The clustered population is mainly associated with regions of recent or ongoing star formation and correlates spatially with molecular clouds. The dispersed young population follows a broad lane apparently coinciding spatially with the Gould Belt, while the widespread population consists primarily of active field stars older than 100 Myr. We expect the bi-dimensional picture emerging from this study to grow in depth as soon as the distance and the kinematics of the studied sources will become available from the future Gaia mission.
We report first results on the large-scale distribution of the ROSAT All-Sky Survey (RASS) X-ray sources in a 5000 deg^2 field centered on Orion. Our final aim is to study the properties of different widespread populations in the Orion Complex close
to the Gould Belt (GB) in order to trace the star formation history in the solar neighbourhood.
We present a spectroscopic/photometric analysis of the rapid rotator KIC8429280, discovered by ourselves as a very young star and observed by the Kepler mission. We use spectroscopic/photometric ground-based data to derive stellar parameters, and we
adopt a spectral subtraction technique to highlight the chromospheric emission in the cores of Halpha, CaII H&K and IRT lines. We fit a robust spot model to the high-precision Kepler photometry spanning 138 days. Model selection and parameter estimation is performed in a Bayesian manner using a Markov chain Monte Carlo method. We find that KIC8429280 is a cool (K2V) star with an age of ~50 Myr, based on its Li content, that has passed its T Tau phase and is spinning up approaching the ZAMS. Its high level of chromospheric activity is indicated by the radiative losses in CaII H&K and IRT, Halpha, and Hbeta lines. Furthermore, its Balmer decrement and the flux ratio of CaII IRT lines imply that these lines are mainly formed in optically-thick sources analogue to solar plages. The analysis of the Kepler data uncovers evidence of at least 7 enduring spots. Since the stars inclination is rather high, ~70{deg}, the assignment of the spots to the northern/southern hemisphere is not unambiguous. We find at least 3 solutions with nearly the same level of residuals. The distribution of the active regions is such that the spots are located around 3 latitude belts, i.e. the equator and +-(50{deg}-60{deg}), with the high-latitude spots rotating slower than the low-latitude ones. The equator-to-pole differential rotation ~0.27 rad/d is at variance with some recent mean-field models of differential rotation in rapidly rotating MS stars, which predict a much smaller latitudinal shear. Our results are consistent with the scenario of a higher differential rotation, which changes along the magnetic cycle.
We present the results of contemporaneous spectroscopic and photometric monitoring of the young solar-type star HD171488 (Prot~1.337 d) aimed at studying surface inhomogeneities at photospheric/chromospheric levels. Echelle FOCES spectra (R~40000) an
d Johnson photometry have been performed in 2006. Spectral type, rotational velocity, metallicity, and gravity were determined using a code developed by us. The metallicity was measured from the analysis of iron lines. The spectral subtraction technique was applied to the most relevant chromospheric diagnostics included in the FOCES spectral range (CaII IRT, Halpha, HeI-D3, Hbeta, CaII H&K). A model with two large high-latitude spots is sufficient to reproduce the B/V light curves and the radial velocity modulation, if a temperature difference between photosphere and spots of 1500 K is used. A Doppler imaging analysis of photospheric lines confirms a similar spot distribution. With the help of an analogous geometric two-spot model, we are able to reproduce the modulations in the residual chromospheric emissions adopting different values of ratios between the flux of plages and quiet chromosphere (5 for Halpha and 3 for CaII). Facular regions of solar type appear to be the main responsible for the modulations of chromospheric diagnostics. Both the spot/plage model and the cross-correlation between the light curve and the chromospheric line fluxes display a lead effect of plages with respect to spots (20-40 deg in longitude). The active regions of the rapidly rotating star HD171488 are similar to the solar ones in some respect, because the spot temperature is close to that of sunspot umbrae and the plage flux-contrast is consistent with the average solar values. The main differences with respect to the Sun are larger sizes and higher latitudes.
We present the first results of a ground-based programme conducted on 1-4m class telescopes. Our sample consists of 1097 active and presumably young stars, all of them being optical counterparts of RASS X-ray sources in the northern hemisphere. We co
ncentrate on the 704 optically brightest (V_Ticho<=9.5 mag) candidates. We acquired high-res spectroscopy in the Halpha/Li spectral regions for 426 of such stars without relevant literature data. We describe the sample and the observations and we start to discuss its physical properties. We used a cross-correlation technique and other tools to derive accurate radial/rotational velocities and to perform a spectral classification for both single and SB2 stars. The spectral subtraction technique was used to derive chromospheric activity levels and Li abundances. We estimated the fraction of young single stars and multiple systems in stellar soft X-ray surveys and the contamination by more evolved systems, like RS CVns. We classified stars on the basis of Li abundance and give a glimpse of their sky distribution. The sample appears to be a mixture of young Pleiades-/Hyades- like stars plus an older Li-poor population (~1-2 Gyr). 7 stars with Li abundance compatible with the age of IC 2602 (~30 Myr) or younger were detected as well, although 2 appear to be Li-rich giants. The discovery of a large number of Li-rich giants is another outcome of this survey. The contamination of soft X-ray surveys by old systems in which the activity level is enhanced by tidal synchronisation is not negligible, especially for K-type stars. 5 stars with Li content close to the primordial abundance are probably associated with known moving groups in the solar neighbourhood. Some of them are PTTS candidates according to their positions in the HR diagram.
Our aim is investigating surface inhomogeneities of the young late-type star SAO51891, from photosphere to upper chromosphere, analyzing contemporaneous high-resolution spectra and broad-band photometry. The FOCES@CAHA spectral range is used to deter
mine spectral classification and derive vsini and Vrad. The Li abundance is measured to estimate the age. The BVRIJHKs bands are used to construct the SED. The variations of our BV fluxes and Teff are used to infer the presence of photospheric spots and observe their behavior over time. The chromospheric activity is studied applying the spectral subtraction technique to Halpha, CaII H&K, Heps, and CaII IRT lines. We find SAO51891 to be a young K0-1V star with Li abundance close to the Pleiades upper envelope, confirming its youth (~100 Myr), also inferred from its kinematical membership to the Local Association. We detect no IR excess from SED analysis, and rotational modulation of luminosity, Teff, CaII, and Heps total fluxes. A spot model with two active regions, ~240 K cooler than the surrounding photosphere, fits our light/Teff curves, and reproduces the small-amplitude Vrad variations. The anti-correlation of light curves and chromospheric diagnostics indicates plages spatially associated with spots. The large amplitude observed in the Heps-flux curve suggests that this line is very sensitive to the plage presence. Finally, SAO51891 is a young active star, lacking significant amounts of circumstellar dust or any evidence for low mass companions. The spots turn out to be larger and warmer than those in less active MS stars. The Vrad variation produced by spots has an amplitude comparable with those induced by Jupiter-mass planets orbiting close to the star. SAO51891 is a good example of star where the detection of planets may be hampered by the high activity level.
