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International audience; We investigate a chromospheric scenario to explain the characteristics of the far-UV emission lines of beta Pictoris, revealed by FUSE spectra of this famous circumstellar disk system. The model assumes a thin region heated up to a few 105 K located close to the stellar photosphere. The resonance lines of C III at 977 Å and O VI at 1032-1037 Å, seen in emission, are produced in this chromosphere-transition region complex. Using complementary data in the mid and near UV (specifically, a resonance doublet of C IV at 1548-1550 Å and Mg II h & k), we show that the whole dataset is remarquably well reproduced by the model. In addition, we investigate the properties of the C III* multiplet at 1176 Å and conclude that this line likely forms in a weak warm wind, originating from the prominent circumstellar disk of beta Pictoris. Finally, radiative losses have been calculated and have provided estimates of the amount of non radiative energy dissipated in the external atmosphere of beta Pictoris, which is a measure of the star's activity level. Such behavior for an A5 V star presents a challenge for both evolution and activity models since the former predict that main-sequence A stars should not be active, while the latter are unable to quantitatively account for the characteristics of the chromospheric heating of beta Pictoris.

International audience; We present the first emission-line spectrum of β Pictoris, as revealed by the Far Ultraviolet Spectroscopic Explorer satellite. Broad emission lines due to highly ionized species, C III and O VI, are well detected above the very low continuum level. The mere presence of such lines in the β Pic spectrum is evidence of a complex stellar environment, including dense and hot regions. We argue that these spectral features may originate from a solar-like extended chromosphere or from magnetospheric accretion.

WOS: 000285342500060, Aims. We present the results of contemporaneous spectroscopic and photometric monitoring of the young solar-type star HD 171488 (P-rot similar or equal to 1.337 days) aimed at studying surface inhomogeneities at both photospheric and chromospheric levels. Methods. Echelle FOCES spectra (R similar or equal to 40 000) and Johnson BV photometry have been performed in August 2006, with a good coverage of rotational phases. Spectral type, rotational velocity, metalicity, and gravity were determined with a code developed by us (ROTFIT) and a library of high-resolution spectra of slowly-rotating reference stars. The metalicity was measured from the analysis of iron lines with the MOOG code. The spectral subtraction technique was applied to the most relevant chromospheric diagnostics included in the FOCES spectral range, namely Ca II IRT, H alpha, He1D(3), H beta, and Ca II H&K lines. Results. A simple model with two large high-latitude spots is sufficient to reproduce the B and V light curves as well as the radial velocity modulation if a temperature difference between photosphere and spots of about 1500 K is used. A Doppler-imaging analysis of photospheric lines basically confirms a similar spot distribution. With the help of an analogous geometric two-spot model, we are able to reproduce the observed modulations in the residual chromospheric emissions adopting different values of ratios between the flux of plages and the quiet chromosphere (about 5 for H alpha and 3 for Ca II diagnostics). Facular regions of solar type appear to be the main responsible features 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 significant lead effect of plages with respect to spots (from 20 degrees to 40 degrees in longitude), as already observed in some active solar-type stars and RS CVn systems. Conclusions. The contemporaneous monitoring of photospheric and chromospheric diagnostics in the young and rapidly rotating solar-type star HD 171488 allowed us to detect active regions which have nearly the same location at both atmospheric layers, with plages slightly leading spots in longitudes. These active regions are similar to the solar ones in some respects, 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., Italian Ministero dell'Istruzione, Universita e Ricerca (MIUR)Ministero dell' Istruzione, dell' Universita e della Ricerca (MIUR); Regione Sicilia; Hungarian OTKAOrszagos Tudomanyos Kutatasi Alapprogramok (OTKA) [K-68626, K-81421], The authors are grateful to the anonymous referee for a careful reading of the paper and valuable comments. We thank the CAHA and OACt teams for the assistance during the observations. This work has been supported by the Italian Ministero dell'Istruzione, Universita e Ricerca (MIUR) and by the Regione Sicilia which are gratefully acknowledged. The ESO Scientific Visitor Programme enabled Z.s.K. and K.B. to visit ESO Garching during the preparation of the paper. Z.s.K. acknowledges the support of the Hungarian OTKA grants K-68626 and K-81421. This research has made use of SIMBAD and VIZIER databases, operated at CDS, Strasbourg, France.

WOS: A1992HL72600010, Space velocities of 146 chromospherically active binary stars have been calculated. Containing F-M spectral types on the main sequence together with G and K giants and subgiants, this very heterogeneous sample has been divided into groups in order to segregate stars which have similar kinematics and ages. After many trials for different criteria, the sample was divided into five groups: two groups for giants, two groups for main-sequence systems, and one group for main-sequence systems. Kinematics of subgiants implies a stellar age of approximately 2-3 Gyr. Young and old groups of giants and main-sequence systems could be characterized by a kinematical age of approximately 1 Gyr and > 5 Gyr, respectively. These ages are estimated approximately according to space velocity distributions and dispersions in velocity space in each group. Inferred ages for the groups above agrees with circularization, rotation activity relation, and stellar evolution theories.

Long-term stellar activity variations can affect the detectability of long-period and Earth-analogue extrasolar planets. We have, for 54 stars, analysed the long-term trend of five activity indicators: log$R'_\mathrm{{HK}}$, the cross-correlation function (CCF) bisector span, CCF full-width-at-half-maximum, CCF contrast, and the area of the Gaussian fit to the CCF; and studied their correlation with the RVs. The sign of the correlations appears to vary as a function of stellar spectral type, and the transition in sign signals a noteworthy change in the stellar activity properties where earlier type stars appear more plage dominated. These transitions become more clearly defined when considered as a function of the convective zone depth. Therefore, it is the convective zone depth (which can be altered by stellar metallicity) that appears to be the underlying fundamental parameter driving the observed activity correlations. In addition, for most of the stars, we find that the RVs become increasingly red-shifted as activity levels increase, which can be explained by the increase in the suppression of convective blue-shift. However, we also find a minority of stars where the RVs become increasingly blue-shifted as activity levels increase. Finally, using the correlation found between activity indicators and RVs, we removed RV signals generated by long-term changes in stellar activity. We find that performing simple cleaning of such long-term signals enables improved planet detection at longer orbital periods.