We present the first results from follow-up optical observations, both photometric and spectroscopic, of stellar X-ray sources, selected from the RasTyc sample, resulting from the cross-correlation of ROSAT All-Sky Survey (RASS) and TYCHO catalogues. In particular, we report on the discovery of six late-type binaries, for which we obtained good radial velocity curves and solved their orbits. We performed an automatic spectral classification of both single-lined and double-lined binaries with codes developed by us and found two binaries composed of two main-sequence stars and four binaries with an evolved (giant or subgiant) component. Filled-in or pure emission Hα profiles indicative of a moderate or high level of chromospheric activity were observed. In nearly all the systems, we also detected a photometric modulation ascribable to surface inhomogeneities that is correlated with the orbital period, suggesting a synchronization between rotational and orbital periods. The position on the HR diagram of the components of the five sources with a known parallax indicates three binaries containing only main-sequence stars and two single-lined systems with a giant component. The kinematical properties of two, or possibly four, of the observed systems are consistent with a young disk population.

Abstract.A study of the local interstellar medium (LISM) using a robust inversion method, similar to current tomography techniques, is applied to compiled data on neutral interstellar absorbers and Hipparcos parallaxes. We present here the 3-D distribution of two neutral species, NaI and HI. Our analysis enables us to obtain a reliable 3-D density distribution of the IS matter in the solar neighbourhood, providing a new basis for the discussion of origin, present state and evolution of the LISM. We show that neutral IS matter is distributed in compact clouds or in cloud complexes with cavities between them. It is now easy to distinguish the so-called Local Bubble and the Loop I cavities and also two tunnels linking the Local Bubble to the outer regions of the galaxy, away from the galactic plane. Better accuracy is achieved for NaI, as to a larger number of lines-of-sight and target stars than are available for HI. A rather detailed NaI 3-D density distribution is obtained with a 40 pc smoothing length. The extended high-density regions in the NaI and HI maps are correlated which is not the case for the diffuse regions. The density ratio ρHI/ρNaI is lower or equal to 9.1.10 8 ± 3.10 8 for extended high density clouds.

Aims. We present the first results of an ambitious ground-based observation programme conducted on 1-4 meter class telescopes. Our sample consists of 1097 active and presumably young stars, all of them being optical (Tycho Catalogue) counterparts of ROSAT All-Sky Survey X-ray sources in the northern hemisphere. In this paper, we concentrate on the optically brightest (V T 9. m 5) candidates (704 objects). We acquired high-resolution optical spectroscopy in the Hα and/or lithium spectral regions for 426 of such stars without relevant data in the literature. We describe the star sample and the observations and we start to discuss the physical properties of the investigated stars. Methods. We used a cross-correlation technique and other tools developed by us to derive accurate radial and rotational velocities and to perform an automatic spectral classification for both single stars and double-lined systems. The spectral subtraction technique was used to derive chromospheric activity levels and lithium 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 CVn binaries. We classified stars on the basis of their lithium abundance and give a glimpse of their sky distribution.Results. The sample appears to be a mixture of quite young Pleiades-like and Hyades-like stars plus an older lithium-poor population probably born within the last 1-2 Gyr. Seven stars with a lithium abundance compatible with the age of IC 2602 (about 30 Myr) or even younger were detected as well, although two appear to be lithium-rich giants.The discovery of a large number of highly or moderately lithium-rich giants is another outcome of the present survey. Conclusions. 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. Five stars with lithium content close to the primordial abundance are probably associated with already known moving groups in the solar neighbourhood. Some of them are good post-T Tauri candidates according to their positions in the HR diagram.

Context. Over the last century, the overdensities in the velocity distributions of nearby stars were attributed to stellar kinematic groups or moving groups. Although their reality was initially questioned, their existence is now supported by a confluence of evidence. Aims. To pursue investigations, quantitative methods should be clearly defined to identify new stars belonging to these groups. Here, we present two probabilistic methods to determine the likelihood of kinematic membership for possible candidates in five of the known young stellar kinematic groups -namely, Pleiades, IC 2391, Castor, Ursa Major, and Hyades -in which all are younger than 650 Myr. Methods. We tried different methods to handle kinematic data of their known members. We succeeded in developing two independent procedures able to identify new candidates of these five stellar stream. We tested the robustness of our two approaches by means of extensive Monte Carlo simulations. Results. Our methods are consistent with one another in more than 90% of cases and for almost all the stellar kinematic groups under scrutiny. The IC 2391 supercluster is an exception. Applying our statistical methods to a large sample of young low-mass stars, we confirmed almost all the likely members and good candidates of these stellar streams. We also proposed 39 additional candidates based on the agreement and the high likelihood of age and kinematic membership. Conclusions. These probabilistic methods are very powerful to reliably identify new candidate members of young stellar kinematic groups. However, the kinematic criteria alone are not sufficient to distinguish between coeval stars that are evaporated from open clusters and other field stars trapped by dynamical processes generated by galactic perturbations. The identification of stars belonging to the remnant of a past star-forming event can be possible with the help of additional information, such as indicators of chromospheric activity, age proxies (lithium abundance), and chemical composition.

Context. Star formation in the solar neighborhood is mainly traced by young stars in open clusters, associations and in the field, which can be identified, e.g., by their X-ray emission. The determination of stellar parameters for the optical counterparts of X-ray sources is crucial for a full characterization of these stars. Aims. This work extends the spectroscopic study of the RasTyc sample, obtained by the cross-correlation of the TYCHO and ROSAT All-Sky Survey catalogs, to stars fainter than V = 9.5 mag and it is aimed to the identification of sparse populations of young stars in the solar neighborhood. Methods. We acquired 625 high-resolution spectra for 443 presumabily young stars with four different instruments in the Northern hemisphere. The radial and rotational velocity (v sin i) of our targets are measured by means of the cross-correlation technique, which is also helpful to discover single-lined (SB1), double-lined spectroscopic binaries (SB2), and multiple systems. We use the code ROTFIT for performing an MK spectral classification and for determining the atmospheric parameters (T eff , log g, [Fe/H]) and v sin i of the single stars and SB1 systems. For these objects, the spectral subtraction of slowly rotating templates is used to measure the equivalent widths of the Hα and Li i 6708Å lines, which enables us to derive their chromospheric activity level and lithium abundance. We make use of Gaia DR1 parallaxes and proper motions for locating the targets in the HR diagram and for computing the space velocity components of the youngest objects. Results. We find a remarkable fraction (at least 35 %) of binaries and multiple systems. On the basis of the lithium abundance, the sample of single stars and SB1 systems appears to be mostly (∼ 60 %) composed of stars younger than the members of the UMa cluster. The remaining sources are in the age range between the UMa and Hyades clusters (∼ 20 %) or older (∼ 20 %). In total, we identify 42 very young (PMS-like) stars, which lie above or very close to the Pleiades upper envelope of the lithium abundance. A significant fraction (∼ 12 %) of evolved stars (giants and subgiants) is also present in our sample. Some of them (∼ 36 %) are also lithium rich (A(Li)> 1.4).

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Context. During the study of a large set of late-type stellar X-ray sources, we discovered a high fraction of multiple systems. Aims. In this paper we investigate the orbital elements and kinematic properties of three new spectroscopic triple systems, along with the spectral types and astrophysical parameters (T eff , log g, v sin i, log N(Li)) of their components. Methods. We conducted follow-up optical observations of these systems, both photometric and spectroscopic ones at high resolution. We used a synthetic approach and the cross-correlation method to derive most of the stellar parameters. Results. We estimated reliable radial velocities and deduced the orbital elements of the inner binaries. The comparison of the observed spectra with synthetic composite ones, obtained as the weighted sum of three spectra of non-active reference stars, allowed us to determine the stellar parameters for each component of these systems. We found all are only composed of main sequence stars. Conclusions. These three systems are certainly stable hierarchical triples composed of short-period inner binaries plus a tertiary component in a long-period orbit. From their kinematics and/or lithium content, these systems turn out to be fairly young.

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