A&A 399, 983–994 (2003) Astronomy DOI: 10.1051/0004-6361:20021867 & c ESO 2003 Astrophysics Nearby young stars? R. Wichmann1;??;???,J.H.M.M.Schmitt1, and S. Hubrig2 1 Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany 2 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei M¨unchen, Germany Received 4 October 2002 / Accepted 29 November 2002 Abstract. We present the results of an extensive all-sky survey of nearby stars of spectral type F8 or later in a systematic search of young (zero-age main sequence) objects. Our sample has been derived by cross-correlating the ROSAT All-Sky Survey and the TYCHO catalogue, yielding a total of 754 candidates distributed more or less randomly over the sky. Follow-up spectroscopy of these candidate objects has been performed on 748 of them. We have discovered a tight kinematic group of ten stars with extremely high lithium equivalent widths that are presumably younger than the Pleiades, but again distributed rather uniformly over the sky. Furthermore, about 43 per cent of our candidates have detectable levels of lithium, thus indicating that these are relatively young objects with ages not significantly above the Pleiades age. Key words. surveys – Galaxy: solar neighbourhood – stars: kinematics – stars: late-type 1. Introduction (Hearty et al. 1999). We therefore prefer the first alternative, since there is little evidence for truly isolated star formation. Traditionally, the study of zero-age main sequence (ZAMS) If then most star-forming associations dissociate rapidly, stars has been a study of open clusters and associations. there must exist many rather young (i.e. ZAMS) stars in the However, there is some reason to expect ZAMS stars to also field. Furthermore we have to keep in mind that the Sun and exist in the field, and in particular also in the solar neighbour- its neighbourhood is located within a huge, young star-forming hood. It is well known that tightly bound, long-lived open clus- complex known as the Gould Belt (GB; for a recent review ters can account for only a few per cent of the total galactic of the GB see cf. P¨oppel 1997). One therefore also expects that star formation rate (cf. Wielen 1971). The inescapable conclu- there are ZAMS field stars in the solar neighbourhood that have sion follows that either most clusters/associations disperse very been formed in the GB over its lifetime and have dispersed into quickly after star formation has started or that most stars are the field since then. born in isolation. This latter view is supported by Guillout et al. (1998) who Indeed a few very small associations are known, like the report the detection of an X-ray active, late-type stellar pop- TW Hya association (Rucinski & Krautter 1983; de la Reza ulation associated with the GB, and apparently distributed in et al. 1989; Gregorio-Hetem et al. 1992; Webb et al. 1999), a “disrupted disk”-like structure. According to their model, the η Chamaeleontis cluster (Mamajek et al. 1999), or the young stars are not located exclusively at the outer edge of one in front of the translucent clouds MBM 7 and MBM 55 the GB, where most of its present-day star forming activity takes place, but their distribution extends into the inner parts Send offprint requests to: R. Wichmann, of the GB. e-mail: [email protected] This model is also supported by a study of lithium-rich ? Based on observations collected at the European Southern stars detected towards the Lupus dark clouds (Wichmann et al. Observatory, Chile (ESO No. 62.I-0650, 66.D-0159(A), 67.D- 1999). They were able to construct a distance distribution of 0236(A)). these stars by combining v sin i data with photometrically de- ?? Visiting Astronomer, Kitt Peak National Observatory, National termined rotational periods, and assuming a mean inclination Optical Astronomy Observatory, which is operated by the Association angle of π/4. This distance distribution (Fig. 8 in Wichmann of Universities for Research in Astronomy, Inc. (AURA) under coop- et al. 1999) shows a peak at about the distance of the Lupus erative agreement with the National Science Foundation. ??? Visiting Astronomer, German-Spanish Astronomical Centre, clouds, but also a marked tail extending to lower distances, i.e. Calar Alto, operated by the Max-Planck-Institute for Astronomy, inwards of the GB. Heidelberg, jointly with the Spanish National Commission for As a consequence one expects to find young stars also in the Astronomy. solar neighbourhood and the purpose of this paper is to report Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20021867 984 R. Wichmann et al.: Nearby young stars Fig. 1. H–R diagram showing the RASS-TYCHO sample and (dashed lines) our selection box. Fig. 2. Histogram showing the distribution of distances in our sample. on a systematic all-sky survey for young (ZAMS) stars in the Likewise, the distribution of distances of our sample stars solar vicinity, up to about 50 pc distance. is shown in Fig. 2. If possible, parallaxes from HIPPARCOS (ESA 1997b) are used, otherwise we use TYCHO parallaxes (694 stars actually have HIPPARCOS parallaxes). The distance 2. Candidate selection distribution (Fig. 2) has a peak at 20–30 pc, with a tail extend- ing to about 200 pc. The few stars at very large distances are Since most young stars show strong X-ray emission, the can- giants that were originally included in our sample as the result didate sample for this survey was selected from a cross- of incorrect TYCHO parallaxes, but Fig. 2 demonstrates clearly correlation of the Rosat All-Sky Survey (RASS) with the that the bulk of our sample stars is located closer than 50 pc. TYCHO catalogue (ESA 1997a). We used the RassTych sam- To get some idea on the completeness of the sample, one ple constructed by Guillout et al. (1999), and enhanced it our- can plot the data from Fig. 2 as a cumulative distribution. This selves by cross-correlating it with the HIPPARCOS catalogue distribution turns out to be similar to the same distribution de- (ESA 1997a). The resulting sample is similar to the RassHip rived from all HIPPARCOS stars out to a distance of about sample shown in Fig. 5b of Guillout et al. (1999). From this 30 5 pc for our complete sample. Therefore we conclude that sample, we selected stars with (B V) > 0:54 with (TYCHO) our± sample is complete out to that distance. Since our sample − parallax errors better than 3:5 σ.Thecutoff at (B V) > 0:54 mostly consists of G stars, we conclude that for these we are − corresponds approximately to spectral type F8. We then pro- limited by the effective X-ray horizon and not the flux limit of ceeded by constructing an H–R diagram of the stars (Fig. 1), the HIPPARCOS catalogue. and discarded stars very far above the main sequence (giants) and very far below (erroneous parallaxes), resulting in a total sample size of 754 stars. Our candidate selection is visualized 3. Spectroscopic observations and data reduction in Fig. 1, where the selection criteria and the resulting region Spectroscopic follow-up observations were carried out at three in the H–R diagram are outlined by dashed lines. different observatories: ESO (La Silla, Chile), KPNO (Kitt The restriction on spectral type was imposed mainly be- Peak, USA), and DSAZ (Calar Alto, Spain). At ESO, the cause there seems to be no reliable age indicator for earlier 1.52 m ESO telescope with the FEROS echelle spectrograph spectral types, as Li I is ionized away. The reason for the re- (R = 48 000, 3550–9210 Å) was used. Observations at KPNO striction on the parallax error was that we wanted to have some were performed using the 0.9 m Coud´e Feed telescope with the information on the location in the H–R diagram, and also on Coud´e spectrograph (camera 5, grating A, Ford 3K 1K CCD, the 3D space velocity (which requires parallax information to R = 25 000, 6415–6730 Å). At the DSAZ, we used× the FOCES convert proper motions into space velocities). echelle spectrograph (R = 40 000, 3730–9550 Å) on the 2.2 m The distribution of Johnson (B V)J colours in our sam- telescope. The observing dates are summarized in Table 1. In ple is very similar to the one shown− in Fig. 4 of Guillout et al. total, we were able to observe 748 out of the 754 candidate (1999). Because of the magnitude limit of the TYCHO cata- stars in our sample. The remaining 6 stars (HD 4635, HD 4741, logue, our sample is strongly biased towards G-type stars. This HD 7924, HD 8049, HD 133002, and HD 209943) could not be implies that in our survey we are likely to miss most of the observed because of bad weather. young late-type stars in the field (assuming a standard IMF). In In addition to our target stars, two radial velocity standard our sample, there are 473 stars earlier than K0 ((B V) < 0:81), stars were usually observed each night. To determine rotational 257 K-stars (0:81 (B V) < 1:4), and only 24 M-stars− (in the velocities, on each run several stars with known rotational ve- range M0–M1.5).≤ − locity were observed to be used as calibrators. R. Wichmann et al.: Nearby young stars 985 Table 1. Observing runs for this project. (*) Completely lost because Table 2. Breakdown of the sample studied by us (total = 748 stars).