New triple systems in the RasTyc sample of stellar X-ray sources

et al. 2009

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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.

Section: Effective Temperature Gravity and Metallicitymentioning

confidence: 68%

“…The membership in SKG can be analysed more objectively using probabilistic methods (Klutsch 2008;Klutsch et al 2009, in preparation). We computed the probability that a star with heliocentric velocities U, V, and W has a galactic motion compatible with one of the five aforementioned comoving groups of stars.…”

Section: Moving Groups Candidatesmentioning

confidence: 99%

A spectroscopic survey of the youngest field stars in the solar neighbourhood

Guillout¹,

Klutsch²,

et al. 2009

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“…We remark that the other values of parallax listed in Table 1 do not significantly change the values of the space motion components. We also applied a kinematic method developed by Klutsch et al (2008) to determine the membership probability of five young SKGs, namely, the IC 2391 supercluster, the Pleiades moving group or Local Association, the Castor moving group, the Ursa Major group or Sirius supercluster, and the Hyades supercluster. We find a space velocity fully consistent with the young-disk population and a high membership probability (64%) of the LA (20−150 Myr).…”

Section: Radial Velocity and Space Motionmentioning

confidence: 99%

The young active star SAO 51891 (V383 Lacertae)

Biazzo

Marilli

et al. 2009

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Aims. The aim of this work is to investigate the surface inhomogeneities of a young, late-type star, SAO 51891, at different atmospheric levels, from the photosphere to the upper chromosphere, analyzing contemporaneous optical high-resolution spectra and broad-band photometry. Methods. The full spectral range of FOCES@CAHA (R 40 000) is used to perform the spectral classification and to determine the rotational and radial velocities. The lithium abundance is measured to obtain an age estimate. The BVRIJHK s photometric bands are used to construct the spectral energy distribution (SED). The variations in the observed BV fluxes and effective temperature 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 Hα, Ca ii H & K, H , and Ca ii IRT lines.Results. We find SAO 51891 to be a young K0-1V star with a lithium abundance close to the Pleiades upper envelope, confirming its youth (∼100 Myr), which is also inferred from its kinematical membership of the Local Association. No infrared excess is detected from analysis of its SED, limiting the amount of remaining circumstellar dust. We detect a rotational modulation of the luminosity, effective temperature, Ca ii H & K, H , and Ca ii IRT total fluxes. A simple spot model with two main active regions, about 240 K cooler than the surrounding photosphere, fits the observed light and temperature curves very well. The small-amplitude radial velocity variations are also well reproduced by our spot model. The anti-correlation of light curves and chromospheric diagnostics indicates chromospheric plages spatially associated with the spots. The largest modulation amplitude is observed for the H flux suggesting that this line is very sensitive to the presence of chromospheric plages. Conclusions. SAO 51891 is a young active star, lacking significant amounts of circumstellar dust or any evidence of low mass companions, and displays the typical phenomena produced by magnetic activity. The spots are larger and warmer than those in less active main-sequence stars. If some debris is still present around the star, it will only be detectable by future far-infrared and sub-mm observations (e.g., Herschel or ALMA). The RV variation produced by the starspots has an amplitude comparable with those induced by Jupiter-mass planets orbiting close to the host star. SAO 51891 is another good example of an active star in which the detection of planets may be hampered by the high activity level.

“…Afterwards, BD+44 3670 and BD+45 598 were recognized as members of the Columba association (∼30 Myr; Zuckerman et al 2011) and of the β Pic moving group (∼12 Myr; Moór et al 2011), respectively. Klutsch (2008) noticed that an almost uniform spatial density of young stars in the optically bright RasTyc sample. At the same time, the spatial density in the northern hemisphere (Paper I) is about one order of magnitude lower, on average, than that in the southern hemisphere (the SACY survey).…”

Section: Introductionmentioning

confidence: 97%

Discovery of new members of the nearby young stellar association in Cepheus

Klutsch

Guillout

et al. 2020

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Context. Young field stars are hardly distinguishable from older ones because their space motion rapidly mixes them with the stellar population of the Galactic plane. Nevertheless, a careful target selection allows for young stars to be spotted throughout the sky. Aims. We aim to identify additional sources associated with the four young comoving stars that we discovered towards the CO Cepheus void and to provide a comprehensive view of the Cepheus association. Methods. Based on multivariate analysis methods, we have built an extended sample of 193 young star candidates, which are the optical and infrared counterparts of ROSAT All-Sky Survey and XMM-Newton X-ray sources. From optical spectroscopic observations, we measured their radial velocity with the cross-correlation technique. We derived their atmospheric parameters and projected rotational velocity with the code ROTFIT. We applied the subtraction of inactive templates to measure the lithium equivalent width, from which we infer their lithium abundance and age. Finally, we studied their kinematics using the second Gaia data release. Results. Our sample is mainly composed of young or active stars and multiple systems. We identify two distinct populations of young stars that are spatially and kinematically separated. Those with an age between 100 and 300 Myr are mostly projected towards the Galactic plane. In contrast, 23 of the 37 sources younger than 30 Myr are located in the CO Cepheus void, and 21 of them belong to the stellar kinematic group that we previously reported in this sky area. We report a total of 32 bona fide members and nine candidates for this nearby (distance = 157 ± 10 pc) young (age = 10–20 Myr) stellar association. According to the spatial distribution of its members, the original cluster is already dispersed and partially mixed with the local population of the Galactic plane.