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Highlights  of  Spanish  Astrophysics  VI,  Proceedings  of  the  IX  Scientific  Meeting  of  the  Spanish  Astronomical  Society  held  on  September  13  -­  17,  2010,  in  Madrid,  Spain.  M.  R.  Zapatero  Osorio  et  al.  (eds.)   Spectroscopic properties of nearby late-type stars,members of stellar kinematic groups 1 Universidad Aut´onoma de Madrid, Dpto. F´ısica Te´orica, M´odulo 15, Facultad deCiencias, Campus de Cantoblanco, E-28049 Madrid, Spain, [email protected] Universidad Complutense de Madrid, Facultad Ciencias F´ısicas, E-28040 Madrid, Spain3 Centro de Astrobiolog´ıa (CSIC-INTA), ESAC Campus, P.O. BOX 78, E-28691, Villanuevade la Ca˜ Nearby late-type stars are excellent targets to look for young objects in stellar associations The study of these groups goes back more than one century ago however, their origin is still misunderstood. Although their existence have been confirmed by statistical studies of large sample of stars, the identification of a group of stars as member of moving groups, is not an easy task, list of members often change with time and most members have been identified by means of kinematics criteria which is not sufficient since many old stars can share the same spatial motion of those stars in moving groups. In this contribution we attempt to identify unambiguous moving groups members, among a sample of nearby-late type stars. High resolution echelle spectra is used to i) derive accurate radial velocities which allow us to study the stars’ kinematics and make a first selection of moving groups members; and ii) analyze several age-related properties for young late-type stars index). The different age-estimators are compared and the moving group membership of the kinematically selected candidates are discussed.
The dispersal of young stars out into the field and whether or not there are kinematically-related groups of stars in the solar neighbourhood is one of the open problems in Galacticastrophysics. Although stars do not form isolated but within clusters and associations, mostof them end up in the field.
Once scattered through the Galaxy, these young stars are difficult to identify. assumed that clusters form a halo of evaporated stars. During itsdisintegration, the whole group can not be identified although within some regions one can Spectroscopic properties of nearby late-type stars found stars moving in the same direction and at the same rate. This idea gave rise to theconcept of moving group or Stellar Kinematic Group (hereafter SKG).
The idea of a moving group as a group of stars sharing a common origin has been the subject of an intense discussion. It is now, however, well established that the “classical”moving groups (e.g. Hyades, Ursa Major) are in reality a mixture of two different populations:a group of coeval stars (related to the halo of an evaporating cluster) and a second groupwith a dynamical (resonant) origin, e.g., Identifying stars with a common origin (i.e. same kinematics, same age, same chemical composition) is only possible if a combination of techniques are used. Nearby-late type starsare excellent targets for this kind of study since i) their spectra is full of narrow absorptionlines, allowing determination of accurate radial velocities, and ii) it is unlikely that an oldstar by chance shares chromospheric indices or a lithium abundance similar to those of youngsolar-like stars.
In a recent work, studied a sample of 405 nearby late-type stars, attempting to identifyunambiguous SKGs members by analysing high-resolution (R ∼ 57000) echelle spectra ob-tained in 2-3 meters class telescopes. The study is focused in nearby (distances less than25 pc), main-sequence (luminosity class V/IV-V), late-type (spectral types FGK) stars. TheHipparcos catalogue is used as a reference. To identify stars in SKGs, analyzed boththe kinematics and the spectroscopic age indicators.
Radial velocities were measured by cross-correlating, using the IRAF routine fxcor, the spec-tra of the program stars with spectra of radial velocity standard stars of similar spectraltypes. For those known spectroscopic binaries, the radial velocity of the centre of mass of thesystem were considered. Typical uncertainties are between 0.15 and 0.25 km s−1. These ra-dial velocities were used together with Hipparcos parallaxes and Tycho-2 proper motionsto compute the Galactic-spatial velocity components (U, V, W ), as explained in Young stars are assembled in an specific region of the (U, V ) plane (−50 km s−1 < U < 20 km s−1; −30 km s−1 < V < 0 km s−1), although the shape is not a square (see Fig. Possible members of SKGs are selected allowing a dispersion of 8 km s−1 in the U , V com-ponents with respect to the central position of the SKG. The same dispersion is consideredwhen taking the W component into account. The final number of candidates for each SKGis given in Table (column 4).
Members of a given SKG should be coeval and since clusters disperse on time scales of a fewhundred years they should also be moderately young (∼ 50–650 Myr). The classic method (U, V ) plane. Different colours and symbols indicate membership to different SKGs. Large crosses represent the convergence point of the young SKGs shown in the figureas given by The dotted line represents the boundary of the young disc population.
Figure from to compute the stellar age (i.e. evolutionary tracks) does not work good enough for thelatest spectral types. However, late-type stars show other properties which can be used todetermine their age: Lithium abundance: An age estimate of late-type stars can be carried out by comparing A equivalent width with those of stars in well known young open clusters of different ages, e.g., Nevertheless, it should be regarded as an additional age indicatorsince the relation lithium-age is poor constrain and biased towards younger ages.
Age derived from cromospheric activity: There are several observables of the magnetic field of a solar-type star: chromospheric emission lines, e.g. Ca II H, & K or Ca II IRT, or theX-ray emission from the stellar corona, e.g., In addition, there is a strong correlationbetween the stellar rotation and the chromospheric activity in cool stars, In thisway, the stellar age can be estimated which is a measure of the cromospheric emission in the cores of the Ca II H, & K absorption lines, normalised to the total bolometric emission ofthe star.
Spectroscopic properties of nearby late-type stars Table 1: Number of stars identified as possible members and non-members of moderatelyyoung SKGs.
• By searching for X-ray counterparts in the ROSAT catalogue.
• From the rotational period of the star (gyrochronology) The agreement between the different activity-age estimates is overall good, as can be seen in Fig. Chromospheric age shows an enhancement of the star formation rate in thelast 2 Gyr, then the distribution becomes more or less flat. ROSAT ages are biased towardsstars younger than 3–4 Gyr; i.e., older stars have negligible (or undetectable) X-ray emission,and therefore their distribution does not offer information on the stellar formation history.
As far as rotational ages are concerned, there are not enough stars with measured rotationalperiods to draw robust conclusions.
A summary of the ages obtained (percentages of stars according to their ages for each Conclusions and prospects for future work From a total sample of 405 stars, identify 102 stars which share the same kinematics thatthose stars in SKGs (i.e. ∼ 25% of the sample). In addition, 78 stars are classified as otheryoung discs stars (i.e. stars which are in the boundaries of the young disc population butwithout a clear identification to some of the SKGs). From them, 36 have ages that agreewith the accepted ages of the corresponding moving group. That means that only ∼10% ofthe nearby late-type stars can be associated to SKGs. Table summarises the number ofkinematic candidates to the different SKGs and the final number of possible members andno-members of each of the SKGs studied in List of young stars in SKGs can be very useful for further investigations. Some examples include search for solar analogues, substellar companions, study of the flux-flux and rotation-activity-age relationships in groups of stars with different ages or search for cold faintdusty debris discs (e.g. the DUNES project Figure 2: Age distribution for chromospheric-derived ages (upper panel), ROSAT ages (mid-dle panel), and rotational ages (lower panel). Figure from Table 2: Percentages of stars according to their estimated age for each age indicator.
† 50% older than 80–100 Myr; 15% of the stars show no photospheric Li I‡ Rotational periods only available for roughly 17% of the whole sample Spectroscopic properties of nearby late-type stars This work were supported by the Spanish Ministerio de Ciencia e Innovaci´ de Astronom´ıa y Astrof´ısica, under grants AYA2008-01727 and AYA2008-00695 and the ComunidadAut´ onoma de Madrid, under PRICIT project S-2009/ESP-1496 (AstroMadrid). J.M. acknowledges onoma de Madrid (Department of Theoretical Physics).
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