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Oral06 03.Pdf 519 IDENTIFICATION OF MOVING GROUPS IN A SAMPLE OF EARLY-TYPE MAIN-SEQUENCE STARS 1 3 1 1 2 F. Figueras ,A.E.G omez , R. Asiain , B. Chen , F. Comer on , 3 3 4 3 1 S. Grenier , Y. Lebreton , M. Moreno , V. Sabas ,J.Torra 1 Departament d'Astronomia i Meteorologia, Universitat de Barcelona, Spain 2 Europ ean Southern Observatory, Garching b ei Munc hen 3 DASGAL, URA 335 du CNRS, France 4 DepartamentdeF sica i Enginyeria Nuclear, U.P.C., Spain ful compilation of radial velo cities have b een used for ABSTRACT this study. Di erent metho ds have b een used to characterize The kinematic structure of young B5{F5 main- moving groups in the 4-dimensional space of U,V,W, sequence stars in the solar neighb ourho o d is analysed age: a parametric approach SEMMUL algorithm, using b oth accurate observational data { new astro- Celeux & Dieb olt 1986; a non-parametric technic us- metric Hipparcos data and complementary data from ing a Kernel estimator Chen et al. 1997, a metho d the ground { and several robust statistical metho ds. based on the wavelet transform and a metho d based The coherence of the results obtained from the di er- on the aggregation of groups through moving centers. ent metho ds SEMMUL, kernel estimator, wavelets, In Section 2, we describ e the sample's compilation dynamical clusters allows us to de nitively con rm and its mean characteristics. Section 3 contains a the existence of the Eggen's moving groups and to de- brief description of the statistical metho ds used and termine, without any a priori knowledge, their mean the results obtained, and Section 4 is devoted to the characteristics in the 4-dimensional space U,V,W, analysis of the moving groups mean parameters. age. We nd that mean velo city disp ersion in the velo city comp onent in the direction of the galactic rotation for Sirius, Pleiades, Hyades and Praesep e 1 moving groups are in the range 3{8 km s ,avalue 2. THE SAMPLE larger than classically exp ected. The working sample compiled by V. Sabas 1997 has b een selected from the global set of Hipparcos Key words: young stars; galactic kinematics; moving B5{F5 main-sequence stars with apparent magnitude groups. 7.5 or brighter. In order not to bias the results when lo oking for moving group memb ers among the eld stars, the stars known as op en cluster's memb ers have 1. INTRODUCTION b een rejected. To compute reliable physical parame- ters and ages, double or susp ected double stars and stars with p eculiarities in the sp ectra sp ectral typ es The study of the irregularities in the velo city eld from the Inca Catalogue have also b een rejected. of young stars in the solar neighb ourho o d yields im- The radial velo cities have b een taken from the recent p ortant clues to understand the history of star for- compilations of Barbier-Brossat 1997 and Du ot mation and the dynamical pro cesses involved in the et al. 1995, and from the Key-Programmes under- evolution of our Galaxy. The detection and analysis taken to complement the Hipparcos data OHP { of features suchasmoving groups, the vertex devia- Marly Sp ectrograph, ESO { Echelee Sp ectrograph: tion, and gradients of the velo city eld demands the Grenier 1997. Individual ages have b een obtained use of robust statistical metho ds and highly accurate Asiain et al. 1997 interp olation algorithm from astrometric and physical data. Our pro ject attempts Lebreton 1995 stellar evolutionary mo dels, taking b oth to de nitively con rm the presence of moving into account the metallicity, the e ective temp er- groups and, through the kinematic and physical prop- ature b eing computed from Stromgren photometry erties of its memb ers, to constrain mo dels accounting Hauck & Mermillio d 1996, Jordi et al. 1996, the for their origin. stellar luminosity from Hipparcos M and the b olo- v metric correction of Malagnini 1966. Only stars with logage larger than 7.5 have b een retained. A selection of 2588 young B5{F5 main sequence Hip- Distances have b een computed directly from Hip- parcos stars within ab out 250 p c from the Sun, with parcos parallaxes for the stars with = 0:15, known individual ages, computed from Stromgren and with a weighted mean between Hipparcos and photometry and Hipparcos parallaxes, and a care- 520 photometric distance for the rest few stars. Meil- lon 1996 algorithm has b een used to compute the U,V,W comp onents of the helio centric space ve- lo city U directed towards the galactic center, V towards the galactic rotation direction and W to- wards the north galactic p ole and the corresp ond- ing errors considering the Hipparcos correlation co- ecients among variables. Finally, the stars with 1 residual velo city larger than 65 km s have b een rejected b ecause they probably do not b elong to the galactic disk p opulation. The sample contains 2588 stars with a mean velo city comp onents U; V ; W = 1 11:72 0:06; 11:38 0:06; 7:03 0:05 km s o o and a vertex deviation of = 22:5 1:2 . The mean observational errors in the velo city comp onents 1 are ; ; = 2.4,2.3,1.9 km s and the dis- U V W tribution of the relative error in age is presented in Figure 1. Figure 2. The distribution of the total sample in the U,V plane. 300 the U,V,W comp onents of the helio centric space ve- lo city and the individual ages . As will be seen, this last parameter give imp ortant clues to the de - 200 nition of the moving groups physical prop erties 3.1. SEMMUL Algorithm Number of stars 100 SEMMUL algorithm Sto chatique, Estimation, Max- imisation, MULtidimensionnel, develop ed by Celeux & Dieb olt 1986, is a parametric metho d allowing 0 ts inside a sam- 0.00 0.25 0.50 0.75 1.00 the separation of Gaussian comp onen relative error in age ple. It do es not need to adopt a set of initial condi- tions and can work without a predetermined knowl- edge of the numb er of comp onents only needs a max- imum numb er. Adapted by Arenou 1993, it takes Figure 1. Relative errors in age for the total sample into account a Gaussian distribution of the observa- tional errors. computed using Monte Carlo simulations, Asiain et al. 1997. It has b een applied to the whole sample in the four- dimensional space of U,V,W, log and ab out 50 runs have b een p erformed assuming a maximum of 3. STATISTICAL METHODS 6,7, and 8 groups Sabas 1997. The results are pre- sented in Table 1. Figure 2 shows the distribution of the total sample in the U,V plane. De nitively, and as already noticed by Palous and Hauck 1986, the observed velo city 3.2. Wavelet Transform distribution in these sp ectral ranges cannot be de- scrib ed by a unique velo city ellipsoid, indicating that The wavelet transform provides an easily inter- a mixture of several kinematic structures is present. pretable visual representation of a signal sx,y,z. Moreover, it allows to detect structures working at Four di erent statistical metho ds have b een applied di erent scales detection of hierarchic classes, in- to the total sample to determine which are the mean cluding voids in the distribution. The wavelet co e- characteristics of these kinematic structures. They cients are computed from: are based on di erenthyp othesis Gaussian distribu- tion of the sub-structures, Gaussian distribution for the remaining eld without imp osing any prede ned knowledge of the substructures, clustering metho ds O without previous hyp othesis, etc, so the coherence WCx; y ; z ; =gx; y ; z ; sx; y ; z 1 among the obtained results will give us a wide under- standing of the real phenomena. The adopted wavelet function g x; y ; x; is the ra- The p osition of the stars around the Sun do es not dial Mexican Hat Murenzi 1990 which in the 3- provide any discriminant information for the detec- Dimensional space takes the form: tion of moving groups, so the working parameters are 521 Table 1. The means and dispersions of the groups found using the SEMMUL algorithm in the 4-dimensional space: U, 1 V,Wcomponents Units: km s and log . U V W log U V W log 16:8 10:3 6:5 7:2 7:6 4:2 8:0 0:2 6:4 0:6 0:6 0:3 0:4 0:4 0:2 3:4 9:7 23:9 5:5 6:4 4:6 4:4 8:2 0:2 13:8 Pleiades 0:3 0:3 0:3 0:3 0:2 0:2 5:2 9:7 6:0 8:4 4:4 3:1 6:3 8:5 0:2 8:2 0:3 0:2 0:5 0:2 0:2 0:3 5:2 26:5 14:5 5:3 6:4 4:8 6:7 8:6 0:3 10:3 0:4 0:3 0:4 0:3 0:2 0:3 3:5 10:0 2:8 7:0 7:5 5:2 6:5 8:7 0:2 15:1 Sirius 0:4 0:3 0:3 0:3 0:2 0:2 2:3 37:4 15:2 7:9 6:0 5:2 7:6 8:9 0:2 7:7 Hyades 0:4 0:4 0:5 0:3 0:3 0:4 3:7 10:9 12:4 8:0 16:9 13:4 10:2 8:8 0:4 38:5 eld 0:6 0:4 0:3 0:4 0:3 0:2 12:9 2 2 2 2 2 2 x +y +z x +y +z 2 2 2 g x; y ; z ; = 3 e 2 where is the scale variable: its biggest value is con- strained by the extent of the sample and the lowest 1 by the observational errors 2kms .
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