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IRREGULARITIES of the VELOCITY FIELD J. Torra1

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IRREGULARITIES of the VELOCITY FIELD J. Torra1 513 YOUNG STARS: IRREGULARITIES OF THE VELOCITY FIELD 1 2 1 3 J. Torra ,A.E.G omez , F. Figueras , F. Comer on , 2 4 1 1 S. Grenier , M.O. Mennessier , M. Mestres ,D.Fern andez 1 Departament d'Astronomia i Meteorologia. Universitat de Barcelona, Spain 2 Observatoire de Paris-Meudon, D.A.S.G.A.L., France 3 Europ ean Southern Observatory, Garching b ei Munc hen, Germany 4 GRAAL. Universit e de Montp ellier I I, France interpreted as a lo cal e ect of the spiral arm Lind- ABSTRACT blad 1973, by energetic event on the galactic disk Olano 1982, Elmegreen 1982, corrugation waves in the galactic disk Nelson 1976 or by an halo-disk We present here a rst review of the structure and interaction Comer on 1992, Lepine & Duvert 1995. kinematics of the lo cal system of young stars. Hip- parcos astrometric data for a large sample of O and The velo city eld of young stars is describ ed to rst B stars have b een complemented with a careful com- order by the Oort's formulae Ogoro dnikov 1965 pilation of available radial velo cities and Stromgren relating the observed radial velo city, distance and photometry thus providing reliable distance, spatial prop er motion with the solar motion and the gra- velo city and age for each star in the sample. The dients of the velo city eld. Several works have b een fundamental parameters characterizing the structure devoted to the determination of the Oort's constants of the Gould Belt, inclination, size and age are deter- considering only di erential rotation or including mined. The velo city eld is studied by means of the other e ects such as spiral structure or expanding classical rst order approach. Oort's constants A, B, motions Lindblad 1980, Cr ez e et al. 1973, Byl et al. C, K are determined for several subsamples selected 1981, Comer on et al. 1991 among others. by age and the results explained by the presence of the Gould Belt. 2. THE CATALOGUE Key words: galactic kinematics; young stars; Gould Belt. Our initial sample contains 6922 O-B typ e stars Hip- parcos Internal proposal, INCA060, of which 5846 b elong to the Hipparcos Survey. The sources of ra- 1. INTRODUCTION dial velo cities have b een the Barbier-Brossat 1997 and the Du ot et al. 1995 compilations. Priority has b een given to the rst one, and only stars with O and B stars in the solar neighb ourho o d have b een quality A, B or C in the last compilation have b een traditionally used as prob es of galactic structure and considered Grenier 1997. Using these sources, only kinematics. Their intrinsic brightness allows their 3182 stars of our total sample have known radial ve- observations to great distances from the Sun, and lo city, this parameter b eing, at present, the most im- due to their youth, their motions can be exp ected p ortant limitation to undertake the study of the ve- to hold imp ortant clues to the pro cesses that formed lo city eld in the solar neighb ourho o d. them. An imp ortant characteristic of the distribu- Stromgren photometry has b een used to compute tion of young stars were recognized more than one photometric distances and ages. 3031 stars from our century ago when J. Herschel realized that the lo- sample have complete photometry in the Hauck & cal system of young stars shows a concentration of Mermillio d 1996 catalogue. From these, we have stars along a great circle tilted resp ect to the galac- rejected the stars b eing double or multiple from Hip- tic equator. Since the pioneering work by Gould in parcos <10 arcsec and m<3 mag, the variable 1874-1879 a great number of studies have b een de- stars m > 0:6 mag and the stars showing p ecu- voted to the Gould Belt trying to explain its struc- liarities in the sp ectral typ e or in the photometric ture and kinematics, characterized by the existence indices Masana 1994. Crawford 1978 calibration of a expanding motion Lesh 1968, Stothers & Frogel, has b een used to derive photometric distances which 1974, Frogel & Stothers 1977. More recently Westin are compared to the Hipparcos distances { computed 1985, Comer on et al. 1991, Comer on 1992, as the inverse of the parallax { in Figure 1. Al- Palous 1995 and Lindblad et al. 1997 have studied though no systematic trend seems to b e present, an the Belt and determined its orientation, age and kine- exp ected large scattering is observed for stars at dis- matics. Several mo dels have b een presented in order tances higher than 200 pc. Other photometric cal- to explain the origin of the Gould Belt: it has b een 514 1000 250 200 750 150 500 100 Number of stars Hipparcos distance (pc) 250 50 0 0 0 250 500 750 1000 6.0 6.5 7.0 7.5 8.0 8.5 9.0 Photometric distance (pc) Log(Age) Figure1. Comparison between photometric and Hippar- Figure 2. Age distribution of the working sample 2139 cos distances for the working sample 2139 stars. Pho- stars. Ages computedfrom Bressan et al. 1993 evolu- tometric distances computed from Crawford 1978 cali- tionary models. brations. 200 ibrations Balona et al. 1984, Jakobsen 1985 have b een also considered for the computation of photo- metric distances, thus checking that the kinematic results do not dep end of the calibration used. 150 Individual ages have b een computed from the evo- lutionary mo dels of Bressan et al. 1993 for solar following Asiain et al. 1997 interp o- comp osition 100 lation algorithm, which considers, as input parame- ters, the T and log g derived from the photometric e Number of stars indices Mo on et al. 1985, grids. The stars show- ing a relative error on age larger than 100 p er cent 50 have b een rejected these are stars near or b elow the ZAMS for which the obtained age is not reliable. The histograms of the age distribution and the com- puted individual errors are presented in Figure 2 and 0 ely. Figure 3 resp ectiv 0 102030405060708090100 Relative error in Age According to the physical information available we Figure3. Distribution of the relative errors in age 2139 have de ned twoworking samples: stars. Sample 1: 2139 stars with good = 100 stars clearly showing the galactic b elt and the in- p er cent ages and photometric distances. Useful clined and assymetric structure of the Gould Belt. to determine the fundamental parameters of the Gould Belt. Wehave not tried to individually separate the stars b elonging to the Gould Belt from the ones in the Sample 2: 1539 stars, a subsample of Sample 1, galactic plane; otherwise we will follow the metho d containing the stars with known radial velo cities develop ed by Comer on et al. 1994, well suited for having 7 km/s, well suited for the analysis V r large samples, which allows a statistical determina- of the systematic velo city eld. tion of the parameters characterizing the geometry of b oth b elts. We assume that the density distribution of the sample in the celestial sphere can b e writen as: 3. FUNDAMENTAL PARAMETERS OF THE GOULD BELT l; b= l; b+ l; b 1 G g We can see in Figure 4 where X,Y,Z are the galac- tic helio centric co ordinates with X directed towards the galactic center, Y in the direction of the galactic where and are the density distributions around G g rotation and Z directed towards the north galactic the Gould Belt and the galactic equator resp ectively. p ole, the well known spatial distribution of young For each one of these distributions we assume that: 515 500 a) individual ages, to b e complete, our sample is limited to stars brighter than 6.5 mag. 0 In Table 1 we show the results obtained for several subsamples of sample 1 selected by age and photo- Assuming q > 0 as an indicator of −500500 metric distance. e see that it is de- b) the presence of the Gould Belt, w ned by i =17 20 and = 278 290 , in good t with previous results. We found the high- 0 agreemen Z (pc) est q value, 0.94, for stars younger than 30 million years and distances between 400 and 600 pc; some ears establish the limits −500500 older stars up to 60 million y the Gould Belt at 400 pc, and very few, if any, c) of stars older than 60 million years indicate also a limit Tsioumis & Fricke 1979 considered the 0 of 400 p c. extent to b e 450{600 p c, and Westin 1985 consid- ered 250{500 p c dep ending on the galactic longitude. e can see the relationship b etween age −500 In Figure 4 w −1500 −1000 −500 0 500 1000 1500 and structure in the X, Z plane. Wewant to remark X (pc) that when the metho d is applied to the complete sam- Figure4. Spatial distribution in the X,Z plane: a total ple of OB Survey stars with V 7:9 see Figure 5 the same conclusions are reached, although in this sample of 2139 stars; b 1120 stars with 60 Myr; c case only sp ectral typ es are available as indicators of 1019 stars with >60 Myr. evolution. 2 sin 4. SYSTEMATIC MOTION OF YOUNG STARS / exp 2 2 2 sin =2 4.1.
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