1 STRUCTURE AND EVOLUTION OF NEARBY OB ASSOCIATIONS 1 1 1 1 1 1 1;2 P.T. de Zeeuw , A.G.A. Brown , J.H.J. de Bruijne , R. Ho ogerwerf , J. Lub , R.S. Le Po ole , A. Blaauw 1 Sterrewacht Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands 2 Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands The study of OB asso ciations is also imp ortant in ABSTRACT the context of the evolution of the Galaxy. The kine- matics of the nearest OB asso ciations provides in- sightinto the prop erties and origin of the Gould Belt We present the rst results of a comprehensive cen- system. Furthermore, OB asso ciations are resp onsi- sus of the stellar content of the nearby OB asso cia- ble for large bubbles in the interstellar medium lled tions based on Hipparcos p ositions, prop er motions with hot gas e.g., Mac Low & McCray 1988. More and parallaxes for 12842 candidate memb er stars dis- detailed knowledge of their stellar content is imp or- tributed over 21 elds on the sky. We use a new tant for understanding the energetics and dynamical metho d to identify moving groups in these elds see evolution of the bubbles. Ultimately, establishing the de Bruijne et al., these pro ceedings. Previously, as- prop erties of the nearby asso ciations is a prerequisite trometric memb ership in nearly all the nearby OB for the interpretation of observations of extragalactic asso ciations was known only for stars with sp ectral star forming regions and starburst galaxies. typ es earlier than B5. The Hipparcos measurements now allow us to identify memb ers down to late F. This census provides a rm basis for studies of galac- The Solar neighb ourho o d contains a number of OB tic and extragalactic star forming regions. asso ciations, so that detailed studies are p ossible. The most reliable memb ership determinations for OB asso ciations are based on prop er motion studies. Al- though these groups are unb ound, their expansion Key words: OB asso ciations; HR diagram; luminos- 1 velo cities are only a few km s e.g., Mathieu 1986, ity calibration; stars: early-typ e so that the common space motion is p erceived as a motion of the memb ers towards a convergent p oint on the sky e.g., Blaauw 1946; Bertiau 1958. Mem- b ership determinations have b een carried out byvar- 1. INTRODUCTION ious investigators see Blaauw 1964, 1991 and refer- ences therein. Due to their large extent on the sky, OB asso ciations are not amenable to prop er motion Ever since sp ectral classi cations for the bright stars studies with photographic plates. Instead, one had b ecame available it was evident that O and B stars to rely on prop er motions from large scale surveys are not distributed randomly on the sky, but in- with meridian circles. This resulted in much uncer- stead are concentrated in lo ose groups Blaauw 1964 taintyonmemb ership of stars of sp ectral typ e later and references therein. Ambartsumian 1947 found than B5. As a result, our knowledge of these young that the stellar mass density in these groups, which stellar groups remained rather limited. were subsequently called OB asso ciations, is usu- 3 ally less than 0:1M pc . Bok 1934 had already In order to remedy this problem the SPECTER con- shown that suchlow-density stellar groups are unsta- sortium was formed in Leiden in 1982. It success- ble against Galactic tidal forces, so that the observed fully prop osed the observation by Hipparcos of can- OB asso ciations must be young, a conclusion sup- didate memb ers of nearby OB asso ciations. An ex- p orted by the ages derived from Hertzsprung{Russell tensive program of ground-based observations was diagrams. These groups are prime sites for the study carried out in anticipation of the release of the Hip- of star formation pro cesses and of the interaction of parcos data see de Zeeuw et al. 1994. This in- early-typ e stars with the interstellar medium see, cluded Walraven V B LU W photometry de Geus e.g., Blaauw 1964, 1991 for reviews. Detailed knowl- et al. 1989; Brown et al. 1994, mm and radio ob- edge of the stellar content and structure of OB asso- servations of the interstellar medium surrounding the ciations allows us to address fundamental questions asso ciations de Geus 1992; Brown et al. 1995, and on the formation of stars in giant molecular clouds. sp ectroscopy aimed at obtaining precise radial and What is the initial mass function? What are the char- rotational velo cities Verschueren et al. 1997; Brown acteristics of the initial binary p opulation? What &Verschueren 1997. Theoretical work included the is the star formation eciency? Do all stars in a distinguishing features of the formation of b ound and group form at the same time? What pro cess causes unb ound groups Verschueren & David 1989, and the distinction b etween the formation of b ound op en the reliability of the so-called kinematic ages of OB clusters and unb ound asso ciations? How is angular asso ciations Brown et al. 1997a. momentum redistributed during star formation? oral05-06 2 Figure1. Proper motions for 532 members of the Sco OB2 association selectedfrom 4156 candidate stars in our Hipparcos sample for the areabounded by the dashed lines. The dotted lines are the schematic boundaries between the three classical subgroups Upper Scorpius, Upper Centaurus Lupus, and Lower Centaurus Crux. prop er motion memb ers, and 3 of the 10 additional Here we present the preliminary results of the photometric memb ers. We have added another 148 SPECTER census of the nearby asso ciations based on memb ers, including one M giant. Figure 1 illustrates the Hipparcos prop er motions and trigonometric par- the motions in the three elds, and shows that assign- allaxes. Wehave develop ed a new pro cedure to iden- ing b oundaries to the subgroups is not trivial. The tify moving groups in the Hipparcos Catalog de numb ers given ab ove are based on the preliminary Bruijne et al. 1997, and have applied it to 21 elds b oundaries used by de Zeeuw et al. 1994. Analy- for which we have Hipparcos data. The elds are sis of the entire Hipparcos Catalog, and inclusion of centered on 18 known asso ciations and 3 susp ected radial velo city data, is needed to determine the full groups, all within 800 p c from the Sun. A list of the extent of Sco OB2, and the division in subgroups. elds and their b oundaries, with the numberofob- jects, can be found in de Zeeuw et al. 1994. The dataset also includes 153 candidate runaway OB stars Prior to the Hipparcos mission, we collected precise as well as 49 stars from the disp ersed Cas{Tau as- intermediate band Walraven V B LU W photometry so ciation. Below we discuss a few examples of the for 2243 of the 4156 Hipparcos stars in the three Sco results obtained so far. Other examples are given in OB2 elds de Geus et al. 1989, 1990. We have de Bruijne et al. 1997 and Ho ogerwerf et al. 1997. measurements for 276 of the total of 532 Hipparcos memb ers. Figure 2 shows the Walraven [B U ]versus [B L] diagram for the previously susp ected memb ers top and the Hipparcos memb ers b ottom. Figure 2 2. SCORPIO{CENTAURUS shows that the Hipparcos measurements allow identi- cation of new memb ers to much later sp ectral typ es, all the way to the mid F regime! The removal of inter- The Sco OB2, or Scorpio{Centaurus, asso ciation con- lop ers in the pre-Hipparcos memb ership lists results tains three classical subgroups which di er in age inamuch tighter correlation near the S-turn of the Blaauw 1946, Blaauw 1964; de Geus et al. 1989: main sequence around [B L]; [B U ] 0:2; 0:4. Upp er Scorpius, Upp er Centaurus Lupus, and Lower Stellar evolution moves stars towards the lower left: Centaurus Crux. Our memb ership selection for Up- this causes the slightdownward curve at the top of p er Scorpius is describ ed in detail by de Bruijne et al. the main sequence. The remaining spread is caused 1997. We have similarly analysed the other two by i p eculiar sp ectra with, e.g., emission features elds. For Upp er Centaurus Lupus 19 of the 29 clas- cf. 48 Lib and Oph, ii undetected duplic- sical prop er motion memb ers, and 30 of the 101 pre- ity/multiplicity; and iii stellar rotation: high values viously known photometric memb ers were selected. of v sin i move stars towards the lower left. This e ect Wehave discovered another 139 memb ers. For Lower < < is strongest for B7 { B9 stars 0:05 [B L] 0:13. Centaurus Crux we con rm 15 of the classical 22 oral05-06 3 -0.2 0 0.2 log Teff 0.4 log g 0.6 0 0.2 0.4 0.6 0 0.1 0.2 0.3 0 0.1 0.2 0.3 0 0.1 0.2 0.3 Figure 2. De-reddened Walraven colour-colour diagrams for the three classical subgroups of Sco OB2. a 84/91 pre- Hipparcos `members' of Upper Scorpius top and 106/178 new members bottom for which we have Walraven photometry. Speci c stars are indicated.