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Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/913
| Optical polarimetry, high--resolution spectroscopy and IR analysis of the Chamaeleon I dark cloud
We present optical polarimetry and high resolution spectroscopy of asample of stars toward the Chamaeleon I dark cloud. We use ourpolarimetry which includes 33 stars to study the wavelength dependenceof the degree and position angle of polarization. From fits to thenormalized wavelength dependence of interstellar polarization, we deriveestimates of lambdaMax ranging from 4500 \AA to 6700 \AA, andPMax ranging from 3 to 8%. The values of lambdaMaxwere found to be well correlated with the IRAS 100 micron intensity,while PMax was found to increase with E_{B-V}. Highresolution spectra of the Ca II, CH, and CH^{+} lines were obtained for10 stars, which show two components of Ca II in absorption at 3.0
| On the distance to the Chamaeleon I and II associations
Constraints on the distances to the dark clouds Chamaeleon I and II areinvestigated in detail. A compilation of photometric data, spectraltypes and absolute magnitudes for field stars towards each cloud ispresented, and results are used to examine the distribution of reddeningwith distance along each line of sight. The distances to starsassociated with reflection nebulae in each cloud are examined in detail.On the basis of these results, we deduce the most probable distance ofCha I to be 160+/-15pc, and that of Cha II to be 178+/-18pc. Anexamination of the mean fluxes of T Tauri stars in each cloud providesindependent evidence to suggest that Cha II is significantly moredistant than Cha I. Both clouds appear to be embedded in a macroscopicsheet-like structure extending over much of the Chamaeleon-Musca-Cruxregion. The Chamaeleon III and DC\ts 300.2--16.9 clouds are probablypart of the same structure, with probable distances ~ 140--160pc.
| The ROSAT all-sky survey catalogue of optically bright OB-type stars.
For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars.
| A Study of the Chamaeleon-I Dark Cloud and T-Association - Part Six - Interstellar Polarization Grain Alignment and Magnetic Field
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.268....1W&db_key=AST
| The distribution of interstellar dust in the solar neighborhood
We surveyed the IRAS data base at the positions of the 1808 O6-B9.5stars in The Bright Star Catalog for extended objects with excessemission at 60 microns, indicating the presence of interstellar dust atthe location of the star. Within 400 pc the filling factor of theinterstellar medium, for dust clouds with a density greater than 0.5/cucm is 14.6 + or - 2.4%. Above a density of 1.0/cu cm, the densitydistribution function appears to follow a power law index - 1.25. Whenthe dust clouds are mapped onto the galactic plane, the sun appears tobe located in a low-density region of the interstellar medium of widthabout 60 pc extending at least 500 pc in the direction of longitudes 80deg - 260 deg, a feature we call the 'local trough'.
| A study of the Chamaeleon dark cloud and T-association. I - Extinction, distance and membership
The results of an observing program to investigate the nature and degreeof reddening of a number of stars in the field towards the dark cloudand T-association in Chamaeleon are presented. The observations compriseoptical (UBVRI) and infrared (JHK) photometry, and optical spectroscopy.New and previously published data are combined to assess membershipcriteria for 110 stars, and separate catalogues of association membersand field stars are presented. Optical identifications are listed for 27IRAS point sources in the region. The reddening/distance relation forfield stars indicates a distance to the cloud of 140 + or - 12 pc. Thisresult confirms that the extinction law is anomalous towards some starsin the cloud, notably the embedded A0 ZAMS star HD 97300, with values ofup to 5.5 for the ratio of total-to-selective extinction. A comparisonof 21-cm data with the reddening of background field stars indicatesthat the ratio N(H I)/E(B-V) is significantly less than the interstellarmean, indicating that most of the hydrogen in the cloud is molecular.
| The local system of early type stars - Spatial extent and kinematics
Published uvby and H-beta photometric data and proper motions arecompiled and analyzed to characterize the structure and kinematics ofthe bright early-type O-A0 stars in the solar vicinity, with a focus onthe Gould belt. The selection and calibration techniques are explained,and the data are presented in extensive tables and graphs and discussedin detail. The Gould belt stars of age less than 20 Myr are shown togive belt inclination 19 deg to the Galactic plane and node-lineorientation in the direction of Galactic rotation, while the symmetricaldistribution about the Galactic plane and kinematic properties (purecircular differential rotation) of the belt stars over 60 Myr oldresemble those of fainter nonbelt stars of all ages. The unresolveddiscrepancy between the expansion observed in the youngest nearby starsand the predictions of simple models of expansion from a point isattributed to the inhomogeneous distribution of interstellar matter.
| Radial velocities of bright southern stars. I - 139 B-type HR and FK stars
Radial velocity determinations for 139 southern B stars in the BrightStar, FK4, and FK4 Supplement catalogues and for 53 bright southernlate-type stars are presented, based on coudespectrograms taken with theESO 1.5 m telescope at La Silla. The programs, observations,measurements, and reductions are described. Among the data presented arethe visual magnitude, spectral class, mean error, E/T variabilityparameter, velocity of interstellar lines, and rotation class.
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