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Optical polarization study in the open cluster NGC 6250
We present (UBVRI) multicolour linear polarimetric data for 32 of thebrightest stars in the area of the open cluster NGC 6250, with the aimof studying the properties of the interstellar medium (ISM) towards thecluster. Our data yield a mean polarization per cent, close to thepolarization value produced by the ISM with normal efficiency for a meancolour excess of EB- V = 0.37. Our analysis indicates thatthe observed visual absorption in NGC 6250 is caused by a nearby dustlayer (within 300 pc) producing a polarization with an angle close tothe Galactic plane (?GP ~ 38°). In addition, thereare at least two more dust layers along the line of sight between theSun and the cluster producing a change in the observed polarization,making our results compatible with Neckel and Klare's results. Theobservations show differences between the orientation of the localmagnetic field of the nearby dust layer and the one that is polarizingalong the way to the cluster (??v = 20°).The internal dispersion of the polarization values for the members ofNGC 6250 seems to be compatible with the presence of intracluster dust.The majority of observed stars do not present evidence of intrinsicpolarization in their light. In this work, we also show (as in severalprevious papers) how polarimetry is an excellent technique foridentifying non-member stars.Based on observations obtained at Complejo Astronómico ElLeoncito (CASLEO), operated under agreement between the Conicet and theNational Universities of La Plata, Córdoba, and San Juan,Argentina.E-mail: cfeinstein@fcaglp.unlp.edu.ar
| Diffuse interstellar bands in RAVE survey spectra
We have used spectra of hot stars from the RAVE Survey in order toinvestigate the visibility and properties of five diffuse interstellarbands previously reported in the literature. The RAVE spectroscopicsurvey for Galactic structure and kinematics records CCD spectracovering the 8400-8800 Å wavelength region at 7500 resolvingpower. The spectra are obtained with the UK Schmidt at the AAO, equippedwith the 6dF multi-fiber positioner. The DIB at 8620.4 Å is by farthe strongest and cleanest of all DIBs occurring within the RAVEwavelength range, with no interference by underlying absorption stellarlines in hot stars. It correlates so tightly with reddening that itturns out to be a reliable tool to measure it, following the relationEB-V = 2.72 (± 0.03)× EW~(Å), validthroughout the general interstellar medium of our Galaxy. The presenceof a DIB at 8648 Å is confirmed. Its intensity appears unrelatedto reddening, in agreement with scanty and preliminary reports availablein the literature, and its measurability is strongly compromised bysevere blending with underlying stellar He I doublet at 8649 Å.The two weak DIBs at 8531 and 8572 Å do not appear real and shouldactually be blends of underlying stellar lines. The very weak DIB at8439 Å cannot be resolved within the profile of the much strongerunderlying hydrogen Paschen 18 stellar line.Table 1 is only available in electronic form at http://www.aanda.org
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Uvby-beta observations of 528 type B stars with V between the 8th and 9th magnitude
The paper presents uvby-beta measurements of 528 type B stars selectedfrom the SAO Catalog on the basis of two criteria: the spectral types inthe range B3-B5 and mV between the 8th and the 9th magnitude. Reddeningindependent (c1) values are estimated from the spectral classificationand compared to the observed values. No systematic trend with observed(b-y), H-beta, or spectral type appears to be present, but the range of(c1) residuals is surprisingly large. A rather large part of the starshas small beta values, smaller than for the BIa supergiants. Only twoare classified as O stars and most of them have the suffix e, ne, ornne. Most beta values for the O type stars are slightly above the upperlimit of 2.585 m.
| Five-colour photometry of early-type stars in the direction of galactic X-ray sources
The results of five-color (Walraven system) photometry of 551 O- andB-type stars located in 17 fields of a few square degrees aroundgalactic X-ray sources are presented. From a comparison ofreddening-free combinations of color indices with theoretical values,calculated for model atmospheres of Kurucz (1979), effective temperatureand surface gravity for these stars are derived. In addition theirabsolute magnitude are determined by combining these parameters with theresults of evolutionary calculations of massive stars. These effectivetemperatures are in good agreement with the temperature scale ofBohm-Vitense (1981) for stars of luminosity classes II to V. For thesupergiants the effective temperatures are about 40 percent higher. Forstars of luminosity classes III to V the absolute magnitudes agree wellwith the results of independent luminosity calibrations of spectraltypes, but for brighter stars they deviate systematically. Thephotometric data are also used to study the interstellar reddening inthe direction of the X-ray sources.
| Extinction law in dust clouds and the young southern cluster NGC 6250 - Further evidence for high values of R
Abstract image available at:http://adsabs.harvard.edu/abs/1977AJ.....82..902H
| Southern open star clusters VI. UBV-Hbeta photometry of 18 clusters from Centaurus to Sagittarius.
Abstract image available at:http://adsabs.harvard.edu/abs/1975A&AS...20..155M
| Photometric standard sequences in Norma iII = 320° - 340°
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