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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.
| Reddening and the Extinction Law at High Galactic Latitude
We present near-infrared (JHKL) photometry of 103 southern stars locatedbehind translucent interstellar clouds at high Galactic latitude. Ourdata are combined with visual photometry and spectral type informationfrom the literature in a detailed analysis of the wavelength dependenceof interstellar extinction by dust in these high-latitude clouds. Weinvestigate the shape of the near-infrared extinction curve and evaluatethe total line-of-sight extinction (AV) and ratio oftotal-to-selective extinction (RV) in each line of sight.Sources of uncertainty in RV arising from photometric errorsand spectral classification errors are carefully assessed and quantifiedas functions of the line-of-sight reddening. We detect appreciabledifferences in our results compared with the average extinction curvefor dust in the diffuse interstellar medium (ISM) close to the Galacticplane. Assuming a power-law form in the near-infrared, we find the meanfor high-latitude clouds to be well described by~λ-2.3, somewhat steeperthan that for the diffuse ISM(~λ-1.8). Our sampleincludes a substantial number of lines of sight with low RVvalues (47% with RV<2.8) relative to the diffuse ISMaverage of 3.05+/-0.15. We conclude that many high-latitude clouds haveenhanced abundances of relatively small grains.
| 3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.Tables 1 and 2 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/411/447
| 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.
| Spectropolarimetry of the Type II Supernovae 1997ds, 1998A, and 1999gi
We present single-epoch spectropolarimetry of the Type II supernovae(SNe II) 1997ds, 1998A, and 1999gi. SN 1997ds and SN 1998A were bothobserved during the early photospheric phase, less than 50 days afterexplosion, while spectropolarimetry of SN 1999gi was obtained near thestart of the transition to the nebular phase, about 110 days afterexplosion. Uncorrected for interstellar polarization (ISP), SN 1997ds ischaracterized by pV=0.85%+/-0.02%, SN 1998A haspV=0.24%+/-0.05%, and SN 1999gi is polarized atpV=5.72%+/-0.01%. SN 1997ds and SN 1999gi exhibit distinctpolarization modulations (up to Δptot=1.6% in SN 1997dsand Δptot=1.0% in SN 1999gi) at the wavelengths of thestrongest spectral line features. While no spectral polarizationfeatures were observed in SN 1998A, the data are insensitive topolarization features at the levels confirmed in the other two objects.The low continuum polarization inferred for SN 1997ds and SN 1998A andthe amplitude of (or limits on) the polarization modulations areconsistent with those measured at similar epochs for SN 1987A and theType II plateau SN 1999em and supports the growing consensus thatcore-collapse events with hydrogen envelopes substantially intact at thetime of explosion are not significantly aspherical during the earlyphotospheric phase. The spectral shape of the high continuumpolarization of SN 1999gi closely resembles a ``Serkowski'' ISP curve(characterized by pmax=5.8%, θ=154deg, andλmax=5300 Å) and is inconsistent with thewavelength-independent nature of electron scattering expected for anaspherical SN atmosphere. Since Galactic reddening is minimal along thisline of sight, the majority of the observed polarization in SN 1999gi isbelieved to be due to ISP of the host galaxy, although significant (upto p~2%) intrinsic polarization cannot be ruled out. The potential powerof SN spectropolarimetry to study the properties of interstellar dust inexternal galaxies is described and applied to the SN 1999gi data, whereit is shown that if the polarization is indeed predominantlyinterstellar in origin, then RV=3.0+/-0.2 for the dust alongthis line of sight in NGC 3184.
| Observations of Molecular Hydrogen in the Carina Nebula
We present a study of molecular-hydrogen absorption lines in thecontinuum spectra of three early-type stars in the Carina Nebula, HD93129a, HD 93250, and HD 303308, made with the Berkeley Extreme andFar-Ultraviolet Spectrometer (BEFS) on the ORFEUS telescope in 1993September. The instrument possesses an intermediate spectral resolvingpower (λ/Δλ=3000) over the wavelength rangeλλ390-1170.We derive column densities for theH2 rotational levels up to J''=6 and estimate thekinetic temperature and the UV radiation fields at the surface of theclouds in the nebula. All three stars show strong H2absorption features, indicating that the Carina Nebula contains abundantmolecular hydrogen. The UV radiation field strength seems to becorrelated with the angular distance of the target star from η Car,implying that the clouds might be located close to η Car. We derivethe absolute distances between the clouds and η Car, which are ingood agreement with the angular positions of these stars. We alsoexamine the morphology of the Carina Nebula based on the present result,as well as previous CO observations, and derive the conversion factorN(H2)/W(12CO) in this region.
| ORFEUS II Echelle Observations of Molecular Hydrogen in the Galactic Halo
The far-ultraviolet spectra of HD 219188, HD 94473, and HD 18100, threeearly-type stars in the Galactic halo, were obtained with the echellespectrometer on the ORFEUS II telescope in 1996 November. We deriveH2 column densities for the rotational levels up toJ''=6 and estimate excitation temperatures and H2dissociation rates for the clouds toward HD 219188 and HD 94473. Theresults indicate that strong radiation fields might exist near theseclouds, but these radiation fields do not seem to act as major heatingsources. No significant H2 absorption is seen toward HD18100. The correlation between our H2 column densities andpublished extinction maps confirms that the molecular clouds areassociated with dust clouds that lie between the observed stars and theSun. We estimate the conversion factor between N(H2) andW(12CO) to be XCO=4+/-2x1019cm-2 (K km s-1)-1 for the clouds alongthe line of sight to HD 94473, consistent with previous estimates towithin a factor of a few.
| Magnetic fields around BOK globules: CCD polarimetry of CB 4
The small Bok globule CB 4 was probed using a CCD imaging polarimeter inorder to create a detailed map of the magnetic field associated withthis cloud. Stars as faint as 17th mag at V band were measuredpolarimetrically with uncertainties less than 1%. Sky transmissionvariations were minimized via a system of synchronous polaroid rotationand bidirectional charge shifting. In all, 80 stars behind the peripheryof the globule were accurately analyzed polarimetrically. Thelarge-scale (1-2 pc) magnetic field direction around CB 4 was found tobe very uniform (P.A. = 63.3 deg +/- 1.1 deg). Double-Gaussian fittingof the polarization position angle histogram gave a dispersion of 10 degabout the primary field direction. Possible field-line compression wasfound inward of approximately 0.2 pc from the cloud center. Noappreciable twisting of field lines was found. By plotting stellarseparations against differences of polarization angles, CB 4 was foundto have a magnetic field decorrelation length of approximately 0.1 pc,similar to the size of the visually opaque cire, but much smaller thanthe size of the bright optical rim or CO half-power contour ofapproximately 0.5 pc. The magnetic field decorrelation length may berelated to a characteristic transient clumping size, or perhaps even toclumps of a more permanent nature.
| Photometric and spectroscopic analysis of high galactic latitude molecular clouds. II - High-resolution spectroscopic observations of NA I, CA II, CA I, CH, and CH(+1)
High-resolution spectroscopic observations in the wavelengths of thespecies Na I, Ca II, and Ca I and the molecular species CH and CH(+)have been performed toward stars behind a variety of high galacticlatitude molecular clouds (HLCs). Seventeen new detections of molecularabsorption are reported. The sizes, densities, and molecular contentfound for the HLCs are consistent with an interpretation of HLCmolecular cores as extremely compact structures of enhanced molecularcontent which may have evolved from larger H I shells.
| Photometric and spectroscopic analysis of high Galactic latitude molecular clouds. I - Distances and extinctions of stars toward 25 selected regions
The paper presents new photoelectric photometry in the UBV, uvby, andH-beta systems for 320 stars at high Galactic latitudes, selected fortheir proximity to the densest portions of a variety of high Galacticlatitude atomic and molecular clouds. Low-resolution digital spectra areemployed to classify 115 of the stars. Several novel techniques aredeveloped for automatic digital spectral classification, and fordereddening Stromgren uvby-beta and Johnson UBV,H-beta photometry. Anempirical determination of the errors in the derived dereddened colors,and techniques for converting between the two photometric systems arepresented on the basis of the analysis of a statistically significantsample of 1480 stars with complete data sets in both photometric systemsand with an accurate MK spectral type. New constraints on the distancesto several clouds are presented.
| CCD measurements of visual binaries
CCD measurements of visual double stars were obtained with the ESO 1.5 mdanish reflector. All binaries observed are candidates for the HIPPARCOSInput Catalogue. More than 400 observations have been made in four clearnights. The accuracy obtained is comparable to the accuracy of thephotographic technique, but the observing and reduction times are oneorder of magnitude smaller.
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