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Physical parameters and wind properties of galactic early B supergiants
We present optical studies of the physical and wind properties, plus CNOchemical abundances, of 25 O9.5-B3 Galactic supergiants. We employnon-LTE, line blanketed, extended model atmospheres, which provide amodest downward revision in the effective temperature scale of early Bsupergiants of up to 1-2 kK relative to previous non-blanketed results.The so-called "bistability jump" at B1 (Teff 21 kK)from Lamers et al. is rather a more gradual trend (with large scatter)from v&infy;/vesc3.4 for B0-0.5 supergiantsabove 24 kK to v&infy;/vesc 2.5 for B0.7-1supergiants with 20 kK ≤ Teff ≤ 24 kK, andv&infy;/vesc 1.9 for B1.5-3 supergiants below20 kK. This, in part, explains the break in observed UV spectralcharacteristics between B0.5 and B0.7 subtypes as discussed by Walbornet al. We compare derived (homogeneous) wind densities with recentresults for Magellanic Cloud B supergiants and generally confirmtheoretical expectations for stronger winds amongst Galacticsupergiants. However, winds are substantially weaker than predictionsfrom current radiatively driven wind theory, especially at mid-Bsubtypes, a problem which is exacerbated if winds are already clumped inthe Hα line forming region. In general, CNO elemental abundancesreveal strongly processed material at the surface of Galactic Bsupergiants, with mean N/C and N/O abundances 10 and 5 times higher thanthe Solar value, respectively, with HD 2905 (BC0.7 Ia) indicating thelowest degree of processing in our sample, and HD 152236 (B1.5Ia+) the highest.
| Correlation patterns between 11 diffuse interstellar bands and ultraviolet extinction
We relate the equivalent widths of 11 diffuse interstellar bands,measured in the spectra of 49 stars, to different colour excesses in theultraviolet. We find that most of the observed bands correlatepositively with the extinction in the neighbourhood of the2175-Åbump. Correlation with colour excesses in other parts of theextinction curve is more variable from one diffuse interstellar band toanother; we find that some diffuse bands (5797, 5850 and 6376 Å)correlate positively with the overall slope of the extinction curve,while others (5780 and 6284 Å) exhibit negative correlation. Wediscuss the implications of these results on the links between thediffuse interstellar band carriers and the properties of theinterstellar grains.
| The Complex Interstellar Na I Absorption toward h and χ Persei
Recent high spatial and spectral resolution investigations of thediffuse interstellar medium (ISM) have found significant evidence forsmall-scale variations in the interstellar gas on scales <=1 pc. Tobetter understand the nature of small-scale variations in the ISM, wehave used the Kitt Peak National Observatory (KPNO) WIYN Hydramultiobject spectrograph, which has a mapping advantage over thesingle-axis, single-scale limitations of studies using high propermotion stars and binary stars, to obtain moderate-resolution (~12 kms-1) interstellar Na I D absorption spectra of 172 starstoward the double open cluster h and χ Persei. All of the sightlines toward the 150 stars with spectra that reveal absorption from thePerseus spiral arm show different interstellar Na I D absorptionprofiles in the Perseus arm gas. Additionally, we have utilized the KPNOcoudé feed spectrograph to obtain high-resolution (~3 kms-1) interstellar Na I D absorption spectra of 24 of thebrighter stars toward h and χ Per. These spectra reveal an evengreater complexity in the interstellar Na I D absorption in the Perseusarm gas and show individual components changing in number, velocity, andstrength from sight line to sight line. If each of these individualvelocity components represents an isolated cloud, then it would appearthat the ISM of the Perseus arm gas consists of many small clouds.Although the absorption profiles vary even on the smallest scales probedby these high-resolution data (~30", ~0.35 pc), our analysis revealsthat some interstellar Na I D absorption components from sight line tosight line are related, implying that the ISM toward h and χ Per isprobably composed of sheets of gas in which we detect variations due todifferences in the local physical conditions of the gas.
| HD 183143: A Hypergiant
We present spectroscopic evidence that the luminosity of HD 183143 ishigher by one magnitude than thought previously. The star is yet anotherB6-8 Ia-0 white hypergiant of the Galaxy. Its absolute visual magnitudeis close to -8 mag, and its distance is close to 2 kpc. We describespectroscopic manifestations of the nonstationary behavior of itsatmosphere and wind.
| Toward an adequate method to isolate spectroscopic families of diffuse interstellar bands
We divide some of the observed diffuse interstellar bands (DIBs) intofamilies that appear to have the spectral structure of single species.Three different methods are applied to separate such families, exploringthe best approach for future investigations of this type. Starting witha statistical treatment of the data, we found that statistical methodsby themselves give insufficient results. Two other methods of dataanalysis (`averaging equivalent widths' and `investigating the figureswith arranged spectrograms') were found to be more useful as tools forfinding the spectroscopic families of DIBs. On the basis of thesemethods, we suggest some candidates as `relatives' of 5780- and5797-Å bands.
| A Method for Simultaneous Determination of AV and R and Applications
A method for the simultaneous determination of the interstellarextinction (AV) and of the ratio of total to selectiveextinction (R), derived from the 1989 Cardelli, Clayton, & Mathisfitting of the interstellar extinction law, is presented and applied toa set of 1900 color excesses derived from observations of stars inUBVRIJHKL. The method is used to study the stability of AVand R within selected regions in Perseus, Scorpius, Monoceros, Orion,Sagittarius, Ophiuchus, Carina, and Serpens. Analysis shows that R isapproximately constant and peculiar to each sector, with mean valuesthat vary from 3.2 in Perseus to 5.6 in Ophiuchus. These results aresimilar to published values by Aiello et al., He et al., Vrba &Rydgren, O'Donnell, and Cardelli, Clayton, & Mathis.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| Membership in the Region of the Double Cluster h and χ Persei Working from Proper Motions and Positions: Distance Moduli and Extinction in That Galactic Direction
A segregation between cluster members and field stars in the region ofthe double cluster h and χ Persei is accomplished working fromproper motions and positions as kinematic random variables. Theextinction RV and the distance moduli are determined once thephysical cluster members are found.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| The Star Formation History and Mass Function of the Double Cluster h and χ Persei
The h and χ Per ``double cluster'' is examined using wide-field(0.98d×0.98d) CCD UBV imaging supplemented by optical spectra ofseveral hundred of the brightest stars. Restricting our analysis to nearthe cluster nuclei, we find identical reddenings [E(B-V)=0.56+/-0.01],distance moduli (11.85+/-0.05), and ages (12.8+/-1.0 Myr) for the twoclusters. In addition, we find an initial mass function slope for eachof the cluster nuclei that is quite normal for high-mass stars,Γ=-1.3+/-0.2, indistinguishable from a Salpeter value. We derivemasses of 3700 Msolar (h) and 2800 Msolar (χ)integrating the present-day mass function from 1 to 120Msolar. There is evidence of mild mass segregation within thecluster cores. Our data are consistent with the stars having formed at asingle epoch; claims to the contrary are very likely due to theinclusion of the substantial population of early-type stars located atsimilar distances in the Perseus spiral arm, in addition tocontamination by G and K giants at various distances. We discuss theuniqueness of the double cluster, citing other examples of suchstructures in the literature but concluding that the nearly identicalnature of the two cluster cores is unusual. We fail to settle thelong-standing controversy regarding whether or not the double cluster isthe core of the Per OB1 association and argue that this may beunanswerable with current techniques. We also emphasize the need forfurther work on the pre-main-sequence population of this nearby andhighly interesting region.
| Chemical composition of B-type supergiants in the OB 8, OB 10, OB 48, OB 78 associations of M 31
Absolute and differential chemical abundances are presented for thelargest group of massive stars in M 31 studied to date. These resultswere derived from intermediate resolution spectra of seven B-typesupergiants, lying within four OB associations covering a galactocentricdistance of 5-12 kpc. The results are mainly based on an LTE analysis,and we additionally present a full non-LTE, unified model atmosphereanalysis of one star (OB 78-277) to demonstrate the reliability of thedifferential LTE technique. A comparison of the stellar oxygen abundancewith that of previous nebular results shows that there is an offset ofbetween ~ 0.15-0.4 dex between the two methods which is criticallydependent on the empirical calibration adopted for the R_23 parameterwith [O/H]. However within the typical errors of the stellar and nebularanalyses (and given the strength of dependence of the nebular results onthe calibration used) the oxygen abundances determined in each methodare fairly consistent. We determine the radial oxygen abundance gradientfrom these stars, and do not detect any systematic gradient across thisgalactocentric range. We find that the inner regions of M 31 are not, aspreviously thought, very ``metal rich''. Our abundances of C, N, O, Mg,Si, Al, S and Fe in the M 31 supergiants are very similar to those ofmassive stars in the solar neighbourhood.
| CCD uvbybeta photometry of young open clusters. I. The double cluster h and chi Persei
We present CCD uvbybeta photometry for stars in the nuclei of the youngdouble cluster h and chi Persei. We find that the reddening is highlyvariable through the h Per nucleus, increasing from west to east, withvalues ranging from E(b-y) = 0.328+/-0.022 in the western part to E(b-y)= 0.465+/-0.024 in the south-east. Towards chi Per the reddening isfairly constant, with E(b-y) = 0.398+/-0.025. Both clusters share acommon distance modulus of 11.7+/-0.1 mag, and an age of log t =7.10+/-0.05 years. Tables 6 and 7 are only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/394/479
| Far-ultraviolet extinction and diffuse interstellar bands
We relate the equivalent widths of the major diffuse interstellar bands(DIBs) near 5797 and 5780Å with different colour excesses,normalized by E(B-V), which characterize the growth of interstellarextinction in different wavelength ranges. It is demonstrated that thetwo DIBs correlate best with different parts of the extinction curve,and the ratio of these diffuse bands is best correlated with thefar-ultraviolet (UV) rise. A number of peculiar lines of sight are alsofound, indicating that the carriers of some DIBs and the far-UVextinction can be separated in certain environments, e.g. towards thePer OB2 association.
| UBVI and Hα Photometry of the h and χ Persei Cluster
UBVI and Hα photometry is presented for 17319 stars in vicinity ofthe young double cluster h and χ Persei. Our photometry extends overa 37'×1deg field centered on theassociation. We construct reddening contours within the imaged field. Wefind that the two clusters share a common distance modulus of11.75+/-0.05 and ages of logage(yr)=7.1+/-0.1. From the V-Hαcolor, a measure of the Hα emission strength, we conduct a surveyfor emission line objects within the association. We detect a sample of33 Be stars, eight of which are new detections. We present a scenario ofevolutionary enhancement of the Be phenomenon to account for the peak inBe fraction toward the top of the main sequence in the population of hand χ Persei and similar young clusters.
| Photometric study of the double cluster h & chi Persei
We present uvbybeta CCD photometry of the central region of the doublecluster h & chi Persei. We identify ~350 stars, of which 214 werenot included in Oosterhof's catalogue. Our magnitude limit V=16.5 allowsus to reach early F spectral type and obtain very accurate fits to theZAMS. We derive reddening values of E(b-y) = 0.44+/-0.02 for h Perseiand E(b-y) = 0.39+/-0.05 for chi Persei. From the ZAMS fitting, wederive distance moduli V0-MV = 11.66+/-0.20 andV0-MV = 11.56+/-0.20 for h and chi Perseirespectively. These values are perfectly compatible with both clustersbeing placed at the same distance and having identical reddenings. Theshift in the main-sequence turnoff and isochrone fitting, however, showthat there is a significant age difference between both clusters, withthe bulk of stars in h Persei being older than chi Persei. There is,however, a significant population of stars in h Persei which are youngerthan chi Persei. All this argues for at least three different epochs ofstar formation, corresponding approximately to log t = 7.0, 7.15 and 7.3Tables 2, 8 and 9 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/372/477. Based onobservations obtained with the Jacobus Kapteyn Telescope operated on theisland of La Palma by the Isaac Newton Group, in the SpanishObservatorio Roque de los Muchachos of the Instituto deAstrofísica de Canarias.
| The OB Zoo: A Digital Atlas of Peculiar Spectra
A digital atlas of 20 high-luminosity, peculiar OB spectra in the3800-4900 Å range is presented. The atlas is organized anddiscussed in terms of the following four categories: WN-A or WNL stars,OB Iape or very late WN (WNVL) stars, iron stars, and B-supergiantluminous blue variables (LBVs). Several objects in the earliercategories are also active or quiescent LBVs. Some (but not all) ofthese objects have been well studied, and extensive references areprovided, as are comprehensive spectral-line identifications. Severalnew morphological relationships among the objects have been recognizedthrough this presentation. In particular, attention is drawn to theoccurrence of spatial pairing between nearly identical, unusual spectra,which may have implications for a particular mode of massive-starformation. This small sample includes one or both members of at leastfive such pairs. Physical explanations of these peculiar, likelytransitional spectra and the relationships among them are essential fora complete understanding of massive stellar evolution.
| 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
| Radial velocities. Measurements of 2800 B2-F5 stars for HIPPARCOS
Radial velocities have been determined for a sample of 2930 B2-F5 stars,95% observed by the Hipparcos satellite in the north hemisphere and 80%without reliable radial velocity up to now. Observations were obtainedat the Observatoire de Haute Provence with a dispersion of 80Ä,mm(-1) with the aim of studying stellar and galactic dynamics.Radial velocities have been measured by correlation with templates ofthe same spectral class. The mean obtained precision is 3.0 km s(-1)with three observations. A new MK spectral classification is estimatedfor all stars. Based on observations made at the Haute ProvenceObservatory, France and on data from The Hipparcos Catalogue, ESA.Tables 4, 5 and 6 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr or viahttp://cdsweb.u-strasbg.fr/Abstract.htm
| The wind momentum-luminosity relationship of galactic A- and B-supergiants
The Balmer lines of four A Ia-supergiants (spectral type A0 to A3) andfourteen B Ia and Ib-supergiants (spectral type B0 to B3) in the solarneighbourhood are analyzed by means of NLTE unified model atmospheres todetermine the properties of their stellar winds, in particular theirwind momenta. As in previous work for O-stars (Puls et al. \cite{pul96})a tight relationship between stellar wind momentum and luminosity(``WLR'') is found. However, the WLR varies as function of spectraltype. Wind momenta are strongest for O-supergiants, then decrease fromearly B (B0 and B1) to mid B (B1.5 to B3) spectral types and becomestronger again for A-supergiants. The slope of the WLR appears to besteeper for A- and mid B-supergiants than for O-supergiants. Thespectral type dependence is interpreted as an effect of ionizationchanging the effective number and the line strength distributionfunction of spectral lines absorbing photon momentum around the stellarflux maximum. This interpretation needs to be confirmed by theoreticalcalculations for radiation driven winds. The ``Pistol-Star'' in theGalactic Centre, an extreme mid B-hypergiant recently identified as oneof the most luminous stars (Figer et al. \cite{fig99}) is found tocoincide with the extrapolation of the mid B-supergiant WLR towardshigher luminosities. However, the wind momentum of the Luminous BlueVariable P Cygni, a mid B-supergiant with extremely strong mass-loss, is1.2 dex higher than the WLR of the ``normal'' supergiants. Thissignificant difference is explained in terms of the well-known stellarwind bi-stability of supergiants very close to the Eddinton-limit inthis particular range of effective temperatures. A-supergiants in M31observed with HIRES at the Keck telescope have wind momenta compatiblewith their galactic counterparts. The potential of the WLR as a new,independent extragalactic distance indicator is discussed. It isconcluded that with ten to twenty objects, photometry with HST andmedium resolution spectroscopy with 8m-telescopes from the grounddistance moduli can be obtained with an accuracy of about 0fm1 out tothe Virgo and Fornax clusters of galaxies.
| Galactic B-supergiants: A non-LTE model atmosphere analysis to estimate atmospheric parameters and chemical compositions
A non-LTE model atmosphere analysis of moderate resolution (R ~ 5 000)spectra of 46 Galactic B-type supergiants is presented. Standardtechniques are adopted, viz. plane-parallel geometry and radiative andhydrostatic equilibrium. Spectroscopic atmospheric parameters (T_eff,log g & v_turb) and chemical abundances (He, C, N, O, Mg & Si)are estimated, both as a test of the validity of such an approach and inan attempt to provide consistent results for supergiants covering asignificant range of spectral types. The values of the estimatedatmospheric parameters and their dependence on the physics adopted inthe model atmospheres calculations are discussed. The absolute metalabundances are compared to those of main sequence B-type stars and, ingeneral, their chemical compositions appear to be similar. Theabundances for He, C, N & O are considered in some detail and arediscussed in the context of possible evolutionary histories for thisstellar sample. Specifically, it is found that the supergiant sample canbe subdivided into a number of evolutionarily distinct groups. The lowermass objects are predominantly chemically near-normal i.e. theirphotospheres show little or no evidence for chemical processing, whereasthe higher mass supergiants have CNO ratios which are indicative of CNand possibly NO-cycle burning. An attempt is made to quantify thedifference in nitrogen and carbon abundances between the high and lowmass targets but this is hampered by theoretical uncertainties. Thepossibilities that the most highly processed supergiants may have eitherlarger rotational velocities or have undergone mass transfer within abinary system are discussed.
| The interstellar clouds of ADAMS and Blaauw revisited: an HI absorption study - II.
Not Available
| The interstellar clouds of ADAMS and Blaauw revisited: an HI absorption study - I.
Not Available
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| Cross-correlation characteristics of OB stars from IUE spectroscopy
We present a catalogue of homogeneous measures of the linewidthparameter, v_esin i, for 373 O-type stars and early B supergiants(including the separate components of 25 binary and three triplesystems), produced by cross-correlating high-resolution,short-wavelength IUE spectra against a `template' spectrum of tauSco. Wealso tabulate terminal velocities. There are no O supergiants in oursample with v_esin i<65 km s^-1, and only one supergiant earlier thanB5 has v_esin i<50 km s^-1, confirming that an important linebroadening mechanism in addition to rotation must be present in theseobjects. A calibration of the area under the cross-correlation peakagainst spectral type is used to obtain estimates of continuum intensityratios of the components in 28 spectroscopically binary or multiplesystems. At least seven SB2 systems show evidence for the `Struve-Sahadeeffect', a systematic variation in relative line strength as a functionof orbital phase. The stellar wind profiles of the most rapid rotator inour sample, the O9III:n* star HD 191423 (v_esin i=436km s^-1), show itto have a `wind-compressed disc' similar to that of HD 93521; this starand other rapid rotators are good candidates for studies of non-radialpulsation.
| The Peculiar Population of Hot Stars at the Galactic Center
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...456..206T&db_key=AST
| CNO Abundances and the Evolutionary Status of Galactic, A-Type Supergiants
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...449..839V&db_key=AST
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| An IUE survey of interstellar H I LY alpha absorption. 1: Column densities
We measure Galactic interstellar neutral hydrogen column densities byanalyzing archival interstellar Ly alpha absorption line data toward 554B2 and hotter stars observed at high resolution with the IUE satellite.This study more than doubles the number of lines of sight with measuresof N(H I) based on Ly alpha. We have included the scattered lightbackground correction algorithm of Bianchi and Bohlin in our datareduction. We use the correlation between the Balmer discontinuity(c1) index and the stellar Ly alpha absorption in order toassess the effects of stellar Ly alpha contamination. Approximately 40%of the B stars with measured (c1) index, exhibit seriousstellar Ly alpha contamination. One table contains the derived values ofthe interstellar N(H I) for 393 stars with at most small amounts ofstellar contamination. Another lists the observed values of total N(H I)for 161 stars with suspected stellar Ly alpha contamination and/oruncertain stellar parameters.
| An IUE survey of interstellar H I LY alpha absorption. 2: Interpretations
We present an analysis of interstellar neutral hydrogen column densitiesobtained from archival Ly alpha absorption line data toward the completesample of B2 and hotter stars observed at high spectral resolution withthe IUE satellite. The full sample includes 554 stars and more thandoubles the number of lines of sight previously observed. Many of the B2and B1.5 stars exhibit contamination from stellar Ly alpha absorption,and were excluded from the subsequent analysis. The final working sampleincludes 393 stars. We present statistical averages for a number ofquantities derived for the sample. The stars range in distance from 0.12to 11 kpc with an average distance of 2.1 kpc. Values for averagesight-line density range from 0.017 to 8.62 atoms cm-3, withan overall average of 0.23 atoms cm-3. Neutral hydrogen anddust are well correlated, with N(H I)/E(B - V) = 4.93 x 1021cm-2 mag-1, and N(H I)/E(Bump) = 2.17 x1021 cm-2 mag-1. There is an increasein the H I to dust ratios for the densest sight lines. Evidently, the HI to dust ratios in dense clouds are higher than in the lower densitymedium between the clouds and in interarm directions. The effect isprobably an indication of dust modification in the dense clouds. The H Ito dust ratios are approx. 17% smaller for the sight lines to O starscompared to the ratios found toward B stars. We have identified sightlines with anomalously large and small values of N(H I)/E(B - V) and H(HI)/E(Bump). Some of these sight lines are well-known examples ofpeculiar extinction (i.e., HD 147933 and HD 37061 (NU Ori)). Others areless well known and may provide important new examples of regions withhighly modified dust. The number of stars with anomalously low H I todust ratios is disproportionately larger than the number of stars withanomalously high H I to dust ratios. The distribution of the gas awayform the Galactic plane for the sample of objects is complex. For thesubset of 375 stars whose lines of sight have a statistically lowprobablility of intersecting a large cloud, the distribution may beroughly approximated by an exponential density distribution, with amidplane density of 0.366 atoms cm-3, a scale height of 195pc, and a random logarithmic scattering parameter sigma p of0.159 dex.
| The chemical composition of B-supergiant atmospheres
Not Available
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| Sað Açýklýk: | 02h19m13.94s |
| Yükselim: | +57°10'09.2" |
| Görünürdeki Parlaklýk: | 6.694 |
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