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Millimeter-wave survey of molecular clouds around the W5-East triggered star-forming region
Aims. We examined the physical properties of molecular clouds(morphology, column density, number density, mass) to investigate themechanism of triggered star formation by UV radiation from a massivestar. Methods: We made extensive, high-resolution maps of molecularclouds associated and interacting with the W5-East Hii region using the 45-m telescope at the Nobeyama Radio Observatory(HPBW = 15.6 arcsec) in 13CO (J = 1-0) and C18O (J= 1-0) to reveal details in the high-density regions of the molecularclouds. In addition, to investigate the spatial distributions of youngstellar objects (YSOs) in the W5-East H ii region, wemapped the spatial distributions of Class I and II candidates. Results:We identified eight 13CO molecular clouds (three of them areknown bright-rimmed clouds) and nine 18O clumps. The massesof the clouds and clumps range from 460 to 36 000 Mȯ andfrom 55 to 740 Mȯ, respectively. The peak13CO column densities of the clouds facing the H ii regionare twice as large as the others. They have steep density gradientstoward the H ii region, indicating interactions with the H ii region. Weselected 55 Class I candidates and 778 Class II candidates associatedwith the W5-East H ii region from the previousSpitzer IRAC/MIPS survey. Most Class I candidates are located aroundintegrated intensity peaks of 13CO, whereas most Class IIcandidates are distributed along the front sides of the BRC arcs closeto the exciting star. Conclusions: The alignments of the YSO candidatesand the molecular clouds in order of age indicate that triggered starformation occurs in the W5-East H ii region as aresult of gas compression by strong UV radiation. Based on the columndensities of 13CO and the spatial distribution of YSOcandidates, we identified a new bright-rimmed cloud candidate on thewest side of the W5-East H ii region.
| Clustered and Triggered Star Formation in W5: Observations with Spitzer
We present images and initial results from our extensive Spitzer SpaceTelescope imaging survey of the W5 H II region with the Infrared ArrayCamera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS). Wedetect dense clusters of stars, centered on the O stars HD 18326, BD +60586, HD 17505, and HD 17520. At 24 μm, substantial extended emissionis visible, presumably from heated dust grains that survive in thestrongly ionizing environment of the H II region. With photometry ofmore than 18,000 point sources, we analyze the clustering properties ofobjects classified as young stars by their IR spectral energydistributions (a total of 2064 sources) across the region using aminimal-spanning-tree algorithm. We find ~40%-70% of infrared excesssources belong to clusters with >=10 members. We find that within theevacuated cavities of the H II regions that make up W5, the ratio ofClass II to Class I sources is ~7 times higher than for objectscoincident with molecular gas as traced by 12CO emission andnear-IR extinction maps. We attribute this contrast to an age differencebetween the two locations and postulate that at least two distinctgenerations of star formation are visible across W5. Our preliminaryanalysis shows that triggering is a plausible mechanism to explain themultiple generations of star formation in W5 and merits furtherinvestigation.
| On the Formation of Perseus OB1 at High Galactic Latitudes
The Per OB1 association, which contains the remarkable double cluster hand χ Per, is unusual in not having a giant molecular cloud in itsvicinity. We show from Hipparcos data that the luminous members of thisassociation exhibit a bulk motion away from the Galactic plane, suchthat their average velocity increases with height above the Galacticplane. We find HAeBe and T Tauri stars toward probable remnant molecularclouds associated with Per OB1. These star-forming regions lie wellbeyond the location of the luminous member stars at heights of 280-400pc above the Galactic plane, far higher than that previously found forembedded clusters. We argue that the observed motion of the luminousmember stars is most naturally explained if many formed from moleculargas pushed and accelerated outward by an expanding superbubble, drivenpresumably by stellar winds and perhaps also by supernova explosions. Alarge shell of atomic hydrogen gas and dust that lies just beyond theremnant molecular clouds, believed to be driven by just such asuperbubble, may comprise the swept-up remains of the parental giantmolecular cloud from which this association formed. In support of thispicture, we find a weak trend for the younger O star members to lie athigher Galactic latitudes than the older supergiant members. Thestar-forming regions located at even larger heights above the Galacticplane presumably correspond to more recent episodes of star formation ator near the periphery of this superbubble.
| Automatic Detection of Expanding H I Shells in the Canadian Galactic Plane Survey Data
The identification of expanding H I shells is difficult because of theirvariable morphology. In this paper we present an automatic detector forH I shells, based on the more stable dynamical characteristics ofexpanding bubbles with radii <40 pc. The detection is performed intwo stages. First, artificial neural networks are trained to recognizethe dynamical signature of an expanding bubble in the velocity spectraof 21 cm data. The second stage consists of subsequent validations basedon the potential bubble's morphology. The technique is tested on 11known bubbles, and 10 of them are successfully detected. Conducting asystematic detection on a 48deg×9deg regionin the Perseus arm, we obtain 7100 detections with spatial distributionfollowing the stellar distribution of the Galactic disk. The estimatedradius and expansion velocity distributions for objects with R<=10 pcagree with the distributions predicted by models of adiabaticallyexpanding bubble populations. The fraction of the Perseus arm volumeoccupied by the detected objects, which can be interpreted as the smallbubbles' contribution to the Galactic porosity Q, is calculated toQR<40pc=0.007+0.025-0.003. Four newbubble cases and eight serious candidates, related to known progenitors,are proposed.
| Binary and Multiple O-Type Stars in the Cassiopeia OB6 Association
We present the results of time-resolved spectroscopy of 13 O-type starsin the Cas OB6 stellar association. We conducted a survey for radialvelocity variability in search of binary systems, which are expected tobe plentiful in young OB associations. Here we report the discovery oftwo new single-lined binaries, and we present new orbital elements forthree double-lined binaries (including one in the multiple-star systemHD 17505). One of the double-lined systems is the eclipsing binarysystem DN Cas, and we present a preliminary light-curve analysis thatyields the system inclination, masses, and radii. We compare the spectraof the single stars and the individual components of the binary starswith model synthetic spectra to estimate the stellar effectivetemperatures, gravities, and projected rotational velocities. We alsomake fits of the spectral energy distributions to derive E(B-V),R=AV/E(B-V), and angular diameter. A distance of 1.9 kpcyields radii that are consistent with evolutionary models. We find that7 of 14 systems with spectroscopic data are probable binaries,consistent with the high binary frequency found for other massive starsin clusters and associations.
| Triggered Star Formation in the W5 H II Region
Young, massive stars can have a profound effect on the surroundinginterstellar medium. The triggering of star formation by an H II regionexpanding into nearby molecular material is an example of this type ofinteraction. This paper presents a multiwavelength study of the W5star-forming region investigating the possibility of triggered starformation. The ionizing sources and the morphologies of the ionized andmolecular gas and the dust are examined. A population of YSOs isidentified. Statistical evidence for triggering of these YSOs isdiscussed in terms of the spatial dependence of the star formationefficiency and the clustering and correlation of the YSO sources withrespect to the molecular gas and the H II region. Evidence for directinteractions between the components is discussed, on both large scalesand the scale of individual objects. Timescales for the expansion of theH II region, triggering processes, and the ages of the YSOs are comparedto establish a plausible sequence of events.
| On the Absolute Magnitudes of the O Stars
The conclusion published in 1992 by Garmany & Stencel from a studyof northern OB associations, that the absolute magnitudes of the O starsshow ``a large scatter ... intrinsic to the classification system,'' iscritically examined. It is found that the differences between theirderived absolute magnitudes of O stars and this author's 1973calibration exhibit large systematic effects in several associations,ranging from -0.74 to +1.02 mag with substantially smaller dispersions.Of course, when these results are combined, the scatter equals the fullrange of the systematic effects. To investigate the possibility ofdistance errors, the Garmany & Stencel B0-B2.5 stars in the sameassociations are subjected to the same analysis. The results for the Bstars show no significant systematic differences, eliminating errors inthe association distances derived by Garmany & Stencel from the Bstars as the source of the differences found for the O stars. It isnoteworthy that the dispersions in the absolute magnitudes of the Bstars within a given association are similar to or larger than those ofthe O stars. An examination of the distribution on the sky of the starsshows that the O and B stars in the discrepant associations aregenerally not colocated; such was already known to be the case for theimportant Perseus OB1 association. It is suggested that despite theirefforts to improve them, significant problems remain with theassociation memberships adopted by Garmany & Stencel; the relativelysmall dispersions of the O star absolute magnitudes even in thediscrepant cases indicate that they belong to different, usually moredistant associations near the lines of sight to the B associations withwhich they have been mistakenly connected. Several individual cases ofunrecognized multiple systems and classification errors are also foundin the Garmany & Stencel sample. It is concluded that the scatter inthe absolute magnitudes of the O stars is not as large as found byGarmany & Stencel, and not larger than that of the B stars.
| Photometry and Classification of Stars Along the Camelopardalis and Perseus Border
Seven-color photometry in the Vilnius system for 309 stars down to 12.5mag in the area along the galactic equator near the Camelopardalis andPerseus border is presented. Photometric spectral and luminosity classesof the stars are determined.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| A Search for Interstellar Bubbles surrounding Massive Stars in Perseus OB1
We have examined the interstellar medium in the vicinity of massivestars belonging to the Per OB1 association based on neutral hydrogen 21cm observations obtained with the 100 m radio telescope at Effelsberg(HPBW=8.4′) and complementary data from the Leiden-Dwingeloo H ISurvey (HPBW=36'). The higher angular resolution H I observationsallowed us to discover probable wind-blown bubbles related to fourmassive stars in the association, namely, HD 14442 [O5n(f)p], HD 14947[O5If+], HD 13022 [O9.5II-III((n))], and HD 13338 [O9.5V], while thedetection of a wind-blown bubble associated with HD 16691 [O5If+] isless conclusive. A clear H I shell coincident in position with two B1IIIstars (HD 15233 and Hilt 311) was also detected. Some of these featuresalso have infrared and/or molecular counterparts. The energetics of thestructures related to each massive star is analyzed. The new H Iinterstellar bubbles appear to be similar to the ones found surroundingWolf-Rayet stars and other Of stars. The large-scale maps obtained usingthe lower angular resolution H I data show that most of the early-typestars belonging to Per OB1 are placed in a region of low H I emission.The association could have blown a H I shell of about 350×550 pcin size. This large H I shell has an infrared counterpart.
| Speckle Interferometry of New and Problem HIPPARCOS Binaries
The ESA Hipparcos satellite made measurements of over 12,000 doublestars and discovered 3406 new systems. In addition to these, 4706entries in the Hipparcos Catalogue correspond to double star solutionsthat did not provide the classical parameters of separation and positionangle (rho,theta) but were the so-called problem stars, flagged ``G,''``O,'' ``V,'' or ``X'' (field H59 of the main catalog). An additionalsubset of 6981 entries were treated as single objects but classified byHipparcos as ``suspected nonsingle'' (flag ``S'' in field H61), thusyielding a total of 11,687 ``problem stars.'' Of the many ground-basedtechniques for the study of double stars, probably the one with thegreatest potential for exploration of these new and problem Hipparcosbinaries is speckle interferometry. Results are presented from aninspection of 848 new and problem Hipparcos binaries, using botharchival and new speckle observations obtained with the USNO and CHARAspeckle cameras.
| Interstellar extinction.
Not Available
| A large-scale, interstellar Faraday-rotation feature of unknown origin
The disk of the Milky Way contains free electrons and magnetic fieldswhich contribute significantly to the energetics of the interstellarmedium. The concentrations of electrons and magnetic fields are too lowto be detected by direct methods, but may be investigated using Faradayrotation, a wavelength-dependent shift in linear polarization angleinduced by a magneto-ionic medium. Structures in polarization anglearising from Faraday rotation have been detected recently at long radiowavelengths. These structures are disorganized and filamentary, probablyarising from interstellar gas in the vicinity of the Sun. Here we reporta more distant, highly ordered Faraday-rotation structure of ellipticalshape, with its long axis parallel to the plane of the Galaxy. Thefeature appears to be located in an inter-arm region of the Milky Way,between the spiral arm containing the Sun and the next outer (Perseus)spiral arm. Within the elliptical region, small-scale structure whichcharacterizes the turbulence seen in adjacent regions of theinterstellar medium is absent. The origin of this magneto-ionic featureis uncertain, but it must arise from an organization of themagnetic-field and electron-density distributions on a scale of theorder of 50 parsecs (165 light years).
| 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.
| Wolf-Rayet stars and O-star runaways with HIPPARCOS. I. Kinematics
Reliable systemic radial velocities are almost impossible to secure forWolf-Rayet stars, difficult for O stars. Therefore, to study the motions- both systematic in the Galaxy and peculiar - of these two relatedtypes of hot, luminous star, we have examined the Hipparcos propermotions of some 70 stars of each type. We find that (a) both groupsfollow Galactic rotation in the same way, (b) both have a similarfraction of ``runaways'', (c) mean kinetic ages based on displacementand motion away from the Galactic plane tend to slightly favour thecluster ejection over the the binary supernova hypothesis for theirformation, and (d) those with significant peculiar supersonic motionrelative to the ambient ISM, tend to form bow shocks in the direction ofthe motion. Based on data from the ESA Hipparcos astrometry satellite.Table~1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Metal line strengths of blue stragglers towards the young galactic association Perseus OB1.
We present equivalent width measurements of lines of HeI, CIII, NIII,OII, MgII and SiIV from high resolution optical spectra of eight bluestragglers towards the young galactic association Perseus OB1, togetherwith similar data from ultraviolet spectra. We have carried outintercomparisons of the observed line strengths between target stars ofsimilar effective temperatures, and comparisons between the target starsand data for other O-type stars, in order to search for possibleabundance anomalies in our target stars. We find firm evidence that twostars, HD 12323 and HD 13268, exhibit CNO bicycle processed material attheir surfaces, and we outline evolutionary scenarios to explain thisphenomenon. One further star, HD 12993, also shows marginal evidence forprocessed material at its surface. There is no evidence for abundanceanomalies in the five remaining stars, and we consider that they may notbe association members. Hence their identification as blue stragglers isin doubt.
| 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.
| Blue stragglers in the young galactic association Perseus OB1.
Moderate spectral resolution, very high signal-to-noise optical spectrahave been obtained for eight blue stragglers towards the youngassociation Per OB1. These have been used to estimate projectedrotational and radial velocities. The line profiles of hydrogen andneutral and ionized helium have been analysed using non-LTE modelatmosphere calculations to deduce effective temperatures and logarithmicsurface gravities. Helium abundances have also been derived, althoughfor some targets this quantity was not well constrained. In addition,stellar radii, luminosities, and masses have been estimated both byspectroscopic methods and by comparison with evolutionary tracks. Thespectroscopic estimates were based on absolute magnitudes deduced fromeither assuming a unique distance modulus, or from a spectraltype-absolute magnitude calibration; the latter leads to a range ofdistance moduli of 1.4 magnitudes. Both sets of spectroscopic masseswere systematically smaller than the evolutionary masses, butluminosities deduced from spectral types gave a smaller and lessscattered set of mass discrepancies. None of these stars is definitelygenerically related to Per OB1 and they may therefore represent a moreheterogeneous group than has previously been thought, containingpossibly one helium and nitrogen enriched spectroscopic binary, twostars belonging to other associations/clusters, two runaway stars(nitrogen enhanced and possibly helium rich), and a group of O-typestars beyond Per OB1 containing two very fast rotators which may behelium enriched.
| Interstellar extinction toward the CAS OB6 association: Where is the dust?
We have completed a multiband (ultraviolet, optical, and near-infrared)study of the interstellar extinction properties of nine massive stars inIC 1805 and IC 1848, which are both part of Cas OB6 in the Perseusspiral arm. Our analysis includes determination of absolute extinctionover the wavelength range from 3 micrometers to 1250 A. We haveattempted to distinguish between foreground dust and dust local to CasOB6. This is done by quantitatively comparing extinction laws of theleast reddened sightlines (sampling mostly foreground dust) versus themost reddened sightlines (sampling a larger fraction of the dust in theCas OB6 region). We have combined previous investigations to betterunderstand the evolution of the interstellar medium in this active starforming region. We found no variation of extinction curve behaviorbetween moderately reddend and heavily reddened Cas OB6 stars. None ofthe curves show any significant deviation from theCardelli-Clayton-Mathis (CCM) Rupsilon-dependent extinction.They are all consistent with that seen from diffuse dust. Most or all ofthe dust along the line of sight may be foreground to Cas OB6. Massivestar forming regions can show significant deviations from CCM behaviorwhich have been attributed to processing of the dust grains. Any dustlocal to the association must exist far from the hot stars in IC 1805and IC 1848. A previous episode of star formation may have alreadycleared out the region of most of the gas and dust. Evidence for thiscan be seen in H I and IRAS data of the region.
| Tracing the Roots of Interstellar Mid Infrared Emission
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&A...275..549J&db_key=AST
| Environment Dependence of Interstellar Extinction Curves
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&A...274..439J&db_key=AST
| Galactic OB associations in the northern Milky Way Galaxy. I - Longitudes 55 deg to 150 deg
The literature on all OB associations was reviewed, and their IRAS pointsource content was studied, between galactic longitude 55 and 150 deg.Only one third of the 24 associations listed by Ruprecht et al. (1981)have been the subject of individual studies designed to identify thebrightest stars. Distances to all of these were recomputed using themethod of cluster fitting of the B main sequence stars, which makes itpoossible to reexamine the absolute magnitude calibration of the Ostars, as well as for the red supergiant candidate stars. Also examinedwas the composite HR diagram for these associations. Associations withthe best defined main sequences, which also tend to contain very youngclusters, referred to here as OB clusters, have extremely few evolved Band A or red supergiants. Associations with poorly defined mainsequences and few OB clusters have many more evolved stars. They alsoshow an effect in the upper HR diagram referred to as a ledge byFitzpatrick and Garmany (1990) in similar data for the Large MagellanicCloud. It is suggested that the differences in the associations are notjust observational selection effects but represent real differences inage and formation history.
| Infrared dust and millimeter-wave carbon monoxide emission in the Orion region
The far-infrared dust emission seen by the IRAS satellite in the Orionregion is analyzed as a function of the local radiation field intensity,and the dust temperature and opacity are compared with (C-12)O and(C-13)O emission. The infrared radiation is interpreted within theframework of a single-component large grain model and a multicomponentgrain model consisting of subpopulations of grains with size-dependenttemperatures. A strong dependence of the 100-micron optical depthderived is found using the large grain model on the averageline-of-sight dust temperature and radiation field. In the hotenvironment surrounding high-luminosity sources and H II regions, alldust along the line-of-sight radiates at 100 microns, and thedust-to-gas ratio, based on the 100-micron opacity and I(/C-13/O),appears to be in agreement with the standard value, about 1 percent bymass. A relationship is found between the inferred dust-to-gas ratio andthe radiation field intensity responsible for heating the dust which canbe used to estimate the gas column density from the dust opacity derivedfrom the 60- and 100-micron IRAS fluxes.
| The upper main sequence of OB associations. II - The single-lined O stars: Spectral classification of northern stars and lines of C and N
The properties of the stars close to the upper main sequence of OBassociations are studied in order to provide constraints on the theoryof evolution of massive stars. The spectral classification of northernsingle-lined O stars, most of them belonging to the associations CygOB1, Per OB1 and Cas OB6, is presented. The classification is performedaccording to Conti's classification scheme for O stars, amended byMathys (1988). The behavior of the C and N lines of the stars isstudied, in order to determine whether CNO-processed material is visibleat their surface. Three new ON stars have been discovered: BD + 36 deg4063 (O9.71), HD 13268 (O8V), and HD 110360 (O7V).
| An analysis of the shapes of ultraviolet extinction curves. II - The far-UV extinction
In this paper the properties of interstellar extinction in thefar-ultraviolet region are examined utilizing IUE extinction curves fora primary data sample of 45 reddened Milky Way OB stars. These resultsare combined with those derived for the 2175 A bump in a previous study.It is found that IUE extinction curves can be represented by linearcombinations of a Lorentzian-like 2175 A bump profile, a well-determinedFUV curvature term, and an underlying linear component. The parametersof the linear component are strongly correlated and therefore only fivefree parameters are required to fit all of the curves in our sample.Three parameters describe the 2175 A bump, one parameter describes thelinear background, and one parameter describes the strength of the FUVcurvature term. The shape of the FUV curvature is found to be identical,to within the observational errors, for all the curves in the sample -which spans virtually the entire range of extinction curve morphologiesobserved in the Milky Way.
| Atlas of the wavelength dependence of ultraviolet extinction in the Galaxy
The paper presents a collection of 115 extinction curves derived fromlow-dispersion IUE spectra. The spectra have been reduced with the useof techniques designed to reduce the effects of random noise and theinfluence of residual spectral features due to classification mismatch.The magnitudes of other instrumental and interpretational uncertaintiesare estimated. The extinction curves are presented with normalization toE(B-V) = 1 and the FUV portion (below 170 nm) is also shown for E(13-17)= 1. The atlas includes examples of extinction originating in thediffuse medium and several major nebulae and dense clouds.
| ICCD speckle observations of binary stars. III - A survey for duplicity among high-velocity stars
An analysis of speckle interferometry data for 182 stars has resulted inthe detection of 10 binaries, four of which are newly resolved systems.After correcting for selection effects, the data are found to becompatible with a total frequency for high-velocity long-period doubleswhich is as large as that for low-velocity stars. Based on spectroscopicparallaxes and visual magnitudes, these binaries are found to be within100 pc of the sun, with eight having linear separations of less than 20AU. Four of the binaries are determined to have periods of less than 20yr.
| Properties of blue stragglers in young OB associations
The properties of the blue stragglers (BS) belonging to the associationsSco OB1, Cyg OB1, Car OB1, Cen OB1, and Per OB1 as well as of the earlyO stars belonging to Cas OB6 are studied on the basis of an extensivesurvey of the literature. Eleven of the thirteen BS for which therelevant information exists show an enhanced atmospheric abundance of N,which is interpreted as supporting the view (Maeder, 1987) that BS areprobably quasi-homogeneously evolved stars where the products of the CNOcycle have become observable at the surface. The mechanism inducing theturbulent diffusion responsible for the homogeneization of the stellarinterior does not clearly appear from the existing data. Tidal forces inclose binaries may play a role in some cases, but except in the case ofPer OB1 there is no convincing evidence that single BS have abnormallylarge rotational velocities. Finally the scenario of quasi-homogeneousevolution of the BS is not in contradiction with the observed propertiesof their likely descendents, the Wolf-Rayet stars, which are present inthe associations under consideration. The results presented in thispaper point toward the need for more observational data.
| Observational constraints on the carriers of the ultraviolet extinction bump
Data from the Infrared Astronomical Satellite is used to investigate theorigin of the ultraviolet (UV) extinction bump. Correlations of the dusttemperature with the parameters of the UV bump, as derived from IUEobservations, are presented. Significant correlations of the 60/100brightness ratio with the area under the UV bump and the height of theUV bump are found. These last results suggest that the particlesresponsible for the UV bump are very sensitive to the strength of theradiation field. However, no significant correlation is found betweenthe dust temperature and the width of the UV bump, as would have beenexpected on the basis of recent physical models.
| Mid-infrared excess and ultraviolet extinction
Recent suggestions that the mid-infrared excess and the 2174 Aextinction bump have the same origin are investigated. For this purpose,correlations between the mid infrared excess and the ultravioletextinction parameters are examined, using a sample of 61 stars reportedto have peculiar extinction properties. No correlation is found betweenthe mid-infrared excess and the ultraviolet extinction bump or the farultraviolet extinction rise. This result suggests that the carriers ofthe ultraviolet bump and the mid-infrared emission are different.
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