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| Guilt by Association: The 13 Micron Dust Emission Feature and Its Correlation to Other Gas and Dust Features
A study of all full-scan spectra of optically thin oxygen-richcircumstellar dust shells in the database produced by the ShortWavelength Spectrometer on ISO reveals that the strength of severalinfrared spectral features correlates with the strength of the 13 μmdust feature. These correlated features include dust features at 19.8and 28.1 μm and the bands produced by warm carbon dioxide molecules(the strongest of which are at 13.9, 15.0, and 16.2 μm). The databasedoes not provide any evidence for a correlation of the 13 μm featurewith a dust feature at 32 μm, and it is more likely that a weakemission feature at 16.8 μm arises from carbon dioxide gas ratherthan dust. The correlated dust features at 13, 20, and 28 μm tend tobe stronger with respect to the total dust emission in semiregular andirregular variables associated with the asymptotic giant branch than inMira variables or supergiants. This family of dust features also tendsto be stronger in systems with lower infrared excesses and thus lowermass-loss rates. We hypothesize that the dust features arise fromcrystalline forms of alumina (13 μm) and silicates (20 and 28 μm).Based on observations with the ISO, a European Space Agency (ESA)project with instruments funded by ESA member states (especially thePrincipal Investigator countries: France, Germany, the Netherlands, andthe United Kingdom) and with the participation of the Institute of Spaceand Astronautical Science (ISAS) and the National Aeronautics and SpaceAdministration (NASA).
| High-Resolution Spectroscopy of the Yellow Hypergiant ρ Cassiopeiae from 1993 through the Outburst of 2000-2001
We present an overview of the spectral variability of the peculiarF-type hypergiant ρ Cas, obtained from our long-term monitoringcampaigns over the past 8.5 yr with four spectrographs in the northernhemisphere. Between 2000 June and September an exceptional variabilityphase occurred when the V brightness dimmed by about a full magnitude.The star recovered from this deep minimum by 2001 April. It is the thirdoutburst of ρ Cas on record in the last century. We observe TiOabsorption bands in high-resolution near-IR spectra obtained with theUtrecht Echelle Spectrograph during the summer of 2000. TiO formation inthe outer atmosphere occurred before the deep brightness minimum.Atmospheric models reveal that the effective temperature decreases by atleast 3000 K, and the TiO shell is driven supersonically withM~=5.4×10-2 Msolar yr-1. Strongepisodic mass loss and TiO have also been observed during the outburstsof 1945-1947 and 1985-1986. A detailed analysis of the exceptionaloutburst spectra is provided, by comparing with high-resolution opticalspectra of the early M-type supergiants μ Cep (Ia) and Betelgeuse(Iab). During the outburst, central emission appears above the localcontinuum level in the split Na D lines. A prominent optical emissionline spectrum appears in variability phases of fast wind expansion. Theradial velocity curves of Hα and of photospheric metal absorptionlines signal a very extended and velocity-stratified dynamic atmosphere.The outburst spectra indicate the formation of a low-temperature,optically thick circumstellar gas shell of 3×10-2Msolar during 200 days, caused by dynamic instability of theupper atmosphere of this pulsating massive supergiant near the Eddingtonluminosity limit. We observe that the mass-loss rate during the outburstis of the same order of magnitude as has been proposed for the outburstsof η Carinae. We present calculations that correctly predict theoutburst timescale, whereby the shell ejection is driven by the releaseof hydrogen ionization recombination energy.
| The Evolutionary State of Stars in the NGC 1333S Star Formation Region
We present 2 μm near-IR spectroscopic observations of a sample of 33objects in the NGC 1333S active star-forming cluster centered on thepre-main-sequence star SSV 13. We have previously studied this regionphotometrically in the optical and near-IR, and with the addition ofthese near-IR spectra, we further probe the pre-main-sequence clustermembership and evolutionary state. From the atomic and molecularabsorption features observed, together with the earlier photometry, wederive spectral types, effective temperatures, masses, and ages of thestars and conclude that almost all (90%) the stars observed in thissample are pre-main-sequence objects. This result significantly refinesthe evolutionary information obtained from photometric evidence alone.Comparison with theoretical evolutionary tracks and isochrones suggeststhat our survey has sampled sources with masses in the range 0.2-2Msolar and stellar ages between 7×104 and1×108 yr with a preponderance of sources around3×106 yr. This implies the presence of low- tointermediate-mass T Tauri stars of evolutionary designation Class I toClass III. We conclude that star formation seems to have occurred inlikely several bursts rather than occurring coevally. Star formation insuch a region as NGC 1333S is likely significantly affected by the largenumber of active molecular outflows in the region, which could provide amechanism for cloud turbulence and the onset of subsequent starformation.
| The association of IRAS sources and 12CO emission in the outer Galaxy
We have revisited the question of the association of CO emission withIRAS sources in the outer Galaxy using data from the FCRAO Outer GalaxySurvey (OGS). The availability of a large-scale high-resolution COsurvey allows us to approach the question of IRAS-CO associations from anew direction - namely we examined all of the IRAS sources within theOGS region for associated molecular material. By investigating theassociation of molecular material with random lines of sight in the OGSregion we were able to construct a quantitative means to judge thelikelihood that any given IRAS-CO association is valid and todisentangle multiple emission components along the line of sight. Thepaper presents a list of all of the IRAS-CO associations in the OGSregion. We show that, within the OGS region, there is a significantincrease ( ~ 22%) in the number of probable star forming regions overprevious targeted CO surveys towards IRAS sources. As a demonstration ofthe utility of the IRAS-CO association table we present the results ofthree brief studies on candidate zone-of-avoidance galaxies with IRAScounterparts, far outer Galaxy CO clouds, and very bright CO clouds withno associated IRAS sources. We find that ~ 25% of such candidate ZOAGsare Galactic objects. We have discovered two new far outer Galaxystar-forming regions, and have discovered six bright molecular cloudsthat we believe are ideal targets for the investigation of the earlieststages of sequential star formation around HII regions. Finally, thispaper provides readers with the necessary data to compare othercatalogued data sets with the OGS data.Tables 1, 2 and A1 are only available in electronic form at the CDS viaanonymous ftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/1083
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veraenderliche Sterne e.V.
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| Near-Infrared Integral Field Spectroscopy and Mid-Infrared Spectroscopy of the Starburst Galaxy M82
We present new infrared observations of the central regions of thestarburst galaxy M82. The observations consist of near-infrared integralfield spectroscopy in the H and K bands obtained with the MPE 3Dinstrument and of λ=2.4-45 μm spectroscopy from the ShortWavelength Spectrometer (SWS) onboard the Infrared Space Observatory.These measurements are used, together with data from the literature, to(1) reexamine the controversial issue of extinction, (2) determine thephysical conditions of the interstellar medium (ISM) within thestar-forming regions, and (3) characterize the composition of thestellar populations. Our results provide a set of constraints fordetailed starburst modeling, which we present in a companion paper. Wefind that purely foreground extinction cannot reproduce the globalrelative intensities of H recombination lines from optical to radiowavelengths. A good fit is provided by a homogeneous mixture of dust andsources, and with a visual extinction of AV=52 mag. The SWSdata provide evidence for deviations from commonly assumed extinctionlaws between 3 and 10 μm. The fine-structure lines of Ne, Ar, and Sdetected with SWS imply an electron density of ~300 cm-3, andabundance ratios Ne/H and Ar/H nearly solar and S/H about one-fourthsolar. The excitation of the ionized gas indicates an average effectivetemperature for the OB stars of 37,400 K, with little spatial variationacross the starburst regions. We find that a random distribution ofclosely packed gas clouds and ionizing clusters and an ionizationparameter of ~10-2.3 represent well the star-forming regionson spatial scales ranging from a few tens to a few hundreds of parsecs.From detailed population synthesis and the mass-to-K-light ratio, weconclude that the near-infrared continuum emission across the starburstregions is dominated by red supergiants with average effectivetemperatures ranging from 3600 to 4500 K and roughly solar metallicity.Our data rule out significant contributions from older, metal-richgiants in the central few tens of parsecs of M82. Based on observationswith ISO, an ESA project with instruments funded by ESA Member States(especially the PI countries: France, Germany, Netherlands, and theUnited Kingdom) and with the participation of ISAS and NASA. The SWS isa joint project of SRON and MPE.
| Speckle Interferometry of New and Problem Hipparcos Binaries. II. Observations Obtained in 1998-1999 from McDonald Observatory
The Hipparcos satellite made measurements of over 9734 known doublestars, 3406 new double stars, and 11,687 unresolved but possible doublestars. The high angular resolution afforded by speckle interferometrymakes it an efficient means to confirm these systems from the ground,which were first discovered from space. Because of its coverage of adifferent region of angular separation-magnitude difference(ρ-Δm) space, speckle interferometry also holds promise toascertain the duplicity of the unresolved Hipparcos ``problem'' stars.Presented are observations of 116 new Hipparcos double stars and 469Hipparcos ``problem stars,'' as well as 238 measures of other doublestars and 246 other high-quality nondetections. Included in these areobservations of double stars listed in the Tycho-2 Catalogue andpossible grid stars for the Space Interferometry Mission.
| 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
| Studies of Yellow Semiregular(SRd) Variables
We have used the Hipparcos database of epoch photometry to study thevariability of several yellow semiregular(SRd) variables. For some ofthe stars (V847 Cas, RW Cep, BM Sco, CE Vir), the results wereinconclusive. For SX Lac and TY Vir, the periods found were consistentwith the General Catalogue of Variable Stars (190 and 50 daysrespectively). For UU Her, the known periods of about 45 and 72 dayswere recovered. For WY And, RU Cep, and SX Her, reliable periods werefound; we used archival data to construct (O-C) diagrams to study boththe random and systematic period changes in these three stars.
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| Silicate and hydrocarbon emission from Galactic M supergiants
Following our discovery of unidentified infrared (UIR) band emission ina number of M supergiants in h and chi Per, we have obtained 10-μmspectra of a sample of 60 galactic M supergiants. Only three newsources, V1749 Cyg, UW Aql and IRC+40 427, appear to show the UIR bands;the others show the expected silicate emission or a featurelesscontinuum. The occurrence of UIR-band emission in M supergiants istherefore much higher in the h and chi Per cluster than in the Galaxy asa whole. Possible explanations for the origin and distribution of UIRbands in oxygen-rich supergiants are discussed. We use our spectra toderive mass-loss rates ranging from 10^-8 to 10^-4 M_solar yr^-1 for thenew sample, based on the power emitted in the silicate feature. Therelationship between mass-loss rate and luminosity for M supergiants isdiscussed, and correlations are explored between their mid-infraredemission properties.
| Infrared giants vs. supergiants. II. CO observations
We report systematic observations of millimeter CO emission from asample of 109 oxygen-rich evolved stars (AGB and supergiants),colour-selected from the IRAS Point Source Catalog (0.69 200). In most cases, the observed spread in the valuesof this ratio can be explained by a large range of luminosities. Thisleads to a new criterion to identify AGB stars: an object with {cal R}< 150 must have a low mass progenitor. Here we study the correlationsbetween {cal R} and various physical properties of the sources. Mostsources with high values of {cal R} also have low galactic latitudes,small IRAS variability indices, and early spectral types (typicallyM1-M5). Conversely, there is no dependence on the IRAS colours, nor onthe intensity of silicate 10;microns emission. However, a few AGB starsexhibit large {cal R}; other factors than luminosity are required toexplain these values. Different hypotheses, such as the possiblepresence of a chromosphere, a low (12) C abundance or a variablemass-loss rate, are examined. Considering the global high OH detectionrate ( ~ 67%), we studied the correlations with CO and OH emission. Thedetection of OH seems to be a useful discriminator of mechanisms thatenhance {cal R}.
| The yellow hypergiants
We list the main stellar data of known hypergiants and similarlyluminous stars, and then concentrate on a review of the yellowhypergiants. These stars are post-red supergiants evolving alongblueward loops in the Hertzsprung-Russell diagram. Their properties,their location in the Hertzsprung-Russell diagram and their occasionalmass ejections are related to a region of atmospheric instability in theH-R diagram, the Yellow Void. The `bouncing against the border of theVoid' of three objects: ρ Cas, HR8752 and IRC+10420, is described.The apparent atmospheric instability of yellow hypergiants is related tothe atmospheric pulsations. There are indications that the approach tothe Void is associated with an increased amplitude of the pulsation andwith enhanced mass loss. The observed small-scale motion field is onlyapparently strongly supersonic; the observed large stochastic velocitiesare the quasi-stochastically varying thermal motions in the many hotsheets that occur in the wakes of many small shocks, while the realhydrodynamic velocity component is small and subsonic. This shock-wavefield is also responsible for the observed rate of mass loss and foremission in the wings of Hα . Most yellow hypergiants haveenvelopes containing gas and dust, but a thick extended envelope,presumably dissipating and showing bipolar outflow, is only known aroundIRC+10420. At the interface of the bipolar wind and the interstellarmedium one or more stationary shocks may develop as is observed in thecase of IRC+10420 and suspected with ρ Cas.
| Classification and Identification of IRAS Sources with Low-Resolution Spectra
IRAS low-resolution spectra were extracted for 11,224 IRAS sources.These spectra were classified into astrophysical classes, based on thepresence of emission and absorption features and on the shape of thecontinuum. Counterparts of these IRAS sources in existing optical andinfrared catalogs are identified, and their optical spectral types arelisted if they are known. The correlations between thephotospheric/optical and circumstellar/infrared classification arediscussed.
| Medium-Resolution Spectra of Normal Stars in the K Band
An Atlas of 115 medium-resolution K-band (2.0--2.4 mu m) stellarspectra, spanning spectral types O--M and luminosity types I--V, ispresented. K-band spectra are also presented for one N- and one J-typecarbon star. A time series of spectra is presented for an S-type Miravariable. All the spectra are at a resolution of ~3000 (1.4 cm-1) andhave had the terrestrial absorption removed by dividing by a featurelessspectrum. The spectra are plotted with the major spectral featuresidentified and are available digitally.
| Professional-amateur collaboration in variable star research: V. Observations of yellow semi-regular variables
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| The luminous starburst galaxy UGC 8387
We present broad-band J, H, and K images and K-band spectroscopy of theluminous starburst galaxy UGC 8387. The images show a disturbedmorphology, tidal tails, and a single elongated nucleus. Near infraredcolor maps constructed from the images reveal that the nucleus region ishighly reddened. Strong emission from the central 3 arcseconds in the2.166 micrometer Brackett gamma, 2.122 micrometer H2 v = 1-0 S(1), and2.058 micrometer He I lines is present in the K-band spectrum. From theBrackett gamma and published radio fluxes, we find an optical depthtoward the nucleus of tauV approximately 24. The CO bandheads produce strong absorption in the spectral region long-ward of 2.3micrometers. We measure a 'raw' CO index of 0.17 +/- 0.02 mag,consistent with a population of K2 supergiants of K4 giants. The nuclearcolors, however, are not consistent with an obscured population ofevolved stars. Instead, the red colors are best explained by an obscuredmixture of stellar and warm dust emission. The amount of dust emissionpredicted by the near-infrared colors exceeds that expected fromcomparisons to galactic H II regions. After correcting the spectrum ofUGC 8387 for dust emission and extinction, we obtain a CO index ofgreater than or equal to 0.25 mag. This value suggests the stellarcomponent of the 2.2 micrometer light is dominated by young supergiants.The infrared excess, LIR/LLy alpha derived for UGC8387 is lower than that observed in galactic H II regions and M82. Thisimplies that either the lower or upper mass cutoff of the initial massfunction must be higher than those of local star-forming regions andM82. The intense nuclear starburst in this galaxy is presumably theresult of merger activity; and we estimate the starburst age to be atleast a few times 107 yr.
| An Infrared Color-Magnitude Relationship
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110.2910H&db_key=AST
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| Ploarimetric and Photometric Observations of W Cephei and RW Cephei
The results of the observations of RW Cep during five years and of W Cepfor three years are presented. A symmetry of light curve of RW Cep isremarked in relation to the light minimum of 1979. The intrinsicpolarization of these supergiants is increasing in or near light minima.
| A near-infrared spectroscopic study of the luminous merger NGC 3256. 1: Constraints on the initial mass function of the starburst
Near-infrared spectroscopy of the central regions of the merger galaxyNGC 3256 is presented. The central 3 sec of the nucleus show strongrecombination lines of hydrogen, helium, and a prominent CO bandabsorption at 2.3 micrometers. These features suggest there is a largepopulation of OB stars and red supergiants associated with a vigorousepisode of star formation activity. The relative strength of severalrecombination lines and the slope of the 2.2 micrometers continuumindicate that the nucleus of NGC 3256 is affected by an extinctionAk = 0.6 mag. A stellar population model is used forinterpreting the observational properties of the galaxy. We show thatthe Br-gamma equivalent width and the strength of the CO band at 2.3micrometers provide a simple tool for determining the age of a starburstevent. A chi squared analysis is presented which allows both the age ofthe burst and the slope of the initial mass function to be determined ina unique way. The analysis shows that the starburst started between 12and 27 Myr ago. Given the dynamical mass observed in the nucleus of NGC3256, our analysis yields an IMF index (psi(m) proportional tom-alpha)alpha less than 2.2 which is shallower than recentlyderived IMFs in the solar neighborhood and the Magellanic Clouds.
| Characterization and proportion of very cold C-rich circumstellar envelopes
The paper develops a method for identification of very cold C-richcircumstellar envelopes (CSEs) in IR regions colder than region VII(i.e., larger value of IRAS flux ratio S25/S12), by reviewing thedifferent criteria and by identifying C-rich CSEs in a sample of 228bright IRAS sources. By using the results of OH surveys and catalogstogether with the results of additional HCN/CO millimeter observationsperformed on critical classes of objects, a C/O classification isproposed for 94 percent of the sources with a good probability. It isshown that the low-resolution spectra by themselves are a reasonable andeasy way to identify C-rich late AGB CSEs. For post-AGB object with coldCSEs, HCN and OH observations or visible/near IR spectroscopy arenecessary.
| Observation of gradual brightening of P Cygni due to stellar evolution
An analysis of modern and historical photometric measurements of thestar P Cygni is reported. A steady change of apparent brightness isfound, and it is argued that it is due to evolution of the star. Thechange is about twice as fast as standard models predict, but thedifference may be due to misestimation of the star's mass or inadequatetreatment of atmospheric expansion in the stellar models.
| A 2.18 to 2.28 micron study of the stellar population in the nucleus of M82
A 2.18-2.28 spectrum of the stellar nucleus of M82 at a low resolution(= 640) with high signal-to-noise ratio (greater than 100) is presentedin order to examine the properties of the stars found in the central 50pc of the starburst galaxy M82. It is concluded from a comparison ofthis spectrum to those of field giants and supergiants that theabsorption features in the spectrum of M82 at 2.227 and 2.238 micronsare significantly stronger relative to the 2.207- and 2.263-micronfeatures. Potential explanations for this and other peculiarities in thestellar population of the nucleus are investigated using both observedand synthesized stellar spectra. Spectra of solar-neighborhoodlate-class stars demonstrate that the ratios of these strong absorptionfeatures remain constant over a large range in effective temperature andgravity.
| On the interpretation of the spectrum of FU Orionis
The paper addresses several difficulties with the accretion diskinterpretation of the spectra of FU Orionis objects. It is argued thatthe double absorption lines observed in FU Ori and other members of thegroup, which have been claimed to be the signature of a self-luminousKeplerian disk, can be produced equally well by a single rotating star.The properties of the absorption-line spectrum of FU Ori as observed ata resolution of about 50,000 are described and compared to simple modelsof a single rotator with a stratified atmosphere. It is shown that theline structure of FU Ori can be reproduced by a model consisting of anemission-line shell overlaid by a cooler absorbing layer, the twosuperimposed upon a macroturbulent G-type supergiant spectrum. In thepresent model, the appearance of line doubling is due to the appearanceof weak emission cores in the metallic absorption lines, whilecurve-of-growth effects account for the observed dependence of reversalintensity upon absorption line strength.
| The metallicity and luminosity of RV Tauri variables from medium-resolution spectra
Results are reported from spectroscopic observations of members of theRV Tauri class of pulsating variable stars at moderate resolution, andfrom photometric observations of these stars with the B and V filters ofthe Johnson UBV system and the first four filters of the Wingeight-color narrowband near-IR system. Of the 22 variables observed, TiOabsorption was detected in eight, and 10 variables displayedhydrogen-line emission at some phase. Spectral classifications wereassigned relative to the MK process. Color excesses were determined forthose variables for which spectroscopic and photometric data wereobtained within a few days of each other. The spectra obtained at aresolution of 2.5 A were fitted with synthetic spectra to determine thestellar atmospheric parameters. The range in metallicity among thevariables was found to be quite extensive, -0.3 to -0.7, and may reflecta mixture of stellar populations or masses.
| 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.
| A Complete Survey of OH / Infrared Objects from the IRAS LRS Sources Within a Domain of the Color Diagram
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1992A&A...254..133L&db_key=AST
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