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A Multiwavelength Optical Emission Line Survey of Warm Ionized Gas in the Galaxy
We report on observations of several optical emission lines toward avariety of newly revealed faint, large-scale Hα-emitting regionsin the Galaxy. The lines include [N II] λ6583, [N II]λ5755, [S II] λ6716, [O III] λ5007, and He Iλ5876 obtained with the Wisconsin Hα Mapper (WHAM) towardsight lines that probe superbubbles, high-latitude filamentary features,and the more diffuse warm ionized medium (WIM). Our observations includemaps covering thousands of square degrees toward the well-knownOrion-Eridanus bubble, a recently discovered60deg×20deg bipolar superbubble centered inPerseus, plus several classical H II regions surrounding OB stars andhot evolved stellar cores. We use the emission-line data to explore thetemperature and ionization conditions within the emitting gas and theirvariations between the different emission regions. We find that in thediffuse WIM and in the faint high-latitude filamentary structures theline ratios of [N II]/Hα and [S II]/Hα are generally high,while [O III]/Hα and He I/Hα are generally low compared tothe bright classical H II regions. This suggests that the gas producingthis faint widespread emission is warmer, in a lower ionization state,and ionized by a softer spectrum than gas in classical H II regionssurrounding O stars, the presumed ionization source for the WIM. Inaddition, we find differences in physical conditions between the largebubble structures and the more diffuse WIM, suggesting that theionization of superbubble walls by radiation from interior Oassociations does not account entirely for the range of conditions foundwithin the WIM, particularly the highest values of [N II]/Hα and[S II]/Hα.
| Cloud Fragmentation and Proplyd-like Features in H II Regions Imaged by the Hubble Space Telescope
We have analyzed Hubble Space Telescope ACS and WFPC2 new and archivalimages of eight H II regions to look for new protoplanetary disks(proplyds) similar to those found in the Orion Nebula. We find a wealthof features similar in size (although many are larger) to the brightcusps around the Orion Nebula proplyds. None of them, however, containsa definitive central star. From this, we deduce that the new cusps maynot be proplyds but instead fragments of molecular cloud material. Outof all the features found in the eight H II regions examined, only one,an apparent edge-on silhouette in M17, may have a central star. Thisfeature might join the small number of bona fide proplyds found outsidethe Orion Nebula, in M8, M20, and possibly M16. In line with the resultsfound recently by Smith et al., the paucity of proplyds outside theOrion Nebula can be explained by their transient nature, as well as bythe specific environmental conditions under which they can be observed.Several fragments are seen as dark silhouettes against a brightbackground. We have reanalyzed those found in IC 2944 by Reipurth et al.and found new, similar ones in M16. None of these fragments contains acentral star, and we exclude the possibility that they are disks.Reipurth et al. concluded that the IC 2944 silhouettes are not starforming. We argue here that their assumption of a constant optical depthfor these fragments is not physical and that it is more likely thatthese fragments are star forming, a condition that is supported,although not proved, by their shapes and distributions. The process ofcloud fragmentation and photoevaporation produces a large number ofsmall fragments, while the size hierarchy expected in a photoevaporativeenvironment would not favor small fragments. The size distributionsobserved will constrain any future theories of cloud fragmentation. Onebright microjet candidate is found in M17, protruding from a large,limb-brightened fragment. A second, larger, jetlike feature, similar inshape and size to a Herbig-Haro jet, is found in Pismis 24. No centralstar appears to be associated with either of these jet candidates.Based on observations made with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS5-26555.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| The origin of massive O-type field stars. I. A search for clusters
We present a study aimed at clarifying the birthplace for 43 massiveO-type field stars. In this first paper we present the observationalpart: a search for stellar clusters near the target stars. We derivestellar density maps at two different resolving scales, viz. 0.25pc and 1.0 pc from NTT and TNG imaging and the 2MASS catalogue.These scales are typical for cluster sizes. The main result is that thelarge majority of the O-type field population are isolated stars: only12% (5 out of 43) of the O-type field stars is found to harbour asmall-scale stellar cluster. We review the literature and aim atcharacterizing the stellar field of each O-type field star with theemphasis on star formation and the presence of known young stellarclusters. An analysis of the result of this paper and a discussion ofthe O-type field population as products of a dynamical ejection event ispresented in an accompanying paper.Based on observations collected at the European Southern Observatory,Chile, and at the Italian Telescopio Nazionale Galileo (TNG) operated onthe island of La Palma by the Centro Galileo Galilei of the CNAA(Consorzio Nazionale per l'Astronomia e l'Astrofisica) at the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias.Table 2 and Figs. 4 to 17 are available in electronic form athttp://www.edpsciences.org
| A molecular cloud complex above the galactic plane.. I. Extended CO observations of the NGC 281 region
We have mapped the entire extent of the molecular cloud complexassociated with NGC281, both in 12CO (J=1-0) and 13CO (J=1-0),using the Taeduk Radio Astronomy Observatory (TRAO) 14 m telescope. Wealso observed a few selected fields with bright 13COemission, in CS (J=2-1) and HCO+ (J=1-0). This region isdistinguished by its vertical height of ~320 pc above the midplane ofthe Perseus spiral arm, at a distance of 2.9 kpc. The molecular gas inthe mapped region is found to extend far beyond the Hii region NGC 281 (Sh184). The southern boundary of the CO emission closely follows thesouthern boundary of the 100 μm dust emission. The molecular cloudcomplex consists of two main clouds bisected by the Hii region, as wellas several other clouds distributed over a range of 90 pc along thegalactic longitude direction. Except for one cloud atVLSR=-43.9 km s-1, all these clouds occupy asmall, contiguous velocity range centered at VLSR=-30 kms-1. Although these clouds are weakly connected, they clearlyshow distinct, knotted features along the galactic longitude direction.We assigned names to each cloud (A-H), derive their various physicalparameters, and discuss their physical properties. These clouds do notshow any specific, systematic behavior in their velocity field, exceptfor a small gradient along galactic longitude (for A-E). The brightest12CO and 13CO emissions are detected toward cloudA, which is located farthest from the two main clouds. We found starformation to be ongoing in clouds A and B, though these are not asactive as D and E. The associated newborn stars have FIR propertiessimilar to those of Herbig Ae/Be stars, and several outflows are alsoseen. We estimated the cloud masses in three different ways, and discussthe discrepancies between the resulting values. We estimate the totalmass of the mapped region to be ~3.7×104Msolar, using a conversion factor from CO luminosity to gasmass. About half of the clouds seem to be gravitationally bound.
| High-mass binaries in the very young open cluster NGC 6231. Implication for cluster and star formation
New radial-velocity observations of 37 O- and B stars in the very youngopen cluster NGC 6231 confirm the high frequency of short-periodspectroscopic binaries on the upper main sequence. Among the 14 O-typestars, covering all luminosity classes from dwarfs to supergiants, 8 aredefinitively double-lined systems and all periods but one are shorterthan 7 days. Several additional binaries have been detected among theearly B-type stars. NGC 6231 is an exceptional cluster to constrain thescenarios of cluster- and binary-star formation over a large range ofstellar masses. We discuss the evidences, based on NGC 6231 and 21 otherclusters, with a total of 120 O-type stars, for a clear dichotomy in themultiplicity rate and structure of very young open clusters containingO-type stars in function of the number of massive stars. However, wecannot answer the question whether the observed characteristics resultfrom the formation processes or from the early dynamical evolution.
| Oxygen and helium abundances in Galactic Hii regions - II. Abundance gradients
The absolute integrated fluxes that we obtained (Paper I) for 34 Hiiregions, with Galactocentric distances RG in the 6.6-17.7kpcrange, in the emission lines [Oii] λλ3726 and 3729, [Oiii]λλ4363 and 5007, Hei λ5876, Hα and Hβ,are analysed to derive the extinctions, the electron densities andtemperatures, the ionic abundances O+/H+,O++/H+ and He+/H+ and theO/H abundances. The electron temperature has been derived from the[Oiii] λ4363/λ5007 ratio in six Hii regions withRG between 6.6 and 14.8kpc. These new measurements ofTe, which are in good agreement with those from radiorecombination lines, widen the RG range for which reliableTe measurements exist. Combining our newTe([Oiii]) results with the radio values, we obtain thefollowing temperature relationship: TeK]=(372+/-38)RG+4260+/-350. The O+/H+and O++/H+ abundances have been obtained assuminga two-temperature Hii region model. Our O/H relationship for5kpc=180) measured in the Galaxyby Peimbert et al.
| Oxygen and helium abundances in Galactic Hii regions - I. Observations
Absolute integrated line fluxes of Hii regions have been measured usinga Fabry-Perot spectrophotometer. We describe the observations andcalibration procedures. Fluxes are given for 36 Hii regions withGalactocentric distances ranging from 6.6 to 17.7kpc. Several emissionlines have been measured, mainly [Oii] λλ3726 and 3629,Hβ, [Oiii] λ5007, Hei λ5876 and Hα. The veryfaint [Oiii] λ4363 line has been measured in six regions,allowing a direct determination of the electron temperature. Newphotometric distances have been derived based on data from theliterature. A discussion of these results in terms of extinction,electron density and temperature, and oxygen and helium abundances isgiven in Paper II.
| Far-Infrared Study of IRAS 00494+5617 and IRAS 05327-0457
High angular resolution far-infrared observations at 143 and 185 μm,using the Tata Institute of Fundamental Research 1 m balloon-bornetelescope, are presented for two Galactic star-forming complexesassociated with IRAS 00494+5617 and IRAS 05327-0457. The latter map alsoreveals the cold dust in OMC-3. Both these regions are illuminated atthe edges by high-mass stars with substantial UV flux. The HIRES(High-Resolution processing using Maximum Correlation Method) processedIRAS maps at 12, 25, 60, and 100 μm have also been presented forcomparison. The present study is aimed at quantifying the role of thenearby stars vis-à-vis embedded young stellar objects in theoverall heating of these sources. Based on the FIR observations at 143and 185 μm carried out simultaneously with almost identical angularresolution, reliable dust temperature and optical depth maps have beengenerated for the brighter regions of these sources. Radiative transfermodeling in spherical geometry has been carried out to extract physicalparameters of these sources by considering the observationalconstraints, such as spectral energy distribution, angular size atdifferent wavelengths, dust temperature distribution, etc. It isconcluded that for both IRAS 00494+5617 and IRAS 05327-0457, theembedded energy sources play the major role in heating them with finitecontribution from the nearby stars. The best-fit model for IRAS00494+5617 is consistent with a simple two-phase clump-interclumppicture with ~5% volume filling factor (of clumps) and a densitycontrast of ~80.
| Two-colour photometry for 9473 components of close Hipparcos double and multiple stars
Using observations obtained with the Tycho instrument of the ESAHipparcos satellite, a two-colour photometry is produced for componentsof more than 7 000 Hipparcos double and multiple stars with angularseparations 0.1 to 2.5 arcsec. We publish 9473 components of 5173systems with separations above 0.3 arcsec. The majority of them did nothave Tycho photometry in the Hipparcos catalogue. The magnitudes arederived in the Tycho B_T and V_T passbands, similar to the Johnsonpassbands. Photometrically resolved components of the binaries withstatistically significant trigonometric parallaxes can be put on an HRdiagram, the majority of them for the first time. Based on observationsmade with the ESA Hipparcos satellite.
| Photoelectric Photometry of Stars in the Orion Standard Region
Magnitudes and color indices in the Vilnius seven-color photometricsystem are given for 108 stars in the Orion standard region around theOrion Belt and the star lambda Orionis. New spectral and luminosityclasses, estimated from the photometric data, are given for some of thestars.
| 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.
| Trapezium Type Multiple Stars-Instable Systems
Not Available
| ICCD speckle observations of binary stars. XIX - an astrometric/spectroscopic survey of O stars
We present the results of a speckle interferometric survey made with theCHARA speckle camera and 4 m class telescopes of Galactic O-type starswith V less than 8. We can detect with the speckle camera binaries inthe angular separation range 0.035-1.5 arcsec with delta M less than 3,and we have discovered 15 binaries among 227 O-type systems. We combinedour results on visual binaries with measurements of wider pairs from theWashington Double Star Catalog and fainter pairs from the HipparcosCatalog, and we made a literature survey of the spectroscopic binariesamong the sample. We then investigated the overall binary frequency ofthe sample and the orbital characteristics of the known binaries.Binaries are common among O stars in clusters and associations but lessso among field and especially runaway stars. There are many triplesystems among the speckle binaries, and we discuss their possible rolein the ejection of stars from clusters. The period distribution of thebinaries is bimodal in log P, but we suggest that binaries with periodsof years and decades may eventually be found to fill the gap. The massratio distribution of the visual binaries increases toward lower massratios, but low mass ratio companions are rare among close,spectroscopic binaries. We present distributions of the eccentricity andlongitude of periastron for spectroscopic binaries with ellipticalorbits, and we find strong evidence of a bias in the longitude ofperiastron distribution.
| The NGC 281 west cluster. I. Star formation in photoevaporating clumps.
The NGC281 West molecular cloud is an excellent test case for studyingstar formation in the clumpy interface between a \hii region and a giantmolecular cloud. We present here a study based on new high resolutionradio and near-infrared data. Using the IRAM 30-meter telescope, we havemapped the interface in the \cotwo, \coone, and \cs transitions withFWHP beamwidths <= 22''. We have imaged the same region with the VLAin the 20, 6 and 2 cm continuum bands to obtain complementary maps ofthe ionized gas distribution with angular resolutions <= 13''. Inaddition, we have obtained near-infrared J and K'-band images to detectyoung stars in the interface. The 30-meter data shows the molecular gasis concentrated into three clumps with masses of 570, > 210, and 300\msun and average volume densities of 1.4, >1, and 2 x 10(4) \cm. Wedetect \cs emission in two of the clumps, indicating peak densities inexcess of 5x 10(5) \cm are attained in the clumps. A comparison of the\co line data with the 20 cm continuum image suggests that the molecularclumps are being photoevaporated through their direct exposure to the UVradiation from neighboring OB stars. The luminosity and extent of theobserved 20 cm emission is in good agreement with theoreticalpredictions. We use models of photoevaporative flows to estimate thepressure exerted on the clumps by the ionized gas and find that itexceeds the internal, turbulent pressure of the clumps by a factor of a2.5. Although a pressure equilibrium is not excluded given theuncertainties inherent in determining the pressures of the ionized andmolecular gases, our best estimates of the clumps and flow parametersfavor the the existence of low velocity shocks (1.5 \kms) in the clumps.The clumps exhibit broad, non-gaussian lineshapes and complexkinematical structures suggestive of shocks. Further evidence for shocksis found in a comparison of position-velocity diagrams with publishednumerical simulations of imploding spherical clumps. We discuss thepossibility that the knots of \cs emission may trace gas compressed byconverging shock waves. The K'-band observations show a rich cluster ofprimarily low mass stars in the \hii/molecular interface, which we argueis divided into two distinct sub-clusters. We associate one sub-clusterwith the two clumps nearest the OB stars, and the second sub-clusterwith the third clump. The two clumps nearest the OB stars contain anembedded population, suggesting that star formation is ongoing. Wediscuss the impact photoevaporation is having on star formation in thesetwo clumps. We find that photoevaporation is dispersing the moleculargas from which the cluster is forming and estimate that the moleculargas will be completely evaporated in 5 Myr. Deep K'-band imaging of thetwo clumps show that the stars are detected primarily on the sides ofthe clumps facing the OB stars and in the adjoining \hii region. Weexamine three explanations for this asymmetry: displacement of the clumpcenters from the cluster center by the acceleration of the molecular gasthrough photoevaporation (i.e. the rocket effect), unveiling of young,embedded stars by ionization-shock fronts, and the triggered formationof stars by shocks advancing into the clumps. If shock compression isindeed ongoing in the clumps, then we argue that there is a good casefor shock triggered star formation.
| IC 1590, A Young Cluster Embedded in the Nebulosity of NGC 281
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....113.2116G&db_key=AST
| JHK Photometry of Three BOK Globules in NGC 281
IC1590 is a very sparse, young, galactic cluster, centered about the O6Vtrapezium system HD5005, embedded in the HII region NGC281, and obscuredin the south by a dust cloud. According to star counts, the clusternucleus has a diameter of 6 parsec and contains only 45 +/- 9 memberswith M_v <= 3.6 mag. An IR survey was initiated in order to detectany other probable cluster members which are obscured by the dust. ThreeBok globules (at various distances from the center) are visible againstthe nebulosity and have been investigated in the JHK bandpasses, usingthe USNO HgCdTe IR camera. Among the many field stars observed are threeobjects which are probable early--type members according to UBV data.Only one IR source was found and is located in the globule closest toHD5005. The question of whether this star is a very heavily reddenedcluster member or a background object will be discussed as well as thesignificance and implications of these results.
| Two-dimensional Ultraviolet Spectral Typing of O-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...460..906P&db_key=AST
| Derivation of the Galactic rotation curve using space velocities
We present rotation curves of the Galaxy based on the space-velocitiesof 197 OB stars and 144 classical cepheids, respectively, which rangeover a galactocentric distance interval of about 6 to 12kpc. Nosignificant differences between these rotation curves and rotationcurves based solely on radial velocities assuming circular rotation arefound. We derive an angular velocity of the LSR of{OMEGA}_0_=5.5+/-0.4mas/a (OB stars) and {OMEGA}_0_=5.4+/-0.5mas/a(cepheids), which is in agreement with the IAU 1985 value of{OMEGA}_0_=5.5mas/a. If we correct for probable rotations of the FK5system, the corresponding angular velocities are {OMEGA}_0_=6.0mas/a (OBstars) and {OMEGA}_0_=6.2mas/a (cepheids). These values agree betterwith the value of {OMEGA}_0_=6.4mas/a derived from the VLA measurementof the proper motion of SgrA^*^.
| 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 atlas of ultraviolet P Cygni profiles
We have selected spectra of 232 stars from the International UltravioletExplorer (IUE) archives for inclusion in an atlas intended for varioususes but tailored especially for the study of stellar winds. The atlascovers the range in spectral types from O3 to F8. The full atlas coversthe reduced and normalized high resolution spectra from the IUE long-and short-wavelength spectrographs. Here we discuss the selection of thestars and the data reduction, and we present in velocity units theprofiles of lines formed in the stellar winds. The selected lines covera wide range of ionizations, allowing a comparison of the profiles fromdifferent ions in the wind of each star and a comparison of thedifferent wind lines as a function spectral type and luminosity. We alsopresent the basic data on the program stars to facilitate study of thedependence of wind features on stellar parameters such as luminosity,temperature, escape velocity, and v sin i. We provide an overview of thecharacteristic behavior of the wind lines in the H-R diagram. Thecomplete spectra are available in digital form through the NASAAstrophysics Data System (ADS). We offer a description of the electronicdatabase that is available through the ADS and guidelines for obtainingaccess to that database.
| 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.
| On the Connection Between Radial Systems of Dark Globules and Stellar Associations
We study the connection between radial systems of dark globules andstellar associations. It is shown that of the 17 systems of type 1 inTable 1 of [1] 16 radial systems are connected with known associations.A new association is found (missing from the catalogs) connected withthe remaining system (System No. 2). Four systems of the six systems oftype 2 (Table 2 of [1]) are connected with known associations. A newmethod of determining the distance to associations is proposed, usingthe mean linear thickness of dark globules of radial systems connectedwith these associations as the criterion for distance. Using this methodwe make the distance to the association Cyg OB 9 more precise and answerthe question whether several radial systems belong to the correspondingstellar associations.
| Multi-wavelength study of NGC 281 A
We present a study of the molecular cloud NGC 281 A and the associatedcompact and young star cluster NGC 281 (AS 179). Optical photometryleads to a new distance of 3500 pc for the star cluster which is in goodagreement with the kinematical distance of the adjacent molecular cloudNGC 281 A. The exciting star HD 5005 of the optical nebulosity is aTrapezium system with O6 III as photometric spectral type for thecomponent HD 5005 AB. For the age of the star cluster we estimated avalue of about 3 x 106 yr. The (12)CO (2 to 1), (13)CO (2 to1), and (12)CO (3 to 2) emission shows that the molecular cloud NGC 281A consists of two cloud fragments. The western fragment is more compactand massive than the eastern fragment and contains an NH3 core. Thiscore is associated with the IRAS source 00494+5617, an H2O maser, and1.3 millimeter dust continuum radiation. Both cloud fragments containaltogether 22 IRAS point sources which mostly share the properties ofyoung stellar objects. They have luminosities between 150 and 8800 solarluminosity. The maxima of the 60 and 100 micrometers HIRES mapscorrespond to the maxima of the (12)CO (3 to 2) emission. The NGC 281 Aregion shares many properties with the Orion Trapezium-BN/KL region themain differences being a larger separation between the cluster centroidand the new site of star formation as well as a lower mass andluminosity of the molecular cloud and the infrared cluster.
| Stellar Counts in NGC 281
Not Available
| The development of H I dissociation zones around new H II regions
Results of computations for a model of the time development of H Iphotodissociation zones in the molecular gas around O and early Bmain-sequence stars and their associated H II regions are presented. Thecomputations are for a grid of values of T(eff) from 20 to 45 kK and ofambient gas particle densities from 30 to 3000 atoms/cu cm. The modelcalculations show that in the case of atomic zones around H II regions,in low-density gas, the unshocked H I zone will persist for up to halfthe main-sequence lifetime of the star, whereas in high-density gas thezone will last only a few percent of the stellar lifetime. A star withT(eff) at the lower end of the range will dissociate to a radius severaltimes the radius of the H II region, whereas the maximum width of thedissociation zone around the star at the early end of the range will beless than the ionized radius. The sizes and masses of both H II regionsand their H I zones increase with decreasing density of the surroundinggas.
| 12CO(3->2) measurements of NGC 281 (S 184).
Not Available
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