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Публикации по объекту
Water masers in the Local Group of galaxies
We compare the number of detected 22 GHz H2O masers in the Local Groupgalaxies M 31, M 33, NGC 6822, IC 10, IC 1613, DDO 187, GR8, NGC 185,and the Magellanic Clouds with the water maser population of the MilkyWay. To accomplish this we searched for water maser emission in the twoLocal Group galaxies M 33 and NGC 6822 using the Very Large Array (VLA)and incorporated results from previous studies. We observed 62 Hiiregions in M 33 and 36 regions with Hα emission in NGC 6822.Detection limits are 0.0015 and 0.0008 L_ȯ for M 33 and NGC 6822,respectively (corresponding to 47 and 50 mJy in three channels with 0.7km s-1 width). M 33 hosts three water masers above ourdetection limit, while in NGC 6822 no maser source was detected. We findthat the water maser detection rates in the Local Group galaxies M 31, M33, NGC 6822, IC 1613, DDO 187, GR8, NGC 185, and the Magellanic Cloudsare consistent with expectations from the Galactic water masers if oneconsiders the different star formation rates of the galaxies. However,the galaxy IC 10 exhibits an overabundance of masers, which may resultfrom a compact central starburst.
| Understanding Radio-selected Thermal Sources in M33: Ultraviolet, Optical, Near-Infrared, Spitzer Mid-Infrared, and Radio Observations
We present ultraviolet, optical, near-infrared, Spitzer mid-infrared,and radio images of 14 radio-selected objects in M33. These objects arethought to represent the youngest phase of star cluster formation. Wehave detected the majority of cluster candidates in M33 at allwavelengths. From the near-IR images, we derived ages 2-10 Myr,KS-band extinctions (AKS) of 0-1 mag,and stellar masses of 103-104 Msolar.We have generated spectral energy distributions (SEDs) of each clusterfrom 0.1 to 160 μm. From these SEDs, we have modeled the dustemission around these star clusters to determine the dust masses(1-103 Msolar) and temperatures (40-90 K) of theclusters' local interstellar medium. Extinctions derived from theJHKS, Hα, and UV images are similar to within a factorof 2 or 3. These results suggest that 11 of the 14 radio-selectedobjects are optically visible young star clusters with a surrounding HII region, that 2 are background objects, possibly active galacticnuclei (AGNs), and that 1 is a Wolf-Rayet star with a surrounding H IIregion.
| The Impact of Space Experiments on our Knowledge of the Physics of the Universe
With the advent of space experiments it was demonstrated that cosmicsources emit energy practically across all the electromagnetic spectrumvia different physical processes. Several physical quantities givewitness to these processes which usually are not stationary; thosephysical observable quantities are then generally variable. Thereforesimultaneous multifrequency observations are strictly necessary in orderto understand the actual behaviour of cosmic sources. Space experimentshave opened practically all the electromagnetic windows on the Universe.A discussion of the most important results coming from multifrequencyphotonic astrophysics experiments will provide new inputs for theadvance of the knowledge of the physics, very often in its more extremeconditions. A multitude of high quality data across practically thewhole electromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. With thesedata we are attempting to explain the physics governing the Universeand, moreover, its origin, which has been and still is a matter of thegreatest curiosity for humanity. In this paper we will try to describethe last steps of the investigation born with the advent of spaceexperiments, to note upon the most important results and open problemsstill existing, and to comment upon the perspectives we can reasonablyexpect. Once the idea of this paper was well accepted by ourselves, wehad the problem of how to plan the exposition. Indeed, the exposition ofthe results can be made in different ways, following several points ofview, according to: - a division in diffuse and discrete sources; -different classes of cosmic sources; - different spectral ranges, whichimplies in turn a sub-classification in accordance with differenttechniques of observations; - different physical emission mechanisms ofelectromagnetic radiation; - different vehicles used for launching theexperiments (aircraft, balloons, rockets, satellites, observatories). Inorder to exhaustively present The Impact of Space Experiments on ourKnowledge of the Physics of the Universe it would then have beennecessary to write a kind of Encyclopaedia of the Astronomical SpaceResearch, which is not our desire. On the contrary, since our goal is toprovide an useful tool for the reader who has not specialized in spaceastrophysics and for the students, we decided to write this paper in theform of a review, the length of which can be still consideredreasonable, taking into account the complexity of the argumentsdiscussed. Because of the impossibility of realizing a complete pictureof the physics governing the Universe, we were obliged to select how toproceed, the subjects to be discussed the more or the less, or those tobe rejected. Because this work was born in the Ph.D. thesis of one of us(LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomicaltradition' used there, namely: the spectral energy ranges. Although suchenergy ranges do not determine physical objects (even if in many casessuch ranges are used to define the sources as: radio, infrared, optical,ultraviolet, X-ray, γ-ray emitters), they do determine themethods of study, and from the technical point of view they define thetechnology employed in the relative experiments. However, since then wehave decided to avoid a deep description of the experiments, satellites,and observatories, simply to grant a preference to the physical results,rather than to technologies, however fundamental for obtaining thoseresults. The exposition, after an introduction (Section 1) and somecrucial results from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deduced fromspace experiments (Section 3), the physics of cosmic rays from ground-and space-based experiments (Section 4), and the physics of discretecosmic sources deduced from space experiments (Section 5). In this firstpart of the paper we have used the logic of describing the main resultsobtained in different energy ranges, which in turn characterize theexperiments on board space vehicles. Within each energy range we havediscussed the contributions to the knowledge of various kind of cosmicsources coming from different experiments. And this part is mainlyderived by the bulk of the introductory part of LSG's Ph.D. thesis. Inthe second part of the paper, starting from Section 6, we have preferredto discuss several classes of cosmic sources independently of the energyranges, mainly focussing the results from a multifrequency point ofview, making a preference for the knowledge of the physics governing thewhole class. This was decided also because of the multitude of new spaceexperiments launched in the last fifteen years, which would haverendered almost impossible a discussion of the results divided intoenergy ranges without weakening the construction of the entire puzzle.We do not pretend to cover every aspect of every subject consideredunder the heading of the physics of the universe. Instead a crosssection of essays on historical, modern, and philosophical topics areoffered and combined with personal views into tricks of the spaceastrophysics trade. The reader is, then, invited to accept this papereven though it obviously lacks completeness and the arguments discussedare certainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length. Some parts of itcould seem, in certain sense, to belong to an older paper, in which the`news' is not reported. But this is owed to our own choice, just in fullaccord with the goals of the text: we want to present those resultswhich have, in our opinion, been really important, in the development ofthe science. These impacting results do not necessarily constitute thelast news. This text was formally closed just on the day of the launchof the INTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.
| Multicolor Photometry of 145 of the H II Regions in M33
This paper is the first in a series presenting CCD multicolor photometryfor 145 H II regions, selected from 369 candidate regions fromBoulesteix et al., in the nearby spiral galaxy M33. The observations,which covered the whole area of M33, were carried out with the BeijingAstronomical Observatory 60/90 cm Schmidt telescope, in 13intermediate-band filters, covering a range of wavelengths from 3800 to10000 Å. This procedure provides a series of maps that can beconverted into a multicolor map of M33, in pixels of 1.7"×1.7".Using aperture photometry we obtain the spectral energy distributions(SEDs) for these H II regions. We also give their identification charts.Using the relationship between the Beijing-Arizona-Taiwan-Connecticutintermediate-band system used for the observations and the UBVRIbroadband system, the magnitudes in the B and V bands are then derived.Histograms of the magnitudes in V and in B-V are plotted, and thecolor-magnitude diagram is also given. The distribution of magnitudes inthe V band shows that the apparent magnitude of almost all the regionsis brighter than 18, corresponding to an absolute magnitude of -6.62 foran assumed distance modulus of 24.62, which corresponds to a singlemain-sequence O5 star, while the distribution of color shows that thesample is blue, with a mode close to -0.05, as would be expected from arange of typical young clusters.
| Neon Abundances in the H II Regions of M33
We present neon abundances for 25 H II regions of M33, measured fromline profiles of the mid-infrared transitions of [Ne II] and [Ne III]taken with the Infrared Space Observatory Short-Wavelength Spectrometer.The distribution of neon abundances as a function of galactocentricradius is best described as a step, -0.15 dex relative to the solar neonabundance from 0.7 to 4.0 kpc and -0.35 dex from 4.0 to 6.7 kpc, withestimated intrinsic scatter of 0.07 dex. The nearly flat neon abundancedistribution differs from the steep oxygen abundance gradient found byprevious investigators. Unless the oxygen abundance determinations arewrong, the chemical evolution of the galaxy has been radially dependent.
| The Excitation and Metallicity of Galactic H II Regions from Infrared Space Observatory SWS Observations of Mid-Infrared Fine-Structure Lines
We present mid-infrared Infrared Space Observatory Short-WavelengthSpectrometer (ISO-SWS) observations of the fine-structure emissionslines [Ne II] 12.8 μm, [Ne III] 15.6 μm, [Ne III] 36.0 μm, [ArII] 6.99 μm, [Ar III] 8.99 μm, [S III] 18.7 μm, [S III] 33.5μm, and [S IV] 10.5 μm and the recombination lines Brα andBrβ in a sample of 112 Galactic H II regions and 37 nearbyextra-Galactic H II regions in the LMC, SMC, and M33. We selected oursources from archival ISO-SWS data as those showing prominent [Ne II]12.8 μm or [Ne III] 15.6 μm emissions. The Galactic sources have awide range in galactocentric distance (0kpc<~Rgal<~18kpc), which enables us to study excitation and metallicity variationsover large Galactic scales. We detect a steep rise in the [Ne III] 15.6μm/[Ne II] 12.8 μm, [Ar III] 8.99 μm/[Ar II] 6.99 μm, and [SIV] 10.5 μm/[S III] 33.5 μm excitation ratios from the innerGalaxy outward, and a moderate decrease in metallicity, from ~2Zsolar in the inner Galaxy to ~1 Zsolar in theouter disk. The extra-Galactic sources in our sample show low gasdensity, low metallicity, and high excitation. We find a goodcorrelation between [Ne III] 15.6 μm/[Ne II] 12.8 μm and [Ar III]8.99 μm/[Ar II] 6.99 μm excitation ratios in our sample. Theobserved correlation is well reproduced by theoretical nebular modelsthat incorporate new-generation wind-driven non-LTE model stellaratmospheres for the photoionizing stars. In particular, the non-LTEatmospheres can account for the production of [Ne III] emission in the HII regions. We have computed self-consistent nebular and stellaratmosphere models for a range of metallicities (0.5-2Zsolar). We conclude that the increase in nebular excitationwith galactocentric radius is due to an increase in stellar effectivetemperature (as opposed to a hardening of the stellar spectral energydistributions due to the metallicity gradient). We estimate anintegrated [Ne III] 15.6 μm/[Ne II] 12.8 μm ratio for the Galaxyof 0.8, which puts it well inside the range of values for starburstgalaxies. The good fit between observations and our models support theconclusion of Thornley and coworkers that the low [Ne III] 15.6μm/[Ne II] 12.8 μm ratios observed in extra-Galactic sources aredue to global aging effects. Based on observations with ISO, an ESAproject with instruments funded by ESA member states (especially the PIcountries: France, Germany, the Netherlands, and the United Kingdom)with the participation of ISAS and NASA.
| Evolved Massive Stars in the Local Group. II. A New Survey for Wolf-Rayet Stars in M33 and Its Implications for Massive Star Evolution: Evidence of the ``Conti Scenario'' in Action
We expect the evolution of massive stars to be strongly influenced bymass loss and hence to be sensitive to metallicity. It should bepossible to test this ``Conti scenario'' be comparing the populations ofevolved massive stars among the Local Group galaxies, but suchinvestigations have been hampered by incompleteness. In a previouspaper, we presented results of a new survey for red supergiants (RSGs)in selected regions of the Local Group galaxies M33, M31, and NGC 6822.In the present paper, we survey eight fields in M33 for Wolf-Rayet stars(WRs), using interference-filter imaging with a CCD to selectcandidates. Follow-up spectroscopy is used to confirm 22 newly found WRstars, 21 of WN type. We establish that our survey would readily detectWRs as weak-lined as any known, and we conclude that our survey isessentially complete. This survey confirms suspicions that the previousphotographic surveys were only 50% complete for WN-type WRs and allowsus to combine the data with equally complete samples on other LocalGroup galaxies. We find that the relative number of WC- and WN-type WRscorrelates extremely well with metallicity, varying by a factor of 3with galactocentric distance within the plane of M33, and continuing thetrend to lower and higher metallicity galaxies. The WC/WN ratio within 3kpc of the sun is slightly above this trend, and we argue that WN starsare underrepresented in this sample. The WC/WN ratio is anomalously highin IC 10, given its low metallicity, and we demonstrate that this is notbecause of selection effects but is likely caused by IC 10's currentstatus as a starburst system. We examine the spectral properties of WCstars within these galaxies, confirming the previously reported trendsthat the spectral lines are stronger and broader in regions of lowermetallicity. We suggest that the different WC spectral subclasses do notprimarily indicate different physical properties for these stars butrather are simply a reflection of the effect that the initial metalabundances has had on the stellar wind structure. Finally, we comparethe luminous RSGs with WRs in these galaxies. We find that there is avery strong correlation of the relative numbers of RSGs and WRs withmetallicity, in the sense predicted by Maeder, Lequeux, & Azzopardi:at lower metallicities the fraction of luminous (M_bol < -7) RSGs ishigher, with a factor of 6 change within the disk of M33 [Delta log(O/H) = 0.35 dex], and a factor of ~10 change from M31 (or the innerportions of M33) to NGC 6822 [Delta log (O/H) = 0.5 dex]. This is easilyexplained by the Conti scenario in terms of massive stars spendingproportionately less of their He-burning lifetimes as RSGs rather thanWRs at higher metallicities and hence higher mass-loss rates. Finally,we note that the presence of luminous RSGs and WRs stars is extremelywell correlated for the OB associations in M31 and M33: where one findsone, one finds the other. To the extent that an association is strictlycoeval, this argues that some stars of 15 M_ȯ and above indeed dogo through both RSG and WR stages. The presence of WR stars of both WNand WC types in the same associations as luminous RSGs further suggeststhat some WCs, at least, have gone through the RSG phase. We include anAppendix providing a complete catalog of confirmed WR stars in LocalGroup galaxies beyond the Magellanic Clouds. Some of the observationsreported in this paper were obtained at the Multiple Mirror TelescopeObservatory, a facility operated jointly by the University of Arizonaand the Smithsonian Institution.
| The ultraviolet color gradient in the late-type spiral galaxy M33
The ultraviolet surface brightness and color distributions for thelate-type spiral galaxy M33 are derived from images at 1520 and 2490 Athat were obtained by the Ultraviolet Imaging Telescope (UIT) on theAstro Spacelab mission. Although the surface brightness shows a generaldecline with radius, the dominant spiral arms cause significantdeviations from an exponential fit. Colors in elliptical annuli become0.2-0.3 mag bluer with increasing radius. Our measures of individual HII regions and sections of spiral arms follow this same trend. Interarmregions are redder than the arms. A metallicity gradient affecting onlythe colors of stars is inadequate in accounting for the observed colorgradient. A plausible explanation invokes a combination of an LMC-typereddening curve and a radial gradient in the internal reddening in M33.
| The molecular hydrogen content of NGC 604 and other M33 H II region complexes
Six bright H II regions in M33 have been searched for H2 line emission.Previously detected fluorescent H2 emission in the first-ranked M33 H IIregion, NGC 604, has been mapped, making it possible to estimate themass of its molecular cloud (of order 3 x 10 to the 6th solar masses).This result implies a ratio I(CO)/N(H2) lower by an order of magnitudethan that of galactic molecular clouds. The lack of shocked H2 in NGC604 implies that the ionized stellar-wind bubbles seen in the nebula arenot interacting strongly with the molecular material. The dynamics ofNGC 604 appear to be dominated by gravitational interaction.
| Large-scale properties of interstellar dust and gas in M33
IRAS observations of M33 are used to study the large-scalecharacteristics of interstellar dust. At the four IRAS wavelengths, theM33 emission can be separated into a diffuse disk and a brightpoint-source component. The point-source emission is shown to beassociated with star-formation regions and to contribute half of thetotal IR luminosity of M33. The diffuse disk emission displays spectralcharacteristics similar to the Galactic cirrus. The results suggest thatlarge particles are selectively destructed by the more intenseinterstellar radiation field near the center of M33. A steep radialdecrease of the dust-to-(atomic) gas ratio is found. The results areconsistent with the grain model of Draine and Anderson (1985).
| Kinematics and composition of H II regions in spiral galaxies. I - M33
Moderate-dispersion, 0l.7 A/pixel spectra of H II regions in M33 weretaken, and the velocity as well as the excitation were measured byobserving the spectral region around H-beta. Velocity measurements for55 H II regions and excitation measurements for 42 regions arepresented. The velocity data are used to measure the systemic velocityat -172 + or - 6 km/s, the inclination at 56 + or - 1 deg, the positionangle of the major axis at 23 + or - 1 deg, the rotation curve of the HII regions, and their line-of-sight velocity dispersion at 9 + or - 4km/s. These data are used to derive the M/L ratio from radii of 0.5-3.7kpc. The excitation measurements are used to derive a metallicitygradient for M33 and to quantify the dispersion in the excitation valuesat a given radius.
| A multifrequency radio continuum survey of M33. II - Thermal and non-thermal emission
The galactic radio continuum emission of M33 at 17.4, 11.1, 6.3, and 2.8cm has been separated into thermal and nonthermal components. The radialprofiles of the thermal radio emission and SNR candidates are found tobe much steeper than those of the nonthermal emission and its linearlypolarized fraction. The M33 results are discussed with respect to thethermal fraction, the origin of cosmic ray electrons, and starformation.
| Correlations between integrated parameters and H-alpha velocity widths in giant extragalactic H II regions - A new appraisal
Investigations of relationships between diameters (or luminosities) andvelocity widths of H-alpha line profiles in giant extragalactic H IIregions (GEHR) have firmly established that these parameters arecorrelated. However, three independent studies on the subject disagreeon the slopes of these relations. It is shown that all measurements ofthe velocity width of integrated H-alpha profiles of GEHRs are entirelyconsistent. Discrepancies in the relations are explained by differentsamples and by the use of different parameters such as distances togalaxies and diameters of GEHRs. Assembling all observations of H-alphavelocity widths, new values of slopes and zero-points are derived forthe relations between luminosities (or diameters) and velocity widths.
| The spectra of extragalactic Wolf-Rayet stars
Spectra of 93 WR stars in M31, M33, NGC 6822, and IC 1613, obtained withthe IIDS and Cryogenic Camera on the KPNO 4-m telescope and with thephoton-counting Reticon system on the MMT, are examined in order tostudy the massive-star content of these galaxies and the properties oftheir most massive stars. The M33 WN and WC stars are shown to followthe same linewidth/line-strength relation as do the Galactic and LMCstars, while the WR stars in NGC 6822 resemble the SMC stars. It is alsonoted that the M31 WR stars are relatively weak lined for theirlinewidths.
| A multifrequency radio continuum survey of M33. I - Observations
Radio continuum surveys of M33 with the Effelsberg 100-m telescope at21.1 cm, 17.4 cm, 11.1 cm, 6.3 cm and 2.8 cm are presented. The data aredisplayed as distributions of total intensity and linear polarization.Thirty-eight point sources in a 2 x 2 deg field around M33 are seenwhich are either constituents of the galaxy or background sources. Achain of up to nine sources may be connected with the known large-scalealignment of quasars and radio galaxies to the southwest of M33.
| HII regions in M33. II - Radio continuum survey
A new WSRT survey of M33 in the continuum at 1.4 GHz is presented. Withan angular resolution of 25 x 49 arcsec HPBW and a rms noise of 0.2 mJyper beam area (at the field center), 112 radio sources with H-alphanebulosities have been identified. From a comparison of this survey dataand 5 GHz observations obtained with the VLA and WSRT, the spectralindices of 17 sources have been determined. Only three of these sourceshave nonthermal spectra. A compact source is discovered 1 arcmin southof the nucleus of M33. This source has a flat radio spectrum and noH-alpha counterpart. This source is either a heavily obscured compactHII region or, more likely, a source similar to the Crab Nebula.
| Turbulent gas motions in giant H II regions. II - The luminosity-velocity dispersion relation
The relation between luminosity L(H-alpha) and velocity dispersion sigmais reinvestigated in view of H-alpha and O III 5007-A forbidden linemeasurements of the velocity dispersion for 43 giant, extragalactic H IIregions. A correlation of L(H-alpha) that varies as sigma (H-alpha) tothe 6.6th power is obtained; O III-line velocity dispersions are notedto obey a similar relation, but are somewhat smaller, and the slope ofboth relations is steeper than expected for gravitationally boundsystems.
| H II regions in M33. III - Physical properties
The properties of the radio H II regions in M33 are investigatedutilizing a calibrated H-alpha survey of a large number of H II regionsin the inner part of M33 (Donas, 1977) and a forbidden O III/H-betasurvey obtained by Boulesteix et al. (1981). The luminosity function ofthe H II region in the inner part of M33, the extinction in the H IIregion, and the dust content in these nebulae are determined. Theassociation of H II regions with H I concentrations is brieflydiscussed.
| Mass loss and R 136a-type stars
Empirical and semiempirical expressions for the rate of mass loss,derived for O and B stars, have been examined vis-a-vis R 136a. The bestexpression has been used to predict the rate of mass loss for some R136a-type objects. The change in predicted values, when these objectsare not single stars but are composed of a dense cluster of similarstars has been discussed.
| IUE UV spectra of extragalactic H II regions. I - The catalogue and the atlas
More than 150 spectrograms of giant H II complexes in spiral, bluecompact and irregular galaxies, excluding the Magellanic Clouds,obtained within the period March 1, 1978 and April 1, 1982 with theInternational Ultraviolet Explorer (IUE) in the wavelength range 1150 Ato 3200 A are cataloged. Many of the observations are presented ingraphic form. A comparison of repeated observations for two objects in M33 and M 101 is made to investigate the reliability of singleobservations. General considerations on the UV spectra of giantextragalactic H II regions are given and a comparison of the spectra ofH II regions at various distances from the centers of M 33 and M 101 ispresented.
| Astrophysical properties of a luminous Wolf-Rayet type object in the core of the extragalactic H II region IC 132 from an analysis of its lambda-lambda 1200-6000 A spectrum
Abstract image available at:http://adsabs.harvard.edu/abs/1983MNRAS.203..157D
| Chemical compositions of H II regions in the Triangulum spiral, M33
Measurements of 12 H II regions secured with the Robinson-Wampler ImageTube Scanner at the Shane Telescope, Lick Observatory cover the spectralrange 3700-7600 A. The distances of these regions from the nucleus rangefrom 1 to 6 kpc. These data are analysed to establish plasma diagnosticsand chemical compositions. In a manner similar to that previouslyemployed for studies of the Magellanic Clouds, theoretical models areused as interpolation devices to establish ionization correction factorsfor S, Cl and Ar. Except for helium, the N(element)/N(H) ratios fall offwith increasing radial distance with closely similar rates.Consequently, the ratios not only of nitrogen, but also neon, sulphurand argon, with respect to oxygen, remain essentially constant. Thefollowing log N(element)/N(oxygen) are found: N = -1.25, Ne = -1.5, Cl =-3.6 and Ar = -2.4.
| Nitrogen and oxygen abundances in galaxies
Nitrogen and oxygen abundances are computed for galactic andextragalactic low-temperature H II regions by using a semiempiricalrelation between line intensities and temperature. The results areinterpreted in terms of galactic evolution on the basis of a chemicalmodel. The study confirms that certain relations previously obtained forhigh-excitation underabundant H II regions extend to low-excitationoverabundant regions. It is found that the nuclear regions of spiral andMarkarian galaxies of H II type present a large range of abundances andconstitute a very heterogeneous class and that there are smalldifferences in the initial mass function among galaxies. In particular,irregular galaxies are shown to have an initial mass function that hasthe same slope for stars more massive than 1 solar mass but containsmore lower-mass stars than the local function.
| Observations of three Wolf-Rayet stars in M33
Spectra of three objects in M33 identified as Wolf-Rayet stars areexamined which were obtained over the wavelength range from 3450 to 5100A with an image photon-counting system mounted on the digitalspectrograph at the Cassegrain focus of the 5-m Hale telescope. Theobject WR-13 is found to be embedded in an H II region and is classifiedas a WN star of subclass WN5 or WN6. The spectrum of WR-16 is shown toconfirm the previous result that this object is a WC7 star. Anearly-type companion is inferred, and it is suggested that WR stars inM33 are similar to those in the Galaxy. A previously unclassified objectobserved in the M33 H II region IC 132 is identified as a WN4 star.
| An optical study of M 33. I - Morphology of the gas
A general catalogue of 369 distinct H II regions has been compiled fromwide-field photographs obtained with a focal reducer and narrow-bandinterference filters. Ring-like emission nebulae have been observed asfar as 35 min of arc from the nucleus. It is proposed that these ringsrepresent a late stage in the life of expanding ionized regions. Generalhistograms related to the distribution of H II regions, intensities anddiameters are plotted. The number surface density of the faint regionsis rather constant over the whole galaxy, while for the strongerregions, it is distinctly greater in the central part. A most-probablevalue of 13 sec of arc is found for the apparent diameter of the H IIregions. A detailed spatial comparison is made with radio continuumsources, Wolf-Rayet stars, H I clouds, and clusters of hot stars. Spiralstructure is drawn based on the H II and H I observations. A very goodcorrelation is found between H II arms and the distribution of H Iclouds. Outer extensions of H emission are related to the spiralstructure.
| Radio observations of H II regions in external galaxies. I. H II regions in M33.
Abstract image available at:http://adsabs.harvard.edu/abs/1974A&A....32..363I
| Evidence for Composition Gradients across the Disks of Spiral Galaxies
Abstract image available at:http://adsabs.harvard.edu/abs/1971ApJ...168..327S
| The Spectra of the Emission Nebulosities in Messier 33.
Abstract image available at:http://adsabs.harvard.edu/abs/1942ApJ....95...52A
| The Rotation of the Spiral Nebula Messier 33.
Abstract image available at:http://adsabs.harvard.edu/abs/1942ApJ....95....5M
| Emission Nebulosities in the Spiral Nebula Messier 33
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