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On the Distance and Molecular Environment of Westerlund 2 and HESS J1023-575
The extended TeV gamma-ray source HESS J1023-575 is coincident with themassive, young stellar cluster Westerlund 2 (Wd2) and its surrounding HII region RCW 49. On the basis of an analysis of the CO emission and 21cm absorption along the line of sight to Wd2, it is argued that thiscluster, and by assumption the TeV source as well, must be associatedwith a giant molecular cloud in the far side of the Carina arm with amass of 7.5×105 Msolar. Analysis of thespatial and velocity structure of the cloud reveals clear evidence ofinteraction with Wd2. The cloud's kinematic distance of 6.0+/-1.0 kpc isshown to be consistent with distances inferred from both the radius-linewidth relation of molecular clouds and the foreground gas column derivedfrom 230 X-ray sources in Wd2.
| Cleaning Up η Carinae: Detection of Ammonia in the Homunculus Nebula
We report the first detection of ammonia in the Homunculus Nebula aroundη Carinae, which is also the first detection of emission from apolyatomic molecule in this or any other luminous blue variable (LBV)nebula. Observations of the NH3 (J,K)=(3,3) inversiontransition made with the Australia Telescope Compact Array revealemission at locations where infrared H2 emission had beendetected previously, near the strongest dust emission in the core of theHomunculus. We also detect ammonia emission from the so-called strontiumfilament in the equatorial disk. The presence of NH3 aroundη Car hints that molecular shells around some Wolf-Rayet stars couldhave originated in prior LBV eruptions, rather than in cool redsupergiant winds or the ambient interstellar medium. Combined with thelack of any CO detection, NH3 seems to suggest that theHomunculus is nitrogen-rich like the ionized ejecta around η Car. Italso indicates that the Homunculus is a unique laboratory in which tostudy unusual molecule and dust chemistry, as well as their rapidformation in a nitrogen-rich environment around a hot star. We encouragefuture observations of other transitions like NH 3 (1, 1) and(2, 2), related molecules like N2H+, and renewedattempts to detect CO.
| X-Ray Emission from Wind-blown Bubbles. III. ASCA SIS Observations of NGC 6888
We present ASCA SIS observations of the wind-blown bubble NGC 6888.Owing to the higher sensitivity of the SIS for higher energy photonscompared to the ROSAT PSPC, we are able to detect aT~8×106 K plasma component in addition to theT~1.3×106 K component previously detected in PSPCobservations. No significant temperature variations are detected withinNGC 6888. García-Segura & Mac Low's analytical models of WRbubbles constrained by the observed size, expansion velocity, and massof the nebular shell underpredict the stellar wind luminosity and cannotreproduce simultaneously the observed X-ray luminosity, spectrum,surface brightness profile, and SIS count rate of NGC 6888's bubbleinterior. The agreement between observations and expectations frommodels may be improved if one or more of the following ad hocassumptions are made: (1) the stellar wind luminosity was weaker in thepast, (2) the bubble is at a special evolutionary stage and the nebularshell has recently been decelerated to 1/2 of its previous expansionvelocity, and (3) the heat conduction between the hot interior and thecool nebular shell is suppressed. Chandra and XMM-Newton observationswith high spatial resolution and high sensitivity are needed toaccurately determine the physical conditions of NGC 6888's interior hotgas for critical comparisons with bubble models.
| An XMM-Newton look at the Wolf-Rayet star WR 40. The star itself, its nebula and its neighbours
We present the results of an XMM-Newton observation of the field of theWolf-Rayet star WR 40. Despite a nominal exposure of 20 ks and the highsensitivity of the satellite, the star itself is not detected: we thusderive an upper limit on its X-ray flux and luminosity. Joining thisresult to recent reports of a non-detection of some WC stars, we suggestthat the X-ray emission from single normal Wolf-Rayet stars could oftenbe insignificant despite remarkable instabilities in the wind. On thebasis of a simple modelling of the opacity of the Wolf-Rayet wind of WR40, we show that any X-ray emission generated in the particular zonewhere the shocks are supposed to be numerous will indeed have littlechance to emerge from the dense wind of the Wolf-Rayet star. We alsoreport the non-detection of the ejecta nebula RCW 58 surrounding WR 40.Concerning the field around these objects, we detected 33 X-ray sources,most of them previously unknown: we establish a catalog of these sourcesand cross-correlate it with catalogs of optical/infrared sources.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member States andthe USA (NASA).
| A unique Galactic planetary nebula with a [WN] central star
We report the discovery of the first probable Galactic [WN] central starof a planetary nebula (CSPN). The planetary nebula candidate was foundduring our systematic scans of the AAO/UKST Hα Survey of the MilkyWay. Subsequent confirmatory spectroscopy of the nebula and central starreveals the remarkable nature of this object. The nebular spectrum showsemission lines with large expansion velocities exceeding 150 kms-1, suggesting that perhaps the object is not a conventionalplanetary nebula. The central star itself is very red and is identifiedas being of the [WN] class, which makes it unique in the Galaxy. A largebody of supplementary observational data supports the hypothesis thatthis object is indeed a planetary nebula and not a Population IWolf-Rayet star with a ring nebula.
| Determination of temperature of the ionizing stars of H II regions
The determination of temperature (T_eff) of the ionizing stars of H Iiregions was considered. In this work we used photoionization models forH Ii regions ionized by a single star to show that the index R=log ([OIi]lambda lambda 3726+3729/[O Iii]lambda 5007) can be used to estimateT_eff. The relation R vs. T_eff proved to be rather independent of thechemical abundances, but strongly dependent on the ionization parameterof the nebula. In order to check the reliability of using R fortemperature determination, we compared the values of T_eff obtained viathe index R for a sample of H Ii regions with data available in theliterature with independent estimations.
| An Inside-Out View of Bubbles
Fast stellar winds can sweep up ambient media and form bubbles. Theevolution of a bubble is largely controlled by the content and physicalconditions of the shocked fast wind in its interior. This hot gas wasnot clearly observed until the recent advent of Chandra and XMM-NewtonX-ray observatories. To date, diffuse X-ray emission has beenunambiguously detected from two circumstellar bubbles blown by WR stars,four planetary nebulae, and two superbubbles blown by young clusters.Model fits to the X-ray spectra show that the circumstellar bubbles aredominated by hot gas with low temperatures ( < 3×10^6 K), whilethe interstellar bubbles contain significant fractions of hotter gas (< 5×10^6 K) . In all cases, large discrepancies in the X-rayluminosity are found between observations and conventional models ofbubbles. Future theoretical models of bubbles need to re-examine thevalidity of heat conduction and take into account realistic microscopicprocesses such as mass loading from dense clumps/knots and turbulentmixing. Chandra ACIS-S observation of NGC 6888 will shed light on theseastrophysical processes.
| Molecular gas around the Wolf-Rayet star WR 18 (WN4)
Optically observed ring nebulae and Hi cavities around Wolf-Rayet starshave enabled us to obtain information on the history of mass-lossassociated with these massive evolved stars. However, such studies haveleft a number of unanswered questions regarding the amount of mass-lossand the conditions of the stars during a sequence of mass-loss phases.Here we discuss the molecular gas environments of the WR star WR 18,which has an associated optical ring nebula NGC 3199. Our observationsshow that significant amounts of molecular gas appear close to andassociated with the star. Mapping of molecular CO near the star showsthat molecular materials appear to substantially avoid areas of opticalemission and, instead, form a distorted clumpy shell interior to NGC3199. Molecular emission lines are broader than lines seen in theinterstellar medium and suggest the shell is composed of ejecta. Thisis further corroborated by the enhanced abundances of moleculescontaining C, N and O. Implications of the observations for theevolution of WR 18 are discussed.
| Ring nebulae around massive stars throughout the Hertzsprung-Russell diagram
Massive stars evolve across the H-R diagram, losing mass along the wayand forming a variety of ring nebulae. During the main sequence stage,the fast stellar wind sweeps up the ambient interstellar medium to forman interstellar bubble. After a massive star evolves into a red giantor a luminous blue variable, it loses mass copiously to form acircumstellar nebula. As it evolves further into a WR star, the fast WRwind sweeps up the previous mass loss and forms a circumstellar bubble.Observations of ring nebulae around massive stars not only arefascinating, but also are useful in providing templates to diagnose theprogenitors of supernovae from their circumstellar nebulae. In thisreview, I will summarize the characteristics of ring nebulae aroundmassive stars throughout the H-R diagram, show recent advances in X-rayobservations of bubble interiors, and compare supernovae's circumstellarnebulae with known types of ring nebulae around massive stars.
| First Detections of Molecular Gas Associated with the Wolf-Rayet Ring Nebula NGC 3199
This paper presents the first observations of molecular gas associatedwith the Wolf-Rayet ring nebula NGC 3199 around the WR star WR 18. Thisincludes first observations of the molecules HCN, HCO+, CN,and HNC seen in any Wolf-Rayet ring nebula. Our observations immediatelysuggest the presence of high-density molecular gas (>104cm-3) in the nebula with significant amounts of associatedmolecular gas, which is in the form of clumpy ejecta and/or interstellarmaterial. Molecular CO gas was mapped across the optically brightportion of the nebula and out into the diffuse ionized component usingthe 12CO J=1-->0 line. CO gas is not seen within theoptically bright rim of NGC 3199 but adjacent to it. The opticalemission rim therefore appears to mark regions of photodissociation.Velocity components in the CO data are consistent with those seen inhigh-resolution optical spectra of the Hα line but extend beyondthe visible emission. A prior suggestion of the formation of the nebulavia a bow shock appears unlikely since Hipparcos measurements show theproper motion of WR 18 is almost at right angles to the directionrequired for the bow shock model. Instead, line splitting toward thenorth of the nebula suggests that a possible blowout of the Wolf-Rayetwind through surrounding ejecta may be responsible for some of thevelocity features observed. Preliminary estimates of molecularabundances in the nebula seen toward the central star are significantlyhigher than for the interstellar medium and are similar to those inplanetary nebulae, although CN is distinctly underabundant in comparisonto the very high values found in many planetary nebulae. The abundancesfound are consistent with the idea that at least a portion of themolecular material is associated with ejecta from the central star.Based on observations collected at the Swedish-ESO SubmillimetreTelescope (SEST) at the European Southern Observatory, La Silla, Chile.The Swedish-ESO Submillimetre Telescope is operated jointly by theEuropean Southern Observatory (ESO) and the Swedish National Facilityfor Radio Astronomy, Onsala Space Observatory, at Chalmers University ofTechnology.
| Australia Telescope Compact Array Observations of the OH Star Roberts 22: Resolved Images of Hydroxyl Emission
We have imaged the unusual OH/IR object G284-0.8 (Roberts 22) in the OHtransitions at 1612, 1665, and 1667 MHz using the Australia TelescopeCompact Array in Narrabri, NSW, Australia. The angular resolution of theOH images we present here is 6" (12,000 AU at a distance of 2 kpc). Wefind OH emission of extent 1.5" (2800 AU) located inside the opticalbipolar flow region (size 8") discovered in the early eighties by Allenet al. and imaged recently by Sahai et al. with the Hubble SpaceTelescope. We present astrometry of Roberts 22, providing registrationof the OH and Hubble Space Telescope images to within ~0.35". Wedescribe the OH velocity field in Roberts 22 and discuss the kinematicproperties of the source and its evolutionary status.
| First Results from the Combination of the AAO/UKST and Marseille Hα Surveys
We have combined the observational data of the AAO/UKST and MarseilleHα surveys to extract preliminary new results about ionisedstructures of our Galaxy. This includes the detection of a new largebubble located in the far part of the Carina arm.
| First Observations of Complex Molecules in a Wolf-Rayet Ring Nebula
We report the first observations of complex molecules in the ring nebulasurrounding a Wolf-Rayet (WR) star. The molecules HCN, HNC,HCO+ and CN have all been detected together with12CO in observations made of the ring nebula NGC3199 aroundthe WR star WR18 made at the Swedish-ESO Submillimeter Telescope (SEST).This is the first time that the HNC, HCN, HCO+ and CNmolecules have been detected in the environment around a Wolf-Rayetstar. The existence of such molecules indicates the presence of dense(>104cm-3) clumps of neutral materials close tothese hot luminous stars. This is a much higher density than impliedfrom optical emission-line data and suggests extreme density variationsexist within the ring nebula. Clumps of molecular material are likely tocontain a significant neutral gas mass. We discuss the origin of themolecular gas, particularly with respect to the likely prior evolutionof the central WR star. We also consider the implications of theobservations in terms of the effects of strong stellar winds andionization from the WR star on the chemistry of molecular gas in itsvicinity. APM acknowledges the support of a NASA ADP grant NAG 5-6584.IPAC,
| Integrated photometry of galactic H Ii regions
Integrated photoelectric measurements of the equivalent width W_Hβ, the [O Iii]/Hβ ratio and the Hβ emission line flux arepresented for 31 southern hemisphere galactic H Ii regions. The Lymancontinuum photon fluxes are obtained for some of these objects. Theintegrated [O Iii]/Hβ ratios have not shown any statisticallysignificant deviation from non-integrated measurements found in theliterature.
| An Empirical Test and Calibration of H II Region Diagnostics
We present spectrophotometry in the 3600-9700 Å region for asample of 39 H II regions in the Galaxy and Magellanic Clouds, for whichindependent information is available on the spectral types and effectivetemperatures of the ionizing stars. The spectra have been used toevaluate nebular diagnostics of stellar temperature, metal abundance,and ionization parameter, and to compare the observed behavior of theline indices with predictions of nebular photoionization models. Weobserve a strong degeneracy between forbidden-line sequences produced bychanges in stellar Teff and metal abundance, which severelycomplicates the application of many forbidden-line diagnostics toextragalactic H II regions. Our data confirm however that the Edmunds& Pagel [O II]+[O III] abundance index and the Vílchez &Pagel η' index provide more robust diagnostics of metalabundance and stellar effective temperature, respectively. A comparisonof the fractional helium ionization of the H II regions with stellartemperature confirms the reliability of the spectral type versusTeff calibration for the relevant temperature rangeTeff<=38,000 K. We use empirical relations between thenebular hardness indices and Teff to reinvestigate the casefor systematic variations in the stellar effective temperatures and theupper initial mass functions of massive stars in extragalactic H IIregions. The data are consistent with a significant softening of theionizing spectra (consistent with cooler stellar temperatures) withincreasing metal abundance, especially for Z<=Zsolar.However, unresolved degeneracies between Z and Teff stillcomplicate the interpretation of this result.
| Calibration of Nebular Emission-Line Diagnostics. I. Stellar Effective Temperatures
We present a detailed comparison of optical H II region spectra tophotoionization models based on modern stellar atmosphere models. Weexamine both spatially resolved and integrated emission-line spectra ofthe H II regions DEM L323, DEM L243, DEM L199, and DEM L301 in the LargeMagellanic Cloud. The published spectral classifications of the dominantstars range from O7 to WN3, and morphologies range from Strömgrensphere to shell structure. Two of the objects include SNR contamination.The overall agreement with the predictions is generally within 0.2 dexfor major diagnostic line ratios. An apparent pattern in the remainingdiscrepancies is that the predicted electron temperature is ~1000 Khotter than observed. [Ne III] intensities are also slightlyoverpredicted, which may or may not be related. We model the shockemission for the SNR-contaminated objects and find excellent agreementwith the observations for composite shock and photoionized spectra. DEML301's emission apparently results from both shocks and density-boundedphotoionization. The existence of contaminating shocks can be difficultto ascertain in the spatially integrated spectra. Our analysis of thecomplex DEM L199 allows a nebular emission-line test of unprecedenteddetail for WR atmospheres. Surprisingly, we find no nebular He IIλ4686 emission, despite the fact that both of the dominant WN3stars should be hot enough to fully ionize He I in their atmospheres.The nebular diagnostics are again in excellent agreement with the data,for stellar models not producing He+-ionizing photons. Theoptical diagnostics are furthermore quite insensitive to the ionizingenergy distribution for these early WR stars. We confirm that the η'emission-line parameter is not as useful as hoped for determining theionizing stellar effective temperature, T*. Both empiricallyand theoretically, we find that it is insensitive forT*>~40 kK and that it also varies spatially. Theshock-contaminated objects show that η' will also yield a spuriouslyhigh T* in the presence of shocks. It is furthermoresensitive to shell morphology. We suggest [Ne III]/Hβ as anadditional probe of T*. Although it is abundance dependent,[Ne III]/Hβ has higher sensitivity to T*, is independentof morphology, and is insensitive to shocks in our objects. Theseobservations should be useful data points for a first empiricalcalibration of nebular diagnostics of T*, which we attemptfor LMC metallicity.
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
| High-resolution radio observations of RCW 49.
High-resolution radio continuum observations of the southern HII complexRCW49 are presented. The observations were carried out with the MolongloObservatory Synthesis Telescope (MOST) at 0.843GHz, and the AustraliaTelescope Compact Array (ATCA) at 1.38 and 2.38GHz. Two wind-blownshells have been discovered in the core region of RCW49, the moremassive of which surrounds the Westerlund 2 cluster and exhibits aremarkable blister structure. The second shell, surrounding therecently-discovered star WR20b, is an example of a Wolf-Rayet ringnebula.
| A survey of nebulae around galactic wolf-rayet stars in the southern sky, 2.
We present the second half of a charge coupled device (CCD) narrow-bandimaging survey of galactic Wolf-Rayet stars located in the southernhemisphere as listed by van der Hucht et al. (1981). Images of 50Wolf-Rayet stars were taken using a wide-field CCD and narrowbandinterference filters centered on H alpha and (O III) 5007 A wavelengths.The first half of the survey (Marston, Chu, & Garcia-Segura 1993,hereafter Paper I) revealed six new ring nebulae residing aroundWolf-Rayet stars. Here we reveal a possible 11 new rings and theexistence of multiple rings associated with two previously known nebula,RCW 118 and G2.4+1.4 and around the stars WR 16 and WR 43. Combining ourresults with those of Miller & Chu (1993) and Paper I, 92% of thevan der Hucht catalog of Wolf-Rayet stars have now been surveyed. Of the38 possible ring nebulae found in our surveys to date, 22 reside aroundWN subtype Wolf-Rayet stars, 13 around WC stars, one around a triplet ofWolf-Rayet stars and one around a WO star (WR 102). One ring existsaround a WN/WC star (WR 98). A bias toward rings being observed aroundW-R + OB binaries is noted. Such pairings are generally bright, and thedetection of a ring around them may merely be a function of theircombined luminosity. Several Wolf-Rayet stars are shown to be surroundedby multiple rings (two or three) which suggests that a number ofejections of stellar material have taken place during their evolution.
| The Wolf-Rayet nebula NGC 3199 - an interstellar snow plough?
The Wolf-Rayet nebula NGC 3199 has a highly asymmetric morphology, witha very bright hemisphere near the exciting star HD 89358 and a muchfainter and more extended other hemisphere. This nebula is modeled interms of the distorted bubble produced by a moving star blowing a strongstellar wind into a surrounding uniform interstellar medium; this modelis fitted to the morphology and observed kinematic data. The excitingstar appears to be moving at about 60 km/s into local interstellar gasof density of about 10/cu cm, and has a mass-loss rate of about 0.000027solar mass/yr. This latter mass-loss rate is in excellent agreement withobserved mass-loss rates from Wolf-Rayet stars.
| Kinematics of star forming regions
The cubic approximation of a general two-dimensional velocity field isused to study the radial velocities of 218 star forming regions up to 10kpc from the sun. It is found that departures from the Galactic circularflow are generally related to complexes of star forming regions 1-2 kpcin size. It is suggested that complexes are a consequence of clumpedstar formation and subsequent expansion of multisupernova remnants. Theresults imply that the negative departure from circular motion observedin complexes P(1)-P(4) may be due to the influence of the Perseus spiralarm.
| The nature of the Wolf-Rayet nebula NGC 3199
A deep print of a photograph of the WR nebula NGC 3199 shows thepresence of a faint outer shell forming an ellipse with dimensions 26 x20 arcmin. The exciting star is very asymmetrically located with respectto the centroid of this ellipse and is located near a bright nebulararc. A multiple shell structure to the whole of the nebulosity is alsosuggested. No high-speed knots, similar to those found previously in thecomparable nebula RCW 104, are present along three slit positions of aspectrograph. The spectral observations, however, do clearly indicatethat radiative ionization is dominant in the bright nebular arc. It issuggested that, although the expected stellar wind from the WR star HD89358 could form the larger expanding bubble of NGC 3199, there is noevidence for any localized effects of it within the bright nebularfilaments.
| The velocity field of the outer Galaxy in the Southern Hemisphere. II - CO observations of galactic nebulae
CO observations of 308 objects (77 percent) from a catalogue (Brand etal., 1985) of galactic emission and reflection nebulae are presented; COwas detected in the direction of 234 nebulae (76 percent). For 194 ofthese objects (63 percent) the emission could actually be associatedwith the nebula. Fifteen objects (5 percent) have associated CO emissionwith velocity in excess of 50 km/s (VLSR). These objects areof crucial importance as they are potentially very distant, and nearlyall of them have been newly identified. In the course of this surveyseveral sources with interesting line profiles were found. Eighteen ofthem exhibit wings and/or plateaus and are potential CO outflow sources.
| The velocity field of the outer Galaxy in the Southern Hemisphere. I - Catalogue of nebulous objects
A multifrequency program to measure the velocity field of the outerGalaxy in the Southern Hemisphere (l = 230-305 deg) is outlined. Thispaper, the first in a series, presents a catalog of HII regions andreflection nebulae used as a basis for the study. Most of the entriesare previously uncatalogued; the emphasis is on small objects likely tobe very distant from the sun.
| CO (J = 2-1) observations of molecular clouds associated with H II regions from the southern hemisphere
The results of a search for CO (J = 2-1) emission at 47 positions towardsouthern H II regions, performed using the 1.4-m CoudeAuxiliaryTelescope and the 3.6-m telescope at ESO and the 2.5-m DuPont telescopeat Las Campanas Observatory during 1981-1982, are presented in tables,spectra, and maps. Negative results are reported for 19 positions, andno systematic velocity difference is found between the molecular andionized gas. Several objects are characterized in detail.
| Galactic ring nebulae associated with Wolf-Rayet stars. VIII Summary and atlas
Photographs for ten galactic ring nebulae associated with WR stars arepresented, and references are given to published photographs for othersuch nebulae. Every WR ring nebula suggested in the literature isdiscussed and reevaluated. Fabry-Perot data have been obtained for twonew nebulae in order to diagnose their formation mechanisms. Sevenstellar wind-blown bubble type (W type), two stellar ejecta type, fourshell-structured H II region type (Rs type), and twoamorphous region type nebulae are acknowledged. Most W-type nebulae areassociated with early-type WN stars, while most Rs-typenebulae surround WC stars. The implications of statistical results forthese ring nebulae are discussed.
| The galactic abundance gradient
Chemical abundances in a large and representative sample of galactic HII regions covering a wide range in galactocentric radius RGwere measured using radio and optical spectroscopy. Accurate electrontemperatures in 67 H II regions spanning the range RG =3.5-13.7 kpc were determined using radio recombination lines and thesetemperatures were applied to optical spectra of 33 of the same H IIregions in order to determine the abundances of O, N, S, Ne, Ar, andHe(+). Among other results, it is found that some H II regions haveelectron temperatures below 5000 K and that the radio-determinedelectron temperatures agree well with those obtained from the opticalline ratios, in the light of standard models of H II regions. A gradientof H II region electron temperature with distance from the galacticcenter is found which equals +433 + or - 40 K/kpc, while the oxygenabundance gradient is -0.07 + or - 0.015 dex/kpc. The nitrogen abundancegradient is similar to that of oxygen, -0.09 + or 0.015 dex/kpc, whilethe sulfur abundance gradient (-0.01 + or - 0.02 dex/kpc) issignificantly flatter than that of oxygen. No significant gradient inHe(+)/H(+) is detected. In addition, evidence indicates that theabundance gradients may be steeper over the inner regions of thegalactic disk.
| Galactic ring nebulae associated with Wolf-Rayet stars. VI - NGC 3199, anon /MR 26/, RCW 58, and RCW 104
Narrow-band interference filter photographs and high resolutionFabry-Perot spectra have been obtained for four galactic ring nebulaeassociated with Wolf-Rayet stars - NGC 3199, anon (MR 26), RCW 58, andRCW 104. All of these four nebulae show interaction between the stellarwind and the ambient interstellar medium. NGC 3199, anon (MR 26), andRCW 104 are classified as W-type nebulae. RCW 58, having a prominentring of stellar ejecta, is classified as an E-type nebula. For mostW-type nebulae, the kinetic energy in the shell is only about 1 percentof the total mechanical energy input from the stellar wind, while theratio of the shell momentum to the total momentum injected by thestellar wind is about 0.5 and apparently increases with the nebular age.
| On the Correlation of Discrete Gamma-Ray Sources with Supernova Remnants
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| Compact H II regions and OB star formation
Studies of compact H II regions and their association with OB starformation are reviewed. Difficulties in observing compact objects aresummarized, radio observations of compact H II regions are examined, andIR observations of sources associated with star formation are discussed.Attention is also given to molecular masers associated with compact H IIregions and emission from molecular clouds containing such regions. Therelevance of compact objects to OB star formation is considered,particularly in relation to the tendency of OB stars to form in groupsand the formation of individual objects.
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