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Stellar Winds and Embedded Star Formation in the Galactic Center Quintuplet and Arches Clusters: Multifrequency Radio Observations
A multifrequency, multiconfiguration study has been made of the compactradio sources in the Galactic center Quintuplet and Arches stellarclusters using the Very Large Array. Ten radio sources have beendetected in the Quintuplet cluster. The majority of these radio sourceshave rising spectral indices and are positionally coincident with youngmassive stars that are known to have powerful stellar winds. We concludethat the three most compact of these sources are produced by stellarwind emission; thus, mass-loss rates can be derived and have an averagevalue of 3×10-5 Msolar yr-1. Theremainder of the sources are likely to be a combination of stellar windemission and free-free emission from surrounding ionized gas. In threecases, the radio sources have no stellar counterpart, and the radioemission is thought to arise from compact or ultracompact H II regions.If so, these sources would be the first detections of embedded massivestars to be discovered in the Galactic center clusters. The radio nebulaassociated with the Pistol star resembles the nebula surrounding theluminous blue variable star η Car and may be related to the stellarwind of the Pistol star. Ten compact radio sources are also detected inthe Arches cluster and are interpreted to be stellar wind sources,consistent with previous findings. Several of the sources show moderatevariability (10%-30%) in their flux density, possibly related to anonthermal component in the wind emission. A number of radio sources inboth clusters have X-ray counterparts, which have been interpreted to bethe shocked, colliding winds of massive binary systems.
| The Young Cluster NGC 2362
An Hα emission survey of the young cluster NGC 2362 resulted inthe detection of 130 Hα emission stars in an11'×11' field approximately centered on thefourth magnitude O9 Ib multiple star τ CMa. The survey was carriedout using the wide-field grism spectrograph on the University of Hawaii2.2 m telescope and the Gemini Multi-Object Spectrograph (GMOS) onGemini North. Deep optical VRCIC (to V~23.0) andnear-infrared (NIR) photometry (JHK) to K~16 were obtained for severalfields within the cluster. Spectra covering the 6000-8000 Å regionat a resolution of R~3000 (adequate for the determination of Li Iλ6708 line strengths) were also acquired for ~200pre-main-sequence (PMS) candidates with GMOS. Ages and masses for theHα emitters in NGC 2362 were inferred from the isochrones andevolutionary tracks of D'Antona & Mazzitelli, as well as those ofBaraffe et al. An estimated cluster age of ~1.8 Myr follows from themodels of D'Antona & Mazzitelli and 3.5-5.0 Myr from those ofBaraffe et al. The fraction of the T Tauri star (TTS) population that iscomposed of weak-line emitters, f(WTTS), is 0.91, compared with 0.43 forthe TTS population of NGC 2264. On the basis of W(Hα) alone, thefraction of TTSs still undergoing accretion is 5%-9%, comparable to theinner disk fraction determined from JHKL-band excesses by Haisch andcoworkers (12%). Approximately 15% of the PMS sample in this studyexhibits possible NIR excess, having EH-K>0.1 mag. Giventhe lack of NIR excess and strong Hα emission from the majority ofcluster members, it is inferred that the inner disk regions of the TTSpopulation have dissipated significantly. The mean level ofchromospheric activity among the WTTS population of NGC 2362 islog(LHα/Lbol)=-3.65, significantly greaterthan that of the low-mass population of the 600 Myr old Hyades cluster,log(LHα/Lbol)=-3.90. The total mass of theHα emitters and the OB stellar population of NGC 2362 defines alower limit for the cluster mass of ~300 Msolar. Allowancefor A- and F-type stars still on the radiative track, multiplicity,outlying members, and the low-mass population lying below thecompleteness limit of the Hα emission survey increases this lowerlimit to well over 500 Msolar. The derived relaxation,disruption, and evaporation timescales for the cluster imply that NGC2362 will likely survive beyond the age of the Pleiades, but statisticsof galactic cluster lifetimes favor its disruption well before the ageof the Hyades.
| LkHα 101 and the Young Cluster in NGC 1579
The central region of the dark cloud L1482 is illuminated by LkHα101, a heavily reddened (AV~10 mag) high-luminosity(>=8×103 Lsolar) star having an unusualemission-line spectrum plus a featureless continuum. About 35 muchfainter (mostly between R=16 and >21) Hα emitters have been foundin the cloud. Their color-magnitude distribution suggests a median ageof about 0.5 Myr, with considerable dispersion. There are also at leastfive bright B-type stars in the cloud, presumably of about the same age;none show the peculiarities expected of HAeBe stars. Dereddened, theirapparent V magnitudes lead to a distance of about 700 pc. Radioobservations suggest that the optical object LkHα 101 is in fact ahot star surrounded by a small H II region, both inside an opticallythick dust shell. The level of ionization inferred from the shape of theradio continuum corresponds to a Lyman continuum luminosity appropriatefor an early B-type zero-age main-sequence star. The V-I color isconsistent with a heavily reddened star of that type. However, theoptical spectrum does not conform to this expectation: the absorptionlines of an OB star are not detected. Also, the [O III] lines of an H IIregion are absent, possibly because those upper levels are collisionallydeexcited at high densities. There are several distinct contributors tothe optical spectrum of LkHα 101. The Hα emission line isvery strong, with wings extending to about +/-1700 km s-1,which could be produced by a thin overlying layer of hot electronscatterers. There is no sign of P Cygni type mass ejection. Lines of SiII are narrower, while the many Fe II lines are still narrower and aredouble with a splitting of about 20 km s-1. Lines of [Fe II],[O I], and [S II] are similarly sharp but are single, at the samevelocity as the Fe II average. Work by Tuthill et al. allowed theinference, from K-band interferometry, that the central source isactually a small horseshoe-shaped arc about 0.05" (35 AU) across. Atipped annulus of that size in rotation about a 15 Msolarstar would produce double spectrum lines having about the splittingobserved for Fe II. The totality of observational evidence encouragesthe belief that LkHα 101 is a massive star caught in an earlyevolutionary state.
| Interstellar H3+ Line Absorption toward LkHα 101
We present a detection of three lines of the H+3ion in the near-infrared spectrum of the Herbig Be star LkHα 101.H+3 is the principal initiator of gas-phasechemistry in interstellar clouds and can be used to constrain theionization rate or the path length through interstellar material alongthe line of sight. Essentially all of the observedH+3 column of (2.2+/-0.3)×1014cm-2 toward LkHα 101 originates in the same dense, darkcloud; less than 1 mag of the ~11 total magnitudes of visual extinctionis attributable to diffuse material. Constraints on the density[1×104cm-3
| Cosmic Rays Acceleration in Wolf-Rayet Stellar Winds
Popescu et al (2004) gave a model for the observed cosmic rays between5×1015 and 3×1018 eV. Their source ispresumed to be the supernova of stars that explode in their winds. Theobserved cosmic rays abundance at the source are affected by spallationin the supernova shell, by the difference in ionization degree (beingone or two times ionized) at the injection in the supernova shock, thestars with initial masses 15MSun≤M≤30MSunhaving a different contribution to them than the stars with30MSun≤M≤50MSun, this being 2:1 for theelements with Z≥6. Still, the abundances after these corrections aredifferent by a factor Zi/ZHe, where Ziis the atomic number for the element i. This paper is dedicated to theexplanation of this factor and its physical meanings by consideringthat, prior to the shock injection, the wind particles are radiativeaccelerated.
| A Cluster of Compact Radio Sources in NGC 2024 (Orion B)
We present deep 3.6 cm radio continuum observations of the H II regionNGC 2024 in Orion B obtained using the Very Large Array in its Aconfiguration,1 with 0.2" angularresolution. We detect a total of 25 compact radio sources in a region of4'×4'. We discuss the nature of thesesources and its relation to the infrared and X-ray objects in theregion. At least two of the radio sources are obscured proplyds whosemorphology can be used to restrict the location of the main ionizingsource of the region. This cluster of radio sources is compared withothers that have been found in regions of recent star formation.
| N2H+(1-0) survey of massive molecular cloud cores
We present the results of N2H+(1-0) observationsof 35 dense molecular cloud cores from the northern and southernhemispheres where massive stars and star clusters are formed. Lineemission has been detected in 33 sources, for 28 sources detailed mapshave been obtained. Peak N2H+ column densities liein the range: 3.6x 1012-1.5x 1014 cm-2.Intensity ratios of (01-12) to (23-12) hyperfine components are slightlyhigher than the LTE value. The optical depth of (23-12) component towardpeak intensity positions of 10 sources is ~ 0.2-1. In many cases thecores have elongated or more complex structures with several emissionpeaks. In total, 47 clumps have been revealed in 26 sources. Their sizeslie in the range 0.3-2.1 pc, the range of virial masses is ~ 30-3000Msun. Mean N2H+ abundance for 36 clumpsis 5x 10-10. Integrated intensity maps with axial ratios<2 have been fitted with a power-law radial distribution ~r-p convolved with the telescope beam. Mean power-law indexfor 25 clumps is close to 1.3. For reduced maps where positions of lowintensity are rejected mean power-law index is close to unitycorresponding to the ~ r-2 density profile providedN2H+ excitation conditions do not vary insidethese regions. In those cases where we have relatively extensive andhigh quality maps, line widths of the cores either decrease or stayconstant with distance from the center, implying an enhanced dynamicalactivity in the center. There is a correlation between total velocitygradient direction and elongation angle of the cores. However, the ratioof rotational to gravitational energy is too low (4x10-4-7.1x 10-2) for rotation to play a significantrole in the dynamics of the cores. A correlation between mean linewidths and sizes of clumps has been found. A comparison with physicalparameters of low-mass cores is given.
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141
| Imaging the Disk around the Luminous Young Star LkHα 101 with Infrared Interferometry
The Herbig Ae/Be star LkHα 101 has been imaged at high angularresolution at a number of wavelengths in the near-infrared (from 1 to ~3μm), using the Keck I Telescope, and also observed in themid-infrared (11.15 μm), using the UC Berkeley Infrared SpatialInterferometer (ISI). The resolved circular disk with a central hole orcavity reported by Tuthill, Monnier, & Danchi is confirmed. This isconsistent with an almost face-on view (inclination of <~35°)onto a luminous pre-main-sequence or early main-sequence objectsurrounded by a massive circumstellar disk. With a multiple-epoch studyspanning almost 4 years, relative motion of the binary companion hasbeen detected, together with evidence of changes in the brightnessdistribution of the central disk/star. The resolution of the LkHα101 disk by ISI mid-infrared interferometry constitutes the first suchmeasurement of a young stellar object in this wavelength region. Theangular size was found to increase only slowly from 1.6 to 11.15 μm,inconsistent with standard power-law temperature profiles usuallyencountered in the literature, supporting instead models with a hotinner cavity and relatively rapid transition to a cool or tenuous outerdisk. The radius of the dust-free inner cavity is consistent with amodel of sublimation of dust in equilibrium with the stellar radiationfield. Measurements from interferometry have been combined withpublished photometry, enabling an investigation of the energetics andfundamental properties of this prototypical system.
| The Milky Way in Molecular Clouds: A New Complete CO Survey
New large-scale CO surveys of the first and second Galactic quadrantsand the nearby molecular cloud complexes in Orion and Taurus, obtainedwith the CfA 1.2 m telescope, have been combined with 31 other surveysobtained over the past two decades with that instrument and a similartelescope on Cerro Tololo in Chile, to produce a new composite CO surveyof the entire Milky Way. The survey consists of 488,000 spectra thatNyquist or beamwidth (1/8deg) sample the entire Galactic plane over astrip 4deg-10deg wide in latitude, and beamwidthor 1/4deg sample nearly all large local clouds at higher latitudes.Compared with the previous composite CO survey of Dame et al. (1987),the new survey has 16 times more spectra, up to 3.4 times higher angularresolution, and up to 10 times higher sensitivity per unit solid angle.Each of the component surveys was integrated individually using clippingor moment masking to produce composite spatial and longitude-velocitymaps of the Galaxy that display nearly all of the statisticallysignificant emission in each survey but little noise. The composite mapsprovide detailed information on individual molecular clouds, suggestrelationships between clouds and regions widely separated on the sky,and clearly display the main structural features of the molecularGalaxy. In addition, since the gas, dust, and Population I objectsassociated with molecular clouds contribute to the Galactic emission inevery major wavelength band, the precise kinematic information providedby the present survey will form the foundation for many large-scaleGalactic studies. A map of molecular column density predicted fromcomplete and unbiased far-infrared and 21 cm surveys of the Galaxy wasused both to determine the completeness of the present survey and toextrapolate it to the entire sky at |b|<32deg. The closeagreement of the observed and predicted maps implies that only ~2% ofthe total CO emission at |b|<32deg lies outside ourcurrent sampling, mainly in the regions of Chamaeleon and the GumNebula. Taking into account this small amount of unobserved emission,the mean molecular column density decreases from~3×1020 cm-2 at |b|=5deg to~0.1×1020 cm-2 at |b|=30deg thisdrop is ~6 times steeper than would be expected from a plane-parallellayer, but is consistent with recent measurements of the mean molecularcolumn density at higher latitudes. The ratio of the predicted molecularcolumn density map to the observed CO intensity map provides acalibration of the CO-to-H2 mass conversion factorX≡NH2/WCO. Out of the Galacticplane (|b|>5deg), X shows little systematic variation withlatitude from a mean value of (1.8+/-0.3)×1020cm-2 K-1 km-1 s. Given the large skyarea and large quantity of CO data analyzed, we conclude that this isthe most reliable measurement to date of the mean X value in the solarneighborhood.
| J=1-0 HCN toward bright far-infrared sources in the outer Galaxy
Results of the J=1-0 HCN observations toward 34 bright far-infraredsources selected from the IRAS Point Source Catalog are reported.Together with 17 sources observed in this line earlier (Pirogov et al.,1996) they form a complete sample of the sources with flux densitiesS(100 mu m)>500 Jy and delta > 0degr in the outer Galaxy. The HCNdata are compared with the HCO(+) , NH_3, CS and CO data taken fromliterature. Prominent correlations with nearly similar slopes of ~ 1 arerevealed between line integrated intensities of the molecules known tobe high density tracers (HCN, HCO(+) , NH_3 and CS). The correlationsbecome higher after adding the data for dark clouds, small globules andcirrus cores implying similar excitation and formation mechanisms of theconsidered molecules. Collisional excitation in regions with differentdensities as well as different molecular abundances and velocitydispersions in different types of cores seem to be important inproducing these correlations. The following relations hold on theaverage over ~ 3 orders of magnitude of integrated intensities:I(HCN)>~ I(HCO(+) ~ ) I(CS) > I(NH_3) where ammonia integratedintensities are several times lower than HCN ones. Correlations are alsofound between HCN and CO integrated intensities for the sample sourcesas well as between HCN line widths and those of other species. The HCNlines have the same widths as the HCO(+) ones and are larger than CS andespecially NH_3 line widths. Weak correlations are found between HCNline widths and luminosities of IRAS sources as well as between HCNintegrated intensities, IRAS flux densities at 100mu m and luminositiesof IRAS sources divided by distance squared. The sources with mostintense HCN lines have associated water masers and molecular outflowswhile the lack of associated maser and outflow implies weak or no HCNemission. In order to reproduce the anomalies of the J=1-0 HCN hyperfinestructure (R12 < 0.6) detected in several sources togetherwith suprathermal line widths the model calculations are performed. Twomodels have been considered: a microturbulent one with a smooth densitydistribution and a clumpy model. It is found that in the parameter spaceexplored a clumpy model is preferable in comparison with amicroturbulent one due to the absence of self-reversals on calculatedprofiles and the possibility of reproducing HCN profile anomalies andsuprathermal line widths. Detailed clumpy model simulations have beenperformed to fit the observed HCN and H(13) CN spectra toward S140 IRS1.The best fit model parameters are calculated, including radialdependencies of clump density and volume filling factor.
| Distribution of gas, dust and the lambda 6613 Å DIB carrier in the Perseus OB2 association
We present a study of the spatial distribution of the lambda 6613 ÄDIB carrier in the Perseus OB2 association based on high resolutionobservations toward lines of sight representing different interstellarenvironments. We determined that in the studied region, the lambda 6613Ä DIB carrier is concentrated in two distinct clouds withvelocities of 1.4 (+/- 0.4) and 12.0 (+/- 0.9) km s(-1) . We comparedthe lambda 6613 Ä DIB carrier's velocity with the Na I velocitydistribution derived from our survey measurements, as well as with CO,OH, H I and Ca Ii measurements from the literature. We conclude that thebehaviour of the carrier of the lambda 6613 Ä DIB follows theoverall expansion motion of the gas in the association. The DIB velocityis directly linked to that of Ca Ii and H I. The DIB total columndensity is proportional to the total column density of Ca Ii and H Imaking those atoms good tracers of the lambda 6613 Ä DIB carrier.Those new results support the assumption that the lambda 6613 Ä DIBwould arise from a gas phase molecule, possibly single-ionized(Sonnentrucker et al. 1997). We also conclude that the DIB carrier isdistributed in shell structures over the whole association. We finallyshow from the DIB velocity structure that the DIB carrier, gas and dustare well mixed toward the association but that the DIB shells have anangular extent twice larger than that of the dust. Based on observationswith OHP 1.52m Telescope and Aurelie spectrograph.
| Interstellar isotope ratios from mm-wave molecular absorption spectra
We have measured galactic lambda3 mm absorption spectra of {HCO(+) ,HCN, HNC, and CS toward compact extragalactic continuum sources in orderto derive the isotopic abundance ratios (12) C/(13) C, (14) N/(15) N,(16) O/(18) O, and (32) S/(34) S in local diffuse clouds. For carbon,our data confirm recent results for the local ISM: we find (12) C/(13) C= 59+/-2. For nitrogen, we find 14N/15N =237(-21,+27) consistent with the Solar value of 270, but substantiallysmaller than the values inferred from HCN emission in dense clouds. Forsulfur we find (32) S/(34) S ~ 19+/-8 consistent with the Solar value of23. We also find one striking individual anomaly: toward 3C111(B0415+379), H ^12CN/{H ^13,CN} = 170+/- 50 in one kinematic component.We attribute this to the fractionation of ^13C into ^13CO which may beso great that HCN is starved for ^13C.
| IRAS LRS Spectra of the Sharpless H II Regions
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJS..100..389C&db_key=AST
| Radio Recombination Line Observations of Partially Ionized Gas in Galactic H II Regions
Abstract image available at:http://adsabs.harvard.edu/abs/1995ApJ...448..727O
| Millimeter-wave continuum measurements of young stars
New continuum observations at 1.3 millimeters of 121 young stars arepresented; the majority of these objects have not been previouslyobserved. The stars which are detected are interpreted assuming that theemission is thermal radiation from small particles entrained incircumstellar disks. Several new conclusions are drawn.
| A VLA survey of optically visible galactic H II regions
Radio continuum observations of 109 optically visible Galactic H IIregions were obtained with the Very Large Array. Contour plots of theimages of those that were well resolved and tables of radio continuumflux density for all detected objects are presented. The measured totalflux densities matched well with the results of single-dish surveys, andthus it is expected that the problem of missing 'zero-spacing flux' hasnot contributed greatly to the morphologies seen in the resultingimages. The relative numbers of objects of each morphological type werevery similar to the numbers seen in the survey of ultracompact H IIregions by Wood and Churchwell (1989) even though the objects in thisnew survey are on average 50 times larger (in radius) and thereforepresumably more evolved.
| Small Galactic H II regions. III - Images and spectrophotometry of the ionized gas
Narrow-band H-alpha and N II images and long-slit and aperturespectroscopy are presented for a sample of small Galactic H II regionslocated from 6 to 15 kpc from the Galactic center. Emission-line fluxesare measured from H-beta to S II 6717,31 for selected areas throughouteach nebula. Reddening is determined from the ratio of H-alpha to H-betaand density from the ratio of the S II lines. Abundances of S(+), N(+),and O(2+) relative to H(+) are determined for those regions for whichelectron temperatures exist in the literature. Variations of OIII/H-beta, N II/H-alpha, S II/H-alpha, and N II/S II within the nebulaeand from nebula to nebula as a function of distance from the enter ofthe Galaxy are examined. The status of the H II regions and theirrelationship to the molecular clouds are briefly discussed. These dataprovide a useful data base for studies of the interaction between H IIregions and their natal clouds, studies of the evolution of H IIregions, and studies of variations of elements and physical conditionswithin the Galaxy.
| 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.
| Small Galactic H II regions. II - The molecular clouds and star formation
CO maps of molecular clouds associated with 11 small Galactic H IIregions are presented and compared with IR images obtained by IRAS. Themolecular masses of the clouds are computed and compared with the massesof the stellar content. The mapped clouds have masses of 1000-60,000solar and are typical of the more numerous, smaller Galactic molecularclouds. All of the clouds have recently made massive OB stars, and manyhave complex spatial and kinematic structures. The coincidence of IRASsources and CO peaks suggests that many of the clouds have sites of starformation other than the optically visible H II region. Star-formationefficiencies are uncertain, with values for the clouds ranging from 0.02to 0.6 with an average value of 0.2. There is no trend of the upperstellar mass limit with Galactic radius and with molecular cloud mass.
| Small Galactic H II regions. I - Spectral classifications of massive stars
By studying the stellar content of star-forming regions with differentcharacteristics, such as gas cloud size, one can determine factors thataffect the star-formation process. This paper is part of a study of thestellar content and natal cloud characteristics of a sample ofrelatively small Galactic star-forming regions. Spectral classificationsbased on moderate dispersion spectra of the optically visible stars inthe regions are presented. The H-alpha, radio, and far-infraredluminosities of the nebulas are used as a check for additional embeddedor unidentitied hot stars. A histogram of the most massive star perstar-forming unit shows that there is a range in upper mass limits forthe sample and that one is statistically sampling a mass functionintermediate between that of Selpeter and that of Miller-Scalo.
| Fabry-Perot H-alpha observations of Galactic H II regions
The radial velocity and linewidth of H-alpha emission from 284 objectslisted in Galactic H II region catalogs were observed using aFabry-Perot spectrometer. A few of these objects are supernova remnantsor planetary nebula. The rest of this sample encompasses most of the HII regions that are visible optically from the Northern Hemisphere.These observations are compared to previous H-alpha observations as wellas to radio recombination line and CO observations. The averageradial-velocity difference V(CO) - V(H-alpha) is 0.50 + or - 0.48 km/sand the (1-sigma) dispersion is 6.44 km/s. The implications of thesevalues are discussed in terms of a very simple kinematic and extinctionH II region model. Total H-alpha luminosities for the sample areestimated, and the Galactic H II region luminosity function is comparedto that recently found for other galaxies.
| An HCN J = 1-0 line survey of molecular clouds associated with H II regions from the Sharpless catalog - Results of observations
Not Available
| A CO survey of the dark nebulae in Perseus, Taurus, and Auriga
A new SIS receiver with extremely low noise temperature, used on theColumbia 1.2-m telescope has permitted mapping CO rapidly with fullsampling. Results are presented of a survey for which the angularresolution of the telescope was reduced to 0.5 deg, allowing theobservations for the complete region of 750 square degrees to befinished within four months, while retaining sufficient resolution tosee significant substructure. Most positions with emission are in theTaurus-Auriga dark nebulae, a cloud associated with IC 348 and NGC 1333,and a cloud associated with the California nebula (NGC 1499) and NGC1579, which overlaps the northern Taurus-Auriga nebulae but is separatedfrom them in velocity. Also seen were several small clouds at Galacticlatitude -25 deg to -35 deg southwest of the Taurus clouds, and theL1558 and L1551 clouds in the south.
| The H II region surrounding LK H-alpha 101 and small-scale structure in the nearby atomic hydrogen
New VLA observations of the H II region surrounding Lk H-alpha 101 andof the fine-scale structure in the extended H I cloud associated withthe region are presented. The H II region measures about 0.3 pc inoverall diameter, comprises 0.09 solar mass, and shows evidence of asubstantial density gradient. The H I emission is characterized by aprominent ridge on the north side of the H II region and by numerousclumps of emission extending to radii between 1 and 2 pc. This distanceis significantly less than the radius of the total H I cloud as revealedby lower resolution observations and may represent the extent of thepenetration of the UV-driven dissociation front into dense molecularconcentrations. Close to the star the advancement of the dissociationinto such clumps may be abetted by the process of 'multiple pumping' inthe Lyman bands. This process also accounts for the formation of asubstantial H I zone around a relatively young H II region. The expectedshocked shell of H I outside the ionization boundary was not detected.
| Environs of H II regions
The time evolution of conditions in the neutral gas ahead of theionization front of the expanding H II region of an early-type star isexplored in a review of recent observations and analyticalinvestigations. The initial conditions in the molecular cloud(containing H2, He, CO, and dust grains) are defined, and four stagesare characterized: formation of an extended C(+) region, rapiddissociation of H2 in a narrow region, formation of a broad H I zone,and narrowing of the H I zone. Diagrams, graphs, and maps of sample dataare provided, and unresolved problems are discussed.
| Catalog of CO radial velocities toward galactic H II regions
This is a catalog of 242 molecular cloud complexes which are associatedwith optical H II regions. CO observations were made toward all but fiveof the H II regions in the Sharpless catalog and toward 62 additionalsuspected H II regions, 33 of which are previously uncataloged. Radialvelocities are tabulated for each molecular cloud complex found to beassociated with an H II region. The CO antenna temperature and linewidth are given for the most intense CO line seen toward each source.The catalog also summarizes previous CO observations as well as theoptical distances to the stars exciting the H II regions. Radio-quiet HII regions (those with 1.4 GHz flux densities less than 100 mJy) arefound to be well correlated with objects having no associated CO. A listof kinematically distinct complexes is tabulated to facilitateinvestigations of the motions of the complexes.
| A High Resolution Search for Small-Scale Structure in Sharpless HII Regions at 4.995-GHZ - Part Three - Description of Selected Sources
Abstract image available at:http://adsabs.harvard.edu/abs/1981A&A...100...42F
| A high-resolution search for small-scale structure in Sharpless H II regions at 4.995 GHz. II - General properties of the entire sample. III - Description of selected sources
A statistical study is presented of the properties of a sample of 75optically visible H II regions from the Sharpless Catalog observed at afrequency of 4.994 GHz by the Westerbork Synthesis Radio Telescope, andthe most interesting small-diameter H II components detected in thesurvey are discussed in detail. The optical and radio properties of theregions are compared, and results are presented of high-andlow-resolution observations. The nebulae are divided into five classesaccording to optical morphology and the presence of obscuration, whichclasses can be interpreted as representing subsequent evolutionarystages of a single H II region. It is also found that the change of theelectron density due to H II region expansion occurs at a higher ratefor H II regions ionized by stars of an earlier spectral type, and thatthe rate of change of the mean electron density for a given type isapproximately hyperbolic. Small-diameter, high-density components arefound preferentially in small, isolated nebulae with high mean electrondensities, while the ratio of the Lyman continuum flux contained in thesmall-diameter component to that contained in the entire H II regioncorrelates with the mean density of the region. Of the 22 objectsdiscussed individually, 12 can be classified as compact H II regionswith an internal source of excitation, and the remaining sources can beattributed to increases in the electron density at ionization frontslocated at the edges of dense molecular clouds. From an analysis ofradio, infrared and molecular line observations of several sources, itis concluded that a single early-type star embedded in the H II regionin the presence of dust can be the energy source of the observedemission.
| 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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