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3He in the Milky Way Interstellar Medium: Ionization Structure
The cosmic abundance of the 3He isotope has importantimplications for many fields of astrophysics. We are using the 8.665 GHzhyperfine transition of 3He+ to determine the3He/H abundance in Milky Way H II regions and planetarynebulae. This is one in a series of papers in which we discuss issuesinvolved in deriving accurate 3He/H abundance ratios from theavailable measurements. Here we describe the ionization correction weuse to convert the 3He+/H+ abundance,y+3, to the 3He/H abundance,y3. In principle the nebular ionization structure cansignificantly influence the y3 derived for individualsources. We find that in general there is insufficient informationavailable to make a detailed ionization correction. Here we make asimple correction and assess its validity. The correction is based onradio recombination line measurements of H+ and4He+, together with simple core-halo sourcemodels. We use these models to establish criteria that allow us toidentify sources that can be accurately corrected for ionization andthose that cannot. We argue that this effect cannot be very large formost of the sources in our observational sample. For a wide range ofmodels of nebular ionization structure we find that the ionizationcorrection factor varies from 1 to 1.8. Although larger corrections arepossible, there would have to be a conspiracy between the density andionization structure for us to underestimate the ionization correctionby a substantial amount.

A Search for Formaldehyde 6 cm Emission toward Young Stellar Objects. II. H2CO and H110α Observations
We report the results of our second survey for Galactic H2COmaser emission toward young stellar objects. Using the GBT and the VLAin the A configuration we observed 58 star-forming regions anddiscovered the fifth H2CO 6 cm maser region in the Galaxy(G23.71-0.20). We have discussed the detection toward G23.71-0.20 in aprevious paper. Here we present all the other results from our survey,including detection of H2CO absorption features toward 48sources, detection of the H110α recombination line toward 29sources, detection (including tentative detections) of the carbonrecombination line C110α toward 14 sources, subarcsecond angularresolution images of 6 cm continuum emission toward five sources, andobservations of the H2CO masers in IRAS 18566+0408 and NGC7538. In the case of NGC 7538, we detected the two main H2COmaser components, and our observations confirm variability of theblueshifted component recently reported by Hoffman et al. Thevariability of both maser components in NGC 7538 could be caused by ashock wave that reached the redshifted component approximately 14 yrbefore reaching the blueshifted component. If that were the case, wewould expect to detect an increase in the flux density rate of change ofthe blueshifted component sometime after the year 2009. Our data alsosupport the use of H2CO/H110α observations as a tool toresolve the kinematic distance ambiguity of massive star-forming regionsin the Galaxy.

Magnetic Fields at the Periphery of Ultracompact H II Regions from Carbon Recombination Line Observations
Several indirect evidences indicate a magnetic origin for the nonthermalwidth of spectral lines observed toward molecular clouds. In thisLetter, I suggest that the origin of the nonthermal width of carbonrecombination lines (CRLs) observed from photodissociation regions(PDRs) near ultracompact H II regions is magnetic and that the magnitudeof the line width is an estimate of the Alfvén speed. Themagnetic field strengths estimated based on this suggestion compare wellwith those measured toward molecular clouds with densities similar toPDR densities. I conclude that multifrequency CRL observations have thepotential to form a new tool to determine the field strength nearstar-forming regions.

Helium abundance and ionization structure in the Orion nebula: radio recombination lines observations
Results of the Ori A HII region mapping based onhydrogen (H), helium (He) and carbon (C) Radio Recombination lines (RRL)are presented. Observations were made with the same angular resolution(2') using the 32 m VLBI dish of Medicina (Italy, 22.4 GHz) and thePushchino RT-22 dish (Russia, 36.5 GHz). The behaviour of the ionizedhelium abundance, y^+, with distance from the center shows that theHe+ zone size is smaller than that of H^+. Such a behaviouris different for the core and for the envelope, as well as for differentdirections from the center. The helium abundance,N(He)/N(H)=10.0(± 0.8)%, is measured. Derived line radialvelocities, their widths and y+ data support the well-known"blister-type" structure of this HII region. LTE electron temperatures(7800-9600 K) are also measured.Appendices (Figs. 15, 16 and Sect. 4.1 "Carbon RRLs") are only availablein electronic form at http://www.aanda.org

The Galactic distribution of magnetic fields in molecular clouds and HII regions
Aims.Magnetic fields exist on all scales in our Galaxy. There is acontroversy about whether the magnetic fields in molecular clouds arepreserved from the permeated magnetic fields in the interstellar medium(ISM) during cloud formation. We investigate this controversy usingavailable data in the light of the newly revealed magnetic fieldstructure of the Galactic disk obtained from pulsar rotation measures(RMs). Methods: We collected measurements of the magnetic fields inmolecular clouds, including Zeeman splitting data of OH masers in cloudsand OH or HI absorption or emission lines of clouds themselves. Results: The Zeeman data show structures in the sign distribution of theline-of-sight component of the magnetic field. Compared to thelarge-scale Galactic magnetic fields derived from pulsar RMs, the signdistribution of the Zeeman data shows similar large-scale fieldreversals. Previous such examinations were flawed in the over-simplifiedglobal model used for the large-scale magnetic fields in the Galacticdisk. Conclusions: .We conclude that the magnetic fields in the cloudsmay still "remember" the directions of magnetic fields in the GalacticISM to some extent, and could be used as complementary tracers of thelarge-scale magnetic structure. More Zeeman data of OH masers in widelydistributed clouds are required.Tables 1 and 2 are only available in electronic form athttp://www.aanda.org

The Fragmenting Superbubble Associated with the H II Region W4
New observations at high latitudes above the H II region W4 show thatthe structure formerly identified as a chimney candidate, an opening tothe Galactic halo, is instead a superbubble in the process offragmenting and possibly evolving into a chimney. Data at high Galacticlatitudes (b>5deg) above the W3/W4 star-forming region at1420 and 408 MHz Stokes I (total power) and 1420 MHz Stokes Q and U(linear polarization) reveal an egg-shaped structure with morphologicalcorrelations between our data and the Hα data of Dennison,Topasna, and Simonetti. Polarized intensity images show depolarizationextending from W4 up the walls of the superbubble, providing strongevidence that the radio continuum is generated by thermal emissioncoincident with the Hα emission regions. We conclude that theparts of the H II region hitherto known as W4 and the newly revealedthermal emission are all ionized by the open cluster OCl 352. At anassumed distance of 2.35 kpc, the ovoid structure is 164 pc wide andextends 246 pc above the midplane of the Galaxy. The shell's emissiondecreases in total intensity and polarized intensity in variouslocations, appearing to have a break at its top and another on one side.Using a geometric analysis of the depolarization in the shell's walls,we estimate that a magnetic field line-of-sight component of 3-5 μGexists in the shell. We explore the connection between W4 and theGalactic halo, considering whether sufficient radiation can escape fromthe fragmenting superbubble to ionize the kiloparsec-scale Hα loopdiscovered by Reynolds, Sterling and Haffner.

Spitzer Observations of the Giant Molecular Cloud W3
We present new images of the giant molecular cloud W3 obtained with theInfrared Array Camera (IRAC) and the Multiband Imaging Photometer forSpitzer (MIPS) on board the Spitzer Space Telescope. The imagesencompass the star forming regions W3 Main, W3(OH), and a region that werefer to as the Central Cluster, which encloses the emission nebula IC1795. We present a star count analysis of the point sources detected inW3. The star count analysis shows that the stellar population of theCentral Cluster, when compared to that in the background, contains anover density of sources. The Central Cluster also contains an excess ofsources with colors consistent with Class II young stellar objects(YSOs). An analysis of the color-color diagrams also reveals a largenumber of Class II YSOs in the Central Cluster. Our results suggest thatan earlier epoch of star formation created the Central Cluster, createda cavity, and triggered the active star formation in the W3 Main andW3(OH) regions. We also detect a new outflow and its candidate excitingstar.

The WENSS & Dwingeloo surveys and the Galactic magnetic field
Aims.We investigate the structure of the Galactic magnetic field in the2nd Galactic quadrant using radio continuum polarization data from the325 MHz WENSS (WEsterbork Northern Sky Survey) survey in combinationwith earlier single-dish observations. Methods: .We studygradients in polarization angle along Galactic longitude and latitude inthe region 130° ⪉ l ⪉ 173° and -5° ⪉ b ⪉31°. These gradients were determined with a new method that wedeveloped to efficiently and reliably fit linear gradients to periodicdata like polarization angles. Since the WENSS data were obtained with asynthesis array they suffer from the "missing short spacing" problem. Wehave tried to repair this by adding an estimate of the large-scaleemission based on the single-dish (Dwingeloo) data obtained by Brouw andSpoelstra. Combining all available data we derive a rotation measure mapof the area, from which we estimate all 3 components of the magneticfield vector. Results: .In the part of WENSS where large-scalestructure in polarized intensity is relatively unimportant, we find thatthe magnetic field is predominantly perpendicular to the line-of-sight,and parallel to the Galactic plane. The magnetic field components alongthe line-of-sight and along Galactic latitude have comparable values,and the strength of these components is much smaller than the strengthof the total magnetic field. Our observations also cover part of theso-called "fan" region, an area of strong polarized intensity, wherelarge-scale structure is missing from our WENSS data. We tentativelyshow that Faraday rotation occurring in front of the Perseus arm iscausing both the WENSS RM and the RM towards the fan region observed inprevious single-dish surveys, and we suggest that the fan is formed bylocal emission that originates in front of the emission we see inWENSS.

Enhanced density and magnetic fields in interstellar OH masers
Aims.We have observed the 6030 and 6035 MHz transitions of OH inhigh-mass star-forming regions to obtain magnetic field estimates inboth maser emission and absorption. Methods: .Observations weretaken with the Effelsberg 100 m telescope. Results: .Ourobservations are consistent with previous results, although we do detecta new 6030 MHz maser feature near -70 km s-1 in the vicinityof W3(OH). In absorption we obtain a possible estimate of -1.1 ±0.3 mG for the average line-of-sight component of the magnetic field inthe absorbing OH gas in K3-50 and submilligauss upper limits for theline-of-sight field strength in DR 21 and W3. Conclusions: .Theseresults indicate that the magnetic field strength in the vicinity of OHmasers is higher than that of the surrounding, non-masing material,which in turn suggests that the density of masing OH regions is higherthan that of their surroundings.

High spatial resolution mid-infrared spectroscopy of the starburst galaxies NGC 3256, II Zw 40 and Henize 2-10
Aims.In order to show the importance of high spatial resolutionobservations of extra-galactic sources when compared to observationsobtained with larger apertures such as ISO, we present N-band spectra(8-13 μm) of some locations in three starburst galaxies. Inparticular, we show the two galactic nuclei of the spiral galaxy NGC3256, the compact IR supernebula in the dwarf galaxy II Zw 40 and thetwo brightest IR knots in the central starburst of the WR galaxy He2-10. Methods: .The spectra were obtained with TIMMI2 on the ESO3.6 m telescope. An inventory of the spectra in terms of atomicfine-structure lines and molecular bands is presented. Results:.We show the value of these high spatial resolution data in constrainingproperties such as the extinction in the mid-IR, metallicity or stellarcontent (age, IMF, etc.). We have constrained the stellar content of theIR compact knot in II Zw 40 by using the mid-IR fine-structure lines andsetting restrictions on the nebular geometry. We have constructed a newmid-/far-IR diagnostic diagram based on the 11.2 μm PAH andcontinuum, accessible to ground-based observations. We find thatextra-galactic nuclei and star clusters observed at high spatialresolution (as is the case of the TIMMI2 observations) are closer inPAH/far-IR to compact H II regions, while galaxies observed by largeapertures such as ISO are closer to exposed PDRs such as Orion. This islikely due to the aperture difference. We find a dependence between thepresence of PAHs and the hardness of the radiation field as measured bythe [S IV]/[Ne II] ratio that may be explained by the PAH-dustcompetition for FUV photons or the relative contribution of thedifferent phases of the interstellar medium.

Luminosity functions of YSO clusters in Sh-2 255, W3 main and NGC 7538 star forming regions
We have conducted deep near-infrared surveys of the Sh-2 255, W3 Mainand NGC 7538 massive star forming regions using simultaneous observations of the JHKs -band with the near-infrared camera SIRIUS onthe UH 88-inch telescope. The near-infrared surveys cover a total areaof ~72 square arcmin of three regions with 10-sigma limiting magnitudesof ~19.5, 18.4 and 17.3 in J, H and Ks-band, respectively.Based on the colour-colour and colour- magnitude diagrams and theirclustering properties, the candidate young stellar objects areidentified and their luminosity functions are constructed in Sh-2 255,W3 Main and NGC 7538. A large number of previously unreported redsources (H - K > 2) have also been detected around these regions. Weargue that these red stars are most probably pre-main sequence starswith intrinsic colour excesses. The detected young stellar objects showa clear clustering pattern in each region: the Class I-like sources aremostly clustered in molecular cloud region, while the Class II-likesources in or around more evolved optical H II regions. We find thatthe slopes of the Ks -band luminosity functions of Sh-2 255, W3 Main andNGC 7538 are lower than the typical values reported for the youngembedded clusters and their stellar populations are primarily composedof low mass pre-main sequence stars. From the slopes of the Ks -bandluminosity functions, we infer that Sh-2 255, W3 Main and NGC 7538 starforming regions are rather young (age 1 Myr).

Star formation associated with H II regions
Star formation associated with H II regions is briefly reviewed. Specialemphasis is laid on our series of observational studies on bright-rimmedclouds (BRCs), in which we found a phenomenon called "small-scalesequential star formation." In addition a new hypothesis is advocated onthe two modes of star formation associated with H II regions, i.e., thecluster and dispersed modes. The former gives birth to a rich clusterand in the associated H II region BRCs are formed only at a later stageof its evolution in the peripheries. In the latter mode no clusters oronly loose ones are formed, but BRCs can appear at earlier stages ininner part of the H II region. Presumably these modes depend on theinitial density distribution of the natal molecular cloud.

Hierarchical Triggering of Star Formation by Superbubbles in W3/W4
It is generally believed that expanding superbubbles and mechanicalfeedback from massive stars trigger star formation, because there arenumerous examples of superbubbles showing secondary star formation attheir edges. However, although these systems show an age sequence, theydo not provide strong evidence of a causal relationship. The W3/W4Galactic star-forming complex suggests a three-generation hierarchy: thesupergiant shell structures correspond to the oldest generation; thesetriggered the formation of IC 1795 in W3, the progenitor of a molecularsuperbubble that in turn triggered the current star-forming episodes inthe embedded regions W3-North, W3-Main, and W3-OH. We present UBVphotometry and spectroscopic classifications for IC 1795, which show anage of 3-5 Myr. This age is intermediate between the reported 6-20 Myrage of the supergiant shell system and the extremely young ages(104-105 yr) for the embedded knots of theultracompact H II regions, W3-North, W3-Main, and W3-OH. Thus, an agesequence is indeed confirmed for the entire W3/W4 hierarchical system.This therefore provides some of the first convincing evidence thatsuperbubble action and mechanical feedback are indeed a triggeringmechanism for star formation.

Massive Stars in Clusters and the Field
What is the relation between field massive stars and clusters? Do theyrepresent an extreme in the universal, power-law relation for stellarclustering? Or do they represent a substantially different mode of starformation? What is the origin of the clustering law itself? We examinethe massive star population of the Small Magellanic Cloud and find acontinuous, power-law relation between field stars and clusters. Thisimplies that the fraction of field massive stars ranges from about 35%to 7% for most astrophysical situations, with a weak dependence on thegalaxy size and/or star formation rate. We also examine the starformation history of the Galactic complex W3/W4, which is a system ofthree generations of hierarchical, triggered star formation. This lendssome of the strongest evidence to date that superbubbles indeed triggerstar formation. We speculatively link this hierarchical process to thepower-law clustering of stars.

Emission of CO, C I, and C II in W3 Main
We used the KOSMA 3m telescope to map the core 7' × 5' of theGalactic massive star forming region W3 Main in the two fine structurelines of atomic carbon and four mid-J transitions of CO and13CO. The maps are centered on the luminous infrared sourceIRS 5 for which we obtained ISO/LWS data comprising four high-J COtransitions, [C II], and [O I] at 63 and 145 μm. In combination witha KAO map of integrated line intensities of [C II] \citep{howe1991},this data set allows us to study the physical structure of the molecularcloud interface regions where the occurence of carbon is believed tochange from C+ to C0, and to CO. The molecular gasin W3 Main is warmed by the far ultraviolet (FUV) field created by morethan a dozen OB stars. Detailed modelling shows that most of theobserved line intensity ratios and absolute intensities are consistentwith a clumpy photon dominated region (PDR) of a few hundred unresolvedclumps per 0.84 pc beam, filling between 3 and 9% of the volume, with atypical clump radius of 0.025 pc (2.2''), and typical mass of 0.44Mȯ. The high-excitation lines of CO stem from a 100-200K layer, as do the [C I] lines. The bulk of the gas mass is however atlower temperatures.

The Profiles of the 3-12 Micron Polycyclic Aromatic Hydrocarbon Features
We present spectra of the 3.3 μm and 11.2 μm polycyclic aromatichydrocarbon (PAH) features of a large number of stellar sources,planetary nebulae, reflection nebulae, H II regions, and galaxies,obtained with Infrared Space Observatory Short Wavelength Spectrometer.Clear variations are present in the profiles of these features. Most ofthe sources show a symmetric 3.3 μm feature peaking at ~3.290 μm,while only very few show an asymmetric 3.3 μm feature peaking at aslightly longer wavelength. The profiles of the 11.2 μm feature aredistinctly asymmetric. The majority of the sources has a 11.2 μmfeature peaking between 11.20 and 11.24 μm, with a very steep bluerise and a low tail-to-top ratio. A few sources show a 11.2 μmfeature with a peak position of ~11.25 μm, a less steep blue rise,and a high tail-to-top ratio. The sources are classified independentlyon the basis of the 3.3 and 11.2 μm feature profiles and peakpositions. Correlations between these classes and those based on the 6-9μm features (Peeters et al.) are found. In particular, sources withthe most common profiles in the 6-9 μm region also show the mostcommon 3.3 and 11.2 μm feature profiles. However, the uncommonprofiles do not correlate with each other. Also, these classificationsdepend on the type of object. In general, H II regions, nonisolatedHerbig AeBe stars and young stellar objects show the same profiles forall 3-12 μm features. Many planetary nebulae and post-asymptoticgiant branch stars show uncommon feature profiles. The three galaxies inour sample show the same profiles as the H II regions for all but the11.2 μm feature, being similar to that of evolved stars. The observedpronounced contrast in the spectral variations for the CH modes (3.3 and11.2 μm bands) versus the CC modes (6.2, 7.7, and 8.6 μm bands) isstriking: the peak wavelengths of the features attributed to CC modesvary by ~15-80 cm-1, while for the CH modes the variationsare ~4-6.5 cm-1. We summarize existing laboratory data andtheoretical calculations of the modes emitting in the 3-12 μm regionof PAH molecules and complexes. In contrast to the 6.2 and 7.7 μmcomponents, which are attributed to PAH cations, the 3.3 μm featureappears to originate in neutral and/or negatively charged PAHs. Weattribute the variations in peak position and profile of these IRemission features to the composition of the PAH family. The variationsin FWHM of the 3.3 μm feature remains an enigma, while those of the11.2 μm can be explained by anharmonicity and molecular structure.The possible origin of the observed contrast in profile variationsbetween the CH modes and the CC modes is highlighted.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of ISASand NASA.

Deep Near-Infrared Observations of the W3 Main Star-forming Region
We present a deep JHKs-band imaging survey of the W3 Mainstar-forming region, using the near-infrared camera SIRIUS mounted onthe University of Hawaii 2.2 m telescope. The near-infrared surveycovers an area of ~24 arcmin2 with 10 σ limitingmagnitudes of ~19.0, 18.1, and 17.3 in the J, H, and Ksbands, respectively. We construct JHK color-color and J versus J-H and Kversus H-K color-magnitude diagrams to identify young stellar objectsand estimate their masses. Based on these color-color andcolor-magnitude diagrams, a rich population of young stellar objects isidentified that is associated with the W3 Main region. A large number ofpreviously unreported red sources (H-K>2) have also been detectedaround W3 Main. We argue that these red stars are most probablypre-main-sequence stars with intrinsic color excesses. We find that theslope of the Ks-band luminosity function (KLF) of W3 Main islower than the typical values reported for young embedded clusters. Thederived slope of the KLF is the same as that found in 1996 by Megeathand coworkers, from which analysis indicated that the W3 Main region hasan age in the range of 0.3-1 Myr. Based on the comparison between modelsof pre-main-sequence stars and the observed color-magnitude diagram, wefind that the stellar population in W3 Main is primarily composed oflow-mass pre-main-sequence stars. We also report the detection ofisolated young stars with large infrared excesses that are most probablyin their earliest evolutionary phases.

Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much ofmodern astrophysics. However, a quantitative prediction of the starformation rate and the initial distribution of stellar masses remainselusive. For several decades it has been thought that the star formationprocess is primarily controlled by the interplay between gravity andmagnetostatic support, modulated by neutral-ion drift (known asambipolar diffusion in astrophysics). Recently, however, bothobservational and numerical work has begun to suggest that supersonicturbulent flows rather than static magnetic fields control starformation. To some extent, this represents a return to ideas popularbefore the importance of magnetic fields to the interstellar gas wasfully appreciated. This review gives a historical overview of thesuccesses and problems of both the classical dynamical theory and thestandard theory of magnetostatic support, from both observational andtheoretical perspectives. The outline of a new theory relying on controlby driven supersonic turbulence is then presented. Numerical modelsdemonstrate that, although supersonic turbulence can provide globalsupport, it nevertheless produces density enhancements that allow localcollapse. Inefficient, isolated star formation is a hallmark ofturbulent support, while efficient, clustered star formation occurs inits absence. The consequences of this theory are then explored for bothlocal star formation and galactic-scale star formation. It suggests thatindividual star-forming cores are likely not quasistatic objects, butdynamically collapsing. Accretion onto these objects varies depending onthe properties of the surrounding turbulent flow; numerical models agreewith observations showing decreasing rates. The initial massdistribution of stars may also be determined by the turbulent flow.Molecular clouds appear to be transient objects forming and dissolvingin the larger-scale turbulent flow, or else quickly collapsing intoregions of violent star formation. Global star formation in galaxiesappears to be controlled by the same balance between gravity andturbulence as small-scale star formation, although modulated by coolingand differential rotation. The dominant driving mechanism instar-forming regions of galaxies appears to be supernovae, whileelsewhere coupling of rotation to the gas through magnetic fields orgravity may be important.

High-Resolution Continuum Imaging at 1.3 and 0.7 Centimeters of the W3 IRS 5 Region
High-resolution images of the hypercompact H II (HC H II) regions in W3IRS 5 taken with the Very Large Array (VLA) at 1.3 and 0.7 cm arepresented. Four HC H II regions were detected with sufficientsignal-to-noise ratios to allow the determination of relevantparameters, such as source position, size, and flux density. The sourcesare slightly extended in our ~0.2" beams; the deconvolved radii are lessthan 240 AU. A comparison of our data with VLA images taken at epoch1989.1 shows proper motions for sources IRS 5a and IRS 5f. Between1989.1 and 2002.5, we find a proper motion of 210 mas at a positionangle of 12° for IRS 5f and a proper motion of 190 mas at a positionangle of 50° for IRS 5a. At the assumed distance to W3 IRS 5,1.83+/-0.14 kpc, these offsets translate to proper motions of ~135 and~122 km s-1, respectively. These sources are eithershock-ionized gas in an outflow or ionized gas ejected from high-massstars. We find no change in the positions of IRS 5d1/d2 and IRS 5b, andwe show through a comparison with archival NICMOS 2.2 μm images thatthese two radio sources coincide with the infrared double constitutingW3 IRS 5. These sources contain B or perhaps O stars. The flux densitiesof the four sources have changed compared to the epoch 1989.1 results.In our epoch 2002.5 data, none of the spectral indices obtained fromflux densities at 1.3 and 0.7 cm are consistent with optically thinfree-free emission; IRS 5d1/d2 shows the largest increase in fluxdensity from 1.3 to 0.7 cm. This may be an indication of free-freeoptical depth within an ionized wind, a photoevaporating disk, or anaccretion flow. It is less likely that this increase is caused by dustemission at 0.7 cm.

Linear Polarization Observations of Water Masers in W3 IRS 5
We present a magnetic field mapping of water maser clouds in thestar-forming region W3 IRS 5, which has been made on the basis of thelinear polarization VLBI observation. Using the Very Long Baseline Arrayat 22.2 GHz, 16 of 61 detected water masers were found to be linearlypolarized with polarization degrees up to 13%. Although 10 polarizedfeatures were widely distributed in the whole W3 IRS 5 water maserregion, they had position angles similar to the magnetic field vectors(~75° east from the north). The magnetic field vectors are roughlyperpendicular to the spatial alignments of the maser features. They areconsistent with the hourglass model of the magnetic field, which waspreviously proposed to explain the magnetic field in the whole W3 Mainregion (r~0.1 pc). They are, on the other hand, not aligned to thedirections of maser feature proper motions observed previously. Thisimplies that the W3 IRS 5 magnetic field was controlled by a collapse ofthe W3 Main molecular cloud rather than by the outflow originated fromW3 IRS 5.

Astral magnetic fields-as observed in starforming nurseries, in stars, and in the Solar system
``Cherchez le champ magnétique'' is trendy in astronomy. Themagnetic field takes the hourglass shape in gravitationally contractingyoung stellar objects, helically wraps up the jets of protostars, guidesparticles in stellar coronae, explodes in stellar flares, causes unrestin pulsar quakes and creates planetary aurorae. In stars and pulsars,dipolar dynamo magnetic fields play a dominant rôle in the gasdynamics. In planetary disks, the field is toroidal or archimedeanspiral. Remanent magnetism is found in meteorites and asteroids. Theastral magnetic fields can reach 1018 G in magnetars,dwarfing anything we can do on Earth in stable conditions(107 G) or in pulsed conditions (1010 G).Magnetism plays a physical rôle in starforming clouds and itsconcomittent structures, from stellar nurseries to protostars, down toplanets and asteroids. Starting with star-forming clouds (~10 pc=32light-years=31×1016 m), this reviews coversprotostellar systems (~1 cpc), circumstellar space (~1 mpc), masers,interplanetary space (~1 μpc), pulsars, stars, planets (~1 npc),asteroids (~30 km; ~1 ppc), and meteorites (~0.3 m; ~10 apc).

The Line-of-Sight Velocities of OB Associations and Molecular Clouds in a Wide Solar Neighborhood: The Streaming Motions of Stars and Gas in the Perseus Arm
We compare the line-of-sight velocities of stars in OB associationswithin 3 kpc of the Sun and the molecular clouds closest to them. ThePerseus arm is used as an example to show that the line-of-sightvelocity ranges in which OB-association stars, molecular clouds, and HII regions ionized by these association stars are observed overlap. Thestreaming motions produced by density waves in the (l, VLSR)plane were found to lead to an inversion of the distances to objects ofthe Perseus arm and the interarm space beyond this arm.

Magnetic Fields in Shocked Regions: Very Large Array Observations of H2O Masers
We present VLA observations of the Zeeman effect in 22 GHzH2O masers in several high-mass star-forming regions. Thesemasers are believed to arise from collisional pumping in postshockenvironments. Therefore, the Zeeman effect data provide the most directmeasurements of magnetic field strengths in high-density(n>~108 cm-3) postshock gas, where the fieldenergy determines other physical conditions. Our observations yieldsignificant magnetic field detections in W3 IRS 5, W3(OH), W49 N, and OH43.8-0.1. In these sources, we detect line-of-sight field strengthsranging from 13 to 49 mG. For some regions, the detected fields providea 2-3 point sampling of the magnetic field, indicating the nature offield variations on arcsecond scales. These field strengths areconsistent with a shock-driven maser model having relatively low speed(20 km s-1), C-type shocks. We examine the balance betweenmagnetic field energy and turbulent kinetic energy in the masingregions. These energies appear close to equilibrium.

X-Ray Detection of the Ionizing Stars in Ultracompact H II Regions
We present observations of the W3 complex of massive star formationregions using the Chandra X-Ray Observatory. In the W3 core region, ourobservations resolve the emission previously observed with ASCA into 101compact sources. The integrated spectrum is consistent with thatobserved earlier by ASCA and shows significant emission at energieslarger than 2.5 keV. Comparing our Chandra data with existingnear-infrared images, we find X-ray counterparts for W3 IRS 2, W3 IRS2a, and W3 IRS 3a that are believed to be the ionizing stars for the HII regions W3A and W3B. We also detect X-ray emission coincident withthe massive protostar candidate W3 IRS 5. Comparing our Chandra datawith existing radio continuum data, we find that for radio continuumcomponents A, B, C, D, G, and H, one or several X-ray sources arelocated at the peak radio position and/or the geometric center of the HII region. We postulate that the X-ray sources are the young massivestars that are also responsible for the ionization of the compact andultracompact H II regions in the W3 core. Our observations show thatvery young massive stars are emitters of relatively hard X-rays and thatthey can be detected with Chandra even in a high-density environment.

The rich 6 to 9 vec mu m spectrum of interstellar PAHs
IR spectroscopy provides a valuable tool for the characterisation andidentification of interstellar molecular species. Here, we present 6-9μm spectra of a sample of reflection nebulae, HII regions, YSOs,evolved stars and galaxies that show strong unidentified infrared bands,obtained with the SWS spectrograph on board ISO. The IR emissionfeatures in this wavelength region show pronounced variations. 1) The6.2 μm feature shifts from 6.22 to 6.3 μm and clearly showsprofile variations. 2) The 7.7 μm complex is comprised of at leasttwo subpeaks peaking at 7.6 and one longwards of 7.7 μm. In somecases the main peak can apparently shift up to 8 μm. Two sources donot exhibit a 7.7 μm complex but instead show a broad emissionfeature at 8.22 μm. 3) The 8.6 μm feature has a symmetric profilein all sources and some sources exhibit this band at slightly longerwavelengths. For the 6.2, 7.7 and 8.6 μm features, the sources havebeen classified independently based on their profile and peak position.The classes derived for these features are directly linked with eachother. Sources with a 6.2 μm feature peaking at ~ 6.22 μm exhibita 7.7 μm complex dominated by the 7.6 μm component. In contrast,sources with a 6.2 μm profile peaking longwards of 6.24 μm show a7.7 μm complex with a dominant peak longwards of 7.7 μm and a 8.6μm feature shifted toward the red. Furthermore, the observed 6-9μm spectrum depends on the type of object. All ISM-like sources and afew PNe and Post-AGB stars belong to the first group while isolatedHerbig AeBe stars, a few Post-AGB stars and most PNe belong to thesecond group. We summarise existing laboratory data and theoreticalquantum chemical calculations of the modes emitting in this wavelengthregion of PAH molecules. We discuss the variations in peak position andprofile in view of the exact nature of the carrier. We attribute theobserved 6.2 μm profile and peak position to the combined effect of aPAH family and anharmonicity with pure PAHs representing the 6.3 μmcomponent and substituted/complexed PAHs representing the 6.2 μmcomponent. The 7.6 μm component is well reproduced by both pure andsubstituted/complexed PAHs but the 7.8 μm component remains anenigma. In addition, the exact identification of the 8.22 μm featureremains unknown. The observed variations in the characteristics of theIR emission bands are linked to the local physical conditions. Possibleformation and evolution processes that may influence the interstellarPAH class are highlighted. 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) andwith the participation of ISAS and NASA.

Gaussian Spectral Line Profiles of Astrophysical Masers
Calculations are performed to demonstrate the deviations from Gaussianthat occur in the spectral line profiles of a linear maser as a resultof the amplification process. Near-Gaussian profiles are presented forbright, interstellar 22 GHz water masers obtained from high-resolutionVery Long Baseline Array observations of W3 IRS 5. For the profiles tobe so close to Gaussian, the calculations indicate that these masersmust originate in quite hot gas with temperatures greater than 1200 K-aconclusion that is supportive of C-type shocks as the origin of thesemasers. In addition, the degree of saturation of these masers must beless than approximately one-third, from which it follows that thebeaming angles are less than about 10-4 sr and the actualluminosities are modest. If spectral profiles that are as close toGaussian as the profiles presented in this initial investigation arefound to occur widely, they can be valuable diagnostics for theenvironments of astrophysical masers.

ISO spectroscopy of compact H II regions in the Galaxy. I. The catalogue
Infrared spectra between 2.3 and 196 μm were taken towards a sampleof 45 compact H II regions using the two spectrometers (SWS and LWS) onboard ISO. The primary goal is to determine the distribution of elementabundances in the Galaxy, although there are also many other uses ofthis database. The spectra contain a wealth of information on theionized gas and the associated photodissociation regions through theatomic fine-structure lines and on the dust properties via the dustemission bands and the continuum. Significant variations are found fromsource to source in both spectral shape and content. The sample of H IIregions spans a wide range in galactocentric distance (from 0 to 22 kpc)enabling to investigate the variations of the nebular properties acrossthe Galactic plane. The observations and the data reduction aredescribed in detail in the present paper. The ISO spectral catalogue ofcompact H II regions contains the combined SWS-LWS spectra for each ofthe sources, the fluxes of the atomic fine-structure lines and hydrogenrecombination lines, and an inventory of the spectra in terms ofmolecular lines, dust and ice bands. Based on observations with ISO, anESA project with instruments funded by ESA Member States (especially thePI countries: France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA.

High-Mass, OB Star Formation in M51: Hubble Space Telescope Hα and Paα Imaging
We have obtained Hα and Paα emission-line images coveringthe central 3'-4' of M51 using the WFPC2 and NICMOS instruments on theHubble Space Telescope to study the high-mass stellar population. The0.1"-0.2" pixels provide 4.6-9 pc resolution in M51, and theHα/Paα line ratios are used to obtain extinction estimates.A sample of 1373 Hα emission regions is cataloged using anautomated and uniform measurement algorithm. Their sizes are typically10-100 pc. The luminosity function for the Hα emission regions isobtained over the range LHα=1036 to2×1039 ergs s-1. The luminosity function isfitted well by a power law with dN/dlnL~L-1.01. The power lawis significantly truncated, and no regions were found with observedLHα above 2×1039 ergs s-1(uncorrected for extinction). (The maximum seen in ground-based studiesis approximately a factor of 5 higher, very likely because of theblending of multiple regions.) The extinctions derived here increase themaximum intrinsic luminosity to above 1040 ergss-1. The logarithmically binned luminosity function is alsosomewhat steeper (α=-1.01) than that found from ground-basedimaging (α=-0.5 to -0.8)-probably also a result of our resolvingregions that were blended in the ground-based images. The two-pointcorrelation function for the H II regions exhibits strong clustering onscales <=2", or 96 pc. To analyze the variations of H II regionproperties vis-à-vis the galactic structure, the spiral arm areaswere defined independently from millimeter-CO and optical continuumimaging. Although the arms constitute only 25% of the disk surface area,the arms contain 45% of the cataloged H II regions. The luminosityfunction is somewhat flatter in spiral arm regions than in the interarmareas (-0.72 to -0.95) however, this is very likely the result ofincreased blending of individual H II regions in the arms that havehigher surface density. No significant difference is seen in the sizesand electron densities of the H II regions in spiral arm and interarmregions. For 209 regions that had >=5 σ detections in bothPaα and Hα, the observed line ratios indicate visualextinctions in the range AV=0-6 mag. The mean extinction wasAV=3.1 mag (weighting each region equally), 2.4 mag(weighting each by the observed Hα luminosity), and 3.0 mag(weighting by the extinction-corrected luminosity). On average, theobserved Hα luminosities should be increased by a factor of ~10,implying comparable increases in global OB star cluster luminosities andstar formation rates. The full range of extinction-corrected Hαluminosities is between 1037 and 2×1040 ergss-1. The most luminous regions have sizes >=100 pc, so itis very likely that they are blends of multiple regions. This is clearbased on their sizes, which are much larger than the maximum diameter(<=50 pc) to which an H II region might conceivably expand within the~3×106 yr lifetime of the OB stars. It is alsoconsistent with the observed correlation (L~D2) between themeasured luminosities and sizes of the H II regions. We thereforegenerated a subsample of 1101 regions with sizes <=50 pc, which ismade up of those regions that might conceivably be ionized by a singlecluster. Their extinction-corrected luminosities range between2×1037 and 1039 ergs s-1, orbetween two-thirds of M42 (the Orion Nebula) and W49 (the most luminousGalactic radio H II region). The upper limit for individual clusters istherefore conservatively <=1039 ergs s-1,implying QLyc,up~=7×1050 s-1(with no corrections for dust absorption of the Lyman continuum or UVthat escapes to the diffuse medium). This corresponds to cluster masses<=5000 Msolar (between 1 and 120 Msolar). Thetotal star formation rate in M51 is estimated from theextinction-corrected Hα luminosities to be ~4.2 Msolaryr-1 (assuming a Salpeter initial mass function between 1 and120 Msolar), and the cycling time from the neutralinterstellar medium into these stars is 1.2×109 yr. Wedevelop a simple model for the UV output from OB star clusters as afunction of the cluster mass and age in order to interpret constraintsprovided by the observed luminosity functions. The power-law index atthe high-luminosity end of the luminosity function (α=-1.01)impliesN(Mcl)/dMcl~M-2.01cl. Thisimplies that high-mass star formation, cloud disruption due to OB stars,and UV production are contributed to by a large range of cluster masseswith equal effects per logarithmic interval of cluster mass. Thehigh-mass clusters (~1000 Msolar) have a mass such that theinitial mass function is well sampled up to ~120 Msolar, butthis cluster mass is <=1% of that available in a typical giantmolecular cloud. We suggest that OB star formation in a cloud coreregion is terminated at the point that radiation pressure on thesurrounding dust exceeds the self-gravity of the core star cluster andthat this is what limits the maximum mass of standard OB star clusters.This occurs at a stellar luminosity-to-mass ratio of ~500-1000(L/M)solar, which happens for clusters >=750Msolar. We have modeled the core collapse hydrodynamicallyand have found that a second wave of star formation may propagateoutward in a radiatively compressed shell surrounding the core starcluster-this triggered, secondary star formation may be the mechanismfor formation of the super-star clusters seen in starburst galaxies.

High-Resolution Mid-Infrared Imaging and Spectroscopic Observations of a Massive Star Forming Region W51 IRS 2
Mid-infrared (7.8-13.5μm) imaging and low-resolution spectroscopicobservations were made for the massive star-forming region W51 IRS 2with spatial resolution of 1". IRS 2 was resolved into seven sourcesbased on the continuum and the emission line maps. The nature of thesesources was investigated using their thermal dust emission, the 9.7μm absorption feature, and/or three fine-structure emission lines of[Ne II] at 12.8 μm, [Ar III] at 8.99 μm, and [S IV] at 10.51μm. Four of the sources were identified as ultracompact H II regionsand one as an embedded protostar candidate. The spectral types of theionizing sources are derived from the line ratios to be all around O9,which is much later than those (O5.5 and O7.5) derived from the radiocontinuum fluxes. The observed line ratios of (ultra-)compact H IIregions in IRS 2 and in the literature follow a single excitation track,but it is different from that predicted from model calculations. Wepropose two possible resolutions for the discrepancy: (1) the employedmodels for the stellar atmosphere are not sufficiently accurate in theUV range, or (2) the (ultra-)compact H II regions are ionized by stellarclusters.

VLA High-Sensitivity 4HE Imaging of Galactic H II Regions
We have carried out 8.3 GHz hydrogen and helium radio recombination lineand continuum observations of three Galactic H II regions with the NRAO140 Foot Telescope and the Very Large Array. These observations measurethe spatial variations in the singly ionized helium-to-hydrogenabundance ratio, 4He+/H+(y+), and probe for any localized enhancements iny+. The three observed regions (W43, NGC 6334A, and K3-50)are morphologically complex, and high-resolution, wide fieldobservations are crucial to measure the helium abundances on scales thatwould be associated with individual massive stars or clusters of stars.The nearly constant y+ value across the W43 source(+>W43=7.7%+/-1%) indicates that any ionizationcorrection over the spatial scales investigated is small. Suchionization corrections are important for calculating total abundances ofthe light element 3He, which is measured in the Galaxy via asingly ionized hyperfine transition. Localized enhancements in they+ value have been detected in both NGC 6334A(y+=20%+/-4%) and K3-50A (y+=24%+/-5%). The4He abundance results are discussed along with othermeasurements of the physical properties of these H II regions, such asthe kinematics and excitation.

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