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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.

Low-excitation blobs in the Magellanic Clouds
Aims.We study an unknown, or very poorly known, interstellar H IIcomponent in the Magellanic Clouds. This is the first study ever devotedto this class of objects, which we call low-excitation blobs (LEBs). Methods: We used low-dispersion spectroscopy carried out at ESO toobtain emission line intensities of Hα, Hβ, and [O III](λλ 4959 + 5007) for 15 objects in the Large MagellanicCloud and 14 objects in the Small Magellanic Cloud. Results aredisplayed in excitation ([O III]/Hβ ratio) versus Hβluminosity diagrams. Results: We show the presence of an LEB componentin the Magellanic Clouds and study its relationship with the alreadyknown class of high-excitation blobs (HEBs). The newly found LEBs arelower excitation counterparts of HEBs and are powered by less massiveexciting stars. Further study of LEBs is expected to provide new piecesof information for a better understanding the low mass end of the upperinitial mass function in the Magellanic Clouds.

A Survey of the SO JK=10-01 Transition toward Massive Star-forming Regions
We present the most extensive survey to date of the SO ground statetransition JK=10-01 (30 GHz). Weobserved 49 regions of massive star formation with theMax-Planck-Institut für Radioastronomie (MPIfR) 100 m telescope. Inaddition, the SO JK=43-32 transitionwas observed with the IRAM 30 m telescope toward a subsample of sevensources. Emission in the SO JK=10-01transition most likely arises from the parsec-scale molecular clumpssurrounding ultracompact H II regions. In general, the SOJK=10-01 line has a Gaussian shape andappears to be optically thin. We derive SO column densities between0.58×1014 and 12×1014 cm-2and SO abundances in the range0.18×10-9-4.4×10-9. Our observationsshow that the SO ground state transition is an effective tool to studymassive star-forming clumps, and the data presented in this paperprovide a useful supplement for future multitransition SO studies.

Distances to Anomalous X-Ray Pulsars Using Red Clump Stars
We identify ``red clump stars''-core helium-burning giants-among 2MASSstars and use them to measure the run of reddening with distance in thedirection of each of the Galactic anomalous X-ray pulsars (AXPs). Wecombine this with extinction estimates from X-ray spectroscopy to inferdistances and find that the locations of all AXPs are consistent withbeing in Galactic spiral arms. We also find that the 2-10 keVluminosities implied by our distances are remarkably similar for allAXPs, being all around ~1.3×1035 ergs s-1.Furthermore, using our distances to estimate effectiveblackbody-emitting radii, we find that the radii are tightlyanticorrelated with pulsed fraction and somewhat less tightlyanticorrelated with blackbody temperature. We find no obviousrelationship of any property with the dipole magnetic field strengthinferred from the spin-down rate.

Radio Recombination Lines in Galactic H II Regions
We report radio recombination line (RRL) and continuum observations of asample of 106 Galactic H II regions made with the NRAO 140 Foot (43 m)radio telescope in Green Bank, West Virginia. We believe this to be themost sensitive RRL survey ever made for a sample this large. Most of oursource integration times range between 6 and 90 hr, yielding typical rmsnoise levels of ~1.0-3.5 mK. Our data result from two differentexperiments performed, calibrated, and analyzed in similar ways. A C IIsurvey was made at the 3.5 cm wavelength to obtain accurate measurementsof carbon radio recombination lines. When combined with atomic (C I) andmolecular (CO) data, these measurements will constrain the composition,structure, kinematics, and physical properties of the photodissociationregions that lie on the edges of H II regions. A second survey was madeat the 3.5 cm wavelength to determine the abundance of 3He inthe interstellar medium of the Milky Way. Together with measurements ofthe 3He+ hyperfine line, we get high-precision RRLparameters for H, 4He, and C. Here we discuss significantimprovements in these data with both longer integrations and newlyobserved sources.

Compact H II Regions: What Lies Within?
This paper presents both stellar mass and H II region diagnostics basedon dusty, radiation-pressure-dominated photoionization models forcompact and ultracompact H II regions, and compares these withobservational constraints. These models successfully reproduce theobserved relationship between the density and the thickness of theionized layer. The absorption of ionizing photons in the dusty ionizedplasma makes denser ionized regions thinner than simple photoionizationmodels would predict, improving the fit with the observations. Themodels provide a good fit to observed diagnostic plots involving ratiosof infrared emission lines, all accessible with the IRS instrument ofthe Spitzer Space Telescope. These give the effective temperature to anaccuracy of about 2500 K and the mass of the ionizing star to aprecision of about +/-30%. The S IV/S III ratio is sensitive toforeground extinction as well as to stellar effective temperature ormass. From this ratio, we determine that the mean extinction to observedcompact H II regions is typically AV~30 mag. The electrontemperature depends on the chemical abundances, the pressure, and theeffective temperature of the exciting star. We use these models torederive the slope of the Galactic abundance gradient, with the resultthat dlog(O/H)/dRG=0.06+/-0.01 dex kpc-1, bringingthe Galactic abundance gradient derived from compact H II regions intocloser agreement with those based on other techniques. The shape of thefar-IR SED of compact H II regions can be used to constrain the meanpressure or density in the H II region. The Spitzer MIPS instrumentshould be very helpful in this regard.

Abundance Gradients in the Galaxy
Six H II regions at galactocentric distances of R=10-15 kpc have beenobserved in the far-IR emission lines of [O III] (52 μm, 88 μm),[N III] (57 μm), and [S III] (19 μm) using the Kuiper AirborneObservatory. These observations have been combined with Very Large Arrayradio continuum observations of these sources to determine theabundances of O++, N++, and S++relative to hydrogen. In addition, eight of the most recent sets ofmeasurements of ionic line strengths in H II regions have beenreanalyzed in order to attempt to reconcile differences in opticalversus far-IR abundance determinations. We have in total 168 sets ofobservations of 117 H II regions in our analysis. The new analysisincluded updating the atomic constants (transition probabilities andcollision cross sections), recalculation of some of the physicalconditions in the H II regions (ne and Te), andthe use of new photoionization models to determine stellar effectivetemperatures of the exciting stars. We also use the most recent dataavailable for the distances for these objects, although for most westill rely on kinematic distance determinations. Our analysis findslittle indication of differences between optical and infraredobservations of the nitrogen abundances, but some differences are seenin the oxygen and sulfur abundances. A very significant offset continuesto be seen between optical and infrared measurements of the N/Oabundance ratio.

The 12C/13C Isotope Gradient Derived from Millimeter Transitions of CN: The Case for Galactic Chemical Evolution
New measurements of 12C/13C ratios in Galacticmolecular clouds have been conducted using the N=1-->0 transition ofthe CN radical. This species is unique in that it has extensivehyperfine structure that can be accurately used to correct for linesaturation effects. Combined with the past observations of Savage andcoworkers, the ratios derived from CN are the most extensive data set todate for molecular clouds, and they include sources that lie in therange of 0.09-16.41 kpc in distance from the Galactic center(DGC). The ratios derived from CN indicate a gradient withGalactic distance of12C/13C=6.01DGC+12.28. This gradientagrees rather closely with those derived from measurements of CO andH2CO. The least-squares fit to all data points for the threemolecules is 12C/13C=6.21DGC+18.71. CO,CN, and H2CO are synthesized from quite varied reactions, andany 13C fractionation must follow different pathways forthese three species. The relatively good agreement between the12C/13C ratios of the three molecules, as well astheir lack of correlation with gas kinetic temperature, suggests thatchemical fractionation and isotope-selective photodissociation both donot play a substantial role in influencing such ratios. Therefore, the12C/13C gradient found in the Galaxy is a trueindicator of Galactic chemical evolution. The apparent discrepancybetween the solar system (12C/13C=89) and localinterstellar medium values (12C/13C~68) of thisratio may be a result of 13C enrichment since the formationof the solar system, as predicted by recent models.

Polycyclic Aromatic Hydrocarbons as a Tracer of Star Formation?
Infrared (IR) emission features at 3.3, 6.2, 7.7, 8.6, and 11.3 μmare generally attributed to IR fluorescence from (mainly)far-ultraviolet (FUV) pumped large polycyclic aromatic hydrocarbon (PAH)molecules. As such, these features trace the FUV stellar flux and arethus a measure of star formation. We examined the IR spectralcharacteristics of Galactic massive star-forming regions and of normaland starburst galaxies, as well as active galactic nuclei (AGNs) andultraluminous infrared galaxies (ULIRGs). The goal of this study is toanalyze whether PAH features are a good qualitative and/or quantitativetracer of star formation, and hence to evaluate the application of PAHemission as a diagnostic tool in order to identify the dominantprocesses contributing to the infrared emission from Seyfert galaxiesand ULIRGs. We develop a new mid-infrared (MIR)/far-infrared (FIR)diagnostic diagram based on our Galactic sample and compare it to thediagnostic tools of Genzel and coworkers and Laurent and coworkers, withthese diagnostic tools also applied to our Galactic sample. This MIR/FIRdiagnostic is derived from the FIR normalized 6.2 μm PAH flux and theFIR normalized 6.2 μm continuum flux. Within this diagram, theGalactic sources form a sequence spanning a range of 3 orders ofmagnitude in these ratios, ranging from embedded compact H II regions toexposed photodissociation regions (PDRs) and the (diffuse) interstellarmedium (ISM). However, the variation in the 6.2 μm PAHfeature-to-continuum ratio is relative small. Comparison of ourextragalactic sample with our Galactic sources revealed an excellentresemblance of normal and starburst galaxies to exposed PDRs. WhileSeyfert 2 galaxies coincide with the starburst trend, Seyfert 1 galaxiesare displaced by at least a factor of 10 in 6.2 μm continuum flux, inaccordance with general orientation-dependent unification schemes forAGNs. ULIRGs show a diverse spectral appearance. Some show a typical AGNhot dust continuum. More, however, either are starburst-like or showsigns of strong dust obscuration in the nucleus. One characteristic ofthe ULIRGs also seems to be the presence of more prominent FIR emissionthan either starburst galaxies or AGNs. We discuss the observedvariation in the Galactic sample in view of the evolutionary state andthe PAH/dust abundance and discuss the use of PAHs as quantitativetracers of star formation activity. Based on these investigations, wefind that PAHs may be better suited as a tracer of B stars, whichdominate the Galactic stellar energy budget, than as a tracer of massivestar formation (O stars).

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.

The Unidentified Infrared Bands in the Diffuse Interstellar Medium across the Galaxy Based on the Infrared Telescope in Space Mid-Infrared Spectrometer Observation
We present the results of observations of the unidentified infrared(UIR) bands in the diffuse Galactic emission across the Galaxy by theMid-Infrared Spectrometer (MIRS) on board the Infrared Telescope inSpace (IRTS). While previous studies on the UIR bands in the Milky Waywere limited to the inner Galactic plane, we extend the observing areato the outer Galactic plane. In this paper we analyze the data of fourareas of 8deg×8deg around the Galactic plane(|b|<=4deg -12deg<=l<=-4deg,44deg<=l<=52deg,-136deg<=l<=-128deg, and168deg<=l<=176deg) and investigate the UIRband intensity relative to the far-infrared (FIR) intensity, as well asthe variation in the band profile. Together with the good correlationbetween the UIR band and the FIR intensities in the four regions, wehave found a systematic variation in the UIR-to-FIR ratio such that theratio becomes larger in the outer Galactic plane than in the innerGalactic plane. In addition, the 8.6 and 11.3 μm UIR bands were foundto be stronger relative to the 6.2 and 7.7 μm bands in the outerGalactic plane, which may be related to differences in the structure orphysical conditions of the band carriers. We have also found smallshifts (~0.1 μm) in the peak wavelength of each UIR band to shorterwavelengths from the inner Galactic plane to the outer Galactic plane.Possible interpretations of these variations are discussed.

Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).

Far-infrared loops in the 2nd Galactic Quadrant
We present the results of an investigation of the large-scale structureof the diffuse interstellar medium in the 2nd Galactic Quadrant(90°≤l≤180°). 145 loops were identified on IRAS-basedfar-infrared maps. Our catalogue lists their basic physical properties.The distribution clearly suggests that there is an efficient processthat can generate loop-like features at high Galactic latitudes.Distances are provided for 30 loops. We also give an observationalestimate of the volume filling factor of the hot gas in the Local Arm,4.6%≤f2nd<6.4%.Appendices A-C are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/131

The Overlooked H II Region DA 568
During the inspection of Canadian Galactic Plane Survey 1420 MHzcontinuum images of the outer Galaxy we have rediscovered the large,bright, H II region DA 568. Intriguingly, this region does not appear inany of the commonly used catalogs of H II regions and radio sources inthe outer Galaxy. After demonstrating how DA 568 was essentiallyoverlooked, we present new radio, infrared, and molecular lineobservations of the region. We show that, far from being a minoroverlooked H II region, DA 568 is actually the most prominent part of amuch larger star-forming complex in the Perseus arm of our Galaxy. Thecomplex spans ~45×20 pc, contains over 104Msolar of ionized and molecular gas, and includes the BFS 10H II region and numerous associated IRAS sources. The star-formingcomplex is apparently quite evolved with most of the original giantmolecular cloud having been severely disrupted by the action of DA 568.

A New Distance Technique for Galactic Plane Objects
We present a new method based on H I column densities for determinationof distances within the disk of the Galaxy. The technique is useful forall Galactic plane objects, including H II regions and supernovaremnants (SNRs), provided a line-of-sight velocity can be assigned tothe object. Our method uses 21 cm spectral-line data to find the atomichydrogen column density to an object, and beyond it to the Galacticedge. A model of the smooth large-scale Galactic distribution of H Imaterial seen in emission (which principally traces the smooth structureof the Galaxy) is constructed. Our model accounts for scale-heightflaring with increasing Galactocentric radius and includes the Galacticwarp, which is prominent in the first and second quadrants of theGalaxy. The model's ability to trace the observed distribution of H I isdemonstrated on lines of sight toward SNR DA 530 (l=93.3d,b=7deg) and H II region Sh 121 (l=90.2d, b=1.7d). We thenapply the new technique to 29 Sharpless H II regions with knownphotometric distances across the second quadrant. We measureline-of-sight velocities for the H II regions from associated12CO emission, using 1' resolution 12CO (J=1-0)data from the Canadian Galactic Plane Survey. Our distance method yieldsdistances to these objects that are consistent with their photometricdistances and which are markedly smaller than the kinematic distancesfound from a flat Galactic rotation curve.

A radio continuum and infrared study of Galactic H II regions
We present observations of the 4.8 and 8.6 GHz continuum emissiontowards 11 southern H Ii regions made with the Australian TelescopeCompact Array. The observed objects were selected from the InfraredSpace Observatory (ISO) spectral catalogue of compact H Ii regions\citep{peeters:catalogue}. The morphologies observed for practically allthe sources are consistent with them being ionized by a cluster ofstars, rather than by a single star. The linear diameters of the regionsrange from 0.03 pc to 3 pc, the electron densities from 300 to 2.5x104 cm-3, and the Lyman continuum photon flux from1047 to 1050 s-1. We confirm theexistence of a relation between the density and size of H Ii regionswhich can be fit by a power law shallower than that predicted by theclassic Strömgren theory. The radio observations provide, inaddition, information about the distribution of the ionized gas withinthe ISO apertures. As a result of the combined radio and infrared study,estimates of the extinction in the infrared and the metal content of thenebular gas were calculated. In this analysis, we also included several(ultra)compact H Ii regions previously observed by the Very Large Array.Values for extinction in the K-band between ~ 0 and 6 mag are found.The elemental abundances of nitrogen, neon, argon and sulphur were foundto decrease with Galactocentric distance. Finally, the degree ofionization of the nebulae is confirmed to be correlated with the metalcontent.

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.

New infrared star clusters in the Northern and Equatorial Milky Way with 2MASS
We carried out a survey of infrared star clusters and stellar groups onthe 2MASS J, H and Ks all-sky release Atlas in the Northernand Equatorial Milky Way (350deg < l < 360deg, 0deg < l < 230 deg). Thesearch in this zone complements that in the Southern Milky Way (Dutra etal. \cite{Dutra03}a). The method concentrates efforts on the directionsof known optical and radio nebulae. The present study provides 167 newinfrared clusters, stellar groups and candidates. Combining the twostudies for the whole Milky Way, 346 infrared clusters, stellar groupsand candidates were discovered, whereas 315 objects were previouslyknown. They constitute an important new sample for future detailedstudies.

The association of IRAS sources and 12CO emission in the outer Galaxy
We have revisited the question of the association of CO emission withIRAS sources in the outer Galaxy using data from the FCRAO Outer GalaxySurvey (OGS). The availability of a large-scale high-resolution COsurvey allows us to approach the question of IRAS-CO associations from anew direction - namely we examined all of the IRAS sources within theOGS region for associated molecular material. By investigating theassociation of molecular material with random lines of sight in the OGSregion we were able to construct a quantitative means to judge thelikelihood that any given IRAS-CO association is valid and todisentangle multiple emission components along the line of sight. Thepaper presents a list of all of the IRAS-CO associations in the OGSregion. We show that, within the OGS region, there is a significantincrease ( ~ 22%) in the number of probable star forming regions overprevious targeted CO surveys towards IRAS sources. As a demonstration ofthe utility of the IRAS-CO association table we present the results ofthree brief studies on candidate zone-of-avoidance galaxies with IRAScounterparts, far outer Galaxy CO clouds, and very bright CO clouds withno associated IRAS sources. We find that ~ 25% of such candidate ZOAGsare Galactic objects. We have discovered two new far outer Galaxystar-forming regions, and have discovered six bright molecular cloudsthat we believe are ideal targets for the investigation of the earlieststages of sequential star formation around HII regions. Finally, thispaper provides readers with the necessary data to compare othercatalogued data sets with the OGS data.Tables 1, 2 and A1 are only available in electronic form at the CDS viaanonymous ftp to\ cdsarc.u-strasbg.fr ( or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/1083

Large-Scale Turbulence in Molecular Clouds
Principal component analysis of 12CO (J=1-0) emission is usedto diagnose the presence of large-scale, global velocity fluctuations inmolecular clouds. We search for, and find, large-scale flows of atomicmaterial in which the globally turbulent molecular clouds are embedded.This is consistent with the picture of molecular clouds existing asshort-lived, turbulent density fluctuations within larger scale atomicflows. Large-scale driving of turbulence in molecular clouds, byconverging flows of atomic material, can reconcile recent numericaldeterminations of the dissipation rate of turbulent energy with theobservations, that otherwise, for small-scale driving, leads tounacceptably high cloud luminosities. On the other hand, lack oflarge-scale shock signatures in the molecular gas, as expected forlarge-scale driving, may, if not due to an observational bias, requirethat the large-scale flows are themselves driven by energy injectionoccurring on smaller scales within molecular clouds.

The Distance to Supernova Remnant CTB 109 Deduced from Its Environment
We conducted a study of the environment around the supernova remnant(SNR) CTB 109. We found that the SNR is part of a large complex of H IIregions extending over an area of 400 pc along the Galactic plane at adistance of about 3 kpc at the closer edge of the Perseus spiral arm. Atthis distance, CTB 109 has a diameter of about 24 pc. We demonstratedthat including spiral shocks in the distance estimation is an ultimaterequirement to determine reliable distances to objects located in thePerseus arm. The most likely explanation for the high concentration of HII regions and SNRs is that the star formation in this part of thePerseus arm is triggered by the spiral shock.

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.

Expanding shells of shocked neutral hydrogen around compact H II regions
By comparing radial velocities of radio bright compact H Ii regions withtheir H I absorption profiles, we discovered expanding shells of neutralhydrogen around them. These shells are revealed by absorption of theradio continuum emission from the H Ii regions at velocities indicatinggreater distances than the observed radial velocity. We believe thatthese shells are shock zones at the outer edge of the expanding ionizedregion. Additionally we found evidence for a velocity inversion insidethe Perseus arm caused by a spiral shock, which results in a deepabsorption line in the spectra of compact H Ii regions behind it.

The stellar content, metallicity and ionization structure of H II regions
Observations of infrared fine-structure lines provide direct informationon the metallicity and ionization structure of H Ii regions andindirectly on the hardness of the radiation field ionizing thesenebulae. We have analyzed a sample of Galactic and Magellanic Cloud H Iiregions observed by the Infrared Space Observatory (ISO) to examine theinterplay between stellar content, metallicity and the ionizationstructure of H Ii regions. The observed [S Iv],10.5/[S Iii],18.7 μmand [Ne Ii]I 15.5/[Ne Ii],12.8 μm line ratios are shown to be highlycorrelated over more than two orders of magnitude. We have compared theobserved line ratios to the results of photoionization models usingdifferent stellar energy distributions. The derived characteristics ofthe ionizing star depend critically on the adopted stellar model as wellas the (stellar) metallicity. We have compared the stellar effectivetemperatures derived from these model studies for a few well-studied HIi regions with published direct spectroscopic determinations of thespectral type of the ionizing stars. This comparison supports ourinterpretation that stellar and nebular metallicity influences theobserved infrared ionic line ratios. We can explain the observedincrease in degree of ionization, as traced by the [S Iv]/[S Iii] and[Ne Iii]/[Ne Ii] line ratios, by the hardening of the radiation fielddue to the decrease of metallicity. The implications of our results forthe determination of the ages of starbursts in starburst galaxies areassessed. Based on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, The Netherlands and the UK) and with the participationof ISAS and NASA.

Spatial distribution of emission in unidentified infrared bands from the Midcourse Space Experiment survey
The Midcourse Space Experiment (MSX) has surveyed the Galactic plane infour infrared bands between 6 and 25 mu m. Two of these bands coverseveral Unidentified Infrared emission Bands (UIBs). With the aim ofextracting the spatial distribution of the UIB emission on a largescale, a scheme has been developed to model the MSX data with emissionin the UIBs alongwith the underlying thermal continuum from theinterstellar dust. In order to test this scheme, a sample of fiveGalactic compact H II regions (Sh-61, Sh-138, Sh-152, Sh-156, Sh-186;Zavagno & Ducci \cite{Zavagno01}) for which imaging data in someindividual UIBs is available from ISOCAM measurements, has been studied.The results of this comparative study on a small angular scale are asfollows: (i) the morphological details extracted from our scheme agreevery well with those from the superior ISOCAM measurements; (ii) theintegrated strength of UIBs extracted from the MSX database correlatesextremely well with the sum of the strengths of individual UIBs measuredfrom ISOCAM. This tight correlation is very encouraging and promises thepotential of MSX database for the study of large-scale spatialdistribution of UIB emission (and the carriers of UIBs) in the entireGalactic plane.

Spatial Distribution of Dust Grains within H II Regions
We discuss the dust distribution within photoionized regions. Assuming ageometry with a central dust cavity, which is strongly suggested by theliterature, we can estimate the cavity radius from the ratio of theinfrared and radio fluxes by using a simple transfer model of Lymancontinuum photons. We apply the method to a sample of the Galactic H IIregions. The estimated typical radius of the dust cavity of the Galacticcompact H II regions is about 30% of the Strömgren radius. Takinginto account uncertainties in both the observational data and the model,we can reject a dust distribution lacking a central cavity. Therefore,the dust cavity model is supported independently of the previous works.We discuss the formation mechanism of such a dust cavity and itsdetectability by present and future infrared facilities.

Interstellar Turbulence. II. Energy Spectra of Molecular Regions in the Outer Galaxy
The multivariate tool of principal component analysis (PCA) is appliedto 23 fields in the FCRAO CO Survey of the Outer Galaxy. PCA enables theidentification of line profile differences, which are assumed to begenerated from fluctuations within a turbulent velocity field. Thevariation of these velocity differences with spatial scale within amolecular region is described by a singular power law,δv=cLα, which can be used as a powerfuldiagnostic to turbulent motions. For the ensemble of 23 fields, we finda mean value <α>=0.62+/-0.11. From a recent calibration of thismethod using fractal Brownian motion simulations (Brunt & Heyer),the measured velocity difference-size relationship corresponds to anenergy spectrum, E(k), which varies as k-β, whereβ=2.17+/-0.31. We compare our results to both decaying and forcedhydrodynamic simulations of turbulence. We conclude that energy must becontinually injected into the regions to replenish that lost bydissipative processes such as shocks. The absence of large, widelydistributed shocks within the targeted fields suggests that the energyis injected at spatial scales less than several parsecs.

The PAH emission spectra of Large Magellanic Cloud H II regions
A set of ISOPHOT spectra from a sample of H Ii regions in the LargeMagellanic Cloud (LMC) is presented. In all the spectra, emission bandsarising from Polycyclic Aromatic Hydrocarbons (PAHs) are clearlypresent. These features are observed to vary considerably in relativestrength to each other from source to source and even within 30 Doradus.The LMC spectra have been compared with ISO-SWS spectra from Galactic HIi regions and with the ISOCAM observation towards a quiescent molecularcloud in the SMC (Reach et al. \cite{Reach}). A correlation is foundbetween the I7.7/I11.2 versusI6.2/I11.2 and theI8.6/I11.2 versus I6.2/I11.2ratios. A segregation between the sources in the different types ofenvironment (Milky Way - LMC - SMC) is present. Furthermore, within theLMC observations, a clear distinction between 30 Doradus and non-30Doradus pointings is found. We discuss the variations in the relativestrength of the PAH features in view of the different physicalenvironments and highlight the relation with the PAH/dust ratio and theextinction curve. We conclude that 1) the same conditions responsiblefor the observed trends in the relative PAH-feature strengths alsoaffect the carrier of the 2175 Å bump leading to the differencesin strength of the latter, and 2) the molecular structure is the majorcause of the observed variations in the relative strength of the PAHfeatures. In the SMC and 30 Doradus compact PAH species dominate, whilePAHs with an open, uneven structure are the dominant ones in Galactic HIi regions and the non-30 Dor LMC sources. Based on observations withISO, an ESA project with instruments funded by ESA Member States(especially the PI countries: France, Germany, The Netherlands and theUK) and with the participation of ISAS and NASA.

ISO spectroscopy of compact H II regions in the Galaxy. II. Ionization and elemental abundances
Based on the ISO spectral catalogue of compact H II regions by\cite{peeters:catalogue}, we present a first analysis of the hydrogenrecombination and atomic fine-structure lines originated in the ionizedgas. The sample consists of 34 H II regions located at galactocentricdistances between RGal,= 0 and 15 kpc. The SWS H Irecombination lines between 2 and 8 μm are used to estimate theextinction law at these wavelengths for 14 H II regions. An extinctionin the K band between 0 and ~ 3 mag has been derived. The fine-structurelines of N, O, Ne, S and Ar are detected in most of the sources. Most ofthese elements are observed in two different ionization stages probing arange in ionization potential up to 41 eV. The ISO data, by itself orcombined with radio data taken from the literature, is used to derivethe elemental abundances relative to hydrogen. The present data thusallow us to describe for each source its elemental abundance, its stateof ionization and to constrain the properties of the ionizing star(s).The main results of this study are as follows. The ionization ratiosAr++/Ar+, N++/N+,S+3/S++ and Ne++/Ne+, whichmeasure the degree of ionization and to first order, the hardness of thestellar radiation, seem to increase with RGal. Theseionization ratios correlate well with each other, implying that thespectral hardening affects equally the full range of ionizing energies.A Galactocentric gradient of N/O (Delta log N/O = - 0.056+/- 0.009 dexkpc-1) is observed in the sense of a decreasing abundanceratio with RGal in agreement with previous studies. Abundancegradients for neon and argon are derived of the form Delta log Ne/H = -0.039+/- 0.007 dex kpc-1 and Delta log Ar/H = -0.045+/- 0.011dex kpc-1. These elemental gradients could be enlarged by theexisting Galactic Te gradient. Adopting a Tegradient of approximately 330 K kpc-1, the slopes in the Ne/Hand Ar/H gradients become -0.06 and -0.07 dex kpc-1,respectively. Lower limits for the sulphur and oxygen abundances arederived. Nitrogen abundances are derived for 16 sources. Based onobservations with ISO, an ESA project with instruments funded by ESAMember States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.}

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.

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Right ascension:23h05m09.90s
Apparent magnitude:99.9

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