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A Sino-German λ6 cm polarization survey of the Galactic plane. I. Survey strategy and results for the first survey region
Aims.Polarization measurements of the Galactic plane at λ6 cmprobe the interstellar medium (ISM) to larger distances compared tomeasurements at longer wavelengths, enabling us to investigateproperties of the Galactic magnetic fields and electron density. Methods: We are conducting a new λ6 cm continuum and polarizationsurvey of the Galactic plane covering 10°≤ l≤230° and|b|≤5°. Missing large-scale structures in the U and Q maps arerestored based on extrapolated polarization K-band maps from the WMAPsatellite. The λ6 cm data are analyzed together with maps inother bands. Results: We discuss some results for the first surveyregion, 7°×10° in size, centered at (l,b)=(125.5 °,0°). Two new passive Faraday screens, G125.6-1.8 and G124.9+0.1,were detected. They cause significant rotation of backgroundpolarization angles but little depolarization. G124.9+0.1 was identifiedas a new faint HII region at a distance of 2.8 kpc. G125.6-1.8, with asize of about 46 pc, has neither a counterpart in enhanced Hαemission nor in total intensity. A model combining foreground andbackground polarization modulated by the Faraday screen was developed.Using this model, we estimated the strength of the ordered magneticfield along the line of sight to be 3.9 μG for G124.9+0.1, andexceeding 6.4 μG for G125.6-1.8. We obtained an estimate of 2.5 and6.3 mK kpc-1 for the average polarized and total synchrotronemissivity towards G124.9+0.1. The synchrotron emission beyond thePerseus arm is quite weak. A spectral curvature previously reported forSNR G126.2+1.6 is ruled out by our new data, which prove a straightspectrum. Conclusions: .The new λ6 cm survey will play animportant role in improving the understanding of the properties of themagneto-ionic ISM. The magnetic fields in HII regions can be measured.Faraday screens with very low electron densities but large rotationmeasures were detected indicating strong and regular magnetic fields inthe ISM. Information about the local synchrotron emissivity can beobtained.

HCN versus HCO+ as Dense Molecular Gas Mass Tracers in Luminous Infrared Galaxies
It has recently been argued that the HCN J=1-0 line emission may not bean unbiased tracer of dense molecular gas (n>~104cm-3) in luminous infrared galaxies (LIRGs;LFIR>1011 Lsolar) and thatHCO+ J=1-0 may constitute a better tracer instead, castingdoubt onto earlier claims supporting the former as a good tracer of suchgas. In this paper new sensitive HCN J=4-3 observations of four suchgalaxies are presented, revealing a surprisingly wide excitation rangefor their dense gas phase that may render the J=1-0 transition fromeither species a poor proxy of its mass. Moreover, the well-knownsensitivity of the HCO+ abundance to the ionization degree ofmolecular gas (an important issue omitted from the ongoing discussionabout the relative merits of HCN and HCO+ as dense gastracers) may severely reduce the HCO+ abundance in thestar-forming and highly turbulent molecular gas found in LIRGs, whileHCN remains abundant. This may result in the decreasingHCO+/HCN J=1-0 line ratios with increasing IR luminosityfound in LIRGs, and it casts doubts on HCO+ rather than HCNas a good dense molecular gas tracer. Multitransition observations ofboth molecules are needed to identify the best such tracer and itsrelation to ongoing star formation, and to constrain what may be aconsiderable range of dense gas properties in such galaxies.

A Cometary Bow Shock and Mid-Infrared Emission Variations Revealed in Spitzer Observations of HD 34078 and IC 405
We present new infrared observations of the emission/reflection nebulaIC 405 obtained with the Spitzer Space Telescope. Infrared images in thefour IRAC bands (3.6, 4.5, 5.8, and 8.0 μm) and two MIPS bands (24and 70 μm) are complemented by IRS spectroscopy (5-30 μm) of twonebular filaments. The IRAC (8.0 μm) and MIPS imaging shows evidenceof a bow shock associated with the runaway O9.5 V star, HD 34078,created by the interaction between the star and nebular material. Theratio of emission at 24 to 70 μm is higher in the immediate vicinityof HD 34078 than in the outer filaments, providing evidence for elevateddust temperatures (Td>~90 K) in the shock region. Thenebular imaging reveals that the morphology is band dependent, withvarying contributions from aromatic emission features, H2,and dust emission. Nebular spectroscopy is used to quantify thesecontributions, showing several aromatic emission bands between 6-14μm, the S(5), S(3), S(2), and S(1) pure rotational emission lines ofH2, and atomic fine-structure lines of Ne, S, and Ar. Thelow-dispersion spectra provide constraints on the ionization state ofthe large molecules responsible for the aromatic infrared features.H2 rotational temperatures of the two bright nebularfilaments are determined from the observed line strengths. An averageT(H2)~400 K is inferred, with evidence for additionalnonuniform excitation by UV photons in the intense radiation field of HD34078. The photoexcitation hypothesis is supported by direct measurementof the far-UV H2 fluorescence spectrum, obtained with FUSE.Based in part on observations made with the Spitzer Space Telescope,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under a contract with NASA.

The cooling of atomic and molecular gas in DR21
Aims. We present an overview of a high-mass star formation regionthrough the major (sub-)mm, and far-infrared cooling lines to gaininsight into the physical conditions and the energy budget of themolecular cloud. Methods: We used the KOSMA 3 m telescope to map thecore (10'× 14') of the Galactic star-forming region DR21/DR21 (OH)in the Cygnus X region in the two fine structure lines of atomic carbon(C I ^3P_1-^3P0 and ^3P_2-^3P_1), in four mid-J transitionsof CO and 13CO, and in CS J=7-6. These observations werecombined with FCRAO J=1-0 observations of 13CO andC18O. Five positions, including DR21, DR21 (OH), and DR21FIR1, were observed with the ISO/LWS grating spectrometer in the [O I]63 and 145 μm lines, the [C II] 158 μm line, and four high-J COlines. We discuss the intensities and line ratios at these positions andapply the local thermal equilibrium (LTE) and non-LTE analysis methodsin order to derive physical parameters such as mass, density andtemperature. The CO line emission was modeled up to J=20. Results: From non-LTE modeling of the low- to high-J CO lines, we identify twogas components, a cold one at temperatures of T_kin˜ 30-40 K andone with T_kin˜ 80-150 K at a local clump density of about n(H2) ~10^4-106 cm-3. While the cold quiescent componentis massive, typically containing more than 94% of the mass, the warm,dense, and turbulent gas is dominated by mid- and high-J CO lineemission and its large line widths. The medium must be clumpy with avolume-filling of a few percent. The CO lines are found to be importantin cooling the cold molecular gas, e.g. at DR21 (OH). Near the outflowof the UV-heated source DR21, the gas cooling is dominated by lineemission of atomic oxygen and of CO. Atomic and ionised carbon play aminor role.

Diffuse Far-Ultraviolet Observations of the Taurus Region
Diffuse far-ultraviolet (FUV; 1370-1670 Å) flux from the Taurusmolecular cloud region has been observed with the SPEAR/FIMS imagingspectrograph. An FUV continuum map of the Taurus region, similar to thevisual extinction maps, shows a distinct cloud core and halo region. Thedense cloud core, where the visual extinction Av>1.5,obscures the background diffuse FUV radiation, while scattered FUVradiation is seen in and beyond the halo region, whereAv<1.5. The total intensity of H2 fluorescencein the cloud halo is IH2=6.5×104photons cm-2 s-1 sr-1 in the 1370-1670Å wavelength band. A synthetic model of the H2fluorescent emission fits the present observation best with a hydrogendensity nH=50 cm-3, H2 column densityN(H2)=0.8×1020 cm-2, and incidentFUV intensity IUV=0.2. H2 fluorescence is not seenin the core, presumably because the required radiation flux to inducefluorescence is unable to penetrate the core region.

Molecular hydrogen formation on porous dust grains
Recent laboratory experiments on interstellar dust analogues have shownthat H2 formation on dust-grain surfaces is efficient in arange of grain temperatures below 20 K. These results indicate thatsurface processes may account for the observed H2 abundancein cold diffuse and dense clouds. However, high abundances ofH2 have also been observed in warmer clouds, includingphoton-dominated regions (PDRs), where grain temperatures may reach 50K, making the surface processes extremely inefficient. It was suggestedthat this apparent discrepancy can be resolved by chemisorption sites.However, recent experiments indicate that chemisorption processes maynot be efficient at PDR temperatures. Here we consider the effect ofgrain porosity on H2 formation, and analyse it using arate-equation model. It is found that porosity extends the efficiency ofthe recombination process to higher temperatures. This is because Hatoms that desorb from the internal surfaces of the pores may re-adsorbmany times and thus stay longer on the surface. However, thisporosity-driven extension may enable efficient H2 formationin PDRs only if porosity also contributes to significant cooling of thegrains, compared to non-porous grains.

Fluorescent Molecular Hydrogen Emission in IC 63: FUSE, Hopkins Ultraviolet Telescope, and Rocket Observations
We present far-ultraviolet observations of IC 63, an emission/reflectionnebula illuminated by the B0.5 IV star γ Cas, located 1.3 pc fromthe nebula. Molecular hydrogen fluorescence was detected first in IC 63by IUE and later at shorter wavelengths by ORFEUS. Here we present FarUltraviolet Spectroscopic Explorer (FUSE) observations toward threelocations in the nebula, complemented by Hopkins Ultraviolet Telescope(HUT) data on the central nebular position. In addition, we present asounding rocket calibration of a FUSE spectrum of γ Cas. Molecularhydrogen fluorescence is detected in all three FUSE pointings. Theintensity of this emission, as well as the contributions from otherspecies, are seen to vary with position. The absolute flux calibrationof the sounding rocket data allows us to reliably predict the radiationfield incident on IC 63. We use these data to test models of thefluorescent process. Our modeling resolves the perceived discrepancybetween the existing ultraviolet observations and achieves asatisfactory agreement with the H2 rotational structureobserved with FUSE.

A Multiwavelength Study of IC 63 and IC 59
IC 63 and IC 59 are two nearby arc-shaped nebulae with relatively simplegeometries and minimal obscuring material. The two regions, in spite ofa similar projected distance from their ionizing star, have verydifferent observational properties, both in continuum emission and inthe presence and strength of line emission from molecular species. Thispaper conducts a multiwavelength study of the two regions using archiveddata from a variety of sources, including the Canadian Galactic PlaneSurvey and the Infrared Space Observatory. The multiwavelengthmorphology and structure of the two nebulae are studied in detail,particularly the ionization fronts in IC 63. The possibility oftriggered star formation in IC 63 is investigated and determined to bespurious. H2 and polycyclic aromatic hydrocarbon emission isdetected in both IC 63 and IC 59, confirming the presence of molecularhydrogen in IC 59. The averaged line ratios are similar in the tworegions, but variations are seen within each region. Temperatures anddensities were calculated from the S(3) and S(5) pure rotational linesof molecular hydrogen. We derived a temperature of 630 K in IC 63,comparable to previous results, and a column density of5.8×1017 cm-2, somewhat lower than previousvalues. New results for IC 59 show values of 590 K and3.4×1017 cm-2, slightly cooler and withlower column density than IC 63. The contrast in appearance between IC63 and IC 59 is consistent with a difference in actual (rather thanprojected) distances and a small variation in temperature and columndensity.

Far-infrared detection of methylene
We present a clear detection of CH2 in absorption towards themolecular cloud complexes Sagittarius B2 andW49 N using the ISO Long Wavelength Spectrometer.These observations represent the first detection of its low excitationrotational lines in the interstellar medium. Towards Sagittarius B2, wedetect both ortho and para transitions allowing a determination of thetotal CH2 column density ofN(CH2)=(7.5±1.1)×1014cm-2. We compare this with the related molecule, CH, todetermine [CH/CH2]=2.7±0.5. Comparison with chemicalmodels shows that the CH abundance along the line of sight is consistentwith diffuse cloud conditions and that the high [CH/CH2]ratio can be explained by including the effect of grain-surfacereactions.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands and the UK) with the participation of ISAS and NASA.

HUT, FUSE, and Rocket Observations of IC 63: Molecular Hydrogen Fluorescence
We present far-ultraviolet spectroscopy of the photodissociation regionIC 63. These observations mark the first time that the entire electronictransition band of H2 has been studied as a whole and the first look atthe resolved rotational structure of molecular hydrogen below 1200 A inIC 63. Models of molecular hydrogen fluorescence in IC 63 haveoverpredicted the absolute flux at short wavelengths by as much as anorder of magnitude. We present a fluorescence model that resolves thediscrepancies between mid and far-ultraviolet emission by employing twotemperature components, self-absorption out of the ground vibrationallevel of H2, and a flux calibrated input spectrum of the excitingsource, gamma-Cas (HD 5394). A scattered light spectrum of gamma-Casobtained by the Far Ultraviolet Spectroscopic Explorer (FUSE) at anangular separation of 1' has been flux calibrated using the results of arecent sounding rocket flight to observe gamma-Cas with afar-ultraviolet sensitive CCD. The spectrum of IC 63 acquired with theHopkins Ultraviolet Telescope allows us to test this model with spectralcoverage across the entire 900-1650 A window spanned by fluorescent H2emission. This model can be tested in finer detail by a comparison withhigh-resolution spectra of IC 63 from FUSE. Our model finds relativelygood agreement with the observed emission line strengths seen in thespectra.

Organic molecules in protoplanetary disks around T Tauri and Herbig Ae stars
The results of single-dish observations of low- and high-J transitionsof selected molecules from protoplanetary disks around two T Tauri stars(LkCa 15 and TW Hya) and twoHerbig Ae stars (HD 163296 and MWC480) are reported. Simple molecules such as CO,13CO, HCO+, CN and HCN are detected. Several linesof H2CO are found toward the T Tauri star LkCa15 but not in other objects. No CH3OH has beendetected down to abundances of 10-9-10-8 withrespect to H2. SO and CS lines have been searched for withoutsuccess. Line ratios indicate that the molecular emission arises fromdense (106-108 cm-3) and moderatelywarm (T ˜ 20-40 K) intermediate height regions of the diskatmosphere between the midplane and the upper layer, in accordance withpredictions from models of the chemistry in disks. The sizes of thedisks were estimated from model fits to the 12CO 3-2 lineprofiles. The abundances of most species are lower than in the envelopearound the solar-mass protostar IRAS 16293-2422.Freeze-out in the cold midplane and photodissociation by stellar andinterstellar ultraviolet photons in the upper layers are likely causesof the depletion. CN is strongly detected in all disks, and the CN/HCNabundance ratio toward the Herbig Ae stars is even higher than thatfound in galactic photon-dominated regions, testifying to the importanceof photodissociation by radiation from the central object in the upperlayers. DCO+ is detected toward TW Hya,but not in other objects. The high inferredDCO+/HCO+ ratio of ˜0.035 is consistent withmodels of the deuterium fractionation in disks which include strongdepletion of CO. The inferred ionization fraction in the intermediateheight regions as deduced from HCO+ is at least10-11-10-10, comparable to that derived for themidplane from recent H2D+ observations. Comparisonwith the abundances found in cometary comae is made.Tables 3-5 are only available in electronic form athttp://www.edpsciences.org

ISO Spectroscopy of Gas and Dust: From Molecular Clouds to Protoplanetary Disks
Observations of interstellar gas-phase and solid-state species in the2.4 200 m range obtained with the spectrometers on board the InfraredSpace Observatory (ISO) are reviewed. Lines and bands caused by ices,polycyclic aromatic hydrocarbons, silicates, and gas-phase atoms andmolecules (in particular H2, CO, H2O, OH, andCO2) are summarized and their diagnostic capabilitiesillustrated. The results are discussed in the context of the physicaland chemical evolution of star-forming regions, includingphoton-dominated regions, shocks, protostellar envelopes, and disksaround young 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.

Carbon budget and carbon chemistry in Photon Dominated Regions
We present a study of small carbon chains and rings in Photon DominatedRegions (PDRs) performed at millimetre wavelengths. Our sample consistsof the Horsehead nebula (B33), the ρ Oph L1688 cloud interface, andthe cometary-shaped cloud IC 63. Using the IRAM 30-m telescope, the SESTand the Effelsberg 100-m telescope, we mapped the emission ofC2H, c-C3H2 and C4H, andsearched for heavy hydrocarbons such as c-C3H,l-C3H, l-C3H2,l-C4H2 and C6H. The large scale mapsshow that small hydrocarbons are present until the edge of all PDRs,which is surprising as they are expected to be easily destroyed by UVradiation. Their spatial distribution reasonably agrees with thearomatic emission mapped in mid-IR wavelength bands. C2H andc-C3H2 correlate remarkably well, a trend alreadyreported in the diffuse ISM (Lucas & Liszt \cite{Lucas2000}). Theirabundances relative to H2 are relatively high and comparableto the ones derived in dark clouds such as L134N or TMC-1, known asefficient carbon factories. The heavier species are however onlydetected in the Horsehead nebula at a position coincident with thearomatic emission peak around 7 μm. In particular, we report thefirst detection of C6H in a PDR. We have run steady-state PDRmodels using several gas-phase chemical networks (UMIST95 and the NewStandard Model) and conclude that both networks fail in reproducing thehigh abundances of some of these hydrocarbons by an order of magnitude.The high abundance of hydrocarbons in the PDR may suggest that thephoto-erosion of UV-irradiated large carbonaceous compounds couldefficiently feed the ISM with small carbon clusters or molecules. Thisnew production mechanism of carbon chains and rings could overcome theirdestruction by the UV radiation field. Dedicated theoretical andlaboratory measurements are required to understand and implement theseadditional chemical routes.Appendix A is only available in electronic form athttp://www.edpsciences.org

Some empirical estimates of the H2 formation rate in photon-dominated regions
We combine recent ISO observations of the vibrational ground state linesof H2 towards Photon-Dominated Regions (PDRs) withobservations of vibrationally excited states made with ground-basedtelescopes in order to constrain the formation rate of H2 ongrain surfaces under the physiconditions in the layers responsible forH2 emission. We briefly review the data available for fivenearby PDRs. We use steady state PDR models in order to examine thesensitivity of different H2 line ratios to the H2formation rate R_f. We show that the ratio of the 0-0 S(3) to the 1-0S(1) line increases with Rf but that one requires independentestimates of the radiation field incident upon the PDR and the densityin order to infer Rf from the H2 line data. Weconfirm earlier work by \cite{habart2003a} on the Oph W PDR which showedthat an H2 formation rate higher than the standard value of 3× 10-17 cm3 s-1 inferred from UVobservations of diffuse clouds is needed to explain the observedH2 excitation. From comparison of the ISO and ground-baseddata, we find that moderately excited PDRs such as Oph W, S140 and IC 63require an H2 formation rate of about five times the standardvalue whereas the data for PDRs with a higher incident radiation fieldsuch as NGC 2023 and the Orion Bar can be explained with the standardvalue of Rf. We compare also the H21-0 S(1) lineintensities with the emission in PAH features and find a rough scalingof the ratio of these quantities with the ratio of local density toradiation field. This suggests but does not prove that formation ofH2 on PAHs is important in PDRs. We also consider someempirical models of the H2 formation process with the aim ofexplaining these results. Here we consider both formation on classicalgrains of size roughly 0.1 μm and on very small (˜10 Å)grains by either direct recombination from the gas phase (Eley-Ridealmechanism) or recombination of physisorbed H atoms with atoms in achemisorbed site. We conclude that indirect chemisorption where aphysisorbed H-atom scans the grain surface before recombining with achemisorbed H-atom is most promising in PDRs. Moreover small grainswhich dominate the total grain surface and spend most of their time atrelatively low (below 30 K for χ ≤ 3000) temperatures may be themost promising surface for forming H2 in PDRs.

A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).

The Trumpler 14 photodissociation region in the Carina Nebula
We report the results of observations of the fine-structure emissionlines [C II] 158 μm and [O I] 63 μm using FIFI on the KuiperAirborne Observatory (KAO) and the Long Wavelength Spectrometer (LWS) onboard ISO, towards the molecular cloud associated with the stellarcluster Trumpler 14 (Tr 14) in the Carina Nebula. These data arecompared with selected CO and CS transitions obtained with the SEST aswell as IRAS and MSX images to produce a detailed view of the morphologyand the physical conditions prevailing in the photodissociation region(PDR) at the interface between the ionized gas and the molecular dustlane. The relative intensity distribution observed for the varioustracers is consistent with the stratification expected for a molecularcloud seen edge-on and exposed to a radiation field of ~ 104G_0, which is dominated by the most massive stars of Tr 14. The grainphotoelectric heating efficiency, \epsilon, is estimated to be ~5 x10-3 and is comparable to other galactic PDRs. The moleculargas has a complicated velocity structure with a high velocity dispersionresulting from the impact of the stellar winds arising from Tr 14. Thereis evidence of small-scale clumping with a very low volume fillingfactor. Despite the rich concentration of massive O stars in Tr 14 wefind that the parameters of the PDR are much less-extreme than those ofthe Orion and M 17 massive star-forming regions.

A multiwavelength study of the S 106 region. II. Characteristics of the photon dominated region
The O star S 106 IR powers a bright, spatially extended 10'x 3' (1.75x0.5 pc at a distance of 600 pc) photon dominated region (PDR) traced byour observations of FIR fine structure lines and submm moleculartransitions. The [C II] 158 mu m, [C I] 609 and 370 mu m, CO 7->6,and CO 4->3 measurements probe the large scale (1.2 pc) PDR emission,whereas [O I] 63 mu m, CN N=3->2, and CS J=7->6 observations arefocused on the immediate ( ~ 1' (0.2 pc)) environment of S 106 IR. A hot(T>200 K) and dense (n>3 *E5 cm-3) gascomponent (emission peaks of [C I] 158 mu m, CO 7->6, and CO 4->3)is found at S 106 IR. Cooler gas associated with the bulk emission ofthe molecular cloud is characterized by two emission peaks (one close(20'' east) to S 106 IR and one 120'' to the west) seen in the [C I] andlow-J (Jup<4) CO emission lines. In the immediateenvironment of the star, the molecular and [C I] lines showhigh-velocity emission due to the interaction of the cloud with thestellar wind of S 106 IR. The intensities of the FIR lines measuredwith the KAO are compared to those observed with the ISO LWS towards twopositions, S 106 IR and 120'' west. We discuss intensities and lineratios of the observed species along a cut through the molecular cloud/HII region interface centered on S 106 IR. The excitation conditions(Tex, opacities, column densities) are derived from an LTEanalysis. We find that the temperature at the position of S 106 IRobtained from the [C I] excitation is high (>500 K), resulting insubstantial population of the energetically higher3P2 state; the analysis of the mid- and high-J COexcitation confirms the higher temperature at S 106 IR. At thisposition, the [O I] 63 mu m line is the most important cooling line,followed by other atomic FIR lines ([O III] 52 mu m, [C II] 158 mu m)and high-J CO lines, which are more efficient coolants compared to [C I]2->1 and 1->0. We compare the observed line ratios toplane-parallel PDR model predictions and obtain consistent results forUV fluxes spanning a range from 102 to 103.5G0 and densities around 105 cm-3 onlyat positions away from S 106 IR. Towards S 106 IR, we estimate a densityof at least 3*E5 at temperatures between 200 and 500 K fromnon-LTE modelling of the CO 16->15/14->13 ratio and the CO 7->6intensity. Our new observations support the picture drawn in the firstpart of this serie of papers that high-density (n>105cm-3) clumps with a hot PDR surface are embedded in low- tomedium density gas (nle10 4 cm-3).Appendix A is only available in electronic form athttp://www.edpsciences.org

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 ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141

Rocket Observations of Far-Ultraviolet Dust Scattering in NGC 2023
The reflection nebula NGC 2023 was observed by a rocket-borne long-slitimaging spectrograph in the 900-1400 Å bandpass on 2000 February11. A spectrum of the star, as well as that of the nebular scatteredlight, was recorded. Through the use of a Monte Carlo modeling process,the scattering properties of the dust were derived. The albedo is low,0.2-0.4, and decreasing toward shorter wavelengths, while the phasefunction asymmetry parameter is consistent with highlyforward-scattering grains, g~0.85. The decrease in albedo, while theoptical depth increases to shorter wavelengths, implies that the far-UVrise in the extinction curve is due to an increase in absorptionefficiency.

The Photophysics of the Carrier of Extended Red Emission
Interstellar dust contains a component that reveals its presence byemitting a broad unstructured band of light in the 540-950 nm wavelengthrange, referred to as extended red emission (ERE). The presence ofinterstellar dust and ultraviolet photons are two necessary conditionsfor ERE to occur. This is the basis for suggestions that attribute EREto an interstellar dust component capable of photoluminescence. In thisstudy, we have collected all published ERE observations withabsolute-calibrated spectra for interstellar environments, where thedensity of ultraviolet photons can be estimated reliably. In each case,we determined the band-integrated ERE intensity, the wavelength of peakemission in the ERE band, and the efficiency with which absorbedultraviolet photons are contributing to the ERE. The data show thatradiation is not only driving the ERE, as expected for aphotoluminescence process, but is modifying the ERE carrier, asmanifested by a systematic increase in the ERE band's peak wavelengthand a general decrease in the photon conversion efficiency withincreasing densities of the prevailing exciting radiation. The overallspectral characteristics of the ERE and the observed high quantumefficiency of the ERE process are currently best matched by the recentlyproposed silicon nanoparticle (SNP) model. Using the experimentallyestablished fact that ionization of semiconductor nanoparticles quenchestheir photoluminescence, we proceeded to test the SNP model bydeveloping a quantitative model for the excitation and ionizationequilibrium of SNPs under interstellar conditions for a wide range ofradiation field densities. With a single adjustable parameter, the crosssection for photoionization, the model reproduces the observations ofERE intensity and ERE efficiency remarkably well. The assumption thatabout 50% of the ERE carriers are neutral under radiation conditionsencountered in the diffuse interstellar medium leads to a prediction ofthe single-photon ionization cross section of SNPs with average diameter3.5 nm of <=3.4×10-15 cm2. The shift ofthe ERE band's peak wavelength toward larger values with increasingradiation density requires a change of the size distribution of theactively luminescing ERE carriers through a gradual removal of thesmaller particles by size-dependent photofragmentation. We propose thatheat-assisted Coulomb decay of metastable, multiply charged SNPs is sucha process, which selectively removes the smaller components of anexisting SNP size distribution.

The Location of the Dense and Ionized Gas in the NGC 2023 Photon-dominated Region
The VLA and the BIMA array were used to obtain high-resolution (10"-20")observations of C+, traced by the C91α recombinationline at 8.6 GHz, and the dense molecular gas, traced by HCN andHCO+(1-0), of the photon-dominated region (PDR) associatedwith the reflection nebula NGC 2023. Using the VLA, continuum emissionis detected at 8.6 GHz from a faint H II region associated with HD37903. The C91α emission originates from a 0.4 pc long filament,extending from the east to the south of the exciting star HD 37903.Within the filament three C91α clumps can be distinguished, eachassociated with filamentary vibrationally excited H2 emissionin the direction toward HD 37903. The HCO+ emission has aclumpy appearance superimposed on a more extended component. C91αis, in general, closer to the exciting star than HCO+emission as expected from PDR models. The morphologies ofHCO+ and HCN are quite similar. Based on the C91α linewidth toward one of the clumps, a limit of 170 K on the kinetictemperature in the ionized carbon layer can be derived. This value isconsistent with PDR models with H2 densities of about105 cm-3. However, this result suggestssurprisingly low limits on the turbulence in the PDR. We detected acompact 3 mm continuum source in the PDR, which appears to be a cold``core'' of density 107 cm-3, 0.03 pc diameter,and 6 Msolar. We conclude that it may have formed within thePDR. In an appendix, observations of the C91α recombination linetoward five additional PDRs using the Effelsberg 100 m telescope aredescribed.

A New Probe of the Molecular Gas in Galaxies: Application to M101
Recent studies of nearby spiral galaxies suggest that photodissociationregions (PDRs) are capable of producing much of the observed H I ingalaxy disks. In that case, measurements of the observed H I columndensity and the far-ultraviolet (FUV) photon flux responsible for thephotodissociation process provide a new probe of the volume density ofthe local underlying molecular hydrogen. We develop the method and applyit to the giant Scd spiral galaxy M101 (NGC 5457). The H I columndensity and amount of FUV emission have been measured for a sample of 35candidate PDRs located throughout the disk of M101 using the Very LargeArray and the Ultraviolet Imaging Telescope. We find that, aftercorrection for the best-estimate gradient of metallicity in theinterstellar medium (ISM) of M101 and for the extinction of theultraviolet emission, molecular gas with a narrow range of density from30-1000 cm-3 is found near star-forming regions at all radiiin the disk of M101 out to a distance of 12'~26 kpc, close tothe photometric limit of R25~13.5′. In this picture,the ISM is virtually all molecular in the inner parts of M101. Thestrong decrease of the H I column density in the inner disk of thegalaxy at RG<10 kpc is a consequence of a strong increasein the dust-to-gas ratio there, resulting in an increase of theH2 formation rate on grains and a corresponding disappearanceof hydrogen in its atomic form.

Molecular abundance variations in the Magellanic Clouds
We have observationally studied the effect of metallicity andfar-ultraviolet (FUV) radiation on the physical conditions and themolecular abundances in interstellar clouds in the Small and the LargeMagellanic Clouds (SMC and LMC, respectively). Spectral line emissionfrom a number of molecules was observed in a sequence of clouds withpositions in and between the 30 Doradus (30Dor) and the southern part ofthe N159 region in the LMC, and in one cloud (N27, also denoted LIRS 49)located in the SMC bar. Physical conditions and molecular abundanceswere estimated from the observational data by excitation and radiativetransfer calculations. A comparison of the molecular abundances inclouds in the SMC, the LMC, and the Galaxy is presented. We also reportthe first detection of hydrogen sulphide (ortho-H2S) in anextragalactic source, detections of methanol (CH3OH) inthermal emission and methyl acetylene (CH3CCH), and atentative detection of thio-formaldehyde (H2CS) in N159W. Theabundances (relative to H_2) of molecular species (except CO) in the LMCsources and in N27 are estimated to be typically 5x10(-10) , and1x10(-10) , respectively. These values apply to the gas volume definedby the CO line emission. Relative to Galactic clouds, the abundances inN159W (our reference cloud) are five to twenty times lower. In two ofthe clouds: N27 and the centremost cloud in 30Dor (30Dor-10), thederived abundances deviate significantly from those in the other cloudsin our sample, by being on the average six and eight times lower,respectively. In N27, the most likely explanation is the lowermetallicity in the SMC, whereas the underabundance in 30Dor-10 isprobably mainly caused by a more rapid photodissociation due to the moreintense FUV radiation in this area. An alternative explanation for theunderabundances in both N27 and 30Dor-10 would be a higher H/H_2 ratioinside these molecular clouds. The ethynyl radical (C_2H), with anestimated average abundance of 5x10(-9) in seven clouds in the LMC and3x10(-9) in two clouds in the SMC, is the most abundant observed tracemolecule after the CO isotopomers. Qualitatively, the high C_2Habundance can be explained as reflecting the C(+) -rich and FUVphoton-rich environment, i.e., a chemistry characteristic forphoton-dominated regions. For N27 we have, using HCO(+) and H(13) CO(+)data, estimated the gas-phase (12) C/(13) C ratio to be 40-90, a rangethat encompasses the values found in N159W and in Galactic disc clouds.In all clouds in our sample, the number density estimates from anexcitation analysis of CS, SO, HCO(+) , HCN and H_2CO are in the range(1-100)*E(4) cm(-3) . CO data gives a lower limit of a few*E(3) cm(-3) .However, the average densities (estimated from the virial mass) aresignificantly lower, typically a fewx10(2) cm(-3) , suggesting that theclouds (as probed by trace molecules) are very clumpy withvolume-filling factors <<1. In N159W, where our data-base is byfar most extensive, the number density and the kinetic temperature ofmolecular hydrogen in the dense part of the gas are estimated to be(1-10)*E(5) cm(-3) and 25+/-10 K, respectively. The correspondingnumbers in N27 are, although based on less data than in N159W,(5-50)x10(4) cm(-3) and 15+/-5 K. Thus, the metallicity differencebetween the LMC and the SMC does not seem to affect the density and thetemperature of the gas dramatically. In the SMC, the CO(J=1-0)/HCO(+)(J=1-0) line intensity ratio follows the same trend with respect to thestar-formation activity as in the LMC: a lower ratio is found in cloudswith a more vigorous star-formation activity. A similar trend is alsoexhibited by the CO(J=1-0)/C_2H(N=1-0) line intensity ratio in the LMC.Based on observations using the Swedish-ESO Submillimetre Telescope(SEST) at the European Southern Observatory (ESO), La Silla, Chile.Figs. 3-14 are only available in the electronic version of this paper.

Large-Scale C I Emission from Molecular Clouds with Associated Ultraviolet Sources
We present large-scale observations of the ^3P_1-->^3P_0 transitionof neutral carbon, as well as observations of ^12CO, ^13CO, and C^18O,in four molecular clouds with nearby ultraviolet sources: W3, L1630/NGC2024, S140, and Cep A. The observations cover approximately 30^'x30^'regions with a 3' beam. A typical C^0 column density is about 1.6x10^17cm^-2. The overall extent and morphology of the C I emission is similarto that of the ^12CO and ^13CO J=2-->1 emission. There is a strongcorrelation of C I and ^13CO line intensities. The column densities andline strengths of the C I lines imply that these lines arise in gas atthe edge of the molecular cloud that is dissociated by ultravioletradiation. The correlation of C I and ^13CO intensity can arise as aresult of a combination of column density variations, volume densityvariations, and unresolved cloud structure. In the latter case, however,any unresolved structures containing both C I and CO need not be morethan a few times smaller than the telescope beam. Taken together withC^+ observations, the C I data imply that only about half of thegas-phase carbon in molecular clouds is in CO. The observations indicatethat the large ^13CO/C^18O abundance ratio (~20) seen at the edges ofmolecular clouds results from isotope-selective photodissociation ofC^18O.

The Ionization Fraction in Dense Molecular Gas. II. Massive Cores
We present an observational and theoretical study of the ionizationfraction in several massive cores located in regions that are currentlyforming stellar clusters. Maps of the emission from the J=1-->0transitions of C^18O, DCO^+, N_2H^+, and H^13CO^+, as well as theJ=2-->1 and 3-->2 transitions of CS, were obtained for each core.Core densities are determined via a large velocity gradient analysiswith values typically of ~10^5 cm^-3. With the use of observations toconstrain variables in the chemical calculations, we derive electronfractions for our overall sample of five cores directly associated withstar formation and two apparently starless cores. The electronabundances are found to lie within a small range, -6.9

Far-Ultraviolet Fluorescence of Molecular Hydrogen in IC 63
We present observations of H 2 fluorescence at wavelengthsbetween 1000 and 1200 A from the bright reflection nebula IC 63.Observations were performed with the Berkeley spectrograph aboard theORFEUS II mission. To the best of our knowledge, this is the firstdetection of astrophysical H 2 fluorescent emission at thesewavelengths (excluding planetary atmospheres). The shape of the spectrumis well described by the model of Sternberg. The absolute intensity,however, is fainter by a factor of 10 than an extrapolation fromobservations at longer ultraviolet wavelengths. Of the mechanisms thatmight help to reconcile these observations, optical depth effects in thefluorescing H 2 itself are the most promising (or at leastthe most difficult to rule out).

Carbon and Oxygen Depletion and Extinction in the Translucent Cloud toward HD 24534 (X Persei)
Recent studies of the gas-phase abundances of carbon and oxygen indiffuse clouds have suggested that the depletions of both elements areinvariant and independent of extinction curve properties. We show,however, that in the line of sight toward HD 24534 (X Persei), thedepletion of carbon is at least a factor of 3 greater than in thediffuse clouds previously surveyed. For HD 24534, the interstellargas-phase C/H ratio is lower than the mean of diffuse sight lines by afactor of about 3.5. We conclude that the carbon depletion may beenhanced in translucent molecular clouds and also that carbon begins toshow an enhanced level of depletion at lower extinctions than oxygen,which is also discussed. We argue that it is imperative to measureabundances and depletions in translucent and denser clouds in order tounderstand the interaction between gas and dust as the thicknesses anddensities of clouds increase.

New chemical models for dense portions of NGC 7023
We model the steady-state chemistry of two of the dense portions in NGC7023, one of the most famous photon-dominated regions, with an extensivegas-phase chemical network including sulphur-bearing molecules. Ourresults are in reasonable agreement with observation, and we calculatethe abundances of many additional molecules not included in an earliermodel, such as CS, which has been detected in the source. Our resultsfor CS show a severe overproduction, unless a significant depletion ofthe gas-phase elemental sulphur abundance is assumed. Additionalpredictions of abundances for molecules not yet detected in theseregions, especially sulphur-bearing species, are provided.

A radio continuum and H i study of IC 63, IC 59, and IRAS 00556+6048: nebulae in the vicinity of gamma CAS
Sh 185, which is associated with the B0-B0.5IV star gamma Cas andcontains the two nebulae IC 63 and IC 59, has been observed with theDRAO synthesis telescope in continuum emission at 408 and 1420MHz, andin H i line emission. Continuum emission is clearly detected for IC 63and weakly detected for IC 59. The emission from both nebulae isthermal, with the masses of ionized gas being 0.08 and 0.07 Msolar,respectively. The masses of H i detected are 0.15 Msolar for IC 63, and0.64 Msolar for IC 59. Infrared emission was detected in the IRAS surveyfrom both IC 63 and IC 59, and also from a point source, IRAS00556+6048, located between the two clouds. The infrared and radioluminosities of IC 63 and IC 59 are consistent with the heating andionization being produced by gamma Cas. However, the time required toproduce the observed H i through photodissociation is less than anyreasonable age for gamma Cas by orders of magnitude. This suggests thatthe nebulae are density-bounded and the production of H i throughphotodissociation by radiation from gamma Cas has mostly ceased. Theexception occurs in a small region of molecular gas in IC 63, whereextended red emission and molecular hydrogen fluorescent emission arestill observed. H i is also detected near IRAS 00556+6048. This H iappears to be associated with the infrared source, and with a velocityof -34kms^-1 suggests that the infrared source is not associated with Sh185. The exciting star for IRAS 00556+6048 may be another example of adissociating star.

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