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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.
| 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.
| 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
| 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).
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141
| 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.
| 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).
| 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.
| Near-Infrared Spectra of IC 59/63 and NGC 1535: Comparing Infrared and Ultraviolet Observations of H 2
We present observations of near-infrared H2 line emission toward thereflection/emission nebulae, IC 59 and IC 63, and the planetary nebula,NGC 1535. Each source has been observed previously in the ultraviolet,where H2 was detected in emission toward IC 63 and in absorption towardNGC 1535. In IC 63, we have detected the 1.601 mu m v = 6--4 Q(1), 2.121mu m v = 1--0 S(1), and 2.247 mu m v = 2--1 S(1) lines of H2 arisingfrom a near-infrared fluorescent cascade following ultraviolet continuumpumping. The detection marks the first time that both infrared andultraviolet portions of the H2 fluorescent cascade have been measured ina region exposed to far-ultraviolet continuum photons. Furthermore, wealso report 1--0 S(1) and 2--1 S(1) fluorescent emission toward IC 59, asource previously thought to display no H2 fluorescence and considereddevoid of molecules based on ultraviolet and CO observations. Toward NGC1535, we find no H2 emission in the near-infrared, in spite of thereported ultraviolet H2 absorption.
| Physical and chemical structure of the IC 63 nebula. 1: Millimeter and far-infrared observations
We present results of a (sub)millimeter and far-infrared study of thereflection/emission nebula IC 63, located close to the BO.5p star gammaCas. The source has been mapped in the (12)CO 2 - 1 and 3 - 2, (13)CO 2- 1, and CS 2 - 1 lines and shows a small molecular cloud less than 1'x2' in extent, which coincides with the brightest optical nebulosity andIRAS 100 micrometer emission. IC 63 is therefore an excellent example ofa nearby (d approximately = 230 pc), edge-on photon-dominated region(PDR). Various other molecules have been observed at the peak positionthrough their rotational transitions, in order to probe the physicalparameters and to derive abundances. The measured CO, HCO(+) HCN, CS andH2CO line ratios suggest that the cloud is warm, T approximately = 50 K,and dense, n (H2) approximately = 5 x 104/cu cm. Excitationof molecules by electrons may play a significant role in this PDR. Onthe basis of these physical conditions, column densities have beendetermined from the observed line strengths. Several different methodsare discussed to constrain the H2 column density, including the use ofmeasured submillimeter continuum fluxes. The resulting abundances ofspecies such as CN and CS are similar to those found in cold, darkclouds like TMC-1 and L134N. However, the abundances of other simplemolecules such as HNC, HCO(+) and possibly C2H are lower by factors ofat least three, probably because of the enhanced photodissociation ratesat a distance of 1.3 pc from a B star. Surprisingly, only the abundanceof the H2S molecule appears enhanced. More complex, volatile moleculessuch as CH3OH CH3CN and HNCO, and the sulfur-oxides SO and SO2 have notbeen found in this cloud. Limited observations of molecules in thereflection nebulea NGC 2023 are presented as well, and the resultingmolecular abundances are compared with those found for IC 63.
| Spectroscopy of extended red emission in reflection nebulae
This paper reports the results of a spectroscopic survey of reflectionnebulae, aimed at studying the characteristics of the broad emissionfeature responsible for the extended red emission (ERE) observed inthese objects. The ERE band was detected and its strength, centralwavelength, and width were measured in 12 nebulae, while upper limitswere recorded in 12 further objects. The maximum ERE band intensity wasfound to vary from object to object within an extremely narrow range,while the associated scattered light intensities differed by nearly twoorders of magnitude. The ERE band is interpreted as arising from thephotoluminescence of hydrogenated amorphous carbon (HAC) grains whichbecome rehydrogenated and gain luminescence efficiency in narrow H2photodissociation zones. These zones are probably thin shells around theexciting stars. Observed changes in the central wavelengths and widthsof the ERE bands detected in different parts of a given nebula, and fromnebula to nebula, support the HAC model and are explained as arisingfrom variations in the degree of hydrogenation to the solid grains.
| A Star-Hop in Cassiopeia
Not Available
| UV fluorescence of molecular hydrogen and red dust emission in the Gamma Cassiopeiae nebula IC 63
Resonant fluorescence of molecular hydrogen has been observed in thereflection nebula IC 63 with the IUE satellite. Both the noncoherentresonant scattering and the dissociative continuum are shown to agreewell with molecular theory. Extended red emission is also found in thisnebula and may be physically related to the dissociation of H2.
| Time variability of Gamma Cassiopeiae in X-rays
A 22,000 s observation with Exosat of the X-ray source 4U 0053 +60/Gamma Cassiopeiae reveals oscillations in the X-ray flux with aperiod of about 6000 s. The source also exhibits chaotic variabilitywith time scales down to a few tens of seconds. This observationstrengthens the hypothesis that the X-ray source is a neutron star in awide orbit around the Be star Gamma Cas.
| Studies of bright diffuse galactic nebulae with special regard to their spatial distribution.
Not Available
| A general study of diffuse galactic nebulae.
Not Available
| Photographic nebulosities and star clusters connected with the Milky Way.
Not Available
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