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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.
| 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.
| Modeling the Neutral Hydrogen Interstellar Medium: A Better Kinematic Distance Tool
An advanced approach to the kinematic distance method is developed. Themethod is applicable to second- and third-quadrant Galactic objects withknown velocities. It is based on fitting a model of the density andvelocity features in an isothermal H I disk to observed H I data. Thevelocity field of the gas is modeled with a power law for basic circularrotation, underlying noncircular motions from a two-armed density wavepattern. With a reasonable number of adjustable parameters andconstraints the model reproduces observations toward many Galacticobjects, and accurate distances are found from the modeled velocityfield. High-resolution H I spectral line data from the Canadian GalacticPlane Survey (CGPS; Taylor et al.) are used to discriminate clouds fromthe intercloud medium (the ``stratum'') for which the model is intended.The ability of the model to reproduce these data is demonstrated in one-[Tb(v)] and two- [Tb(l, v)] dimensional fits.Distances to 22 H II regions and SNRs calculated by the fitted velocityfield compare extremely well with other kinematically independentdistances.
| Two new Perseus arm supernova remnants discovered in the Canadian Galactic Plane Survey
We report the discovery of two new second-quadrant supernova remnants,G96.0+2.0 and G113.0+0.2, in the data of the Canadian Galactic PlaneSurvey. The two SNRs are residents of the Perseus spiral arm atdistances of 4.0 kpc and 3.1 kpc, respectively. The distances weredetermined kinematically by associating the objects with neutralhydrogen and molecular material. G96.0+2.0 is most likely located at theedge of a large stellar wind bubble with a systemic velocity of about-44 km s-1. It consists of a relatively bright shell wherethe shock is encountering the wall of H I and slowly fades away towardsthe interior of the stellar wind bubble. The visible part of the remnanthas a diameter of about 30 pc and a radio spectral index of α ≈-0.66 (S ˜ ν^α), indicating that it is a shell-typeremnant in an early stage of development. The SNR is most likely theremnant of a type Ib/c supernova explosion. G113.0+0.2 is located in anarea of confusing thermal emission not far from the radio-brightsupernova remnant Cassiopeia A. It has an unusual elongated structureconsisting of a long polarized filament and a more complex headstructure that is interacting with a small molecular cloud; it residesin a butterfly-shaped H I cavity, probably a stellar wind bubble. It isabout 36 pc long and 15 pc wide at a position angle of 70° with theGalactic Plane. A pulsar with a relatively low period derivative, givingit a characteristic age of 10 million years, is located close to thecentre of the radio continuum emission at a Perseus arm distance.Whether the pulsar is the result of the same supernova explosion thatcreated G113.0+0.2 or if it was left behind by an earlier supernova thatalso shaped the stellar wind bubble remains uncertain.
| Wind-Blown Bubbles around Massive Stars. The effects of stellar wind and photo-ionization on the circumstellar Medium
We simulate the evolution of the circumstellar medium around a massivestar. We take the relevant input parameters for our simulation from astellar evolution calculation: mass loss rate, wind velocity and thenumber of ionizing photons as a function of time. By pursuing thecalculation through the various stages of massive star evolution, usinga realistic mass loss history as input, we simulate the creation andevolution of a wind-blown bubble around the star up to the time of thesupernova explosion. Unlike most previous work on this subject, weinclude the effects of photo-ionization. From our calculations we canconclude that photo-ionization affects both the size andmass-distribution of the circumstellar bubble during the main sequenceand red supergiant stage.
| Morphological Analysis of H I Features. I. Metric Space Technique
This is the first of two papers on the morphological analysis of H Ifeatures. In this first paper, we use the so-called metric spacetechnique, developed by F. C. Adams and J. Wiseman. The metric spacetechnique is an image analysis, mathematical formalism used toquantitatively compare astrophysical maps according to complexity.Instead of comparing maps on a pixel-by-pixel basis, we compare themaps' one-dimensional ``output functions,'' which characterize specificmorphological/physical aspects of the maps. The tool is used to analyze28 H I features of known origin taken from the Canadian Galactic PlaneSurvey (CGPS), where the maps are scaled at 18" per pixel (resolution of1cosδ arcmin). Technical and mathematical improvements to theformalism are presented. After classifying the 28 maps according tocomplexity, we searched for correlations between this complexity rankingand other quantifiable aspects of the H I features such as age, area, HI area, distance, flux from the ionizing star(s), fractal dimension, H Imass, and |z| (the absolute value of the height of the objects, above orbelow the Galactic plane). The most interesting correlations are (1) thehigher the flux of UV photons, the more complex is the photodissociatedH I feature, and (2) the older the supernova remnant, the more complexthe H I associated with it. There is no correlation between the fractaldimension of the maps and their complexity or their physicalcharacteristics, thus showing that the metric space technique could beused as a solution to the degeneracy of the fractal dimension.
| 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.
| Distances to Cepheid open clusters via optical and K-band imaging
We investigate the reddening and main-sequence-fitted distances to 11young, Galactic open clusters that contain Cepheids. Each clustercontains or is associated with at least one Cepheid variable star.Reddening to the clusters is estimated using the U-B:B-V colours of theOB stars and the distance modulus to the cluster is estimated via B-V:Vand V-K:V colour-magnitude diagrams. Our main-sequence fitting assumesthat the solar-metallicity zero-age main sequence of Allen appliesuniversally to all the open clusters, although this point iscontroversial at present. In this way we proceed to calibrate theCepheid period-luminosity (PL) relation and find MV=-2.87× logP- 1.243 +/- 0.09, MK=-3.44 × logP- 2.21 +/-0.10 and absolute distance moduli to the Large Magellanic Cloud (LMC) of18.54 +/- 0.10 from the V-band and 18.48 +/- 0.10 from the K-band givingan overall distance modulus to the LMC of μ0= 18.51 +/-0.10. This is in good agreement with the previous Cepheid PL-K result ofLaney & Stobie at μ0= 18.51 +/- 0.09 and with theHipparcos parallax-calibrated Cepheid PL-K estimate of Feast &Catchpole at μ0= 18.66 +/- 0.10 when no account is takenof the LMC metallicity.We also find that the two-colour U-B:B-V diagrams of two importantclusters are not well fitted by the standard main-sequence line. In onecase, NGC 7790, we find that the F stars show a UV excess and in thesecond case, NGC 6664, they are too red in U-B. Previous spectroscopicestimates of the metallicity of the Cepheids in these clusters appear tosuggest that the effects are not due to metallicity variations. Otherpossible explanations for these anomalies are positional variations inthe dust reddening law and contamination by foreground or backgroundstars.
| 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 Line-of-Sight Velocities of OB Associations and Molecular Clouds in a Wide Solar Neighborhood: The Streaming Motions of Stars and Gas in the Perseus Arm
We compare the line-of-sight velocities of stars in OB associationswithin 3 kpc of the Sun and the molecular clouds closest to them. ThePerseus arm is used as an example to show that the line-of-sightvelocity ranges in which OB-association stars, molecular clouds, and HII regions ionized by these association stars are observed overlap. Thestreaming motions produced by density waves in the (l, VLSR)plane were found to lead to an inversion of the distances to objects ofthe Perseus arm and the interarm space beyond this arm.
| Line profile variability in the spectra of Oef stars. I. BD + 60deg 2522
We report the analysis of a long term observing campaign designed tomonitor the spectroscopic variability of the Oef star BD+60deg 2522. We find that the double-peaked He {II} lambda4686 emission line undergoes strong profile variability on time scalesof 2-3 day. However, the time scale as well as the pattern of thesevariations turn out to be epoch dependent and the phenomenon is thusmost likely not ruled by a single stable clock. On the other hand, theabsorption lines in the spectrum of BD +60deg 2522 displayline profile variability on time scales of a few hours that might berelated to non-radial pulsations. We tentatively propose that thebeating of several non-radial pulsation modes triggers transientlarge-scale density perturbations in a confined stellar wind thatproduce the 2-3 day time scale variability.Based on observations collected at the Observatoire de Haute Provence,France.
| The effect of HII regions on rotation measure of pulsars
We have obtained new rotation measures for 11 pulsars observed with theEffelsberg 100-m radio telescope, in the direction of the Perseus arm.Using a combination of 34 published and the 11 newly measured pulsarrotation measures we study the magnetic field structure towards thePerseus arm. We find that two pulsars towards l ~ 149deg(Region 1) and four pulsars towards l ~ 113deg (Region 2)lie behind HII regions which seriously affect the pulsar rotationmeasures. The rotation measure of PSR J2337+6151 seems to be affected byits passage through the supernova remnant G114.3+0.3. For Region 1, weare able to constrain the random component of the magnetic field to 5.7mu G. For the large-scale component of the Galactic magnetic field wedetermine a field strength of 1.7+/-1.0 mu G. This average field isconstant on Galactic scales lying within the Galactic longitude range of85deg < l < 240deg and we find no evidencefor large scale field reversal upto 5-6 kpc. We highlight the greatimportance to include the effects of foreground emission in anysystematic study.
| Bubble Nebulae around Ultraluminous X-Ray Sources
The nature of extra-nuclear ultraluminous X-ray sources (ULX) in nearbygalaxies continues to be an enigma, since their adopted isotropichigh-energy output would surpass the Eddington limit of even the mostmassive stellar black holes. Many ultraluminous X-ray sources aresurrounded by emission nebulae that show indications of both shockionization and X-ray ionization. Relatively compact X-ray ionizednebulae can be used to independently infer the luminosities, and thus toexclude possible beaming effects into our line of sight. Largerbubble-like nebulae reach several hundred parsec diameters and provideimportant information on the formation and/or mass loss history of ULX.We point out the close relationship to microquasars and the previouslyunique SS 433 system with its radio nebula W 50.
| An Inside-Out View of Bubbles
Fast stellar winds can sweep up ambient media and form bubbles. Theevolution of a bubble is largely controlled by the content and physicalconditions of the shocked fast wind in its interior. This hot gas wasnot clearly observed until the recent advent of Chandra and XMM-NewtonX-ray observatories. To date, diffuse X-ray emission has beenunambiguously detected from two circumstellar bubbles blown by WR stars,four planetary nebulae, and two superbubbles blown by young clusters.Model fits to the X-ray spectra show that the circumstellar bubbles aredominated by hot gas with low temperatures ( < 3×10^6 K), whilethe interstellar bubbles contain significant fractions of hotter gas (< 5×10^6 K) . In all cases, large discrepancies in the X-rayluminosity are found between observations and conventional models ofbubbles. Future theoretical models of bubbles need to re-examine thevalidity of heat conduction and take into account realistic microscopicprocesses such as mass loading from dense clumps/knots and turbulentmixing. Chandra ACIS-S observation of NGC 6888 will shed light on theseastrophysical processes.
| Ring nebulae around massive stars throughout the Hertzsprung-Russell diagram
Massive stars evolve across the H-R diagram, losing mass along the wayand forming a variety of ring nebulae. During the main sequence stage,the fast stellar wind sweeps up the ambient interstellar medium to forman interstellar bubble. After a massive star evolves into a red giantor a luminous blue variable, it loses mass copiously to form acircumstellar nebula. As it evolves further into a WR star, the fast WRwind sweeps up the previous mass loss and forms a circumstellar bubble.Observations of ring nebulae around massive stars not only arefascinating, but also are useful in providing templates to diagnose theprogenitors of supernovae from their circumstellar nebulae. In thisreview, I will summarize the characteristics of ring nebulae aroundmassive stars throughout the H-R diagram, show recent advances in X-rayobservations of bubble interiors, and compare supernovae's circumstellarnebulae with known types of ring nebulae around massive stars.
| Hubble Space Telescope Observations of the Windblown Nebula NGC 7635
We present Hubble Space Telescope observations of the northern part ofNGC 7635, a circular shell around the O6.5 IIIf star BD +60°2522.The nebula, which lies within the large emission-line region S162, isnotable not only for its symmetric shell, but also for a complex of``cometary'' knots close to the central star. Our observations includespectra taken with the Space Telescope Imaging Spectrograph andnarrowband images taken with the Wide Field Planetary Camera 2. The highspatial resolution of these data reveals the knots to be the ionizededges of a much larger mass of neutral material, with strongphotoevaporative flows toward the central star. The cometary appearanceof the knots is produced by the intersection of two ridges, one in theplane of the sky and the other 65° relative to it. Stratification inthe emission from the shell can also be seen, the result of shockheating as material is swept into the expanding shell. We also see forthe first time a small loop of emission between the central star and thecometary knot complex. We propose that this was formed by the collisionbetween the strong stellar wind and the photoevaporative flow from theclosest and brightest of these knots. Based on observations made withthe NASA/ESA Hubble Space Telescope, obtained at the Space TelescopeScience Institute (STScI), which is operated by the Association ofUniversities for Research in Astronomy (AURA), Inc., under NASA contractNAS 5-26555.
| Analysis and Models of Photoionized Structures Seen in Hubble Space Telescope Images of NGC 7635
We present the analysis of photoionized structures seen in Hubble SpaceTelescope Wide Field Planetary Camera 2 images of NGC 7635 and thesurrounding H II region S162. We exploit the high spatial resolutionimages to make a census of the total ionizing flux from the O6.5 IIIfstar BD +60°2522. We also construct detailed models of twostructures with a photoionization code, reproducing the line emissionseen in spatial profiles extracted directly from the images. Wedemonstrate that the density distribution of the models producesdeviations in the spectrally derived elemental abundances at a levelexceeding typical observational errors. Our results also show a largediscrepancy between the star's effective temperature used for ourbest-fit models (34,320 K) and that expected based on its spectral type(>40,000 K) from a hot-star calibration taken from the literature. Wesuggest that this is due to the use of plane-parallel stellaratmospheres in constructing that calibration. Based on observations madewith the NASA/ESA Hubble Space Telescope, obtained at the SpaceTelescope Science Institute (STScI), which is operated by theAssociation of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555.
| 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.
| 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.
| Iron abundance in H II regions
Optical CCD spectra are used to determine the \element{Fe} abundances atseveral positions inside seven bright Galactic H Ii regions. Theobserved [Fe Iii] line ratios are compared with the predictions ofdifferent sets of collision strengths and transition probabilities forthis ion to select the atomic data providing the best fit to theobservations. The values found for the \element[++]{Fe} and\element[+]{Fe} abundances, along with ionization correction factors forthe contribution of \element[3+]{Fe}, obtained from available grids ofphotoionized models, imply that the Fe/O ratio in the ionized gas isbetween 2% and 30% of solar. The \element{Fe} abundances derived foreach area are correlated both with the degree of ionization and thecolour excess. A possible explanation is suggested, namely the presenceof a population of small grains, probably originating from thefragmentation of larger grains. These small grains would release\element{Fe} atoms into the gas after the absorption of energeticphotons; the small grains surviving this destruction process would beswept out of the ionized region by the action of radiation pressure orstellar winds. An indication of a further and more efficient destructionagent is given by the high \element{Fe} abundance derived for a positionsampling the optical jet H 399 in M20, where dust destruction due to shock waves has presumablytaken place. Based on observations made with the Isaac Newton Telescope,operated on the island of La Palma by the Isaac Newton Group in theSpanish Observatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias.
| Extended Red Emission in the Evil Eye Galaxy (NGC 4826)
NGC 4826 (M64) is a nearby Sab galaxy with an outstanding, absorbingdust lane (called the Evil Eye) asymmetrically placed across itsprominent bulge. In addition, its central region is associated withseveral regions of ongoing star formation activity. We obtained accuratelow-resolution (4.3 Å pixel-1) long-slit spectroscopy(KPNO 4 m) of NGC 4826 in the 5300-9100 Å spectral range, with aslit of 4.4′ length, encompassing the galaxy's bulge size,positioned across its nucleus. The wavelength-dependent effects ofabsorption and scattering by the dust in the Evil Eye are evident whencomparing the observed stellar spectral energy distributions (SEDs) ofpairs of positions symmetrically located with respect to the nucleus,one on the dust lane side and one on the symmetrically opposite side ofthe bulge, under the assumption that the intrinsic (i.e., unobscured)radiation field is to first-order axisymmetric. We analyzed the SEDratios for a given number of pairs of positions through themultiple-scattering radiative transfer model of Witt & Gordon. As amain result, we discovered strong residual extended red emission (ERE)from a region of the Evil Eye within a projected distance of about 13"from the nucleus, adjacent to a broad, bright H II region, interceptedby the spectrograph slit. ERE is an established phenomenon well-coveredin the literature and interpreted as originating from photoluminescenceby nanometer-sized clusters, illuminated by UV/optical photons of thelocal radiation field. In the innermost part of the Evil Eye, the EREband extends from about 5700 to 9100 Å, with an estimated peakintensity of ~3.7×10-6 ergs s -1Å-1 cm-2 sr-1 near 8300 Åand with an ERE to scattered light band integrated intensity ratio,I(ERE)/I(sca), of about 0.7. At farther distances, approaching thebroad, bright H II region, the ERE band and peak intensity shift towardlonger wavelengths, while the ERE band-integrated intensity, I(ERE),diminishes and, eventually, vanishes at the inner edge of this H IIregion. The radial variation of I(ERE) and I(ERE)/I(sca) does not matchthat of the optical depth of the model derived for the dust lane. Bycontrast, the radial variation of I(ERE), I(ERE)/I(sca) and of the EREspectral domain seems to depend strongly on the strength and hardness ofthe illuminating radiation field. In fact, I(ERE) and I(ERE)/I(sca)diminish and the ERE band shifts toward longer wavelengths when both thetotal integrated Lyman continuum photon rate,Q(H0)TOT, and the characteristic effectivetemperature, Teff, of the illuminating OB stars increase.Q(H0)TOT and Teff are estimated fromthe extinction-corrected Hα (λ=6563 Å) line intensityand line intensity ratios [N II] (λ6583)/Hα and [SII](λλ6716+6731)/Hα, respectively, and areconsistent with model and observed values typical of OB associations.Unfortunately, we do not have data shortward of 5300 Å, so thatthe census of the UV/optical flux is incomplete. The complex radialvariation of the ERE peak intensity and peak wavelength of I(ERE) andI(ERE)/I(sca) with optical depth and strength of the UV/opticalradiation field is reproduced in a consistent way through thetheoretical interpretation of the photophysics of the ERE carrier bySmith & Witt, which attributes a key role to the experimentallyestablished recognition that photoionization quenches the luminescenceof nanoparticles. When examined within the context of ERE observationsin the diffuse interstellar medium (ISM) of our Galaxy and in a varietyof other dusty environments, such as reflection nebulae, planetarynebulae, and the Orion Nebula, we conclude that the ERE photonconversion efficiency in NGC 4826 is as high as found elsewhere but thatthe size of the actively luminescing nanoparticles in NGC 4826 is abouttwice as large as those thought to exist in the diffuse ISM of ourGalaxy.
| Iron abundance in Galactic H II regions
Some recent results point to a possible discrepancy between the Feabundance in M42 as derived from the emission of the different Fe ions,but it is argued that the uncertainties are currently too high to decidewhether the discrepancy is real. The results of a recent analysis of theFe abundance in 7 Galactic H II regions are discussed as well as theimplications they could have for dust in H II regions.
| Crystalline silicon nanoparticles as carriers for the Extended Red Emission
In an attempt to determine the carrier of the Extended Red Emission(ERE), we have investigated a series of amorphous and crystallinematerials: natural coal, amorphous hydrogenated carbon, amorphoushydrogenated silicon carbide, porous silicon, and crystalline siliconnanoparticles. The photoluminescence (PL) behavior of various samples ofthese materials upon excitation with UV light was studied at roomtemperature focusing on both the wavelength dependence of thephotoluminescence and the PL yield. For some samples the yield is by fartoo low, other samples do not comply with the characteristic wavelengthrange of ERE. Only the samples of nanocrystalline silicon (poroussilicon and silicon nanoparticles) reveal PL properties that arecompatible with the astronomical observations. Besides this experimentalevidence, we will supply additional arguments leading to the conclusionthat silicon nanoparticles should be seriously considered as anattractive carrier for the Extended Red Emission.
| Hα Emission Stars in the Cepheus OB3 Region
Hα emission objects were searched for, using the objective-prismSchmidt plates in a 36 deg^2 sky area covering the Cepheus OB3association. Hundred and eight emission stars have been found, 68 ofwhich are new findings to our knowledge. The properties of the IRASpoint sources identified with Hα emission stars indicate that mostof the Hα emission stars are T Tauri stars or related objects.Hα emission stars associated with IRAS sources and thoseunassociated have different distributions, suggesting an age sequenceamong them. Altogether, they show a ring- or shell-like surfacedistribution, which almost coincides with that of heated dusts, andthese distributions appear to be encompassed by an HI shell.
| An Empirical Test and Calibration of H II Region Diagnostics
We present spectrophotometry in the 3600-9700 Å region for asample of 39 H II regions in the Galaxy and Magellanic Clouds, for whichindependent information is available on the spectral types and effectivetemperatures of the ionizing stars. The spectra have been used toevaluate nebular diagnostics of stellar temperature, metal abundance,and ionization parameter, and to compare the observed behavior of theline indices with predictions of nebular photoionization models. Weobserve a strong degeneracy between forbidden-line sequences produced bychanges in stellar Teff and metal abundance, which severelycomplicates the application of many forbidden-line diagnostics toextragalactic H II regions. Our data confirm however that the Edmunds& Pagel [O II]+[O III] abundance index and the Vílchez &Pagel η' index provide more robust diagnostics of metalabundance and stellar effective temperature, respectively. A comparisonof the fractional helium ionization of the H II regions with stellartemperature confirms the reliability of the spectral type versusTeff calibration for the relevant temperature rangeTeff<=38,000 K. We use empirical relations between thenebular hardness indices and Teff to reinvestigate the casefor systematic variations in the stellar effective temperatures and theupper initial mass functions of massive stars in extragalactic H IIregions. The data are consistent with a significant softening of theionizing spectra (consistent with cooler stellar temperatures) withincreasing metal abundance, especially for Z<=Zsolar.However, unresolved degeneracies between Z and Teff stillcomplicate the interpretation of this result.
| A CO Survey of Giant Molecular Clouds near Cassiopeia A and NGC 7538
With the 1.2 m telescope at the Center for Astrophysics we haveundertaken a survey of the J=1-->0 CO line toward the conspicuouscomplex of giant molecular clouds (GMCs) in the Perseus arm in theregion of Cas A and NGC 7538. The survey, at a spectral resolution of250 kHz (0.65 km s-1), on a uniform grid of points spacedevery 0.125d in latitude and longitude, covers 55 deg2between l=107deg and 116°. A smaller area of ~15deg2 between l=108deg and 113° was similarlysurveyed in 13CO. In the region covered in both isotopicspecies we find at least seven GMCs with masses on the order of105 Msolar. An intensity-weighted radius gives amore meaningful measure of cloud size than the simple geometrical areaand is best used to estimate the virial mass. The ratio of total cloudluminosities in CO and 13CO, S12/S13,ranges from 6 to 10, with a mean of 8.5. The distribution of moleculargas is very similar in CO and 13CO, and thevelocity-integrated intensities at each point are closely correlated. Inthe (l,v)-diagram the Perseus arm is kinematically separated from thelocal arm by an interarm gap that is nearly free of CO; the contrast inmolecular gas surface density between the Perseus arm and the interarmgap is apparently at least 20.
| The abundances of O, S, Cl, N, Ar, He and C in seven Galactic H bt II regions regions
An analysis of CCD spectra in the range lambda lambda4200 -8775,obtained at several positions of seven bright Galactic H Ii regions,with galactocentric distances in the range 6-10 kpc, is presented.Temperatures, densities and ionic abundances are calculated for all thepositions, and the relative trends followed by these parameters areestablished. It is shown that all these H Ii regions share the sameabundances of \element{O}, \element{S}, \element{Cl}, \element{N},\element{Ar} and \element{He}, within their expected uncertainties, aresult which is consistent with a flat or small abundance gradient inthe Galactic disc within +/-2 kpc from the Sun, as suggested by somestudies of the abundances in H Ii regions and B-type stars. The same setof homogeneously derived ionic abundances is also used to test theaccuracy of the fractional ionization predicted by available grids ofmodels for photoionized nebulae. Based on observations made with theIsaac Newton Telescope, operated on the island of La Palma by the IsaacNewton Group in the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofísica de Canarias.
| ^3He in the Milky Way Interstellar Medium: Abundance Determinations
We are making precise determinations of the abundance of the lightisotope of helium, ^3He. The ^3He abundance in Milky Way sourcesprovides strong constraints for theories of stellar evolution, chemicalevolution, and cosmology. Here we report on new ^3He^+/H^+ abundancedeterminations for a sample of 21 Galactic H II regions observed for thepast 14 yr with the NRAO 140 Foot telescope. The abundances are based onimproved ^3He^+ observations, as well as new models for source densitystructure. The models are constrained by the observed ^3He^+ emission,the strength of the H and ^4He recombination lines, and the continuumemission at 3.5 cm wavelength. Density structure serves only to increasethe actual ^3He^+/H^+ abundance ratio over that derived using a uniformdensity sphere assumption. The density structure models allow one toidentify sources that are ``simple'' in that their structure correctionsare negligible. These objects provide a pool of H II regions that can inprinciple yield accurate ^3He^+/H^+ abundances. For the seven simple HII regions in our sample there is no evidence for any systematic trendof the ^3He^+/H^+ abundance with any other parameter. For these sources,we derive an average abundance ratio by number of^3He^+/H^+=(1.6+/-0.5)x10^-5 (statistical error).
| Optical Spectroscopy of Galactic Cirrus Clouds: Extended Red Emission in the Diffuse Interstellar Medium
We present initial results from the first optical spectroscopic surveyof high-latitude Galactic cirrus clouds. The observed shape of thecirrus spectrum does not agree with that of scattered ambient Galacticstarlight. This mismatch can be explained by the presence of extendedred emission (ERE) in the diffuse interstellar medium, as found in manyother astronomical objects, probably caused by photoluminescence ofhydrocarbons. The integrated ERE intensity, I_{{ERE}}~1.2x10^{-5} ergs s^{-1} cm ^{-2} sr ^{-1} , is roughly one-third of the scattered lightintensity, consistent with recent color measurements of diffuse Galacticlight. The peak of the cirrus ERE ( lambda 0 ~ 6000 A) is shifted towardshort (bluer) wavelengths compared with the ERE in sources excited byintense ultraviolet radiation, such as H II regions ( lambda0~8000 A); such a trend is seen in laboratory experiments onhydrogenated amorphous carbon films.
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