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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.
| The Chemical Evolution of Helium
We report on measurements of the 4He abundance toward theouter Galaxy H II region S206 with the NRAO Green Bank Telescope.Observations of hydrogen and helium radio recombination lines between 8and 10 GHz were made toward the peak radio continuum position in S206.We derive 4He/H=0.08459+/-0.00088 (random)+/-0.0010 (knownsystematic), 20% lower than optical recombination line results. It isdifficult to reconcile the large discrepancy between the optical andradio values even when accounting for temperature, density, andionization structure or for optical extinction by dust. Using only M17and S206 we determine ΔY/ΔZ=1.41+/-0.62 in the Galaxy,consistent with standard chemical evolution models. High heliumabundances in the old stellar population of elliptical galaxies can helpexplain the increase in UV emission with shorter wavelength between 2000and 1200 Å, called the ``UV upturn'' or UVX. Our lower values ofΔY/ΔZ are consistent with a normal helium abundance athigher metallicity and suggest that other factors, such as a variablered giant branch mass loss with metallicity, may be important. Whencombined with 4He abundances in metal-poor galaxy H IIregions, Magellanic Cloud H II regions, and M17 that have beendetermined from optical recombination lines, including the effects oftemperature fluctuations, our radio 4He/H abundance ratio forS206 is consistent with a helium evolution of ΔY/ΔZ=1.6. Alinear extrapolation to zero metallicity predicts a 4He/Hprimordial abundance ratio about 5% lower than that given by theWilkinson Microwave Anisotropy Probe and standard big bangnucleosynthesis. The measured 4He abundances may besystematically underestimated by a few percent if clumping exists inthese H II regions.
| 1051 erg less: the Galactic H II region OA 184
Aims.The identification of the object OA 184 as a Galactic SupernovaRemnant (SNR) is re-examined, and evidence to the contrary is presented.The aim is to show definitively that OA 184 is actually a Galactic H IIregion, and to estimate some physical parameters for the nebula (e.g.temperature, density, and magnetic field in the ionized shell).Methods: .To determine the broad spectral properties of OA 184 amultiwavelength approach is used, with historical and new radio data,and existing X-ray and IR data on OA 184 considered. Radio continuum 408and 1420 MHz Canadian Galactic Plane Survey (CGPS) data, Effelsberg 2695MHz data, and new Urumqi 5 GHz radio observations in Stokes I, Q, and Uare presented. As well, we present CGPS H I line data, and eight radiorecombination lines (RRL, H103-110α) observed for this study withthe Green Bank Telescope. Results: .An integrated radio spectrumof index α=-0.14 to -0.2 (S(Jy) ∝ να) isdetermined from four radio frequencies. RRL emission appears atvLSR=-25.8 km s-1 and at the intensity predictedfor free-free thermal emission. Q and U maps at λ~6 cm show OA184 as a depolarizing source, affecting a background filament ofpolarized non-thermal Galactic emission. An H II shell of thickness lessthan 0.2° is indicated by this depolarization. CGPS ~1-arcminuteresolution H I line maps are presented, and the systemic velocity of theISM immediately surrounding OA 184 is -26.8 km s-1. Thedistance of OA 184 is 2.2±0.4 kpc, obtained from a new distancemethod based on a Galactic H I modelling procedure. A simple model as ashell of ionized hydrogen 4 pc thick is considered, and the RRLobservations are used to estimate (under conditions of non-thermodynamicequilibrium) the electron temperature Te=8300 K and densityne=5.3 cm-3. The line of sight magnetic field inthe ionized shell is found to be ≃-2.5~μG (directed away from the Sun). Conclusions: .We conclude that OA 184 is a Galactic H II regionenergized by the lone O7.5V star BD+41°1144.
| 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.
| Abundance Gradients in the Galaxy
Six H II regions at galactocentric distances of R=10-15 kpc have beenobserved in the far-IR emission lines of [O III] (52 μm, 88 μm),[N III] (57 μm), and [S III] (19 μm) using the Kuiper AirborneObservatory. These observations have been combined with Very Large Arrayradio continuum observations of these sources to determine theabundances of O++, N++, and S++relative to hydrogen. In addition, eight of the most recent sets ofmeasurements of ionic line strengths in H II regions have beenreanalyzed in order to attempt to reconcile differences in opticalversus far-IR abundance determinations. We have in total 168 sets ofobservations of 117 H II regions in our analysis. The new analysisincluded updating the atomic constants (transition probabilities andcollision cross sections), recalculation of some of the physicalconditions in the H II regions (ne and Te), andthe use of new photoionization models to determine stellar effectivetemperatures of the exciting stars. We also use the most recent dataavailable for the distances for these objects, although for most westill rely on kinematic distance determinations. Our analysis findslittle indication of differences between optical and infraredobservations of the nitrogen abundances, but some differences are seenin the oxygen and sulfur abundances. A very significant offset continuesto be seen between optical and infrared measurements of the N/Oabundance ratio.
| A Millimeter Continuum Survey for Massive Protoclusters in the Outer Galaxy
Our search for the earliest stages of massive star formation turned up12 massive pre-protocluster candidates plus a few protoclusters. Forthis search, we selected 47 FIR-bright IRAS sources in the outer Galaxy.We mapped regions of several square arcminutes around the IRAS source inthe millimeter continuum in order to find massive cold cloud corespossibly being in a very early stage of massive star formation. Massesand densities are derived for the 128 molecular cloud cores found in theobtained maps. We present these maps together with near-infrared,mid-infrared, and radio data collected from the 2MASS, MSX, and NVSScatalogs. Further data from the literature on detections of high-densitytracers, outflows, and masers are added. The multiwavelength data setsare used to characterize each observed region. The massive cloud cores(M>100 Msolar) are placed in a tentative evolutionarysequence depending on their emission at the investigated wavelengths.Candidates for the youngest stages of massive star formation areidentified by the lack of detections in the above-mentionednear-infrared, mid-infrared, and radio surveys. Twelve massive coresprominent in the millimeter continuum fulfill this requirement. Sinceneither FIR nor radio emission have been detected from these cloudcores, massive protostars must be very deeply embedded in these cores.Some of these objects may actually be pre-protocluster cores: an up tonow rare object class, where the initial conditions of massive starformation can be studied.
| 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.
| On the oxygen abundance in our Galaxy
The compilation of published spectra of Galactic H II regions withavailable diagnostic [OIII]lambda 4363 line information has been carriedout. Our list contains 71 individual measurements of 13 H II regions inthe range of galactocentric distances from 6.6 to 14.8 kpc. The oxygenabundances in all the H II regions were recomputed in the same way,using the classic T_e-method. The oxygen abundance at the solargalactocentric distance traced by those H II regions is in agreementwith the oxygen abundance in the interstellar medium in the solarvicinity derived with high precision from the interstellar absorptionlines towards stars. The derived radial oxygen abundance distributionwas compared with that for H II regions from the Shaver et al.(\cite{Shaver83}) sample which is the basis of many models for thechemical evolution of our Galaxy. It was found that the original Shaveret al.'s oxygen abundances are overestimated by 0.2-0.3 dex. Oxygenabundances in H II regions from the Shaver et al. sample have beenredetermined with the recently suggested P-method. The radialdistribution of oxygen abundances from the Shaver et al. sampleredetermined with the P-method is in agreement with our radialdistribution of (O/H)T_e abundances.
| Multi-frequency GMRT observations of the H II regions S 201, S 206, and S 209. Galactic temperature gradient
We present radio continuum images of three Galactic H Ii regions, S 201,S 206, and S 209 near 232, 327, and 610 MHz using the Giant MeterwaveRadio Telescope (GMRT). The GMRT has a mix of short and long baselines,therefore, even though the data have high spatial resolution, the mapsare still sensitive to diffuse extended emission. We find that all threeH Ii regions have bright cores surrounded by diffuse envelopes. We usethe high resolution afforded by the data to estimate the electrontemperatures and emission measures of the compact cores of these H Iiregions. Our estimates of electron temperatures are consistent with alinear increase of electron temperature with Galacto-centric distancefor distances up to ~ 18 kpc (the distance to the most distant H Iiregion in our sample).
| On the oxygen abundances in H II regions of the Galaxy
Oxygen abundances in H II regions of the Galaxy were derived with theclassical Te-method within the framework of the two-zone H IIregion model using published spectro-photometric data (69 spectra of 11H II regions in the range of galactocentric distances from 6.6 to 14.8kpc). The derived radial distribution of the oxygen abundance wascompared with that from Shaver et al. (1983), which is widely used inconstructing the model of the chemical evolution of the Galaxy. It wasfound that the oxygen abundances given by Shaver et al. areoverestimated by 0.2-0.3 dex.
| The cosmological density of baryons from observations of 3He+ in the Milky Way
Primordial nucleosynthesis after the Big Bang can be constrained by theabundances of the light elements and isotopes 2H,3He, 4He and 7Li (ref. 1). The standardtheory of stellar evolution predicts that 3He is alsoproduced by solar-type stars, so its abundance is of interest not onlyfor cosmology, but also for understanding stellar evolution and thechemical evolution of the Galaxy. The 3He abundance instar-forming (HII) regions agrees with the present value for the localinterstellar medium, but seems to be incompatible with the stellarproduction rates inferred from observations of planetary nebulae, whichprovide a direct test of stellar evolution theory. Here we develop ourearlier observations, which, when combined with recent theoreticaldevelopments in our understanding of light-element synthesis anddestruction in stars, allow us to determine an upper limit for theprimordial abundance of 3He relative to hydrogen:3He/H = (1.1 +/- 0.2) × 10-5. The primordialdensity of all baryons determined from the 3He data is inexcellent agreement with the densities calculated from othercosmological probes. The previous conflict is resolved because mostsolar-mass stars do not produce enough 3He to enrich theinterstellar medium significantly.
| The electron temperatures of HII regions S 201, S 206 and S 209: multi-frequency GMRT observations.
Three Galactic HII regions, viz., S 201, S 206, and S 209 have beenimaged at three frequencies, viz., 232, 327, and 610 MHz using the GMRT.The resolutions of these images are typically 15" at 232, 10" at 327,and 6" at 610 MHz. These are the highest resolution low frequency imagesof these HII regions. We found that all three HII regions havecore--envelope morphologies. We use the high resolution afforded by thedata to estimate the electron temperatures of the compact cores of theseHII regions. These estimates of the electron temperatures are consistentwith an increase in the temperature with Galacto-centric distance; aneffect attributed to a decrease in the heavy elements abundances atlarge Galacto-centric distances.
| Effect of Dust Extinction on Estimating the Star Formation Rate of Galaxies: Lyman Continuum Extinction
We reexamine the effect of Lyman continuum (λ<=912 Å)extinction (LCE) by dust in H II regions in detail and discuss how itaffects the estimation of the global star formation rate (SFR) ofgalaxies. To clarify the first issue, we establish two independentmethods for estimating a parameter of LCE (f), which is defined as thefraction of Lyman continuum photons contributing to hydrogen ionizationin an H II region. One of those methods determines f from the set ofLyman continuum flux, electron density, and metallicity. In theframework of this method, as the metallicity and/or the Lyman photonflux increase, f is found to decrease. The other method determines ffrom the ratio of infrared flux to Lyman continuum flux. Importantly, weshow that f<~0.5 via both methods in many H II regions of the Galaxy.Thus, it establishes that dust in such H II regions absorbs significantamount of Lyman continuum photons directly. To examine the second issue,we approximate f to a function of only the dust-to-gas mass ratio (i.e.,metallicity), assuming a parameter fit for the Galactic H II regions. Wefind that a characteristic f, which is defined as f averaged over agalaxywide scale, is 0.3 for the nearby spiral galaxies. This relativelysmall f indicates that a typical increment factor due to LCE forestimating the global SFR (1/f) is large (~3) for the nearby spiralgalaxies. Therefore, we conclude that the effect of LCE is notnegligible relative to other uncertainties of estimating the SFR ofgalaxies.
| On the oxygen abundance determination in HII regions. High-metallicity regions
This is our second paper devoted to the problem of line intensity -oxygen abundance calibration starting from the idea of McGaugh(\cite{mcg91}) that the strong oxygen lines ([OII] lambda lambda 3727,3729 and [OIII] lambda lambda 4959, 5007) contain the necessaryinformation to determine accurate abundances in HII regions. In theprevious study (Pilyugin 2000) the corresponding relations were obtainedfor the low-metallicity HII regions (12+log O/H <= 7.95, the lowerbranch of the O/H - R23 diagram). The high-metallicity HIIregions (12+log O/H >= 8.2, the upper branch of the O/H -R23 diagram) are considered in the present study. A relationof the type O/H=f(P, R23) between oxygen abundance and thevalue of abundance index R23, introduced by Pagel et al.(\cite{pag79}), and the excitation parameter P (which is defined here asthe contribution of the radiation in [OIII] lambda lambda 4959, 5007lines to the ``total" oxygen radiation) has been derived empiricallyusing the available oxygen abundances determined via measurement of atemperature-sensitive line ratio [OIII]4959,5007/[OIII]4363(Te-method). By comparing oxygen abundances inhigh-metallicity HII regions derived with the Te-method andthose derived with the suggested relations (P-method), it was found thatthe precision of oxygen abundance determination with the P-method isaround 0.1 dex (the mean difference for the 38 HII regions considered is~ 0.08 dex) and is comparable to that of the Te-method. Arelation of the type Te=f(P, R23) between electrontemperature and the values of abundance index R23 and theexcitation parameter P was derived empirically using the availableelectron temperatures determined via measurement oftemperature-sensitive line ratios. The maximum value of differencesbetween electron temperatures determined via measurement oftemperature-sensitive line ratios and those derived with the suggestedrelation is around 1000 K for HII regions considered here, the meanvalue of differences for 38 HII regions is ~ 500 K, which is the sameorder of magnitude as the uncertainties of electron temperaturedeterminations in high-metallicity HII regions via measuredtemperature-sensitive line ratios.
| Photometric Classification of Stars and the Interstellar Extinction near the Camelopardalis and Perseus Border
157 stars down to 11th mag in a 3degr times 4degr area at l = 149degr ,b = --0.3degr have been measured in the Vilnius photometric system.Magnitudes V, color indices, spectral classes, absolute magnitudes,color excesses and distances of the stars are determined. Theinterstellar extinction in all the area appears at a distance of about100 pc, grows monotonically and at a distance of 500 pc it reaches ~ 2mag in the area of the dark cloud L 1391 and 0.7--1.7 mag in theremaining area. At larger distances the extinction is affected by aselection effect.
| Far infrared observations of the southern Galactic star forming complex around IRAS 09002-4732
The Galactic star forming region in the southern sky, associated withIRAS 09002-4732 has been mapped simultaneously in two far infrared bands(lambda eff = 148 & 209 mu m), with ~ 1 arcmin angularresolution. Fifteen sources, including IRAS 08583-4719, 08589-4714,09002-4732 and 09014-4736 have been detected, some of which are wellresolved. Taking advantage of similar beams in the two bands, a reliabledust temperature [T(148/209)] map has been obtained, which detectscolder dust (< 30 K) in this region. The HIRES processed IRAS maps at12, 25, 60 and 100 mu m, have also been used for comparison. The opticaldepth maps, tau 200 and tau 100, generated fromthese far-IR data quantify the spatial distribution of the dust. Thediffuse emission from this entire region has been found to be 35% of thetotal FIR luminosity. The slope of the IMF in the mass range 4-16Msun has been estimated to be-1.25+0.75-0.65 for this star forming complex.Radiative transfer models in spherical geometry have been explored tofit available observations of the 4 IRAS sources and extract variousphysical parameters for corresponding dust-gas clouds. Whereas aconstant (r0) radial density distribution is favoured in IRAS08583-4719, 08589-4714 and 09002-4732, the r-1 law isinferred for IRAS 09014-4736. The dust composition is found to besimilar (Silicate dominated) in all four IRAS sources modelled. Theluminosity per unit mass is found to be in the narrow range of 44 - 81Lsun/Msun for these star forming regions.
| Mid-and far-infrared studies of galactic compact H II regions
Several Galactic H II region complexes have been observed in the farinfrared (FIR) wavebands using the TIFR 1-m balloon borne telescope, andin the mid infrared (MIR) using the Infrared Space Observatory (ISO).Many of these FIR maps show interesting structural details. The MIRobservations have clearly detected PAH features in two compact H IIregions and one molecular clump. A self consistent radiative transferscheme (1-D) has been developed which has been successful in extractingimportant details like geometric sizes, radial density distribution,dust composition, etc, from these and other similar measurements. A newscheme (2-D) has been developed in cylindrical geometry to model IRAS19181 + 1349, which has been resolved into two sources. The modelparameters in this scheme have been constrained by the observed spectralenergy distribution (SED) and radial profiles at MIR & FIRwavebands. Non-equilibrium processes have also been incorporated inspherical geometry, to explain the PAH emission features. This schemehas successfully explained high resolution spectra (ISO-SWS) of severalGalactic compact H II regions.
| Oxygen and Helium Abundances in Galactic H II Regions: Abundance Gradients
Absolute integrated fluxes have been obtained for 34 H II regions, withGalactocentric distances RG in the 6.6-17.7 kpc range, in theemission lines O II λ3726 and λ3729, O III λ4363and λ5007, HeI\ λ5876, Hα and Hβ. These fluxesare analyzed to derive the physical parameters, the ionic abundancesO+/H+, O++/H+ andHe+/H+ and the O/H abundances. Accurate electrontemperatures have been derived in six H II regions with RGbetween 6.6 kpc and 14.8 kpc, widening the RG range for whichreliable Te measurements exist. Our O/H relationship for 5kpc=180) measured in the Galaxy.
| A survey of the 6.7 GHz methanol maser emission from IRAS sources. I. Data
We report the first results of a search for 6.7 GHz methanol masers inthe direction of 1399 IRAS objects north of declination-20deg with the flux densities greater than 100 Jy at 60 mu mand the flux density ratio F60/F25>1.Observations were made with the sensitivity of 1.7 Jy and the velocityresolution of 0.04 km s-1 using the 32-m Toruń radiotelescope. Maser emission was found in 182 sources, including 70 newdetections. 32 new sources were identified with objects of radioemission associated with star-forming regions. Comparison of the presentdata set with other observations suggests that about 65% of methanolmasers exhibit moderate or strong variations on time-scales of about 4and 8 years. Table 2 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/Abstract.html
| Oxygen and helium abundances in Galactic Hii regions - II. Abundance gradients
The absolute integrated fluxes that we obtained (Paper I) for 34 Hiiregions, with Galactocentric distances RG in the 6.6-17.7kpcrange, in the emission lines [Oii] λλ3726 and 3729, [Oiii]λλ4363 and 5007, Hei λ5876, Hα and Hβ,are analysed to derive the extinctions, the electron densities andtemperatures, the ionic abundances O+/H+,O++/H+ and He+/H+ and theO/H abundances. The electron temperature has been derived from the[Oiii] λ4363/λ5007 ratio in six Hii regions withRG between 6.6 and 14.8kpc. These new measurements ofTe, which are in good agreement with those from radiorecombination lines, widen the RG range for which reliableTe measurements exist. Combining our newTe([Oiii]) results with the radio values, we obtain thefollowing temperature relationship: TeK]=(372+/-38)RG+4260+/-350. The O+/H+and O++/H+ abundances have been obtained assuminga two-temperature Hii region model. Our O/H relationship for5kpc=180) measured in the Galaxyby Peimbert et al.
| Oxygen and helium abundances in Galactic Hii regions - I. Observations
Absolute integrated line fluxes of Hii regions have been measured usinga Fabry-Perot spectrophotometer. We describe the observations andcalibration procedures. Fluxes are given for 36 Hii regions withGalactocentric distances ranging from 6.6 to 17.7kpc. Several emissionlines have been measured, mainly [Oii] λλ3726 and 3629,Hβ, [Oiii] λ5007, Hei λ5876 and Hα. The veryfaint [Oiii] λ4363 line has been measured in six regions,allowing a direct determination of the electron temperature. Newphotometric distances have been derived based on data from theliterature. A discussion of these results in terms of extinction,electron density and temperature, and oxygen and helium abundances isgiven in Paper II.
| Far-Infrared Observations of the Galactic Star-forming Regions Associated with IRAS 00338+6312 and RAFGL 5111
Two Galactic star-forming regions, one in a very early phase ofevolution and another evolved one, associated with the IRAS sources00338+6312 and 03595+5110 (RAFGL 5111), respectively, have been studiedin detail. These sources have been mapped simultaneously in twofar-infrared bands (lambda_eff=143 and 185 μm), with ~1.5′angular resolution, using the Tata Institute of Fundamental Research(TIFR) 100 cm balloon-borne telescope. The HIRES-processed IRAS maps at12, 25, 60, and 100 μm have been used for comparison. Whereas IRAS00338+6312 is resolved only in the TIFR bands, RAFGL 5111 is very wellresolved in both the TIFR bands as well as in at least three IRAS bands.The neighboring fainter source IRAS 04004+5114 has also been resolved inthe TIFR bands. Taking advantage of the identical beams in the two TIFRbands at 143 and 185 mum, dust color temperature, T(143/185), andoptical depth, tau_150, maps have been generated for RAFGL 5111. Thesemaps show interesting structural details. Radiative-transfer modeling inspherical geometry has been carried out for individual sources toextract information about the cloud size, the type of the embeddedsource, the radial density distribution, the optical depth, thegas-to-dust ratio, and the dust grain composition. The best-fit modelsare in good agreement with the observed spectral energy distribution(SED), radio continuum data, and so on. Another scheme of radiativetransfer through the interstellar dust-gas cloud including the heavierelements has been used to predict ionic nebular line emission, which isin reasonable agreement with the measurements for RAFGL 5111. Animportant conclusion from the present study is that, for all threesources (IRAS 00338+6312, 03595+5110, and 04004+5114, a faint source inthe neighborhood of RAFGL 5111), the best fit to the observed SED isobtained for a uniform density [n(r)~r^0] cloud.
| J=1-0 HCN toward bright far-infrared sources in the outer Galaxy
Results of the J=1-0 HCN observations toward 34 bright far-infraredsources selected from the IRAS Point Source Catalog are reported.Together with 17 sources observed in this line earlier (Pirogov et al.,1996) they form a complete sample of the sources with flux densitiesS(100 mu m)>500 Jy and delta > 0degr in the outer Galaxy. The HCNdata are compared with the HCO(+) , NH_3, CS and CO data taken fromliterature. Prominent correlations with nearly similar slopes of ~ 1 arerevealed between line integrated intensities of the molecules known tobe high density tracers (HCN, HCO(+) , NH_3 and CS). The correlationsbecome higher after adding the data for dark clouds, small globules andcirrus cores implying similar excitation and formation mechanisms of theconsidered molecules. Collisional excitation in regions with differentdensities as well as different molecular abundances and velocitydispersions in different types of cores seem to be important inproducing these correlations. The following relations hold on theaverage over ~ 3 orders of magnitude of integrated intensities:I(HCN)>~ I(HCO(+) ~ ) I(CS) > I(NH_3) where ammonia integratedintensities are several times lower than HCN ones. Correlations are alsofound between HCN and CO integrated intensities for the sample sourcesas well as between HCN line widths and those of other species. The HCNlines have the same widths as the HCO(+) ones and are larger than CS andespecially NH_3 line widths. Weak correlations are found between HCNline widths and luminosities of IRAS sources as well as between HCNintegrated intensities, IRAS flux densities at 100mu m and luminositiesof IRAS sources divided by distance squared. The sources with mostintense HCN lines have associated water masers and molecular outflowswhile the lack of associated maser and outflow implies weak or no HCNemission. In order to reproduce the anomalies of the J=1-0 HCN hyperfinestructure (R12 < 0.6) detected in several sources togetherwith suprathermal line widths the model calculations are performed. Twomodels have been considered: a microturbulent one with a smooth densitydistribution and a clumpy model. It is found that in the parameter spaceexplored a clumpy model is preferable in comparison with amicroturbulent one due to the absence of self-reversals on calculatedprofiles and the possibility of reproducing HCN profile anomalies andsuprathermal line widths. Detailed clumpy model simulations have beenperformed to fit the observed HCN and H(13) CN spectra toward S140 IRS1.The best fit model parameters are calculated, including radialdependencies of clump density and volume filling factor.
| ^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).
| A Finding List of Faint UV-bright Stars in the Galactic Plane. IV.
One hundred fifty-five UV-bright stars have been identified on threetwo-color 48 inch (1.2 m) Oschin Schmidt plates as part of thecontinuing examination of Sandage survey plates centered on the Galacticplane. The three 6.6dx6.6d field plates examined were centered atGalactic longitude (l) and latitude (b) coordinates of (123, -6), (141,0), and (148, 0), respectively. The sources identified range in U-Bcolor from near U-B~-0.1 to U-B~-1.2, and in magnitude from m_B~10 to~20. Some potentially interesting sources are discussed, and findingcharts are provided for all UV sources listed. Correlations withpreviously identified objects in the literature are also provided.
| Helium-3: Status and Prospects
We report on our continuing efforts to determine 3He abundances in H IIregions and planetary nebulae. Our detections of 3He in some PNe showthat some stars produce large amounts of 3He. However the H II regionabundances show no evidence for this production. From our sample of >40 H II regions, the subsample which should yield the most reliableabundances has 3He/H abundances which scatter between 1-2 x 10-5. Thereis no trend with either galactocentric distance or metallicity. Even ifwe do not understand the underlying mechanisms, we see empirically thatstars neither produce nor destroy 3He in a major way. We thus suggestthat the level of the "3He Plateau" (3He/H = $$1.5_{-0.5}^{+1.0}$$ x10-5) is a reasonable estimate for the primordial 3He.
| 3He in the Milky Way Interstellar Medium: Summary of Relevant Observations
We summarize spectral line and continuum observations of a sample of 21Galactic H II regions made at 3.46 cm wavelength during the past 14years as part of a program to measure the abundance of 3He in the MilkyWay interstellar medium. Converting the observed 3He+ column densityinto an abundance ratio relative to hydrogen depends on the density andionization structure of each nebula. The observations needed to modelour sources were made with the NRAO 140 foot (43 m), NRAO VLA, and MPIfR100 m telescopes. Here we describe and compile our measurements of the3He+ emission line as well as those of various recombination transitionsof H, He, and C. We also summarize the continuum database which includessingle-dish observations and interferometer maps.
| Infrared emission and H-R diagram of bright far-infrared sources during star formation
Young stellar objects (YSOs) were studied using a set of bright FIRsources selected from IRAS-PSC and associated with high-mass YSOs. Amodified blackbody photospheric model, double-shell dust emission model,and a detailed radiation transfer model were used to fit the observedinfrared fluxes, and some physical parameters, such as the photospherictemperature of the central star, the circumstellar extinction, andthermal structure of dust shell were derived. Using the derivedphotospheric temperatures and corresponding stellar luminosities, wehave determined the positions on an extended H-R diagram and theevolution properties of the investigated bright FIR sources. We havefound that there is a trend of continuous increase of the infraredspectral slope S (12-60) from 12 to 60 microns along the direction tomain sequence stars at the same mass line of H-R diagram. For the YSOs,which are evolving at the same time, there is also a trend of increaseof S (12-60) with mass. We analyzed the nature of the bright FIRsources. It seems that the extended H-R diagram presented here is a goodmeans to study this kind of objects, which are deeply embedded inmolecular clouds and dust envelopes.
| All-Sky 4.85 GHz Flux Measurements of H II Regions
We present an all-sky flux catalog of 760 H II regions with angularsizes ranging up to 10' at 4.85 GHz. The data were compiled in aself-consistent manner from existing images of the Green Bank andParkes-MIT-NRAO 4.85 GHz radio continuum surveys. Nearly 35% of these HII regions have fluxes measured from these surveys for the first time.We compared our results to the previously published source catalogs thatfit the same data. The new flux measurements agree within 5%--7% ofthese values and fall well within the formal errors. The diameters fallwithin 16% of the previously published fits to the same data, which iswithin the formal uncertainties of these values.
| Derivation of the Galactic rotation curve using space velocities
We present rotation curves of the Galaxy based on the space-velocitiesof 197 OB stars and 144 classical cepheids, respectively, which rangeover a galactocentric distance interval of about 6 to 12kpc. Nosignificant differences between these rotation curves and rotationcurves based solely on radial velocities assuming circular rotation arefound. We derive an angular velocity of the LSR of{OMEGA}_0_=5.5+/-0.4mas/a (OB stars) and {OMEGA}_0_=5.4+/-0.5mas/a(cepheids), which is in agreement with the IAU 1985 value of{OMEGA}_0_=5.5mas/a. If we correct for probable rotations of the FK5system, the corresponding angular velocities are {OMEGA}_0_=6.0mas/a (OBstars) and {OMEGA}_0_=6.2mas/a (cepheids). These values agree betterwith the value of {OMEGA}_0_=6.4mas/a derived from the VLA measurementof the proper motion of SgrA^*^.
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