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The brightest stars of the σ Orionis cluster
Context: The very young σ Orionis cluster (~3 Ma) is a cornerstonein understanding the formation of stars and substellar objects down toplanetary masses. However, its stellar population is far from beingcompletely known. Aims: This study's purpose is to identify andcharacterise the most massive stars of σ Orionis to complementcurrent and future deep searches for brown dwarfs and planetary-massobjects in the cluster. Methods: I have cross-correlated thesources in the Tycho and 2MASS catalogues in a region of 30 arcminradius with its centre in the O-type star σ Ori A. In this area, Istudied the membership in the Ori OB 1b association of the brighteststars in the optical using astrometric, X-ray, and both infrared andoptical photometric data from public catalogues, and spectroscopic datafrom the literature. Results: A list of 26 young stars, fourcandidate young stars, and 16 probable foreground stars has arisen fromthe study. Seven young stars probably harbour discs (four are new).There is no mass dependence of the disc frequency in the cluster. I havederived the first mass spectrum for σ Orionis from 1.1 to 24M_ȯ (α = +2.0+0.2-0.1; roughlySalpeter-like). I also provide additional proof of the existence ofseveral spatially superimposed stellar populations in the direction ofσ Orionis. Finally, the cluster may be closer and older thanpreviously thought.
| Low sulfur depletion in the Horsehead PDR
Aims.We present 3.65'' × 3.34'' angular-resolution IRAM Plateau deBure Interferometer (PdBI) observations of the CS J = 2-1 line towardthe Horsehead Photodissociation Region (PDR), complemented with IRAM-30msingle-dish observations of several rotational lines of CS,C34S and HCS^+. We analyse the CS and HCS+photochemistry, excitation and radiative transfer to obtain theirabundances and the physical conditions prevailing in the cloud edge.Since the CS abundance scales to that of sulfur, we determine the gasphase sulfur abundance in the PDR, an interesting intermediate mediumbetween translucent clouds (where sulfur remains in the gas phase) anddark clouds (where large depletions have been invoked). Methods:.A nonlocal non-LTE radiative transfer code including dust and cosmicbackground illumination adapted to the Horsehead geometry has beendeveloped to carefuly analyse the CS, C34S, HCS+and C18O rotational line emission. We use this model toconsistently link the line observations with photochemical models todetermine the CS/HCS^+/S/S+ structure of the PDR.Results: .Densities of n(H_2)≃(0.5{-}1.0) × 105cm-3 are required to reproduce the CS and C34S J =2-1 and 3-2 line emission. CS J = 5-4 lines show narrower line widthsthan the CS low-J lines and require higher density gas components notresolved by the 10'' IRAM-30m beam. These values are larger thanprevious estimates based in CO observations. We found χ(CS) = (7± 3) × 10-9 and χ(HCS+) = (4± 2) × 10-11 as the averaged abundances in thePDR. According to photochemical models, the gas phase sulfur abundancerequired to reproduce these values is S/H = (3.5 ± 1.5) ×10-6, only a factor ⪉4 less abundant than the solarsulfur elemental abundance. Since only lower limits to the gastemperature are constrained, even lower sulfur depletion values arepossible if the gas is significantly warmer. Conclusions: .Thecombination of CS, C34S and HCS+ observationstogether with the inclusion of the most recent CS collisional andchemical rates in our models implies that sulfur depletion invoked toaccount for CS and HCS+ abundances is much smaller than inprevious studies.
| SCUBA observations of the Horsehead nebula - what did the horse swallow?
We present observations taken with the Submillimetre Common-UserBolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT) ofthe Horsehead nebula in Orion (B33), at wavelengths of 450 and 850μm. We see bright emission from that part of the cloud associatedwith the photon-dominated region (PDR) at the `top' of the horse's head,which we label B33-SMM1. We characterize the physical parameters of theextended dust responsible for this emission, and find that B33-SMM1contains a more dense core than was previously suspected, with a mass of~2Msolar in a region of 0.31 × 0.13pc, and a peakvolume density of ~6 × 105cm-3. We comparethe SCUBA data with data from the Infrared Space Observatory (ISO) andfind that the emission at 6.75μm is offset towards the west,indicating that the mid-infrared emission is tracing the PDR while thesubmillimetre emission comes from the molecular cloud core behind thePDR. We calculate the virial balance of this core and find that it isnot gravitationally bound but is being confined by the external pressurefrom the HII region IC434, and that it will either be destroyed by theionizing radiation, or else may undergo triggered star formation.Furthermore, we find evidence for a lozenge-shaped clump in the `throat'of the horse, which is not seen in emission at shorter wavelengths. Welabel this source B33-SMM2 and find that it is brighter at submillimetrewavelengths than B33-SMM1. We calculate the physical parameters of SMM2and find it has a mass of ~4Msolar in a region 0.15 ×0.07pc, with a peak volume density of ~2 ×106cm-3 and peak column density of ~9 ×1022cm-2. SMM2 is seen in absorption in the6.75-μm ISO data, from which we obtain an independent estimate of thecolumn density in excellent agreement with that calculated from thesubmillimetre emission. We calculate the stability of this core againstcollapse and find that it is in approximate gravitational virialequilibrium. This is consistent with it being a pre-existing core inB33, possibly pre-stellar in nature, but that it may also eventuallyundergo collapse under the effects of the HII region.
| Submillimeter imaging spectroscopy of the Horsehead nebula
We present 15 arcsecond resolution single-dish imaging of the Horseheadnebula in the Ci (1-0) and CO (4-3) lines, carried out using the CHAMParray at the Caltech Submillimeter Observatory (CSO). The data are usedtogether with supporting observations of the (2-1) transitions of the COisotopologues to determine the physical conditions in the atomic andmolecular gas via Photon Dominated Region (PDR) modeling. The CO(4-3)/(2-1) line ratio, which is an excellent tracer of the direction ofthe incoming UV photons, increases at the western and northern edges ofthe nebula, confirming that the illumination is provided mostly by thestars σ and ζ Orionis. The observed line intensities areconsistent with PDR models with an H nuclei volume density of 3{-} 7 × 104 cm-3. The models predict akinetic temperature of 12 K and a C18O fractional abundancewith respect to H atoms of 2.4 × 10-7 in the shieldedregion, which in turn imply a total molecular mass of 24 Mȯ in theC18O filament. The outer halo, devoid of C18O, buttraced by the Ci emission has a comparable density and contributesadditional 13 Mȯ of material, resulting in an upper limit of 37Mȯ for the total molecular mass of the nebula.
| Large Area Mapping at 850 μm. IV. Analysis of the Clump Distribution in the Orion B South Molecular Cloud
We present results from a survey of a 1300 arcmin2 region ofthe Orion B South molecular cloud, including NGC 2024, NGC 2023, and theHorsehead Nebula (B33), obtained using the Submillimetre Common-UserBolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT).Submillimeter continuum observations at 450 and 850 μm are discussed.Using an automated algorithm, 57 discrete emission features (``clumps'')are identified in the 850 μm map. The physical conditions withinthese clumps are investigated under the assumption that the objects arein quasi-hydrostatic equilibrium. The best-fit dust temperature for theclumps is found to be Td=18+/-4 K, with the exception ofthose associated with the few known far-infrared sources residing in NGC2024. The latter internally heated sources are found to be much warmer.In the region surrounding NGC 2023, the clump dust temperatures agreewith clump gas temperatures determined from molecular line excitationmeasurements of the CO molecule. The bounding pressure on the clumpslies in the range log(k-1P cm3K-1)=6.1+/-0.3. The cumulative mass distribution is steep atthe high-mass end, as is the stellar initial mass function. Thedistribution flattens significantly at lower masses, with a turnoveraround 3-10 Msolar.
| Treasures of Orion.
Not Available
| Density structure of the Horsehead nebula photo-dissociation region
We present high angular resolution images of the H2 1-0 S(1) lineemission obtained with the Son of ISAAC (SOFI) at the New TechnologyTelescope (NTT) of the Horsehead nebula. These observations are analysedin combination with Hα line emission, aromatic dust, CO and dustcontinuum emissions. The Horsehead nebula illuminated by the O9.5V starσ Ori (χ 60) presents a typical photodissociation region(PDR) viewed nearly edge-on and offers an ideal opportunity to study thegas density structure of a PDR. The H2 fluorescent emission observationsreveal extremely sharp and bright filaments associated with theilluminated edge of the nebula which spatially coincides with thearomatic dust emission. Analysis of the H2 fluorescent emission,sensitive to both the far-UV radiation field and the gas density, inconjunction with the aromatic dust and Hα line emission, bringsnew constraints on the illumination conditions and the gas density inthe outer PDR region. Furthermore, combination of this data withmillimeter observations of CO and dust continuum emission allows us totrace the penetration of the far-UV radiation field into the cloud andprobe the gas density structure throughout the PDR. From comparison withPDR model calculations, we find that i) the gas density follows a steepgradient at the cloud edge, with a scale length of 0.02 pc (or 10'') andnH 104 and 105 cm-3 inthe H2 emitting and inner cold molecular layers respectively; and ii)this density gradient model is essentially a constant pressure model,with P 4 × 106 K cm-3. The constraintsderived here on the gas density profile are important for the study ofphysical and chemical processes in PDRs and provide new insight into theevolution of interstellar clouds. Also, this work shows the stronginfluence of the density structure on the PDR spatial stratification andillustrates the use of different tracers to determine this densitystructure.
| Are PAHs precursors of small hydrocarbons in photo-dissociation regions? The Horsehead case
We present maps at high spatial and spectral resolution in emissionlines of CCH, c-C3H2, C4H, 12CO and C18O of theedge of the Horsehead nebula obtained with the IRAM Plateau de BureInterferometer (PdBI). The edge of the Horsehead nebula is aone-dimensional Photo-Dissociation Region (PDR) viewed almost edge-on.All hydrocarbons are detected at high signal-to-noise ratio in the PDRwhere intense emission is seen both in the H2 ro-vibrational lines andin the PAH mid-infrared bands. C18O peaks farther away fromthe cloud edge. Our observations demonstrate that CCH, c-C3H2 and C4Hare present in UV-irradiated molecular gas, with abundances nearly ashigh as in dense, well-shielded molecular cores. PDR models i) need alarge density gradient at the PDR edge to correctly reproduce the offsetbetween the hydrocarbons and H2 peaks; and ii) fail to reproduce thehydrocarbon abundances. We propose that a new formation path of carbonchains, in addition to gas phase chemistry, should be considered inPDRs: because of intense UV-irradiation, large aromatic molecules andsmall carbon grains may fragment and feed the interstellar medium withsmall carbon clusters and molecules in significant amounts.
| 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
| ISOCAM and molecular observations of the edge of the Horsehead nebula
We present ISOCAM observations (5-18 mu m) of the Horsehead nebula,together with observations of the (J=1-0) and (J=2-1) transitions of12CO, 13CO and C18O taken at the IRAM30-m telescope. The Horsehead nebula presents a typicalphotodissociation region illuminated by the O9.5 V system sigma Ori.The ISOCAM emission is due to very small particles transiently heated tohigh temperature each time they absorb a UV photon. A very sharpfilament (width: ~ 10'' or ~ 0.02 pc) is detected by ISOCAMat the illuminated edge of the nebula. This filament is due to acombined effect of steep increase of the column density and extinctionof incident radiation, on typical sizes below ~ 0.01 pc. Both thethree-dimensional shape and the local density of the illuminatedinterface are strongly constrained. The dense material forming the edgeof the Horsehead nebula appears illuminated edge-on by sigma Ori, andthe particles located beyond the border should not be affected by theincident radiation field. This structure may be due to dense filamentsin the parental cloud which have shielded the material located in theirshadow from the photo-dissociating radiations. The measurement of thepenetration depth of the incident radiation from the infrared data (~0.01 pc) gives a density of a few 104 cm-3 justbehind the bright filament. This value is comparable to the estimate ofthe density beyond the edge and deduced from our molecular observations,and also to the density behind the ionization front calculated in thestationary case. The material behind the illuminated edge could also benon-homogeneous, with clump sizes significantly smaller than theobserved penetration depth of ~0.01 pc. In that case no upper limit onthe average density just behind the illuminated edge can be given.
| Observations of Star-Forming Regions with the Midcourse Space Experiment
We have imaged seven nearby star-forming regions, the Rosette Nebula,the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5,with the Spatial Infrared Imaging Telescope on the Midcourse SpaceExperiment (MSX) satellite at 18" resolution at 8.3, 12.1, 14.7, and21.3 μm. The large angular scale of the regions imaged (~7.2-50deg2) makes these data unique in terms of the combination ofsize and resolution. In addition to the star-forming regions, twocirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the southGalactic pole (MSXBG 239) were also imaged. Point sources have beenextracted from each region, resulting in the identification over 500 newsources (i.e., no identified counterparts at other wavelengths), as wellas over 1300 with prior identifications. The extended emission from thestar-forming regions is described, and prominent structures areidentified, particularly in W3 and Orion. The Rosette Nebula isdiscussed in detail. The bulk of the mid-infrared emission is consistentwith that of photon-dominated regions, including the elephant trunkcomplex. The central clump, however, and a line of site toward thenorthern edge of the cavity show significantly redder colors than therest of the Rosette complex.
| Looking into the Horsehead
We present the first interferometric CO (1-0) map of the HorseheadNebula, made with the Berkeley-Illinois-Maryland Association array. Themap has an angular resolution of about 10", corresponding to about 0.02pc. The CO form of the Horsehead closely matches its appearance invisible dust, with the CO emission dominated by a bright, thin ridgealong its western side. The molecular cloud exhibits a pronouncedvelocity gradient of 5 km s-1 pc-1 from thenortheast to the southwest, with the southwestern portion beingredshifted. The mass of the cloud is 27 Msolar. We find anunusual U-shaped feature aligned with the ``horse's nose'' for which wediscuss several interpretations. The northern portion of the U may tracea small outflow driven by the embedded young star B33-1. We discuss theformation, evolution, and ultimate fate of this well-known nebula.
| Fractal structure of the Horsehead nebula (B 33)
Analysis of the CCD image of the Horsehead nebula (B 33), taken in the Halpha (6561 Å) using the 2.34 m Vainu Bappu Telescope (VBT) atKavalur, India, is performed to test its fractal structure. Ten samplereadings of the box dimension of this image were taken using a fractalanalysis software, giving an average value of 1.6965725. The sampledimensions were found to be different from the topological dimension ofone. Importantly, the box dimension of B 33 was not found to besignificantly different from that of the Julia set (box dimension1.679594) with c = -0.745429 + 0.113008i. This provides compellingevidence to show that the structure of the Horsehead nebula is not onlyfractal, but also that its geometry can be described by the Juliafunction f(z) = z2 + c, where both z and c are complexnumbers.
| Star Formation in Bright-rimmed Clouds. I. Millimeter and Submillimeter Molecular Line Surveys
We present the results of the first detailed millimeter andsubmillimeter molecular line survey of bright-rimmed clouds, observed atFCRAO in the CO (J=1-->0), C18O (J=1-->0),HCO+ (J=1-->0), H13CO+ (J=1-->0),and N2H+ (J=1-->0) transitions, and at theHeinrich Hertz Telescope in the CO (J=2-->1), HCO+(J=3-->2), HCO+ (J=4-->3), H13CO+(J=3-->2), and H13CO+ (J=4-->3) molecularline transitions. The source list is composed of a selection ofbright-rimmed clouds from the catalog of such objects compiled bySugitani et al. We also present observations of three Bok globules donefor comparison with the bright-rimmed clouds. We find that theappearance of the millimeter CO and HCO+ emission isdominated by the morphology of the shock front in the bright-rimmedclouds. The HCO+ (J=1-->0) emission tends to trace theswept-up gas ridge and overdense regions, which may be triggered tocollapse as a result of sequential star formation. Five of the sevenbright-rimmed clouds we observe seem to have an outflow; however, onlyone shows the spectral line blue-asymmetric signature that is indicativeof infall in the optically thick HCO+ emission. We alsopresent evidence that in bright-rimmed clouds the nearby shock front mayheat the core from outside-in, thereby washing out the normally observedline infall signatures seen in isolated star-forming regions. We findthat the derived core masses of these bright-rimmed clouds are similarto other low- and intermediate-mass star-forming regions.
| Evolution of very small particles in the southern part of Orion B observed by ISOCAM
We present ISOCAM observations (5-18 mu m) of the southern part of OrionB, including the reflection nebula NGC 2023 and the Horsehead nebulailluminated by the B star HD 37903 and the O star sigma Orionis,respectively. Due to the limited radiation field, the emission in thesewavelengths is due to very small particles which are heated each timethey absorb a UV photon. A filamentary structure is detected at smallangular scales (down to the angular resolution of 6'') on topof a smoother background. The particular case of the Horsehead nebulasuggests that the filaments in general result from the illuminatedsurfaces of dense structures, while the smoother background comes fromlower density matters probably ionised. Striking spatial variations ofthe infrared colour (5-8.5 mu m/12-18 mu m) are also detected.Spectroscopic observations show that they are due to variations of theintensity of the aromatic features (especially at 7.7 mu m) relative toa continuum emission increasing in intensity towards longer wavelengths.The contribution of the continuum relative to the aromatic featuresappears significantly higher at the illuminated surfaces of densestructures than in lower density matter. This effect could be thesignature of the evolution of the very small particles from shieldedmolecular material to photo-dissociated and photo-ionised matter. Wealso show that size segregation due to grain dynamics in uni-directionalradiation fields may play a major role.
| Hα Emission Stars and Herbig-Haro Objects in the Vicinity of Bright-rimmed Clouds
Bright-rimmed clouds (BRCs) found in H II regions are probably sites oftriggered star formation due to compression by ionization/shock fronts.Many BRCs harbor IRAS point sources of low dust temperature. They alsofrequently contain a small cluster of near-IR stars that is elongatedalong the axis of the BRC. Here we present the results of our Hαgrism spectroscopy and narrowband imaging observations of BRCs in searchof candidate pre-main-sequence stars of the T Tauri, Herbig Ae/Be andrelated types, and Herbig-Haro (HH) objects. We have detected a largenumber (460) of Hα emission stars down to a limiting magnitude ofabout R=20 in and around all but two of the 28 BRCs observed. Thepresent study has, for the first time, reached down nearly to thefaintest classical T Tauri star population in OB associations. A totalof 12 new HH objects have been found. Most are of small apparent size,emphasizing the need for deep searches at high spatial resolution, butHH 588 associated with BRC 37 represents a huge HH complex composed oftwo-staged bow shocks on both sides of a tiny central knot. These starsand HH objects are concentrated near the tip of bright rims (i.e., inthe head of the BRCs and just outside the rims) and often make looseaggregates similar to those of near-IR stars, thus supporting ourhypothesis of ``small-scale sequential star formation.'' The presence ofsuch a large number of Hα emission stars in the immediatevicinities of BRCs implies that second-generation formation of low-massstars is relatively extensive and further supports the notion ofcohabitation of high- and low-mass populations in OB associations.
| The Distribution of Atomic Carbon in the Orion Giant Molecular Cloud 1
The large-scale distribution of the C I(3P1-3P0, 492 GHz) emissionline from Orion A and B giant molecular clouds has been imaged with theMount Fuji submillimeter-wave telescope. The total area observed isabout 15 deg2. The overall spatial and velocity structure ofthe C I line is found to be similar to that of the 13CO(J=1-0) line. The derived column density ratio N(C I)/N(CO) shows alarge variation ranging from 0.2 to 2.9 toward the cloud edges, whereasit is relatively constant between 0.1 and 0.2 toward the interior regionof the entire Orion clouds in both massive star-forming and dark cloudregions. This almost constant ratio suggests that C I can coexist withCO even in the deep inside of the molecular cloud.
| CO mapping and multi-line-analysis of Cepheus B
We present large scale (approx. 15x15 arcmin) on-the-fly maps of theCepheus B molecular cloud in the J=3-2 and 2-1 transitions of12CO, 13CO and C18O obtained with theKOSMA 3m-telescope. We use these maps to study the spatial variation ofthe excitation conditions of the molecular gas. Additional Ciobservations allow to refine our analysis. The gas temperature isdominated by UV radiation from identified adjacent and embedded sourcesplus additional heating by a hot core in the north-west of the cloud. Aspherical clump PDR model allows to reproduce the observed line ratiosat most positions. Only 12CO line ratios at the hot coreposition are inconsistent with this scenario and hence give evidence foradditional heating mechanisms. Local volume densities are ~ 2*104 cm-3 while the average volume density ofCepheus B is less than 103 cm-3; thus, Cepheus Bis highly clumped and the volume of these clumps fills only 2% to 4% ofthe whole cloud. Deep penetration of FUV radiation into the clumpymedium is consistent with the detection of [Ci] emission from within thecloud. Abundance ratios of atomic carbon to CO are >= 0.2. The13CO/C18O integrated line intensity ratio risessignificantly above the isotopic element ratio at the cloud edges whereC18O intensities are low. Possible explanations for thiscommon observation in terms of fractionation and selectivephotodissociation on clump surfaces in a clumpy cloud are discussed.
| The Properties of Molecular Hydrogen toward the Orion Belt Stars from Observations by the Interstellar Medium Absorption Profile Spectrograph
Absorption features from the Lyman and Werner bands of interstellarmolecular hydrogen were recorded by the Interstellar Medium AbsorptionProfile Spectrograph (IMAPS) at λ/Δλ=80,000 in thespectra of δ Ori A and ɛ Ori. The objective was to find andstudy more examples of an unusual phenomenon found for one of thevelocity components of H2 in the spectrum of ζ Ori byJenkins & Peimbert (1997). Specifically, they detected a gradualshift in velocity and broadening for features arising from progressivelyhigher rotational excitations J. This effect appears to be absent in thespectra of both δ and ɛ Ori, which are only a few degreesaway in the sky from ζ Ori. The absence of atomic material at alarge negative velocity in the spectra of δ and ɛ Ori (andits presence in ζ Ori) supports a proposal by Jenkins &Peimbert that the line of sight to ζ intercepts a bow shock facingaway from us, perhaps created by the collision of windlike material withsome foreground obstruction. One edge of the molecular cloud complexLynds 1630 is situated close to ζ Ori in the sky, but we presentsome evidence that seems to indicate that the cloud is more distant, inwhich case it could not serve as the obstruction. However, it ispossible that the outermost extension of a high-speed jet from a starforming within the cloud can explain the high-velocity material and theshock front created by it. For both stars, the H2 absorptionfeatures are separated into two velocity components. Total H2column densities toward δ and ɛ Ori are5.5×1014 and 1.9×1016 cm-2,respectively. When these values are compared to the column densities ofH I, the fractions of H atoms bound in molecular form2N(H2)/[2N(H2)+N(HI)]=7×10-6 for δ and1.3×10-4 for ɛ. The rotation temperatures of themolecules with J>2 toward ɛ Ori indicate that the gas is in thegeneral vicinity of the stars that emit UV fluxes capable ofrotationally pumping the molecules. For the strongest component ofH2 toward δ Ori, the pumping rate is lower andconsistent with a general UV flux level in the plane of the Galaxy.
| New Herbig-Haro objects and giant outflows in Orion
We present the results of a photographic and CCD imaging survey forHerbig-Haro (HH) objects in the L1630 and L1641 giant molecular cloudsin Orion. The new HH flows were initially identified from a deepHα film from the recently commissioned AAO/UKST Hα Survey ofthe southern sky. Our scanned Hα and broad-band R images highlightboth the improved resolution of the Hα survey and the excellentcontrast of the Hα flux with respect to the broad-band R.Comparative IVN survey images allow us to distinguish between emissionand reflection nebulosity. Our CCD Hα, [Sii], continuum and I-bandimages confirm the presence of a parsec-scale HH flow associated withthe Ori I-2 cometary globule, and several parsec-scale strings of HHemission centred on the L1641-N infrared cluster. Several smalleroutflows display one-sided jets. Our results indicate that, fordeclinations south of -6 deg in L1641, parsec-scale flows appear to bethe major force in the large-scale movement of optical dust andmolecular gas.
| Spectroscopy Between the Stars
The emission and absorption spectra of interstellar molecules arereviewed with special consideration of recent observational andtechnical advances in the shorter submillimeter wave region of theelectromagnetic spectrum. Single-dish observations have contributed inthe past probably most of the information about the structure ofinterstellar molecular clouds.
| A Jet-driven, Extreme High-Velocity Outflow Powered by a Cold, Low-Luminosity Protostar near NGC 2023
We have discovered an extreme high-velocity bipolar CO outflow in thevicinity of NGC 2023, with total outflow velocities of ~200 km s^-1. Atvery high velocities this outflow is jetlike with an opening angle<=4 deg, while it shows a separate outflow lobe at low velocities.The outflow is bipolar and exhibits a clear mirror symmetry, whichsuggests that the source powering the outflow is episodic or precessing.The dynamical timescales for the outflow are <=3000 yr. We identifythe source driving the CO jet with a deeply embedded low-luminositysubmillimeter double source (separation ~23"), where the primarycomponent lies on the symmetry axis of the outflow and has all thesignatures of a ``class 0'' protostellar object. Analysis of moleculardata and (sub)millimeter photometry suggests that the driving source iscold and compact, with a luminosity of <~10 L_solar and a total massof 1.8-4.6 M_solar. It has no near-IR counterpart, it drives anextremely young outflow, and it emits a large fraction of its luminosityin the submillimeter regime. Both millimeter sources have low dustemissivity, beta~0.8-1.3, similar to what is found for other class 0objects, while the surrounding molecular cloud core appears to have abeta~2.0, the canonical value for ``normal'' interstellar dust in thesubmillimeter regime.
| The giant molecular clouds Orion A and B.
Not Available
| The large system of molecular clouds in Orion and Monoceros
Emission is noted over about one-eighth of an 850-sq deg region centeredon Orion and Monoceros that has been surveyed in the J = 1 to 0 line ofCO; most of the emission arises from giant molecular clouds associatedwith Orion A and B, and Mon R2. A much smaller area was surveyed forC-13O emission. A comparison of cloud masses obtained by threeindependent methods indicates that CO luminosity is as accurate ameasure of cloud mass as other indicators. The possible relationshipsamong clouds in the survey are discussed, including the conjecture thatthe overall Orion complex of clouds is a much larger system thanpreviously considered, incorporating most of the clouds in the presentsurvey.
| Photoelectric photometry of diffuse gaseous nebulae in the H-alpha line
Results are given of the H-alpha photometry of diffuse gaseous nebulaeobserved with a nebula photometer attached to the 60-cm reflector of thePic du Midi Observatory. Isophote maps are given for the followingobjects: the Orion nebula, the Rosette nebula, IC 410, IC 434 and NGC2024, and M 8 (NGC 6523); several scans are presented for NGC 6514, NGC7000, and Barnard's Loop. In cases where the spherical modelapproximation may be performed, the variation of electron density withradius was determined; for the Rosette nebula a steeper decrease towardthe center than previously established was found.
| Cloud structure in the galactic plane - A cosmic bubble bath
Evidence is presented for two types of structures in the interstellarmedium: loops and dust filaments. H II regions showing well defined loopstructures are described, and the presence and formation of dust shellsin external galaxies are discussed. It is suggested that both types ofstructures can be explained as old supernova shells in various stages ofdissipation and reentry to the galactic plane, with the dust componentbeing formed either during supernova expansion or by Rayleigh-Taylorinstability while falling into the plane. Twenty-eight possible shellstructures are described, which have been observed as emission regions,absorption lanes, or radio features, and include Cyg 1 through 5, Mon 1and 2, and Barnard's Loop (Ori 1).
| A Comparison between Radio and Optical Radial Velocities of H N Regions
Abstract image available at:http://adsabs.harvard.edu/abs/1970A&A.....7..322G
| Observations of Four H II Regions at a Wavelength of 2.8 Centimeters
Abstract image available at:http://adsabs.harvard.edu/abs/1968ApJ...154..833M
| Galactic H II Regions. II. Observations of Their Hydrogen 109α Recombination-Line Radiation at the Frequency 5009 MHz
Abstract image available at:http://adsabs.harvard.edu/abs/1967ApJ...147..490M
| Einige Beziehungen der RW-Aurigae-Sterne zur interstellaren Materie.
Not Available
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