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The Unusual Hydrocarbon Emission from the Early Carbon Star HD 100764: The Connection between Aromatics and Aliphatics
We have used the Infrared Spectrograph (IRS) on the Spitzer SpaceTelescope to obtain spectra of HD 100764, an apparently single carbonstar with a circumstellar disk. The spectrum shows emission featuresfrom polycyclic aromatic hydrocarbons (PAHs) that are shifted to longerwavelengths than normally seen, a characteristic of ``class C'' systemsin the classification scheme of Peeters et al. All seven of the knownclass C PAH sources are illuminated by radiation fields that are coolerthan those which typically excite PAH emission features. The observedwavelength shifts are consistent with hydrocarbon mixtures containingboth aromatic and aliphatic bonds. We propose that the class C PAHspectra are distinctive because the carbonaceous material has not beensubjected to a strong ultraviolet radiation field, allowing relativelyfragile aliphatic materials to survive.
| Infrared Extinction toward Nearby Star-forming Regions
We present an independent estimate of the interstellar extinction lawfor the Spitzer IRAC bands, as well as a first attempt at extending thelaw to the 24 μm MIPS band. The source data for these measurementsare observations of five nearby star-forming regions: the Orion A cloud,NGC 2068/2071, NGC 2024/2023, Serpens, and Ophiuchus. Color excessratios EH-Ks/EKs-[λ]were measured for stars without infrared excess dust emission fromcircumstellar disks/envelopes. For four of these five regions, theextinction laws are similar at all wavelengths and differ systematicallyfrom a previous determination of the extinction law, which was dominatedby the diffuse ISM, derived for the IRAC bands. This difference could bedue to the difference in the dust properties of the dense molecularclouds observed here and those of the diffuse ISM. The extinction law atlonger wavelengths toward the Ophiuchus region lies between that to theother four regions studied here and that for the ISM. In addition, weextended our extinction law determination to 24 μm for Serpens andNGC 2068/2071 using Spitzer MIPS data. We compare these results againstseveral ISO extinction law determinations, although in each case thereare assumptions which make absolute comparison uncertain. However, ourwork confirms a relatively flatter extinction curve from 4 to 8 μmthan the previously assumed standard, as noted by all of these recentstudies. The extinction law at 24 μm is consistent with previousmeasurements and models, although there are relatively largeuncertainties.
| Observations of spatial and velocity structure in the Orion molecular cloud
Observations are reported of H2 IR emission in the S(1) v=1-0 line at2.121 μm in the Orion Molecular Cloud, OMC1, using the GriFinstrument on the Canada-France-Hawaii Telescope. GriF uses acombination of adaptive optics and Fabry-Perot interferometry, yieldinga spatial resolution of 0.15´´ to 0.18´´ andvelocity discrimination as high as 1 km s-1. 193 bright H2emission regions can be identified in OMC1. The general characteristicsof these features are described in terms of radial velocities,brightness and spatial displacement of maxima of velocity andbrightness, the latter to yield the orientation of flows in the plane ofthe sky. Strong spatial correlation between velocity and bright H2emission is found and serves to identify many features as shocks.Important results are: (i) velocities of the excited gas illustrate thepresence of a zone to the south of BN-IRc2 and Peak 1, and the west ofPeak 2, where there is a powerful blue-shifted outflow with an averagevelocity of -18 km s-1. This is shown to be the NIRcounterpart of an outflow previously identified in the radio,originating from either source I or source n. (ii) There is a band ofweak radial velocity features (<5 km s-1) in Peak 1. (iii)A small proportion of the flows may represent sites of low mass starformation and one region shows evidence of multiple flows which mayindicate multiple low mass star formation within OMC1.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) which is operated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique of France, and the University of Hawaii. TablesA.1-A.3 are only available in electronic form at http://www.aanda.org
| Molecular and Atomic Excitation Stratification in the Outflow of the Planetary Nebula M27
High-resolution spectroscopy with FUSE and HST STIS of atomic andmolecular velocity stratification in the nebular outflow of M27challenge models of the abundance kinematics in planetary nebulae. Thesimple picture of a very high speed (~1000 km s-1),high-ionization, radiation-driven stellar wind surrounded by a slower(~10 km s-1) mostly molecular outflow, with low-ionizationand neutral atomic species residing at the wind interaction interface,is not supported. Instead, we find vibrationally excited H2intermixed with mostly neutral atomic species at a transition velocity(33 km s-1) between a fast (33-65 km s-1)low-ionization zone and a slow (<~33 km s-1)high-ionization zone. Lyα fluorescence of H2 has beendetected, but far-UV continuum fluorescence has not. The diffuse nebularmedium is inhospitable to molecules and dust. Maintaining a modestequilibrium abundance of H2 [N(H2)/N(HI)<<1] in the diffuse nebular medium requires a source ofH2, mostly likely the clumpy nebular medium. The stellar SEDshows no sign of reddening [E(B-V)<0.01], but paradoxicallyHα/Hβ indicates E(B-V)~0.1. The enhancement ofHα/Hβ in the absence of dust may result from a two-stepprocess of H2 ionization by Lyman continuum photons followedby dissociative recombination [H 2 + γ -->H+2 + e- --> H(1s) + H(nl)], whichultimately produces fluorescence of Hα and Lyα. In theoptically thin limit at the inferred radius of the velocity transition,we find that dissociation of H2 by stellar Lyman continuumphotons is an order of magnitude more efficient than spontaneousdissociation by far-UV photons. The importance of this H2destruction process in H II regions has been overlooked.
| A SCUBA survey of Orion - the low-mass end of the core mass function
We have re-analysed all of the Submillimetre Common User Bolometer Array(SCUBA) archive data of the Orion star-forming regions. We have puttogether all of the data taken at different times by different groups.Consequently, we have constructed the deepest submillimetre maps ofthese regions ever made. There are four regions that have been mapped:Orion A North and South, and Orion B North and South. We find that twoof the regions, Orion A North and Orion B North, have deeper sensitivityand completeness limits, and contain a larger number of sources, so weconcentrate on these two. We compare the data with archive data from theSpitzer Space Telescope to determine whether or not a core detected inthe submillimetre is pre-stellar in nature. We extract all of thepre-stellar cores from the data and make a histogram of the core masses.This can be compared to the stellar initial mass function (IMF). We findthe high-mass core mass function (CMF) follows a roughly Salpeter-likeslope, just like the IMF, as seen in previous work. Our deeper mapsallow us to see that the CMF turns over at, ~1.3Msolar abouta factor of 4 higher than our completeness limit. This turnover hasnever previously been observed, and is only visible here due to our muchdeeper maps. It mimics the turnover seen in the stellar IMF at~0.1Msolar. The low-mass side of the CMF is a power law withan exponent of, 0.35 +/- 0.2 which is consistent with the low-mass slopeof the young cluster IMF of 0.3 +/- 0.1. This shows that the CMFcontinues to mimic the shape of the IMF all the way down to the lowercompleteness limit of these data at ~0.3Msolar.
| HCN versus HCO+ as Dense Molecular Gas Mass Tracers in Luminous Infrared Galaxies
It has recently been argued that the HCN J=1-0 line emission may not bean unbiased tracer of dense molecular gas (n>~104cm-3) in luminous infrared galaxies (LIRGs;LFIR>1011 Lsolar) and thatHCO+ J=1-0 may constitute a better tracer instead, castingdoubt onto earlier claims supporting the former as a good tracer of suchgas. In this paper new sensitive HCN J=4-3 observations of four suchgalaxies are presented, revealing a surprisingly wide excitation rangefor their dense gas phase that may render the J=1-0 transition fromeither species a poor proxy of its mass. Moreover, the well-knownsensitivity of the HCO+ abundance to the ionization degree ofmolecular gas (an important issue omitted from the ongoing discussionabout the relative merits of HCN and HCO+ as dense gastracers) may severely reduce the HCO+ abundance in thestar-forming and highly turbulent molecular gas found in LIRGs, whileHCN remains abundant. This may result in the decreasingHCO+/HCN J=1-0 line ratios with increasing IR luminosityfound in LIRGs, and it casts doubts on HCO+ rather than HCNas a good dense molecular gas tracer. Multitransition observations ofboth molecules are needed to identify the best such tracer and itsrelation to ongoing star formation, and to constrain what may be aconsiderable range of dense gas properties in such galaxies.
| A Cometary Bow Shock and Mid-Infrared Emission Variations Revealed in Spitzer Observations of HD 34078 and IC 405
We present new infrared observations of the emission/reflection nebulaIC 405 obtained with the Spitzer Space Telescope. Infrared images in thefour IRAC bands (3.6, 4.5, 5.8, and 8.0 μm) and two MIPS bands (24and 70 μm) are complemented by IRS spectroscopy (5-30 μm) of twonebular filaments. The IRAC (8.0 μm) and MIPS imaging shows evidenceof a bow shock associated with the runaway O9.5 V star, HD 34078,created by the interaction between the star and nebular material. Theratio of emission at 24 to 70 μm is higher in the immediate vicinityof HD 34078 than in the outer filaments, providing evidence for elevateddust temperatures (Td>~90 K) in the shock region. Thenebular imaging reveals that the morphology is band dependent, withvarying contributions from aromatic emission features, H2,and dust emission. Nebular spectroscopy is used to quantify thesecontributions, showing several aromatic emission bands between 6-14μm, the S(5), S(3), S(2), and S(1) pure rotational emission lines ofH2, and atomic fine-structure lines of Ne, S, and Ar. Thelow-dispersion spectra provide constraints on the ionization state ofthe large molecules responsible for the aromatic infrared features.H2 rotational temperatures of the two bright nebularfilaments are determined from the observed line strengths. An averageT(H2)~400 K is inferred, with evidence for additionalnonuniform excitation by UV photons in the intense radiation field of HD34078. The photoexcitation hypothesis is supported by direct measurementof the far-UV H2 fluorescence spectrum, obtained with FUSE.Based in part on observations made with the Spitzer Space Telescope,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under a contract with NASA.
| A Study of the Isolated Dark Globule DC 314.8-5.1: Extinction, Distance, and a Hint of Star Formation
The southern dark globule DC 314.8-5.1 contains a reflection nebulailluminated by a normal B9 V star, HD 130079. This serendipitousassociation provides the opportunity to evaluate the distance to theglobule with greater accuracy than would otherwise be possible, subjectto accurate accounting for the effects of interstellar extinction andreddening. It is shown that the dust in the line of sight has opticalproperties characterized by elevated values of the ratio of extinctionto reddening (RV=AV/EB-V) and thewavelength of maximum polarization (λmax), signifyinggrowth that most probably results from grain-grain coagulation withinthe globule. Taking this into account yields a distance of 342+/-50 pc,significantly lower than previous estimates that assume the standarddiffuse interstellar medium average extinction law. Comparison of thisresult with the loci of other major sources of extinction along the lineof sight suggests that the globule is isolated; it appears to not bephysically associated with the adjacent Circinus molecular cloud andstar formation complex G317.0-4.0. Estimates are made of the mean numberdensity (nH>~9×103 cm-3) andmass (30-100 Msolar) of DC 314.8-5.1. A stellar census of theregion using the Two Micron All Sky Survey suggests that it is not asite of vigorous star formation, although two (out of 387) sources areidentified that appear to be good candidates for young stellar objectstatus on the basis of their near-infrared colors. A deep mid-infraredsurvey will be needed to determine whether DC 314.8-5.1 is, indeed,starless (with respect to indigenous birth) or a site of sedentarylow-mass star formation. The globule may also prove to be a valuablelaboratory for future study of the interaction of dense molecular gasand dust in a quiescent core with the relatively soft UV radiation fieldemanating from the embedded B9 V star.
| Abundances in planetary nebulae: Hb 5
The ISO spectra of the bilobal planetary nebula Hb 5are presented. These spectra are combined with the spectra in the visualwavelength region to obtain a complete, extinction corrected, spectrum.The chemical composition of the nebula is then calculated in severalways. First by directly calculating and adding individual ionabundances, assuming that all the ionic lines are formed in an ionizedregion surrounding the ionizing star. Secondly by building an"end-to-end model" nebula in which we have included a neutral region anda photodissociation region (PDR) beyond the ionized nebula. In this waywe attempt to interpret the molecular hydrogen lines observed by ISO ina more self-consistent way. In the final analysis, the model is found tobe basically heuristic, but gives new insights about the PDR and the PN.The implications of these are discussed.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France,Germany, the Netherlands and the UK) and with theparticipation of ISAS and NASA.
| A method for detection of structure
Context: .In order to understand the evolution of molecular clouds it isimportant to identify the departures from self-similarity associatedwith the scales of self-gravity and the driving of turbulence.Aims: . A method is described based on structure functions fordetermining whether a region of gas, such as a molecular cloud, isfractal or contains structure with characteristic scale sizes. Methods: .Using artificial data containing structure it is shown thatderivatives of higher order structure functions provide a powerful wayto detect the presence of characteristic scales should any be presentand to estimate the size of such structures. The method is applied toobservations of hot H2 in the Kleinman-Low nebula, north of theTrapezium stars in the Orion Molecular Cloud, including both brightnessand velocity data. The method is compared with other techniques such asFourier transform and histogram techniques. Results: . It is foundthat the density structure, represented by H2 emission brightness in theK-band (2-2.5 μm), exhibits mean characteristic sizes of 110, 550,1700 and 2700 AU. The velocity data show the presence of structure at140, 1500 and 3500 AU. Compared with other techniques such as Fouriertransform or histogram, the method appears both more sensitive tocharacteristic scales and easier to interpret.
| High spatial resolution mid-infrared spectroscopy of the starburst galaxies NGC 3256, II Zw 40 and Henize 2-10
Aims.In order to show the importance of high spatial resolutionobservations of extra-galactic sources when compared to observationsobtained with larger apertures such as ISO, we present N-band spectra(8-13 μm) of some locations in three starburst galaxies. Inparticular, we show the two galactic nuclei of the spiral galaxy NGC3256, the compact IR supernebula in the dwarf galaxy II Zw 40 and thetwo brightest IR knots in the central starburst of the WR galaxy He2-10. Methods: .The spectra were obtained with TIMMI2 on the ESO3.6 m telescope. An inventory of the spectra in terms of atomicfine-structure lines and molecular bands is presented. Results:.We show the value of these high spatial resolution data in constrainingproperties such as the extinction in the mid-IR, metallicity or stellarcontent (age, IMF, etc.). We have constrained the stellar content of theIR compact knot in II Zw 40 by using the mid-IR fine-structure lines andsetting restrictions on the nebular geometry. We have constructed a newmid-/far-IR diagnostic diagram based on the 11.2 μm PAH andcontinuum, accessible to ground-based observations. We find thatextra-galactic nuclei and star clusters observed at high spatialresolution (as is the case of the TIMMI2 observations) are closer inPAH/far-IR to compact H II regions, while galaxies observed by largeapertures such as ISO are closer to exposed PDRs such as Orion. This islikely due to the aperture difference. We find a dependence between thepresence of PAHs and the hardness of the radiation field as measured bythe [S IV]/[Ne II] ratio that may be explained by the PAH-dustcompetition for FUV photons or the relative contribution of thedifferent phases of the interstellar medium.
| 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.
| [Si II], [Fe II], [C II], and H2 Emission from Massive Star-forming Regions
We calculate the [Si II] 34.8 μm, [Fe II] 26.0 μm, and [C II] 158μm infrared fine-structure emission that may arise from ionized gas(H II regions) and/or associated neutral gas (photodissociation regions[PDRs]) in massive star-forming regions. Assuming thermal pressurebalance between an H II region and a PDR, the relative amounts offine-structure line emission from the H II region and PDR depend on theelectron density and resulting thermal pressure in the H II region,gas-phase abundances of the emitting species, and the UV spectrum fromthe stellar population producing the H II region. For normalmetallicity, we find that [C II] emission is always dominated by PDRs,while [Si II] and [Fe II] are dominated by PDRs for H II regions withelectron density ne>~10 cm-3. We also calculatethe H2 0-0 S(0), 0-0 S(1), 0-0 S(2), and 0-0 S(3) purerotational line emission arising from the PDR at the atomic-to-molecularinterface. The overall intensity of the H2 line emissiondirectly traces warm molecular mass, while H2 line ratiosconstrain the PDR temperature, gas density, and far-ultraviolet fieldstrength. Models of the integrated emission of [Si II], [Fe II], [C II],and H2 from Galactic and extragalactic star-forming regionsare presented for use in interpreting observations with Spitzer, ISO,SOFIA, and the Herschel Space Observatory. We compare our results withobservations of the Galactic source NGC 2023 (an individual HII/photodissociation region in Orion), the inner regions of the MilkyWay, and the central regions of the nearby star-forming spiral galaxyNGC 7331. We also compare our results with recently published similarwork by Abel and coworkers.
| Far-ultraviolet scattering by dust in Orion
We have modelled diffuse far-ultraviolet (FUV) spectrum observed by theFar Ultraviolet Spectroscopic Explorer (FUSE) near M42 as the scatteringof the starlight from the Trapezium stars by dust in front of thenebula. The dust grains are known to be anomalous in Orion withRV= 5.5 and these are the first measurements of the FUVoptical properties of the grains outside of `normal' Milky Way dust. Wefind an albedo varying from 0.3 +/- 0.1 at 912 Å to 0.5 +/- 0.2 at1020 Åwhich is consistent with theoretical predictions.
| Detection of CO+ in the Nucleus of M82
We present the detection of the reactive ion CO+ toward theprototypical starburst galaxy M82. This is the first secure detection ofthis short-lived ion in an external galaxy. Values of[CO+]/[HCO+]>0.04 are measured across the inner650 pc of the nuclear disk of M82. Such high values of[CO+]/[HCO+] have previously only been measuredtoward the atomic peak in the reflection nebula NGC 7023. This detectioncorroborates the scenario in which the molecular gas reservoir in theM82 disk is heavily affected by the UV radiation from recently formedstars. Comparing the column densities measured in M82 with those foundin prototypical Galactic photon-dominated regions (PDRs), we need ~20clouds along the line of sight to explain our observations. We havecompleted our model of the molecular gas chemistry in the M82 nucleus.Our PDR chemical model successfully explains the[CO+]/[HCO+] ratios measured in the M82 nucleusbut fails by an order of magnitude to explain the large measuredCO+ column densities [~(1-4)×1013cm-2]. We explore possible routes to reconcile the chemicalmodel and the observations.
| 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.
| Molecular hydrogen formation on porous dust grains
Recent laboratory experiments on interstellar dust analogues have shownthat H2 formation on dust-grain surfaces is efficient in arange of grain temperatures below 20 K. These results indicate thatsurface processes may account for the observed H2 abundancein cold diffuse and dense clouds. However, high abundances ofH2 have also been observed in warmer clouds, includingphoton-dominated regions (PDRs), where grain temperatures may reach 50K, making the surface processes extremely inefficient. It was suggestedthat this apparent discrepancy can be resolved by chemisorption sites.However, recent experiments indicate that chemisorption processes maynot be efficient at PDR temperatures. Here we consider the effect ofgrain porosity on H2 formation, and analyse it using arate-equation model. It is found that porosity extends the efficiency ofthe recombination process to higher temperatures. This is because Hatoms that desorb from the internal surfaces of the pores may re-adsorbmany times and thus stay longer on the surface. However, thisporosity-driven extension may enable efficient H2 formationin PDRs only if porosity also contributes to significant cooling of thegrains, compared to non-porous grains.
| The Excitation of Extended Red Emission: New Constraints on Its Carrier from Hubble Space Telescope Observations of NGC 7023
The carrier of the dust-associated photoluminescence process causing EREin many dusty interstellar environments remains unidentified. Severalcompeting models are more or less able to match the observed broad,unstructured ERE band. We now constrain the character of the ERE carrierfurther by determining the wavelengths of the radiation that initiatesthe ERE. Using the imaging capabilities of the HST, we have resolved thewidth of narrow ERE filaments appearing on the surfaces of externallyilluminated molecular clouds in the bright reflection nebula NGC 7023and compared them with the depth of penetration of radiation of knownwavelengths into the same cloud surfaces. We identify photons withwavelengths shortward of 118 nm as the source of ERE initiation, not tobe confused with ERE excitation, however. There are strong indicationsfrom the well-studied ERE in the Red Rectangle Nebula and in thehigh-|b| Galactic cirrus that the photon flux with wavelengths shortwardof 118 nm is too small to actually excite the observed ERE, even with100% quantum efficiency. We conclude, therefore, that ERE excitationresults from a two-step process. The first, involving far-UV photonswith E>10.5 eV, leads to the creation of the ERE carrier, most likelythrough photoionization or photodissociation of an existing precursor.The second, involving more abundant near-UV/optical photons, consists ofthe optical pumping of the previously created carrier, followed bysubsequent deexcitation via photoluminescence. The latter process canoccur many times for a single particle, depending upon the lifetime ofthe ERE carrier in its active state. While none of the previouslyproposed ERE models can match these new constraints, we note that underinterstellar conditions most PAH molecules are ionized to the dicationstage by photons with E>10.5 eV and that the electronic energy levelstructure of PAH dications is consistent with fluorescence in thewavelength band of the ERE. Therefore, PAH dications deserve furtherstudy as potential carriers of the ERE.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555. These observations are associated with program9471.
| Probing turbulence with infrared observations in OMC1
A statistical analysis is presented of the turbulent velocity structurein the Orion Molecular Cloud at scales ranging from 70 AU to3×104 AU. Results are based on IR Fabry-Perotinterferometric observations of shock and photon-excited H2 in theK-band S(1) v=1{-}0 line at 2.121 μm and refer to the dynamicalcharacteristics of warm perturbed gas. Data consist of a spatiallyresolved image with a measured velocity for each resolution limitedregion (70 AU× 70 AU) in the image. The effect of removal ofapparent large scale velocity gradients is discussed and the conclusiondrawn that these apparent gradients represent part of the turbulentcascade and should remain within the data. Using our full data set,observations establish that the Larson size-linewidth relation is obeyedto the smallest scales studied here extending the range of validity ofthis relationship by nearly 2 orders of magnitude. The velocityprobability distribution function (PDF) is constructed showing extendedexponential wings, providing evidence of intermittency, furthersupported by the skewness (third moment) and kurtosis (fourth moment) ofthe velocity distribution. Variance and kurtosis of the PDF of velocitydifferences are constructed as a function of lag. The variance shows anapproximate power law dependence on lag, with exponent significantlylower than the Kolmogorov value, and with deviations below 2000 AU whichare attributed to outflows and possibly disk structures associated withlow mass star formation within OMC1. The kurtosis shows strong deviationfrom a Gaussian velocity field, providing evidence of velocitycorrelations at small lags. Results agree accurately with semi-empiricalsimulations in Eggers & Wang (1998). In addition, 170 individual H2emitting clumps have been analysed with sizes between 500 and 2200 AU.These show considerable diversity with regard to PDFs and variancefunctions (related to second order structure functions) displaying avariety of shapes of the PDF and different values of the scalingexponent within a restricted spatial region. However, a regionassociated with an outflow from a deeply embedded O-star shows highvalues of the scaling exponent of the variance function, representing astrong segregation of high and low exponent clumps. Our analysisconstitutes the first characterization of the turbulent velocity fieldat the scale of star formation and provide a dataset which models ofstar-forming regions should aim to reproduce.
| Blue Luminescence and the Presence of Small Polycyclic Aromatic Hydrocarbons in the Interstellar Medium
Blue luminescence (BL) was first discovered in a proto-planetary nebula,the Red Rectangle (RR), surrounding the post-AGB star HD 44179. BL hasbeen attributed to fluorescence by small, 3-4 ringed neutral polycyclicaromatic hydrocarbon (PAH) molecules and was thought to be unique to theRR environment, where such small molecules are actively being producedand shielded from the harsh interstellar radiation by a densecircumstellar disk. In this paper we present the BL spectrum detected inseveral ordinary reflection nebulae illuminated by stars havingtemperatures between 10,000 and 23,000 K. All these nebulae are known toalso exhibit the infrared emission features called aromatic emissionfeatures (AEFs) attributed to large PAHs. We present the spatialdistribution of the BL in these nebulae. In the case of Ced 112, the BLis spatially correlated with mid-IR emission structures attributed toAEFs. These observations provide evidence for grain processing andpossibly for in situ formation of small grains and large molecules fromlarger aggregates. Most importantly, the detection of BL in theseordinary reflection nebulae suggests that the BL carrier is anubiquitous component of the interstellar medium and is not restricted tothe particular environment of the RR.
| Dust Properties in the Far-Ultraviolet in Ophiuchus
We have derived the albedo (a) and phase function asymmetry factor (g)of interstellar dust grains at 1100 Å using archival Voyagerobservations of diffuse radiation in Ophiuchus. We have found that thegrains are highly forward-scattering, with g=0.55+/-0.25 anda=0.40+/-0.10. Even though most of the gas in this direction is in theOphiuchus molecular cloud, the diffuse FUV radiation is almost entirelydue to scattering in a relatively thin foreground cloud. This suggeststhat one cannot assume that the UV background is directly correlatedwith the total amount of gas in any direction.
| High Excitation Molecular Gas in the Magellanic Clouds
We present the first survey of submillimeter 12CO J=4-->3emission in the Magellanic Clouds. The survey is comprised of 156'×6' maps obtained using the AST/ROtelescope toward the molecular peaks of the Large and Small MagellanicClouds. We have used these data to constrain the physical conditions inthese objects, in particular, their molecular gas density andtemperature. We find that there are significant amounts of warmmolecular gas associated with most of these molecular peaks and thathigh molecular gas temperatures are pervasive throughout our sample. Wediscuss whether this may be due to the low metallicities and theassociated dearth of gas coolants in the Clouds and conclude that thepresent sample is insufficient to assert this effect.
| CN and HCN in Dense Interstellar Clouds
We present a theoretical investigation of CN and HCN molecule formationin dense interstellar clouds. We study the gas-phase CN and HCNproduction efficiencies from the outer photon-dominated regions (PDRs)into the opaque cosmic-ray-dominated cores. We calculate the equilibriumdensities of CN and HCN and of the associated species C+, C,and CO, as functions of the far-ultraviolet (FUV) optical depth. Weconsider isothermal gas at 50 K, with hydrogen particle densities from102 to 106 cm-3. We study clouds thatare exposed to FUV fields with intensities (at 1000 Å) from1×10-18 to 1×10-14 ergs s-1cm-2 Hz-1 sr-1, or 20 to2×105 times the mean interstellar FUV intensity. Weassume cosmic-ray H2 ionization rates ranging from5×10-17 s-1 to an enhanced value of5×10-16 s-1. We also examine the sensitivityof the density profiles to the gas-phase sulfur abundance.
| H2 Pure Rotational Lines in the Orion Bar
Photodissociation regions (PDRs), where UV radiation dominates theenergetics and chemistry of the neutral gas, contain most of the mass inthe dense interstellar medium of our Galaxy. Observations ofH2 rotational and rovibrational lines reveal that PDRscontain unexpectedly large amounts of very warm (400-700 K) moleculargas. Theoretical models have difficulty explaining the existence of somuch warm gas. Possible problems include errors in the heating andcooling functions or in the formation rate for H2. To date,observations of H2 rotational lines smear out the structureof the PDR. Only by resolving the hottest layers of H2 canone test the predictions and assumptions of current models. Using theTexas Echelon Cross Echelle Spectrograph (TEXES) we mapped emission inthe H2 v=0-0 S(1) and S(2) lines toward the Orion Bar PDR at2'' resolution. We also observed H2 v=0-0 S(4) atselected points toward the front of the PDR. Our maps cover a12'' by 40'' region of the bar where H2rovibrational lines are bright. The distributions of H2 0-0S(1), 0-0 S(2), and 1-0 S(1) line emission agree in remarkable detail.The high spatial resolution (0.002 pc) of our observations allows us toprobe the distribution of warm gas in the Orion Bar to a distanceapproaching the scale length for FUV photon absorption. We use these newobservational results to set parameters for the PDR models described ina companion paper in preparation by Draine et al. The best-fit model canaccount for the separation of the H2 emission from theionization front and the intensities of the ground-state rotationallines, as well as the 1-0 S(1) and 2-1 S(1) lines. This model requiressignificant adjustments to the commonly used values for the dust UVattenuation cross section and the photoelectric heating rate.
| Near-Infrared Imaging of the Star Formation Region AFGL 5142
Near-infrared JHK' and H2 v=1-0 S(1) imaging observations ofthe star-forming region AFGL 5142 are presented. A cluster of youngstars is confirmed to be embedded in the dense molecular cloud core.Many point sources are newly detected. The cluster's K'-magnitudedistribution and [H-K'] color peak have brighter and reddermagnitudes than those outside the cluster. Many of the cluster sourcesexhibit infrared excesses typical of T Tauri stars, Herbig Ae/Be stars,and protostars. The observations also reveal new H2 lineemission around the cluster, exhibiting several jets, filaments, and afaint elliptical structure. The jets are associated with the CO outflowsin the region. The powering sources of the jets, the CO outflows, andthe small faint elliptical structure are identified and analyzedindividually. The strong H2 jets and faint diffuse emissionfurther confirm that the cluster is in an early stage of evolution.
| N88: The physical properties of a compact star-forming region and its neighbourhood in the SMC. II. H and H2 near infrared emission
This paper, the second in a series of papers on the conspicuous compactHII region N88A in the Small Magellanic Cloud, emphasizes the propertiesof the H2 emission as well as of the surrounding stellar content of theentire Henize region N88. Using VJK photometry and stellar evolutionarymodels our analysis of N88 reveals a miscellaneous population of evolvedstars of low and intermediate masses (1.2-8 M_ȯ) in an age range of40 Myr to 6 Gyr. The majority of these stars are found in the cluster HW82 which also contains several main sequence stars. AV inN88A is revisited using near-IR Pδ, Pγ and Pβ Paschenemission lines. Spectroscopic measurements of the H2 emission in Z, J, Hand K bands allow, in conjunction with measurements in the visible, afull spectral coverage from 0.37 μm to 2.45 μm. Long slitspectroscopy through N88 and the nebulosity to the east clearlydistinguishes the ionization zone in N88 characterized by H I and He Iemission with pure H2 emission clearly delineated in the cloud nearby.The principal excitation mechanism for H2 is through fluorescentexcitation in a photodissociation region associated with N88A.
| Which Are the Youngest Protostars? Determining Properties of Confirmed and Candidate Class 0 Sources by Broadband Photometry
We searched the literature to obtain a complete list of known Class 0sources. A list of 95 confirmed or candidate objects was compiled. Tothe best of our knowledge, all published broadband observations from 1μm to 3.5 mm have been collected and are assembled in a catalog.These data were used to determine physical properties (Tbol,Lbol, Lsmm/Lbol, Menv) andfor a uniform classification. Fifty sources possess sufficientobservational data and are classified as Class 0 or Class 0/1 objects.The source properties are compared with different evolutionary models toinfer ages and masses, and their correlations are investigated. About25% of the sources are found to be in a quiet accretion phase or possessa significantly different time evolution of the accretion rate than theaverage. In Taurus, with its isolated star formation mode, this seemsespecially to be the case.
| Modeling the Pan-Spectral Energy Distribution of Starburst Galaxies. I. The Role of ISM Pressure and the Molecular Cloud Dissipation Timescale
In this paper, we combine the stellar spectral synthesis codeSTARBURST99, the nebular modeling code MAPPINGS IIIq, a one-dimensionaldynamical evolution model of H II regions around massive clusters ofyoung stars, and a simplified model of synchrotron emissivity to producepurely theoretical self-consistent synthetic spectral energydistributions (SEDs) for (solar metallicity) starbursts lasting~108 yr. These SEDs extend from the Lyman limit to beyond 21cm. We find that two ISM parameters control the form of the SED: thepressure in the diffuse phase of the ISM (or, equivalently, itsdensity), and the molecular cloud dissipation timescale. In particular,the shape of the far-infrared (dust re-emission) bump is stronglydependent on the mean pressure in the star-forming or starburst galaxy.This can explain the range of far-infrared (FIR) colors seen instarburst galaxies. In the case of objects of composite excitation, suchdiagrams potentially provide a means of estimating the fraction of theFIR emission that is contributed by an active nucleus. We presentdetailed SED fits to Arp 220 and NGC 6240, and we give the predictedcolors for starburst galaxies derived from our models for the IRAS andthe Spitzer Space Telescope MIPS and IRAC instruments. Our modelsreproduce the spread in observed colors of starburst galaxies. From boththe SED fits and the color:color diagrams, we infer the presence of apopulation of compact and ultracompact H II regions around single OBstars or small OB clusters. Finally, we present absolute calibrations toconvert observed fluxes into star formation rates in the UV (GALEX), atoptical wavelengths (Hα), and in the IR (IRAS or Spitzer). We showthat 25 μm fluxes are particularly valuable as star formationindicators, since they largely eliminate one of the parameterscontrolling the IR SED.
| Disk Evolution in the Orion OB1 Association
We analyze multiband photometry of a subsample of low-mass stars in theassociations Ori OB1a and 1b discovered during the Centro deInvestigaciones de Astronomía (CIDA) Orion Variability Survey,which have ages of 7-10 and 3-5 Myr, respectively. We obtainedUBVRCIC photometry at Mount Hopkins for sixclassical T Tauri stars (CTTSs) and 26 weak T Tauri stars (WTTSs) in OriOB1a and for 21 CTTSs and two WTTSs in Ori OB1b. We also obtained L-bandphotometry for 14 CTTSs at Mount Hopkins and 10 and 18 μm photometrywith OSCIR at Gemini for six CTTSs; of these, all six were detected at10 μm, whereas only one was detected at 18 μm. We estimate massaccretion rates from the excess luminosity at U and find that they areconsistent with determinations for a number of other associations, withor without high-mass star formation. The observed decrease of massaccretion rate with age is qualitatively consistent with predictions ofviscous evolution of accretion disks, although other factors can alsoplay a role in slowing accretion rates. We compare the excesses overphotospheric fluxes in H-K, K-L, and K-N with the younger sample ofTaurus and find an overall decrease of disk emission from Taurus to OriOB1b to Ori OB1a. This decrease implies that significant grain growthand settling toward the midplane has taken place in the inner disks ofOri OB1. We compare the spectral energy distribution of the stardetected at both 10 and 18 μm with disk models for similar stellarand accretion parameters. We find that the low fluxes shortward of 18μm of this Ori OB1b star cannot be due to the smaller disk radiusexpected from viscous evolution in the presence of the far-ultravioletradiation fields from the OB stars in the association. Instead, we findthat the disk of this star is essentially a flat disk, with little ifany flaring, indicating a significant degree of dust settling toward themidplane, as expected from dust evolution in protoplanetary disks.
| The CIDA Variability Survey of Orion OB1. I. The Low-Mass Population of Ori OB1a and 1b
We present results of a large-scale, multiepoch optical survey of theOrion OB1 association, carried out with the QUEST camera at theVenezuela National Astronomical Observatory. We identify for the firsttime the widely spread low-mass, young population in the Ori OB1a andOB1b subassociations. Candidate members were picked up by theirvariability in the V band and position in color-magnitude diagrams. Weobtained spectra to confirm membership. In a region spanning ~68deg2, we found 197 new young stars; of these, 56 are locatedin the Ori OB1a subassociation and 141 in Ori OB1b. The spatialdistribution of the low-mass young stars is spatially coincident withthat of the high-mass members but suggests a much sharper edge to theassociation. Comparison with the spatial extent of molecular gas andextinction maps indicates that the subassociation Ori OB1b isconcentrated within a ringlike structure of radius ~2°(~15 pc at 440pc), centered roughly on the star ɛ Ori in the Orion belt. The ringis apparent in 13CO and corresponds to a region with anextinction AV>=1. The stars exhibiting strong Hαemission, an indicator of active accretion, are found along this ring,whereas the center is populated with weak Hα-emitting stars. Incontrast, Ori OB1a is located in a region devoid of gas and dust. Weidentify a grouping of stars within a ~3 deg2 area located inOri OB1a, roughly clustered around the B2 star 25 Ori. The Herbig Ae/Bestar V346 Ori is also associated with this grouping, which could be anolder analog of σ Ori. Using several sets of evolutionary tracks,we find an age of 7-10 Myr for Ori OB1a and of ~4-6 Myr for Ori OB1b,consistent with previous estimates from OB stars. Indicators such as theequivalent width of Hα and near-IR excesses show that the numberof accreting low-mass stars decreases sharply between Ori OB1b and OriOB1a. These results indicate that although a substantial fraction ofaccreting disks remain at ages ~5 Myr, inner disks are essentiallydissipated by 10 Myr.Based on observations obtained at the Llano del Hato NationalAstronomical Observatory of Venezuela, operated by Centro deInvestigaciones de Astronomía (CIDA) for the Ministerio deCiencia y Tecnología, and at the Fred Lawrence WhippleObservatory (FLWO) of the Smithsonian Institution.Based on observations obtained at the 3.5 m WIYN Telescope. The WIYNObservatory is a joint facility of the University of Wisconsin-Madison,Indiana University, Yale University, and the National Optical AstronomyObservatory (NOAO).
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