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Star Formation in the Era of the Three Great Observatories
This paper summarizes contributions and suggestions as presented at theChandra Workshop Star Formation in the Era of Three Great Observatoriesconducted in July 2005. One of the declared goals of the workshop was toraise recognition within the star formation research community about thesensible future utilization of the space observatories Spitzer, Hubble,and Chandra in their remaining years of operation to tackle imminentquestions of our understanding of stellar formation and the earlyevolution of stars. A white paper was generated to support thecontinuous and simultaneous usage of observatory time for star formationresearch. The contents of this paper have been presented and discussedat several other meetings during the course of 2005 and January 2006.
| Millimetre continuum observations of southern massive star formation regions - II. SCUBA observations of cold cores and the dust grain emissivity index (β)
We report the results of a submillimetre continuum emission surveytargeted towards 78 star formation regions, 72 of which are devoid ofmethanol maser and UC HIIregions, identified in the Swedish ESOSubmillimetre Telescope (SEST)/SEST IMaging Bolometer Array (SIMBA)millimetre continuum survey of Hill et al. At least 45 per cent of thelatter sources, dubbed `mm-only', detected in this survey are alsodevoid of the mid-infrared MSXemission. The 450- and 850-μm continuumemission was mapped using the Submillimetre Common User Bolometer Array(SCUBA) instrument on the James Clerk Maxwell Telescope (JCMT). Emissionis detected towards 97 per cent of the 78 sources targeted as well astowards 28 other SIMBA sources lying in the SCUBA fields.In total, we have identified 212 cores in this submillimetre survey,including 106 previously known from the SIMBA survey. Of the remaining106 sources, 53 result from resolving a SIMBA source into multiplesubmillimetre components, whilst the other 53 sources are submillimetrecores, not seen in the SIMBA. Additionally, we have identified twofurther mm-only sources in the SIMBA images. Of the total 405 sourcesidentified in the SIMBA survey, 255 are only seen at millimetrewavelengths.We concatenate the results from four (sub)millimetre continuum surveysof massive star formation, together with the Galactic plane map ofPierce-Price et al. in order to determine the dust grain emissivityindex β for each of the sources in the SIMBA source list. Weexamine the value of β with respect to temperature, as well as forthe source classes identified in the SIMBA survey, for variation of thisindex. Our results indicate that β is typically 2, which isconsistent with previous determinations in the literature, but for aconsiderably larger sample than previous work.
| Faint emission lines in the Galactic HII regions M16, M20 and NGC 3603*
We present deep echelle spectrophotometry of the Galactic HII regionsM16, M20 and NGC 3603. The data have been taken with the Very LargeTelescope Ultraviolet-Visual Echelle Spectrograph in the 3100-10400Å range. We have detected more than 200 emission lines in eachregion. Physical conditions have been derived using different continuumand line intensity ratios. We have derived He+,C++ and O++ abundances from pure recombinationlines as well as collisionally excited lines (CELs) for a large numberof ions of different elements. We have obtained consistent estimationsof the temperature fluctuation parameter, t2, using differentmethods. We also report the detection of deuterium Balmer lines up toDδ (M16) and to Dγ (M20) in the blue wings of the hydrogenlines, which excitation mechanism seems to be continuum fluorescence.The temperature fluctuation paradigm agrees with the results obtainedfrom optical CELs, and the more uncertain ones from far-infraredfine-structure CELs in NGC 3603, although, more observations coveringthe same volume of the nebula are necessary to obtain solid conclusions.
| Molecular line mapping of the giant molecular cloud associated with RCW 106 - I. 13CO
We present the first paper in a series detailing the results of13CO observations of a ~1 deg2 region of the giantmolecular cloud (GMC) complex associated with the HII region RCW 106.The 13CO observations are also the first stage of amultimolecular line study of the same region. These observations wereamongst the first made using the new on-the-fly mapping capability ofthe Australia Telescope National Facility Mopra Telescope. In theconfiguration used, the instrument provided a full width at half-maximum(FWHM) beam size of 33 arcsec and a velocity resolution of 0.17kms-1. The gas emission takes the form of a string of knots,oriented along an axis that extends from the north-west (NW) to thesouth-east (SE) of the field of the observations, and which issurrounded by a more extended, diffuse emission. We analyse the 2Dintegrated 13CO emission using the CLUMPFIND algorithm andidentify 61 clumps. We compare the gas data in the GMC with the dustdata provided by 21-μm Midcourse Space Experiment (MSX) and 1.2-mmSwedish European Southern Observatory Submillimetre Telescope (SEST)images that we both regridded to the cell spacing of the Mopra data andsmoothed to the same resolution. The 13CO emission is morediffuse and extended than the dust emission revealed at the latter twowavebands, which both have a much higher contrast between the peaks andthe extended emission. From comparison of their centre positions, wefind that only ~50 per cent of the 13CO clump fits to thedata are associated with any dust clumps. Using the clump fits, thetotal local thermodynamic equilibrium gas mass above the 3σ levelmeasured from the molecular data is 2.7 ×105Msolar, whereas that measured from the smoothed1.2-mm SEST dust data is 2.2 × 105Msolar.
| A CH3CN and HCO+ survey towards southern methanol masers associated with star formation
We present the initial results of a 3-mm spectral-line survey towards 83methanol maser-selected massive star-forming regions. Here, we reportobservations of the J=5-4 and 6-5 rotational transitions of methylcyanide (CH3CN) and the J=1-0 transition of HCO+and H13CO+.CH3CN emission is detected in 58 sources (70per cent of oursample). We estimate the temperature and column density for 37 of theseusing the rotational diagram (RD) method. The temperatures we deriverange from 28-166K, and are lower than previously reported temperatures,derived from higher J transitions. We find that CH3CN isbrighter and more commonly detected towards ultracompact HII (UCHII)regions than towards isolated maser sources. Detection ofCH3CN towards isolated maser sources strongly suggests thatthese objects are internally heated and that CH3CN is excitedprior to the UCHII phase of massive star formation.HCO+ is detected towards 82 sources (99per cent of oursample), many of which exhibit asymmetric line profiles compared toH13CO+. Skewed profiles are indicative of inwardor outward motions, however, we find approximately equal numbers of red-and blue-skewed profiles among all classes. Column densities are derivedfrom an analysis of the HCO+ and H13CO+line profiles.80 sources have mid-infrared (mid-IR) counterparts: 68 seen in emissionand 12 seen in absorption as `dark clouds'. Seven of the 12 dark cloudsexhibit asymmetric HCO+ profiles, six of which are skewed tothe blue, indicating infalling motions. CH3CN is also commonin dark clouds, where it has a 90per cent detection rate.
| Optique active et optique adaptative.
Not Available
| Caroline Herschel's catalogue of nebulae
Not Available
| 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.
| Modeling the Pan-Spectral Energy Distribution of Starburst Galaxies. II. Control of the H II Region Parameters
We examine, from a theoretical viewpoint, how the physical parameters ofH II regions are controlled in both normal galaxies and in starburstenvironments. These parameters are the H II region luminosity function,the time-dependent size, the covering fraction of molecular clouds, thepressure in the ionized gas, and the ionization parameter. The factorsthat control them are the initial mass function (IMF) of the excitingstars, the cluster mass function, the metallicity, and the mean pressurein the surrounding interstellar medium. We investigate the sensitivityof the Hα luminosity to the IMF, and find that this can translateto more than a factor 2 variation in derived star formation rates. Themolecular cloud dissipation timescale is estimated from a case study ofM17 to be ~1 Myr for this object. Based on H II luminosity functionfitting for nearby galaxies, we suggest that the H II region clustermass function is fitted by a lognormal form peaking at ~100Msolar. The cluster mass function continues the stellar IMFto a higher mass regime. The pressure in the H II regions is controlledby the mechanical luminosity flux from the central cluster. Since thisis closely related to the ionizing photon flux, we show that theionization parameter is not a free variable, and that the diffuseionized medium may be composed of many large, faint, and old H IIregions. Finally, we derive theoretical probability distributions forthe ionization parameter as a function of metallicity and compare theseto those derived for SDSS galaxies.
| A Remnant Disk around a Young Massive Star
While the formation of low-mass stars has become a well-studied process,it is still difficult to verify a similar evolutionary sequence formassive stars. Although several young stages from massive starless coresto massive protostellar candidates with jets and outflows have beenobserved, massive star/disk systems whose properties can be inferreduniquely are rare. The final stage of this sequence, i.e., a newbornmassive star that is still surrounded by a remnant disk, is missing.This is probably a consequence of the rapid evolution of these systemsand the early destruction of the disk in the vicinity of a massive star.We report on an optically visible young massive star (IRS 15) within M17that displays a huge IR excess. This fortunate coincidence offers therare opportunity to investigate the star as well as its circumstellarenvironment in great detail. We have performed both optical and infraredphotometry and spectroscopy of the stellar source; in addition, itscircumstellar environment has been investigated by mid-infrared imaging.Our data suggest that IRS 15 is a star of about 26 Msolarsurrounded by a huge remnant disk of about half a Jupiter mass of dust.From this, we corroborate that massive stars can form by disk accretionand conclude that also their circumstellar disks evolve like those oflow-mass stars.
| High-Mass Star Formation. II. The Mass Function of Submillimeter Clumps in M17
We have mapped a ~5.5×5.5 pc portion of the M17 massivestar-forming region in both 850 and 450 μm dust continuum emissionusing the Submillimeter Common-User Bolometer Array (SCUBA) on the JamesClerk Maxwell Telescope (JCMT). The maps reveal more than 100 dustyclumps with deconvolved linear sizes of ~0.05-0.2 pc and masses of~0.8-120 Msolar, most of which are not associated with knownmid-infrared point sources. Fitting the clump mass function with adouble power law gives a mean power-law exponent ofαhigh=-2.4+/-0.3 for the high-mass power law,consistent with the exponent of the Salpeter stellar mass function. Weshow that a lognormal clump mass distribution with a peak at ~4Msolar produces as good a fit to the clump mass function asdoes a double power law. This 4 Msolar peak mass is wellabove the peak masses of both the stellar initial mass function and themass function of clumps in low-mass star-forming regions. Despite thedifference in intrinsic mass scale, the shape of the M17 clump massfunction appears to be consistent with the shape of the core massfunction in low-mass star-forming regions. Thus, we suggest that theclump mass function in high-mass star-forming regions may be a scaled upversion of that in low-mass regions, instead of its extension to highermasses.
| Balmer and Paschen Jump Temperature Determinations in Low-Metallicity Emission-Line Galaxies
We have used the Balmer and Paschen jumps to determine the temperaturesof the H+ zones of a total sample of 47 H II regions. TheBalmer jump was used on MMT spectrophotometric data of 22low-metallicity H II regions in 18 blue compact dwarf (BCD) galaxies andof one H II region in the spiral galaxy M101. The Paschen jump was usedon spectra of 24 H II emission-line galaxies selected from the DataRelease 3 of the Sloan Digital Sky Survey (SDSS). To derive thetemperatures, we have used a Monte Carlo technique varying the electrontemperature in the H+ zone, the extinction of the ionized gasand that of the stellar population, the relative contribution of theionized gas to the total emission, and the star formation history to fitthe spectral energy distribution of the galaxies. For the MMT spectra,the fit was done in the wavelength range 3200-5200 Å, whichincludes the Balmer discontinuity, and for the SDSS spectra, in thewavelength range 3900-9200 Å, which includes the Paschendiscontinuity. We find for our sample of H II regions that thetemperatures of the O2+ zones determined from thenebular-to-auroral line intensity ratio of doubly ionized oxygen [O III]λλ(4959+5007)/λ4363 do not differ, in a statisticalsense, from the temperatures of the H+ zones determined fromfitting the Balmer and Paschen jumps and the spectral energydistributions (SEDs). We cannot rule out small temperature differencesof the order of 3%-5%.
| 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.
| Investigation of Diffuse Hard X-Ray Emission from the Massive Star-forming Region NGC 6334
Chandra ACIS-I data of the molecular cloud and H II region complex NGC6334 were analyzed. The hard X-ray clumps detected with ASCA (Sekimotoand coworkers) were resolved into 792 point sources. After removing thepoint sources, an extended X-ray emission component was detected over a5×9 pc2 region, with the 0.5-8 keV absorption-correctedluminosity of 2×1033 ergs s-1. Thecontribution from faint point sources to this extended emission wasestimated as at most ~20%, suggesting that most of the emission isdiffuse in nature. The X-ray spectrum of the diffuse emission wasobserved to vary from place to place. In tenuous molecular cloud regionswith hydrogen column density of (0.5-1)×1022cm-2, the spectrum can be represented by a thermal plasmamodel with temperatures of several keV. The spectrum in dense cloudcores exhibits harder continuum, together with higher absorption of morethan ~3×1022 cm-2. In some of such highlyobscured regions, the spectra show extremely hard continua equivalent toa photon index of ~1, and favor a nonthermal interpretation. Theseresults are discussed in the context of thermal and nonthermal emission,both powered by fast stellar winds from embedded young early-type starsthrough shock transitions.
| Cloud Fragmentation and Proplyd-like Features in H II Regions Imaged by the Hubble Space Telescope
We have analyzed Hubble Space Telescope ACS and WFPC2 new and archivalimages of eight H II regions to look for new protoplanetary disks(proplyds) similar to those found in the Orion Nebula. We find a wealthof features similar in size (although many are larger) to the brightcusps around the Orion Nebula proplyds. None of them, however, containsa definitive central star. From this, we deduce that the new cusps maynot be proplyds but instead fragments of molecular cloud material. Outof all the features found in the eight H II regions examined, only one,an apparent edge-on silhouette in M17, may have a central star. Thisfeature might join the small number of bona fide proplyds found outsidethe Orion Nebula, in M8, M20, and possibly M16. In line with the resultsfound recently by Smith et al., the paucity of proplyds outside theOrion Nebula can be explained by their transient nature, as well as bythe specific environmental conditions under which they can be observed.Several fragments are seen as dark silhouettes against a brightbackground. We have reanalyzed those found in IC 2944 by Reipurth et al.and found new, similar ones in M16. None of these fragments contains acentral star, and we exclude the possibility that they are disks.Reipurth et al. concluded that the IC 2944 silhouettes are not starforming. We argue here that their assumption of a constant optical depthfor these fragments is not physical and that it is more likely thatthese fragments are star forming, a condition that is supported,although not proved, by their shapes and distributions. The process ofcloud fragmentation and photoevaporation produces a large number ofsmall fragments, while the size hierarchy expected in a photoevaporativeenvironment would not favor small fragments. The size distributionsobserved will constrain any future theories of cloud fragmentation. Onebright microjet candidate is found in M17, protruding from a large,limb-brightened fragment. A second, larger, jetlike feature, similar inshape and size to a Herbig-Haro jet, is found in Pismis 24. No centralstar appears to be associated with either of these jet candidates.Based on observations made with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS5-26555.
| A Chandra ACIS Study of 30 Doradus. II. X-Ray Point Sources in the Massive Star Cluster R136 and Beyond
We have studied the X-ray point-source population of the 30 Doradus (30Dor) star-forming complex in the Large Magellanic Cloud using highspatial resolution X-ray images and spatially resolved spectra obtainedwith the Advanced CCD Imaging Spectrometer (ACIS) on board the ChandraX-Ray Observatory. Here we describe the X-ray sources in a17'×17' field centered on R136, the massivestar cluster at the center of the main 30 Dor nebula. We detect 20 ofthe 32 Wolf-Rayet stars in the ACIS field. The cluster R136 is resolvedat the subarcsecond level into almost 100 X-ray sources, including manytypical O3-O5 stars, as well as a few bright X-ray sources previouslyreported. Over 2 orders of magnitude of scatter in LX is seen among R136O stars, suggesting that X-ray emission in the most massive starsdepends critically on the details of wind properties and the binarity ofeach system, rather than reflecting the widely reported characteristicvalue LX/Lbol~=10-7. Such a canonicalratio may exist for single massive stars in R136, but our data are tooshallow to confirm this relationship. Through this and future X-raystudies of 30 Dor, the complete life cycle of a massive stellar clustercan be revealed.
| Diameters of Open Star Clusters
The present paper presents a tabulation of data on all 600 Galactic openclusters for which it is currently possible to calculate lineardiameters. As expected, the youngest ``clusters,'' with ages <15 Myr,contain a significant (>=20%) admixture of associations. Amongintermediate-age clusters, with ages in the range 15 Myr to 1.5 Gyr, themedian cluster diameter is found to increase with age. Small, compactclusters are rare among objects with ages >1.5 Gyr. Open clusterswith ages >1 Gyr appear to form what might be termed a ``clusterthick disk,'' part of which consists of objects that were probablycaptured gravitationally by the main body of the Galaxy.
| Modeling the NIR-silhouette massive disk candidate in M 17
Aims.The physical properties of the massive disk candidate in thestar-forming region M 17 are analyzed. Methods: .Making use ofthe rare configuration in which the gas and dust structure is seen insilhouette against the background radiation at λ=2.2~μm, wedetermine the column density distribution from a high-resolutionNAOS/CONICA image. The influence of scattered light on the massdetermination is analyzed using 3D radiative transfer calculations.Further upper flux limits derived from observations with the Spitzertelescope at MIR wavelengths are used together with the NACO image toestimate the flux from the central object. For a range of stellar radii,stellar surface temperatures, and dust grain sizes, we apply threedifferent models to account for the observed fluxes. The stability ofthe disk against self-gravitational forces is analyzed calculating theratio of the gravitational acceleration by the central object and thedisk, and the deviations from a Keplerian profile. Results: .Wefind that the column density is consistent with a central sourcesurrounded by a rotationally symmetric distribution of gas and dust. Theextent of the symmetric disk part is about 3000 AU, with a warpedpoint-symmetrical extension beyond that radius, and therefore largerthan any circumstellar disk yet detected. The modeling yields a radialdensity powerlaw exponent of -1.1 indicating a flat radial densitydistribution, and a large e-folding scale height ratio H/R of about 0.5.The mass of the entire disk estimated from the column density isdiscussed depending on the assumed distance and the dust model andranges between 0.02 and 5 Mȯ. We conclude that unless astar is located close to the disk in the foreground, scattered lightwill have little influence on the mass determination. We present aSpitzer image taken at λ=7.8~μm with the disk seen in emissionand identify polycyclic aromatic hydrocarbon (PAH) emission on the disksurface excited by the nearby massive stars as a possible source. Our 3Dradiative transfer calculations for the scattered light image of thecentral source through an edge-on disk indicate that the ellipticalshape seen in the NACO image does not require the assumption of a binarysystem and that it is consistent with a single object. We derive stellarmain sequence masses of several Mȯ, 50Mȯ, or 10 Mȯ, depending on ourassumptions that the extinction of the stellar flux is dominated (i) bythe outer disk, (ii) by an inner disk comparable to the disks aroundintermediate-mass stars, or (iii) by an inner disk with dominating hotdust emission. We find that even for a star-disk mass ratio of 1, onlythe outer parts of the circumstellar disk may be influenced byself-gravity effects due to the large e-folding scale height ratio.
| VLT K-band spectroscopy of massive young stellar objects in (ultra-)compact HII regions
High-quality K-band spectra of strongly reddened point sources, deeplyembedded in (ultra-)compact H II, have revealed a population of 20 youngmassive stars showing no photospheric absorption lines, but sometimesstrong Brγ emission. The Brγ equivalent widths occupy a widerange (from about 1 to over 100 Å); the line widths of 100-200 kms-1 indicate a circumstellar rather than a nebular origin.The K-band spectra exhibit one or more features commonly associated withmassive young stellar objects (YSOs) surrounded by circumstellarmaterial: a very red colour (J-K) 2, CO bandhead emission,hydrogen emission lines (sometimes doubly peaked), and Fe II and/or MgII emission lines. The large number of objects in our sample allows amore detailed definition and thorough investigation of the properties ofthe massive YSOs. In the (K, J-K) colour-magnitude diagram (CMD) themassive YSO candidates are located in a region delimited by the OBzero-age main sequence, Be stars, Herbig Ae and Be stars, and B[e]supergiants. The massive YSO distribution in the CMD suggests that themajority of the objects are of similar spectral type as the Herbig Bestars, but some of them are young O stars. The spectral properties ofthe observed objects do not correlate with the location in the CMD. TheCO emission must come from a relatively dense ( 1010cm-3) and hot (T 2000-5000 K) region, sufficientlyshielded from the intense UV radiation field of the young massive star.The hydrogen emission is produced in an ionised medium exposed to UVradiation. The best geometrical solution is a dense and neutralcircumstellar disk causing the CO bandhead emission, and an ionisedupper layer where the hydrogen lines are produced. We present argumentsthat the circumstellar disk is more likely a remnant of the accretionprocess than the result of rapid rotation and mass loss such as inBe/B[e] stars.
| A SCUBA imaging survey of ultracompact HII regions. The environments of massive star formation
We present a SCUBA submillimetre (450 and 850 μm) survey of theenvironment of 105 IRAS point sources, selected from the Wood &Churchwell (1989a) and Kurtz et al. (1994) radio ultracompact (UC) Hiiregion surveys. We detected a total of 155 sub-mm clumps associated withthe IRAS point sources and identified three distinct types of object:ultracompact cm-wave sources that are not associated with any sub-mmemission (sub-mm quiet objects), sub-mm clumps that are associated withultracompact cm-wave sources (radio-loud clumps); and sub-mm clumps thatare not associated with any known ultracompact cm-wave sources(radio-quiet clumps). 90% of the sample of IRAS point sources were foundto be associated with strong sub-mm emission. We consider the sub-mmcolours, morphologies and distance-scaled fluxes of the sample of sub-mmclumps and show that the sub-mm quiet objects are unlikely to representembedded UC Hii regions unless they are located at large heliocentricdistances. Many of the 2.5 arcmin SCUBA fields contain more than onesub-mm clump, with an average number of companions (the companion clumpfraction) of 0.90. The clumps are more strongly clustered than othercandidate HMPOs and the mean clump surface density exhibits a brokenpower-law distribution with a break at 3 pc. We demonstrate that thesub-mm and cm-wave fluxes of the majority of radio-loud clumps are inexcellent agreement with the standard model of ultracompact Hii regions.We speculate on the nature of the radio-quiet sub-mm clumps and, whilstwe do not yet have sufficient data to conclude that they are in a pre-UCHii region phase, we argue that their characteristics are suggestive ofsuch a stage.
| L-band (3.5 μm) IR-excess in massive star formation. II. RCW 57/NGC 3576
Context: .We present a JHKsL survey of the massive star forming regionRCW 57 (NGC 3576) based on L-band data at 3.5 μm taken with SPIREX(South Pole Infrared Explorer), and 2MASS JHKs data at 1.25-2.2 μm.This is the second of two papers, the first one concerning a similarJHKsL survey of 30 Doradus. Aims: .Colour-colour andcolour-magnitude diagrams are used to detect sources with infraredexcess. This excess emission is interpreted as coming from circumstellardisks, and hence gives the cluster disk fraction (CDF). Based on the CDFand the age of RCW 57, it is possible to draw conclusions on theformation and early evolution of massive stars. Methods: .Theinfrared excess is detected by comparing the locations of sources inJHKsL colour-colour and L vs. (K_s-L) colour-magnitude diagrams to thereddening band due to interstellar extinction. Results: .A totalof 251 sources were detected. More than 50% of the 209 sources includedin the diagrams have an infrared excess. Conclusions: .Comparisonwith other JHKsL surveys, including the results on 30 Doradus from thefirst paper, support a very high initial disk fraction (>80%) evenfor massive stars, although there is an indication of a possible fasterevolution of circumstellar disks around high mass stars. 33 sources onlyfound in the L-band indicate the presence of heavily embedded, massiveClass I protostars. We also report the detection of diffuse PAHsemission throughout the RCW 57 region.
| An empirical calibration of sulphur abundance in ionised gaseous nebulae
We have derived an empirical calibration of the abundance of S/H as afunction of the S{23} parameter, defined using the bright sulphur linesof [SII] and [SIII]. Contrary to the case for the widely used O{23}parameter, the calibration remains single valued up to the abundancevalues observed in the disk HII regions. The calibration is based on alarge sample of nebulae for which direct determinations of electrontemperatures exist and the sulphur chemical abundances can be directlyderived. ICFs, as derived from the [SIV] 10.52 μ emission line (ISOobservations), are shown to be well reproduced by Barker's formula for avalue of α = 2.5. Only about 30% of the objects in the samplerequire ICFs larger than 1.2. The use of the proposed calibration opensthe possibility of performing abundance analysis with red to IRspectroscopic data using S/H as a metallicity tracer.
| Calibration of the PRONAOS/SPM submillimeter photometer
Context: . Aims: .PRONAOS is a stratospheric balloon-borne projectdedicated to submillimeter astronomy. Sensitive to low emissiongradients, PRONAOS allowed pointed observations in four photometricbands between 200 μm and 1.2 mm with an angular resolution of 2' to3.5'. We present here the PRONAOS calibration strategy and the accuracyachieved. Methods: .An absolute calibration scheme based on anInternal Calibration System provided original photometric data. Theaccurate determination of the spectral dependence of the measured fluxesis a key to the understanding of the cold universe. Results: . Wedemonstrate that the PRONAOS absolute photometric accuracy is 8%(1-σ) and that the channel to channel relative accuracy is 6%(1-σ). This calibration is checked against observations of Saturn.We show that the PRONAOS calibration is in good agreement with thepresent knowledge of the Saturn submillimeter emission. PRONAOSobservations also agrees with the observations of several compact andextended sources by the COBE/DIRBE or the ISOPHOT instruments.Conclusions: .
| ISM properties in low-metallicity environments
We present new ISOCAM mid-infrared spectra of three starbursting nearbydwarf galaxies, NGC 1569, NGC 1140 and II Zw 40 and the 30 Dor region ofthe LMC and explore the properties of the ISM in low-metallicityenvironments, also using additional sources from the literature. Weanalyse the various components of the ISM probed by the mid-infraredobservations and compare them with other Galactic and extragalacticobjects. The MIR spectra of the low-metallicity starburst sources aredominated by the [ Ne iii] λ 15.56~μ m and [ S iv] λ10.51~μ m lines, as well as a steeply rising dust continuum. PAHbands are generaly faint, both locally and averaged over the fullgalaxy, in stark contrast to dustier starburst galaxies, where the PAHfeatures are very prominant and even dominate on global scales. Thehardness of the modeled interstellar radiation fields for the dwarfgalaxies increases as the presence of PAH band emission becomes lesspronounced. The [ Ne iii] /[ Ne ii] ratios averaged over the full galaxyare strikingly high, often >10. Thus, the hard radiation fields arepronounced and pervasive. We find a prominent correlation between thePAHs/VSGs and the [ Ne iii] /[ Ne ii] ratios for a wide range ofobjects, including the low metallicity galaxies as well as Galactic H iiregions and other metal-rich galaxies. This effect is consistent withthe hardness of the interstellar radiation field playing a major role inthe destruction of PAHs in the low metallicity ISM. We see a PAHs/VSGsand metallicity correlation, also found by Engelbracht et al. (2005,ApJ, 628, 29) for a larger survey. Combined effects of metallicity andradiation field seem to be playing important roles in the observedbehavior of PAHs in the low metallicity systems.
| Faint C and O recombination lines in H II regions and large telescopes: The case of S311
We present preliminary results on the analysis of very deep echelle UVES(8m VLT) spectra of the Galactic H II region S311 (NGC 2467) locatedoutside the Solar circle. The data cover from 3100 to 10450 Å witha resolution R ~ 8800. We have detected and measured more than 300emission lines, some of them are faint recombination lines ofheavy-element ions. We have derived the O abundances from OI and OIIrecombination lines and the C abundance from CII lines and an IonizationCorrection Factor (ICF). This kind of observations will permit us toderive the Galactic abundance gradient of C and O from nebularrecombination lines, which are almost independent on the realtemperature structure of the nebula.
| Oxygen Recombination Line Abundances in Gaseous Nebulae
The determination of the heavy element abundances from giantextragalactic H II regions has been generally based on collisionallyexcited lines. We will discuss the reasons to study the characteristicsof recombination lines, and then use these lines to determine chemicalabundances. Of these lines the oxygen (specifically the O II) lines arethe most important; and, of them, the lines of multiplet 1 of O II arethe most accessible. It has often been assumed that by measuring theintensity of a single line within a multiplet the intensities of all thelines in the multiplet can be determined; in recent studies we havefound that the intensity ratios of lines within a multiplet can dependon density; we will present empirical density-intensity relationshipsfor multiplet 1 based on recent observations of H II regions andplanetary nebulae. From observations of H II regions we find that thecritical density for collisional redistribution of the multiplet 1 O IIrecombination lines amounts to 2800+/-500 cm-3. We point out that theO/H recombination abundances of H II regions in the solar vicinity arein excellent agreement with the O/H solar value, while the abundancesderived from collisionally excited lines are not. We present acalibration of Pagel's method in the 8.2 < 12 + log O/H < 8.8range based on O recombination lines.
| Infrared Detectors for Astrophysics
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| No high-mass protostars in the silhouette young stellar object M17-SO1
The birth of very massive stars is not well understood, in contrast tothe formation process of low-mass stars like our Sun. It is not evenclear that massive stars can form as single entities; rather, they mightform through the mergers of smaller ones born in tight groups. Therecent claim of the discovery of a massive protostar in M17 (a nearbygiant ionized region) forming through the same mechanism as low-massstars has therefore generated considerable interest. Here we show thatthis protostar has an intermediate mass of only 2.5 to 8 solar masses(Msolar), contrary to the earlier claim of20Msolar (ref. 8). The surrounding circumstellar envelopecontains only 0.09Msolar and a much more extended localmolecular cloud has 4-9Msolar.
| First science with SINFONI
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