Swedish-ESO Submillimetre Telescope Research Articles

CHEMICAL AND PHYSICAL CONDITIONS IN MOLECULAR CLOUD CORE DC 000.4–19.5 (SL42) IN CORONA AUSTRALIS

Chemical reactions in starless molecular clouds are heavily dependent on interactions between gas phase material and solid phase dust and ices. We have observed the abundance and distribution of molecular gases in the cold, starless core DC 000.4-19.5 (SL42) in Corona Australis using data from the Swedish ESO Submillimeter Telescope. We present column density maps determined from measurements of C18O(J=2-1,1-0) and N2H+(J=1-0) emission features. Herschel data of the same region allow a direct comparison to the dust component of the cloud core and provide evidence for gas phase depletion of CO at the highest extinctions. The dust color emperature in the core calculated from Herschel maps ranges from roughly 10.7 to 14.0 K. This range agrees with the previous determinations from Infrared Space Observatory and Planck observations. The column density profile of the core can be fitted with a Plummer-like density distribution approaching n(r) ~ r^{-2} at large distances. The core structure deviates clearly from a critical Bonnor-Ebert sphere. Instead, the core appears to be gravitationally bound and to lack thermal and turbulent support against the pressure of the surrounding low-density material: it may therefore be in the process of slow contraction. We test two chemical models and find that a steady-state depletion model agrees with the observed C18O column density profile and the observed N(C18O) versus AV relationship.

DISCOVERY OF TWO NEW CLASS II METHANOL MASER TRANSITIONS IN G 345.01+1.79

We have used the Swedish ESO Submillimetre Telescope (SEST) to search for new class II methanol maser transitions towards the southern source G345.01+1.79. Over a period of 5 days we observed 11 known or predicted class II methanol maser transitions. Emission with the narrow line width and characteristic velocity of class II methanol masers (in this source) was detected in 8 of these transitions, two of which have not previously been reported as masers. The new class II methanol maser transitions are the 13(-3)-12(-4)E transition at 104.1 GHz and the 5(1)-4(2)E transition at 216.9 GHz. Both of these are from transition series for which there are no previous known class II methanol maser transitions. This takes the total number of known class II methanol maser series to 10, and the total number of transitions (or transition groups) to 18. The observed 104.1 GHz maser suggests the presence of two or more regions of masing gas with similar line of sight velocities, but quite different physical conditions. Although these newly discovered transitions are likely to be relatively rare, where they are observed combined studies using the Australia Telescope Compact Array and the Atacama Large Millimeter Array offer the prospect to be able to undertake multi-transition methanol maser studies with unprecedented detail.

Gas and star formation in the Circinus galaxy

We present a detailed study of the Circinus galaxy, investigating its star formation, dust and gas properties, both in the inner and outer disc. To achieve this, we obtained high-resolution Spitzer mid-infrared images with the IRAC (3.6, 5.8, 4.5 and 8.0 μm) and MIPS (24 and 70 μm) instruments and sensitive H i data from the Australia Telescope Compact Array and the 64-m Parkes telescope. These were supplemented by CO maps from the Swedish-ESO Submillimetre Telescope. Because Circinus is hidden behind the Galactic plane, we demonstrate the careful removal of foreground stars as well as large- and small-scale Galactic emission from the Spitzer images. We derive a visual extinction of AV = 2.1 mag from the spectral energy distribution of the Circinus galaxy and total stellar and gas masses of 9.5 × 1010 and 9 × 109 M⊙, respectively. Using various wavelength calibrations, we find obscured global star formation rates between 3 and 8 M⊙ yr−1. Star-forming regions in the inner spiral arms of Circinus, which are rich in H i, are beautifully unveiled in the Spitzer 8 μm image. The latter is dominated by polycyclic aromatic hydrocarbon (PAH) emission from heated interstellar dust. We find a good correlation between the 8 μm emission in the arms and regions of dense H i gas. The (PAH 8 μm)/24 μm surface brightness ratio shows significant variations across the disc of Circinus.

Ammonia towards dust clumps in the giant molecular cloud associated with RCW 106

AbstractHigh-mass stars are known to be born within giant molecular clouds (GMCs); However, the exact processes involved in forming a high-mass star are still not well understood. It is clear that high-mass stars do not form in isolation, and that the processes surrounding high-mass star formation may affect the environment of the entire molecular cloud. We are studying the GMC associated with RCW 106 (G333), which is one of the most active massive-star formation regions in the Galactic plane. This GMC, located at l = 333° b = − 0.5°, has been mapped in over 20 molecular line transitions with the Mopra radio telescope (83-110 GHz), in Australia, and with the Swedish-ESO Submillimeter Telescope (SEST) in the 1.2 mm cool dust continuum. The region is also within the Spitzer GLIMPSE infrared survey (3.6, 4.5, 5.8, and 8.0 μm) area. We have decomposed the dust continuum using a clump-finding algorithm (CLUMPFIND), and are using the multiple molecular line traditions from the Mopra radio telescope to classify the type and stage of star formation taking place therein. Having accurate physical temperatures of the star forming clumps is essential to constrain other parameters to within useful limits. To achieve this, we have obtained pointed NH3 observations from the Tidbinbilla 70-m radio telescope, in Australia, towards these clumps.

Massive star formation in the GMC G345.5+1.0: spatial distribution of the dust emission

We attempt to make a complete census of massive-star formation within all of GMC G345.5+1.0. This cloud is located one degree above the galactic plane and at 1.8 kpc from the Sun, thus there is little superposition of dust along the line-of-sight, minimizing confusion effects in identifying individual clumps. We observed the 1.2 mm continuum emission across the whole GMC using the Swedish-ESO Submillimetre Telescope Imaging Bolometer Array mounted on the SEST. Observations have a spatial resolution of 0.2 pc and cover 1.8 deg\times 2.2 deg in the sky with a noise of 20 mJy/beam. We identify 201 clumps with diameters between 0.2 and 0.6 pc, masses between 3.0 and 1.3\times10^3 Msun, and densities between 5\times10^3 and 4\times10^5 cm^-3. The total mass of the clumps is 1.2\times10^4 Msun, thus the efficiency in forming these clumps, estimated as the ratio of the total clump mass to the total GMC mass, is 0.02. The clump mass distribution for masses between 10 and 10^3 Msun is well-fitted by a power law dN/dM proportional to M^-alpha, with a spectral mass index alpha of 1.7+/-0.1. Given their mass distribution, clumps do not appear to be the direct progenitors of single stars. Comparing the 1.2 mm continuum emission with infrared images taken by the Midcourse Space Experiment (MSX) and by the SPITZER satellite, we find that at least 20% of the clumps are forming stars, and at most 80% are starless. Six massive-star forming regions embedded in clumps and associated with IRAS point sources have mean densities of ~10^5 cm^-3, luminosities >10^3 Lsun, and spectral energy distributions that can be modeled with two dust components at different mean temperatures of 28+/-5 and 200+/-10 K.

Multiwavelength observations of the supernova remnant G349.7+0.2 interacting with a molecular cloud

We present molecular-line observations at millimetre, centimetre and infrared wavelengths of the region containing OH(1720 MHz) masers in the supernova remnant (SNR) G349.7+0.2, using the Australia Telescope (AT) Mopra antenna, the Swedish-ESO Submillimeter Telescope, the AT Compact Array and the UNSW Infrared Fabry-Perot narrow-band filter installed on the Anglo-Australian Telescope. Several molecular transitions were observed between 1.6 and 3 mm to constrain the physical parameters of the molecular cloud interacting with the SNR and to investigate the effects of the SNR shock on the gas chemistry. We detected shock-excited near-infrared H2 emission towards the centre of the SNR, revealing highly clumped molecular gas and a good correlation with published mid-infrared images from the Spitzer Space Telescope. An excellent correlation between the H2 clumps and OH(1720 MHz) maser positions supports the shock excitation of the OH(1720 MHz) maser emission. Furthermore, we detected OH absorption at 1665 and 1667 MHz which shows a good correlation with the shocked H2 emission and the masers. We found maser emission at 1665 MHz near the OH(1720 MHz) masers in this SNR, which is found to be associated with a GLIMPSE source SSTGLMC G349.7294+00.1747. We also detected 1665 and 1667 MHz OH masers, and weak 4.8 GHz H2CO absorption towards the ultracompact HII region IRAS 17147-3725 located to the southeast of the SNR. We found no 4.7- or 6-GHz excited-state OH masers or 6-GHz CH3OH maser towards either the SNR or the HII region.

The large-scale atomic and molecular gas in the Circinus galaxy

We have used the Australia Telescope Compact Array (ATCA) and the Swedish-ESO Submillimetre Telescope (SEST) to map the large-scale atomic and molecular gas in the nearby (4 Mpc) Circinus galaxy. The ATCA HI mosaic of Circinus exhibits the warps in position angle and inclination revealed in the single-pointing image of Jones et al., both of which appear to settle beyond the inner 30 kpc which was previously imaged. The molecular gas has been mapped in both the CO J = 1 → 0 and J = 2 → 1 transitions down to a column density of N H2 ≥10 21 cm -2 (3σ), where we derive a total molecular gas mass of M H2 ≈ 2 x 10 9 M ⊙ . Within a radius of 3 kpc, i.e. where CO was clearly detected, the molecular fraction climbs steeply from ≈0.7 to unity (where N H2 = 4 x 10 22 cm -2 , cf. N H1 = 10 21 cm -2 ) with proximity to the nucleus. Our Hi mosaic gives an atomic gas mass of M HI ≈ 6 x 10 9 M ⊙ , which is 70 per cent of the fully mapped single dish value. Combining the atomic and molecular gas masses gives a total gas mass of M gas ≡ M HI + M H2 ≈ 1 x 10 10 M ⊙ , cf. the total dynamical mass of ≈ 3 x 10 11 M ⊙ within the inner 50 kpc of our mosaiced image. The total neutral gas mass to dynamical mass ratio is therefore 3 per cent, consistent with the SAS3 (Third Small Astronomy Satellite) classification of Circinus. The high (molecular) gas mass fraction of M H2 /M dyn ≈ 50 per cent found by Curran et al., only occurs close to the central ≈0.5 kpc and falls to ≤10 per cent within and outwith this region, allaying previous concerns regarding the validity of applying the Galactic N H2 /Ι CΟ conversion ratio to Circinus. The rotation curve, as traced by both the H i and CO, exhibits a steep dip at ≈ 1 kpc, the edge of the atomic/molecular ring, within which the starburst is occurring. We find the atomic and molecular gases to trace different kinematical features and believe that the fastest part (≥130km s -1 ) of the subkpc ring consists overwhelmingly of molecular gas. Beyond the inner kpc, the velocity climbs to settle into a solid body rotation of ≈150 km s-1 at ≥10kpc. Most of the starlight emanates from within this radius and so much of the dynamical mass, which remains climbing to the limit of our data (≥50 kpc), must be due to the dark matter halo.

A Multiwavelength Study of Young Massive Star Forming Regions. II. The Dust Environment

We present observations of 1.2-mm dust continuum emission, made with the Swedish ESO Submillimeter Telescope, towards eighteen luminous IRAS point sources, all with colors typical of compact HII regions and associated with CS(2-1) emission, thought to be representative of young massive star forming regions. Emission was detected toward all the IRAS objects. We find that the 1.2-mm sources associated with them have distinct physical parameters, namely sizes of 0.4 pc, dust temperatures of 30 K, masses of 2x10^3 Msun, column densities of 3x10^23 cm^-2, and densities of 4x10^5 cm^-3. We refer to these dust structures as massive and dense cores. Most of the 1.2-mm sources show single-peaked structures, several of which exhibit a bright compact peak surrounded by a weaker extended envelope. The observed radial intensity profiles of sources with this type of morphology are well fitted with power-law intensity profiles with power-law indices in the range 1.0-1.7. This result indicates that massive and dense cores are centrally condensed, having radial density profiles with power-law indices in the range 1.5-2.2. We also find that the UC HII regions detected with ATCA towards the IRAS sources investigated here (Paper I) are usually projected at the peak position of the 1.2-mm dust continuum emission, suggesting that massive stars are formed at the center of the centrally condensed massive and dense cores.

The interaction of young massive stars with their environment

Using the 15-m Swedish ESO Sub-millimeter Telescope (SEST), CO, HCN, and HCO+ observations of the galactic star-forming region NGC 6334 FIR II are presented, complemented by [C i] 3PP0 and 3PP1 data from the Atacama Pathfinder Experiment (APEX 12-m telescope). Embedded in the extended molecular cloud and associated with the H ii region NGC 6334–D, there is a molecular “void”. [C i] correlates well with 13CO and other molecular lines and shows no rim brightening relative to molecular cloud regions farther off the void. While an interpretation in terms of a highly clumped cloud morphology is possible, with photon dominated regions (PDRs) reaching deep into the cloud, the data do not provide any direct evidence for a close association of [C i] with PDRs. Kinetic temperatures are ~40-50 K in the molecular cloud and 200 K toward the void. CO and [C i] excitation temperatures are similar. A comparison of molecular and atomic fine structure line emission with the far infrared and radio continuum as well as the distribution of 2.2m H2 emission indicates that the well-evolved H ii region expands into a medium that is homogeneous on pc-scales. If the H2 emission is predominantly shock excited, both the expanding ionization front (classified as subsonic, “D-type”) and the associated shock front farther out (traced by H2) can be identified, observationally confirming for the first time a classical scenario that is predicted by evolutionary models of Hii regions. Integrated line intensity ratios of the observed molecules are determined, implying a mean C18O/C17O abundance ratio of 4.13 ± 0.13 that reflects the 18O/17O isotope ratio. This ratio is consistent with values determined in nearby clouds. Right at the edge of the void, however, the oxygen isotope ratio might be smaller.

Dust and CO emission towards the centers of normal galaxies, starburst galaxies and active galactic nuclei

Aims. The amount of interstellar matter in a galaxy determines its evolution, star formation rate and the activity phenomena in the nucleus. We therefore aimed at obtaining a data base of the 12 CO line and thermal dust emission within equal beamsizes for galaxies in a variety of activity stages. Methods. We have conducted a search for the 12 CO (1–0) and (2–1) transitions and the continuum emission at 1300 µ mt owards the centers of 88 galaxies using the IRAM 30 m telescope (MRT) and the Swedish ESO Submillimeter Telescope (SEST). The galaxies are selected to be bright in the far infrared (S 100 µm > 9 Jy) and optically fairly compact (D25 ≤ 180 �� ). We have applied optical spectroscopy and IRAS colours to group the galaxies of the entire sample according to their stage of activity into three sub-samples: normal, starburst and active galactic nuclei (AGN). The continuum emission has been corrected for line contamination and synchrotron contribution to retrieve the thermal dust emission. For the latter we have determined the radio spectral indices of the individual sources and extrapolated the synchrotron emission corresponding to our millimeter beams to 1300 µm. Results. We present new observational data for the 12 CO (1–0) and (2−1) transitions and the thermal dust emission at 1300 µm for 88 galaxies. In conjunction with our previous data, the new observations are used to compile an updated catalogue for a total of 160 galaxies.

Synthesis Imaging of Dense Molecular Gas in the N113 HiiRegion of the Large Magellanic Cloud

We present aperture synthesis imaging of dense molecular gas in the Large Magellanic Cloud, taken with the prototype millimeter receivers of the Australia Telescope Compact Array (ATCA). Our observations of the N113 HII region reveal a condensation with a size of ~6" (1.5 pc) FWHM, detected strongly in the 1-0 lines of HCO+, HCN and HNC, and weakly in C_2H. Comparison of the ATCA observations with single-dish maps from the Mopra Telescope and sensitive spectra from the Swedish-ESO Submillimetre Telescope indicates that the condensation is a massive clump of ~10^4 solar masses within a larger ~10^5 solar mass molecular cloud. The clump is centered adjacent to a compact, obscured HII region which is part of a linear structure of radio continuum sources extending across the molecular cloud. We suggest that the clump represents a possible site for triggered star formation. Examining the integrated line intensities as a function of interferometer baseline length, we find evidence for decreasing HCO+/HCN and HCN/HNC ratios on longer baselines. These trends are consistent with a significant component of the HCO+ emission arising in an extended clump envelope and a lower HCN/HNC abundance ratio in dense cores.

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 survey targeted towards 78 star formation regions, 72 of which are devoid of methanol maser and UC HII regions, identified in the Swedish ESO Submillimetre Telescope (SEST)/SEST IMaging Bolometer Array (SIMBA) millimetre continuum survey of Hill et al. At least 45 per cent of the latter sources, dubbed ‘mmonly’, detected in this survey are also devoid of the mid-infrared MSX emission. The 450- and 850-μm continuum emission was mapped using the Submillimetre Common User Bolometer Array (SCUBA) instrument on the James Clerk Maxwell Telescope (JCMT). Emission is detected towards 97 per cent of the 78 sources targeted as well as towards 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 remaining 106 sources, 53 result from resolving a SIMBA source into multiple submillimetre components, whilst the other 53 sources are submillimetre cores, not seen in the SIMBA. Additionally, we have identified two further mmonly sources in the SIMBA images. Of the total 405 sources identified in the SIMBA survey, 255 are only seen at millimetre wavelengths. We concatenate the results from four (sub)millimetre continuum surveys of massive star formation, together with the Galactic plane map of Pierce-Price et al. in order to determine the dust grain emissivity index β for each of the sources in the SIMBA source list. We examine the value of β with respect to temperature, as well as for the source classes identified in the SIMBA survey, for variation of this index. Our results indicate that β is typically 2, which is consistent with previous determinations in the literature, but for a considerably larger sample than previous work.

The nuclear molecular clouds of NGC 4945

The 3-mm transitions of HCN, HCO+ and HNC in the dense molecular clouds near the nucleus of the southern starburst galaxy NGC 4945 have been observed with three antennas of the Australia Telescope Compact Array (ATCA). Molecular-line emission was detected within the velocity range (300–800 km s−1) seen in single-dish spectra, although emission with extent greater than about 15 arcsec would not have been detected because of the lack of sufficiently small Compact Array (CA) baselines. Imaging the results at velocity intervals of 22.5 km s−1 and with a restoring beam of dimension 5.6 × 3.5 arcsec2 yielded images of typical deconvolved dimensions 7 × 3 arcsec2 (130 × 60 pc for an assumed galaxy distance of 4 Mpc). The positions of the images vary systematically with velocity, consistent with the result expected for a rotating molecular cloud ensemble viewed edge-on and inclined at a position angle (PA) of 45°. Although the brightest images occurred at velocities of the two major features (~430 and ~710 km s−1) in Swedish-ESO Submillimetre Telescope (SEST) spectra, the images at intervening velocities were unexpectedly faint or even in absorption. The results have been interpreted in terms of an extended cloud component not detected by the CA in conjunction with absorption against the nuclear continuum emission. Some of the HCO+ images are dominated by absorption and the overall results do not truly represent the cloud structure. The HNC results appear to contain little absorption, and the emission integrated over velocity yielded an elliptical image centred on an H2O ‘megamaser’ located at the galaxy's nucleus. The HNC position—velocity distribution along the major axis is consistent with an edge-on circumnuclear molecular ring with a rotational velocity of 135 km s−1. However, the ring radius of ~60 km s−1 is significantly less than previously derived from carbon monoxide (CO) observations. It can be reconciled with the CO results if the HNC ring traces high-density gas concentrated around the inside of the molecular ring, whereas the CO traces low-density gas that extends to higher radii. The HNC feature appears to be associated with features revealed in published infrared (IR) studies of the distribution of dust and star formation regions in the nuclear region of NGC 4945. Limited results on the variation of HNC/HCN abundance ratios within the nuclear molecular clouds support previous conclusions that the clouds are at an advanced evolutionary stage within a starburst period.

Radio, millimeter and optical monitoring of GRB 030329 afterglow: constraining the double jet model

We present radio, millimeter and optical observations of the afterglow of GRB030329. UBVR_{C}I_{C} photometry is presented for a period of 3 hours to 34 days after the burst. Radio monitoring at 1280 MHz has been carried out using the GMRT for more than a year. Simultaneous millimeter observations at 90 GHz and 230 GHz have been obtained from the Swedish-ESO Submillimeter Telescope (SEST) and the IRAM-PdB interferometer over more than a month following the burst. We use these data to constrain the double jet model proposed by Berger et al. (2003) for this afterglow. We also examine whether instead of the two jets being simultaneously present, the wider jet could result from the initially narrow jet, due to a fresh supply of energy from the central engine after the ``jet break''.

TheSpitzerc2d Survey of Large, Nearby, Interstellar Clouds. I. Chamaeleon II Observed with MIPS

We present maps of over 1.5 deg2 in Chamaeleon (Cha) II at 24, 70, and 160 μm observed with the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS) and a 1.2 deg2 millimeter map from SIMBA on the Swedish-ESO Submillimetre Telescope (SEST). The c2d Spitzer Legacy Team's data reduction pipeline is described in detail. Over 1500 24 μm sources and 41 70 μm sources were detected by MIPS with fluxes greater than 10 σ. More than 40 potential YSOs are identified with a MIPS and 2MASS color-color diagram and by their spectral indices, including two previously unknown sources with 24 μm excesses. Our new SIMBA millimeter map of Cha II shows that only a small fraction of the gas is in compact structures with high column densities. The extended emission seen by MIPS is compared with previous CO observations. Some selected interesting sources, including two detected at 1 mm, associated with Cha II are discussed in detail, and their SEDs are presented. The classification of these sources using MIPS data is found to be consistent with that of previous studies.

Millimetre continuum observations of southern massive star formation regions -- I. SIMBA observations of cold cores

We report the results of a 1.2-mm continuum emission survey toward 131 star-forming complexes suspected of undergoing massive star formation. These regions have previously been identified as harbouring a methanol maser and/or a radio continuum source [ultracompact (UC) H II region], the presence of which is in most instances indicative of massive star formation. The 1.2-mm emission was mapped using the SIMBA instrument on the 15-m Swedish ESO Submillimetre Telescope (SEST). Emission is detected toward all of the methanol maser and UC H II regions targeted, as well as towards 20 others lying within the fields mapped, implying that these objects are associated with cold, deeply embedded objects. Interestingly, there are also 20 methanol maser sites and nine UC H II regions within the fields mapped which are devoid of millimetre continuum emission. In addition to the maser and UC H II regions detected, we have also identified 253 other sources within the SIMBA maps. All of these (253) are new sources, detected solely from their millimetre continuum emission. These ‘mm-only’ cores are devoid of the traditional indicators of massive star formation, (i.e. methanol/OH maser, UC H II regions or IRAS point sources). At least 45 per cent of these mm-only cores are also without mid-infrared Mid-course Space Experiment (MSX) emission. The ‘mm-only’ core may be an entirely new class of source that represents an earlier stage in the evolution of massive stars, prior to the onset of methanol maser emission. Or, they may harbour protoclusters which do not contain any high-mass stars (i.e. below the H II region limit). In total, 404 sources are detected, representing four classes of sources which are distinguished by the presence of the different combination of associated tracer/s. Their masses, estimated assuming a dust temperature of 20 K and adopting kinematic distances, range from 0.5 × 10 1 to 3.7 × 10 4 M� , with an average mass for the sample of 1.5 × 10 3 M� . The H2 number density (nH2 )o fthe source sample ranges from 1.4 × 10 3 to 1.9 × 10 6 cm −3 , with an average of 8.7 × 10 4 cm −3 . The average radius of the sample is 0.5 pc. The visual extinction ranges from 10 to 500 mag with an average of 80 mag, which implies a high degree of embedding. The surface density (� )v aries from 0.2 to 18.0 kg m −2 with an average of 2.8 kg m −2 . Analysis of the millimetre-only sources shows that they are less massive ( ¯

The Nuclear Starburst in NGC 4945

AbstractNGC 4945 is with D ~3-4 Mpc one of the nearest starburst galaxies known and a goldmine for molecular cloud research. A multi-line mm-wave study has been carried out towards its nuclear region with the Swedish-ESO Sub-millimetre Telescope (SEST). The study covers the frequency range from 82 GHz to 354 GHz and includes 80 transitions of 19 molecules, including rare isotope-bearing species. Applying a Large Velocity Gradient (LVG) code to the data, H2 densities and column densities of 22 molecular species are calculated. Many of these species indicate the presence of a prominent high density interstellar gas component characterized by nH2 ~105cm-3. Abundances of molecular species are calculated and compared with abundances observed toward the starburst galaxies NGC 253 and M 82 and galactic sources. Apparent is an ‘overabundance’ of HNC and CN in the nuclear environment of NGC 4945. NGC 4945 is the second known starburst galaxy with an HNC/HCN abundance ratio ≥1. Carbon, nitrogen, oxygen and sulfur isotope ratios are also determined. The data indicate that high 18O/17O, low 16O/18O and 14N/15N and perhaps also low 32S/34S ratios (6.4±0.3, 195±45, 105±25 and 13.5±2.5, respectively) are characteristic properties of a starburst environment in an advanced evolutionary stage.

A Near-Infrared/Millimeter-Wave Study of Six Fourth-Quadrant High-Mass Star Formation Regions

We present near-infrared and millimeter-wave observations of six high-mass star-forming regions in the fourth quadrant: RCW 108, G333.6, RCW 117, RCW 122, NGC 6334 I, and G351.6. These regions have heliocentric distances of 1.3–3.0 kpc and total luminosities ranging from 0.5 to 3 × 105 L⊙. Millimeter maps taken in transitions of C18O, HC3N, 12C16O, and SiO with the Swedish-ESO Submillimetre Telescope 15 m telescope detail the structure and kinematics of the clouds. Near-infrared K-band images of the same regions obtained with the MPG/ESO 2.2 m telescope are used to map the stellar surface density. The C18O (1 → 0) line shows extended high column density gas, with column densities ranging from 1022 to 3 × 1023 cm-2; in contrast, the HC3N (15 → 14) emission shows eight sharply peaked dense cores. We examine these cores for recent and ongoing star formation by searching for high-velocity, non-Gaussian wings on the CO (1 → 0) and SiO (2 → 1) lines, peaks in the surface density of stars, and positional coincidences with IRAS point sources, as well as masers and H II regions reported in the literature. All of the cores show evidence of ongoing star formation. NGC 6334 I(N) stands out as a unique case in which an outflow has formed and masers have appeared, but no IRAS source, K-band cluster, or UC H II region has yet appeared; our data support previous claims that this is a rare example of a core in the earliest stages of forming a cluster of high- and low-mass stars.

Molecular gas in the galaxy M 83

We present the kinematics of the molecular gas in the barred spiral galaxy M 83 (NGC 5236). The study is based on 12CO( 1–0 and 2–1) observations with the Swedish-ESO Submillimetre Telescope (SEST). Iso-velocity maps of the entire optical disk, 10′ 10′ or 13 13 kpc, are produced. They show the pattern of an inclined, rotating disk, but also the effects of streaming motions along the spiral arms. A dynamical mass of about 6 10 is estimated by fitting the rotation curve of an exponential disk model to these data. The gas constitutes about 13% of the disk mass. The pattern speed is determined from the residual velocity pattern. The locations of various resonances are discussed. The molecular gas velocity dispersion is determined, and a trend of decreasing dispersion with increasing galactocentric radius is found. A total gas (H2 + + He) mass surface density map is presented, and compared to the critical density for star formation of an isothermal gaseous disk. The critical density is exceeded in the spiral arms, but not in the interarm regions. The locations of Giant Molecular Associations (GMAs) and regions are consistent with this scenario of dynamically induced star formation.

The relationship of CO abundance to extinction andN(H$\mathsf{_{2}}$):

We have studied the ratio of 13 CO and C 18 O column densities to the extinction A J of background field stars in the direction of three globules: B 133, B 335, L 466. Selected positions in the globules were observed in the 12 CO($J=1-0$) line and in $J=1-0$ and $J=2-1$ transitions of 13 CO and C 18 O using the 15-m Swedish-ESO Submillimetre Telescope (SEST) in Chile. One of the clouds, L 466, was also mapped in the C 18 O($J=1-0$) line. In addition to the N ( 13 CO)/ A J and N (C 18 O)/ A J ratios also the ratios of N ( 13 CO) and N (C 18 O) to A V and N (H 2 ) are given. These ratios were found to vary from cloud to cloud so that they are larger in B 335 than in B 133 and L 466. These variations are thought to primarily arise from variations of the column density ratio of CO and its isotopes to H 2 . The alternative explanation in terms of the N (H 2 )/ A J ratio being larger in starforming regions (B 335) than in more quiescent regions lacking star formation (B 133 and L 466) appears less likely. We also discuss the evidence for N (CO)/ A J variations on the basis of previous observations in star forming and non-star forming dark clouds.