CO Clumps Research Articles

TheHerschelview of the on-going star formation in the Vela-C molecular cloud

As part of the Herschel guaranteed time key program 'HOBYS', we present the photometric survey of the star forming region Vela-C, one of the nearest sites of low-to-high-mass star formation in the Galactic plane. Vela-C has been observed with PACS and SPIRE in parallel mode between 70 um and 500 um over an area of about 3 square degrees. A photometric catalogue has been extracted from the detections in each band, using a threshold of 5 sigma over the local background. Out of this catalogue we have selected a robust sub-sample of 268 sources, of which 75% are cloud clumps and 25% are cores. Their Spectral Energy Distributions (SEDs) have been fitted with a modified black body function. We classify 48 sources as protostellar and 218 as starless. For two further sources, we do not provide a secure classification, but suggest they are Class 0 protostars. From SED fitting we have derived key physical parameters. Protostellar sources are in general warmer and more compact than starless sources. Both these evidences can be ascribed to the presence of an internal source(s) of moderate heating, which also causes a temperature gradient and hence a more peaked intensity distribution. Moreover, the reduced dimensions of protostellar sources may indicate that they will not fragment further. A virial analysis of the starless sources gives an upper limit of 90% for the sources gravitationally bound and therefore prestellar. We fit a power law N(logM) prop M^-1.1 to the linear portion of the mass distribution of prestellar sources. This is in between that typical of CO clumps and those of cores in nearby star-forming regions. We interpret this as a result of the inhomogeneity of our sample, which is composed of comparable fractions of clumps and cores.

DENSE CLUMPS IN GIANT MOLECULAR CLOUDS IN THE LARGE MAGELLANIC CLOUD: DENSITY AND TEMPERATURE DERIVED FROM13CO(J= 3-2) OBSERVATIONS

In order to precisely determine temperature and density of molecular gas in the Large Magellanic Cloud, we made observations of optically thin $^{13}$CO($J=3-2$) transition by using the ASTE 10m telescope toward 9 peaks where $^{12}$CO($J=3-2$) clumps were previously detected with the same telescope. The molecular clumps include those in giant molecular cloud (GMC) Types I (with no signs of massive star formation), II (with HII regions only), and III (with HII regions and young star clusters). We detected $^{13}$CO($J=3-2$) emission toward all the peaks and found that their intensities are 3 -- 12 times lower than those of $^{12}$CO($J=3-2$). We determined the intensity ratios of $^{12}$CO($J=3-2$) to $^{13}$CO($J=3-2$), $R^{12/13}_{3-2}$, and $^{13}$CO($J=3-2$) to $^{13}$CO($J=1-0$), $R^{13}_{3-2/1-0}$, at 45$\arcsec$ resolution. These ratios were used for radiative transfer calculations in order to estimate temperature and density of the clumps. The parameters of these clumps range kinetic temperature $T\mathrm{_{kin}}$ = 15 -- 200 K, and molecular hydrogen gas density $n(\mathrm{H_2})$ = 8$\times 10^2$ -- 7$\times 10^3$ cm$^{-3}$. We confirmed that the higher density clumps show higher kinetic temperature and that the lower density clumps lower kinetic temperature at a better accuracy than in the previous work. The kinetic temperature and density increase generally from a Type I GMC to a Type III GMC. We interpret that this difference reflects an evolutionary trend of star formation in molecular clumps. The $R^{13}_{3-2/1-0}$ and kinetic temperature of the clumps are well correlated with H$\alpha$ flux, suggesting that the heating of molecular gas $n(\mathrm{H_2})$ = $10^3$ -- $10^4$ cm$^{-3}$ can be explained by stellar FUV photons.

SPITZERSPECTRAL LINE MAPPING OF PROTOSTELLAR OUTFLOWS. II. H2EMISSION IN L1157

We present an analysis of Spitzer-IRS spectroscopic maps of the L1157 protostellar outflow in the H2 pure-rotational lines from S(0) to S(7). The aim of this work is to derive the physical conditions pertaining to the warm molecular gas and study their variations within the flow. The mid-IR H2 emission follows the morphology of the precessing flow, with peaks correlated with individual CO clumps and H2 2.12{\mu}m ro-vibrational emission. More diffuse emission delineating the CO cavities is detected only in the low-laying transitions, with J(lower) less or equal to 2. The H2 line images have been used to construct 2D maps of N(H2), H2 ortho-to-para ratio and temperature spectral index beta, in the assumption of a gas temperature stratification where the H2 column density varies as T^(beta). Variations of these parameters are observed along the flow. In particular, the ortho-to-para ratio ranges from 0.6 to 2.8, highlighting the presence of regions subject to recent shocks where the ortho-to-para ratio has not had time yet to reach the equilibrium value. Near-IR spectroscopic data on ro-vibrational H2 emission have been combined with the mid-IR data and used to derive additional shock parameters in the brightest blue- and red-shifted emission knots. A high abundance of atomic hydrogen (H/H2 about 0.1-0.3) is implied by the observed H2 column densities, assuming n(H2) values as derived by independent SiO observations. The presence of a high fraction of atomic hydrogen, indicates that a partially-dissociative shock component should be considered for the H2 excitation in these localized regions. However, planar shock models, either of C- or J-type, are not able to consistently reproduce all the physical parameters derived from our analysis of the H2 emission. Globally, H2 emission contributes to about 50% of the total shock radiated energy in the L1157 outflow.

ASTE CO (3-2) MAPPING TOWARD THE WHOLE OPTICAL DISK OF M 83: PROPERTIES OF INTER-ARM GIANT MOLECULAR-CLOUD ASSOCIATIONS

We present a new on-the-fly (OTF) mapping of CO(J=3-2) line emission with the Atacama Submillimeter Telescope Experiment (ASTE) toward the 8' x 8' (or 10.5 x 10.5 kpc at the distance of 4.5 Mpc) region of the nearby barred spiral galaxy M 83 at an effective resolution of 25''. Due to its very high sensitivity, our CO(J=3-2) map can depict not only spiral arm structures but also spur-like substructures extended in inter-arm regions. This spur-like substructures in CO(J=3-2) emission are well coincident with the distribution of massive star forming regions traced by Halpha luminosity and Spitzer/IRAC 8 um emission. We have identified 54 CO(J=3-2) clumps as Giant Molecular-cloud Associations (GMAs) employing the CLUMPFIND algorithm, and have obtained their sizes, velocity dispersions, virial masses, and CO luminosity masses. We found that the virial parameter alpha, which is defined as the ratio of the virial mass to the CO luminosity mass, is almost unity for GMAs in spiral arms, whereas there exist some GMAs whose alpha are 3 -- 10 in the inter-arm region. We found that GMAs with higher $\alpha$ tend not to be associated with massive star forming regions, while other virialized GMAs are. Since alpha mainly depends on velocity dispersion of the GMA, we suppose the onset of star formation in these unvirialized GMAs with higher alpha are suppressed by an increase in internal velocity dispersions of Giant Molecular Clouds within these GMAs due to shear motion.

THE INFLUENCE OF FAR-ULTRAVIOLET RADIATION ON THE PROPERTIES OF MOLECULAR CLOUDS IN THE 30 DOR REGION OF THE LARGE MAGELLANIC CLOUD

We present a complete 12CO J = 1 → 0 map of the prominent molecular ridge in the Large Magellanic Cloud (LMC) obtained with the 22 m ATNF Mopra Telescope. The region stretches southward by ∼2° (or 1.7 kpc) from 30 Doradus, the most vigorous star-forming region in the Local Group. The location of this molecular ridge is unique insofar as it allows us to study the properties of molecular gas as a function of the ambient radiation field in a low-metallicity environment. We find that the physical properties of CO-emitting clumps within the molecular ridge do not vary with the strength of the far-ultraviolet radiation field. Since the peak CO brightness of the clumps shows no correlation with the radiation field strength, the observed constant value for CO-to-H2 conversion factor along the ridge seems to require an increase in the kinetic temperature of the molecular gas that is offset by a decrease in the angular filling factor of the CO emission. We find that the difference between the CO-to-H2 conversion factor in the molecular ridge and the outer Milky Way is smaller than has been reported by previous studies of the CO emission: applying the same cloud identification and analysis methods to our CO observations of the LMC molecular ridge and CO data from the outer Galaxy survey by Dame et al., we find that the average CO-to-H2 conversion factor in the molecular ridge is XCO ≃ (3.9 ± 2.5) × 1020 cm−2 (K km s−1)−1, approximately twice the value that we determine for the outer Galaxy clouds. The mass spectrum and the scaling relations between the properties of the CO clumps in the molecular ridge are similar, but not identical, to those that have been established for Galactic molecular clouds.

ANALYTICAL THEORY FOR THE INITIAL MASS FUNCTION. II. PROPERTIES OF THE FLOW

Recently, Hennebelle and Chabrier (2008) derived an analytical theory for the mass spectrum of non self-gravitating clumps associated with overdensities in molecular clouds and for the initial mass function of gravitationally bound prestellar cores, as produced by the turbulent collapse of the cloud. In this companion paper, we examine the effects of the non-isothermality of the flow, of the turbulence forcing and of local fluctuation of the velocity dispersion, on the mass function. In particular, we investigate the influence of a polytropic equation of state and of the effective adiabatic exponent $\gamma$ and find that it has a drastic influence on the low mass part of the IMF. We also consider a barotropic equation of state (i.e. a piecewise polytropic eos) that mimics the thermal behaviour of the molecular gas and compare the prediction of our theory with the results of numerical simulations and with the observationally-derived IMF, for cloud parameters which satisfy Larson's type relations. We find that for clouds whose density is, at all scales, almost an order of magnitude larger than the density inferred for the CO clumps in the Galaxy, a good agreement is obtained between the theory and the observed IMF, suggesting that star formation preferentially occurs in high density environments. We derive an analytical expression for the IMF which generalizes the expression previously obtained for the isothermal case. This easy-to-implement analytical IMF should serve as a template to compare observational or numerical results with the theory.

A large-scale CO survey of the Rosette Molecular Cloud: assessing the effects of O stars on surrounding molecular gas

We present a new large-scale survey of the J= 3–2 12CO emission covering 4.8 deg2 around the Rosette Nebula. The results reveal the complex dynamics of the molecular gas in this region. We identify about 2000 compact gas clumps having a mass distribution given by dN/dM∼M−1.8, with no dependence of the power-law index on distance from the central O stars. A detailed study of a number of the clumps in the inner region shows that most exhibit velocity gradients in the range 1–3 km s−1 pc−1, generally directed away from the exciting nebula. The magnitude of the velocity gradient decreases with distance from the central O stars, and we compare the apparent clump acceleration with a photoionized gas acceleration model. For most clumps outside the central nebula, the model predicts lifetimes of a few 105 yr. In one of the most extended of these clumps, however, a near-constant velocity gradient can be measured over 1.7 pc, which is difficult to explain with radiatively driven models of clump acceleration. As well as the individual accelerated clumps, an unresolved limb-brightened rim lies at the interface between the central nebular cavity and the Rosette Molecular Cloud. Extending over 4 pc along the edge of the nebula, this region is thought to be at an earlier phase of disruption than the accelerating compact globules. Blueshifted gas clumps around the nebula are in all cases associated with dark absorbing optical globules, indicating that this material lies in front of the nebula and has been accelerated towards us. Redshifted gas shows little evidence of associated line-of-sight dark clouds, indicating that the dominant bulk molecular gas motion throughout the region is expansion away from the O stars. In addition, we find evidence that many of the clumps lie in a molecular ring, having an expansion velocity of 30 km s−1 and radius 11 pc. The dynamical time-scale derived for this structure (∼106 yr) is similar to the age of the nebula as a whole (2 × 106 yr). The J= 3–2/1–0 12CO line ratio in the clumps decreases with radial distance from the exciting O stars, from 1.6 at ∼8 pc distance to 0.8 at 20 pc. This can be explained by a gradient in the surface temperature of the clumps with distance, and we compare the results with a simple model of surface heating by the central luminous stars. We identify seven high-velocity molecular flows in the region, with a close correspondence between these flows and embedded young clusters or known young luminous stars. These flows are sufficiently energetic to drive gas turbulence within each cluster, but fall short of the turbulent energy of the Rosette giant molecular cloud by two orders of magnitude. We find 14 clear examples of association between an embedded young star (as seen by Spitzer at 24 μm) and a CO clump in the molecular cloud facing the nebular. The CO morphology indicates that these are photoevaporating circumstellar envelopes. CO clumps without evidence of embedded stars tend to have lower gas velocity gradients. It is suggested that the presence of the young star may extend the lifespan of the externally photoevaporating envelope.

Analytical Theory for the Initial Mass Function: CO Clumps and Prestellar Cores

We derive an analytical theory of the prestellar core initial mass function (IMF) based on an extension of the Press-Schechter statistical formalism. Our approach relies on the general concept of the gravothermal and gravoturbulent collapse of a molecular cloud, with a selection criterion based on the thermal or turbulent Jeans mass, which yields the derivation of the mass spectrum of self-gravitating objects in a quiescent or a turbulent environment. The same formalism also yields the mass spectrum of non-self-gravitating clumps produced in supersonic flows. The mass spectrum of the self-gravitating cores reproduces well the observed IMF. The theory predicts that the shape of the IMF results from two competing contributions, namely, a power law at large scales and an exponential cutoff (lognormal form) centered around the characteristic mass for gravitational collapse. The cutoff exists both in the case of thermal or turbulent collapse, provided that the underlying density field has a lognormal distribution. Whereas pure thermal collapse produces a power-law tail steeper than the Salpeter value, -->dN/dlog M M−x with -->x 1.35, the latter is recovered exactly for the (three-dimensional) value of the spectral index of the velocity power spectrum, -->n 3.8, found in observations and in numerical simulations of isothermal supersonic turbulence. Indeed, the theory predicts that -->x = (n + 1)/(2n − 4) for self-gravitating structures and -->x = 2 − n'/3 for non-self-gravitating structures, where -->n' is the power spectrum index of -->log ρ . We show that, whereas supersonic turbulence promotes the formation of both massive stars and brown dwarfs, it has an overall negative impact on star formation, decreasing the star formation efficiency. This theory provides a novel theoretical foundation to understand the origin of the IMF and provides useful guidance to numerical simulations exploring star formation, while making testable predictions.

Past and Present Star Formation in the SMC: NGC 346 and its Neighborhood

In the quest to understand how star formation occurs and propagates in the low-metallicity environment of the Small Magellanic Cloud (SMC), we acquired deep F555W (∼V) and F814W (∼I) Hubble Space Telescope ACS images of the young and massive star-forming region NGC 346. These images and their photometric analysis provide us with a snapshot of the star formation history of the region. We find evidence for star formation extending from ≈10 Gyr in the past until ≈150 Myr in the field of the SMC. The youngest stellar population (∼3 ± 1 Myr) is associated with the NGC 346 cluster. It includes a rich component of low-mass pre-main-sequence stars mainly concentrated in a number of subclusters spatially colocated with CO clumps previously detected by Rubio and coworkers. Within our analysis uncertainties, these subclusters appear coeval with each other. The most massive stars appear concentrated in the central subclusters, indicating possible mass segregation. A number of embedded clusters are also observed. This finding, combined with the overall wealth of dust and gas, could imply that star formation is still active. An intermediate-age star cluster, BS 90, formed ∼4.3 ± 0.1 Gyr ago, is also present in the region. Thus, this region of the SMC has supported star formation with varying levels of intensity over much of the cosmic time.

Detection of CO Hot Spots Associated with Young Clusters in the Southern Starburst Galaxy NGC 1365

We have used the Submillimeter Array for the first interferometric CO imaging toward the starburst-Seyfert nucleus of the southern barred spiral galaxy NGC 1365, which is one of the four galaxies within 30 Mpc that have L8-1000 mu m >= 10(11) L-.. Our mosaic maps of (CO)-C-12, (CO)-C-13, and (CO)-O-18 (J = 2-1) emission at up to 2 '' ( 200 pc) resolutions have revealed a circumnuclear gas ring and several CO clumps in the central 3 kpc. The molecular ring shows morphological and kinematical signs of bar-driven gas dynamics, and the region as a whole is found to follow the star formation laws of Kennicutt. We have found that some of the gas clumps and peaks in CO brightness temperature, which we collectively call CO hot spots, coincide with the radio and mid-infrared sources previously identified as dust-enshrouded super star clusters. This hot spot-cluster association suggests that either the formation of the most massive clusters took place in large molecular gas concentrations ( of Sigma(mol) similar to 10(3) M-. pc(-2) in 200 pc scales) or the clusters have heated their ambient gas to cause or enhance the CO hot spots. The active nucleus is in the region of weak CO emission and is not associated with distinctive molecular gas properties.

Interferometric COJ= 2–1 Emission Mapping of the Protoplanetary Nebula IRAS 19475+3119

We present ~2'' resolution interferometric maps of the 12CO J = 2-1 emission in the PPN IRAS 19475+3119 obtained with OVRO. These data probe two distinct molecular components, namely, a slowly expanding shell and a fast bipolar outflow.We have used a spatiokinematic model of the 12CO J = 2-1 emission to constrain the properties of these two components. The shell has inner and outer radii of Rin ~ 6.5 × 1016 cm and Rout ~ 2 × 1017 cm and expands at Vexp ~ 11 km s-1. The 12CO J = 2-1 line wing emission arises in a bipolar structure that emerges from two diametrically opposite holes in the slow shell. The bipolar outflow is aligned with one of the two lobe pairs of the quadrupolar optical nebula (at P.A. ≈ 80°). Both the holes and the bipolar outflow are most likely the result of the interaction of fast, collimated post-AGB winds with the shell. The quadrupolar morphology of the optical nebula indicates two distinct bipolar post-AGB winds ejected in two different directions. The elongation of the optical counterpart of the shell (at P.A. ≈ -45°) and two similarly aligned CO clumps suggest that the slow shell has also been affected by the wind interaction. The expansion velocity in the bipolar outflow increases linearly with the distance from the nebula center and reaches Vexp = 30 km s-1 (projected) at the tips of the lobes. This velocity gradient yields a relatively long kinematical age of ~1900 yr, assuming an outflow inclination of i = 30° with respect to the plane of the sky; this age is comparable with the post-AGB lifetime estimated from the shell expansion velocity and inner radius. We derive a mean kinetic temperature of ~14 K and a total mass of ~0.4 M☉. The collimation and linear momentum (P ~ 4 × 1038 g cm s-1) of the outflow are unlikely to result from radiation pressure on dust grains.

Westbrook's Molecular Gun: Discovery of Near-Infrared Microstructures and Molecular Bullets in AFGL 618

We present high-sensitivity near-IR images of a carbon-rich proto-planetary nebula, AFGL 618, obtained with the Infrared Camera and Spectrograph (IRCS) mounted on the 8.2m Subaru Telescope. The deep near-IR images have revealed “bullets” and “horns” extending farther out from the edges of the previously known bipolar nebulosities that consist of dust-scattered star light component and shock-excited line emission component. That these bullets and horns represent the positions from which [Fe II] IR lines arise is strongly suggested from the spatial coincidence between these near-IR microstructures and the optical collimated outflow structure observed by the recent HST/WFPC2 imaging, together with the previous detection of shock-excited, forbidden IR lines of atomic species at those locations. At these positions of the near-IR mincrostructures, we have also discovered CO clumps moving at > 200 km s-1 from our re-analysis of the existing 12CO J = 1 – 0 data obtained with the Berkeley-Illinois-Maryland Association (BIMA) interferometer array. These findings indicate that fast-moving CO clumps seem to be impinging upon the surrounding ambient circumstellar shell, thereby causing shocked emission regions which manifest themselves as the near-IR microstructures at the shock interface. To deepen our understanding of the connection between the near-IR microstructures and the CO outflow structure, we are currently conducting higher resolution observation in CO lines with the BIMA array.

Molecular Gas in Candidate Double‐barred Galaxies. I. The Diverse Morphology and Dynamics of NGC 2273 and NGC 5728

Double bars have been proposed as a means of transporting molecular gas past inner Lindblad resonances into the nuclear regions, where it can fuel active or starburst nuclei. Thus far, the existence of double bars has been determined predominantly through analysis of near-infrared images, which can tell us little about the dynamics and inflow rates of these systems. We have observed two double-bar galaxy candidates (NGC 2273 and NGC 5728) in 12CO J = 1-0 with the Owens Valley Radio Observatory Millimeter Array. Despite the similarity in the near-infrared images of these galaxies, we see rather different nuclear morphologies in the CO maps. NGC 2273 shows evidence of a nuclear gas bar, aligned with the nuclear stellar bar seen in the near-infrared images. Both the nuclear gaseous and the stellar bars are misaligned from the large-scale bar by ~90°, which also allows the possibility that both are the result of stars and gas populating the x2 orbits of the primary bar. Estimates using dynamical friction arguments and star formation rates suggest significant gas inflow rates along the nuclear bar of NGC 2273. Conversely, NGC 5728 does not show any evidence for a nuclear molecular bar but shows an arc of CO clumps that peaks just to the southwest of the dynamical center and curves to the southeast, where it follows the dust lane to the south. Models of double-barred galaxies suggest that these galaxies should contain large amounts of molecular gas in their nuclei. Our calculations suggest that both galaxies contain sufficient amounts of gas in their nuclei, but only NGC 2273 shows evidence for a nuclear gas bar. This may be the result of past episodes of star formation exhausting and dispersing the nuclear gas of NGC 5728, but it is more likely evidence that NGC 5728 has undergone a minor merger event.

Clumpy outer Galaxy molecular clouds and the steepening of the IMF

We report the results of high-resolution (~0.2 pc) CO(1-0) and CS(2-1) observations of the central regions of three star-forming molecular clouds in the far-outer Galaxy (~16 kpc from the Galactic Center): WB89 85 (Sh 2-127), WB89 380, and WB89 437. We used the BIMA array in combination with IRAM 30-m and NRAO 12-m observations. The GMC's in which the regions are embedded were studied by means of KOSMA 3-m CO(2-1) observations. The properties the CO and CS clumps are analyzed and compared with newly derived results of previously published single-dish measurements of local clouds (OrionB South and Rosette). We find that the slopes of the clump mass distributions (-1.28 and -1.49, for WB89 85 and WB89 380, respectively) are somewhat less steep than found for most local clouds, but similar to those of clouds which have been analyzed with the same clumpfind program. We investigate the clump stability by using the virial theorem, including all possible contributions (gravity, turbulence, magnetic fields, and pressure due to the interclump gas). It appears that under reasonable assumptions a combination of these forces would render most clumps stable. Comparing only gravity and turbulence, we find that in the far-outer Galaxy clouds, these forces are in equilibium (virial parameter alpha~1) for clumps down to the lowest masses found (a few Msol). For clumps in the local clouds alpha~1 only for clumps with masses larger than a few tens of Msol. Thus it appears that in these outer Galaxy clumps gravity is the dominant force down to a much lower mass than in local clouds, implying that gravitational collapse and star formation may occur more readily even in the smallest clumps. Although there are some caveats, due to the inhomogeneity of the data used, this might explain the apparently steeper IMF found in the outer Galaxy.

Westbrook’s Molecular Gun: Discovery of Near-Infrared Microstructures in AFGL 618

We present high-sensitivity near-IR images of a carbon-rich proto-planetary nebula, AFGL 618, obtained with the Subaru Telescope. These images have revealed and extending farther out from the edges of the previously known bipolar lobes. The spatial coincidence between these near-IR microstructures and the optical collimated outflow structure, together with the detection of shock-excited, forbidden IR lines of atomic species, strongly suggests that these bullets and horns represent the locations from which [Fe II] IR lines arise. We have also discovered CO clumps moving at greater than 200 km s-1 at the positions of the near-IR bullets by reanalyzing the existing 12CO J = 1-0 interferometry data. These findings indicate that the near-IR microstructures represent the positions of shocked surfaces at which fast-moving molecular clumps interface with the ambient circumstellar shell.

The COJ = 2–1 / J = 1–0 Ratio in the Large Magellanic Cloud

We observed 34 positions throughout the disk of the Large Magellanic Cloud in the CO J = 2-1 emission line with the Tokyo-Onsala-ESO-Calan 60 cm radio telescope. Comparing the spectra with those of the J = 1-0 line at the same angular resolution (9', or 130 pc at 50 kpc), we found that the CO J = 2-1/J = 1-0 intensity ratio (R2-1/1-0) scatters in a range of 0.5-1.3. The luminosity ratio averaged for all observed points is 0.92 ± 0.05. The ratio R2-1/1-0 is approximately unity (0.95 ± 0.06) in 30 Dor, consistent with optically thick and thermalized emission, even in the southern part where massive star formation does not occur yet. This suggests that the high R2-1/1-0 is not primarily due to the UV radiation from young stars but rather to the intrinsic nature of the molecular gas that is relatively dense (103 cm-3) and may be ready to form stars. In addition to a cloud-to-cloud difference of R2-1/1-0, there exists a radial gradient of the ratio of 0.94 ± 0.11 in the inner region (2 kpc from the kinematic center) and 0.69 ± 0.11 in the outer region (2 kpc from the center, excluding the 30 Doradus complex). The higher R2-1/1-0 in the inner galaxy might be due to relatively higher gas densities within CO clumps in molecular clouds and/or higher external heating in that region.

CO Images of the Central Regions of 20 Nearby Spiral Galaxies

We report a CO(J = 1-0) aperture-synthesis survey of the central regions of 20 nearby spiral galaxies. The sample is selected on the basis of inclination, single-dish CO flux, and lack of significant dynamical perturbation. No selection is made on the basis of their nuclear activity, starburst, or infrared luminosity. The observations have been made using the Nobeyama and Owens Valley millimeter arrays, with typical resolutions of 4'' (300 pc) and 20 km s-1, and sensitivities of ~30 mJy beam-1 for a 20 km s-1 channel. In this paper, we present integrated intensity and mean velocity maps, azimuthally averaged radial distributions, and position-velocity plots. Most galaxies in the sample have scale lengths for the radial CO distribution that are much smaller in the nuclear regions than in the outer disks. Typically, the CO emission falls to 1/e of the central peak value at radii of ~500 pc. This is significantly smaller than the scale lengths for the global CO distributions in the galactic disks. CO position-velocity diagrams exhibit a steep rise in rotation velocity, dV/dr ~ 1 km s-1 pc-1, in the central regions of most galaxies. The CO line width typically reaches ~95% of the overall H I line width within a radius of 1 kpc. This steep rise in the rotation velocity mainly reflects the highly concentrated mass distributions in the galactic centers, though it may well be partly due to noncircular motions in barred galaxies. In a number of galaxies, the position-velocity diagrams show a small central hole (d ≲ 100 pc) in CO emission. In many galaxies, we detect CO clumps of subkiloparsec size which may be giant molecular associations (GMAs). In the Appendix, we demonstrate that small holes at the galactic center can be readily detected in position-velocity diagrams even when they are smaller than the spatial resolution of the data.

BIMA Observations of the VLA 1623 Molecular Outflow

We present 12CO J = 1-0 and H13CO+ J = 1-0 observations of the spectacular VLA 1623 outflow that were obtained with the Berkeley-Illinois-Maryland Association (BIMA) interferometer at resolutions of 123 × 44 and 97 × 69, respectively. The 12CO outflow image reveals a clumpy, nonlinear morphology akin to optical jet outflows and Herbig-Haro objects. We identify partially unresolved CO clumps labeled A through H, of which only A and F have associated λ = 2.121 μm H2 emission. One of the two 6 cm continuum sources in the region, VLA 1623E, coincides with CO clump C. The dense ambient gas probed by H13CO+ shows no correlation with the dust continuum peaks in the region, interpreted as evidence of gas-phase depletion onto dust grains. We argue that the H13CO+ peak, which coincides with a water maser, traces the interaction between the outflow and dense gas in the protostellar core. The fractional abundance there is given by X[H13CO+] ≈ 3 × 10-10. The nondetections of H13CN, SO, and SiO allow limits to be placed on their relative abundances at the H13CO+ peak.

IRAS sources beyond the solar circle. VII. The 12C/13C ratio in the far outer Galaxy

We have investigated the 12 C/13 C abundance ratio in the far-outer Galaxy. We have used the IRAM 30-m telescope to obtain the 12 CO and 13 CO(1-0) and (2-1) distributions towards five IRAS sources at about 16-17 kpc from the galactic center. C18 O(1-0) and (2-1) were observed towards the 13 CO peak positions in those clouds. The source with the strongest C18 O emission, WB 89-437, was subsequently observed in 13 C18 O(1-0) and (2-1) and in H2 CS(3-2) and (6-5). To be able to compare our results with published data, we observed the same transitions towards the inner Galaxy source W 33, and towards W 3OH. The ratio of the 13 CO and C18 O column densities is about 14, slightly larger than what was found in local GMCs. This ratio is dominated by excitation and beam filling effects, and is therefore not indicative of the abundance ratios. The ratio C18 O(1-0)/13 C18 O(1-0) directly yields the 12 C/13 C abundance ratio however, for which towards WB 89-437 we find a 3σ lower limit of 201± 15, which means that the 12 C/13 C gradient found in the inner Galaxy continues further out. Our results for W 33 and W 3OH are consistent with earlier observations and give abundance ratios of 43.0± 4.3 and 85± 15, respectively. These J =1-0 measurements are however in contrast to results obtained from the corresponding J =2-1 transitions: we obtain abundance ratios of 104± 60 (WB 89-437), 31± 2 (W 33), and 24± 2 (W 3OH). These differences may be due to the emission of the two transitions originating in different parts of the cloud with different excitation conditions. The 12 CO emission towards WB 89-437 shows strong outflow emission, and that of WB 89-380 is dominated by self-absorption. The sizes of the 13 CO clumps are 1-2 pc and they have peak positions located within 10″(0.5 pc) from the IRAS position. Their (virial) masses are typically several 1000 .

The nearby 2-solar mass BOK globule LBN 11 - Sub-sonic molecular clumps in a magnetic environment

A multiwavelength study of the nearby small Bok globule LBN 11 is performed in order to investigate the relationship between embedded magnetic fields, cloud structure, and star formation. Optical polarimetry of background stars is used to characterize and trace the embedded magnetic field direction. High-dispersion CO isotopic mapping is employed to determine the radial density distribution of the gas, measure cloud rotation, and identify individual gas clumps. CS and SO mapping revealed dense cores within the (C-13)O clumps. Analysis of coadded IRAS images reveal the cloud to be free of current or recent star formation: there are no IR point sources of sufficient flux or proper colors to signify young stars associated with the cloud. The molecular line maps show the cloud to be extremely clumpy. Clumps are found to range in size from 0.2 pc for the largest CO clump to about 0.04 pc for the smallest CS feature. It is concluded that, at least in the envelope of the cloud, the magnetic field and the CO clumps are coupled.