The Class 0 source Barnard 1c

We present our most recent results from an ongoing study of the Class 0 source Barnard 1c in Perseus. This source is of particular interest because it exhibits evidence of strong alignment of grains all the way to the core’s centre, which is contrary to all other low-mass protostellar cores observed to date. Our goal is to clarify the source of poor alignment in other sources by identifying the source of strong alignment in B1c. A central cavity has been identified in N₂H⁺ emission; its anticorrelation with C¹⁸O emission suggests that heating in the centre has released CO from grain mantles, in turn destroying N₂H⁺. We present sensitivity-limited, high spatial resolution polarimetry data from the SubMillimeter Array and discuss the potential implications of these data.     

Chemical effects of H2 formation excitation

The effects of the production on dust grain surfaces of molecular hydrogen in excited states have been investigated. On the assumption that all of the H₂ formed on the surface of grains has a sufficient level of excitation too vercome the energy barriers in the formation reactions for the important OH and CH⁺ radicals, we consider the likely abundances of excited H₂ (H₂ ^*), OH and CH⁺ in various situations. Two different models are employed; the first links the H₂ ^* abundance directly to that of H₂ using a steady-state approximation, whilst the second considers the time-dependence of H₂ ^*. The second model is applied to gas that has been subjected to a strong isothermal shock (specifically, the shock-induced collapse of a diffuse cloud), which results in an extreme (high density, high atomic hydrogen abundance) environment. In general, it is found that the presence of the excited H₂ has only marginal effects on the chemistry of interstellar clouds. However, in the isothermal shock model, the abundances of CH⁺ are significantly enhanced, but only on short timescales, whilst the effects on the OH abundances are smaller, but last longer. We conclude that other than in such exceptional environments there are no obvious chemical signatures of the formation of H₂ ^*. This revised version was published online in July 2006 with corrections to the Cover Date.     

A high galactic latitude HI 21 cm-line absorption survey using the GMRT: II. Results and interpretation

We have carried out a sensitive high-latitude (|b| > 15°) HI 21 cm-line absorption survey towards 102 sources using the GMRT. With a 3σ detection limit in optical depth of ∼ 0.01, this is the most sensitive HI absorption survey. We detected 126 absorption features most of which also have corresponding HI emission features in the Leiden Dwingeloo Survey of Galactic neutral Hydrogen. The histogram of random velocities of the absorption features is well-fit by two Gaussians centered at V_1sr ∼ 0 km s⁻¹ with velocity dispersions of 7.6 ± 0.3 km s⁻¹ and 21 ± 4 km s⁻¹ respectively. About 20% of the HI absorption features form the larger velocity dispersion component. The HI absorption features forming the narrow Gaussian have a mean optical depth of 0.20 ± 0.19, a mean HI column density of (1.46 ± 1.03) × 10²⁰ cm⁻², and a mean spin temperature of 121 ± 69 K. These HI concentrations can be identified with the standard HI clouds in the cold neutral medium of the Galaxy. The HI absorption features forming the wider Gaussian have a mean optical depth of 0.04 ± 0.02, a mean HI column density of (4.3 ± 3.4) × 10¹⁹ cm⁻², and a mean spin temperature of 125 ± 82 K. The HI column densities of these fast clouds decrease with their increasing random velocities. These fast clouds can be identified with a population of clouds detected so far only in optical absorption and in HI emission lines with a similar velocity dispersion. This population of fast clouds is likely to be in the lower Galactic Halo.     

Hard X-ray emission from the SNR G337.2+0.1

We report hard X-ray emission of the non-thermal supernova remnant G337.2+0.1. The source presents centrally filled and diffuse X-ray emission. A spectral study confirms that the column density of the central part of the object is about N _H∼5.9(±1.5)×10²² cm⁻² and its X-ray spectrum is well represented by a single power-law with a photon index Γ=0.96±0.56. Detailed spectral analysis indicates that the outer region is highly absorbed and quite softer than the inner region. Characteristics already observed in other well-known X-ray plerions. Based on the gathered information, we confirm the SNR nature of G337.2+0.1, and suggest that the central region of the source is a pulsar wind nebula (PWN), originated by an energetic though yet undetected pulsar.     

GMRT observations of interstellar clouds in the 21cm line of atomic hydrogen

Nearby interstellar clouds with high (|ν|≥10km s⁻¹) random velocities although easily detected in NaI and CaII lines have hitherto not been detected (in emission or absorption) in the HI 21cm line. We describe here deep Giant Metrewave Radio Telescope (GMRT) HI absorption observations toward radio sources with small angular separation from bright O and B stars whose spectra reveal the presence of intervening high random velocity CaII absorbing clouds. In 5 out of the 14 directions searched we detect HI 21cm absorption features from these clouds. The mean optical depth of these detections is ∼0.09 and FWHM is ∼10km s⁻¹, consistent with absorption arising from CNM clouds.     

On-Going Galaxy Formation

We investigate the process of galaxy formation as can be observed in the only currently forming galaxies - the so-called Tidal Dwarf Galaxies, hereafter TDGs - through observations of the molecular gas detected via its CO (Carbon Monoxide) emission. These objects are formed of material torn off of the outer parts of a spiral disk due to tidal forces in a collision between two massive galaxies. Molecular gas is a key element in the galaxy formation process, providing the link between a cloud of gas and a bona fide galaxy. We have detected CO in 8 TDGs (Braine, Lisenfeld, Duc and Leon, 2000: Nature 403, 867; Braine, Duc, Lisenfeld, Charmandaris, Vallejo, Leon and Brinks: 2001, A&A 378, 51), with an overall detection rate of 80%, showing that molecular gas is abundant in TDGs, up to a few 10⁸ M _⊙. The CO emission coincides both spatially and kinematically with the HI emission, indicating that the molecular gas forms from the atomic hydrogen where the HI column density is high. A possible trend of more evolved TDGs having greater molecular gas masses is observed, in accord with the transformation of HI into H₂. Although TDGs share many of the properties of small irregulars, their CO luminosity is much greater (factor ∼ 100) than that of standard dwarf galaxies of comparable luminosity. This is most likely a consequence of the higher metallicity (≳sim 1/3 solar) of TDGs which makes CO a good tracer of molecular gas. This allows us to study star formation in environments ordinarily inaccessible due to the extreme difficulty of measuring the molecular gas mass. The star formation efficiency, measured by the CO luminosity per Hα flux, is the same in TDGs and full-sized spirals. CO is likely the best tracer of the dynamics of these objects because some fraction of the HI near the TDGs may be part of the tidal tail and not bound to the TDG. Although uncertainties are large for individual objects, as the geometry is unknown, our sample is now of eight detected objects and we find that the ‘dynamical’ masses of TDGs, estimated from the CO line widths, seem not to be greater than the ‘visible’ masses (HI + H₂ + a stellar component). Although higher spatial resolution CO (and HI) observations would help reduce the uncertainties, we find that TDGs require no dark matter, which would make them the only galaxy-sized systems where this is the case. Dark matter in spirals should then be in a halo and not a rotating disk. Most dwarf galaxies are dark matter-rich, implying that they are not of tidal origin. We provide strong evidence that TDGs are self-gravitating entities, implying that we are witnessing the ensemble of processes in galaxy formation: concentration of large amounts of gas in a bound object, condensation of the gas, which is atomic at this point, to form molecular gas and the subsequent star formation from the dense molecular component. This revised version was published online in September 2006 with corrections to the Cover Date.     

Molecular Clouds Toward a New OB Association in Pup-CMa

We have mapped 16 molecular clouds toward a new OB association in the Pup-CMa region to derive their physical properties. The observations were carried out in the ¹²CO (J = 1 – 0) line with the Southern millimetre-wave Telescope at Cerro Tololo, Chile. Distances have been determined kinematically using the rotation curve of Brand with R_⊙ = 8.5 kpc and V_⊙ = 220 km/s. Masses have been derived adopting a CO luminosity to H₂ conversion factor X = 3.8 . 10²⁰ molecules cm^-2 (K km/s)^-1. The observed mean radial velocity of the clouds is comparable with the mean radial velocity of stars composing an OB association in Pup-CMa; it is in favor of the close connection of clouds with these stars. __________ Published in Astrofizika, Vol. 48, No. 4, pp. 491–501 (October–December, 2005).     

Expanding shells in low and high density environments

The gravitational instability of expanding shells evolving in a homogeneous and static medium is discussed. In the low density environment (n = 1 cm^-3), the fragmentation starts in shells with diameters of a few 100 pc and fragment masses are in the range of 5 × 10³ - 10⁶ M _⊙. In the high density environment (n = 10⁵ - 10⁷ cm^-3), shells fragment at diameters of pc producing clumps of stellar masses. The mass spectrum in both environments is approximated by a power law dN/dm ∼ m ^-2.3. This is close to the slope of the stellar IMF. To reproduce the observed mass spectrum of clouds (the spectral index close to ∼ -2.0) we have to assume, that the cloud formation time is independent of the cloud size, similarly to the Jeans unstable medium. This revised version was published online in August 2006 with corrections to the Cover Date.     

Characteristics of Massive Star-forming Molecular Cores: The Spectral Observations of CO,CO and CO and the Statistical Comparison

With the 13.7 m millimeter wave telescope of Purple Mountain Observatory at Qinghai Station, the simultaneous mapping observations at the ¹²CO(J=1-0), ¹³CO(J=1-0) and C¹⁸O(J=1-0) lines were performed towards the 24 Galactic high-mass star-forming cores, which are associated with water masers and have available Spitzer's infrared data. The average mapping range was 8′ × 8′. The C¹⁸O line emission was detected in all the cores, in which 11 cores were observed to the half maximum of their C¹⁸O integrated intensities and the rather extended (5′ − 8′) C¹⁸O maps were obtained, while the others were failed to make such a large scale mapping because of the low SNR or the intrinsically extended morphology of the cores. On the 11 completely mapped dense cores, we analyzed their characteristics and made the statistics and comparisons on the integrated intensity ratios between ¹²CO and ¹³CO (R_12/13), ¹³CO and C¹⁸O(R_13/18), as well as ¹²CO and C¹⁸O(R_12/18). We concluded that as a tracer of dense gas, C¹⁸O is absolutely optically thin and can be used to detect the detailed structures of the cores, and that in general the 3 ratios increase gradually from the core center to the periphery. We found that the integrated intensity ratio R_12/13 ranges from 2 to 6; R_13/18 fluctuates between 4 and 20, but in central regions it is concentrated in the range 6–12 with a small fluctuation; and R_12/18 occupies a wider range 13–90, but it is concentrated between 13 and 50 in the denser regions of the cores.     

Infrared Spectroscopy of Comet Hale-Bopp

High resolution (λ/δλ ∼ 20,000) spectra of comet C/1995 O1 (Hale-Bopp) in the 2–5 μm region were obtained during UT 2–5 March 1997 using CSHELL at the NASA Infrared Telescope Facility (IRTF) on Mauna Kea. The heliocentric and geocentric distances of the comet were ∼1.1 AU and ∼1.5 AU,respectively. We detected emission lines of the gas-phase molecules H₂O, ₄, C₂H₆, C₂H₂, HCN, and CO and derived absolute production rates and relative abundances for all species. We also used the 2-dimensional nature of the CSHELL data to investigate the spatial distribution of the molecules and find evidence that CO was derived at least partly from an extended source in the coma. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chemistry in luminous AGN and starburst galaxies

Molecular line emission is a useful tool for probing the highly obscured inner kpc of starburst galaxies and buried AGNs. Molecular line ratios serve as diagnostic tools of the physical conditions of the gas—but also of its chemical properties. Both provide important clues to the type and evolutionary stage of the nuclear activity. While CO emission remains the main tracer for molecular distribution and dynamics, molecules such as HCN, HNC, HCO⁺, CN and HC₃N are useful for probing the properties of the denser (n≳10⁴ cm⁻³), star-forming gas. Here I discuss current views on how line emission from these species can be interpreted in luminous galaxies. HNC, HCO⁺ and CN are all species that can be associated both with photon dominated regions (PDRs) in starbursts—as well as X-ray dominated regions (XDRs) associated with AGN activity. HC₃N line emission may identify galaxies where the starburst is in the early stage of its evolution.     

The physical conditions in the BHR71 outflows

Highly-collimated outflows are believed to be the earliest stage in outflow evolution, so their study is essential for understanding the processes driving outflows. The BHR71 Bok globule is known to harbour such a highly-collimated outflow, which is powered by a protostar belonging to a protobinary system. Using the APEX telescope on Chajnantor, we mapped the BHR71 highly-collimated outflow in CO(3-2), and observed several bright points of the outflow in the molecular transitions CO(4-3), CO(7-6), ¹³CO(3-2), C¹⁸O(3-2), CH₃OH(7-6) and H₂CO(4-3). We use an LVG code to characterise the temperature enhancements in these regions. These observations are particularly interesting for investigating the interaction of collimated outflows with the ambient molecular cloud. In our CO(3-2) map, the second outflow driven by IRS2, which is the second source of the binary system, is completely revealed and shown to be bipolar. We also measure temperature enhancements in the lobes. The CO and methanol LVG modelling points to temperatures between 30 and 50 K in the two lobes. The methanol emission in the southern lobe bright knot is barely resolved with the APEX single-dish. ALMA will thus be a central tool to study the shock chemistry in these regions.     

A new evolutionary scenario of intermediate-mass star-formation revealed by multi-wavelength observations of OMC-2/3

We have performed millimeter- and submilli- meter-wave survey observations using the Nobeyama millimeter array (NMA) and the Atacama Submillimeter Telescope Experiment (ASTE) in one of the nearest intermediate-mass (IM) star-forming regions: Orion Molecular Cloud-2/3 (OMC-2/3). Using the high-resolution capabilities offered by the NMA (∼several arcsec), we observed dust continuum and H¹³CO⁺(1–0) emission in 12 pre- and proto-stellar candidates identified previously in single-dish millimeter observations. We unveiled the evolutionary changes with variations of the morphology and velocity structure of the dense envelopes traced by the H¹³CO⁺(1–0) emission. Furthermore, using the high-sensitivity capabilities offered by the ASTE, we searched for large-scale molecular outflows associated with these pre- and proto-stellar candidates observed with the NMA. As a result of the CO(3–2) observations, we detected six molecular outflows associated with the dense gas envelopes traced by H¹³CO⁺(1–0) and 3.3 mm continuum emission. The estimated CO outflow momentum increases with the evolutionary sequence from early to late type of the protostellar cores. We also found that the 24 μm flux increases as the dense gas evolutionary sequence. We propose that the enhancement of the 24 μm flux is caused by the growth of the cavity (i.e. the CO outflow destroys the envelope) as the evolutionary sequence. Our results show that the dissipation of the dense gas envelope plays an essential role in the evolution of the IM protostars. The extremely high-sensitivity and high-angular resolution offered by ALMA will reveal unprecedented details of the inner ∼50 AU of these protostars, which will provide us a break through in the classic scenario of IM star/disk formation.     

Molecular cores of the high-latitude cloud MBM 40

Towards the high-latitude cloud MBM 40, we identify 3 dense molecular cores of M0.2–0.5 M, and sizes of 0.2 pc in diameter embedded in the H I cloud of 8 M which is observed to be extended along the northeast–southwest direction. The molecular cloud is located almost perpendicularly to the H I emission. We confirm the previous result of Magnani et al. that MBM 40 is not a site for new star formations. We found a very poor correlation between the H I and the IRAS 100 μm emissions, but the CO (1–0) and 100 μm emissions show a better correlation of W_CO/I₁₀₀=1±0.2 K km s⁻¹ (MJy sr⁻¹)⁻¹. This ratio is larger by a factor of ≥5 than in dense dark clouds, which may indicate that the CO is less depleted in MBM 40 than in dense dark clouds.     

Observations and modeling of infrared and millimeter molecular lines towards GL2591

We have observed C₂H₂ and HCN rovibrational transitions near 13µm in absorption against GL2591. We also have observed rotational transitions at 0.6-3 mm of CS, HCN, H₂CO, and HCO⁺. Analysis of the rotational lines, which arise in the extended cloud around the source, shows that no single density model can explain all the data. Models with density and temperature gradients do much better; in particular models withn(r) r ^–1.5 can reproduce the observed pattern of emission line strengths. The abundances show significant depletion compared to models of gas-phase chemistry. The rovibrational data were analyzed in comparison to the absorption line analysis of CO by Mitchellet al. (1989). Our data are consistent with the C₂H₂ and HCN absorption arising in the same warm (200 K) and hot (1010 K) components seen in CO, but we see little evidence for the cold (38 K) component seen in CO. The rovibrational lines from higher states (J 21) indicate that the hot HCN deviates from LTE, leading to a density of about 3 × 10⁷ cm^–3. Comparison of the two data sets shows that the rovibrational absorption of HCN, rather than arising in the extended envelope, must come from a region with a small angular extent. A model in which early-time gas phase abundances are preserved on grain mantles and released at high temperature can explain the data.     

Maps of hco+ Emission in c/1995 o1 (Hale–Bopp)

On-the-Fly maps of emission from the HCO⁺ J = 3-2 transition at 267.6 GHz were obtained of C/1995 O1 (Hale-Bopp) on 1997 Mar 15.6 UT using the NRAO 12-m telescope with high spatial resolution. Unlike the relatively symmetric and centralized maps of the neutral species CO, HCN and H₂CO, the spatial extent of HCO⁺ emission is very diffuse with a complex structure characterized by at least two physically different regions. The bulk of the HCO⁺ emission peaks in intensity ∼175,000 km anti-sunward from the nuclear position. This peak emission does not fall directly along the anti-sunward direction, but is rotated by ∼10 degrees toward the east from the anti-sunward direction. A substantial void, or decrease, of HCO⁺ emission is observed within ∼ 55,000 km of the nucleus. The HCO⁺ emission in this void is roughly half the intensity of the emission observed 100,000 km away. This decrease of HCO⁺ emission near the nucleus may indicate that production or excitation of HCO⁺ is inhibited, or perhaps that HCO⁺ is easily destroyed in the inner coma, especially within ∼50,000 km of the nucleus. This void roughly coincides with the approximate location and size of the so-called “diamagnetic cavity” in the coma and may mark a significant transition region in the inner coma of Hale-Bopp This revised version was published online in July 2006 with corrections to the Cover Date.     

The warped gas and dust lane in NGC 3718

We present the first observations of molecular line emission in NGC 3718 with the IRAM 30m and the Plateau de Bure Interferometer. This galaxy is an excellent example for a strongly warped gas disk harboring an active galactic nucleus (AGN). An impressive dust lane is crossing the nucleus and a warp is developing into a polar ring. The molecular gas content is found to be typical of an elliptical galaxy with a relatively low molecular gas mass content (∼ 4 × 10⁸ M _⊙). The molecular gas distribution is found to warp from the inner disk together with the HI distribution. The CO data were also used to improve the kinematic modeling in the inner part of the galaxy, based on the so-called tilted ring-model. The nature of NGC 3718 is compared with its northern sky `twin' Centaurus A and the possible recent swallowing of a small-size gas-rich spiral is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

On the origin of the wide HI absorption line towards Sgr A*

We have imaged a region of ∼ 5′ extent surrounding Sgr A* in the HI 21 cm-line absorption using the Very Large Array. A Gaussian decomposition of the optical depth spectra at positions within ∼ 2′ (∼ 5 pc at 8.5 kpc) of Sgr A* detects a wide line underlying the many narrow absorption lines. The wide line has a mean peak optical depth of 0.32 ± 0.12 centered at a mean velocity of V_1sr = −4 ± 15 km s{−1}. The mean full width at half maximum is 119 ± 42 km s⁻¹. Such a wide line is absent in the spectra at positions beyond ∼ 2′ from Sgr A*. The position-velocity diagrams in optical depth reveal that the wide line originates in various components of the circumnuclear disk (radius ∼ 1.3′ ) surrounding Sgr A*. These components contribute to the optical depth of the wide line in different velocity ranges. The position-velocity diagrams do not reveal any diffuse feature which could be attributed to a large number of HI clouds along the line of sight to Sgr A*. Consequently, the wide line has no implications either to a global population of shocked HI clouds in the Galaxy or to the energetics of the interstellar medium as was earlier thought.     

Injection of molecules from the surfaces of grains into the gas phase chemistry of an expanding cloud

We present model results for the chemistry in an expanding cloud or clump in which molecules are injected into the gas phase from grain surfaces when the clump reaches a certain visual extinction A _v during the expansion. We consider separately injection at two different values of A _v, and include a representative large hydrocarbon, C₆H, and sulphur-bearing molecule, H₂SO₄, as well as H₂O and CO. We examine the abundances of certain molecules which have been observed in diffuse and translucent clouds, and compare the results obtained for these abundances with and without an injection during expansion. We also compare our results withpublished observations, and conclude that in most clouds an injection of molecules has occurred.     

Distribution of the Molecular Gas Around SN 1572

The early-stage structure and evolution of a supernova remnant (SNR) depends largely on its ambient interstellar medium, so the interstellar medium becomes the valid probe for investigating the evolution of SNRs. We have observed the ¹²CO (J = 1 − 0) line emission around the remnant of SN 1572 with the 13.7m millimeter-wave telescope at the Qinghai Station of PMO, in order to investigate the distribution of the CO molecular gas around SN 1572 and provide some observational basis for studying the relationship of SN 1572 with its ambient molecular gas and the evolution of this SNR. The observed result indicates that the molecular gas in the velocity range of V_LSR = −69∼ −58 km/s is associated with SN 1572, and this velocity component comes from a large-scale molecular cloud. The molecular gas is distributed along the periphery of the radio shell, continually but not uniformly, and forms a semi-closed molecular shell around the SNR. The enhanced emission exists in its whole eastern half, especially the CO emission is strongest on the northeastern edge. At the emission peak position, the spectral line exhibits a broadened velocity feature (>5 km/s). Combining with available observations in the optical, infrared, X-ray and other wavebands, it is demonstrated that the fast shock wave and ejecta are expanding into the molecular gas on the northeastern edge, and interacting with the dense gas. This interaction will have an important influence on the future evolution of SN 1572.