[en] To explore the formation mechanisms of gas phase CH_3OH in cold starless cores, we have conducted high spectral resolution observations toward the cyanopolyyne peak of Taurus Molecular Cloud-1 (TMC-1 CP) with the IRAM 30 m telescope, the Green Bank Telescope, and the Nobeyama 45 m telescope. The spectral lines of CH_3OH toward TMC-1 CP are found to have a double-peaked profile separated by 0.5 km s"−"1. Since the double-peaked profile is observed for "1"3CH_3OH, it is not due to optical depth and/or self-absorption effects. The spectral line profile of CH_3OH is much different from those of C"3"4S, C_3S, and HC_7N observed toward this source. The H_2 densities of the emitting region of CH_3OH for the blueshifted and redshifted components are derived to be (1.7 ± 0.5) × 10"4 cm"−"3 and (4.3 ± 1.2) × 10"4 cm"−"3, respectively. These densities are similar to or slightly lower than those found for the other molecules. These results suggest a chemical differentiation between CH_3OH and the other molecules, which has indeed been confirmed by mapping observations of the CH_3OH and C"3"4S lines. These results are consistent with the general idea that CH_3OH is formed on dust grains and is liberated into the gas phase by non-thermal desorption. The grain-surface origin of CH_3OH is further confirmed by the CH_3OH/"1"3CH_3OH ratio. Weak shocks caused by accreting diffuse gas to the TMC-1 filament, photoevaporation caused by cosmic-ray induced UV radiation, and the desorption of excess reaction energy in the formation of CH_3OH on dust grains are discussed for the desorption mechanisms

[en] We have carried out an observation of the CCS (J_N = 2₁-1₀) line with the Very Large Array in its D-configuration toward a protostellar core L483 (IRAS 18140-0440). This is a candidate source of the newly found carbon-chain-rich environment called 'Warm-Carbon-Chain-Chemistry (WCCC)', according to the previous observations of carbon-chain molecules. The CCS distribution in L483 is found to consist of two clumps aligned in the northwest-southeast direction, well tracing the CCS ridge observed with the single-dish radio telescope. The most remarkable feature is that CCS is depleted at the core center. Such a CCS distribution with the central hole is consistent with those of previously observed prestellar and protostellar cores, but it is rather unexpected for L483. This is because the distribution of CS, which is usually similar to that of CCS, is centrally peaked. Our results imply that the CCS (J_N = 2₁-1₀) line would selectively trace the outer cold envelope in the chemically less evolved phase that is seriously resolved out with the interferometric observation. Thus, it is most likely that the high abundance of CCS in L483 relative to the other WCCC sources is not due to the activity of the protostar, although it would be related to its younger chemical evolutionary stage, or a short timescale of the prestellar phase.

[en] The millimeter-wave rotational emission lines (4₀₄-3₀₃ and 5₀₅-4₀₄) of protonated carbon dioxide, HCO₂⁺(HOCO⁺), have been detected toward the low-mass Class 0 protostar IRAS 04368+2557 in L1527 with the IRAM 30 m telescope. This is the first detection of HCO₂⁺ except for the Galactic center clouds. The column density of HCO₂⁺ averaged over the beam size (29^'') is determined to be 7.6 x 10¹⁰ cm^-2, assuming a rotational temperature of 12.3 K. The fractional abundance of gaseous CO₂ relative to H₂ is estimated from the column density of HCO₂⁺ with the aid of a simplified chemical model. If the HCO₂⁺ emission only comes from the evaporation region of CO₂ near the protostar (T ∼> 50 K), the fractional abundance of CO₂ is estimated to be higher than 6.6 x 10^-4. This is comparable to the elemental abundance of carbon in interstellar clouds, and hence, the direct evaporation of CO₂ from dust grain is unrealistic as a source of gaseous CO₂ in L1527. A narrow line width of HCO₂⁺ also supports this. On the other hand, the fractional abundance of CO₂ is estimated to be 2.9 x 10^-7, if the source size is comparable to the beam size. These results indicate that gaseous CO₂ is abundant even in the low-mass star-forming region. Possible production mechanisms of gaseous CO₂ are discussed.

[en] We have conducted millimeter-wave observations of deuterated species of various carbon-chain molecules toward a low-mass star-forming region, L1527, which shows extraordinary richness of carbon-chain molecules in a vicinity of the protostar (Warm Carbon Chain Chemistry; WCCC). We have detected the spectral lines of l-C₃D, C₄D, C₄HD, DC₃N, DC₅N, and c-C₃HD, where l-C₃D and C₄HD are detected for the first time in space. The deuterium fractionation ratios are found to be moderate (2% to 7%), although they tend to be higher than those in the starless core, TMC-1. The upper limit to the [CH₂DOH]/[CH₃OH] ratio is also as low as 3%. Therefore, high deuterium fractionation ratios reported for hot corino sources are not seen in L1527. The observed ratios mean that the depletion of CO onto dust grains had not proceeded far in L1527, compared to the hot corino case. This would be consistent with a short timescale of the starless core phase, as suggested for the possible origin of WCCC.

[en] We construct a gas-grain chemical network model which includes carbon isotopes (¹²C and ¹³C) with an emphasis on isotopomer-exchange reactions. Temporal variations of molecular abundances, the carbon isotope ratios (¹²CX/¹³CX), and the isotopomer ratios (¹²C¹³CX/¹³C¹²CX) of CCH and CCS in cold dense cloud cores are investigated by numerical calculations. We confirm that the isotope ratios of molecules, both in the gas phase and grain surfaces, are significantly different depending on whether the molecule is formed from the carbon atom (ion) or the CO molecule. Molecules formed from carbon atoms have CX/¹³CX ratios greater than the elemental abundance ratio of [¹²C/¹³C]. On the other hand, molecules formed from CO molecules have CX/¹³CX ratios smaller than the [¹²C/¹³C] ratio. We reproduce the observed C¹³CH/¹³CCH ratio in TMC-1, if the isotopomer-exchange reaction, ¹³CCH + H ↔ C¹³CH + H + 8.1 K, proceeds with the forward rate coefficient k_f > 10^-11 cm³ s^-1. However, the C¹³CS/¹³CCS ratio is lower than that observed in TMC-1. We then assume the isotopomer-exchange reaction catalyzed by the H atom, ¹³CCS + H ↔ C¹³CS + H + 17.4 K. In the model with this reaction, we reproduce the observed C¹³CS/¹³CCS, CCS/C¹³CS, and CCS/¹³CCS ratios simultaneously.

[en] We have recently discovered a new starless core with bright radio emissions of long carbon-chain molecules in the Lupus molecular cloud, which we have named as Lupus-1A. Toward this source, the peak intensities of the C₆H and C₈H lines are found to be higher than toward TMC-1 by a factor of 2-3. Even the lines of their anions, C₆H^- and C₈H^-, are also brighter than in TMC-1. Moreover, the line of C₄H^- has been detected for the first time in a starless core. The column densities of these long carbon-chain molecules are almost comparable to those in TMC-1, and hence, this source can be regarded as the second 'TMC-1 like cloud'. TMC-1 has long been an outstanding molecular cloud with rich carbon-chain molecules since its discovery in 1976. In spite of extensive efforts, no comparable sources have been found so far. Lupus-1A will be used for hunting of new interstellar molecules as well as understanding of carbon-chain chemistry through critical comparison of physical and chemical properties with TMC-1. This source is important not only for astronomy but also for molecular science as an ideal spectroscopic laboratory because of narrow line shapes and bright intensities.

[en] We have observed the CH₃OH J = 2-1, SiO J = 2-1, C³⁴S J = 2-1, H¹³CO⁺ J = 1-0, HN¹³C J = 1-0, CCH N = 1-0, OCS J = 8-7, and SO J_N = 2₂-1₁ lines toward 20 massive clumps, including Midcourse Space Experiment (MSX) 8 μm dark sources (infrared dark clouds) and MSX 8 μm sources, by using the Nobeyama Radio Observatory 45 m telescope. We have found that the velocity widths of the CH₃OH and C³⁴S lines are broader than those of the H¹³CO⁺ line in the MSX dark sources. On the other hand, they are comparable to the velocity width of the H¹³CO⁺ line in the MSX sources. In addition, the [SiO]/[H¹³CO⁺] abundance ratio is found to be enhanced in the MSX dark sources in comparison with the MSX sources. These results suggest that shocks caused by interaction between an outflow and an ambient dense gas would have substantial impact on the chemical composition of the MSX dark sources. The velocity widths of the CH₃OH and C³⁴S lines relative to that of the H¹³CO⁺ line as well as the [SiO]/[H¹³CO⁺] abundance ratio could be used as good tools for investigating evolutionary stages of massive clumps. On the basis of the results, we discuss the chemical and physical evolution of massive clumps.

[en] We have conducted a spectral line survey in the 3 and 2 mm bands toward two positions in a spiral arm of M51 (NGC 5194) with the Institut de Radioastronomie Millimétrique 30 m telescope. In this survey, we have identified 13 molecular species, including CN, CCH, N₂H⁺, HNCO, and CH₃OH. Furthermore, six isotopologues of the major species have been detected. On the other hand, SiO, HC₃N, CH₃CN, and deuterated species such as DCN and DCO⁺ were not detected. The deuterium fractionation ratios are evaluated to be less than 0.8% and 1.2% for DCN/HCN and DCO⁺/HCO⁺, respectively. By comparing the results of the two positions with different star formation activities, we have found that the observed chemical compositions do not strongly depend on star formation activities. They seem to reflect a chemical composition averaged over the 1 kpc scale region including many giant molecular clouds. Among the detected molecules CN, CCH, and CH₃OH are found to be abundant. High abundances of CN and CCH are consistent with the above picture of a widespread distribution of molecules because they can be produced by photodissociation. On the other hand, it seems likely that CH₃OH is liberated in the gas phase by shocks associated with large-scale phenomena such as cloud-cloud collisions and/or by nonthermal desorption processes such as photoevaporation due to cosmic-ray-induced UV photons. The present result demonstrates a characteristic chemical composition of a giant molecular cloud complex in the spiral arm, which can be used as a standard reference for studying chemistry in active galactic nuclei and starbursts.

[en] NGC 2264-C is a high-mass protocluster where several star formation events are known to have occurred. To investigate whether past protostellar activity has left a chemical imprint in this region, we mapped it in SiO($J=2-<!--\; ???\; -->1$), which is a shock tracer, and several other molecular lines with the Nobeyama 45 m telescope. Our observations show the presence of a complex network of protostellar outflows. The strongest SiO emission lies beyond a radius of ∼0.1 pc with respect to the center of the clump and is characterized by broad ($><!--\; >\; -->10$ km s⁻¹) lines and abundances of $\sim <!--\; ???\; -->1.4\times <!--\; ??\; -->{10}^{-<!--\; ???\; -->8}$ with respect to H₂. Interestingly, SiO appears to be relatively depleted (${\mathrm{\chi <!--\; ??\; -->}}_{\mathrm{S}\mathrm{i}\mathrm{O}}\sim <!--\; ???\; -->4\times <!--\; ??\; -->{10}^{-<!--\; ???\; -->9}$) within this radius, despite it being affected by molecular outflow activity. We attribute this to the fast condensation of SiO back onto dust grains and/or rapid gas-phase destruction of SiO, which is favored by the high density present in this area ($><!--\; >\; -->{10}^6$ cm⁻³). Finally, we identify a peripheral, narrow-line (∼2 km s⁻¹) component where SiO has an abundance of a few times 10⁻¹¹. After considering different options, we conclude that this weak emission may be tracing protostellar shocks from the star formation episode that preceded the current one, which have decelerated over time and eventually resulted in SiO being largely depleted/destroyed. Alternatively, a population of unresolved, low-mass protostars may be responsible for the narrow SiO emission. High-angular resolution observations are necessary to distinguish between these two possibilities, and thus to understand the role of SiO as a chemical tracer of past star formation episodes in massive protoclusters.

[en] The distributions of CCH, C₄H, c-C₃H₂, and HC₅N have been studied in high spatial resolution with the Plateau de Bure Interferometer, where the short-spacing data have been taken with the IRAM 30 m telescope. The distributions show clear central condensation around the protostar, confirming that these molecules are associated with the protostar's environment. The blueshifted and redshifted components are concentrated near the protostar, indicating their existence in the infalling envelope. The intensity distribution of c-C₃H₂ shows a steep increase inward of a radius of 500-1000 AU from the protostar. By comparing the c-C₃H₂ distribution with the H₂ column density distribution from the protostellar envelope model using the DUSTY code, the abundance of c-C₃H₂ is found to be enhanced by a factor of about 10 within the increasing point, where the temperature becomes higher than 20-30 K. This result supports the picture of warm carbon-chain chemistry; carbon-chain molecules and their related molecules are efficiently regenerated by evaporation of CH₄ from dust grains in the warm region (about 25 K). The distributions of CCH and C₄H have extended structures as well as an enhanced component, which implies a contribution of 'remnant' carbon-chain molecules produced in the starless-core phase in addition to the regeneration component. On the other hand, the distributions of CCH, C₄H, and c-C₃H₂ have a slight dip with a radius of 300-600 AU toward the protostar position, indicating that their abundances would decrease toward the central part. The present results provide a new picture of carbon-chain chemistry in the closest vicinity of a low-mass protostar.