Despite the success of the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) method in measuring rate coefficients for neutral-neutral reactions of radicals down close to the very low temperatures prevalent in dense interstellar clouds (ISCs), there are still many reactions of potential importance in the chemistry of these objects for which there have been no measurements of low temperature rate coefficients. One important class of reactions is that between atomic and molecular free radicals and unsaturated hydrocarbons; that is, alkynes and alkenes. Based on semi-empirical arguments and correlations of 'room temperature' rate coefficients, k(298 K), for reactions of this type with the difference between the ionisation energy of the alkyne/alkene and the electron affinity of the radical, we suggest which reactions between the radicals, C(3P), O(3P), N(4S), CH, C2H and CN, and carbon chain molecules (Cn) and cyanopolyynes (HC2nCN and NCC2nCN) are likely to be fast at the temperature of dense ISCs. These reactions and rate coefficients have been incorporated into a purely gas-phase model (osu2005) of ISC chemistry. The results of these calculations are presented and discussed.

Context. Isocyanic acid (HNCO) has been observed in different physical environments in the interstellar medium (ISM) and in external galaxies. HNCO has several metastable isomers with a ground electronic singlet state: HOCN, HCNO, and HONC. The recent detection of fulminic acid (HCNO) in prestellar and protostellar cores and cyanic acid (HOCN) in warm molecular sources (e.g. hot cores) in the Galactic center proves that these species could also be common constituents of the ISM. Aims. To shed some light on the possible formation pathways of these species, we searched for HCNO in the sources where HOCN has been previously detected and vice versa. We have also included the low-mass protostar IRAS 16293-2422, where HNCO is found to be prominent. Methods. Using the new EMIR receivers at the IRAM 30-m telescope, we performed deep searches for three rotational transitions of HOCN and four of HCNO. Results. We report the detection of HOCN in four sources -three dense cores and the lukewarm corino L1527 -where HCNO has been previously observed. HOCN is tentatively detected toward the protostellar binary IRAS 16293-2422. However, HCNO has been detected neither in this source nor in the sources of the Galactic center where HOCN has been previously reported. The derived abundance ratios HCNO/HOCN are close to unity in quiescent clouds, while they are less than 0.01-0.1 in warm clouds. We attempt to explain these results by using both gas-phase and gas-grain chemical models.

Isocyanic acid (HNCO) is a well-known interstellar molecule. Evidence also exists for the presence of two of its metastable isomers in the interstellar medium: HCNO (fulminic acid) and HOCN (cyanic acid). Fulminic acid has been detected toward cold and lukewarm sources, while cyanic acid has been detected both in these sources and in warm sources in the Galactic Center. Gas-phase models can reproduce the abundances of the isomers in cold sources, but overproduce HCNO in the Galactic Center. Here we present a detailed study of a gas-grain model that contains these three isomers, plus a fourth isomer, isofulminic acid (HONC), for four types of sources: hot cores, the warm envelopes of hot cores, lukewarm corinos, and cold cores. The current model is partially able to rationalize the abundances of HNCO, HOCN, and HCNO in cold and warm sources. Predictions for HONC in all environments are also made.

Aims. Chemical networks used for models of interstellar clouds contain many reactions, some of them with poorly determined rate coefficients and/or products. In this work, we report a method for improving the predictions of molecular abundances using sensitivity methods and ab initio calculations. Methods. Based on the chemical network osu.2003, we used two different sensitivity methods to determine the most important reactions as a function of time for models of dense cold clouds. Of these reactions, we concentrated on those between C and C 3 and between C and C 5 , both for their effect on specific important species such as CO and for their general effect on large numbers of species. We then used ab initio and kinetic methods to determine an improved rate coefficient for the former reaction and a new set of products, plus a slightly changed rate coefficient for the latter. Results. Putting our new results in a pseudo-time-dependent model of cold dense clouds, we found that the abundances of many species are altered at early times, based on large changes in the abundances of CO and atomic C. We compared the effect of these new rate coefficients/products on the comparison with observed abundances and found that they shift the best agreement from 3 × 10 4 yr to (1−3) × 10 5 yr.

Aims. With the Green Bank telescope (GBT), seven neutral molecules have been newly detected or confirmed towards either the cold interstellar core TMC-1 or the hot core source Sgr B2(N) within the last 1-2 years. Towards TMC-1, the new molecules seen are cyanoallene (CH 2 CCHCN) and methyl triacetylene (CH 3 C 6 H) while methyl cyanoacetylene (CH 3 CCCN) and methyl cyanodiacetylene (CH 3 C 5 N) were confirmed. Towards Sgr B2(N), the three newly detected molecules are cyclopropenone (c-C 3 H 2 O), ketenimine (CH 2 CNH), and acetamide (CH 3 CONH 2 ); these are mainly seen in absorption and are primarily located in an envelope around the hot core. In this work, we report a detailed study of the gas-phase chemistry of all seven molecules. Methods. Starting with our updated gas-phase chemical reaction network osu.01.2007, we added formation and depletion reactions to treat the chemistry of each of the seven molecules. Some of these were already in our network but with incomplete chemistry, while most were not in the network at all prior to this work. We assumed the standard physical conditions for TMC-1 and assumed that these also hold for the envelope around Sgr B2(N). Standard pseudo-time-dependent calculations were run for each source.Results. For TMC-1, we reproduced the observed fractional abundances of three detected molecules at early times of 10 5−6 yr and came close to reproducing a fourth. For the halo surrounding Sgr B2(N), our results are more ambiguous: only for ketenimine were we able to match the observed abundance very well.