M. Hamberg

Observational tests of interstellar methanol formation

Context. It has been established that the classical gas-phase production of interstellar methanol (CH3OH) cannot explain observed abundances. Instead it is now generally thought that the main formation path has to be by successive hydrogenation of solid CO on interstellar grain surfaces. Aims. While theoretical models and laboratory experiments show that methanol is efficiently formed from CO on cold grains, our aim is to test this scenario by astronomical observations of gas associated with young stellar objects (YSOs). Methods. We have observed the rotational transition quartets J = 2K –1 K of 12 CH3OH and 13 CH3OH at 96.7 and 94.4 GHz, respectively, towards a sample of massive YSOs in different stages of evolution. In addition, the J = 1−0 transitions of 12 C 18 Oa nd 13 C 18 O were observed towards some of these sources. We use the 12 C/ 13 C ratio to discriminate between gas-phase and grain surface origin: If methanol is formed from CO on grains, the ratios should be similar in CH3OH and CO. If not, the ratio should be higher in CH3OH due to 13 C fractionation in cold CO gas. We also estimate the abundance ratios between the nuclear spin types of methanol (E and A). If methanol is formed on grains, this ratio is likely to have been thermalized at the low physical temperature of the grain, and therefore show a relative over-abundance of A-methanol. Results. We show that the 12 C/ 13 C isotopic ratio is very similar in gas-phase CH3OH and C 18 O, on the spatial scale of about 40 �� , towards four YSOs. For two of our sources we find an overabundance of A-methanol as compared to E-methanol, corresponding to nuclear spin temperatures of 10 and 16 K. For the remaining five sources, the methanol E/A ratio is less than unity. Conclusions. While the 12 C/ 13 C ratio test is consistent with methanol formation from hydrogenation of CO on grain surfaces, the result of the E/A ratio test is inconclusive.

Dissociative recombination of highly enriched para-H-3(+)

The determination of the dissociative recombination rate coefficient of H(3) (+) has had a turbulent history, but both experiment and theory have recently converged to a common value. Despite this convergence, it has not been clear if there should be a difference between the rate coefficients for ortho-H(3) (+) and para-H(3) (+). A difference has been predicted theoretically and could conceivably impact the ortho:para ratio of H(3) (+) in the diffuse interstellar medium, where H(3) (+) has been widely observed. We present the results of an experiment at the CRYRING ion storage ring in which we investigated the dissociative recombination of highly enriched ( approximately 83.6%) para-H(3) (+) using a supersonic expansion source that produced ions with T(rot) approximately 60-100 K. We observed an increase in the low energy recombination rate coefficient of the enriched para-H(3) (+) by a factor of approximately 1.25 in comparison to H(3) (+) produced from normal H(2) (ortho:para=3:1). The ratio of the rate coefficients of pure para-H(3) (+) to that of pure ortho-H(3) (+) is inferred to be approximately 2 at low collision energies; the corresponding ratio of the thermal rate coefficients is approximately 1.5 at electron temperatures from 60 to 1000 K. We conclude that this difference is unlikely to have an impact on the interstellar ortho:para ratio of H(3) (+).

Dissociative recombination of protonated methanol

The branching ratios of the different reaction pathways and the overall rate coefficients of the dissociative recombination reactions of CH3OH2+ and CD3OD2+ have been measured at the CRYRING storage ring located in Stockholm, Sweden. Analysis of the data yielded the result that formation of methanol or deuterated methanol accounted for only 3 and 6% of the total rate in CH3OH2+ and CD3OD2+, respectively. Dissociative recombination of both isotopomeres mainly involves fragmentation of the C-O bond, the major process being the three-body break-up forming CH3, OH and H (CD3, OD and D). The overall cross sections are best fitted by sigma = 1.2 +/- 0.1 x 10(-15) E(-1.15 +/- 0.02) cm2 and sigma = 9.6 +/- 0.9 x 10(-16) E(-1.20 +/- 0.02) cm2 for CH3OH2+ and CD3OD2+, respectively. From these values thermal reaction rate coefficients of k(T) = 8.9 +/- 0.9 x 10(-7) (T/300)(-0.59 +/- 0.02) cm3 s(-1) (CH3OH2+) and k(T) = 9.1 +/- 0.9 x 10(-7) (T/300)(-0.63 +/- 0.02) cm3 s(-1) (CD3OD2+) can be calculated. A non-negligible formation of interstellar methanol by the previously proposed mechanism via radiative association of CH3+ and H2O and subsequent dissociative recombination of the resulting CH3OH2+ ion to yield methanol and hydrogen atoms is therefore very unlikely.

ABSOLUTE PHOTODETACHMENT CROSS-SECTION MEASUREMENTS FOR HYDROCARBON CHAIN ANIONS

Absolute photodetachment cross sections have been measured for the hydrocarbon chain anions C(n)H(-), n = 2, 4, and 6, which are relevant for an understanding of molecular clouds in the interstella ...

Dissociative recombination branching ratios and their influence on interstellar clouds

Cross sections and branching ratios for the dissociative recombination (DR) reactions of the astrophysically important ions HN2+, HCO+, DOCO+, and SO2+ at reactant kinetic energies from 1 to 1000 meV have been measured using the CRYRING ion storage ring facility at the Manne Siegbahn Laboratory, Stockholm University. Whereas the break-up of the N-N bond leading to NH + N is the major pathway in the DR of HN2+, the analogous reaction in HCO+ almost exclusively leads to H and CO. In the DR of both DOCO+ and SO2+ three-body break-up was observed. Inclusion of the newly measured branching ratios into a standard model on dark interstellar clouds leads to an improvement of the predictions of such models, especially concerning the abundances of nitrogen compounds. The impact of these newly found branching ratios and reaction rates on the chemistry of different astronomical environments like dark clouds, circumstellar envelopes and planetary ionospheres, is discussed.

Dissociative recombination of fully deuterated protonated acetonitrile, CD3CND+: product branching fractions, absolute cross section and thermal rate coefficient

The dissociative recombination of fully deuterated protonated acetonitrile, CD(3)CND(+), has been investigated at the CRYRING heavy ion storage ring, located at the Manne Siegbahn Laboratory, Stockholm, Sweden. Branching fractions were measured at approximately 0 eV relative collision energy between the ions and the electrons and in 65% of the DR events there was no rupture of bonds between heavy atoms. In the remaining 35%, one of the bonds between the heavy atoms was broken. The DR cross-section was measured between approximately 0 eV and 1 eV relative collision energy. In the energy region between 1 meV and 0.1 eV the cross section data were best fitted by the expression sigma = 7.37 x 10(-16) (E/eV)(-1.23) cm(2), whereas sigma = 4.12 x 10(-16) (E/eV)(-1.46) cm(2) was the best fit for the energy region between 0.1 and 1.0 eV. From the cross section a thermal rate coefficient of alpha(T) = 8.13 x 10(-7) (T/300)(-0.69) cm(3) s(-1) was deduced.

Reassessment of the dissociative recombination of n2h+ at cryring

The dissociative recombination (DR) of N2H+ has been reinvestigated at the heavy ion storage ring CRYRING at the Manne Siegbahn Laboratory in Stockholm, Sweden. Thermal rate coefficients for electr ...

Experimental studies of the dissociative recombination processes for the dimethyl ether ions CD3OCD2+ and (CD3)2OD+

Aims: Determination of branching fractions, cross sections and thermal rate coefficients for the dissociative recombination of CD3OCD2+ (0-0.3 eV) and (CD3)2OD+ (0-0.2 eV) at the low relative kinet ...

The Dissociative Recombination of Protonated Acrylonitrile, CH2CHCNH+, with Implications for the Nitrile Chemistry in Dark Molecular Clouds and the Upper Atmosphere of Titan

Measurements on the dissociative recombination (DR) of protonated acrylonitrile, CH2CHCNH+, have been performed at the heavy ion storage ring CRYRING located in the Manne Siegbahn Laboratory in Sto ...

Dissociative Recombination of Protonated Formic Acid : Implications for Molecular Cloud and Cometary Chemistry

At the heavy ion storage ring CRYRING in Stockholm, Sweden, we have investigated the dissociative recombination of DCOOD2+ at low relative kinetic energies, from similar to 1 meV to 1 eV. The therm ...