Detection of interstellar HCS and its metastable isomer HSC: new pieces in the puzzle of sulfur chemistry

We present the first identification in interstellar space of the thioformyl radical (HCS) and its metastable isomer HSC. These species were detected toward the molecular cloud L483 thanks to observations carried out with the IRAM 30m telescope in the 3 mm band. We derive beam-averaged column densities of 7e12 cm-2 for HCS and 1.8e11 cm-2 for HSC, which translate to fractional abundances relative to H2 of 2e-10 and 6e-12, respectively. Although the amount of sulfur locked by these radicals is low, their detection allows to put interesting constraints on the chemistry of sulfur in dark clouds. Interestingly, the H2CS/HCS abundance ratio is found to be quite low, around 1, in contrast with the oxygen analogue case, in which the H2CO/HCO abundance ratio is around 10 in dark clouds. Moreover, the radical HCS is found to be more abundant than its oxygen analogue, HCO. The metastable species HOC, the oxygen analogue of HSC, has not been yet observed in space. These observational constraints are confronted with the outcome of a recent model of the chemistry of sulfur in dark clouds. The model underestimates the fractional abundance of HCS by at least one order of magnitude, overestimates the H2CS/HCS abundance ratio, and does not provide an abundance prediction for the metastable isomer HSC. These observations should prompt a revision of the chemistry of sulfur in interstellar clouds.Comment: Accepted for publication in A&A Letter

Detection of circumstellar CH2CHCN, CH2CN, CH3CCH and H2CS

We report on the detection of vinyl cyanide (CH2CHCN), cyanomethyl radical (CH2CN), methylacetylene (CH3CCH) and thioformaldehyde (H2CS) in the C-rich star IRC +10216. These species, which are all known to exist in dark clouds, are detected for the first time in the circumstellar envelope around an AGB star. The four molecules have been detected trough pure rotational transitions in the course of a 3 mm line survey carried out with the IRAM 30-m telescope. The molecular column densities are derived by constructing rotational temperature diagrams. A detailed chemical model of the circumstellar envelope is used to analyze the formation of these molecular species. We have found column densities in the range 5 x 10^(12)- 2 x 10^(13) cm^(-2), which translates to abundances relative to H2 of several 10^(-9). The chemical model is reasonably successful in explaining the derived abundances through gas phase synthesis in the cold outer envelope. We also find that some of these molecules, CH2CHCN and CH2CN, are most probably excited trough infrared pumping to excited vibrational states. The detection of these species stresses the similarity between the molecular content of cold dark clouds and C-rich circumstellar envelopes. However, some differences in the chemistry are indicated by the fact that in IRC +10216 partially saturated carbon chains are present at a lower level than those which are highly unsaturated, while in TMC-1 both types of species have comparable abundances.Comment: 9 pages, 5 figures; accepted for publication in A&

Confirmation of circumstellar phosphine

Phosphine (PH3) was tentatively identified a few years ago in the carbon star envelopes IRC+10216 and CRL2688 from observations of an emission line at 266.9 GHz attributable to the J=1-0 rotational transition. We report the detection of the J=2-1 rotational transition of PH3 in IRC+10216 using the HIFI instrument on board Herschel, which definitively confirms the identification of PH3. Radiative transfer calculations indicate that infrared pumping to excited vibrational states plays an important role in the excitation of PH3 in the envelope of IRC+10216, and that the observed lines are consistent with phosphine being formed anywhere between the star and 100 R* from the star, with an abundance of 1e-8 relative to H2. The detection of PH3 challenges chemical models, none of which offers a satisfactory formation scenario. Although PH3 locks just 2 % of the total available phosphorus in IRC+10216, it is together with HCP, one of the major gas phase carriers of phosphorus in the inner circumstellar layers, suggesting that it could be also an important phosphorus species in other astronomical environments. This is the first unambiguous detection of PH3 outside the solar system, and a further step towards a better understanding of the chemistry of phosphorus in space.Comment: Accepted for publication in ApJ Letter

Laboratory And Astronomical Detection Of The Negative Molecular Ion C3N-

The negative molecular ion C3N- has been detected at millimeter wavelengths in a low-pressure laboratory discharge, and then with frequencies derived from the laboratory data in the molecular envelope of IRC+10216. Spectroscopic constants derived from laboratory measurements of 12 transitions between 97 and 378 GHz allow the rotational spectrum to be calculated well into the submillimeter-wave band to 0.03 km s(-1) or better in equivalent radial velocity. Four transitions of C3N- were detected in IRC+10216 with the IRAM 30 m telescope at precisely the frequencies calculated from the laboratory measurements. The column density of C3N- is 0.5% that of C3N, or approximately 20 times greater than that of C4H- relative to C4H. The C3N- abundance in IRC+10216 is compared with a chemical model calculation by Petrie & Herbst. An upper limit in TMC-1 for C3N- relative to C3N (< 0.8%) and a limit for C4H- relative to C4H (< 0.004%) that is 5 times lower than that found in IRC+10216, were obtained from observations with the NRAO 100 m Green Bank Telescope (GBT). The fairly high concentration ofNRFKorean government MEST 2012R1A1A1014646, 2012M4A2026720Southeast Physics Network (SEP-Net)Science and Technology Facilities Council ST/F002858/1, ST/I000976/1Swedish Research Council 2009-4088U.S. NSF AST-0708176, AST-1009799NASA NNX07AH09G, NNG04G177G, NNX11AE09GChandra grant SAO TM8-9009XBiochemistr

Reactivity of OH and CH3OH between 22 and 64 K: Modelling the gas phase production of CH3O in Barnard 1b

In the last years, ultra-low temperature chemical kinetic experiments have demonstrated that some gas-phase reactions are much faster than previously thought. One example is the reaction between OH and CH3OH, which has been recently found to be accelerated at low temperatures yielding CH3O as main product. This finding opened the question of whether the CH3O observed in the dense core Barnard 1b could be formed by the gas-phase reaction of CH3OH and OH. Several chemical models including this reaction and grain-surface processes have been developed to explain the observed abundance of CH$_3$O with little success. Here we report for the first time rate coefficients for the gas-phase reaction of OH and CH3OH down to a temperature of 22 K, very close to those in cold interstellar clouds. Two independent experimental set-ups based on the supersonic gas expansion technique coupled to the pulsed laser photolysis-laser induced fluorescence technique were used to determine rate coefficients in the temperature range 22-64 K. The temperature dependence obtained in this work can be expressed as k(22-64 K) = (3.6+/-0.1)e-12 (T/ 300)^(-1.0+/-0.2) cm3 molecule-1 s-1. Implementing this expression in a chemical model of a cold dense cloud results in CH3O/CH3OH abundance ratios similar or slightly lower than the value of 3e-3 observed in Barnard 1b. This finding confirms that the gas-phase reaction between OH and CH3OH is an important contributor to the formation of interstellar CH3O. The role of grain-surface processes in the formation of CH3O, although it cannot be fully neglected, remains controversial.Comment: Accepted for publication in The Astrophysical Journa

Aromatic cycles are widespread in cold clouds

We report the detection of large hydrocarbon cycles toward several cold dense clouds. We observed four sources (L1495B, Lupus-1A, L483, and L1527) in the Q band (31-50 GHz) using the Yebes 40m radiotelescope. Using the line stack technique, we find statistically significant evidence of benzonitrile (C$_6$H$_5$CN) in L1495B, Lupus-1A, and L483 at levels of 31.8$\,\sigma$, 15.0$\,\sigma$, and 17.2$\,\sigma$, respectively, while there is no hint of C$_6$H$_5$CN in the fourth source, L1527. The column densities derived are in the range (1.7-3.8)$\,\times\,10^{11}$ cm$^{-2}$, which is somewhat below the value derived toward the cold dense cloud TMC-1. When we simultaneously analyze all the benzonitrile abundances derived toward cold clouds in this study and in the literature, a clear trend emerges in that the higher the abundance of HC$_7$N, the more abundant C$_6$H$_5$CN is. This indicates that aromatic cycles are especially favored in those interstellar clouds where long carbon chains are abundant, which suggests that the chemical processes that are responsible for the formation of linear carbon chains are also behind the synthesis of aromatic rings. We also searched for cycles other than benzonitrile, and found evidence of indene (C$_9$H$_8$), cyclopentadiene (C$_5$H$_6$), and 1-cyano cyclopentadiene (1-C$_5$H$_5$CN) at levels of 9.3$\,\sigma$, 7.5$\,\sigma$, and 8.4$\,\sigma$, respectively, toward L1495B, which shows the strongest signal from C$_6$H$_5$CN. The relative abundances between the various cycles detected in L1495B are consistent -- within a factor of three -- with those previously found in TMC-1. It is therefore likely that not only C$_6$H$_5$CN but also other large aromatic cycles are abundant in clouds rich in carbon chains.Comment: Accepted for publication in A&A Letters. Changes with respect to previous version: language edited, error in abstract corrected, and title change

The impact of atmospheric circulation on the chemistry of the hot Jupiter HD 209458b

This is the author accepted manuscript. The final version is available from EDP Sciences via the DOI in this record.We investigate the effects of atmospheric circulation on the chemistry of the hot Jupiter HD 209458b. We use a simplified dynamical model and a robust chemical network, as opposed to previous studies which have used a three dimensional circulation model coupled to a simple chemical kinetics scheme. The temperature structure and distribution of the main atmospheric constituents are calculated in the limit of an atmosphere that rotates as a solid body with an equatorial rotation rate of 1 km/s. Such motion mimics a uniform zonal wind which resembles the equatorial superrotation structure found by three dimensional circulation models. The uneven heating of this tidally locked planet causes, even in the presence of such a strong zonal wind, large temperature contrasts between the dayside and nightside, of up to 800 K. This would result in important longitudinal variations of some molecular abundances if the atmosphere were at chemical equilibrium. The zonal wind, however, acts as a powerful disequilibrium process. We identify the existence of a pressure level of transition between two regimes, which may be located between 100 and 0.1 mbar depending on the molecule. Below this transition layer, chemical equilibrium holds, while above it, the zonal wind tends to homogenize the chemical composition of the atmosphere, bringing molecular abundances in the limb and nightside regions close to chemical equilibrium values characteristic of the dayside, i.e. producing an horizontal quenching effect in the abundances. Reasoning based on timescales arguments indicates that horizontal and vertical mixing are likely to compete in HD 209458b's atmosphere, producing a complex distribution where molecular abundances are quenched horizontally to dayside values and vertically to chemical equilibrium values characteristic of deep layers.M.A., O.V., F.S., and E.H. acknowledge support from the European Research Council (ERC Grant 209622: E3ARTHs). Computer time for this study was provided by the computing facilities MCIA (Mésocentre de Calcul Intensif Aquitain) of the Université de Bordeaux and of the Université de Pau et des Pays de l’Adour. We thank the anonymous referee for a constructive report that helped to improve this manuscript

The abundance and excitation of molecular anions in interstellar clouds

We report new observations of molecular anions with the Yebes 40m and IRAM 30m telescopes toward the cold dense clouds TMC-1 CP, Lupus-1A, L1527, L483, L1495B, and L1544. We detected for the first time C3N- and C5N- in Lupus-1A and C4H- and C6H- in L483. In addition, we report new lines of C6H- toward the six targeted sources, of C4H- toward TMC-1 CP, Lupus-1A, and L1527, and of C8H- and C3N- in TMC-1 CP. Excitation calculations indicate that the lines of anions accessible to radiotelescopes run from subthermally excited to thermalized as the size of the anion increases, with the degree of departure from thermalization depending on the H2 volume density and the line frequency. We noticed that the collision rate coefficients available for the radical C6H cannot explain various observational facts, which advises for a revisitation of the collision data for this species. The observations presented here, together with observational data from the literature, are used to model the excitation of interstellar anions and to constrain their abundances. In general, the anion-to-neutral ratios derived here agree within 50 % (a factor of two at most) with literature values, when available, except for the C4H-/C4H ratio, which shows higher differences due to a revision of the dipole moment of C4H. From the set of anion-to-neutral abundance ratios derived two conclusions can be drawn. First, the C6H-/C6H ratio shows a tentative trend in which it increases with increasing H2 density, as expected from theoretical grounds. And second, it is incontestable that the higher the molecular size the higher the anion-to-neutral ratio, which supports a formation mechanism based on radiative electron attachment. Nonetheless, calculated rate coefficients for electron attachment to the medium size species C4H and C3N are probably too high and too low, respectively, by more than one order of magnitude.Comment: Accepted for publication in A&

Reaction Networks For Interstellar Chemical Modelling: Improvements and Challenges

We survey the current situation regarding chemical modelling of the synthesis of molecules in the interstellar medium. The present state of knowledge concerning the rate coefficients and their uncertainties for the major gas-phase processes -- ion-neutral reactions, neutral-neutral reactions, radiative association, and dissociative recombination -- is reviewed. Emphasis is placed on those reactions that have been identified, by sensitivity analyses, as 'crucial' in determining the predicted abundances of the species observed in the interstellar medium. These sensitivity analyses have been carried out for gas-phase models of three representative, molecule-rich, astronomical sources: the cold dense molecular clouds TMC-1 and L134N, and the expanding circumstellar envelope IRC +10216. Our review has led to the proposal of new values and uncertainties for the rate coefficients of many of the key reactions. The impact of these new data on the predicted abundances in TMC-1 and L134N is reported. Interstellar dust particles also influence the observed abundances of molecules in the interstellar medium. Their role is included in gas-grain, as distinct from gas-phase only, models. We review the methods for incorporating both accretion onto, and reactions on, the surfaces of grains in such models, as well as describing some recent experimental efforts to simulate and examine relevant processes in the laboratory. These efforts include experiments on the surface-catalysed recombination of hydrogen atoms, on chemical processing on and in the ices that are known to exist on the surface of interstellar grains, and on desorption processes, which may enable species formed on grains to return to the gas-phase.Comment: Accepted for publication in Space Science Review

The spatial distribution of an aromatic molecule, C6H5CN, in the cold dark cloud TMC-1

We present a highly sensitive 2D line survey of TMC-1 obtained with the Yebes 40m radio telescope in the Q-band (31.13-49.53 GHz). These maps cover a region of 320 arcsec x 320 arcsec centred on the position of the QUIJOTE line survey with a spatial sampling of 20 arcsec. The region covering 240 arcsec x 240 arcsec, where a longer integration time was used, shows a homogenous sensitivity of 2-4 mK across the band. We present in this work the first determination of the spatial extent of benzonitrile (C6H5CN), which follows that of cyanopolyynes rather well, but differs significantly from that of the radicals CnH and CnN. We definitively conclude that aromatic species in TMC-1 are formed from chemical reactions involving smaller species in the densest zones of the cloud.Comment: Accepted for publication in A&A Letter