Valentine Wakelam

Non-thermal desorption from interstellar dust grains via exothermic surface reactions

Aims. The gas-phase abundance of methanol in dark quiescent cores in the interstellar medium cannot be explained by gas-phase chemistry. In fact, the only possible synthesis of this species appears to be production on the surfaces of dust grains followed by desorption into the gas. Yet, evaporation is inefficient for heavy molecules such as methanol at the typical temperature of 10 K. It is necessary then to consider non-thermal mechanisms for desorption. But, if such mechanisms are considered for the production of methanol, they must be considered for all surface species. Methods. Our gas-grain network of reactions has been altered by the inclusion of a non-thermal desorption mechanism in which the exothermicity of surface addition reactions is utilized to break the bond between the product species and the surface. Our estimated rate for this process derives from a simple version of classical unimolecular rate theory with a variable parameter only loosely constrained by theoretical work. Results. Our results show that the chemistry of dark clouds is altered slightly at times up to 10 6 yr, mainly by the enhancement in the gas-phase abundances of hydrogen-rich species such as methanol that are formed on grain surfaces. At later times, however, there is a rather strong change. Instead of the continuing accretion of most gas-phase species onto dust particles, a steady-state is reached for both gas-phase and grain-surface species, with significant abundances for the former. Nevertheless, most of the carbon is contained in an undetermined assortment of heavy surface hydrocarbons. Conclusions. The desorption mechanism discussed here will be better constrained by observational data on pre-stellar cores, where a significant accretion of species such as CO has already occurred.

The effect of uncertainties on chemical models of dark clouds

The gas-phase chemistry of dark clouds has been studied with a treatment of uncertainties caused both by errors in individual rate coefficients and uncertainties in physical conditions. Moreover, a sensitivity analysis has been employed to attempt to determine which reactions are most important in the chemistry of individual species. The degree of overlap between calculated errors in abundances and estimated observational errors has been used as an initial criterion for the goodness of the model and the determination of a best “chemical” age of the source. For the well-studied sources L134N and TMC-1CP, best agreement is achieved at so-called “early times” of ≈10 5 yr , in agreement with previous calculations but here put on a firmer statistical foundation. A more detailed criterion for agreement, which takes into account the degree of disagreement, is also proposed. Poorly understood but critical classes of reactions are delineated, especially reactions between ions and polar neutrals. Such reactions will have to be understood better before the chemistry can be made more secure. Nevertheless, the level of agreement is low enough to indicate that a static picture of physical conditions without consideration of interactions with grain surfaces is inappropriate for a complete understanding of the chemistry.

Estimation and reduction of the uncertainties in chemical models: application to hot core chemistry

It is not common to consider the role of uncertainties in the rate coefficients used in interstellar gas-phase chemical models. In this paper, we report a new method to determine both the uncertainties in calculated molecular abundances and their sensitivities to underlying uncertainties in the kinetic data utilized. The method is used in hot core models to determine if previous analyses of the age and the applicable cosmic-ray ionization rate are valid. We conclude that for young hot cores (

Sulphur-bearing species in the star forming region L1689N

le 10^4

Sulphur chemistry and molecular shocks: The case of NGC 1333-IRAS 2

~yr), the modeling uncertainties related to rate coefficients are reasonable so that comparisons with observations make sense. On the contrary, the modeling of older hot cores is characterized by strong uncertainties for some of the important species. In both cases, it is crucial to take into account these uncertainties to draw conclusions from the comparison of observations with chemical models.

Chemical sensitivity to the ratio of the cosmic-ray ionization rates of He and H2 in dense clouds

We report observations of the expected main S-bearing species (SO, SO2 and H2S) in the low-mass star forming region L1689N. We obtained large scale (∼300 �� × 200 �� ) maps of several transitions from these molecules with the goal to study the sulphur chemistry, i.e. how the relative abundances change in the different physical conditions found in L1689N. We identified eight interesting regions, where we carried out a quantitative comparative study: the molecular cloud (as refer- ence position), five shocked regions caused by the interaction of the molecular outflows with the cloud, and the two protostars IRAS 16293-2422 and 16293E. In the cloud we carefully computed the gas temperature and density by means of a non-LTE LVG code, while in other regions we used previous results. We hence derived the column density of SO, SO2 and H2S, to- gether with SiO and H2CO - which were observed previously - and their relevant abundance ratios. We find that SiO is the molecule that shows the largest abundance variations in the shocked regions, whereas S-bearing molecules show more moder- ate variations. Remarkably, the region of the brightest SiO emission in L1689N is undetected in SO2 ,H 2 Sa nd H 2CO and only marginally detected in SO. In the other weaker SiO shocks, SO2 is enhanced with respect to SO. We propose a schema in which the different molecular ratios correspond to different ages of the shocks. Finally, we find that SO, SO2 and H2S have significant abundance jumps in the inner hot core of IRAS 16293-2422 and discuss the implications of the measured abundances.

Chemical differentiation along the CepA-East outflows

We present SO and SO2 observations in the region of the low mass protostar IRAS2/NGC1333. The East-West outflow originating from this source has been mapped in four transitions of both SO and SO2. In addition, CS observations published in the literature have been used. We compute the SO, SO2 and CS column densities and the physical conditions at several positions of the outflow using LTE and a non-LTE LVG approximations. The SO2/SO and CS/SO abundance ratios are compared with the theoretical predictions of a chemical model adapted to the physical conditions in the IRAS2 outflow. The SO2/SO abundance ratios are constant in the two lobes of the outflow whereas CS/SO is up to 6 times lower in the shocked gas of the East lobe than in the West one. The comparison with the chemical model allows us to constrain the age of the outflow produced by IRAS2 to >5e3yr. We find low densities and temperatures for the outflow of IRAS2 ( 70 K) from SO and SO2 emission probably because the two molecules trace the cooled entrained material. The East lobe of the outflow shows denser gas compared to the West lobe. We also discuss some constraints on the depleted form of sulphur.

Modeling the ortho-to-para abundance ratio of cyclic C3H2 in cold dense cores

Aim: To determine whether or not gas-phase chemical models with homogeneous and time-independent physical conditions explain the many observed molecular abundances in astrophysical sources, it is crucial to estimate the uncertainties in the calculated abundances and compare them with the observed abundances and their uncertainties. Non linear amplification of the error and bifurcation may limit the applicability of chemical models. Here we study such effects on dense cloud chemistry. Method: Using a previously studied approach to uncertainties based on the representation of rate coefficient errors as log normal distributions, we attempted to apply our approach using as input a variety of different elemental abundances from those studied previously. In this approach, all rate coefficients are varied randomly within their log normal (Gaussian) distribution, and the time-dependent chemistry calculated anew many times so as to obtain good statistics for the uncertainties in the calculated abundances. Results: Starting with so-called high-metal elemental abundances, we found bimodal rather than Gaussian like distributions for the abundances of many species and traced these strange distributions to an extreme sensitivity of the system to changes in the ratio of the cosmic ray ionization rate zeta_He for He and that for molecular hydrogen zeta_H2. The sensitivity can be so extreme as to cause a region of bistability, which was subsequently found to be more extensive for another choice of elemental abundances. To the best of our knowledge, the bistable solutions found in this way are the same as found previously by other authors, but it is best to think of the ratio zeta_He/zeta_H2 as a control parameter perpendicular to the standard control parameter zeta/n_H.

SENSITIVITY OF INTERSTELLAR CHEMISTRY TO THE RATIO OF THE COSMIC-RAY IONIZATION OF HE AND H

We present the results of a multiline survey at mm-wavelengths of the Cepheus A star forming region. Four main flows have been identified: three pointing in the SW, NE, and SE directions and accelerating high density CS clumps. The fourth outflow, revealed by high-sensitivity HDO observations, is pointing towards South and is associated with conditions particularly favourable to a chemical enrichment. The analysis of the line profiles shows that the SiO molecule dominates at the highest velocities and at the highest excitation conditions, confirming its close association with shocks. H2S, SO2, and SO preferentially trace more quiescent regions than SiO, and in particular a lack of bright H2S emission at the highest velocities is found. OCS and H2CS emit at quite high velocities, where the abundances of three shock tracers like SiO, CH3OH, and HDO are higher. These results may indicate that H2S is not the only major sulphur carrier in the grain mantles, and that OCS and H2CS may probably play an important role on the grains; or that alternatively they rapidly form once the mantle is evaporated after the passage of a shock. Finally, the outflow peak emission has been compared with recent time-dependent sulphur chemistry models: the results indicate that, if associated with accurate measurements of the physical conditions, the CH3OH/H2CS column density ratio can be used as an effective chemical clock to date the age of shocked gas.

_2

Aims. We report a detailed attempt to model the ortho-to-para abundance ratio of c-C 3 H 2 so as to reproduce observed values in the cores of the well-known source TMC-1. According to observations, the ortho-to-para ratios vary, within large uncertainties, from a low of near unity to a high of approximately three depending on the core. Methods. We used the osu.2003 network of gas-phase chemical reactions augmented by reactions that specifically consider the formation, depletion, and interconversion of the ortho and para forms of c-C 3 H 2 and its precursor ion c-C 3 H + 3 . We investigated the sensitivity of the calculated ortho-to-para ratio for c-C 3 H 2 to a large number of factors. Results. For the less evolved cores C, CP, and D, we had no difficulty reproducing the observed ortho-to-para ratios of 1-2. In order to reproduce observed ortho-to-para ratios of near three, observed for the evolved cores A and B, it was necessary to include rapid ion-catalyzed interconversion processes.