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

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.Comment: Accepted for publicatio

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" ~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.Comment: Accepted for publication in A&A. Astronomy and Astrophysics in press (2006) in pres

HCN and HNC mapping of the protostellar core Cha-MMS1

Aims. The purpose of this study is to investigate the distributions of the isomeric molecules HCN and HNC and estimate their abundance ratio in the protostellar core Cha-MMS1 located in Chamaeleon {\sc i}. Methods. The core was mapped in the J=1-0 rotational lines of HCN, HNC, and HN13C. The column densities of H13CN, HN13C, H15NC and NH3 were estimated towards the centre of the core. Results. The core is well delineated in all three maps. The kinetic temperature in the core, derived from the NH3 (1,1) and (2,2) inversion lines, is 12.1+/-0.1 K. The HN13C/H13CN column density ratio is between 3 and 4, i.e. similar to values found in several other cold cores. The HN13C/H15NC column density ratio is about 7. In case no 15N fractionation occurs in HNC (as suggested by recent modelling results), the HNC/HN13C abundance ratio is in the range 30-40, which indicates a high degree of 13C fractionation in HNC. Assuming no differential 13C fractionation the HCN and HNC abundances are estimated to be about 7E-10 and about 2E-9, respectively, the former being nearly two orders of magnitude smaller than that of NH3. Using also previously determined column densities in Cha-MMS1, we can put the most commonly observed nitrogenous molecules in the following order according to their fractional abundances: X(NH3) > X(HC3N) > X(HNC) > X(HCN) > X(N2H+). Conclusions. The relationships between molecular abundances suggest that Cha-MMS1 represents an evolved chemical stage, experiencing at present the 'late-time' cyanopolyyne peak. The possibility that the relatively high HNC/HCN ratio derived here is only valid for the $^{13}$C isotopic substitutes cannot be excluded on the basis of the present and other available data.Comment: 9 pages, 8 figures, accepted for publication in Astronomy & Astrophysic

State Immunity and Victims' Rights to Access to Court, Reparation, and the Truth

Recently municipal courts have found that foreign states do not enjoy jurisdictional immunity with respect to civil claims involving serious violations of international law within the forum state's territory during armed conflict. This article assesses the recent judgments' potential impact, taking into account previous court practice and international human rights jurisprudence. It concludes that an exception to immunity in the above circumstances where no alternative judicial remedies exist for the victims has a basis in previous practice and may be required to give effect to international human rights obligations. A recognition by the foreign state of an individual victims' right to bring a claim before that state's courts could provide the victims with reparation in the form of satisfaction. Where no such possibility exists, a limited exception to the rule of state immunity would ensure the victims' right to access to court and to the truth

AN EFFICIENT MECHANISM LEADING TO THE FORMATION OF NEGATIVE IONS IN SPACE

Author Institution: Departments of Physics and Astronomy, and Chemical Physics Program, The Ohio State UniversityRecent spectroscopic studies of carbon chain anions in the gas phase, and more specifically of $C_{7}^{-}$, have brought insight into the long-standing mystery of the unexplained diffuse interstellar bands (DIBs). Previously, negative ions had not been considered highly abundant in interstellar clouds, and the question of efficient mechanisms leading to their formation had not been investigated in great detail. We present a statistical calculation of the rate coefficients for radiative attachment of an electron to small linear carbon clusters containing 4 to 9 atoms. Our conclusion is that for molecules with 6 or more C atoms, the attachment occurs on every collision at the low temperatures of diffuse interstellar clouds