Diffuse Atomic and Molecular Gas near IC443

We present an analysis of results on absorption from Ca II, Ca I, K I, and the molecules CH+, CH, C2, and CN that probes gas interacting with the supernova remnant IC443. The eleven directions sample material across the visible nebula and beyond its eastern edge. Most of the neutral material, including the diatomic molecules, is associated with the ambient cloud detected via H I and CO emission. Analysis of excitation and chemistry yields gas densities that are typical of diffuse molecular gas. The low density gas probed by Ca II extends over a large range in velocities, from -120 to +80 km/s in the most extreme cases. This gas is distributed among several velocity components, unlike the situation for the shocked molecular clumps, whose emission occurs over much the same range but as very broad features. The extent of the high-velocity absorption suggests a shock velocity of 100 km/s for the expanding nebula.Comment: To be published in Ap

A model for atomic and molecular interstellar gas: The Meudon PDR code

We present the revised ``Meudon'' model of Photon Dominated Region (PDR code), presently available on the web under the Gnu Public Licence at: http://aristote.obspm.fr/MIS. General organisation of the code is described down to a level that should allow most observers to use it as an interpretation tool with minimal help from our part. Two grids of models, one for low excitation diffuse clouds and one for dense highly illuminated clouds, are discussed, and some new results on PDR modelisation highlighted.Comment: accepted in ApJ sup

Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide

Ultraviolet absorption from interstellar 12CO and 13CO was detected toward rho Oph A and chi Oph. The measurements were obtained at medium resolution with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Column density ratios, N(12CO)/N(13CO), of 125 \pm 23 and 117 \pm 35 were derived for the sight lines toward rho Oph A and chi Oph, respectively. A value of 1100 \pm 600 for the ratio N(12C16O)/N(12C18O) toward rho Oph A was also obtained. Absorption from vibrationally excited H_2 (v" = 3) was clearly seen toward this star as well. The ratios are larger than the isotopic ratios for carbon and oxygen appropriate for ambient interstellar material. Since for both carbon and oxygen the more abundant isotopomer is enhanced, selective isotopic photodissociation plays the key role in the fractionation process for these directions. The enhancement arises because the more abundant isotopomer has lines that are more optically thick, resulting in more self shielding from dissociating radiation. A simple argument involving the amount of self shielding [from N(12CO)] and the strength of the ultraviolet radiation field premeating the gas (from the amount of vibrationally excited H_2) shows that selective isotopic photodissociation controls the fractionation seen in these two sight lines, as well as the sight line to zeta Oph.Comment: 40 pages, 8 figures, to appear in 10 July 2003 issue of Ap

Photoprocesses in protoplanetary disks

Circumstellar disks are exposed to intense ultraviolet radiation from the young star. In the inner disks, the UV radiation can be enhanced by more than seven orders of magnitude compared with the average interstellar field, resulting in a physical and chemical structure that resembles that of a dense photon-dominated region (PDR). This intense UV field affects the chemistry, the vertical structure of the disk, and the gas temperature, especially in the surface layers of the disk. The parameters which make disks different from traditional PDRs are discussed, including the shape of the UV radiation field, grain growth, the absence of PAHs, the gas/dust ratio and the presence of inner holes. New photorates for selected species, including simple ions, are presented. Also, a summary of available cross sections at Lyman alpha 1216 A is made. Rates are computed for radiation fields with color temperatures ranging from 4000 to 30,000 K, and can be applied to a wide variety of astrophysical regions including exo-planetary atmospheres. The importance of photoprocesses is illustrated for a number of representative disk models, including disk models with grain growth and settling.Comment: A website with the final published version and all photodissociation cross sections and rates can be found at http://www.strw.leidenuniv.nl/~ewine/phot

Non-Thermal Chemistry in Diffuse Clouds with Low Molecular Abundances

High quality archival spectra of interstellar absorption from C I toward 9 stars, taken with the Goddard High Resolution Spectrograph on the Hubble Space Telescope, were analyzed. Our sample was supplemented by two sight lines, 23 Ori and B1 Sco, for which the C I measurements of Federman, Welty, & Cardelli were used. Directions with known CH+ absorption, but only upper limits on absorption from C2 and CN, were considered for our study. This restriction allows us to focus on regions where CH+ chemistry dominates the production of carbon-bearing molecules. Profile synthesis of several multiplets yielded column densities and Doppler parameters for the C I fine structure levels. Equilibrium excitation analyses, using the measured column densities as well as the temperature from H2 excitation, led to values for gas density. These densities, in conjunction with measurements of CH, CH+, C2, and CN column densities, provided estimates for the amount of CH associated with CH+ production, which in turn set up constraints on the present theories for CH+ formation in this environment. We found for our sample of interstellar clouds that on average, 30--40 % of the CH originates from CH+ chemistry, and in some cases it can be as high as 90 %. A simple chemical model for gas containing non-equilibrium production of CH+ was developed for the purpose of predicting column densities for CH, CO, HCO+, CH2+, and CH3+ generated from large abundances of CH+. Again, our results suggest that non-thermal chemistry is necessary to account for the observed abundance of CH and probably that of CO in these clouds.Comment: 45 pages, 3 figures, Accepted for publication in the Astrophysical Journa

Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines

Using UV spectra obtained with FUSE, HST, and/or IUE, we determine interstellar column densities of 12CO, 13CO, and/or C_2 for ten Galactic sight lines with 0.37<E(B-V)<0.72. The N(CO)/N(H_2) ratio varies over a factor of 100 in this sample, due primarily to differences in N(CO). For a given N(H_2), published models of diffuse and translucent clouds predict less CO than is observed. The J=1-3 rotational levels of 12CO are sub-thermally populated in these sight lines, with T_ex typically between 3 and 7 K. In general, there is no significant difference between the excitation temperatures of 12CO and 13CO. Fits to the higher resolution CO line profiles suggest that CO (like CN) is concentrated in relatively cold, dense gas. We obtain C_2 column densities from the F-X (1-0) and (0-0) bands (1314 and 1341 A), the D-X (0-0) band (2313 A), and the A-X (3-0) and (2-0) bands (7719 and 8757 A). Comparisons among those N(C_2) yield a set of mutually consistent f-values for the UV and optical C_2 bands, but also reveal some apparent anomalies within the F-X (0-0) band. Both the kinetic temperature inferred from the C_2 rotational populations (up to J=18) and the excitation temperature T_02(C_2) are generally smaller than the corresponding T_01(H_2). Incorporating additional data for K I, HD, CH, C_2, C_3, CN, and CO from the literature (for a total sample of 74 sight lines), we find that (1) CO is most tightly correlated with CN; (2) the ratios 12CO/H_2 and 13CO/H_2 both are fairly tightly correlated with the density indicator CN/CH (but C_2/H_2 is not); and (3) the ratio 12CO/13CO is somewhat anti-correlated with both CN/CH and N(CO). Sight lines with 12CO/13CO below the average local Galactic value of 12C/13C appear to sample colder, denser gas in which isotope exchange reactions have enhanced 13CO, relative to 12CO.Comment: 78 pages, 24 figures, accepted to ApJ

Magic numbers in heteroatom-containing carbon monocycles

Geometries, electronic structures and energetics of heteroatom-doped carbon clusters of the type CnX+ (${\rm X=B}$, Si; n=9–15) have been investigated by means of the B3LYP (Becke 3-parameter-Lee-Yang-Parr) density functional method. The CnB+ (n=9–15) cations are predicted to be planar monocycles while in the CnSi+ cations a linear form is favored for C9Si+ and structural transition from linear to planar ring-shape structure occurs at n=10. Another difference between the two CnB+ and CnSi+ series is that in the CnB+ cations the boron atom is found to be incorporated into monocyclic structures whereas in the CnSi+ cations the silicon atom is bound to the outside of the carbon monocycle. More generally it is predicted that unlike first-row atoms such as B and N which can be easily networked into monocycles, second-row atoms such as Si, P and S are attached outside the carbon ring in capping position over two carbons. Incremental binding energy diagrams are also produced for the CnB+ and CnSi+ cations. It is shown that maxima of stability appear at n=10 and 14 for the CnB+ cations in very close agreement with the experimental features. In contrast a clear theory-versus-experiment discrepancy has been evidenced in the CnSi+ cluster series where B3LYP results clearly contradict the experiments concerning the relative stability of these species. Possible explanations for this discrepancy are suggested

A comparative ab initio study of the [math] and [math] cations

The [math] and [math] cations have been investigated at ab initio high quality electronic correlation level of theory and with density functional methods. Equilibrium structures, vibrational frequencies and relative energies have been determined for cyclic and linear isomers. We have found that the homoatomic [math] and [math] are unambiguously cyclic while the heteroatomic systems present very low barriers to linearity