16 research outputs found
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
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
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
Magic numbers in heteroatom-containing carbon monocycles
Geometries, electronic structures and energetics of heteroatom-doped carbon clusters of
the type CnX+ (, Si; n=9â15) have been investigated by means of the B3LYP
(Becke 3-parameter-Lee-Yang-Parr) density functional method. The CnB+ (n=9â15)
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
Density functional study of silicon carbide cluster cations C
In this work, we present the results of calculations, based on density functional theory, of the lowest states energy, electronic structure, dipole moments and harmonic vibrational frequencies for various geometrical arrangements of the four-membered silicon-carbide cations. For C4+, the linear and the rhomboidal forms are quasi degenerated. The C3Si+ and Si4+ species exhibit a rhomboidal structure while a linear one is favored for C2Si2+ Finally the CSi3+ ground state is a symmetric T-shaped structure with a central C atom. A comparative analysis of the total atomic charge repartition in the three and four atoms clusters leads to a coherent scheme of the equilibrium geometry found for the ground state of the different clusters. Tendency rules for the reactivity of the silicon atoms are finally presented