118 research outputs found
The role of the charge state of PAHs in ultraviolet extinction
Aims: We explore the relation between charge state of polycyclic aromatic
hydrocarbons (PAHs) and extinction curve morphology. Methods: We fit extinction
curves with a dust model including core-mantle spherical particles of mixed
chemical composition (silicate core, and carbonaceous layers),
and an additional molecular component. We use exact methods to calculate the
extinction due to classical particles and accurate computed absorption spectra
of PAHs in different charge states, for the contribution due to the molecular
component, along a sample of five rather different lines of sight. Results: A
combination of classical dust particles and mixtures of real PAHs
satisfactorily matches the observed interstellar extinction curves. Variations
of the spectral properties of PAHs in different charge states produce changes
consistent with the varying relative strengths of the bump and non-linear
far-UV rise.Comment: 5 pages, 3 figures, Astronomy & Astrophysics Letters, in pres
Photo-desorption of H2O:CO:NH3 circumstellar ice analogs: Gas-phase enrichment
We study the photo-desorption occurring in HO:CO:NH ice mixtures
irradiated with monochromatic (550 and 900 eV) and broad band (250--1250 eV)
soft X-rays generated at the National Synchrotron Radiation Research Center
(Hsinchu, Taiwan). We detect many masses photo-desorbing, from atomic hydrogen
(m/z = 1) to complex species with m/z = 69 (e.g., CHNO, CHO,
CHN), supporting the enrichment of the gas phase.
At low number of absorbed photons, substrate-mediated exciton-promoted
desorption dominates the photo-desorption yield inducing the release of weakly
bound (to the surface of the ice) species; as the number of weakly bound
species declines, the photo-desorption yield decrease about one order of
magnitude, until porosity effects, reducing the surface/volume ratio, produce a
further drop of the yield.
We derive an upper limit to the CO photo-desorption yield, that in our
experiments varies from 1.4 to 0.007 molecule photon in the range ~absorbed photons cm. We apply these findings to a
protoplanetary disk model irradiated by a central T~Tauri star
Chemistry in Evaporating Ices: Unexplored Territory
We suggest that three-body chemistry may occur in warm high density gas
evaporating in transient co\textendash desorption events on interstellar ices.
Using a highly idealised computational model we explore the chemical conversion
from simple species of the ice to more complex species containing several heavy
atoms, as a function of density and of adopted three body rate coefficients. We
predict that there is a wide range of densities and rate coefficients in which
a significant chemical conversion may occur. We discuss the implications of
this idea for the astrochemistry of hot cores.Comment: Accepted in Ap
Dehydrogenated polycyclic aromatic hydrocarbons and UV bump
Recent calculations have shown that the UV bump at about 217.5 nm in the
extinction curve can be explained by a complex mixture of PAHs in several
charge states. Other studies proposed that the carriers are a restricted
population made of neutral and singly-ionised dehydrogenated coronene molecules
(C24Hn, n less than 3), in line with models of the hydrogenation state of
interstellar PAHs predicting that medium-sized species are highly
dehydrogenated. To assess the observational consequences of the latter
hypothesis we have undertaken a systematic study of the electronic spectra of
dehydrogenated PAHs. We use our first results to see whether such spectra show
strong general trends upon dehydrogenation. We used state-of-the-art techniques
in the framework of the density functional theory (DFT) to obtain the
electronic ground-state geometries, and of the time- dependent DFT to evaluate
the electronic excited-state properties. We computed the absorption
cross-section of the species C24Hn (n=12,10,8,6,4,2,0) in their neutral and
cationic charge-states. Similar calculations were performed for other PAHs and
their fullydehydrogenated counterparts. pi electron energies are always found
to be strongly affected by dehydrogenation. In all cases we examined,
progressive dehydrogenation translates into a correspondingly progressive blue
shift of the main electronic transitions. In particular, the pi-pi* collective
resonance becomes broader and bluer with dehydrogenation. Its calculated energy
position is therefore predicted to fall in the gap between the UV bump and the
far-UV rise of the extinction curve. Since this effect appears to be
systematic, it poses a tight observational limit on the column density of
strongly dehydrogenated medium-sized PAHs.Comment: 5 pages, 7 figures, Astronomy & Astrophysics, in pres
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