| 1. |
- Röllig, M., et al.
(författare)
-
A photon dominated region code comparison study
- 2007
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 467:No. 1 (May III 2007), s. 187-206
-
Tidskriftsartikel (refereegranskat)abstract
- Aims.We present a comparison between independent computer codes, modeling the physics and chemistry of interstellar photon dominated regions (PDRs). Our goal was to understand the mutual differences in the PDR codes and their effects on the physical and chemical structure of the model clouds, and to converge the output of different codes to a common solution.Methods. A number of benchmark models have been created, covering low and high gas densities n = 103,105.5 cm-3 and far ultraviolet intensities $\chi$ = 10, 105 in units of the Draine field (FUV: 6
|
|
| 2. |
- Schöier, Fredrik, 1969, et al.
(författare)
-
An atomic and molecular database for analysis of submillimetre line observations
- 2005
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 432, s. 369-379
-
Tidskriftsartikel (refereegranskat)abstract
- Atomic and molecular data for the transitions of a number of astrophysically interesting species are summarized, including energy levels, statistical weights, Einstein A-coefficients and collisional rate coefficients. Available collisional data from quantum chemical calculations and experiments are extrapolated to higher energies (up to $E/k \sim 1000$ K). These data, which are made publically available through the WWW at http://www.strw.leidenuniv.nl/~moldata, are essential input for non-LTE line radiative transfer programs. An online version of a computer program for performing statistical equilibrium calculations is also made available as part of the database. Comparisons of calculated emission lines using different sets of collisional rate coefficients are presented. This database should form an important tool in analyzing observations from current and future (sub)millimetre and infrared telescopes.
|
|
| 3. |
|
|
| 4. |
- Andersson, Stefan, 1973, et al.
(författare)
-
Molecular dynamics simulations of water ice photochemistry
- 2007
-
Ingår i: Molecules in space and laboratory. - 9782901057581
-
Konferensbidrag (refereegranskat)abstract
- Molecular dynamics simulations of the dynamics following the absorption of an ultraviolet photon in amorphous water ice have been performed. Large probabilities of the desorption of H atoms are found. The desorption of OH radicals and H2O molecules is much less probable, but is still nonnegligible.
|
|
| 5. |
- Andersson, Stefan, 1973, et al.
(författare)
-
Photodesorption of water ice: a molecular dynamics study
- 2008
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 491, s. 907-916
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Absorption of ultraviolet radiation by water ice coating interstellar grains can lead to dissociation and desorption of the ice molecules. These processes are thought to be important in the gas-grain chemistry in molecular clouds and protoplanetary disks, but very few quantitative studies exist. Aims. We compute the photodesorption efficiencies of amorphous water ice and elucidate the mechanisms by which desorption occurs. Methods. Classical molecular dynamics calculations were performed for a compact amorphous ice surface at 10 K thought to be representative of interstellar ice. Dissociation and desorption of H2O molecules in the top six monolayers are considered following absorption into the first excited electronic state with photons in the 1300−1500 Å range. The trajectories of the H and OH photofragments are followed until they escape or become trapped in the ice. Results. The probability for H2O desorption per absorbed UV photon is 0.5−1% in the top three monolayers, then decreases to 0.03% in the next two monolayers, and is negligible deeper into the ice. The main H2O removal mechanism in the top two monolayers is through separate desorption of H and OH fragments. Removal of H2O molecules from the ice, either as H2O itself or its products, has a total probability of 2−3% per absorbed UV photon in the top two monolayers. In the third monolayer the probability is about 1% and deeper into the ice the probability of photodesorption falling to insignificant numbers. The probability of any removal of H2O per incident photon is estimated to be 3.7 × 10−4, with the probability for photodesorption of intact H2O molecules being 1.4 × 10−4 per incident photon. When no desorption occurs, the H and OH products can travel up to 70 and 60 Å inside or on top of the surface, respectively, during which they can react with other species, such as CO, before they become trapped.
|
|
| 6. |
- Thi, W. F., et al.
(författare)
-
ISO observation of molecular hydrogen and fine-structure lines in the photodissociation region IC 63
- 2009
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 400:2, s. 622-628
-
Tidskriftsartikel (refereegranskat)abstract
- We wish to constrain the main physical properties of the photodissociation region (PDR) IC63. We present the results of a survey for the lowest pure-rotational lines of H2 with the Short Wavelength Spectrometer and for the major fine-structure cooling lines of OI at 63 and 145μm and CII at 157.7μm with the Long Wavelength Spectrometer on board the Infrared Space Observatory (ISO) in the high-density PDR IC63. The observations are compared with available photochemical models based on optical absorption and/or millimetre emission line data with and without enhanced H2 formation rate on grain surfaces. The cloud density nH is constrained by the fine-structure lines. The models include both collisional excitation and ultraviolet (UV) pumping of the H2 ro-vibrational levels. Molecular pure-rotational lines up to S(5) are detected. The inferred column density of warm H2 at 106 +/- 11K is (5.9 +/- 1.8)+0.9-0.7 × 1021cm-2, while that of the hot component at 685 +/- 68K is (1.2 +/- 0.4) × 1019cm-2. Fine-structure lines are also detected in the far-infrared spectrum of IC63. The fine-structure lines constrain the density of the PDR to be (1-5) × 103cm-3. The impinging UV field on the PDR is enhanced by a factor of 103 compared to the mean interstellar field and is consistent with direct measurements in the UV. PDR models that include an enhanced H2 formation at high dust temperature give higher H2 intensities than models without enhancement. However, the predicted intensities are still lower than the observed intensities.
|
|
| 7. |
- Visser, R., et al.
(författare)
-
The photodissociation and chemistry of CO isotopologues: applications to interstellar clouds and circumstellar disks
- 2009
-
Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 503, s. 323 - 343
-
Tidskriftsartikel (refereegranskat)abstract
- Aims. Photodissociation by UV light is an important destruction mechanism for carbon monoxide (CO) in many astrophysical environments, ranging from interstellar clouds to protoplanetary disks. The aim of this work is to gain a better understanding of the depth dependence and isotope-selective nature of this process.Methods. We present a photodissociation model based on recent spectroscopic data from the literature, which allows us to compute depth-dependent and isotope-selective photodissociation rates at higher accuracy than in previous work. The model includes self-shielding, mutual shielding and shielding by atomic and molecular hydrogen, and it is the first such model to include the rare isotopologues C17O and 13C17O. We couple it to a simple chemical network to analyse CO abundances in diffuse and translucent clouds, photon-dominated regions, and circumstellar disks.Results. The photodissociation rate in the unattenuated interstellar radiation field is 2.6 $\times$ 10-10 s-1, 30% higher than currently adopted values. Increasing the excitation temperature or the Doppler width can reduce the photodissociation rates and the isotopic selectivity by as much as a factor of three for temperatures above 100 K. The model reproduces column densities observed towards diffuse clouds and PDRs, and it offers an explanation for both the enhanced and the reduced N(12CO)/N(13CO) ratios seen in diffuse clouds. The photodissociation of C17O and 13C17O shows almost exactly the same depth dependence as that of C18O and 13C18O, respectively, so 17O and 18O are equally fractionated with respect to 16O. This supports the recent hypothesis that CO photodissociation in the solar nebula is responsible for the anomalous 17O and 18O abundances in meteorites. Grain growth in circumstellar disks can enhance the N(12CO)/N(C17O) and N(12CO)/N(C18O) ratios by a factor of ten relative to the initial isotopic abundances.
|
|
