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

Fine‐Structure Excitation of C + and Si + by Atomic Hydrogen

We present calculations of cross sections for fine-structure excitation in collisions of carbon and silicon ions in the 2P state with atomic hydrogen in the ground state. The results are based on accurate calculations of CH+ and SiH+ molecular potentials, including electronic core correlation and relativistic effects. We find that the energy dependence of the excitation cross sections is largely determined by shape resonances. Our work improves on the results of previous calculations with less accurate potentials. Analytical expressions for the cooling efficiency of C+(2P1/2) and Si+(2P1/2) are given for the temperature interval 15-2000 K.Physic

OH mid-infrared emission as a diagnostic of H

Context. Water is an important molecule in interstellar and circumstellar environments. Previous observations of mid-infrared (IR) rotational lines of OH toward star-forming regions suggest that OH emission may be used to probe the photodissociation of water. Aims. Our goal is to propose a method to quantify H2O photodissociation and measure the local ultraviolet (UV) flux from observations of mid-IR OH lines. Methods. Cross sections for the photodissociation of H2O resolving individual electronic, vibrational, and rotational states of the OH fragment are collected. The state distribution of nascent OH following H2O photodissociation is computed for various astrophysically relevant UV radiation fields (e.g., a single Lyα line or a broadband spectrum). These distributions are incorporated in a new molecular excitation code called GROSBET

Ab Initio Molecular Dynamics Calculations versus Quantum-State-Resolved Experiments on CHD3 + Pt(111): New Insights into a Prototypical Gas–Surface Reaction

The dissociative chemisorption of methane on metal surfaces is of fundamental and practical interest, being a rate-limiting step in the steam reforming process. The reaction is best modeled with quantum dynamics calculations, but these are currently not guaranteed to produce accurate results because they rely on potential energy surfaces based on untested density functionals and on untested dynamical approximations. To help overcome these limitations, here we present for the first time statistically accurate reaction probabilities obtained with ab initio molecular dynamics (AIMD) for a polyatomic gas-phase molecule reacting with a metal surface. Using a general purpose density functional, the AIMD reaction probabilities are in semiquantitative agreement with new quantum-state-resolved experiments on CHD3 + Pt(111). The comparison suggests the use of the sudden approximation for treating the rotations even though CHD3 has large rotational constants and yields an estimated reaction barrier of 0.9 eV for CH4 + Pt(111)

Short- and long-term outcomes of selective use of Frey or extended lateral pancreaticojejunostomy in chronic pancreatitis

BACKGROUND: Surgery is the most effective treatment in patients with painful chronic pancreatitis and a dilated pancreatic duct. Studies reporting the outcomes of selected surgical approach according to the pancreatic head size in these patients are lacking. METHOD: This was a retrospective, observational single-centre study of consecutive patients who underwent either a Frey procedure or extended lateral pancreaticojejunostomy (eLPJ) for pain due to chronic pancreatitis with a dilated main pancreatic duct (5 mm or more) between 2006 and 2017. A Frey procedure was used in patients with pancreatic head enlargement (40 mm or more) and eLPJ (full-length pancreaticojejunostomy, including transection of the gastroduodenal artery) in others. A biliodigestive bypass was added in the case of biliary obstruction. RESULTS: Overall, 140 of 220 patients met the eligibility criteria: 70 underwent a Frey procedure and 70 an eLPJ. Hepaticojejunostomy was added in 17.1 per cent of patients (Frey: 24.3 per cent; eLPJ: 10.0 per cent (P = 0.025)). Major morbidity occurred in 15.0 per cent of patients (Frey: 21.4 per cent; eLPJ: 8.6 per cent (P = 0.033)). After a median 7.8 years of follow-up, the mean (s.d.) decrease in Izbicki pain score was 33 (27) points (34 (28) points after a Frey procedure; 32 (26) points after an eLPJ). Pain relief was reported as 'very much' by 87.5 per cent of patients (Frey: 86.1 per cent; eLPJ: 88.9 per cent) and as 'partial' by 11.1 per cent (Frey: 13.8 per cent; eLPJ: 8.3 per cent). CONCLUSION: Selective-use of either a Frey procedure or eLPJ in patients with symptomatic chronic pancreatitis was-associated with low morbidity and long-term pain relief. Adding a-biliodigestive bypass did not increase morbidity