Spirals in protoplanetary disks from photon travel time

Spiral structures are a common feature in scattered-light images of protoplanetary disks, and of great interest as possible tracers of the presence of planets. However, other mechanisms have been put foward to explain them, including self-gravity, disk-envelope interactions, and dead zone boundaries. These mechanisms explain many spirals very well, but are unable to easily account for very loosely wound spirals and single spiral arms. We study the effect of light travel time on the shape of a shadow cast by a clump orbiting close (within ${\sim}1\,$au) of the central star, where there can be significant orbital motion during the light travel time from the clump to the outer disk and then to the sky plane. This delay in light rays reaching the sky plane gives rise to a variety of spiral- and arc-shaped shadows, which we describe with a general fitting formula for a flared, inclined disk.Comment: Accepted for publication in A&A Letters. Videos available at dl.dropboxusercontent.com/u/3526708/spiralmovies.zi

Photodissociation and photoionisation of atoms and molecules of astrophysical interest

A new collection of photodissociation and photoionisation cross sections for 102 atoms and molecules of astrochemical interest has been assembled, along with a brief review of the basic processes involved. These have been used to calculate dissociation and ionisation rates, with uncertainties, in a standard ultraviolet interstellar radiation field (ISRF) and wavelength-dependent radiation fields. The new ISRF rates generally agree within 30% with our previous compilations, with a few notable exceptions. The reduction of rates in shielded regions was calculated as a function of dust, molecular and atomic hydrogen, atomic C, and self-shielding column densities. The relative importance of shielding types depends on the species in question and the dust optical properties. The new data are publicly available from the Leiden photodissociation and ionisation database. Sensitivity of rates to variation of temperature and isotope, and cross section uncertainties, are tested. Tests were conducted with an interstellar-cloud chemical model, and find general agreement (within a factor of two) with the previous iteration of the Leiden database for the ISRF, and order-of-magnitude variations assuming various kinds of stellar radiation. The newly parameterised dust-shielding factors makes a factor-of-two difference to many atomic and molecular abundances relative to parameters currently in the UDfA and KIDA astrochemical reaction databases. The newly-calculated cosmic-ray induced photodissociation and ionisation rates differ from current standard values up to a factor of 5. Under high temperature and cosmic-ray-flux conditions the new rates alter the equilibrium abundances of abundant dark cloud abundances by up to a factor of two. The partial cross sections for H2O and NH3 photodissociation forming OH, O, NH2 and NH are also evaluated and lead to radiation-field-dependent branching ratios.Comment: Corrected some inconsistent table/figure data. Significant change: Zn photoionisation rate corrected. Accepted for publication by A&

The spin-forbidden vacuum-ultraviolet absorption spectrum of ¹⁴N¹⁵N

Photoabsorption spectra of ${}^{14}{\rm N}{}^{15}{\rm N}$ were recorded at high resolution with a vacuum-ultraviolet Fourier-Transform spectrometer fed by synchrotron radiation in the range 81 to 100 nm. The combination of high column density ($3\times 10^{17}$ cm$^{-2}$) and low temperature (98 K) allowed for the recording of weak spin-forbidden absorption bands exciting levels of triplet character. The triplet states borrow intensity from ${}^1\Pi_u$ states of Rydberg and valence character while causing their predissociation. New predissociation linewidths and molecular constants are obtained for the states $C\,{}^3\Pi_u(v=7,8,14,15,16,21)$, $G\,{}^3\Pi_u(v=0,1,4)$, and $F\,{}^3\Pi_u(v=0)$. The positions and widths of these levels are shown to be well-predicted by a coupled-Schroedinger equation model with empirical parameters based on experimental data on ${}^{14}{\rm N}_2$ and ${}^{15}{\rm N}_2$ triplet levels.Comment: 16 pages, 16 figure

High-resolution Fourier-transform XUV photoabsorption spectroscopy of 14N15N

The first comprehensive high-resolution photoabsorption spectrum of 14N15N has been recorded using the Fourier-transform spectrometer attached to the Desirs beamline at the Soleil synchrotron. Observations are made in the extreme ultraviolet (XUV) and span 100,000-109,000 cm-1 (100-91.7 nm). The observed absorption lines have been assigned to 25 bands and reduced to a set of transition energies, f values, and linewidths. This analysis has verified the predictions of a theoretical model of N2 that simulates its photoabsorption and photodissociation cross section by solution of an isotopomer independent formulation of the coupled-channel Schroedinger equation. The mass dependence of predissociation linewidths and oscillator strengths is clearly evident and many local perturbations of transition energies, strengths, and widths within individual rotational series have been observed.Comment: 14 pages, 8 figures, one data archiv

A MODEL OF ELECTRONICALLY-EXCITED STATES OF N2_2 AND ITS EXTREME-ULTRAVIOLET SPECTRUM.

Author Institution: Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands; Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, AustraliaThe nitrogen molecule is a long-studied and difficult problem in molecular spectroscopy, and many important details of its interaction with radiation remain unexplained. A principal problem of continuing interest concerns the resonant photoabsorption and resultant predissociation of N$_2$ when exposed to extreme-ultraviolet radiation. \vspace{2ex} A model of the relevant excited states of N$_2$ has been developed in order to quantify their interactions and reproduce photoabsorption and photodissociation cross sections between 100\,000 and 118\,500\,cm$^{-1}$ (100 and 84\,nm). This solves the radial Schrodinger equation within a coupled-channels formulation for new diabatic potential-energy curves, homogeneous and heterogeneous state mixing, and electronic transition moments for the optically allowed transitions. The accidental predissociation of ${}^1\Pi_u$ states between $100\,000$ and 112\,500\,cm$^{-1}$ has been quantitatively modelled by spin-orbit coupling these to a set of ${}^3\Pi_u$ and ${}^3\Sigma_u^+$ states which includes unbound members. \vspace{2ex} Following reference to a large experimental database, the model is both accurate and comprehensive and may be used to simulate synthetic cross sections for any temperature or isotopologue. These are suitable for use in high-resolution photochemical models of atmospheric and astrophysical environments

High-resolution Fourier-transform spectroscopy and deperturbation analysis of the A¹Π(v = 1) level in ¹²C¹⁸O

The A1Π(v = 1) level of the 12C18O isotopologue was precisely reinvestigated with two complementary spectroscopic techniques. High resolution B1Σ+ → A1Π(0, 1), (1, 1) and C1Σ+ → A1Π(0, 1) emission bands were recorded in the visible region, 20,700 – 26,100 cm−1, with a 1.71 m Fourier-transform spectrometer (Bruker IFS 125-HR) installed at the University of Rzeszów. The resulting line centre accuracy of isolated and medium to strong lines is 0.005 cm−1. In addition, high-resolution spectra of the A1Π ⟵ X1Σ+(1, 0), B1Σ+⟵ X1Σ+(0, 0) and (1, 0) as well as C1Σ+⟵ X1Σ+(0, 0) bands were recorded between 66,200 and 95,250 cm−1 using the vacuum-ultraviolet Fourier-transform spectrometer installed at the DESIRS beamline of the SOLEIL synchrotron. The wavenumber accuracy for isolated and strong spectral lines is 0.01 cm−1. A data set of 626 spectral lines belonging to seven bands was incorporated into a global deperturbation analysis. Significantly improved deperturbed molecular constants for the A1Π(v = 1), a´3Σ+(v = 10), D1Δ(v = 1), and I1Σ−(v = 2) levels, term values of the B1Σ+(v = 0, 1) and C1Σ+(v = 0) Rydberg states as well as the accompanying spin-orbit and rotation-electronic (L-uncoupling) interaction parameters were obtained. The experimental ro-vibrational term values of the A1Π(v = 1) level and its perturbers were also determined. The mixed composition of interacting states is expressed in terms of their 1Π percentage character

High-Resolution Oscillator Strength Measurements of the v\u27 = 0,1 Bands of the B-X, C-X, and E-X Systems in Five Isotopologues of Carbon Monoxide

We report oscillator strengths for six strong vibrational bands between 105.0 and 115.2 nm, associated with transitions from the v = 0 level of the X 1Σ+ ground state to the v = 0 and 1 levels of the B 1Σ+, C 1Σ+, and E 1Π states, in 12C16O, 12C17O, 12C18O, 13C16O, and 13C18O. These measurements extend the development of a comprehensive database of line positions, oscillator strengths, and linewidths of photodissociating transitions for all astrophysically relevant CO isotopologues. The E–X bands, in particular, play central roles in CO photodissociation and fractionationmodels of interstellar clouds and circumstellar disks including the early solar nebula. The resolving powers of the room-temperature measurements, R = 300,000–400,000, allow for the analysis of individual line strengths within bands; the measurements reveal J-dependences in the branch intensities of the C(v = 0,1)–X(0) and E(v = 0,1)–X(0) bands in all isotopologues. Minimal or no isotopologue dependence was found in the f-values of the C(v = 0,1)–X(0) and E(v = 0,1)–X(0) bands at a ∼5% uncertainty level. Revised dissociation branching ratios for the C(v = 0,1) and E(v = 0,1) levels are computed based on these f-values. The weak isotopologue dependence of the f-values presented here eliminates this mechanism as an explanation for the large 17O enrichments seen in recent laboratory photolysis experiments on CO at wavelengths from 105 to 108 nm

A divergent heritage for complex organics in Isheyevo lithic clasts

Primitive meteorites are samples of asteroidal bodies that contain a high proportion of chemically complex organic matter (COM) including prebiotic molecules such as amino acids, which are thought to have been delivered to Earth via impacts during the early history of the Solar System. Thus, understanding the origin of COM, including their formation pathway(s) and environment(s), is critical to elucidate the origin of life on Earth as well as assessing the potential habitability of exoplanetary systems. The Isheyevo CH/CBb carbonaceous chondrite contains chondritic lithic clasts with variable enrichments in 15N believed to be of outer Solar System origin. Using transmission electron microscopy (TEM-EELS) and in situ isotope analyses (SIMS and NanoSIMS), we report on the structure of the organic matter as well as the bulk H and N isotope composition of Isheyevo lithic clasts. These data are complemented by electron microprobe analyses of the clast mineral chemistry and bulk Mg and Cr isotopes obtained by inductively coupled plasma and thermal ionization mass spectrometry, respectively (MC-ICPMS and TIMS). Weakly hydrated (A) clasts largely consist of Mg-rich anhydrous silicates with local hydrated veins composed of phyllosilicates, magnetite and globular and diffuse organic matter. Extensively hydrated clasts (H) are thoroughly hydrated and contain Fe-sulfides, sometimes clustered with organic matter, as well as magnetite and carbonates embedded in a phyllosilicate matrix. The A-clasts are characterized by a more 15N-rich bulk nitrogen isotope composition (δ15N = 200–650‰) relative to H-clasts (δ15N = 50–180‰) and contain extremely 15N-rich domains with δ15N 15N-rich domains show that the lithic clast diffuse organic matter is typically more 15N-rich than globular organic matter. The correlated δ15N values and C/N ratios of nanoglobules require the existence of multiple organic components, in agreement with the H isotope data. The combined H and N isotope data suggest that the organic precursors of the lithic clasts are defined by an extremely 15N-poor (similar to solar) and D-rich component for H-clasts, and a moderately 15N-rich and D-rich component for A-clasts. In contrast, the composition of the putative fluids is inferred to include D-poor but moderately to extremely 15N-rich H- and N-bearing components. The variable 15N enrichments in H- and A-clasts are associated with structural differences in the N bonding environments of their diffuse organic matter, which are dominated by amine groups in H-clasts and nitrile functional groups in A-clasts. We suggest that the isotopically divergent organic precursors in Isheyevo clasts may be similar to organic moieties in carbonaceous chondrites (CI, CM, CR) and thermally recalcitrant organic compounds in ordinary chondrites, respectively. The altering fluids, which are inferred to cause the 15N enrichments observed in the clasts, may be the result of accretion of variable abundances of NH3 and HCN ices. Finally, using bulk Mg and Cr isotope composition of clasts, we speculate on the accretion regions of the various primitive chondrites and components and the origin of the Solar System’s N and H isotope variability