Convergence of Discretized Light Cone Quantization in the small mass limit

I discuss the slow convergence of Discretized Light Cone Quantization (DLCQ) in the small mass limit and suggest a solution.Comment: 8 pages, 5 Postscript figures, uses boxedeps.te

An ALMA view of CS and SiS around oxygen-rich AGB stars

We aim to determine the distributions of molecular SiS and CS in the circumstellar envelopes of oxygen-rich asymptotic giant branch stars and how these distributions differ between stars that lose mass at different rates. In this study we analyse ALMA observations of SiS and CS emission lines for three oxygen-rich galactic AGB stars: IK Tau, with a moderately high mass-loss rate of $5\times10^{-6}$M$_\odot$ yr$^{-1}$, and W Hya and R Dor with low mass loss rates of $\sim1\times10^{-7}$M$_\odot$ yr$^{-1}$. These molecules are usually more abundant in carbon stars but the high sensitivity of ALMA allows us to detect their faint emission in the low mass-loss rate AGB stars. The high spatial resolution of ALMA also allows us to precisely determine the spatial distribution of these molecules in the circumstellar envelopes. We run radiative transfer models to calculate the molecular abundances and abundance distributions for each star. We find a spread of peak SiS abundances with $\sim10^{-8}$ for R Dor, $\sim10^{-7}$ for W Hya, and $\sim3\times10^{-6}$ for IK Tau relative to H$_2$. We find lower peak CS abundances of $\sim7\times10^{-9}$ for R Dor, $\sim7\times10^{-8}$ for W Hya and $\sim4\times10^{-7}$ for IK Tau, with some stratifications in the abundance distributions. For IK Tau we also calculate abundances for the detected isotopologues: C$^{34}$S, $^{29}$SiS, $^{30}$SiS, Si$^{33}$S, Si$^{34}$S, $^{29}$Si$^{34}$S, and $^{30}$Si$^{34}$S. Overall the isotopic ratios we derive for IK Tau suggest a lower metallicity than solar.Comment: 16 page

Chemical modelling of dust–gas chemistry within AGB outflows – II. Effect of the dust-grain size distribution

Asymptotic giant branch (AGB) stars are, together with supernovae, the main contributors of stellar dust to the interstellar medium (ISM). Dust grains formed by AGB stars are thought to be large. However, as dust nucleation and growth within their outflows are still not understood, the dust-grain size distribution (GSD) is unknown. This is an important uncertainty regarding our knowledge of the chemical and physical history of interstellar dust, as AGB dust forms ∼70 per cent∼70 per cent of the starting point of its evolution. We expand on our chemical kinetics model, which uniquely includes a comprehensive dust–gas chemistry. The GSD is now allowed to deviate from the commonly assumed canonical Mathis, Rumpl & Nordsieck distribution. We find that the specific GSD can significantly influence the dust–gas chemistry within the outflow. Our results show that the level of depletion of gas-phase species depends on the average grain surface area of the GSD. Gas-phase abundance profiles and their possible depletions can be retrieved from observations of molecular emission lines when using a range of transitions. Because of degeneracies within the prescription of GSD, specific parameters cannot be retrieved, only (a lower limit to) the average grain surface area. None the less, this can discriminate between dust composed of predominantly large or small grains. We show that when combined with other observables such as the spectral energy distribution and polarized light, depletion levels from molecular gas-phase abundance profiles can constrain the elusive GSD of the dust delivered to the ISM by AGB outflows

Sulphur-bearing molecules in AGB stars I: The occurrence of hydrogen sulfide

Through a survey of (sub-)millimetre emission lines of various sulphur-bearing molecules, we aim to determine which molecules are the primary carriers of sulphur in different types of AGB stars. In this paper, the first in a series, we investigate the occurrence of H$_2$S in AGB circumstellar envelopes and determine its abundance, where possible. We have surveyed 20 AGB stars with a range of mass-loss rates and of different chemical types using the APEX telescope to search for rotational transition lines of five key sulphur-bearing molecules: CS, SiS, SO, SO$_2$ and H$_2$S. Here we present our results for H$_2$S, including detections, non-detections and detailed radiative transfer modelling of the detected lines. We compare results based on different descriptions of the molecular excitation of H$_2$S and different abundance distributions, including those derived from chemical modelling results. We detected H$_2$S towards five AGB stars, all of which have high mass-loss rates of $\dot{M}\geq 5\times 10^{-6}M_\odot$ yr$^{-1}$ and are oxygen-rich. H$_2$S was not detected towards the carbon or S-type stars that fall in a similar mass-loss range. For the stars in our sample with detections, we find peak o-H$_2$S abundances relative to H$_2$ between $4\times 10^{-7}$ and $2.5\times 10^{-5}$. Overall, we conclude that H$_2$S can play a significant role in oxygen-rich AGB stars with higher mass-loss rates, but is unlikely to play a key role in stars of other chemical types or the lower mass-loss rate oxygen-rich stars. For two sources, V1300 Aql and GX Mon, H$_2$S is most likely the dominant sulphur-bearing molecule in the circumstellar envelope.Comment: 14 pages, 7 figures, accepted in A&

The Relation between Dynamical Mass-to-light Ratio and Color for Massive Quiescent Galaxies out to z ~ 2 and Comparison with Stellar Population Synthesis Models

Article / Letter to editorSterrewach

The impact of stellar companion UV photons on the chemistry of the circumstellar environments of AGB stars

Spherical asymmetries are prevalent within the outflows of AGB stars. Since binary interaction with a stellar or planetary companion is thought to be the underlying mechanism behind large-scale structures, we included the effects of UV radiation originating from a stellar companion in our chemical kinetics model. The one-dimensional model provides a first approximation of its effects on the chemistry throughout the outflow. The presence of a close-by stellar companion can strongly influence the chemistry within the entire outflow. Its impact depends on the intensity of the radiation (set by the stellar radius and blackbody temperature) and on the extinction the UV radiation experiences (set by the outflow density, density structure, and assumed radius of dust formation). Parent species can be photodissociated by the companion, initiating a rich photon-driven chemistry in the inner parts of the outflow. The outcome depends on the balance between two-body reactions and photoreactions. If two-body reactions dominate, chemical complexity within the outflow increases. This can make the abundance profiles of daughters appear like those of parents, with a larger inner abundance and a Gaussian decline. If photoreactions dominate, the outflow can appear molecule-poor. We model three stellar companions. The impact of a red dwarf companion is limited. Solar-like companions show the largest effect, followed by a white dwarf. A stellar companion can also lead to the formation of unexpected species. The outflow’s molecular content, especially combined with abundance profiles, can indicate a stellar companion’s presence. Our results pave the way for further outflow-specific (three-dimensional) model development