Non-local Thermodynamic Equilibrium Radiative Transfer Research Articles

CO2 Thermal Infrared Signature Following a Sprite Event in the Mesosphere

AbstractSprites are a potential thermal infrared radiation source in the stratosphere and mesosphere through molecular vibrational excitation. We developed a plasma‐chemical model to compute the vibrational kinetics induced by a sprite streamer in the 40‐ to 70‐km altitude range until several tens of seconds after the visible flash is over. Then, we computed the consecutive time‐dependent thermal infrared spectra that could be observed from the stratosphere (from a balloon platform), high troposphere (from an aircraft), and low troposphere (aircraft or altitude observatory) using a nonlocal thermodynamic equilibrium radiative transfer model. Our simulations predict a strong production of CO2 in the (001) vibrational level which lasts at least 40 s before falling to background concentrations. This leads to enhanced emissions in the long‐wavelength infrared, around 1,000 cm−1, and midwavelength infrared, around 2,300 cm−1. The maximum sprite infrared signatures (sprite spectra minus background spectra) reach several 10−7 W/sr/cm2/cm−1 after propagation through the mesosphere and stratosphere, to an observer located at 20–40 km of altitude. This maximum signal is about 1 order of magnitude lower if propagated until the troposphere. From the two spectral bands, the 1,000‐cm−1 one could be detected more easily than the 2,300‐cm−1 one, which is more affected by atmospheric absorption (CO2 self‐trapping at all altitudes and H2O mostly in the troposphere). With a sufficiently sensitive instrumentation, mounted in an open stratospheric balloon platform for example, the 1,000‐cm−1 band could be detected from 20–40 km of altitude.

Chemistry in disks

Context. Several sulfur-bearing molecules are observed in the interstellar medium and in comets, in strong contrast to protoplanetary disks where only CS, H2CS, and SO have been detected so far. Aims. We combine observations and chemical models to constrain the sulfur abundances and their sensitivity to physical and chemical conditions in the DM Tau protoplanetary disk. Methods. We obtained 0.5′′ Atacama Large Millimeter Array observations of DM Tau in Bands 4 and 6 in lines of CS, SO, SO2, OCS, CCS, H2CS, and H2S, achieving a ~5 mJy sensitivity. Using the non-Local Thermodynamical Equilibrium radiative transfer code RADEX and the forward-modeling tool DiskFit, disk-averaged CS column densities and upper limits for the other species were derived. Results. Only CS was detected with a derived column density of ~2−6 × 1012 cm−2. We report a first tentative detection of SO2 in DM Tau. The upper limits range between ~1011 and 1014 cm−2 for the other S-bearing species. The best-fit chemical model matching these values requires a gas-phase C/O ratio of ≳1 at r ≳ 50−100 au. With chemical modeling we demonstrate that sulfur-bearing species could be robust tracers of the gas-phase C/O ratio, surface reaction rates, grain size and UV intensities. Conclusions. The lack of detections of a variety of sulfur-bearing molecules in DM Tau other than CS implies a dearth of reactive sulfur in the gas phase, either through efficient freeze-out or because most of the elemental sulfur is in other large species, as found in comets. The inferred high CS/SO and CS/SO2 ratios require a non-solar C/O gas-phase ratio of ≳1, consistent with the recent observations of hydrocarbon rings in DM Tau. The stronger depletion of oxygen-bearing S-species compared to CS is likely linked to the low observed abundances of gaseous water in DM Tau and points to a removal mechanism of oxygen from the gas.

Ammonia in C-rich stars

AbstractHIFI instrument onboard the Herschel satellite provided an unprecedented number of detections of rotational transitions of ammonia in circumstellar envelopes of evolved stars including massive red supergiants, Asymptotic Giant Branch (AGB), and post-AGB stars. The chemistry of ammonia formation in the circumstellar envelopes of evolved stars is poorly understood. The mechanisms proposed for its formation are processes behind the shock front, photochemistry in the inner part of the clumpy envelope, and formation on dust grains. We present results of the non-local thermodynamical equilibrium (non-LTE) radiative transfer modeling of ammonia transitions, mainly of the ground-state rotational one NH3 JK = 10 – 00 at 572.5 GHz, in selected AGB stars, aiming at the quantitative estimation of the NH3 abundance. The model of ammonia includes IR radiative pumping via v2 = 1 vibrational band at 10 μm.

Major impact from a minor merger

Context. Minor mergers are important processes contributing significantly to how galaxies evolve across the age of the Universe. Their impact on the growth of supermassive black holes and star formation is profound – about half of the star formation activity in the local Universe is the result of minor mergers. Aims. The detailed study of dense molecular gas in galaxies provides an important test of the validity of the relation between star formation rate and HCN luminosity on different galactic scales – from whole galaxies to giant molecular clouds in their molecular gas-rich centers. Methods. We use observations of HCN and HCO+ 1−0 with NOEMA and of CO3−2 with the SMA to study the properties of the dense molecular gas in the Medusa merger (NGC 4194) at 1′′ resolution. In particular, we compare the distribution of these dense gas tracers with CO2−1 high-resolution maps in the Medusa merger. To characterize gas properties, we calculate the brightness temperature ratios between the three tracers and use them in conjunction with a non-local thermodynamic equilibrium (non-LTE) radiative line transfer model. Results. The gas represented by HCN and HCO+ 1−0, and CO3−2 does not occupy the same structures as the less dense gas associated with the lower-J CO emission. Interestingly, the only emission from dense gas is detected in a 200 pc region within the “Eye of the Medusa”, an asymmetric 500 pc off-nuclear concentration of molecular gas. Surprisingly, no HCN or HCO+ is detected for the extended starburst of the Medusa merger. Additionally, there are only small amounts of HCN or HCO+ associated with the active galactic nucleus. The CO3−2/2−1 brightness temperature ratio inside “the Eye” is ~2.5 – the highest ratio found so far – implying optically thin CO emission. The CO2−1/HCN 1−0 (~9.8) and CO2−1/HCO+ 1−0 (~7.9) ratios show that the dense gas filling factor must be relatively high in the central region, consistent with the elevated CO3−1/2−1 ratio. Conclusions. The line ratios reveal an extreme, fragmented molecular cloud population inside the Eye with large bulk temperatures (T > 300 K) and high gas densities (n(H2) > 104 cm-3). This is very different from the cool, self-gravitating structures of giant molecular clouds normally found in the disks of galaxies. The Eye of the Medusa is found at an interface between a large-scale minor axis inflow and the central region of the Medusa. Hence, the extreme conditions inside the Eye may be the result of the radiative and mechanical feedback from a deeply embedded, young and massive super star cluster formed due to the gas pile-up at the intersection. Alternatively, shocks from the inflowing gas entering the central region of the Medusa may be strong enough to shock and fragment the gas. For both scenarios, however, it appears that the HCN and HCO+ dense gas tracers are not probing star formation, but instead a post-starburst and/or shocked ISM that is too hot and fragmented to form newstars. Thus, caution is advised in taking the detection of emission from dense gas tracers as evidence of ongoing or imminent star formation.

Circumnuclear Multi-phase Gas in the Circinus Galaxy. I. Non-LTE Calculations of CO Lines

Abstract In this study, we investigate the line emissions from cold molecular gas based on our previous “radiation-driven fountain model,” which reliably explains the spectral energy distribution of the nearest type 2 Seyfert galaxy, the Circinus galaxy. Using a snapshot of the best-fit radiation-hydrodynamic model for the central pc, in which non-equilibrium X-ray-dominated region chemistry is solved, we conduct post-processed non-local thermodynamic equilibrium radiation transfer simulations for the CO lines. We obtain a spectral line energy distribution with a peak around , and its distribution suggests that the lines are not thermalized. However, for a given line of sight, the optical depth distribution is highly non-uniform between and . The CO-to-H2 conversion factor ( ), which can be directly obtained from the results and is not a constant, depends strongly on the integrated intensity and differs from the fiducial value for local objects. exhibits a large dispersion of more than one order of magnitude, reflecting the non-uniform internal structure of a “torus.” In addition, we found that the physical conditions differ between grid cells on a scale of a few parsecs along the observed lines of sight; therefore, a specific observed line ratio does not necessarily represent a single physical state of the interstellar medium.

Efficient Radiative Transfer for Dynamically Evolving Stratified Atmospheres*

Abstract We present a fast multi-level and multi-atom non-local thermodynamic equilibrium radiative transfer method for dynamically evolving stratified atmospheres, such as the solar atmosphere. The preconditioning method of Rybicki & Hummer (RH92) is adopted. But, pressed for the need of speed and stability, a “second-order escape probability” scheme is implemented within the framework of the RH92 method, in which frequency- and angle-integrals are carried out analytically. While minimizing the computational work needed, this comes at the expense of numerical accuracy. The iteration scheme is local, the formal solutions for the intensities are the only non-local component. At present the methods have been coded for vertical transport, applicable to atmospheres that are highly stratified. The probabilistic method seems adequately fast, stable, and sufficiently accurate for exploring dynamical interactions between the evolving MHD atmosphere and radiation using current computer hardware. Current 2D and 3D dynamics codes do not include this interaction as consistently as the current method does. The solutions generated may ultimately serve as initial conditions for dynamical calculations including full 3D radiative transfer.

Measurement and simulation of the temperature evolution of a short pulse laser heated buried layer target

Short pulse laser heated buried layers experiments have been performed on the Orion laser facility to study the time-resolved emission characteristics of plasmas with mass densities ≥ 1 g/cc and electron temperatures > 500 eV. Our streak camera measurements focused on the K-shell emission lines of He-like and H-like aluminum from a buried aluminum layer. The data were analyzed by comparison to synthetic spectra generated with the non-local thermodynamic equilibrium (NLTE) radiation transfer code Cretin, which yielded maximum temperatures of nearly 800 eV at near solid density. The time precise history of the temperature evolution was reproduced with a 1-D radiation hydrodynamic code; however, the known effect of lateral transport of energy out of the focal spot made exacting agree with theory difficult. Thus, we have observed densities of ≥ 1 g/cc and temperatures of > 500 eV using the 1-D analysis, which supports the idea that the aluminum plasma is locally hotter than inferred from our spatially integrating measurements and that modeling requires the inclusion of 2-dimensional effects.

The solar silicon abundance based on 3D non-LTE calculations

We present three-dimensional (3D) non-local thermodynamic equilibrium (non-LTE) radiative transfer calculations for silicon in the solar photosphere, using an extensive model atom that includes recent, realistic neutral hydrogen collisional cross-sections. We find that photon losses in the SiI lines give rise to slightly negative non-LTE abundance corrections of the order -0.01 dex. We infer a 3D non-LTE based solar silicon abundance of 7.51 dex. With silicon commonly chosen to be the anchor between the photospheric and meteoritic abundances, we find that the meteoritic abundance scale remains unchanged compared with the Asplund et al. (2009) and Lodders et al. (2009) results.

Molecular gas kinematics and line diagnostics in early-type galaxies: NGC 4710 and NGC 5866

We present interferometric observations of CO lines (12CO(1-0, 2-1) and 13CO(1-0, 2-1)) and dense gas tracers (HCN(1-0), HCO+(1-0), HNC(1-0) and HNCO(4-3)) in two nearby edge-on barred lenticular galaxies, NGC 4710 and NGC 5866, with most of the gas concentrated in a nuclear disc and an inner ring in each galaxy. We probe the physical conditions of a two-component molecular interstellar medium in each galaxy and each kinematic component by using molecular line ratio diagnostics in three complementary ways. First, we measure the ratios of the position-velocity diagrams of different lines, second we measure the ratios of each kinematic component's integrated line intensities as a function of projected position, and third we model these line ratios using a non-local thermodynamic equilibrium radiative transfer code. Overall, the nuclear discs appear to have a tenuous molecular gas component that is hotter, optically thinner and with a larger dense gas fraction than that in the inner rings, suggesting more dense clumps immersed in a hotter more diffuse molecular medium. This is consistent with evidence that the physical conditions in the nuclear discs are similar to those in photo-dissociation regions. A similar picture emerges when comparing the observed molecular line ratios with those of other galaxy types. The physical conditions of the molecular gas in the nuclear discs of NGC4710 and NGC5866 thus appear intermediate between those of spiral galaxies and starbursts, while the star formation in their inner rings is even milder.

A new nonlocal thermodynamical equilibrium radiative transfer method for cool stars

Context: The solution of the nonlocal thermodynamical equilibrium (non-LTE) radiative transfer equation usually relies on stationary iterative methods, which may falsely converge in some cases. Furthermore, these methods are often unable to handle large-scale systems, such as molecular spectra emerging from, for example, cool stellar atmospheres. Aims: Our objective is to develop a new method, which aims to circumvent these problems, using nonstationary numerical techniques and taking advantage of parallel computers. Methods: The technique we develop may be seen as a generalization of the coupled escape probability method. It solves the statistical equilibrium equations in all layers of a discretized model simultaneously. The numerical scheme adopted is based on the generalized minimum residual method. Result:. The code has already been applied to the special case of the water spectrum in a red supergiant stellar atmosphere. This demonstrates the fast convergence of this method, and opens the way to a wide variety of astrophysical problems.

Scattering line polarization in rotating, optically thick disks

To interpret observations of astrophysical disks it is essential to understand the formation process of the emitted light. If the disk is optically thick, scattering dominated and permeated by a Keplerian velocity field, Non-Local Thermodynamic Equilibrium radiative transfer modeling must be done to compute the emergent spectrum from a given disk model. We investigate Non-local thermodynamic equilibrium polarized line formation in different simple disk models and aim to demonstrate the importance of both radiative transfer effects and scattering as well as the effects of velocity fields. We self-consistently solve the coupled equations of radiative transfer and statistical equilibrium for a two level atom model by means of Jacobi iteration. We compute scattering polarization, that is Q/I and U/I line profiles. The degree of scattering polarization is significantly influenced by the inclination of the disk with respect to observer, but also by the optical thickness of the disk and the presence of rotation. Stokes U shows double-lobed profiles with amplitude which increases with the disk rotation. Our results suggest that the line profiles, especially the polarized ones, emerging from gaseous disks differ significantly from the profiles predicted by simple approximations. The profiles are diverse in shape, but typically symmetric in Stokes Q and antisymmetric in Stokes U. A clear indicator of disk rotation is the presence of Stokes U, which might prove to be a useful diagnostic tool. We also demonstrate that, for moderate rotational velocities, an approximate treatment can be used, where non-local thermodynamic equilibrium radiative transfer is done in the velocity field-free approximation and Doppler shift is applied in the process of spatial integration over the whole emitting surface.

New insights on Be shell stars from modelling their Hα emission profiles

AbstractBe shell stars are believed to be ordinary Be stars seen edge-on, which makes them particularly desirable objects for study since the uncertainty in the inclination of the rotation axis is largely eliminated. We have recently modelled high resolution Hα spectroscopic observations for eight Be shell stars, using the non-local thermodynamical equilibrium radiative transfer codeBedisk(Sigut & Jones 2007) and the new spectral synthesis packageBeray(Sigut 2011). Generally, we confirm that these systems are oriented at high inclination angles, although we find that they are not necessarily as close to edge-on as initially expected.

Herschelspectral mapping of the Helix nebula (NGC 7293)

The Helix Nebula (NGC 7293) is the closest planetary nebulae. Therefore, it is an ideal template for photochemical studies at small spatial scales in planetary nebulae. We aim to study the spatial distribution of the atomic and the molecular gas, and the structure of the photodissociation region along the western rims of the Helix Nebula as seen in the submillimeter range with Herschel. We use 5 SPIRE FTS pointing observations to make atomic and molecular spectral maps. We analyze the molecular gas by modeling the CO rotational lines using a non-local thermodynamic equilibrium (non-LTE) radiative transfer model. For the first time, we have detected extended OH+ emission in a planetary nebula. The spectra towards the Helix Nebula also show CO emission lines (from J= 4 to 8), [NII] at 1461 GHz from ionized gas, and [CI] (2-1), which together with the OH+ lines, trace extended CO photodissociation regions along the rims. The estimated OH+ column density is (1-10)x1e12 cm-2. The CH+ (1-0) line was not detected at the sensitivity of our observations. Non-LTE models of the CO excitation were used to constrain the average gas density (n(H2)=(1-5)x1e5 cm-3) and the gas temperature (Tk= 20-40 K). The SPIRE spectral-maps suggest that CO arises from dense and shielded clumps in the western rims of the Helix Nebula whereas OH+ and [CI] lines trace the diffuse gas and the UV and X-ray illuminated clumps surface where molecules reform after CO photodissociation. [NII] traces a more diffuse ionized gas component in the interclump medium.

A 3D radiative transfer framework

Multi-level non-local thermodynamic equilibrium (NLTE) radiation transfer calculations have become standard throughout the stellar atmospheres community and are applied to all types of stars as well as dynamical systems such as novae and supernovae. Even today spherically symmetric 1D calculations with full physics are computationally intensive. We show that full NLTE calculations can be done with fully 3 dimensional (3D) radiative transfer. With modern computational techniques and current massive parallel computational resources, full detailed solution of the multi-level NLTE problem coupled to the solution of the radiative transfer scattering problem can be solved without sacrificing the micro physics description. We extend the use of a rate operator developed to solve the coupled NLTE problem in spherically symmetric 1D systems. In order to spread memory among processors we have implemented the NLTE/3D module with a hierarchical domain decomposition method that distributes the NLTE levels, radiative rates, and rate operator data over a group of processes so that each process only holds the data for a fraction of the voxels. Each process in a group holds all the relevant data to participate in the solution of the 3DRT problem so that the 3DRT solution is parallelized within a domain decomposition group. We solve a spherically symmetric system in 3D spherical coordinates in order to directly compare our well-tested 1D code to the 3D case. We compare three levels of tests: a) a simple H+He test calculation, b) H+He+CNO+Mg, c) H+He+Fe. The last test is computationally large and shows that realistic astrophysical problems are solvable now, but they do require significant computational resources. With presently available computational resources it is possible to solve the full 3D multi-level problem with the same detailed micro-physics as included in 1D modeling.

Spatially extended OH+emission from the Orion Bar and Ridge

We report the first detection of a Galactic source of OH+ line emission: the Orion Bar, a bright nearby photon-dominated region. Line emission is detected over ~1' (0.12 pc), tracing the Bar itself as well as the Southern tip of the Orion Ridge. The line width of ~4 km/s suggests an origin of the OH+ emission close to the PDR surface, at a depth of A_V ~0.3-0.5 into the cloud where most hydrogen is in atomic form. Steady-state collisional and radiative excitation models require unrealistically high OH+ column densities to match the observed line intensity, indicating that the formation of OH+ in the Bar is rapid enough to influence its excitation. Our best-fit OH+ column density of ~1x10^14 cm^-2 is similar to that in previous absorption line studies, while our limits on the ratios of OH+/H2O+ (>~40) and OH+/H3O+ (>~15) are higher than seen before. The column density of OH+ is consistent with estimates from a thermo-chemical model for parameters applicable to the Orion Bar, given the current uncertainties in the local gas pressure and the spectral shape of the ionizing radiation field. The unusually high OH+/H2O+ and OH+/H3O+ ratios are probably due to the high UV radiation field and electron density in this object. In the Bar, photodissociation and electron recombination are more effective destroyers of OH+ than the reaction with H2, which limits the production of H2O+. The appearance of the OH+ lines in emission is the result of the high density of electrons and H atoms in the Orion Bar, since for these species, inelastic collisions with OH+ are faster than reactive ones. In addition, chemical pumping, far-infrared pumping by local dust, and near-UV pumping by Trapezium starlight contribute to the OH+ excitation. Similar conditions may apply to extragalactic nuclei where OH+ lines are seen in emission.

<3D> NLTE line formation in the atmospheres of red supergiants

Red supergiants with their enormous brightness at J-band are ideal probes of cosmic chemical composition. It is therefore crucial to have realistic models of radiative transfer in their atmospheres, which will permit determination of abundances accurate to 0.15 dex, the precision attainable with future telescope facilities in galaxies as distant as tens of Mpc. Here, we study the effects of non-local thermodynamic equilibrium (NLTE) on the formation of iron, titanium, and silicon lines, which dominate J-band spectra of red supergiants. It is shown that the NLTE radiative transfer models enable accurate derivation of metallicity and effective temperature in the J-band. We also discuss consequences for RSG spectrum synthesis in different spectral windows, including the heavily TiO-blanketed optical region, and atmospheric structure. We then touch upon challenges of NLTE integration with new generation of 3D hydrodynamical RSG models and present the first calculations of NLTE spectra with the mean 3D model of Betelgeuse.

Observation and modelling of main-sequence star chromospheres - XIX. FIES and FEROS observations of dM1 stars

We present 187 high-resolution spectra for 62 different M1 dwarfs from observations obtained with the FIbre-fed Echelle Spectrograph (FIES) on the Nordic Optical Telescope (NOT) and from observations with the Fibre-fed Extended Range Echelle Spectrograph (FEROS) from the European Southern Observatory (ESO) data base. We also compiled other measurements available in the literature. We observed two stars, Gl 745A and Gl 745B, with no Ca ii line core emission and Hα line equivalent widths (EWs) of only 0.171 and 0.188 Å, respectively. We also observed another very low activity M1 dwarf, Gl 63, with an Hα line EW of only 0.199 Å. These are the lowest activity M dwarfs ever observed and are of particular interest for the non-local thermodynamic equilibrium radiative transfer modelling of M1 dwarfs. Thanks to the high signal-to-noise ratio of most of our spectra, we were able to measure the Ca ii H&K full width at half-maximum (FWHM) for most of our stars. We find good correlations between the FWHM values and the mean Ca ii line EW for dM1 stars. Then the FWHM seems to saturate for dM1e stars. Our previous models of M1 dwarfs can reproduce the FWHM for dM1e stars and the most active dM1 stars, but fail to reproduce the observations of lower activity M1 dwarfs. We believe this is due to an effect of metallicity. We also investigate the dependence of the Hα line FWHM as a function of its EW. We find that the models globally agree with the observations including subwarfs, but tend to produce too narrow profiles for dM1e stars. We re-investigate the correlation between the Ca ii line mean EW and the absolute magnitude. With our new data that notably include several M1 subdwarfs, we find a slightly different and better correlation with a slope of −0.779 instead of −0.909. We also re-investigate the variations of the Hα line EW as a function of radius and find that the EW increases continuously with increasing radius. This confirms our previous finding that the level of magnetic activity in M1 dwarfs increases with the radius. For the first time, we investigate the Wilson–Bappu correlation for a given spectral type. We find a rather linear correlation for stars of absolute magnitude greater than 9.6, but below this value the FWHM seems to saturate. In fact, we show that these Wilson–Bappu type correlations are activity–FWHM correlations and are due to the diminishing column mass of the transition region with decreasing activity level.

KINEMATICS OF THE CO GAS IN THE INNER REGIONS OF THE TW Hya DISK

We present a detailed analysis of the spatially and spectrally resolved 12CO J=2-1 and J=3-2 emission lines from the TW Hya circumstellar disk, based on science verification data from the Atacama Large Millimeter/Submillimeter Array (ALMA). These lines exhibit substantial emission in their high-velocity wings (with projected velocities out to 2.1 km/s, corresponding to intrinsic orbital velocities >20 km/s) that trace molecular gas as close as 2 AU from the central star. However, we are not able to reproduce the intensity of these wings and the general spatio-kinematic pattern of the lines with simple models for the disk structure and kinematics. Using three-dimensional non-local thermodynamic equilibrium molecular excitation and radiative transfer calculations, we construct some alternative models that successfully account for these features by modifying either (1) the temperature structure of the inner disk (inside the dust-depleted disk cavity; r < 4 AU); (2) the intrinsic (Keplerian) disk velocity field; or (3) the distribution of disk inclination angles (a warp). The latter approach is particularly compelling because a representative warped disk model qualitatively reproduces the observed azimuthal modulation of optical light scattered off the disk surface. In any model scenario, the ALMA data clearly require a substantial molecular gas reservoir located inside the region where dust optical depths are known to be substantially diminished in the TW Hya disk, in agreement with previous studies based on infrared spectroscopy. The results from these updated model prescriptions are discussed in terms of their potential physical origins, which might include dynamical perturbations from a low-mass companion with an orbital separation of a few AU.

Probing the properties of Be star discs with spectroastrometry and NLTE radiative transfer modelling: β CMi

ABSTRACT While the presence of discs around classical Be stars is well established, their origin is still uncertain. To understand what processes result in the creation of these discs and how angular momentum is transported within them, their physical properties must be constrained. This requires comparing high spatial and spectral resolution data with detailed radiative transfer modelling. We present a high spectral resolution, R∼ 80 000, sub-milliarcsecond precision, spectroastrometric study of the circumstellar disc around the Be star β CMi. The data are confronted with 3D, non-local thermodynamic equilibrium radiative transfer calculations to directly constrain the properties of the disc. Furthermore, we compare the data to disc models featuring two velocity laws: Keplerian, the prediction of the viscous disc model, and angular momentum conserving rotation. It is shown that the observations of β CMi can only be reproduced using Keplerian rotation. The agreement between the model and the observed spectral energy distribution, polarization and spectroastrometric signature of β CMi confirms that the discs around Be stars are well modelled as viscous decretion discs.

Circumstellar molecular composition of the oxygen-rich AGB star IK Tauri

<i>Context. <i/>The interstellar medium is enriched primarily by matter ejected from evolved low and intermediate mass stars. The outflow from these stars creates a circumstellar envelope in which a rich gas-phase chemistry takes place. Complex shock-induced non-equilibrium chemistry takes place in the inner wind envelope, dust-gas reactions and ion-molecule reactions alter the abundances in the intermediate wind zone, and the penetration of cosmic rays and ultraviolet photons dissociates the molecules in the outer wind region.<i>Aims. <i/>Little observational information exists on the circumstellar molecular abundance stratifications of many molecules. Furthermore, our knowledge of oxygen-rich envelopes is not as profound as for the carbon-rich counterparts. The aim of this paper is therefore to study the circumstellar chemical abundance pattern of 11 molecules and isotopologs (<sup>12<sup/>CO, <sup>13<sup/>CO, SiS, <sup>28<sup/>SiO, <sup>29<sup/>SiO, <sup>30<sup/>SiO, HCN, CN, CS, SO, SO<sub>2<sub/>) in the oxygen-rich evolved star IK Tau.<i>Methods. <i/>We have performed an in-depth analysis of a large number of molecular emission lines excited in the circumstellar envelope around IK Tau. The analysis is done based on a non-local thermodynamic equilibrium (non-LTE) radiative transfer analysis, which calculates the temperature and velocity structure in a self-consistent way. The chemical abundance pattern is coupled to theoretical outer wind model predictions including photodestruction and cosmic ray ionization. Not only the integrated line intensities, but also the line shapes are used as diagnostic tool to study the envelope structure.<i>Results. <i/>The deduced wind acceleration is much slower than predicted from classical theories. SiO and SiS are depleted in the envelope, possibly due to the adsorption onto dust grains. For HCN and CS a clear difference with respect to inner wind non-equilibrium predictions is found, either indicating uncertainties in the inner wind theoretical modeling or the possibility that HCN and CS (or the radical CN) participate in the dust formation. The low signal-to-noise profiles of SO and CN prohibit an accurate abundance determination; the modeling of high-excitation SO<sub>2<sub/> lines is cumbersome, possibly related to line misidentifications or problems with the collisional rates. The SiO isotopic ratios (<sup>29<sup/>SiO/<sup>28<sup/>SiO and <sup>30<sup/>SiO/<sup>28<sup/>SiO) point toward an enhancement in <sup>28<sup/>SiO compared to results of classical stellar evolution codes. Predictions for H<sub>2<sub/>O emission lines in the spectral range of the Herschel/HIFI mission are performed.