scholarly journals Circumstellar H2O in M-type AGB stars

2008 ◽  
Vol 4 (S251) ◽  
pp. 163-164
Author(s):  
Matthias Maercker ◽  
Fredrik L. Schöier ◽  
Hans Olofsson
Keyword(s):  
Radiative Transfer ◽  
Radiation Field ◽  
Central Star ◽  
Thermal Excitation ◽  
Circumstellar Envelope ◽  
Agb Stars ◽  
Dust Grains ◽  
Transfer Modelling

AbstractSurprisingly high amounts of H2O have recently been reported in the circumstellar envelope around the M-type AGB star W Hya. However, substantial uncertainties remain, as the required radiative transfer modelling is difficult due to high optical depths, sub-thermal excitation and the sensitivity to the combined radiation field from the central star and dust grains.

scholarly journals AGB Stars and Their Circumstellar Envelopes. I. the VULCAN Code

Universe ◽  
2021 ◽  
Vol 7 (4) ◽  
pp. 80
Author(s):  
Sergio Cristallo ◽  
Luciano Piersanti ◽  
David Gobrecht ◽  
Lucio Crivellari ◽  
Ambra Nanni
Keyword(s):  
Radiative Transfer ◽  
Mass Loss ◽  
Circumstellar Envelope ◽  
Agb Stars ◽  
Dust Grains ◽  
High Complexity ◽  
Circumstellar Envelopes ◽  
Perfect Gas ◽  
Physical Processes ◽  
Transfer Effects

The interplay between AGB interiors and their outermost layers, where molecules and dust form, is a problem of high complexity. As a consequence, physical processes like mass loss, which depend on the chemistry of the circumstellar envelope, are often oversimplified. The best candidates to drive mass-loss in AGB stars are dust grains, which trap the outgoing radiation and drag the surrounding gas. Grains build up, however, is far from being completely understood. Our aim is to model both the physics and the chemistry of the cool expanding layers around AGB stars in order to characterize the on-going chemistry, from atoms to dust grains. This has been our rationale to develop ab initio VULCAN, a FORTRAN hydro code able to follow the propagation of shocks in the circumstellar envelopes of AGB stars. The version presented in this paper adopts a perfect gas law and a very simplified treatment of the radiative transfer effects and dust nucleation. In this paper, we present preliminary results obtained with our code.

scholarly journals Monte-Carlo methods for NLTE spectral synthesis of supernovae

2018 ◽  
Vol 620 ◽  
pp. A156 ◽  
Author(s):  
M. Ergon ◽  
C. Fransson ◽  
A. Jerkstrand ◽  
C. Kozma ◽  
M. Kromer ◽  
...  
Keyword(s):  
Monte Carlo ◽  
Radiative Transfer ◽  
Radiation Field ◽  
Spectral Synthesis ◽  
Time Dependent ◽  
Thermal Excitation ◽  
Chain Model ◽  
Full Time ◽  
Emission Frequency ◽  
Radiative Transfer Problem

We present JEKYLL, a new code for modelling of supernova (SN) spectra and lightcurves based on Monte-Carlo (MC) techniques for the radiative transfer. The code assumes spherical symmetry, homologous expansion and steady state for the matter, but is otherwise capable of solving the time-dependent radiative transfer problem in non-local-thermodynamic-equilibrium (NLTE). The method used was introduced in a series of papers by Lucy, but the full time-dependent NLTE capabilities of it have never been tested. Here, we have extended the method to include non-thermal excitation and ionization as well as charge-transfer and two-photon processes. Based on earlier work, the non-thermal rates are calculated by solving the Spencer-Fano equation. Using a method previously developed for the SUMO code, macroscopic mixing of the material is taken into account in a statistical sense. To save computational power a diffusion solver is used in the inner region, where the radiation field may be assumed to be thermalized. In addition, a statistical Markov-chain model is used to sample the emission frequency more efficiently, and we introduce a method to control the sampling of the radiation field, which is used to reduce the noise in the radiation field estimators. Except for a description of JEKYLL, we provide comparisons with the ARTIS, SUMO and CMFGEN codes, which show good agreement in the calculated spectra as well as the state of the gas. In particular, the comparison with CMFGEN, which is similar in terms of physics but uses a different technique, shows that the Lucy method does indeed converge in the time-dependent NLTE case. Finally, as an example of the time-dependent NLTE capabilities of JEKYLL, we present a model of a Type IIb SN, taken from a set of models presented and discussed in detail in an accompanying paper. Based on this model we investigate the effects of NLTE, in particular those arising from non-thermal excitation and ionization, and find strong effects even on the bolometric lightcurve. This highlights the need for full NLTE calculations when simulating the spectra and lightcurves of SNe.

scholarly journals Probing chemical processes in AGB stars

2008 ◽  
Vol 4 (S251) ◽  
pp. 201-206 ◽  
Author(s):  
Fredrik L. Schöier ◽  
Hans Olofsson
Keyword(s):  
Radiative Transfer ◽  
Mass Loss ◽  
Line Emission ◽  
Chemical Processes ◽  
Agb Stars ◽  
Molecular Line ◽  
Large Sample ◽  
Preliminary Results ◽  
Transfer Modelling ◽  
Non Equilibrium

AbstractWe are conducting multi-transition observations of circumstellar line emission from common molecules such as HCN, SiO, CS, SiS and CN for a large sample of AGB stars with varying photospheric C/O-ratios and mass-loss charachteristics. Our recently published results for SiO and SiS clearly show that major constraints on the relative roles of non-equilibrium chemistry, dust condensation, and photodissociation can be obtained from the study of circumstellar molecular line emission. Presented here are also preliminary results based on detailed radiative transfer modelling of HCN line emission.

scholarly journals From molecules to dust grains: The role of alumina cluster seeds

2019 ◽  
Vol 15 (S350) ◽  
pp. 245-248
Author(s):  
David Gobrecht ◽  
John M.C. Plane ◽  
Stefan T. Bromley ◽  
Leen Decin ◽  
Sergio Cristallo
Keyword(s):  
Bulk Material ◽  
Spectral Feature ◽  
Chemical Kinetic ◽  
Asymptotic Giant Branch ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Circumstellar Envelope ◽  
Agb Stars ◽  
Dust Grains ◽  
Alumina Cluster

AbstractAsymptotic Giant Branch (AGB) stars contribute a major part to the global dust budget in galaxies. Owing to their refractory nature alumina (stoichiometric formula AlO) is a promising candidate to be the first condensate emerging in the atmospheres of oxygen-rich AGB stars. Strong evidence for that is supplied by the presence of alumina in pristine meteorites and a broad spectral feature observed around ∼ 13 μm. The emergence of a specific condensate depends on the thermal stability of the solid, the gas density and its composition. The evaluation of the condensates is based on macroscopic bulk properties. The growth and size distribution of dust grains is commonly described by Classical Nucleation Theory (CNT). We question the applicability of CNT in an expanding circumstellar envelope as CNT presumes thermodynamic equilibrium and requires, in practise, seed nuclei on which material can condense. However, nano-sized molecular clusters differ significantly from bulk analogues. Quantum effects of the clusters lead to non-crystalline structures, whose characteristics (energy, geometry) differ substantially, compared to the bulk material. Hence, a kinetic quantum-chemical treatment involving various transition states describes dust nucleation most accurately. However, such a treatment is prohibitive for systems with more than 10 atoms. We discuss the viability of chemical-kinetic routes towards the formation of the monomer (Al2O3) and the dimer (Al4O6) of alumina.

scholarly journals Maser emission from the CO envelope of the asymptotic giant branch star W Hydrae

2021 ◽  
Vol 654 ◽  
pp. A18
Author(s):  
W. H. T. Vlemmings ◽  
T. Khouri ◽  
D. Tafoya
Keyword(s):  
Mass Loss ◽  
Radiation Field ◽  
Asymptotic Giant Branch ◽  
Small Scale ◽  
High Angular Resolution ◽  
Circumstellar Envelope ◽  
Agb Stars ◽  
Maser Emission ◽  
Co Emission ◽  
Loss Rates

Context. Observation of CO emission around asymptotic giant branch (AGB) stars is the primary method to determine gas mass-loss rates. While radiative transfer models have shown that molecular levels of CO can become mildly inverted, causing maser emission, CO maser emission has yet to be confirmed observationally. Aims. High-resolution observations of the CO emission around AGB stars now have the brightness temperature sensitivity to detect possible weak CO maser emission. Methods. We used high angular resolution observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the small-scale structure of CO J = 3−2 emission around the oxygen-rich AGB star W Hya. Results. We find CO maser emission amplifying the stellar continuum with an optical depth τ ≈−0.55. The maser predominantly amplifies the limb of the star because CO J = 3−2 absorption from the extended stellar atmosphere is strongest towards the centre of the star. Conclusions. The CO maser velocity corresponds to a previously observed variable component of high-frequency H2O masers and with the OH maser that was identified as the amplified stellar image. This implies that the maser originates beyond the acceleration region and constrains the velocity profile since we find the population inversion primarily in the inner circumstellar envelope. We find that inversion can be explained by the radiation field at 4.6 μm and that the existence of CO maser emission is consistent with the estimated mass-loss rates for W Hya. However, the pumping mechanism requires a complex interplay between absorption and emission lines in the extended atmosphere. Excess from dust in the circumstellar envelope of W Hya is not sufficient to contribute significantly to the required radiation field at 4.6 μm. The interplay between molecular lines that cause the pumping can be constrained by future multi-level CO observations.

scholarly journals Diffraction-limited 76 mas speckle-masking interferometry of the carbon star IRC+10 216 and related AGB objects with the SAO 6 m telescope

1999 ◽  
Vol 191 ◽  
pp. 273-278
Author(s):  
G. Weigelt ◽  
T. Blöcker ◽  
K.-H. Hofmann ◽  
R. Osterbart ◽  
Y.Y. Balega ◽  
...  
Keyword(s):  
High Resolution ◽  
Radiative Transfer ◽  
Carbon Star ◽  
Central Star ◽  
Dust Shell ◽  
Bipolar Structure ◽  
The Core ◽  
Transfer Modelling ◽  
Projected Distance ◽  
K Band

We present high-resolution J–, H–, and K–band observations of the carbon star IRC+10 216. The images were reconstructed from 6 m telescope speckle interferograms using the speckle masking bispectrum method. The H image has the unprecedented resolution of 70 mas. The H and K images consist of at least five dominant components within a 0.21 arcsec radius and a fainter asymmetric nebula. The J-, H-, and K—band images seem to have an X-shaped bipolar structure. A comparison of our images from 1995, 1996, 1997, and 1998 shows that the separation of the two brightest components A and B increased from ∼ 193 mas in 1995 to ∼ 246 mas in 1998.The cometary shapes of component A in the H and J images and the 0.79 μm and 1.06 μm HST images suggest that the core of A is not the central star, but the southern (nearer) lobe of the bipolar structure. The position of the central star is probably at or near the position of component B, where the H—K color has its largest value of H—K = 4.2.If the star is located at or near B, then the components A, C, and D are located close to the inner boundary of the dust shell at separations of ∼200 mas ∼30 AU (projected distance) ∼6 stellar radii for a distance of ∼ 150 pc, in agreement with our 2-dimensional radiative transfer modelling.In addition to IRC+ 10 216 we studied the stellar disks and the dust shells of several related objects. Angular resolutions of 24 mas at 700 nm or 57 mas 1.6 μm were achieved.

scholarly journals The spectral evolution of post-AGB stars

1997 ◽  
Vol 180 ◽  
pp. 363-363
Author(s):  
R. D. Oudmaijer ◽  
P.A.M. van Hoof ◽  
L.B.F.M. Waters
Keyword(s):  
Radiative Transfer ◽  
Mass Loss ◽  
Evolutionary Rate ◽  
Central Star ◽  
Parameter Study ◽  
Agb Stars ◽  
Spectral Evolution ◽  
History Of ◽  
Photo Ionization

We have performed a parameter study of the spectral evolution of a typical post-AGB star with particular emphasis on the evolution of the IRAS colours. The models are based on the latest evolutionary tracks by Blöcker (1995, A&A 297, 727 and A&A 299, 755), which are used to define the evolutionary rate and the mass loss history of the central star. The resulting model for the post-AGB star is then used to calculate the spectral evolution with the photo-ionization code CLOUDY (Ferland 1993, Univ. of Kentucky, Int. Report), which includes dust in the radiative transfer.

scholarly journals Polarimetric and radiative transfer modelling of HD 172555

2020 ◽  
Vol 499 (4) ◽  
pp. 5915-5931
Author(s):  
Jonathan P Marshall ◽  
Daniel V Cotton ◽  
Peter Scicluna ◽  
Jeremy Bailey ◽  
Lucyna Kedziora-Chudczer ◽  
...  
Keyword(s):  
Radiative Transfer ◽  
Near Infrared ◽  
Scattered Light ◽  
Position Angle ◽  
Polarization Measurement ◽  
Continuum Emission ◽  
Dust Grains ◽  
Spatially Resolved ◽  
Optical Aperture ◽  
Transfer Modelling

ABSTRACT The debris disc around HD 172555 was recently imaged in near-infrared polarized scattered light by the Very Large Telescope’s Spectro-Polarimetric High-contrast Exoplanet REsearch instrument. Here we present optical aperture polarization measurements of HD 172555 by the HIgh Precision Polarimetric Instrument (HIPPI), and its successor HIPPI-2 on the Anglo-Australian Telescope. We seek to refine constraints on the disc’s constituent dust grains by combining our polarimetric measurements with available infrared and millimetre photometry to model the scattered light and continuum emission from the disc. We model the disc using the 3D radiative transfer code hyperion, assuming the orientation and extent of the disc as obtained from the SPHERE observation. After correction for the interstellar medium contribution, our multiwavelength HIPPI/-2 observations (both magnitude and orientation) are consistent with the recent SPHERE polarization measurement with a fractional polarization p = 62.4  ±  5.2 ppm at 722.3 nm, and a position angle θ = 67°  ±  3°. The multiwavelength polarization can be adequately replicated by compact, spherical dust grains (i.e. from Mie theory) that are around 1.2 μm in size, assuming astronomical silicate composition, or 3.9 μm, assuming a composition derived from radiative transfer modelling of the disc. We were thus able to reproduce both the spatially resolved disc emission and polarization with a single grain composition model and size distribution.

scholarly journals Molecular Data Needs for Modelling AGB Stellar Winds and Other Molecular Environments

Galaxies ◽  
2018 ◽  
Vol 6 (3) ◽  
pp. 86
Author(s):  
Taïssa Danilovich ◽  
Leen Decin ◽  
Marie Van de Sande
Keyword(s):  
Radiative Transfer ◽  
Excited States ◽  
Molecular Data ◽  
Stellar Winds ◽  
Agb Stars ◽  
Vibrationally Excited ◽  
Highly Sensitive ◽  
Transfer Modelling ◽  
First Time ◽  
Modern Era

The modern era of highly sensitive telescopes is enabling the detection of more and more molecular species in various astronomical environments. Many of these are now being carefully examined for the first time. However, to move beyond detection to more detailed analysis such as radiative transfer modelling, certain molecular properties need to be properly measured and calculated. The importance of contributions from vibrationally excited states or collisional (de-)excitations can vary greatly, depending on the specific molecule and the environment being studied. Here, we discuss the present molecular data needs for detailed radiative transfer modelling of observations of molecular rotational transitions, primarily in the (sub-)millimetre and adjacent regimes, and with a focus on the stellar winds of AGB stars.

scholarly journals Interferometric observations of SiO thermal emission in the inner wind of M-type AGB stars IK Tauri and IRC+10011

2019 ◽  
Vol 624 ◽  
pp. A107 ◽  
Author(s):  
J. L. Verbena ◽  
V. Bujarrabal ◽  
J. Alcolea ◽  
M. Gómez-Garrido ◽  
A. Castro-Carrizo
Keyword(s):  
Mass Loss ◽  
Thermal Emission ◽  
Mass Loss Rate ◽  
Asymptotic Giant Branch ◽  
Dust Formation ◽  
Circumstellar Envelope ◽  
Agb Stars ◽  
Dust Grains ◽  
Gas Drive ◽  
Massive Outflow

Context. Asymptotic giant branch (AGB) stars go through a process of strong mass loss that involves pulsations of the atmosphere, which extends to a region in which the conditions are adequate for dust grains to form. Radiation pressure acts on these grains which, coupled to the gas, drive a massive outflow. The details of this process are not clear, including which molecules are involved in the condensation of dust grains. Aims. We seek to study the role of the SiO molecule in the process of dust formation and mass loss in M-type AGB stars. Methods. Using the IRAM NOEMA interferometer we observed the 28SiO and 29SiO J = 3−2, v = 0 emission from the inner circumstellar envelope of the evolved stars IK Tau and IRC+10011. We computed azimuthally averaged emission profiles to compare the observations to models using a molecular excitation and ray-tracing code for SiO thermal emission. Results. We observe circular symmetry in the emission distribution. We also find that the source diameter varies only marginally with radial velocity, which is not the expected behaviour for envelopes expanding at an almost constant velocity. The adopted density, velocity, and abundance laws, together with the mass-loss rate, which best fit the observations, give us information concerning the chemical behaviour of the SiO molecule and its role in the dust formation process. Conclusions. The results indicate that there is a strong coupling between the depletion of gas-phase SiO and gas acceleration in the inner envelope. This could be explained by the condensation of SiO into dust grains.

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