scholarly journals Inner dusty envelope of the AGB stars W Hydrae, SW Virginis, and R Crateris using SPHERE/ZIMPOL

2020 ◽  
Vol 635 ◽  
pp. A200 ◽  
Author(s):  
T. Khouri ◽  
W. H. T. Vlemmings ◽  
C. Paladini ◽  
C. Ginski ◽  
E. Lagadec ◽  
...  
Keyword(s):  
Mass Loss ◽  
Radial Profile ◽  
Stellar Atmospheres ◽  
Continuum Emission ◽  
Asymmetric Distribution ◽  
Evolutionary Stage ◽  
Agb Stars ◽  
Dust Grains ◽  
Dust Density ◽  
Polarised Light

Context. The asymptotic giant branch (AGB) marks the final evolutionary stage of stars with initial masses between ~0.8 and 8 M⊙. During this phase, stars undergo copious mass loss, which contributes significantly to the enrichment of the interstellar medium. The well-accepted mass-loss mechanism requires radiation pressure acting on dust grains that form in the density-enhanced and extended AGB stellar atmospheres. The details of the mass-loss process are not yet well understood, however. For oxygen-rich AGB stars, which are the focus of this study, the dust grains that drive the wind are expected to scatter visible light very efficiently because their sizes are relative large. Aims. We study the distribution of dust in the inner wind of oxygen-rich AGB stars to advance our understanding of the wind-driving process. Methods. We observed light scattered off dust grains that form around three oxygen-rich AGB stars (W Hya, SW Vir, and R Crt) with mass-loss rates between 10−7 and 10−6 M⊙ yr−1 using the extreme-adaptive-optics imager and polarimeter SPHERE/ZIMPOL with three filters centred at 0.65, 0.75 and 0.82 μm. We compared the observed morphologies and the spectral dependence of the scattered light between the three sources and determined the radial profile, per image octant, of the dust density distribution around the closest target, W Hya. Results. We find the distribution of dust to be asymmetric for the three targets. A biconical morphology is seen for R Crt, with a position angle that is very similar to those inferred from interferometric observations of maser emission and of mid-infrared continuum emission. The cause of the biconical outflow cannot be inferred from the ZIMPOL data, but we speculate that it might be the consequence of a circumstellar disc or of the action of strong magnetic fields. The dust grains polarise light more efficiently at 0.65 μm for R Crt and SW Vir and at 0.82 μm for W Hya. This indicates that at the time of the observations, the grains around SW Vir and R Crt had sizes <0.1 μm, while those around W Hya were larger, with sizes ≳0.1 μm. The asymmetric distribution of dust around R Crt makes the interpretation more uncertain for this star, however. We find that polarised light is produced already from within the visible photosphere of W Hya, which we reproduce using models with an inner dust shell that is optically thick to scattering. We fit radiative transfer models to the radial profile of the polarised light observed around W Hya and find a steep dust density profile, with steepness varying considerably with direction. We find the wind-acceleration region of W Hya to extend to at least ~7 R⋆. This is in agreement with theoretical predictions of wind acceleration up to ~12 R⋆, and highlights that ZIMPOL observations probe the crucial region around AGB stars where dust forms and is accelerated.

scholarly journals High-resolution observations of gas and dust around Mira using ALMA and SPHERE/ZIMPOL

2018 ◽  
Vol 620 ◽  
pp. A75 ◽  
Author(s):  
T. Khouri ◽  
W. H. T. Vlemmings ◽  
H. Olofsson ◽  
C. Ginski ◽  
E. De Beck ◽  
...  
Keyword(s):  
Mass Loss ◽  
Polarized Light ◽  
Aluminium Oxide ◽  
Significant Fraction ◽  
Agb Stars ◽  
Dust Grains ◽  
Density Region ◽  
Gas Density ◽  
Stellar Pulsation ◽  
Solar Composition

Context. The outflows of oxygen-rich asymptotic giant branch (AGB) stars are thought to be driven by radiation pressure due to the scattering of photons on relatively large grains, with sizes of tenths of microns. The details of the formation of dust in the extended atmospheres of these stars and, therefore, the mass-loss process, is still not well understood. Aims. We aim to constrain the distribution of the gas and the composition and properties of the dust grains that form in the inner circumstellar environment of the archetypal Mira variable o Cet. Methods. We obtained quasi-simultaneous observations using ALMA and SPHERE/ZIMPOL on the Very Large Telescope (VLT) to probe the distribution of gas and large dust grains, respectively. Results. The polarized light images show dust grains around Mira A, but also around the companion, Mira B, and a dust trail that connects the two sources. The ALMA observations show that dust around Mira A is contained in a high-gas-density region with a significant fraction of the grains that produce the polarized light located at the edge of this region. Hydrodynamical and wind-driving models show that dust grains form efficiently behind shock fronts caused by stellar pulsation or convective motions. The distance at which we observe the density decline (a few tens of au) is, however, significantly larger than expected for stellar-pulsation-induced shocks. Other possibilities for creating the high-gas-density region are a recent change in the mass-loss rate of Mira A or interactions with Mira B. We are not able to determine which of these scenarios is correct. We constrained the gas density, temperature, and velocity within a few stellar radii from the star by modelling the CO v = 1, J = 3−2 line. We find a mass (~3.8 ± 1.3) × 10−4 M⊙ to be contained between the stellar millimetre photosphere, R⋆338 GHz, and 4 R⋆338 GHz. Our best-fit models with lower masses also reproduce the 13CO v = 0, J = 3−2 line emission from this region well. We find TiO2 and AlO abundances corresponding to 4.5% and <0.1% of the total titanium and aluminium expected for a gas with solar composition. The low abundance of AlO allows for a scenario in which Al depletion into dust happens already very close to the star, as expected from thermal dust emission observations and theoretical calculations of Mira variables. The relatively large abundance of aluminium for a gas with solar composition allows us to constrain the presence of aluminium oxide grains based on the scattered light observations and on the gas densities we obtain. These models imply that aluminium oxide grains could account for a significant fraction of the total aluminium atoms in this region only if the grains have sizes ≲0.02 μm. This is an order of magnitude smaller than the maximum sizes predicted by dust-formation and wind-driving models. Conclusions. The study we present highlights the importance of coordinated observations using different instruments to advance our understanding of dust nucleation, dust growth, and wind driving in AGB stars.

scholarly journals Modelling the carbon AGB star R Sculptoris

2018 ◽  
Vol 614 ◽  
pp. A17 ◽  
Author(s):  
M. Brunner ◽  
M. Maercker ◽  
M. Mecina ◽  
T. Khouri ◽  
F. Kerschbaum
Keyword(s):  
Mass Loss ◽  
Spectral Energy Distribution ◽  
Expansion Velocity ◽  
Agb Stars ◽  
Dust Grains ◽  
Dust Shell ◽  
Shell Mass ◽  
Dust Mass ◽  
Dust Grain ◽  
Grain Properties

Context. On the asymptotic giant branch (AGB), Sun-like stars lose a large portion of their mass in an intensive wind and enrich the surrounding interstellar medium with nuclear processed stellar material in the form of molecular gas and dust. For a number of carbon-rich AGB stars, thin detached shells of gas and dust have been observed. These shells are formed during brief periods of increased mass loss and expansion velocity during a thermal pulse, and open up the possibility to study the mass-loss history of thermally pulsing AGB stars. Aims. We study the properties of dust grains in the detached shell around the carbon AGB star R Scl and aim to quantify the influence of the dust grain properties on the shape of the spectral energy distribution (SED) and the derived dust shell mass. Methods. We modelled the SED of the circumstellar dust emission and compared the models to observations, including new observations of Herschel/PACS and SPIRE (infrared) and APEX/LABOCA (sub-millimeter). We derived present-day mass-loss rates and detached shell masses for a variation of dust grain properties (opacities, chemical composition, grain size, and grain geometry) to quantify the influence of changing dust properties to the derived shell mass. Results. The best-fitting mass-loss parameters are a present-day dust mass-loss rate of 2 × 10−10 M⊙ yr−1 and a detached shell dust mass of (2.9 ± 0.3) × 10−5 M⊙. Compared to similar studies, the uncertainty on the dust mass is reduced by a factor of 4. We find that the size of the grains dominates the shape of the SED, while the estimated dust shell mass is most strongly affected by the geometry of the dust grains. Additionally, we find a significant sub-millimeter excess that cannot be reproduced by any of the models, but is most likely not of thermal origin.

Thermodynamics of dust condensation around the dimming Betelgeuse

2020 ◽  
Vol 496 (1) ◽  
pp. L122-L126
Author(s):  
Anuj Gupta ◽  
Sandeep Sahijpal
Keyword(s):  
Chemical Composition ◽  
Solar System ◽  
Mass Loss ◽  
Evolutionary Stage ◽  
Stellar Winds ◽  
Dust Grains ◽  
Thermodynamical Equilibrium ◽  
The Past ◽  
Equilibrium Calculations ◽  
Dust Condensation

ABSTRACT Betelgeuse is one of the brightest red supergiant (RSG) stars because of its proximity to the Solar system. This makes it important when deducing the features and evolutionary phases of RSG stars. Betelgeuse has always been a well-observed target but especially during the past year, because of the reduction in its brightness. It has been speculated that the star is in its last evolutionary stage(s), and that it is soon going to explode. However, in recent work, it has been proposed that the episodic mass loss and dust condensation around the star are major reasons for the reduction in its brightness. In this work, we have performed detailed thermodynamical equilibrium and non-equilibrium calculations of the condensation of dust grains around the cooling envelope of Betelgeuse. Based on the deduced chemical composition, we have ventured to determine the nature of dust that could condense in the stellar winds. The dust grains are essentially found to be oxides of Al, Ca and Ti, and silicates of Al, Ca, Mg and Fe-metal. Further, we have determined the normalized masses of the dust grains of various compositions that could be present around the star and could be causing the reduction in its brightness.

scholarly journals Study of CS, SiO, and SiS abundances in carbon star envelopes: assessing their role as gas-phase precursors of dust

2019 ◽  
Vol 628 ◽  
pp. A62 ◽  
Author(s):  
S. Massalkhi ◽  
M. Agúndez ◽  
J. Cernicharo
Keyword(s):  
Mass Loss ◽  
Gas Phase ◽  
Mass Loss Rate ◽  
Carbon Star ◽  
High Mass ◽  
Agb Stars ◽  
Dust Grains ◽  
Fractional Abundance ◽  
Wide Range ◽  
Loss Rates

Aims. We aim to determine the abundances of CS, SiO, and SiS in a large sample of carbon star envelopes covering a wide range of mass loss rates to investigate the potential role that these molecules could play in the formation of dust in the surroundings of the central AGB star. Methods. We surveyed a sample of 25 carbon-rich AGB stars in the λ 2 mm band, more concretely in the J = 3−2 line of CS and SiO, and in the J = 7−6 and J = 8−7 lines of SiS, using the IRAM 30 m telescope. We performed excitation and radiative transfer calculations based on the large velocity gradient (LVG) method to model the observed lines of the molecules and to derive their fractional abundances in the observed envelopes. We also assessed the effect of infrared pumping in the excitation of the molecules. Results. We detected CS in all 25 targeted envelopes, SiO in 24 of them, and SiS in 17 sources. Remarkably, SiS is not detected in any envelope with a mass loss rate below 10−6 M⊙ yr−1 while it is detected in all envelopes with mass loss rates above that threshold. We found that CS and SiS have similar abundances in carbon star envelopes, while SiO is present with a lower abundance. We also found a strong correlation in which the denser the envelope, the less abundant are CS and SiO. The trend is however only tentatively seen for SiS in the range of high mass loss rates. Furthermore, we found a relation in which the integrated flux of the MgS dust feature at 30 μm increases as the fractional abundance of CS decreases. Conclusions. The decline in the fractional abundance of CS with increasing density could be due to gas-phase chemistry in the inner envelope or to adsorption onto dust grains. The latter possibility is favored by a correlation between the CS fractional abundance and the 30 μm feature, which suggests that CS is efficiently incorporated onto MgS dust around C-rich AGB stars. In the case of SiO, the observed abundance depletion with increasing density is most likely caused by an efficient incorporation onto dust grains. We conclude that CS, SiO (very likely), and SiS (tentatively) are good candidates to act as gas-phase precursors of dust in C-rich AGB envelopes.

scholarly journals Possible Proto-Planetary Nebulae

1989 ◽  
Vol 106 ◽  
pp. 384-390
Author(s):  
M. Parthasarathy
Keyword(s):  
Mass Loss ◽  
Emission Line ◽  
Planetary Nebulae ◽  
Evolutionary Stage ◽  
Galactic Latitude ◽  
Agb Stars ◽  
Recent Past ◽  
High Galactic Latitude ◽  
Dust Shells

AbstractIRAS data for high-galactic-latitude F supergiants, a few luminous F-G stars, and a few peculiar (forbidden) emission-line stars reveal they have dust shells with characteristics similar to those observed in planetary nebulae. The objects described here appear to have experienced severe mass loss in the recent past on the AGB. These stars are most likely post-AGB stars or evolving from the tip of AGB towards the left in the H-R diagram and may be described as possible proto-planetary nebulae. From the IRAS data we find ten additional possible proto-planetary nebulae. From the IRAS data of HD 56126, -53° 5072, HD 168625 and -59° 6723 the luminosities, temperatures and masses of the dust shells are derived. HD 56126 (F5I), HD 168625 (B8Iae), and -59° 6723 (B8Iae) appear to be in an evolutionary stage (post-AGB) similar to that of HD 161796 and HR 4049.

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 Formation of crystalline silicate around oxygen–rich AGB stars

1999 ◽  
Vol 191 ◽  
pp. 239-244 ◽  
Author(s):  
Takashi Kozasa ◽  
Hisato Sogawa
Keyword(s):  
Mass Loss ◽  
Loss Rate ◽  
Mass Loss Rate ◽  
Emission Feature ◽  
High Mass ◽  
Agb Stars ◽  
Dust Grains ◽  
High Mass Loss ◽  
Calculation Results ◽  
History Of

Crystallization of silicate has been investigated within the framework of dust formation in steady state gas outflows around oxygen–rich AGB stars, where silicates are locked not only into homogeneous silicate grains but also into the mantles of heterogeneous grains. Based on the thermal history of dust grains after their formation, the crystallization calculation results in no crystalline silicate for the mass loss rate Ṁ ≤ 2 × 10−5M⊙ yr−1. Only silicate in the mantles of heterogeneous grains can be crystallized for Ṁ ≥ 3 × 10−5M⊙ yr−1, while homogeneous silicate grains remain amorphous. The mass fraction of crystalline silicate increases with increasing Ṁ. The radiation transfer calculations confirm the appearance of an emission feature around 33.5 μm, taking olivine as a representative of crystalline silicates. On the other hand, the 10μm feature appears in absorption, being dominated by homogeneous silicate grains. These trends are consistent with the observations. Thus the crystalline silicate is a diagnostics of high mass loss rate at the late stage of AGB stellar evolution, reflecting the formation process of dust grains.

scholarly journals Properties of dust in the detached shells around U Antilae, DR Serpentis, and V644 Scorpii

2018 ◽  
Vol 620 ◽  
pp. A106 ◽  
Author(s):  
M. Maercker ◽  
T. Khouri ◽  
E. De Beck ◽  
M. Brunner ◽  
M. Mecina ◽  
...  
Keyword(s):  
Mass Loss ◽  
Dust Emission ◽  
Far Infrared ◽  
Asymptotic Giant Branch ◽  
Dust Particles ◽  
Spectral Energy ◽  
Agb Stars ◽  
Dust Grains ◽  
Grain Sizes ◽  
Dust Mass

Context. Asymptotic giant branch (AGB) stars experience strong mass loss driven by dust particles formed in the upper atmospheres. The dust is released into the interstellar medium, and replenishes galaxies with synthesised material from the star. The dust grains further act as seeds for continued dust growth in the diffuse medium of galaxies. As such, understanding the properties of dust produced during the asymptotic giant branch phase of stellar evolution is important for understanding the evolution of stars and galaxies. Recent observations of the carbon AGB star R Scl have shown that observations at far-infrared and submillimetre wavelengths can effectively constrain the grain sizes in the shell, while the total mass depends on the structure of the grains (solid vs. hollow or fluffy). Aims. We aim to constrain the properties of the dust observed in the submillimetre in the detached shells around the three carbon AGB stars U Ant, DR Ser, and V644 Sco, and to investigate the constraints on the dust masses and grain sizes provided by far-infrared and submm observations. Methods. We observed the carbon AGB stars U Ant, DR Ser, and V644 Sco at 870 μm using LABOCA on APEX. Combined with observations from the optical to far-infrared, we produced dust radiative transfer models of the spectral energy distributions (SEDs) with contributions from the stars, present-day mass-loss and detached shells. We assume spherical, solid dust grains, and test the effect of different total dust masses and grain sizes on the SED, and attempted to consistently reproduce the SEDs from the optical to the submm. Results. We derive dust masses in the shells of a few 10−5 M ⊙. The best-fit grain radii are comparatively large, and indicate the presence of grains between 0.1 μm and 2 μm. The LABOCA observations suffer from contamination from 12CO (3 − 2), and hence gives fluxes that are higher than the predicted dust emission at submm wavelengths. We investigate the effect on the best-fitting models by assuming different degrees of contamination and show that far-infrared and submillimetre observations are important to constrain the dust mass and grain sizes in the shells. Conclusions. Spatially resolved observations of the detached shells in the far-infrared and submillimetre effectively constrain the temperatures in the shells, and hence the grain sizes. The dust mass is also constrained by the observations, but additional observations are needed to constrain the structure of the grains.

The contribution of Centaur-emitted dust to the interplanetary dust distribution

2019 ◽  
Vol 490 (2) ◽  
pp. 2421-2429 ◽  
Author(s):  
A R Poppe
Keyword(s):  
Solar System ◽  
Mass Loss ◽  
Kuiper Belt ◽  
Interplanetary Dust ◽  
Outer Solar System ◽  
Dust Grains ◽  
Kuiper Belt Objects ◽  
Dust Density ◽  
Outer Planets ◽  
Current Mass

ABSTRACT Interplanetary dust grains originate from a variety of source bodies, including comets, asteroids, and Edgeworth–Kuiper belt objects. Centaurs, generally defined as those objects with orbits that cross the outer planets, have occasionally been observed to exhibit cometary-like outgassing at distances beyond Jupiter, implying that they may be an important source of dust grains in the outer Solar system. Here, we use an interplanetary dust grain dynamics model to study the behaviour and equilibrium distribution of Centaur-emitted interplanetary dust grains. We focus on the five Centaurs with the highest current mass-loss rates: 29P/Schwassmann-Wachmann 1, 166P/2001 T4, 174P/Echeclus, C/2001 M10, and P/2004 A1, which together comprise 98 per cent of the current mass loss from all Centaurs. Our simulations show that Centaur-emitted dust grains with radii s < 2 μm have median lifetimes consistent with Poynting–Robertson (P–R) drag lifetimes, while grains with radii s > 2 μm have median lifetimes much shorter than their P–R drag lifetimes, suggesting that dynamical interactions with the outer planets are effective in scattering larger grains, in analogy to the relatively short lifetimes of Centaurs themselves. Equilibrium density distributions of grains emitted from specific Centaurs show a variety of structure including local maxima in the outer Solar system and azimuthal asymmetries, depending on the orbital elements of the parent Centaur. Finally, we compare the total Centaur interplanetary dust density to dust produced from Edgeworth–Kuiper belt objects, Jupiter-family comets, and Oort cloud comets, and conclude that Centaur-emitted dust may be an important component between 5 and 15 au, contributing approximately 25 per cent of the local interplanetary dust density at Saturn.

scholarly journals Revealing infrared populations of nearby galaxies using the Spitzer Space Telescope

2009 ◽  
Vol 5 (S262) ◽  
pp. 111-114
Author(s):  
Mikako Matsuura
Keyword(s):  
Mass Loss ◽  
Stellar Evolution ◽  
Asymptotic Giant Branch ◽  
Evolutionary Stage ◽  
Agb Stars ◽  
Spitzer Space Telescope ◽  
Space Telescope ◽  
Chemical Enrichment ◽  
Nearby Galaxies ◽  
Enrichment Process

AbstractDue to their brightness in infrared, asymptotic giant branch (AGB) stars are in important evolutionary stage to be understood at this wavelength. In particular, in next decades, when the infrared optimised telescopes, such as the JWST and the ELT are in operation, it will be essential to include the AGB phase more precisely into the population synthesis models. However, the AGB phase is still one of the remaining major problems in the stellar evolution. This is because the AGB stellar evolution is strongly affected by the mass-loss process from the stars. It is important to describe mass loss more accurately so as to incorporate it into stellar evolutionary models. Recent observations using the Spitzer Space Telescope (SST) enabled us to make a significant progress in understanding the mass loss from AGB stars. Moreover, the SST large surveys contributed to our understanding of the role of AGB stars in chemical enrichment process in galaxies. Here we present the summary of our recent progress.

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