Sulphur-bearing molecules in AGB stars

Context. Sulphur has long been known to form different molecules depending on the chemical composition of its environment. More recently, the sulphur-bearing molecules SO and H2S have been shown to behave differently in oxygen-rich asymptotic giant branch (AGB) circumstellar envelopes of different densities. Aims. By surveying a diverse sample of AGB stars for CS and SiS emission, we aim to determine in which environments these sulphur-bearing molecules most readily occur. We include sources with a range of mass-loss rates and carbon-rich, oxygen-rich, and mixed S-type chemistries. Where these molecules are detected, we aim to determine their CS and SiS abundances. Methods. We surveyed 20 AGB stars of different chemical types using the APEX telescope, and combined this with an IRAM 30 m and APEX survey of CS and SiS emission towards over 30 S-type stars. For those stars with detections, we performed radiative transfer modelling to determine abundances and abundance distributions. Results. We detect CS towards all the surveyed carbon stars, some S-type stars, and the highest mass-loss rate oxygen-rich stars, (Ṁ ≥ 5 × 10−6 M⊙ yr−1). SiS is detected towards the highest mass-loss rate sources of all chemical types (Ṁ ≥ 8 × 10−7 M⊙ yr−1). We find CS peak fractional abundances ranging from ~4 × 10−7 to ~2 × 10−5 for the carbon stars, from ~3 × 10−8 to ~1 × 10−7 for the oxygen-rich stars, and from ~1 × 10−7 to ~8 × 10−6 for the S-type stars. We find SiS peak fractional abundances ranging from ~9 × 10−6 to ~2 × 10−5 for the carbon stars, from ~5 × 10−7 to ~2 × 10−6 for the oxygen-rich stars, and from ~2 × 10−7 to ~2 × 10−6 for the S-type stars. Conclusions. Overall, we find that wind density plays an important role in determining the chemical composition of AGB circumstellar envelopes. It is seen that for oxygen-rich AGB stars both CS and SiS are detected only in the highest density circumstellar envelopes and their abundances are generally lower than for carbon-rich AGB stars by around an order of magnitude. For carbon-rich and S-type stars SiS was also only detected in the highest density circumstellar envelopes, while CS was detected consistently in all surveyed carbon stars and sporadically among the S-type stars.

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The abundance of S- and Si-bearing molecules in O-rich circumstellar envelopes of AGB stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/202037900 ◽

2020 ◽

Vol 641 ◽

pp. A57

Author(s):

S. Massalkhi ◽

M. Agúndez ◽

J. Cernicharo ◽

L. Velilla-Prieto

Keyword(s):

Mass Loss ◽

Mass Loss Rate ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Circumstellar Envelopes ◽

Fractional Abundance ◽

Wide Range ◽

Order Of Magnitude ◽

Large Velocity Gradient ◽

Loss Rates

Aims. We aim to determine the abundances of SiO, CS, SiS, SO, and SO2 in a large sample of oxygen-rich asymptotic giant branch (AGB) 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 these environments. Methods. We surveyed a sample of 30 oxygen-rich AGB stars in the λ 2 mm band using the IRAM 30m telescope. We performed excitation and radiative transfer calculations based on the large velocity gradient method to model the observed lines of the molecules and to derive their fractional abundances in the observed envelopes. Results. We detected SiO in all 30 targeted envelopes, as well as CS, SiS, SO, and SO2 in 18, 13, 26, and 19 sources, respectively. Remarkably, SiS is not detected in any envelope with a mass loss rate below 10−6 M⊙ yr−1, whereas it is detected in all envelopes with mass loss rates above that threshold. From a comparison with a previous, similar study on C-rich sources, it becomes evident that the fractional abundances of CS and SiS show a marked differentiation between C-rich and O-rich sources, being two orders of magnitude and one order of magnitude more abundant in C-rich sources, respectively, while the fractional abundance of SiO turns out to be insensitive to the C/O ratio. The abundance of SiO in O-rich envelopes behaves similarly to C-rich sources, that is, the denser the envelope the lower its abundance. A similar trend, albeit less clear than for SiO, is observed for SO in O-rich sources. Conclusions. The marked dependence of CS and SiS abundances on the C/O ratio indicates that these two molecules form more efficiently in C- than O-rich envelopes. The decline in the abundance of SiO with increasing envelope density and the tentative one for SO indicate that SiO and possibly SO act as gas-phase precursors of dust in circumstellar envelopes around O-rich AGB stars.

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Unravelling the sulphur chemistry of AGB stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318004969 ◽

2018 ◽

Vol 14 (S343) ◽

pp. 379-380

Author(s):

Taïssa Danilovich

Keyword(s):

Mass Loss ◽

Radial Distribution ◽

Loss Rate ◽

Mass Loss Rate ◽

Expansion Velocity ◽

Agb Stars ◽

Circumstellar Envelopes ◽

Carbon Stars

AbstractThere are clear differences in what sulphur molecules form in AGB circumstellar envelopes (CSEs) across chemical types. CS forms more readily in the CSEs of carbon stars, while SO and SO2 have only been detected towards oxygen-rich stars. However, we have also discovered differences in sulphur chemistry based on the density of the CSE, as traced by mass-loss rate divided by expansion velocity. For example, the radial distribution of SO is drastically different between AGB stars with lower and higher density CSEs. H2S can be found in high abundances towards higher density oxygen-rich stars, whereas SiS accounts for a significant portion of the circumstellar sulphur for higher density carbon stars.

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Red Giant Stars: Comparison of Observations and Theory

Symposium - International Astronomical Union ◽

10.1017/s0074180900040171 ◽

1984 ◽

Vol 108 ◽

pp. 195-206

Author(s):

Jeremy Mould

Keyword(s):

Mass Loss ◽

Parameter Space ◽

Loss Rate ◽

Mass Loss Rate ◽

Magellanic Clouds ◽

Asymptotic Giant Branch ◽

Carbon Stars ◽

Giant Stars ◽

Theoretical Investigations ◽

Red Giant

Recent observations in both the field and the clusters of the Magellanic Clouds suggest a higher mass loss rate during or at the end of the asymptotic giant branch phase than previously supposed. Recent theoretical investigations offer an explanation for the frequency of carbon stars in the Clouds, but a rich parameter space remains to be explored, before detailed agreement can be expected.

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Surface brightness fluctuations, tracers of stellar mass-loss?

Proceedings of the International Astronomical Union ◽

10.1017/s1743921310002516 ◽

2009 ◽

Vol 5 (S262) ◽

pp. 48-51

Author(s):

Rosa A. González-Lópezlira ◽

Gustavo Bruzual-A. ◽

Stéphane Charlot ◽

Javier Ballesteros-Paredes ◽

Laurent Loinard

Keyword(s):

Mass Loss ◽

Stellar Population ◽

Loss Rate ◽

Mass Loss Rate ◽

Broad Band ◽

Global Changes ◽

Agb Stars ◽

Near Ir ◽

Surface Brightness Fluctuations ◽

Order Of Magnitude

AbstractWe present optical and IR integrated colors and SBF magnitudes, computed from stellar population synthesis models that include emission from the dusty envelopes surrounding mass-loosing TP-AGB stars. We explore the effects of varying the mass-loss rate by one order of magnitude around the fiducial value, modifying accordingly both the stellar parameters and the output spectra of the TP-AGB stars plus their dusty envelopes. We compare these models to optical and near-IR data of single AGB stars and Magellanic star clusters. Neither broad-band colors nor SBF measurements in the optical or the near-IR can discern global changes in the mass-loss rate of a stellar population. However, we predict that mid-IR SBF measurements can pick out such changes, and actually resolve whether a relation between metallicity and mass-loss exists.

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Wind morphology around cool evolved stars in binaries

Astronomy and Astrophysics ◽

10.1051/0004-6361/202037492 ◽

2020 ◽

Vol 637 ◽

pp. A91 ◽

Cited By ~ 5

Author(s):

I. El Mellah ◽

J. Bolte ◽

L. Decin ◽

W. Homan ◽

R. Keppens

Keyword(s):

Mass Loss ◽

Adaptive Mesh Refinement ◽

Loss Rate ◽

Main Sequence ◽

Mass Loss Rate ◽

Large Fraction ◽

Orbital Plane ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Evolved Stars

Context. The late evolutionary phase of low- and intermediate-mass stars is strongly constrained by their mass-loss rate, which is orders of magnitude higher than during the main sequence. The wind surrounding these cool expanded stars frequently shows nonspherical symmetry, which is thought to be due to an unseen companion orbiting the donor star. The imprints left in the outflow carry information about the companion and also the launching mechanism of these dust-driven winds. Aims. We study the morphology of the circumbinary envelope and identify the conditions of formation of a wind-captured disk around the companion. Long-term orbital changes induced by mass loss and mass transfer to the secondary are also investigated. We pay particular attention to oxygen-rich, that is slowly accelerating, outflows in order to look for systematic differences between the dynamics of the wind around carbon and oxygen-rich asymptotic giant branch (AGB) stars. Methods. We present a model based on a parametrized wind acceleration and a reduced number of dimensionless parameters to connect the wind morphology to the properties of the underlying binary system. Thanks to the high performance code MPI-AMRVAC, we ran an extensive set of 72 three-dimensional hydrodynamics simulations of a progressively accelerating wind propagating in the Roche potential of a mass-losing evolved star in orbit with a main sequence companion. The highly adaptive mesh refinement that we used, enabled us to resolve the flow structure both in the immediate vicinity of the secondary, where bow shocks, outflows, and wind-captured disks form, and up to 40 orbital separations, where spiral arms, arcs, and equatorial density enhancements develop. Results. When the companion is deeply engulfed in the wind, the lower terminal wind speeds and more progressive wind acceleration around oxygen-rich AGB stars make them more prone than carbon-rich AGB stars to display more disturbed outflows, a disk-like structure around the companion, and a wind concentrated in the orbital plane. In these configurations, a large fraction of the wind is captured by the companion, which leads to a significant shrinking of the orbit over the mass-loss timescale, if the donor star is at least a few times more massive than its companion. In the other cases, an increase of the orbital separation is to be expected, though at a rate lower than the mass-loss rate of the donor star. Provided the companion has a mass of at least a tenth of the mass of the donor star, it can compress the wind in the orbital plane up to large distances. Conclusions. The grid of models that we computed covers a wide scope of configurations: We vary the terminal wind speed relative to the orbital speed, the extension of the dust condensation region around the cool evolved star relative to the orbital separation, and the mass ratio, and we consider a carbon-rich and an oxygen-rich donor star. It provides a convenient frame of reference to interpret high-resolution maps of the outflows surrounding cool evolved stars.

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A study of circumstellar envelopes around bright carbon stars. I - Structure, kinematics, and mass-loss rate.

The Astrophysical Journal Supplement Series ◽

10.1086/191804 ◽

1993 ◽

Vol 87 ◽

pp. 267 ◽

Cited By ~ 127

Author(s):

H. Olofsson ◽

K. Eriksson ◽

B. Gustafsson ◽

U. Carlstrom

Keyword(s):

Mass Loss ◽

Loss Rate ◽

Mass Loss Rate ◽

Circumstellar Envelopes ◽

Carbon Stars

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On the role of reduced wind mass-loss rate in enabling exoplanets to shape planetary nebulae

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1551 ◽

2020 ◽

Vol 496 (1) ◽

pp. 612-619

Author(s):

Ahlam Hegazi ◽

Ealeal Bear ◽

Noam Soker

Keyword(s):

Mass Loss ◽

Loss Rate ◽

Mass Loss Rate ◽

Semimajor Axis ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Early Conclusion ◽

Future Evolution ◽

Red Giant ◽

Parent Star

ABSTRACT We use the stellar evolution code MESA–binary and follow the evolution of three exoplanets and two brown dwarfs (BDs) to determine their potential role in the future evolution of their parent star on the red giant branch (RGB) and on the asymptotic giant branch (AGB). We limit this study to exoplanets and BDs with orbits that have semimajor axis of $1 {~\rm au}\lesssim a_0 \lesssim 20 {~\rm au}$, a high eccentricity, $e_0 \gtrsim 0.25$, and having a parent star of mass M*,0 ≥ 1 M⊙. We find that the star HIP 75 458 will engulf its planet HIP 75 458b during its RGB phase. The planet will remove the envelope and terminate the RGB evolution, leaving a bare helium core of mass 0.4 M⊙ that will evolve to form a helium white dwarf. Only in one system out of five, the planet beta Pic c will enter the envelope of its parent star during the AGB phase. For that to occur, we have to reduce the wind mass-loss rate by a factor of about four from its commonly used value. This strengthens an early conclusion, which was based on exoplanets with circular orbits, which states that to have a non-negligible fraction of AGB stars that engulf planets we should consider lower wind mass-loss rates of isolated AGB stars (before they are spun-up by a companion). Such an engulfed planet might lead to the shaping of the AGB mass-loss geometry to form an elliptical planetary nebula.

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An extensive grid of DARWIN models for M-type AGB stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935366 ◽

2019 ◽

Vol 626 ◽

pp. A100 ◽

Cited By ~ 10

Author(s):

S. Bladh ◽

S. Liljegren ◽

S. Höfner ◽

B. Aringer ◽

P. Marigo

Keyword(s):

Mass Loss ◽

Mass Loss Rate ◽

Stellar Atmospheres ◽

Asymptotic Giant Branch ◽

Driving Mechanism ◽

Agb Stars ◽

Wide Range ◽

Order Of Magnitude ◽

Dynamical Properties ◽

Loss Rates

Context. The stellar winds of asymptotic giant branch (AGB) stars are commonly attributed to radiation pressure on dust grains, formed in the wake of shock waves that arise in the stellar atmospheres. The mass loss due to these outflows is substantial, and modelling the dynamical properties of the winds is essential both for studies of individual stars and for understanding the evolution of stellar populations with low to intermediate mass. Aims. The purpose of this work is to present an extensive grid of dynamical atmosphere and wind models for M-type AGB stars, covering a wide range of relevant stellar parameters. Methods. We used the DARWIN code, which includes frequency-dependent radiation-hydrodynamics and a time-dependent description of dust condensation and evaporation, to simulate the dynamical atmosphere. The wind-driving mechanism is photon scattering on submicron-sized Mg2SiO4 grains. The grid consists of ~4000 models, with luminosities from L⋆ = 890 L⊙ to L⋆ = 40 000 L⊙ and effective temperatures from 2200 to 3400 K. For the first time different current stellar masses are explored with M-type DARWIN models, ranging from 0.75 M⊙ to 3 M⊙. The modelling results are radial atmospheric structures, dynamical properties such as mass-loss rates and wind velocities, and dust properties (e.g. grain sizes, dust-to-gas ratios, and degree of condensed Si). Results. We find that the mass-loss rates of the models correlate strongly with luminosity. They also correlate with the ratio L*∕M*: increasing L*∕M* by an order of magnitude increases the mass-loss rates by about three orders of magnitude, which may naturally create a superwind regime in evolution models. There is, however, no discernible trend of mass-loss rate with effective temperature, in contrast to what is found for C-type AGB stars. We also find that the mass-loss rates level off at luminosities higher than ~14 000 L⊙, and consequently at pulsation periods longer than ~800 days. The final grain radii range from 0.25 to 0.6 μm. The amount of condensed Si is typically between 10 and 40%, with gas-to-dust mass ratios between 500 and 4000.

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Radioactive Isotope 26Al in the Interstellar Matter (Resulting From a Mass Loss by AGB Stars)

Symposium - International Astronomical Union ◽

10.1017/s0074180900090537 ◽

1992 ◽

Vol 150 ◽

pp. 411-412

Author(s):

Ju L. Frantsman

Keyword(s):

Mass Loss ◽

Loss Rate ◽

Mass Loss Rate ◽

Asymptotic Giant Branch ◽

Interstellar Matter ◽

Agb Stars ◽

Asymptotic Giant Branch Star ◽

Star Evolution ◽

Calculated Mass ◽

Branch Star

Several calculations of the Asymptotic Giant Branch star evolution have been performed with the aim of explaining the synthesis of interstellar 26Al in these stars. The agreement of theoretically calculated mass of interstellar 26Al and observations is rather satisfactory, the best for the abrupt tenfold jump in the mass-loss rate for the stars reaching the luminosity log(L/L⊙) ⋍ 4.0.

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