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

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

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935069 ◽

2019 ◽

Vol 628 ◽

pp. A62 ◽

Cited By ~ 10

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.

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Sulphur-bearing molecules in AGB stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833317 ◽

2018 ◽

Vol 617 ◽

pp. A132 ◽

Cited By ~ 13

Author(s):

T. Danilovich ◽

S. Ramstedt ◽

D. Gobrecht ◽

L. Decin ◽

E. De Beck ◽

...

Keyword(s):

Chemical Composition ◽

Mass Loss ◽

Loss Rate ◽

Mass Loss Rate ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Circumstellar Envelopes ◽

Carbon Stars ◽

Order Of Magnitude ◽

Transfer Modelling

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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Circumstellar envelopes of semi-regular long-period variables: mass-loss rate estimates and general model fitting of the molecular gas

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936087 ◽

2019 ◽

Vol 629 ◽

pp. A94 ◽

Cited By ~ 2

Author(s):

J. J. Díaz-Luis ◽

J. Alcolea ◽

V. Bujarrabal ◽

M. Santander-García ◽

A. Castro-Carrizo ◽

...

Keyword(s):

Mass Loss ◽

Mass Loss Rate ◽

Asymptotic Giant Branch ◽

Spherically Symmetric ◽

Axially Symmetric ◽

Circumstellar Envelopes ◽

Gas Expansion ◽

Type 3 ◽

Loss Rates

Aims. We aim to study the main properties of a volume-limited unbiased sample of well-characterized semi-regular variables (SRs) in order to clarify important issues that need to be further explained, such as the formation of axially symmetric planetary nebulae (PNe) from spherical circumstellar envelopes (CSEs), which takes place during the mass-loss process along the asymptotic giant branch (AGB) phase. Methods. We present new high-S/N IRAM 30 m observations of the 12CO J = 2–1, 12CO J = 1–0, and 13CO J = 1–0 lines, in a volume-limited sample of SRs for which the HIPPARCOS distances are between 100 and 500 pc and the declinations are above −25°. We analyzed the data by characterizing the main properties of the CSEs. The 12CO J = 2–1 data were used to study the profiles, while the 12CO J = 1–0 data were used to estimate mass-loss rates for the complete sample. Moreover, the 12CO J = 2–1 line has been used to determine the possible structures responsible for such profiles. Results. We have classified the sources into four groups according to the different profiles and final gas expansion velocities. Type 1 and 2 profiles are broad and narrow symmetric lines, respectively. Furthermore, type 1 profiles are more related to previously studied, standard, spherically symmetric envelopes. Type 3 profiles on the contrary are strange profiles with very pronounced asymmetries. Finally, type 4 profiles are those showing two different components: a narrow line profile superimposed on a broad pedestal component. We find that for sources with this latter kind of profile, the variation amplitude is very low, which means that these SRs do not have a well-developed inner envelope differentiated from the outer one. Interestingly, we report a moderate correlation between mass-loss rates and 12CO J = 1–0/12CO J = 2–1 line intensity ratios for O-rich SRs, suggesting a different behaviour between C- and O-rich SRs. Using SHAPE+shapemol, we find a unified simple model based on an oblate spheroid placed in different orientations that may explain all the 12CO profiles in the sample, indicating that the gas expansion is in general predominantly equatorial. Moreover, in order to explain the type 4 profiles, we define an extra component which may somehow be a biconical structure or similar according to the structures already found in this kind of source. Type 1 and 2 profiles, curiously, may also be explained by standard spherically symmetric envelopes, but often requiring anomalously low velocities. Type 3 and 4 profiles however, need axial symmetry to be explained. We conclude that most circumstellar shells around SRs show axial, strongly nonspherical symmetry. More interferometric observations are needed in order to make firm conclusions about mass-loss processes and possible morphologies of SRs.

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Discovery of a complex spiral-shell structure around the oxygen-rich AGB star GX Monocerotis

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935787 ◽

2020 ◽

Vol 636 ◽

pp. A123 ◽

Cited By ~ 2

Author(s):

S. K. Randall ◽

A. Trejo ◽

E. M. L. Humphreys ◽

H. Kim ◽

M. Wittkowski ◽

...

Keyword(s):

Mass Loss ◽

Mass Loss Rate ◽

Close Binary System ◽

Asymptotic Giant Branch ◽

High Mass ◽

Close Binary ◽

Chemical Compositions ◽

Circumstellar Envelopes ◽

Wide Range ◽

High Mass Loss

The circumstellar envelopes of asymptotic giant branch (AGB) stars exhibit a wide range of morphologies and chemical compositions that can be exploited to unravel their mass-loss history as well as binary status. Here, we present ALMA Band 6 observations centred upon the oxygen-rich, high mass-loss rate AGB star GX Mon. The resulting CO (2–1) map reveals an intricate, complex circumstellar spiral-arc structure consistent with hydrodynamical models for an AGB experiencing mass loss in a highly eccentric, close binary system with an orbital period of around 140 years. Several other transitions (including SiO, SiS, SO2, and CS) are detected in the data, however only the SO (5–4) map shows a similar – although much weaker – distribution as imaged for the CO.

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Asteroseismology of luminous red giants with Kepler. II. Dependence of mass loss on pulsations and radiation

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3970 ◽

2020 ◽

Author(s):

Jie Yu ◽

Saskia Hekker ◽

Timothy R Bedding ◽

Dennis Stello ◽

Daniel Huber ◽

...

Keyword(s):

Mass Loss ◽

Mass Loss Rate ◽

Asymptotic Giant Branch ◽

Red Giants ◽

Asymptotic Giant Branch Stars ◽

Chemical Enrichment ◽

Dust Mass ◽

Red Giant ◽

The Masses ◽

Loss Rates

Abstract Mass loss by red giants is an important process to understand the final stages of stellar evolution and the chemical enrichment of the interstellar medium. Mass-loss rates are thought to be controlled by pulsation-enhanced dust-driven outflows. Here we investigate the relationships between mass loss, pulsations, and radiation, using 3213 luminous Kepler red giants and 135000 ASAS–SN semiregulars and Miras. Mass-loss rates are traced by infrared colours using 2MASS and WISE and by observed-to-model WISE fluxes, and are also estimated using dust mass-loss rates from literature assuming a typical gas-to-dust mass ratio of 400. To specify the pulsations, we extract the period and height of the highest peak in the power spectrum of oscillation. Absolute magnitudes are obtained from the 2MASS Ks band and the Gaia DR2 parallaxes. Our results follow. (i) Substantial mass loss sets in at pulsation periods above ∼60 and ∼100 days, corresponding to Asymptotic-Giant-Branch stars at the base of the period-luminosity sequences C′ and C. (ii) The mass-loss rate starts to rapidly increase in semiregulars for which the luminosity is just above the red-giant-branch tip and gradually plateaus to a level similar to that of Miras. (iii) The mass-loss rates in Miras do not depend on luminosity, consistent with pulsation-enhanced dust-driven winds. (iv) The accumulated mass loss on the Red Giant Branch consistent with asteroseismic predictions reduces the masses of red-clump stars by 6.3%, less than the typical uncertainty on their asteroseismic masses. Thus mass loss is currently not a limitation of stellar age estimates for galactic archaeology studies.

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Dust Production around Carbon-Rich Stars: The Role of Metallicity

Universe ◽

10.3390/universe7070233 ◽

2021 ◽

Vol 7 (7) ◽

pp. 233

Author(s):

Ambra Nanni ◽

Sergio Cristallo ◽

Jacco Th. van Loon ◽

Martin A. T. Groenewegen

Keyword(s):

Wind Speed ◽

Galactic Halo ◽

Magellanic Clouds ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Circumstellar Envelopes ◽

Chemical Enrichment ◽

Wide Range ◽

The Universe

Background: Most of the stars in the Universe will end their evolution by losing their envelope during the thermally pulsing asymptotic giant branch (TP-AGB) phase, enriching the interstellar medium of galaxies with heavy elements, partially condensed into dust grains formed in their extended circumstellar envelopes. Among these stars, carbon-rich TP-AGB stars (C-stars) are particularly relevant for the chemical enrichment of galaxies. We here investigated the role of the metallicity in the dust formation process from a theoretical viewpoint. Methods: We coupled an up-to-date description of dust growth and dust-driven wind, which included the time-averaged effect of shocks, with FRUITY stellar evolutionary tracks. We compared our predictions with observations of C-stars in our Galaxy, in the Magellanic Clouds (LMC and SMC) and in the Galactic Halo, characterised by metallicity between solar and 1/10 of solar. Results: Our models explained the variation of the gas and dust content around C-stars derived from the IRS Spitzer spectra. The wind speed of the C-stars at varying metallicity was well reproduced by our description. We predicted the wind speed at metallicity down to 1/10 of solar in a wide range of mass-loss rates.

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Warm CO in evolved stars from the THROES catalogue

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834141 ◽

2019 ◽

Vol 622 ◽

pp. A123 ◽

Cited By ~ 2

Author(s):

J. M. da Silva Santos ◽

J. Ramos-Medina ◽

C. Sánchez Contreras ◽

P. García-Lario

Keyword(s):

Mass Loss ◽

Loss Rate ◽

Mass Loss Rate ◽

Asymptotic Giant Branch ◽

Strong Line ◽

Order Estimate ◽

Evolved Stars ◽

Flux Variability ◽

The Impact ◽

Loss Rates

Context. This is the second paper of a series making use of Herschel/PACS spectroscopy of evolved stars in the THROES catalogue to study the inner warm regions of their circumstellar envelopes (CSEs). Aims. We analyse the CO emission spectra, including a large number of high-J CO lines (from J = 14–13 to J = 45–44, ν = 0), as a proxy for the warm molecular gas in the CSEs of a sample of bright carbon-rich stars spanning different evolutionary stages from the asymptotic giant branch to the young planetary nebulae phase. Methods. We used the rotational diagram (RD) technique to derive rotational temperatures (Trot) and masses (MH2) of the envelope layers where the CO transitions observed with PACS arise. Additionally, we obtained a first order estimate of the mass-loss rates and assessed the impact of the opacity correction for a range of envelope characteristic radii. We used multi-epoch spectra for the well-studied C-rich envelope IRC+10216 to investigate the impact of CO flux variability on the values of Trot and MH2. Results. The sensitivity of PACS allowed for the study of higher rotational numbers than before indicating the presence of a significant amount of warmer gas (∼200 − 900 K) that is not traceable with lower J CO observations at submillimetre/millimetre wavelengths. The masses are in the range MH2 ∼ 10−2 − 10−5 M⊙, anticorrelated with temperature. For some strong CO emitters we infer a double temperature (warm T¯rot ∼ 400 K and hot T¯rot ∼ 820 K) component. From the analysis of IRC+10216, we corroborate that the effect of line variability is perceptible on the Trot of the hot component only, and certainly insignificant on MH2 and, hence, the mass-loss rate. The agreement between our mass-loss rates and the literature across the sample is good. Therefore, the parameters derived from the RD are robust even when strong line flux variability occurs, and the major source of uncertainty in the estimate of the mass-loss rate is the size of the CO-emitting volume.

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Mass-losing AGB Stars in the LMC

Symposium - International Astronomical Union ◽

10.1017/s0074180900002357 ◽

2000 ◽

Vol 177 ◽

pp. 145-151

Author(s):

Jacco Th. Van Loon ◽

Albert A. Zijlstra ◽

Patricia A. Whitelock ◽

Cecile Loup ◽

L.B.F.M. Waters

Keyword(s):

Mass Loss ◽

Optical Depth ◽

Agb Stars ◽

Circumstellar Envelopes ◽

Carbon Stars ◽

Infrared Study ◽

Tentative Evidence ◽

Loss Rates

We show the results of an infrared study of a sample of heavily obscured AGB stars in the LMC. Both carbon-rich and oxygen-rich mass-losing AGB stars can be found at both high and low luminosities, but the percentage of carbon stars decreases with increasing luminosity. The optical depth of the circumstellar envelopes also decreases with increasing luminosity, while the mass-loss rates are (nearly) constant with luminosity. We also show tentative evidence for having found the first post-AGB stars in the LMC.

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Stellar wind models of central stars of planetary nebulae

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937150 ◽

2020 ◽

Vol 635 ◽

pp. A173 ◽

Cited By ~ 1

Author(s):

J. Krtička ◽

J. Kubát ◽

I. Krtičková

Keyword(s):

Mass Loss ◽

White Dwarf ◽

Planetary Nebula ◽

Loss Rate ◽

Mass Loss Rate ◽

Planetary Nebulae ◽

Terminal Velocity ◽

Asymptotic Giant Branch ◽

Central Stars ◽

Loss Rates

Context. Fast line-driven stellar winds play an important role in the evolution of planetary nebulae, even though they are relatively weak. Aims. We provide global (unified) hot star wind models of central stars of planetary nebulae. The models predict wind structure including the mass-loss rates, terminal velocities, and emergent fluxes from basic stellar parameters. Methods. We applied our wind code for parameters corresponding to evolutionary stages between the asymptotic giant branch and white dwarf phases for a star with a final mass of 0.569 M⊙. We study the influence of metallicity and wind inhomogeneities (clumping) on the wind properties. Results. Line-driven winds appear very early after the star leaves the asymptotic giant branch (at the latest for Teff ≈ 10 kK) and fade away at the white dwarf cooling track (below Teff = 105 kK). Their mass-loss rate mostly scales with the stellar luminosity and, consequently, the mass-loss rate only varies slightly during the transition from the red to the blue part of the Hertzsprung–Russell diagram. There are the following two exceptions to the monotonic behavior: a bistability jump at around 20 kK, where the mass-loss rate decreases by a factor of a few (during evolution) due to a change in iron ionization, and an additional maximum at about Teff = 40−50 kK. On the other hand, the terminal velocity increases from about a few hundreds of km s−1 to a few thousands of km s−1 during the transition as a result of stellar radius decrease. The wind terminal velocity also significantly increases at the bistability jump. Derived wind parameters reasonably agree with observations. The effect of clumping is stronger at the hot side of the bistability jump than at the cool side. Conclusions. Derived fits to wind parameters can be used in evolutionary models and in studies of planetary nebula formation. A predicted bistability jump in mass-loss rates can cause the appearance of an additional shell of planetary nebula.

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