The hydrodynamical structure of circumstellar envelopes around
low mass-loss rate, low outflow velocity AGB 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.

Download Full-text

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.

Download Full-text

On the circumstellar envelopes of semi-regular long-period variables

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318005045 ◽

2018 ◽

Vol 14 (S343) ◽

pp. 186-190

Author(s):

J. J. Díaz-Luis ◽

J. Alcolea ◽

V. Bujarrabal ◽

M. Santander-García ◽

M. Gómez-Garrido ◽

...

Keyword(s):

Mass Loss ◽

Loss Rate ◽

Mass Loss Rate ◽

Unexpected Result ◽

Axially Symmetric ◽

Circumstellar Envelopes ◽

Long Period ◽

Short Analysis Time ◽

Line Intensity Ratio ◽

Co Observations

AbstractThe mass loss process along the AGB phase is crucial for the formation of circumstellar envelopes (CSEs), which in the post-AGB phase will evolve into planetary nebulae (PNe). There are still important issues that need to be further explored in this field; in particular, the formation of axially symmetric PNe from spherical CSEs. To address the problem, we have conducted high S/N IRAM 30 m observations of 12COJ = 1−0 and J = 2−1, and 13COJ = 1−0 in a volume-limited unbiased sample of semi-regular variables (SRs). We also conducted Yebes 40 m SiO J = 1−0 observations in 1/2 of the sample in order to complement our 12CO observations. We report a moderate correlation between mass loss rate and the 12CO(1−0)−to−12CO(2−1) line intensity ratio, introducing a possible new method for determining mass loss rates of SRs with short analysis time. We also find that for several stars the SiO profiles are very similar to the 12CO profiles, a totally unexpected result unless these are non-standard envelopes.

Download Full-text

Jsolated Stars of Low Metallicity

Galaxies ◽

10.3390/galaxies6030089 ◽

2018 ◽

Vol 6 (3) ◽

pp. 89

Author(s):

Efrat Sabach

Keyword(s):

Angular Momentum ◽

Mass Loss ◽

Stellar Evolution ◽

Planetary Nebula ◽

Luminosity Function ◽

Loss Rate ◽

Mass Loss Rate ◽

Solar Metallicity ◽

Low Metallicity ◽

Low Mass

We study the effects of a reduced mass-loss rate on the evolution of low metallicity Jsolated stars, following our earlier classification for angular momentum (J) isolated stars. By using the stellar evolution code MESA we study the evolution with different mass-loss rate efficiencies for stars with low metallicities of Z = 0 . 001 and Z = 0 . 004 , and compare with the evolution with solar metallicity, Z = 0 . 02 . We further study the possibility for late asymptomatic giant branch (AGB)—planet interaction and its possible effects on the properties of the planetary nebula (PN). We find for all metallicities that only with a reduced mass-loss rate an interaction with a low mass companion might take place during the AGB phase of the star. The interaction will most likely shape an elliptical PN. The maximum post-AGB luminosities obtained, both for solar metallicity and low metallicities, reach high values corresponding to the enigmatic finding of the PN luminosity function.

Download Full-text

An ALMA view of SO and SO2 around oxygen-rich AGB stars

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa693 ◽

2020 ◽

Vol 494 (1) ◽

pp. 1323-1347 ◽

Cited By ~ 3

Author(s):

T Danilovich ◽

A M S Richards ◽

L Decin ◽

M Van de Sande ◽

C A Gottlieb

Keyword(s):

Mass Loss ◽

Loss Rate ◽

Mass Loss Rate ◽

Energy Levels ◽

High Mass ◽

High Angular Resolution ◽

Circumstellar Envelope ◽

Agb Stars ◽

Abundance Distribution ◽

High Mass Loss

ABSTRACT We present and analyse SO and SO2, recently observed with high angular resolution and sensitivity in a spectral line survey with ALMA, for two oxygen-rich AGB stars: the low mass-loss rate R Dor and high mass-loss rate IK Tau. We analyse 8 lines of SO detected towards both stars, 78 lines of SO2 detected towards R Dor, and 52 lines of SO2 detected towards IK Tau. We detect several lines of 34SO, 33SO, and 34SO2 towards both stars, and tentatively S18O towards R Dor, and hence derive isotopic ratios for these species. The spatially resolved observations show us that the two sulphur oxides are co-located towards R Dor and trace out the same wind structures in the circumstellar envelope. Much of the emission is well reproduced with a Gaussian abundance distribution spatially centred on the star. Emission from the higher energy levels of SO and SO2 towards R Dor provides evidence in support of a rotating inner region of gas identified in earlier work. The new observations allow us to refine the abundance distribution of SO in IK Tau derived from prior observations with single antennas, and confirm that the distribution is shell like with the peak in the fractional abundance not centred on the star. The confirmation of different types of SO abundance distributions will help fine-tune chemical models and allows for an additional method to discriminate between low and high mass-loss rates for oxygen-rich AGB stars.

Download Full-text

Link between Mass-loss and Variability Type for AGB Stars?

Symposium - International Astronomical Union ◽

10.1017/s007418090020332x ◽

1999 ◽

Vol 191 ◽

pp. 395-400 ◽

Cited By ~ 2

Author(s):

Željko Ivezić ◽

Gillian R. Knapp

Keyword(s):

Mass Loss ◽

Time Scales ◽

Loss Rate ◽

Mass Loss Rate ◽

Dust Emission ◽

Agb Stars ◽

Similar Time ◽

Color Distribution ◽

K 12

We find that AGB stars separate in the 25–12 vs. 12-K color-color diagram according to their chemistry (O, S vs. C) and variability type (Miras vs. SRb/Lb). While discrimination according to the chemical composition is not surprising, the separation of Miras from SRb/Lb variables is unexpected.We show that “standard” steady-state radiatively driven models provide excellent fits to the color distribution of Miras of all chemical types. However, these models are incapable of explaining the dust emission from O-rich SRb/Lb stars. The models can be altered to fit the data by postulating different optical properties for silicate grains, or by assuming that the dust temperature at the inner envelope radius is significantly lower (300–400 K) than typical condensation temperatures (800–1000 K), a possibility which is also supported by the detailed characteristics of LRS data. While such lower temperatures are required only for O- and S-rich SRb/Lb stars, they are also consistent with the colors of C-rich SRb/Lb stars.The absence of hot dust for SRb/Lb stars can be interpreted as a recent (order of 100 yr) decrease in the mass-loss rate. The distribution of O-rich SRb/Lb stars in the 25–12 vs. K-12 color-color diagram shows that the mass-loss rate probably resumes again, on similar time scales. It cannot be ruled out that the mass-loss rate is changing periodically on such time scales, implying that the stars might oscillate between the Mira and SRb/Lb phases during their AGB evolution as proposed by Kerschbaum et al. (1996). Such a possibility appears to be supported by recent HST images of the Egg Nebula obtained by Sahai et al. (1997), the discovery of multiple CO winds reported by Knapp et al. (1998), and long-term visual light-curve changes detected for some stars by Mattei (1998).

Download Full-text

Carbon-rich AGB stars in our Galaxy and nearby galaxies as possible sources of PAHs

Proceedings of the International Astronomical Union ◽

10.1017/s1743921308021558 ◽

2008 ◽

Vol 4 (S251) ◽

pp. 197-200

Author(s):

M. Matsuura ◽

G. C. Sloan ◽

J. Bernard-Salas ◽

A. A. Zijlstra ◽

P. R. Wood ◽

...

Keyword(s):

Mass Loss ◽

Loss Rate ◽

Mass Loss Rate ◽

Agb Stars ◽

Carbon Production ◽

Very Large Telescope ◽

Infrared Spectrometer ◽

Low Metallicity ◽

Nearby Galaxies ◽

Array Camera

AbstractWe have obtained infrared spectra of carbon-rich AGB stars in three nearby galaxies – the Large and Small Magellanic Clouds, and the Fornax dwarf spheroidal galaxy. Our primary aim is to investigate gas compositions and mass-loss rate of these stars as a function of metallicity, by comparing AGB stars in several galaxies with different metallicities. C2H2are detectable from AGB stars, and possibly PAHs are subsequently formed from C2H2. Thus, it is worth investigating chemical processes at low metallicity. These stars were observed using the Infrared Spectrometer (irs) onboard theSpitzer Space Telescopewhich covers 5–35 μm region, and the Infrared Spectrometer And Array Camera (isaac) on the Very Large Telescope which covers the 2.9–4.1 μm region. HCN, CH and C2H2molecular bands, as well as SiC and MgS dust features are identified in the spectra. The equivalent width of C2H2molecular bands is larger at lower metallicity, thus PAHs might be abundant in AGB stars at low metallicity. We find no evidence that mass-loss rates depend on metallicity. Chemistry of carbon stars is affected by carbon production during the AGB phase rather than the metallicities. We argue that lower detection rate of PAHs from the interstellar medium of lower metal galaxies is caused by destruction of PAHs in the ISM by stronger UV radiation field.

Download Full-text

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.

Download Full-text

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.

Download Full-text