Unravelling the sulphur chemistry of AGB stars

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.

scholarly journals Sulphur-bearing molecules in AGB stars

2018 ◽  
Vol 617 ◽  
pp. A132 ◽  
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.

The Lithium Abundance and Mass Loss Rate in Galactic Super-Li-Rich Carbon and S Stars

2000 ◽  
Vol 198 ◽  
pp. 377-378
Author(s):  
D. A. Lubowich ◽  
Verne V. Smith ◽  
B. E. Turner ◽  
R. Sahai
Keyword(s):  
Mass Loss ◽  
Loss Rate ◽  
Mass Loss Rate ◽  
Significant Source ◽  
Expansion Velocity ◽  
Agb Stars ◽  
Line Profiles ◽  
Lithium Abundance

The super-Li-rich (SLR) stars are rare AGB stars containing enhanced Li/H ≈ 2000 times the ISM Li/H. In order to determine if mass loss from SLR stars is a significant source of ISM Li, we measured their Li/H and the mass loss rate. From the weak Li I 8126 Â line in 2/4 SLR C stars and 6/8 SLR S stars we obtained Li/H = (0.2 — 5) x 10—6 for SLR C stars and Li/H = (0.2 — 5) x 10—7 for the SLR S stars. We determined dM/dt from the expansion velocity and line profiles of the circumstellar CO. The dM/dt = 1.3 × 10—5M⊙/yr, 2.6 × 10—7M⊙/yr, and 2.6 × 10—8M⊙/yr for the C stars IY Hya, T Ara, and WZ Cas; and average dM/dt = 5.5 × 10—7M⊙/yr for the S stars. Thus mass loss from SLR stars is a significant source of Galactic Li.

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

2002 ◽  
Vol 388 (2) ◽  
pp. 609-614 ◽  
Author(s):  
J. M. Winters ◽  
T. Le Bertre ◽  
L.-Å. Nyman ◽  
A. Omont ◽  
K. S. Jeong
Keyword(s):  
Mass Loss ◽  
Loss Rate ◽  
Mass Loss Rate ◽  
Agb Stars ◽  
Circumstellar Envelopes ◽  
Outflow Velocity ◽  
Low Mass

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

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.

scholarly journals Red Giant Stars: Comparison of Observations and Theory

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.

scholarly journals Mass-losing AGB Stars in the LMC

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.

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

2020 ◽  
Vol 494 (1) ◽  
pp. 1323-1347 ◽  
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.

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

1999 ◽  
Vol 191 ◽  
pp. 395-400 ◽  
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).

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

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.

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