scholarly journals AGB circumstellar envelopes: molecular observations

1999 ◽  
Vol 191 ◽  
pp. 363-372
Author(s):  
V. Bujarrabal
Keyword(s):  
Spatial Distribution ◽  
Chemical Reactions ◽  
Flux Distribution ◽  
Thermal Emission ◽  
Data Interpretation ◽  
Gas Content ◽  
Agb Stars ◽  
Empirical Knowledge ◽  
Circumstellar Envelopes ◽  
Molecular Line

Due to the low excitation requirements and easy observation from the ground, molecular line observations probably constitute our main source of empirical knowledge on circumstellar envelopes (CSEs) around AGB stars. The CO rotational transitions, the most intense ‘thermal’ lines, are efficiently used to determine the total gas content and its spatial distribution in wide samples of objects. Thermal emission from other molecules is mainly useful in order to study their abundances and the chemical reactions taking place in CSEs. Maser lines are easily observed due to their high intensity and the flux distribution in very compact spatial spots and narrow profile spikes, characteristic of the exponential amplification; however the data interpretation is difficult due to the intricate pumping processes. The most important maser lines (of SiO, H2O and OH) arise from very different regions, which allows the study of various components of the CSEs. We will focus here on SiO masers.

scholarly journals Circumstellar spirals/shells/arcs: the message from binary stars

2016 ◽  
Vol 12 (S323) ◽  
pp. 199-206
Author(s):  
Hyosun Kim
Keyword(s):  
Mass Loss ◽  
Binary Stars ◽  
Planetary Nebula ◽  
Fossil Record ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Circumstellar Envelopes ◽  
Molecular Line ◽  
The Past ◽  
Recurrent Patterns

AbstractA consensus has grown in the past few decades that binarity is key to understanding the morphological diversities of the circumstellar envelopes (CSEs) surrounding stars in the Asymptotic Giant Branch (AGB) to Planetary Nebula (PN) phase. The possible roles of binaries in their shaping have, however, yet to be confirmed. Meanwhile, recurrent patterns are often found in the CSEs of AGB stars and the outer halos of PNe, providing a fossil record of the mass loss during the AGB phase. In this regard, recent molecular line observations using interferometric facilities have revealed the spatio-kinematics of such patterns. Numerical simulations of binary interactions producing spiral-shells have been extensively developed, revealing new probes for extracting the stellar and orbital properties from these patterns. I review recent theoretical and observational investigations on the circumstellar spiral-shell patterns and discuss their implications in linking binary properties to the asymmetric ejection events in the post-AGB phase.

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

Universe ◽  
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.

Zooming into the complex dusty envelopes of C-rich AGB stars

2018 ◽  
Vol 14 (S343) ◽  
pp. 456-457
Author(s):  
Foteini Lykou ◽  
Josef Hron ◽  
Daniela Klotz
Keyword(s):  
Temperature Distribution ◽  
Near Infrared ◽  
Angular Resolution ◽  
High Angular Resolution ◽  
Dust Formation ◽  
Agb Stars ◽  
Circumstellar Envelopes ◽  
Recent Advances ◽  
Multi Wavelength

AbstractRecent advances in high-angular resolution instruments (VLT and VLTI, ALMA) have enabled us to delve deep into the circumstellar envelopes of AGB stars from the optical to the sub-mm wavelengths, thus allowing us to study in detail the gas and dust formation zones (e.g., their geometry, chemistry and kinematics). This work focuses on four (4) C-rich AGB stars observed with a high-angular resolution technique in the near-infrared: a multi-wavelength tomographic study of the dusty layers of the circumstellar envelopes of these C-rich stars, i.e. the variations in the morphology and temperature distribution.

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 mm-VLBI observations of SiO masers: Further support for radiative pumping and rotation of the inner shell

2002 ◽  
Vol 206 ◽  
pp. 286-289
Author(s):  
Jean-François Desmurs ◽  
Valentín Bujarrabal ◽  
Francisco Colomer ◽  
Javier Alcolea
Keyword(s):  
Spatial Distribution ◽  
High Spatial Resolution ◽  
Agb Stars ◽  
Polarization Pattern ◽  
Vlbi Observations ◽  
Very High Spatial Resolution ◽  
Order Of Magnitude ◽  
Circumstellar Shells ◽  
Very High ◽  
Better Than

We have performed VLBA observations of the SiO v = 1 and v = 2 J = 1-0 masers in two AGB stars, TX Cam and IRC +10011. We confirm the ring-like spatial distribution, previously found in several AGB objects, as well as the tangential polarization pattern, already reported for TX Cam. Both properties, that seem to be systematic in this kind of objects, are characteristic of radiatively pumped SiO masers. On the contrary, we do not confirm the previous report on the spatial coincidence between the J = 1-0 v = 1 and 2 masers, a result that would have argued in favor of collisional pumping. We find that both lines sometimes arise from nearby spots, typically separated by 1-2 mas, but are rarely coincident. The discrepancy with previous results is explained by the very high spatial resolution of our observations, ∼ 0.5 mas, an order of magnitude better than in the relevant previously published experiment. Moreover, we have been able to measure a probable rotation of the inner shell of a few km/s. Rotation of circumstellar shells is assumed by the most convincing models explaining the drastic change of symmetry between the AGB envelopes (spherical symmetry) and Proto Planetary Nebulae (axial symmetry).

scholarly journals Depleted diffuse bands in circumstellar envelopes of post-AGB stars

2008 ◽  
Vol 4 (S251) ◽  
pp. 69-70
Author(s):  
N. L. J. Cox ◽  
R. Luna ◽  
M. A. Satorre ◽  
D. A. García Hernández ◽  
O. Suárez ◽  
...  
Keyword(s):  
Unknown Origin ◽  
Agb Stars ◽  
Circumstellar Envelopes

AbstractWe report on new results in the search for diffuse bands, signatures of still unknown origin, in the circumstellar envelopes of evolved (post-AGB) stars.

Transmission electron microscopy on metal–organic frameworks – a review

2017 ◽  
Vol 5 (29) ◽  
pp. 14969-14989 ◽  
Author(s):  
Christian Wiktor ◽  
Maria Meledina ◽  
Stuart Turner ◽  
Oleg I. Lebedev ◽  
Roland A. Fischer
Keyword(s):  
Crystal Structure ◽  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Spatial Distribution ◽  
Chemical Reactions ◽  
Metal Organic Frameworks ◽  
Building Blocks ◽  
Transmission Electron ◽  
Metal Organic ◽  
Surface Facets

Versatile materials like MOFs require careful characterization. TEM can be used to determine and identify the crystal structure and surface facets of MOFs, the spatial distribution of guests or building blocks in them and how they are changed in chemical reactions.

Millimetre Observations of Evolved Stars

1996 ◽  
Vol 13 (2) ◽  
pp. 185-186
Author(s):  
Jessica M. Chapman
Keyword(s):  
Mass Loss ◽  
Stellar Atmosphere ◽  
Asymptotic Giant Branch ◽  
High Mass ◽  
Circumstellar Envelope ◽  
Agb Stars ◽  
Circumstellar Envelopes ◽  
Evolved Stars ◽  
High Mass Loss ◽  
Transfer Momentum

Radio emission at centimetre and millimetre wavelengths provides a powerful tool for studying the circumstellar envelopes of evolved stars. These include stars on the asymptotic giant branch (AGB), post-AGB stars and a small number of massive M-type supergiant stars. The AGB stars and M-type supergiants are characterised by extremely high mass-loss rates. The mass loss in such an evolved star is driven by radiation pressure acting on grains which form in the outer stellar atmosphere. The grains are accelerated outwards and transfer momentum to the gas through grain–gas collisions. The outflowing dust and gas thus form an expanding circumstellar envelope through which matter flows from the star to the interstellar medium, at a typical velocity of 15 km s−1. For a recent review of circumstellar mass loss see Chapman, Habing & Killeen (1995).

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