New observations and models of circumstellar CO line emission of AGB stars in theHerschelSUCCESS programme

AbstractCarbon-rich Asymptotic Giant Branch (AGB) stars are major sources of gas and dust in the interstellar medium. During the brief (∼1000 yr) period in the evolution from AGB to the Planetary Nebula (PN) stage, the molecular composition evolves from mainly diatomic and small polyatomic species to more complex molecules. Using the Submillimeter Array (SMA), we have carried out a spectral line survey of CRL 618, covering a frequency range of 281.9 to 359.4 GHz. More than 1000 lines were detected in the ∼60 GHz range, most of them assigned to HC3N and c-C3H2, and their isotopologues. About 200 lines are unassigned. Lines of CO, HCO+, and CS show the fast outflow wings, while the majority of line emission arises from a compact region of ∼1” diameter. We have analyzed the lines of HC3N, c-C3H2, CH3CN, and their isotopologues with rotation temperature diagrams.

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Detection of highly excited OH towards AGB stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935049 ◽

2019 ◽

Vol 623 ◽

pp. L1 ◽

Cited By ~ 5

Author(s):

T. Khouri ◽

L. Velilla-Prieto ◽

E. De Beck ◽

W. H. T. Vlemmings ◽

H. Olofsson ◽

...

Keyword(s):

Rotational Temperature ◽

Zeeman Effect ◽

Line Emission ◽

Asymptotic Giant Branch ◽

High Angular Resolution ◽

Agb Stars ◽

Vibrationally Excited ◽

Maser Effect ◽

Molecular Excitation ◽

The Magnetic Field

Aims. We characterise the gas in the extended atmospheres of the oxygen-rich asymptotic giant branch (AGB) stars W Hya and R Dor using high angular resolution ALMA observations. Methods. We report the detection and investigate the properties of high-excitation Λ-doubling line emission of hydroxyl (OH). Results. The OH lines are produced very close to the central stars and seem optically thin and with no maser effect. We analyse the molecular excitation using a population diagram and find rotational temperatures of ∼2500 K and column densities of ∼1019 cm−2 for both sources. For W Hya, we observe emission from vibrationally excited H2O arising from the same region as the OH emission. Moreover, CO v = 1, J = 3 − 2 emission also shows a brightness peak in the same region. Considering optically thin emission and the rotational temperature derived for OH, we find a CO column density ∼15 times higher than that of OH, within an area of (92 × 84) mas2 centred on the OH emission peak. These results should be considered tentative because of the simple methods employed. The observed OH line frequencies differ significantly from the predicted transition frequencies in the literature, and provide the possibility of using OH lines observed in AGB stars to improve the accuracy of the Hamiltonian used for the OH molecule. We predict stronger OH Λ-doubling lines at millimetre wavelengths than those we detected. These lines will be a good probe of shocked gas in the extended atmosphere and are possibly even suitable as probes of the magnetic field in the atmospheres of close-by AGB stars through the Zeeman effect.

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Abundance of SiC2 in carbon star envelopes

Astronomy and Astrophysics ◽

10.1051/0004-6361/201732038 ◽

2018 ◽

Vol 611 ◽

pp. A29 ◽

Cited By ~ 11

Author(s):

S. Massalkhi ◽

M. Agúndez ◽

J. Cernicharo ◽

L. Velilla Prieto ◽

J. R. Goicoechea ◽

...

Keyword(s):

Silicon Carbide ◽

Gas Phase ◽

Line Emission ◽

Carbon Star ◽

Active Role ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Carbon Stars ◽

Clear Trend ◽

Phase Precursor

Context. Silicon carbide dust is ubiquitous in circumstellar envelopes around C-rich asymptotic giant branch (AGB) stars. However, the main gas-phase precursors leading to the formation of SiC dust have not yet been identified. The most obvious candidates among the molecules containing an Si–C bond detected in C-rich AGB stars are SiC2, SiC, and Si2C. To date, the ring molecule SiC2 has been observed in a handful of evolved stars, while SiC and Si2C have only been detected in the C-star envelope IRC +10216.Aim. We aim to study how widespread and abundant SiC2, SiC, and Si2C are in envelopes around C-rich AGB stars, and whether or not these species play an active role as gas-phase precursors of silicon carbide dust in the ejecta of carbon stars.Methods. We carried out sensitive observations with the IRAM 30 m telescope of a sample of 25 C-rich AGB stars to search for emission lines of SiC2, SiC, and Si2C in the λ 2 mm band. We performed non-LTE excitation and radiative transfer calculations based on the LVG method to model the observed lines of SiC2 and to derive SiC2 fractional abundances in the observed envelopes.Results. We detect SiC2 in most of the sources, SiC in about half of them, and do not detect Si2C in any source except IRC +10216. Most of these detections are reported for the first time in this work. We find a positive correlation between the SiC and SiC2 line emission, which suggests that both species are chemically linked; the SiC radical is probably the photodissociation product of SiC2 in the external layer of the envelope. We find a clear trend where the denser the envelope, the less abundant SiC2 is. The observed trend is interpreted as evidence of efficient incorporation of SiC2 onto dust grains, a process that is favored at high densities owing to the higher rate at which collisions between particles take place.Conclusions. The observed behavior of a decline in the SiC2 abundance with increasing density strongly suggests that SiC2 is an important gas-phase precursor of SiC dust in envelopes around carbon stars.

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Observation of High Doppler Velocity Wings in the Nascent Wind of R Doradus

Communications in Physics ◽

10.15625/0868-3166/30/1/14763 ◽

2020 ◽

Vol 30 (1) ◽

pp. 85

Author(s):

Hoai Thi Do ◽

Nhung Tuyet Pham ◽

Tuan-Anh Pham ◽

Pierre Darriulat ◽

Diep Ngoc Pham ◽

...

Keyword(s):

Current Knowledge ◽

Line Emission ◽

Terminal Velocity ◽

Spectral Lines ◽

Line Of Sight ◽

Doppler Velocity ◽

Agb Stars ◽

Gas Streams ◽

Stellar Pulsations ◽

Molecular Lines

We study the morpho-kinematics in the nascent wind of AGB star R Doradus in the light of high Doppler velocity wings observed in the spectral lines of several species. We probe distances from the star between ∼10 and ∼100 au using ALMA observations of the emission of five different molecular lines. High Doppler velocity enhancements of the line emission are observed in the vicinity of the line of sight crossing the star, reminiscent of those recently interpreted as gas streams in the nascent wind of a similar AGB star, EP Aqr. They are present in both blue-shifted and red- shifted hemispheres but are not exactly back-to-back. They are accelerated at a typical rate of 0.7 km s\(^{−1}\) au\(^{−1}\) up to some 20 km s\(^{−1}\). Important differences are observed between the emissions of different molecules. We exclude an effect of improper continuum subtraction. However, in contrast to EP Aqr, the line of sight plays no particular role in the R Dor morpho-kinematics, shedding doubt on the validity of a gas stream interpretation. We discuss possible interpretations in terms of stellar pulsations or of rotation of the gas in the environment of the star. We conclude that, in the state of current knowledge, no fully convincing picture of the physics governing the production of such high velocities, typically twice as large as the terminal velocity, can be reliably drawn. New high resolution analyses of observations of the nascent wind of oxygen-rich AGB stars are needed to clarify the issue.

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Circumstellar chemistry of Si-C bearing molecules in the C-rich AGB star IRC+10216

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318005410 ◽

2018 ◽

Vol 14 (S343) ◽

pp. 535-537

Author(s):

L. Velilla-Prieto ◽

J. Cernicharo ◽

M. Agúndez ◽

J. P. Fonfría ◽

A. Castro-Carrizo ◽

...

Keyword(s):

Silicon Carbide ◽

Line Emission ◽

Growth Zone ◽

Time Dependent ◽

Circumstellar Envelope ◽

Agb Stars ◽

Millimeter Wavelengths ◽

Stellar Photosphere ◽

Main Components ◽

Dust Seeds

AbstractSilicon carbide together with amorphous carbon are the main components of dust grains in the atmospheres of C-rich AGB stars. Small gaseous Si-C bearing molecules (such as SiC, SiCSi, and SiC2) are efficiently formed close to the stellar photosphere. They likely condense onto dust seeds owing to their highly refractory nature at the lower temperatures (i.e., below about 2500 K) in the dust growth zone which extends a few stellar radii from the photosphere. Beyond this region, the abundances of Si-C bearing molecules are expected to decrease until they are eventually reformed in the outer shells of the circumstellar envelope, owing to the interaction between the gas and the interstellar UV radiation field. Our goal is to understand the time-dependent chemical evolution of Si-C bond carriers probed by molecular spectral line emission in the circumstellar envelope of IRC+10216 at millimeter wavelengths.

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Probing chemical processes in AGB stars

Proceedings of the International Astronomical Union ◽

10.1017/s174392130802156x ◽

2008 ◽

Vol 4 (S251) ◽

pp. 201-206 ◽

Cited By ~ 1

Author(s):

Fredrik L. Schöier ◽

Hans Olofsson

Keyword(s):

Radiative Transfer ◽

Mass Loss ◽

Line Emission ◽

Chemical Processes ◽

Agb Stars ◽

Molecular Line ◽

Large Sample ◽

Preliminary Results ◽

Transfer Modelling ◽

Non Equilibrium

AbstractWe are conducting multi-transition observations of circumstellar line emission from common molecules such as HCN, SiO, CS, SiS and CN for a large sample of AGB stars with varying photospheric C/O-ratios and mass-loss charachteristics. Our recently published results for SiO and SiS clearly show that major constraints on the relative roles of non-equilibrium chemistry, dust condensation, and photodissociation can be obtained from the study of circumstellar molecular line emission. Presented here are also preliminary results based on detailed radiative transfer modelling of HCN line emission.

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