Abundance of SiC2 in carbon star envelopes

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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The Abundance of SiC2 in Carbon Star Envelopes

Proceedings of the International Astronomical Union ◽

10.1017/s1743921317007566 ◽

2017 ◽

Vol 13 (S332) ◽

pp. 261-269

Author(s):

Sarah Massalkhi ◽

M. Agúndez ◽

J. Cernicharo ◽

J. P. Fonfría ◽

M. Santander-García

Keyword(s):

Silicon Carbide ◽

Gas Phase ◽

Carbon Star ◽

Active Role ◽

Agb Stars ◽

Dust Grains ◽

Circumstellar Envelopes ◽

Carbon Stars ◽

Evolved Stars

AbstractSilicon carbide dust grains are ubiquitous in circumstellar envelopes around C-rich AGB stars. However, the main gas-phase precursors leading to the formation of SiC dust have not yet been identified. To date, only three molecules containing an Si–C bond have been identified to have significant abundances in C-rich AGB stars: SiC2, SiC, and Si2C. 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. 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.

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Understanding AGB Carbon Star Nucleosynthesis from Observations

Publications of the Astronomical Society of Australia ◽

10.1071/as03021 ◽

2003 ◽

Vol 20 (4) ◽

pp. 314-323 ◽

Cited By ~ 20

Author(s):

C. Abia ◽

I. Domínguez ◽

R. Gallino ◽

M. Busso ◽

O. Straniero ◽

...

Keyword(s):

Carbon Star ◽

Solar Neighborhood ◽

Asymptotic Giant Branch ◽

Evolutionary Status ◽

Agb Stars ◽

Carbon Stars ◽

The Core ◽

Standard Scenario ◽

Low Mass ◽

Process Elements

AbstractRecent advances in the knowledge of the evolutionary status of asymptotic giant branch (AGB) stars and of the nucleosynthesis processes occurring in them are discussed, and used to interpret abundance determinations for s-process elements, lithium and CNO isotopes in several types of AGB stars. We focus our attention mainly on carbon-rich AGB stars. By combining these different constraints we conclude that most carbon stars in the solar neighborhood are of low mass (M≤3 M⊙), their abundances being a consequence of the operation of thermal pulses and the third dredge-up. However, the observed abundances in carbon stars of the R and J types cannot be explained by this standard scenario. These stars may not be on the AGB, but possibly in the core-He burning phases; their envelopes may have been polluted with nuclear ashes of the core-He flash, followed by CNO re-processing enhancing 13C. Observational evidence suggesting the operation of non-standard mixing mechanisms during the AGB phase is also discussed.

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Abundance Estimates in Carbon Star Envelopes

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318007871 ◽

2018 ◽

Vol 14 (S343) ◽

pp. 460-461

Author(s):

S. Massalkhi ◽

M. Agúndez ◽

J. Cernicharo

Keyword(s):

Observational Study ◽

Interstellar Medium ◽

Gas Phase ◽

Carbon Star ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Circumstellar Envelopes ◽

Condensation Nuclei ◽

Abundance Estimates ◽

Key Questions

AbstractThe synthesis of dust grains mostly takes place in the circumstellar envelopes (CSEs) of asymptotic giant branch (AGB) stars. What are the precursor seeds of condensation nuclei and how do these particles evolve toward the micrometer sized grains that populate the interstellar medium? These are key questions of the NANOCOSMOS project. In this study, we carried out an observational study to constrain what the main gas-phase precursors of dust in C-rich AGB stars are.

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Properties of carbon stars in the solar neighbourhood based on Gaia DR2 astrometry

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936831 ◽

2020 ◽

Vol 633 ◽

pp. A135 ◽

Cited By ~ 2

Author(s):

C. Abia ◽

P. de Laverny ◽

S. Cristallo ◽

G. Kordopatis ◽

O. Straniero

Keyword(s):

Spatial Distribution ◽

Galactic Plane ◽

Accurate Determination ◽

Chemical Properties ◽

Carbon Star ◽

Asymptotic Giant Branch ◽

Solar Neighbourhood ◽

Carbon Stars ◽

Hot Stars ◽

Kinematic Properties

Context. Stars evolving along the asymptotic giant branch (AGB) can become carbon rich in the final part of their evolution. The detailed description of their spectra has led to the definition of several spectral types: N, SC, J, and R. To date, differences among them have been partially established only on the basis of their chemical properties. Aims. An accurate determination of the luminosity function (LF) and kinematics together with their chemical properties is extremely important for testing the reliability of theoretical models and establishing on a solid basis the stellar population membership of the different carbon star types. Methods. Using Gaia Data Release 2 (Gaia DR2) astrometry, we determine the LF and kinematic properties of a sample of 210 carbon stars with different spectral types in the solar neighbourhood with measured parallaxes better than 20%. Their spatial distribution and velocity components are also derived. Furthermore, the use of the infrared Wesenheit function allows us to identify the different spectral types in a Gaia-2MASS diagram. Results. We find that the combined LF of N- and SC-type stars are consistent with a Gaussian distribution peaking at Mbol ∼ −5.2 mag. The resulting LF, however, shows two tails at lower and higher luminosities more extended than those previously found, indicating that AGB carbon stars with solar metallicity may reach Mbol ∼ −6.0 mag. This contrasts with the narrower LF derived in Galactic carbon Miras from previous studies. We find that J-type stars are about half a magnitude fainter on average than N- and SC-type stars, while R-hot stars are half a magnitude brighter than previously found, although fainter in any case by several magnitudes than other carbon types. Part of these differences are due to systematically lower parallaxes measured by Gaia DR2 with respect to HIPPARCOS values, in particular for sources with parallax ϖ < 1 mas. The Galactic spatial distribution and velocity components of the N-, SC-, and J-type stars are very similar, while about 30% of the R-hot stars in the sample are located at distances greater than ∼500 pc from the Galactic plane, and show a significant drift with respect to the local standard of rest. Conclusions. The LF derived for N- and SC-type in the solar neighbourhood fully agrees with the expected luminosity of stars of 1.5−3 M⊙ on the AGB. On a theoretical basis, the existence of an extended low-luminosity tail would require a contribution of extrinsic low-mass carbon stars, while the high-luminosity tail would imply that stars with mass values up to ∼5 M⊙ may become carbon stars on the AGB. J-type stars differ significantly not only in their chemical composition with respect to the N- and SC-types, but also in their LF, which reinforces the idea that these carbon stars belong to a different type whose origin is still unknown. The derived luminosities of R-hot stars means that it is unlikely that these stars are in the red-clump, as previously claimed. On the other hand, the derived spatial distribution and kinematic properties, together with their metallicity values, indicate that most of the N-, SC-, and J-type stars belong to the thin disc population, while a significant fraction of R-hot stars show characteristics compatible with the thick disc.

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SMA Spectral Line Survey of the Proto-Planetary Nebula CRL 618

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318008104 ◽

2018 ◽

Vol 14 (S343) ◽

pp. 483-484

Author(s):

Nimesh A. Patel ◽

Carl Gottlieb ◽

Ken Young ◽

Tomasz Kaminski ◽

Michael McCarthy ◽

...

Keyword(s):

Spectral Line ◽

Planetary Nebula ◽

Line Emission ◽

Asymptotic Giant Branch ◽

60 Ghz ◽

Agb Stars ◽

Complex Molecules ◽

Compact Region ◽

Line Survey ◽

Pn Stage

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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Fundamental problems and basic tests of stellar evolution theory — The case of carbon stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900030503 ◽

1984 ◽

Vol 105 ◽

pp. 3-19

Author(s):

Icko Iben

Keyword(s):

Solar System ◽

Theoretical Work ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Carbon Stars ◽

Core Mass ◽

Convective Envelope ◽

Thermal Pulse ◽

R Process ◽

Neutron Rich Isotopes

Carbon stars are thought to be in the asymptotic giant branch (AGB) phase of evolution, alternately burning hydrogen and helium in shells above an electron-degenerate carbon-oxygen (CO) core. The excess of carbon relative to oxygen at the surfaces of these stars is thought to be due to convective dredge-up which occurs following a thermal pulse. During a thermal pulse, carbon and neutron-rich isotopes are made in a convective helium-burning zone. In model stars of large CO core mass, the source of neutrons for producing the neutron-rich isotopes is the 22Ne(α, n)25Mg reaction and the isotopes are produced in the solar system s-process distribution. In models of small core mass, the 13C(α, n) 16O reaction is thought to be responsible for the release of neutrons, and the resultant distribution of neutron-rich isotopes is expected to vary considerably from one star to the next, with the distribution in isolated instances possibly resembling the solar system distribution of r-process isotopes. After the dredge-up phase following each pulse, the 13C is made by the reactions 12C(p,γ) 13N(β+ v) 13C in a zone of large 12C abundance and small 1H abundance that has been established by semiconvective mixing during the dredge-up phase. There is qualitative accord between the properties of carbon stars in the Magellanic Clouds and properties of model stars, but considerably more theoretical work is required before a quantitative match is achieved.The observed paucity of AGB stars more luminous than MBOL ∼ −6 is interpreted to mean that the AGB lifetime of a star more luminous than this is at least a factor of ten smaller than the AGB lifetime of stars less luminous than this, or, at most 105 yr. Since, with current estimates of the 22Ne(α, n)25Mg reaction rate R22, only AGB model stars more luminous than MBOL ∼ −6 can produce s-process isotopes in the solar system distribution, it is inferred that either (1) the current estimates of R22 are too small by one to two orders of magnitude, allowing less luminous AGB stars to contribute, (2) the solar system distribution is not equivalent to the average Galactic distribution, being rather the consequence of a unique injection into the protosolar nebula of matter from a massive intermediate-mass AGB star, or (3) the estimates of the temperatures in the convective shell that are given by extant models are too low by, sav, 10 or 15 percent.The absence of carbon stars more luminous than MBOL ∼ −6 is suggested to be due primarily to the fact that ∼ 106 yr of AGB evolution is necessary to produce surface C/O > 1, rather than to be due to the burning of dredged-up carbon into nitrogen at the base of the convective envelope during the interpulse quiescent hydrogen-burning phase. Thus, the positive correlation between the nitrogen and helium abundances in planetary nebulae is perhaps primarily a consequence of the second dredge-up episode rather than a consequence of processes occurring during the thermally pulsing phase.

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86 GHz SiO maser survey of late-type stars in the Inner Galaxy

Astronomy and Astrophysics ◽

10.1051/0004-6361/201730717 ◽

2018 ◽

Vol 619 ◽

pp. A35 ◽

Cited By ~ 2

Author(s):

M. Messineo ◽

H.J. Habing ◽

L. O. Sjouwerman ◽

A. Omont ◽

K. M. Menten

Keyword(s):

Detection Rate ◽

Point Sources ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Late Type ◽

Carbon Stars ◽

Infrared Surveys

We present an 86 GHz SiO (v = 1, J = 2 → 1) maser search toward late-type stars located within |b|< 0.°5 and 20° < l < 50°. This search is an extension at longer longitudes of a previously published work. We selected 135 stars from the MSX catalog using color and flux criteria and detected 92 (86 new detections). The detection rate is 68%, the same as in our previous study. The last few decades have seen the publication of several catalogs of point sources detected in infrared surveys (MSX, 2MASS, DENIS, ISOGAL, WISE, GLIMPSE, AKARI, and MIPSGAL). We searched each catalog for data on the 444 targets of our earlier survey and for the 135 in the survey reported here. We confirm that, as anticipated, most of our targets have colors typical of oxygen-rich asymptotic giant branch (AGB) stars. Only one target star may have already left the AGB. Ten stars have colors typical of carbon-rich stars, meaning a contamination of our sample with carbon stars ≲1.7%.

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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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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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Fluorine Abundances in AGB Carbon Stars: New Results?

Publications of the Astronomical Society of Australia ◽

10.1071/as08038 ◽

2009 ◽

Vol 26 (3) ◽

pp. 351-353 ◽

Cited By ~ 2

Author(s):

C. Abia ◽

P. de Laverny ◽

A. Recio-Blanco ◽

I. Domínguez ◽

S. Cristallo ◽

...

Keyword(s):

Asymptotic Giant Branch ◽

Agb Stars ◽

Carbon Stars ◽

Low Mass ◽

Stellar Models ◽

Full Network

AbstractA recent reanalysis of the fluorine abundance in three Galactic Asymptotic Giant Branch (AGB) carbon stars (TX Psc, AQ Sgr and R Scl) by Abia et al. (2009) results in estimates of fluorine abundances systematically lower by ∼0.8 dex on average, with respect to the sole previous estimates by Jorissen, Smith & Lambert (1992). The new F abundances are in better agreement with the predictions of full-network stellar models of low-mass (<3 M⊙) AGB stars.

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