Understanding AGB Carbon Star Nucleosynthesis from Observations

Abia, C.; Domı́nguez, I.; Gallino, R.; Busso, M.; Straniero, O.; Laverny, P. de; Wallerstein, George

doi:10.1071/as03021

Cited by 33 publications

(34 citation statements)

References 49 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Wallerstein & Knapp 1998). In fact, the most effective way of distinguishing between these two spectral types is to compare the intensity of some metallic lines (less prominent in CH-type stars) and, mainly, the intensity of s-element lines which are well known to be enhanced in CH-type stars (Keenan 1993;Abia et al 2003;Goswami 2005). An additional problem concerns the derivation of the temperature subtype.…”

Section: Spectral Classificationmentioning

confidence: 99%

“…In this hypothesis one might argue that the carbon enrichment that we currently observe was a consequence of mass transfer prior to the coalescence. However, it is extremely difficult to form carbon stars from mass transfer at near solar metallicity (Abia et al 2003;Masseron et al 2009, although a few exceptions seem to exist, e.g., BD +57 • 2161, see Začs et al 2005), thus there is very little or no space for the mass transfer scenario. The second scenario (carbon excess in the original gas cloud) is also ruled out because one should expect to find carbon stars of near solar metallicity at earlier evolutionary stages (main sequence, turn-off, sub-giants stars etc.).…”

Section: Evolutionary Statusmentioning

confidence: 99%

See 1 more Smart Citation

The chemical composition of carbon stars. The R-type stars

Zamora¹,

Abia²,

Plez

et al. 2009

A&A

Self Cite

View full text Add to dashboard Cite

Aims. The aim of this work is to shed some light on the problem of the formation of carbon stars of R-type from a detailed study of their chemical composition.Methods. We use high-resolution and high signal-to-noise optical spectra of 23 R-type stars (both early-and late-types) selected from the Hipparcos catalogue. The chemical analysis is made using spectral synthesis in LTE and state-of-the-art carbon-rich spherical model atmospheres. We derive their CNO content (including the 12 C/ 13 C ratio), average metallicity, lithium, and light (Sr, Y, Zr) and heavy (Ba, La, Nd, Sm) s-element abundances. The observed properties of the stars (galactic distribution, kinematics, binarity, photometry and luminosity) are also discussed. Results. Our analysis shows that late-R stars are carbon stars with identical chemical and observational characteristics as the normal (N-type) AGB carbon stars. The s-element abundance pattern derived can be reproduced by low-mass AGB nucleosynthesis models where the 13 C(α, n) 16 O reaction is the main neutron donor. We confirm the results of the sole previous abundance analysis of early-R stars, namely that they are carbon stars with near solar metallicity showing enhanced nitrogen, low 12 C/ 13 C ratios and no s-element enhancements. In addition, we have found that early-R stars have Li abundances larger than expected for post RGB tip giants. We also find that a significant number (∼40%) of the early-R stars in our sample are wrongly classified, probably being classical CH stars and normal K giants. Conclusions. On the basis of the chemical analysis, we confirm the previous suggestion that late-R stars are just misclassified N-type carbon stars in the AGB phase of evolution. Their photometric, kinematic, variability and luminosity properties are also compatible with this. In consequence, we suggest that the number of true R stars is considerably lower than previously believed. This alleviates the problem of considering R stars as a frequent stage in the evolution of low-mass stars. We briefly discuss the different scenarios proposed for the formation of early-R stars. The mixing of carbon during an anomalous He-flash is favoured, although no physical mechanism able to trigger that mixing has been found yet. The origin of these stars still remains a mystery.

show abstract

Section: Spectral Classificationmentioning

confidence: 99%

Section: Evolutionary Statusmentioning

confidence: 99%

The chemical composition of carbon stars. The R-type stars

Zamora¹,

Abia²,

Plez

et al. 2009

A&A

Self Cite

View full text Add to dashboard Cite

show abstract

“…Dust-driven winds (radiation pressure due to large luminosities) then enrich the ISM with these elements. Abia et al (2003) show that solar-metallicity AGB stars with C/O∼1, and a C/O>2 are considered extreme. Nuclear synthesis and evolutionary models show that only low-metallicity AGB stars would produce C/O=10 (Abia et al 2003).…”

Section: Introductionmentioning

confidence: 99%

“…Abia et al (2003) show that solar-metallicity AGB stars with C/O∼1, and a C/O>2 are considered extreme. Nuclear synthesis and evolutionary models show that only low-metallicity AGB stars would produce C/O=10 (Abia et al 2003). Given the low abundance of carbon-rich, nearly-sun-like stars (Fortney 2012), carbon-rich planets around those stars most likely must be linked to the disk evolution rather than to a primordial C/O>1.…”

Section: Introductionmentioning

confidence: 99%

Dust in brown dwarfs and extrasolar planets

Helling

Tootill

Woitke

et al. 2017

A&A

View full text Add to dashboard Cite

Context. Recent observations indicate potentially carbon-rich (C/O>1) exoplanet atmospheres. Spectral fitting methods for brown dwarfs and exoplanets have invoked the C/O ratio as additional parameter but carbon-rich cloud formation modeling is a challenge for the models applied. The determination of the habitable zone for exoplanets requires the treatment of cloud formation in chemically different regimes. Aims. We aim to model cloud formation processes for carbon rich exoplanetary atmospheres. Disk models show that carbon-rich or near-carbon-rich niches may emerge and cool carbon planets may trace these particular stages of planetary evolution. Methods. We extend our kinetic cloud formation model by including carbon seed formation and the formation of, and MgS[s] by gas-surface reactions. We solve a system of dust moment equations and element conservation for a pre-scribed Drift-Phoenix atmosphere structure to study how a cloud structure would change with changing initial C/O 0 =0.43 . . . 10.0. Results. The seed formation efficiency is lower in carbon-rich atmospheres than in oxygen-rich gases due to carbon being a very effective growth species. The consequence is that less particles will make up a cloud for C/O 0 >1. The cloud particles will be smaller in size than in an oxygen-rich atmosphere. An increasing initial C/O ratio does not revert this trend because a much greater abundance of condensible gas species exists in a rich carbon environment. Cloud particles are generally made of a mix of materials: carbon dominates if C/O 0 >1 and silicates dominate if C/O 0 <1. 80-90% carbon is reached only in extreme cases where C/O 0 =3.0 or 10.0. Conclusions. Carbon-rich atmospheres would form clouds that are made of particles of height-dependent mixed compositions, sizes and numbers. The remaining gas-phase is far less depleted than in an oxygen-rich atmosphere. Typical tracer molecules are HCN and C 2 H 2 in combination with a featureless, smooth continuum due to a carbonaceous cloud cover, unless the cloud particles become crystalline.

show abstract

“…Moreover, carbon stars are unlikely to form by mass transfer at near solar metallicity mainly because of the high solar oxygen abundance to be surpassed (Abia et al 2003), although a few exceptions seem to exist (e.g. BD +57 • 2161, see Začs et al 2005).…”

Section: Introductionmentioning

confidence: 99%

Merging in the common envelope and the origin of early R-type stars

Piersanti

Cabezón

Zamora

et al. 2010

A&A

Self Cite

View full text Add to dashboard Cite

Context. Binary systems experiencing one or two common envelope episodes during the red giant branch or the Hertzsprung gap phases can produce a single star, evolving along the Hayashi track, as a final outcome. Even if these objects are expected to be very common in nature, a proper description of their evolution and physical properties is still missing. Moreover, this scenario (red giant merging scenario) has been invoked as the progenitor systems of early-R stars, by assuming that the physical conditions developed as a consequence of the cores merging could produce the mixing into the convective envelope of fresh carbon that was synthesized during the He-flash. Aims. We analyze in detail the red giant merging scenario to verify if the resulting star develops the physical conditions suitable for a dredge-up of C-enriched material from the core to the envelope. Methods. We performed 3D simulations of the merging stars, to check whether He is burnt efficiently during the formation of a selfsustained disk. We therefore did 1D computations of the accretion phase occurring after the merging and of the following evolution up to the settling of quiescent He-burning in the center. We adopted different assumptions on the amount of angular momentum transferred from the disk to the core and on the angular momentum transport. Results. Efficient He-burning does not occur during the merging, because a very high temperature (T > 10 8 K) at the disk/He-core interface develops only for a few minutes. Our computations show that the accretion process is the leading parameter in determining the final properties of the merged object. In particular, the thermal energy delivered by the accreted matter determines the heating of the whole newborn core, thus preventing the developing of highly degenerate physical conditions. This occurrence determines the onset of the He-burning with an He-flash milder and closer to the center, as compared to standard RGB stars. Rotation and different angular momentum transport efficiency plays a secondary role by determining the exact location of the first He-flash. In none of the computed models is material formed in the He-core mixed into the convective envelope, because the H-burning shell, which always active during the He-flashes and later on, acts as a barrier. Conclusions. In the red giant merging scenario, the physical conditions suitable for both a peculiar He-flash and the dredging-up of C-enriched material never occur. Our results speak against the possibility that such an evolutionary scenario could represent the progenitor system of early R-stars.

show abstract

Understanding AGB Carbon Star Nucleosynthesis from Observations

Cited by 33 publications

References 49 publications

The chemical composition of carbon stars. The R-type stars

The chemical composition of carbon stars. The R-type stars

Dust in brown dwarfs and extrasolar planets

Merging in the common envelope and the origin of early R-type stars

Contact Info

Product

Resources

About