Asymptotic Giant Branch evolution at varying surface C/O ratio: effects of changes in molecular opacities

By P. Marigo. 2002

159 citations recorded by the source corpus.

We investigate the eects of molecular opacities on the evolution of TP-AGB stars that experience the third dredge-up, i.e. with surface abundances of carbon and oxygen varying with time. To this aim, a routine is constructed to derive the molecular concentrations through dissociation equilibrium calculations, and estimate the opacities due to H2 ,H 2O, OH, C2, CN, and CO for any given density, temperature and chemical composition of the gas. Then, synthetic TP-AGB models with dredge-up are calculated by either adopting the newly developed routine, or interpolating between xed opacity tables for solar chemical composition. The comparison between the two cases shows that the change in the dominant opacity sources, as the C/O ratio grows from below to above unity, crucially aects the evolution of the eective temperature, i.e. causing a notable cooling of the carbon- rich models (with C/O> 1). From the comparison with observational data, it turns out that TP-AGB models with variable molecular opacities are able to reproduce the observed range of eective temperatures, mass-loss rates, and wind expansion velocities of C-type giants in the solar neighbourhood, otherwise failed if assuming xed molecular opacities for solar-scaled mixtures. Finally, we mention other possibly important evolutionary and observational eects that result from the adoption of the variable opacities, such as: i) signicant shortening of the C-star phase due to the earlier onset of the super-wind; ii) consequent reduction of the carbon yields; iii) reproduction of the observed range of near-infrared colours of C-stars.

Subfields: Astronomy and Astrophysics, Instrumentation, Statistical and Nonlinear Physics

View Paper: https://www.semanticscholar.org/paper/2d892a815e76c09c2986f615680d89849e1a53bb

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In Astronomy and Astrophysics

1. C. Conroy
2. A. Karakas
3. M. Lugaro
4. Benjamin D. Johnson
5. J. Lattanzio
6. S. Charlot
7. L. Girardi
8. A. Bressan

In Instrumentation

1. C. Conroy
2. A. Karakas
3. Benjamin D. Johnson
4. M. Lugaro
5. J. J. Dalcanton
6. S. Charlot
7. L. Girardi
8. P. Hopkins

In Statistical and Nonlinear Physics

1. A. Karakas
2. M. Lugaro
3. J. Lattanzio
4. O. Straniero
5. C. Conroy
6. S. Cristallo
7. Falk Herwig
8. R. Stancliffe

Citations in the corpus

• Updated stellar yields from asymptotic giant branch models (2009, background): “The effect of C-rich opacities on AGB evolution was first discussed by Marigo (2002), who noted that once C/O > 1 the star cools and mass loss increases, shortening the TP-AGB lifetime.”
• THE ACS NEARBY GALAXY SURVEY TREASURY. IX. CONSTRAINING ASYMPTOTIC GIANT BRANCH EVOLUTION WITH OLD METAL-POOR GALAXIES (2010, background): “The opacities of carbonrich stars (C/O > 1), for instance, may be significantly higher, on average, than in oxygen-rich stars (C/O 1), so that the atmospheres of carbon-rich stars are usually less dense (and more extended) than those of oxygen-rich stars (Marigo 2002).”
• EVOLUTION, NUCLEOSYNTHESIS, AND YIELDS OF AGB STARS AT DIFFERENT METALLICITIES. III. INTERMEDIATE-MASS MODELS, REVISED LOW-MASS MODELS, AND THE pH-FRUITY INTERFACE (2015, background): “We demonstrated that the use of low temperature C-bearing opacities has dramatic consequences of the physical evolution of AGB stars (C istallo et al. 2007; see also Marigo 2002 and Ventura & Marigo 2010).”
• Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars - I. Stellar yield tables and the CEMPs (2008, background): “By approximating the opacity of a few key molecules in synthetic stellar models, Marigo (2002) showed how important including this opacity source is.”
• Evolution of asymptotic giant branch stars. I. Updated synthetic TP-AGB models and their basic calibration (2007, methodology): “Moreover, Marigo (2002) has recently demonstrated that the lowtemperature opacities used in present-day TP-AGB calculations are in substantial error, due to non-consideration of the dramatic changes in molecular concentrations that occur in the outer envelope of TP-AGB stars, as their chemical…”
• s-Process Nucleosynthesis in Asymptotic Giant Branch Stars: A Test for Stellar Evolution (2002, background): “Marigo (2002) investigated the effects of molecular opacities on the evolution of AGB stars as the C/O ratio grows from below to above unity and found that in the carbon-rich models, a notable cooling results, and hence, an early onset of the superwind is expected in the framework of the…”
• Evolution of Asymptotic Giant Branch Stars (2005, background): “Most notably, CN has been identified by Marigo (2002) to add important opacity to the atmosphere of C stars.”
• Super and massive AGB stars - IV. Final fates - Initial to final mass relation (2014, background): “When the star becomes carbon rich, the change in molecular chemistry leads to an increase in opacity which results in a cooler and more extended stellar envelope and a higher mass-loss rate (Marigo 2002; Cristallo et al. 2007; Ventura & Marigo 2010).”
• An Empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich asymptotic giant branch stars (2005, background): “Note that the Bertelli et al. (1994) tracks do not account for the molecular opacity efects on the atmospheres of carbon stars which are, however, incorporated in more recent models by Marigo (2002).”
• NEW COOLING SEQUENCES FOR OLD WHITE DWARFS (2010, background): “For the present calculations, we have not considered carbon-enriched molecular opacities (Marigo 2002), which are expected to reduce effective temperatures at the asymptotic giant branch (AGB; Weiss & Ferguson 2009).”
• Evolution of asymptotic giant branch stars. II. Optical to far-infrared isochrones with improved TP- (2007, methodology): “…while Gi00 uses tables for scaled-solar chemical compositions taken from Alexander & Ferguson (1994), in Paper I the opacities are computed “on-the-fly” using the proper chemical mixture of the evolving TP-AGB envelopes, by means of the methods and approximations described in Marigo (2002).”
• THE s-PROCESS IN ASYMPTOTIC GIANT BRANCH STARS OF LOW METALLICITY AND THE COMPOSITION OF CARBON-ENHANCED METAL-POOR STARS (2011, background): “We included an approximate contribution to the molecular opacities (based on the work of Marigo 2002) that varies with envelope composition (Stancliffe & Glebbeek 2008).”
• Evolution of thermally pulsing asymptotic giant branch stars - I. The COLIBRI code (2013, background): “Since Marigo (2002), molecular opacities for C-rich mixtures have been progressively adopted in full TP-AGB models (e.g. Kamath, Karakas & Wood 2012; Ventura & Marigo 2010, 2009; Weiss & Ferguson 2009; Cristallo et al. 2007).”
• Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles. II. CO line survey of evolved stars: derivation of mass-loss rate formulae (2010, unknown): “This is in agreement with the results of Marigo (2002), showing a clear dichotomy in T e ff between both chemical types of AGB stars.”
• A NEW GENERATION OF PARSEC-COLIBRI STELLAR ISOCHRONES INCLUDING THE TP-AGB PHASE (2017, background): “In addition, in the HRD, there is an evident trend of C stars being found at smaller Teff than O-rich stars; this effect is caused by the sudden increase in Rosseland mean opacities as C/O overcomes the value of 1 in stellar atmospheres (Marigo 2002; Marigo et al. 2013).”
• Scaled solar tracks and isochrones in a large region of the Z-Y plane I. From the ZAMS to the TP-A (2008, background): “One fundamental improvement is the adoption ofmolecular opacities coupled to the actual surface C/O ratio (Marigo 2002), in place of tables valid for scaled solar chemical compositions (e.g. Alexander & Ferguson 1994).”
• Modelling the nebular emission from primeval to present-day star-forming galaxies (2016, background): “…of helium in stars more massive than about 8 M (e.g., Maeder 1992; Prantzos et al. 1994; Gustafsson et al. 1999; Henry et al. 2000b), although less massive stars also produce and expel carbon (e.g., van den Hoekj & Groenewegen 1997; Marigo et al. 1996, 1998; Henry et al. 2000a; Marigo 2002).”
• EVOLUTION, NUCLEOSYNTHESIS, AND YIELDS OF LOW-MASS ASYMPTOTIC GIANT BRANCH STARS AT DIFFERENT METALLICITIES (2009, background): “This in turn, shortens the AGB lifetime hence reducing the amount of material returned to the ISM (see also Marigo 2002; Weiss & Ferguson 2009; Ventura & Marigo 2009).”
• MESA ISOCHRONES AND STELLAR TRACKS (MIST). I. SOLAR-SCALED MODELS (2016, background): “The implementation of low temperature molecular opacities in MESA, which have been shown to play an important role in AGB evolution (Marigo 2002; Marigo et al. 2003), is a high priority for the MIST project.”