[Paper Review] The 12CO/13CO ratio in AGB stars of different chemical type-Connection to the 12C/13C ratio and the evolution along the AGB

S. Ramstedt, H. Olofsson|arXiv (Cornell University)|May 25, 2014

Stellar, planetary, and galactic studies1 references56 citations

TL;DR

This study presents the first observational evidence linking the 12CO/13CO ratio in circumstellar envelopes to stellar evolution along the AGB, using detailed radiative transfer modeling of radio line data from 55 AGB stars (M-, S-, and carbon-type). It finds a systematic increase in the 12CO/13CO ratio from M-type (median 13) to S-type (26) to carbon stars (34), with a significantly larger spread in carbon stars, supporting their role as evolutionary endpoints of AGB evolution.

ABSTRACT

The aim of this paper is to investigate the evolution of the 12C/13C ratio along the AGB through the circumstellar 12CO/13CO ratio. This is the first time a sample including a significant number of M- and S-type stars is analysed together with a carbon-star sample of equal size, making it possible to investigate trends among the different types and establish evolutionary effects. The circumstellar 12CO/13CO abundance ratios are estimated through a detailed radiative transfer analysis of single-dish radio line emission observations. First, the 12CO radiative transfer is solved, assuming an abundance (dependent on the chemical type of the star), to give the physical parameters of the gas, i.e. mass-loss rate, gas expansion velocity, and gas temperature distribution. Then, the 13CO radiative transfer is solved using the results of the 12CO model giving the 13CO abundance. Finally, the 12CO/13CO abundance ratio is calculated. The circumstellar 12CO/13CO abundance ratio differs between the three spectral types. This is consistent with what is expected from stellar evolutionary models assuming that the spectral types constitute an evolutionary sequence; however, this is the first time this has been shown observationally for a relatively large sample covering all three spectral types. The median value of the 13CO abundance in the inner circumstellar envelope is 1.6x10^-5, 2.3x10^-5, and 3.0x10^-5 for the M-type, S-type, and carbon stars of the sample, respectively, corresponding to 12CO/13CO abundance ratios of 13, 26, and 34, respectively. Interestingly, the abundance ratio spread of the carbon stars is much larger than for the M- and S-type stars, even when excluding J-type carbon stars, in line with what could be expected from evolution on the AGB. We find no correlation between the isotopologue ratio and the mass-loss rate, as would be expected if both increase as the star evolves.

Motivation & Objective

• To investigate the evolution of the 12C/13C isotopic ratio along the AGB by measuring the circumstellar 12CO/13CO abundance ratio.
• To test whether the spectral types M, S, and carbon stars represent an evolutionary sequence by comparing isotopic ratios across types.
• To assess the influence of mass-loss rate on isotopic ratios, challenging the expectation of a correlation with evolutionary stage.
• To evaluate the reliability of circumstellar 12CO/13CO ratios as tracers of the intrinsic stellar 12C/13C ratio.
• To explore the connection between AGB stars and post-AGB objects by comparing isotopic ratios across evolutionary phases.

Proposed method

• Conducted single-dish radio line observations of multiple 12CO and 13CO transitions across 55 AGB stars of different chemical types.
• Applied a 3D non-LTE radiative transfer code based on the Monte Carlo method to model the radiative transfer in the circumstellar envelope.
• Solved the statistical equilibrium equations and energy balance self-consistently, including thermal dust radiation from SEDs.
• First modeled 12CO emission to derive physical parameters (mass-loss rate, expansion velocity, temperature structure), then used these to derive 13CO abundance.
• Calculated the 12CO/13CO abundance ratio from the derived 13CO abundance and 12CO optical depth.
• Benchmarked the radiative transfer model against similar codes to ensure reliability and consistency.

Experimental results

Research questions

• RQ1 Do the 12CO/13CO ratios in circumstellar envelopes systematically increase from M-type to S-type to carbon-rich AGB stars, as predicted by AGB evolution models?
• RQ2 Is there a correlation between the 12CO/13CO ratio and the mass-loss rate, as would be expected if both increase with evolutionary time?
• RQ3 How do the observed isotopic ratios in AGB stars compare to those in post-AGB stars, and what does this imply about the evolutionary connection between them?
• RQ4 To what extent do the observed 12CO/13CO ratios reflect the intrinsic stellar 12C/13C ratio, and is detailed radiative transfer necessary for accurate abundance determination?
• RQ5 Why is the spread in 12CO/13CO ratios significantly larger in carbon stars compared to M- and S-type stars?

Key findings

• The median 12CO/13CO abundance ratio increases from 13 for M-type stars to 26 for S-type stars and 34 for carbon stars, supporting an evolutionary sequence from M to C stars.
• The 13CO abundance in the inner circumstellar envelope is 1.6 × 10⁻⁵ for M-type, 2.3 × 10⁻⁵ for S-type, and 3.0 × 10⁻⁵ for carbon stars, indicating a systematic increase in 13C abundance with evolutionary stage.
• The spread in 12CO/13CO ratios, quantified as the 90th/10th percentile ratio, is 4 for M-type, 3 for S-type, and 15 for carbon stars, with the latter showing a much broader distribution consistent with AGB evolution and limited ¹²C dredge-up before carbon star formation.
• No significant correlation is found between the 12CO/13CO ratio and the mass-loss rate, challenging the assumption that both increase monotonically with time on the AGB.
• Circumstellar 12CO/13CO ratios derived from detailed radiative transfer modeling are representative of the intrinsic stellar 12C/13C ratio, validating their use as evolutionary tracers.
• The isotopologue ratio distribution in post-AGB stars is more similar to that of M-type AGB stars than to carbon stars, suggesting a possible evolutionary link from M-type stars or a bias toward massive progenitors in post-AGB objects.