Guzman-Ramirez et al. 2015, MNRAS, Witnessing the emergence of a carbon star

Witnessing the emergence of a carbon star

L. Guzman-Ramirez (ESO), E. Lagadec (Obs. Cote d'Azur), R. Wesson (ESO) A.A. Zijlstra (Manchester), A. Mueller (ESO), D. Jones (Univ. La Laguna), H.M.J. Boffin (ESO), G.C. Sloan (Cornell), M.P. Redman (National Univ. Ireland), A. Smette (ESO), A.I. Karakas (ANU), L.-A. Nyman (ESO)

2015, MNRAS, 451, L1

Full manuscript available locally (PDF) or from the arXiv (1504.03349). (The local paper is identical to the published version.)

During the late stages of their evolution, Sun-like stars bring the products of nuclear burning to the surface. Most of the carbon in the Universe is believed to originate from stars with masses up to a few solar masses. Although there is a chemical dichotomy between oxygen-rich and carbon-rich evolved stars, the dredge-up itself has never been directly observed. In the last three decades, however, a few stars have been shown to display both carbon- and oxygen-rich material in their circumstellar envelopes. Two models have been proposed to explain this dual chemistry: one postulates that a recent dredge-up of carbon produced by nucleosynthesis inside the star during the Asymptotic Giant Branch changed the surface chemistry of the star. The other model postulates that oxygen-rich material exists in stable keplerian rotation around the central star. The two models make contradictory, testable, predictions on the location of the oxygen-rich material, either located further from the star than the carbon-rich gas, or very close to the star in a stable disc. Using the FaintObject InfraRed CAmera (FORCAST) instrument on board the Stratospheric Observatory for Infrared Astronomy (SOFIA) Telescope, we obtained images of the carbon-rich planetary nebula BD +30^o 3639 which trace both carbon-rich polycyclic aromatic hydrocarbons and oxygen-rich silicate dust. With the superior spectral coverage of SOFIA, and using a 3D photoionization and dust radiative transfer model we prove that the O-rich material is distributed in a shell in the outer parts of the nebula, while the C-rich material is located in the inner parts of the nebula. These observations combined with the model, suggest a recent change in stellar surface composition for the double chemistry in this object. This is evidence for dredge-up occurring ~10³ yr ago.

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