Witnessing the emergence of a carbon star
Zijlstra, A. A.
Boffin, H. M. J.
Sloan, G. C.
Redman, M. P.
Karakas, A. I.
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Guzman-Ramirez, L. Lagadec, E.; Wesson, R.; Zijlstra, A. A.; Muller, A.; Jones, D.; Boffin, H. M. J.; Sloan, G. C.; Redman, M. P.; Smette, A.; Karakas, A. I.; Nyman, L.-A. (2015). Witnessing the emergence of a carbon star. Monthly Notices of the Royal Astronomical Society: Letters 451 (1), L1-L5
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 degrees 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 similar to 10(3) yr ago.