Witnessing the emergence of a carbon star

• Published in: Monthly notices of the Royal Astronomical Society. Letters Vol. 451; no. 1; pp. L1 - L5
• Main Authors: Guzman-Ramirez, L., Lagadec, E., Wesson, R., Zijlstra, A. A., Müller, A., Jones, D., Boffin, H. M. J., Sloan, G. C., Redman, M. P., Smette, A., Karakas, A. I., Nyman, Lars-Åke
• Format: Journal Article
• Language: English
• Published: Oxford University Press 21.07.2015
• Subjects: planetary nebulae: individual: (BD+30 36 39); circumstellar matter; infrared: stars; ISM: abundances
• ISSN: 1745-3925; 1745-3933
• DOI: 10.1093/mnrasl/slv055

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 Faint Object 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° 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.