  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 disk. 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 (PN) BD+30 3639 which trace both carbon-rich
polycyclic aromatic hydrocarbons (PAHs) and oxygen-rich silicate dust. With the
superior spectral coverage of SOFIA, and using a 3D photoionisation 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 ~1000yr ago.
