Scientific Highlights with the SMA

A Spiral and Bipolar Outflow in CIT 6: Unwinding the Mysteries of a Carbon Star

Protoplanetary nebulae (pPNe) often consist of outer rings (or arcs) and inner bipolar (or multipolar) structures. The coexistence of such geometrically distinct structures is commonly interpreted as the evidence of the shape transition from spherically-symmetric circumstellar envelopes of asymptotic giant branch (AGB) stars to highly-asymmetric PNe. We have used the SubMillimeter Array (SMA) to obtain a high-resolution CO map of CIT 6. It presents both outer spiral-shell pattern and inner nascent bipolar (or possibly multipolar) structure, suggesting that CIT 6 is at the transition from the AGB to post-AGB in particular in a binary system. The dynamical timescale of the spiral-shell pattern of 150-300 years corresponds to the orbital period of a binary system, while the bipolar (or multipolar) structure is an expected outcome of binary interaction. Further details of the observed features (double spiral and one-sided gap) are suggested as the evidence of a highly eccentric orbit (Kim et al. 2015, ApJ, 814, 61).

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Figure 1. Spiral-shell circumstellar pattern of CIT 6 is revealed in our SMA CO J=2-1 channel maps, in blue, shown atop the earlier VLA HC₃N J=4-3 (Claussen et al. 2011) in red. The channel center velocity with respect to the systemic velocity is labeled at the top left corner of each panel. The CO and HC₃N beam sizes are denoted at the bottom right.

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Figure 2. Non-spherical outflow in the central region of CIT 6 at the channel velocities of -19 km s^-1 (blue) and 19 km s^-1 (red). Dashed lines represent the suggested multipolar directions of the observed asymmetric outflow features. The contour levels 8σ, 12σ, and 16σ. The AGB star is located at the coordinate center. The beam is denoted at the bottom left.