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Recent Science Highlights

Persistent Asymmetric Structure of Sagittarius A* on Event Horizon Scales

Constraints on a two point-source model for Sgr A* based on closure phase measurements. Offsets between two point sources that would produce closure phases between +0°9 and +14°9 at all triangles of (u,v) coordinates sampled by our data. For a unit point source centered at the origin, the colored regions indicate the allowed offset of a second point source, with color indicating the maximum value of the flux ratio between the two components. The circle shows an offset of 50 μas, which is approximately the diameter of the predicted shadow in Sgr A*. For two components separated by the shadow diameter, a roughly east-west alignment with the brighter component to the west is required to be consistent with our data.

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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, 61ADS).

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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.

Dispersing Protostellar Envelope around the Keplerian Circumbinary Disk in L1551 NE

We have performed mapping observations of the protostellar envelope surrounding the Class I protostellar binary L1551 NE in the C18O (3-2) line with the ASTE (Atacama Submillimeter Telescope Experiment) telescope, and combined the submillimeter single-dish mapping data with the our previous SMA data. Our previous SMA data reveal a r~300 AU scale Keplerian Disk surrounding the protostellar binary (Takakuwa et al. 2012, ApJ, 754, 52), and our subsequent ALMA observations of L1551 NE unveil an infalling gas motion to feed materials to the protostellar binary in the innermost part of the circumbinary disk (Takakuwa et al. 2014, ApJ, 796, 1). Our new ASTE map shows a r~10000 AU protostellar envelope, and the combined SMA+ASTE image reveals that the Keplerian circumbinary disk is embedded in the protostellar envelope. In contrast to the inner Keplerian rotation, the outer envelope does not show any clear rotation, but shows dispersing gas motion away from the central disk. The dispersing gas motion is caused by the interaction with the molecular outflow driven from the neighboring protostar L1551 IRS 5. The presence of the dispersing gas motion in the protostellar envelope implies that the mass and mass ratio of the protostellar binary of L1551 NE has already been fixed (Takakuwa et al. 2015, ApJ, in press).

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