High-Resolution Co Observation of the Carbon Star Cit 6 Revealing the Spiral Structure and a Nascent Bipolar Outflow

Abstract

CIT 6 is a carbon star in the transitional phase from the asymptotic giant branch (AGB) to the protoplanetary nebulae (pPN). Observational evidences of two point sources in the optical, circumstellar arc segments in an HC 3 N line emission, and a bipolar nebula in near-infrared provide strong support for the presence of a binary companion. Hence, CIT 6 is very attractive for studying the role of companions in the AGB-pPN transition. We have carried out high resolution 12 CO J=2-1 and 13 CO J=2-1 observations of CIT 6 with the Submillimeter Array combined with the Submillimeter Telescope (single-dish) data. The 12 CO channel maps reveal a spiral-shell -2pattern connecting the HC 3 N segments in a continuous form, and an asymmetric outflow corresponding to the near-infrared bipolar nebula. Rotation of the 12 CO channel peak position may be related to the inner spiral winding and/or the bipolar outflow. An eccentric orbit binary is suggested for the presences of an anisotropic mass loss to the west and a double spiral pattern. The lack of interarm emission to the west may indicate a feature corresponding to the periastron passage of a highly eccentric orbit of the binary. Spatially-averaged radial and spectral profiles of 12 CO J=2-1 and 13 CO J=2-1 are compared with simple spherical radiative transfer models, suggesting a change of 12 CO/ 13 CO abundance ratio from ∼ 30 to ∼ 50 inward in the CSE of CIT 6. The millimeter continuum emission is decomposed into extended dust thermal emission (spectral index ∼ −2.4) and compact emission from radio photosphere (spectral index ∼ −2.0).

The large-scale nebular pattern of a superwind binary in an eccentric orbit

“…We note that indications of spiral-shell patterns have been observed in the circumstellar molecular line images of AGB stars with high sensitivity and resolution [3,10,11,[25][26][27][28]. Observations such as these enable detailed investigations of the entire nebular structure on spatial scales ranging from several to several thousand times the typical size of an AGB star and facilitate understandings of the morphological patterns of the AGB, pPN, and PN phases.…”

“…Many of these nebulae consist of outer structures that are nearly spherical (spiral/shell/arc/halo) and inner structures that are highly asymmetric (bipolar/multipolar) [1,2]. The coexistence of such geometrically distinct structures is enigmatic because it hints at the simultaneous presence of both wide and close binary interactions, a phenomenon that has been attributed to stellar binary systems with eccentric orbits [3]. Here we report new high-resolution molecular-line observations of the circumstellar spiral-shell pattern of AFGL 3068, an asymptotic giant branch (AGB) star transitioning to the pPN phase.…”

“…Another carbon star, RW Leonis, has an orbital period in the intermediate range (∼ 300 years) [10]. Its rather broadened outflow and the double spiral feature in the central region are proposed as evidence for an eccentric binary [3].…”

Mass transfer and disc formation in AGB binary systems

“…The low mass secondary in orbit around AGB star can shape the AGB wind into a bipolar outflow structure (Nordhaus & Blackman 2006). An observational example is L2 Puppis Kervella et al 2016). Chen et al (2016b) found that 3D hydrodynamic simulation of L2 Puppis and found very good agreement with the morphology and spectral energy distribution.…”

Mass loss of stars on the asymptotic giant branch

“…The number of examples is steadily increasing, e.g., CW Leo Decin et al 2015), RW LMi (Dinh-V-Trung 2009;Claussen et al 2011;Kim et al 2015), R Scl , R Aqr and W Aql (Mayer et al 2013), and π 1 Gru . A prominent example is that of the dust and gas spiral towards AFGL3068 (Mauron and Huggins 2006;Kim et al 2017), Fig.…”