Tracing the asymmetry in the envelope around the carbon star CIT 6

TL;DR

This study uses high-resolution observations to reveal asymmetric, clumpy, and possibly spiral structures in the envelope of the carbon star CIT 6, indicating complex mass-loss processes and high HC$_3$N abundance.

Contribution

It provides detailed imaging of CIT 6's envelope, showing asymmetric shells and potential spiral structures, and suggests binary interaction influences envelope shaping and chemistry.

Findings

Envelope has asymmetric, clumpy shells with a spiral feature.

Mass loss is time-modulated and highly anisotropic.

High HC$_3$N abundance possibly due to binary interactions.

Abstract

We present high angular resolution observations of HC$_3$N J=5--4 line and 7 mm continumm emission from the extreme carbon star CIT 6. We find that the 7 mm continuum emission is unresolved and has a flux consistent with black-body thermal radiation from the central star. The HC$_3$N J=5--4 line emission originates from an asymmetric and clumpy expanding envelope comprising two separate shells of HC$_3$N J=5--4 emission: (i) a faint outer shell that is nearly spherical which has a radius of 8\arcsec; and (ii) a thick and incomplete inner shell that resembles a one-arm spiral starting at or close to the central star and extending out to a radius of about 5\arcsec. Our observations therefore suggest that the mass loss from CIT 6 is strongly modulated with time and highly anisotropic. Furthermore, a comparison between the data and our excitation modelling results suggests an unusually high abundance of HC$_3$N in its envelope. We discuss the possibility that the envelope might be shaped by the presence of a previously suggested possible binary companion. The abundance of HC$_3$N may be enhanced in spiral shocks produced by the interaction between the circumstellar envelope of CIT 6 and its companion star.