ambient ultraviolet field is no longer blocked by the circumstellar dust and gas. In these outer regions, the production of the radicals C$_2$H and CN drives the growth of acetylenic carbon chains ([@gla1986] 1986; [@nej1987] 1987; [@che1993a] 1993; [@che1993b] 1993; [@mil1994] 1994; [@mil2000] 2000; [@agu2009] 2009; [@cor2009] 2009; [@li2014] 2014).

Owing to its close proximity and brightness, most observational studies of circumstellar envelopes have focused on the prototypical carbon star envelope IRC+10216. In this source, brightness distributions of rotational lines of the radicals C$_2$H and CN, and polyyne and cyanopolyyne chains such as C$_4$H, C$_3$N, HC$_3$N, and HC$_5$N have been mapped using single dish radiotelescopes such as IRAM 30m ([@aud1994] 1994) and radio interferometers such as BIMA ([@bie1993] 1993; [@day1995] 1995), IRAM Plateau de Bure ([@gue1993] 1993, 1997, 1999; [@luc1995] 1995), VLA ([@din2008] 2008), and SMA ([@pat2011] 2011; [@coo2015] 2015). Despite their moderate sensitivity and angular resolution (ranging from 2$''$ to 11$''$), these observations were able to constrain the presence of C$_2$H, CN, and the aforementioned carbon chains to an outer hollow shell of a few arcseconds of width located at a distance between 10$''$ and 20$''$ from the star. This finding implies that whatever the mechanism of formation of 