panel in Fig. \[fig:map\_superimposed\], we note again that the shells and arcs in C$_3$N are well matched by similar structures in HC$_3$N. Also note that the well-defined outer arcs indicated by the lowest contour in the NW and SW directions in C$_3$N are well matched by tenuous, although appreciable, features in HC$_3$N. In this case, HC$_3$N shows shells inward of $\sim$14$''$ that are not seen in its radical C$_3$N, which leads us to conclude that, in the framework of an expanding envelope, the cyanopolyyne HC$_3$N appears in regions where the radical C$_3$N has not yet been formed.

![Brightness distributions of the cyanides HC$_3$N, HC$_5$N, C$_3$N, and CN along a strip intersecting the star at PA = 45$^\circ$. Curves correspond to the maps shown in Fig. \[fig:map\_all\] and have been arbitrarily scaled and shifted in the $y$-axis for display. Horizontal dashed lines indicate the zero of intensity. Grey vertical shadows indicate the positions of brightness maxima in the SW direction from the star, at 11$''$, $15{\mbox{\rlap{.}$''$}}5$, $21{\mbox{\rlap{.}$''$}}5$, and 27$''$.[]{data-label="fig:strip"}](fig_strip){width="\columnwidth"}

Fig. \[fig:strip\] shows in a different way how emission distributions of different species match each other. In this plot we have extracted the surface brightness along a strip intersecting the star at 