An ALMA Spectral Line Survey of Globules B and C in the Helix Nebula

This project seeks to examine the structure of two small, dense clumps of gas, here referred to as Globules B and C, within the ionized region of the expansive, molecule-rich shell of the Helix planetary nebula (NGC 7293). Twelve molecular transition lines were studied, including isotopologues of CO, and transitions of C H, HCN, HCO , and HNC. Analysis of the integrated intensity, column density, and velocity distribution of each molecular line indicates that higher densities are found closest to the central star, with more diffuse gas trailing radially outward. CO appears the most abundant molecule, and is optically thick across both globules. Lower energy transitions of HCN, HNC, and HCO are also found throughout the globules, though centered predominantly around the higher density gas in the heads. The remaining molecular transitions are similarly relegated to the clumpy distribution of dense cores throughout the globules. Analysis of the distribution and spread of these molecules also indicates that Globule B is composed of several smaller globulettes, each with an individual dense head that is connected by trailing gas. Line ratios of molecules detected at multiple transitions suggest that higher energy transitions are more readily excited toward the central star-facing surface of the globule, with lower transitions found more broadly throughout the globules.