An optical absorption study of the Helix Planetary Nebula: NaI and CaII lines, and a search for diffuse bands

TL;DR

This study investigates the Helix planetary nebula through optical absorption spectra, focusing on NaI and CaII lines, and finds strong NaI absorption but no diffuse interstellar bands, indicating neutral or molecular material without DIBs.

Contribution

It provides the first detailed high-resolution spectral analysis of the Helix PN's circumstellar material, searching for diffuse bands and characterizing NaI and CaII absorption features.

Findings

Strong NaI absorption in lines of sight near the PN

No CaII lines observed, indicating Ca depletion

No diffuse interstellar bands detected in the circumstellar material

Abstract

We present the results of an optical absorption study of NGC 7293, the Helix planetary nebula (PN), designed to search for diffuse band (DIB) absorptions perhaps arising in the carbon-rich circumstellar matter of the PN. We made high resolution spectra of 8 stars located close to and behind the Helix. Through a detailed analysis of the NaI and CaII lines, we find that two lines of sight situated close to the PN as mapped in CO and HI exhibit very strong NaI absorption. This absorption is due to the PN neutral or molecular material. No corresponding CaII line is observed in agreement with the very high Ca depletion often observed in PNs. No trace of any DIB features was found in these two circumstellar lines of sight (shortened abstract).