An optical absorption study of the Helix planetary nebula^*

Na I and Ca II lines, and a search for diffuse bands

¹ Groupe d'Astrophysique, UMR 5024 CNRS, Case CC72, Place Bataillon, 34095 Montpellier Cedex 05, France e-mail: mauron@graal.univ-montp2.fr
² Laboratoire d'Astrophysique, Observatoire de Grenoble, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 09, France e-mail: tkendall@obs.ujf-grenoble.fr

Received: 24 May 2004
Accepted: 23 July 2004

Abstract

We present the first results of an optical absorption study of NGC 7293, the Helix planetary nebula (PN), designed to search definitively for diffuse band (DIB) absorptions perhaps arising in the carbon-rich circumstellar matter of the PN. We used the VLT-UVES spectrograph to achieve high resolution () spectra of 8 stars located angularly close to and behind the Helix. These targets were selected through their photometric UBV or 2MASS properties, permitting derivation of spectral types (mainly F-G dwarfs) and distance that place several of them far (700-1500 pc) beyond the Helix (210 pc). Through a detailed analysis of the Na i and Ca ii lines to the 8 targets, we find that two lines of sight situated close to the nebula as mapped in CO and H i exhibit very strong Na i absorption. This absorption is unlikely to arise in the foreground or background interstellar medium which has a relatively low column density, because the Helix is at high galactic latitude . It is much more probable that it is due to the PN neutral or molecular material. This circumstellar origin is reinforced by the fact that no corresponding Ca ii line is observed (as it would usually be from the ISM), which is 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, nor to the other targets. The two circumstellar sightlines discovered in this work open the way to search for molecular species such as C₂ which are not observable at radio wavelengths, and to obtain more information on the rich neutral and molecular content of the Helix.