The offset dependent behavior of narrow optical emission features in the Red Rectangle proto-planetary nebula^⋆

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
e-mail: nadine.wehres@colorado.edu
² Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, 3000 Leuven, Belgium
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 26 January 2011
Accepted: 28 June 2011

Abstract

Context. The Red Rectangle proto-planetary nebula provides a unique laboratory to study the physical conditions and chemical processes in stellar outflows. Snapshots of the ongoing chemical evolution are obtained by monitoring spectra as function of the offset from the central star.

Aims. The focus in this study is on the characterization of narrow optical emission features, that are superimposed on top of extended red emission (ERE). The primary aim is to provide a two-dimensional catalogue of these features for offsets varying from 3′′ to 20′′ from the central star.

Methods. Medium resolution emission spectra for this catalogue have been obtained through optical long-slit measurements using the New Technology Telescope (EMMI-NTT) in La Silla, Chile.

Results. The recorded spectra cover the range between 5550 and 6800 Å. A complete overview of the central band positions and bandwidths (FWHMs) is provided for both stronger (previously reported) and weaker narrow features. Only some bands are omnipresent in the nebula outflows and other bands only appear further away from the central star.

Conclusions. The optical emission bands show intensity variations over the nebula. We suggest that these variations reflect a spatially resolved photochemistry where larger species are photolysed, producing daughter molecules which may be the carriers of the optical emission bands.