A search for diffuse bands in the circumstellar envelopes of post-AGB stars

TL;DR

This study systematically searched for diffuse bands in the circumstellar envelopes of post-AGB stars, finding that these features are likely not formed in their circumstellar environments but originate from interstellar space.

Contribution

It provides the first systematic analysis of diffuse bands in post-AGB stars' circumstellar environments, concluding their absence in such regions under typical conditions.

Findings

Diffuse bands are weak or absent in post-AGB circumstellar envelopes.

Radial velocity analysis confirms interstellar origin of observed features.

Results are consistent across different chemical compositions and spectral types.

Abstract

In this work we present the results of a systematic search for diffuse bands (DBs, hereafter) in the circumstellar envelopes of a carefully selected sample of post-AGB stars. We concentrated on the analysis of 9 of the DBs most commonly found in the interstellar medium. The strength of these features is determined using high resolution optical spectroscopy and the results obtained are compared with literature data on field stars affected only by interstellar reddening. Based on the weak features observed in the subsample of post-AGB stars dominated by circumstellar reddening we conclude that the carrier(s) of these DBs must not be present in the circumstellar environment of these sources, or at least not under the excitation conditions in which DBs are formed. The conclusion is applicable to all the post-AGB stars studied, irrespective of the dominant chemistry or the spectral type of the star considered. A detailed radial velocity analysis of the features observed in individual sources confirms this result, as the Doppler shifts measured are found to be consistent with an interstellar origin.