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

¹ Laboratorio de Astrofísica Experimental, Escuela Politécnica Superior de Alcoy, Universidad Politécnica de Valencia, Plaza de Ferrándiz y Carbonell, 03801 Alcoy, Alicante, Spain e-mail: ralunam@fis.upv.es
² Herschel Science Centre, Research and Scientific Support Department of ESA, European Space Astronomy Centre, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
³ W. J. McDonald Observatory. The University of Texas at Austin. 1 University Station, C1400. Austin, TX 78712-0259, USA
⁴ LUAN, Université de Nice Sophia Antipolis, Parc Valrose, 06108 Nice Cedex 2, France

Received: 27 March 2006
Accepted: 6 November 2007

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 analyzing 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. This 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.