1 Introduction

The far infrared (IR) spectra of carbon-rich evolved objects; i.e., carbon-rich AGB stars (C-stars), post asymptotic giant branch objects (post-AGBs) and planetary nebulae (PNe) are typified by a broad emission feature around 30 $\mu$m. This "30'' $\mu$m feature was first discovered in the far-IR spectra of CW Leo, IC 418 and NGC 6572 by Forrest et al. (1981). Since then this feature has been detected in a wide range of carbon-rich evolved objects from intermediate mass loss C-stars (Yamamura et al. 1998) to post-AGBs and PNe (Omont 1993; Cox 1993; Omont et al. 1995; Jiang et al. 1999; Szczerba et al. 1999; Hony et al. 2001). The feature is commonly found in C-rich post-AGBs and PNe however with varying band shapes and varying feature to continuum ratios (Goebel & Moseley 1985; Waters et al. 2000; Hrivnak et al. 2000; Volk et al. 2002)

Goebel & Moseley (1985) proposed solid magnesium sulfide (MgS) as the possible carrier of the "30'' $\mu$m feature. Their suggestion is based on the coincidence of the emission feature with the sole IR-resonance of MgS (Nuth et al. 1985; Begemann et al. 1994) and the fact that MgS is one of the expected condensates around these objects (Lattimer et al. 1978; Lodders & Fegley 1999). Several authors have taken up on this suggestion and compared observations with laboratory measurements of MgS. These comparisons were further facilitated by the publication of the optical constants of MgS in the IR range by Begemann et al. (1994). These authors found that the far IR excess of CW Leo can be successfully modelled using MgS grains with a broad shape distribution.

More recently, Jiang et al. (1999) and Szczerba et al. (1999) have modelled the spectra taken with the Short Wavelength Spectrometer (SWS) (de Graauw et al. 1996) on-board the Infrared Space Observatory (ISO) (Kessler et al. 1996) of the C-star IRAS 03313+6058 and the post-AGB object IRAS 04296+3429 respectively. They find that for these sources which show a strong "30'' $\mu$m feature, the elemental abundances of Mg and S are consistent with MgS as the carrier of the feature.

Hrivnak et al. (2000) and Volk et al. (2002) have analysed ISO spectra of a sample of post-AGBs. They find that the profile of the "30'' $\mu$m feature varies between sources. Although these authors state that this decomposition is not unique, they find that their "30'' $\mu$m feature is composed of two sub features: one feature peaking near 26 $\mu$m and an other near 30 $\mu$m. Using these two components in varying relative amounts they are able to explain the range of features found in their sample. Based on the discovery of these sub features they consider the carrier(s) of the "30'' $\mu$m feature to be unidentified.

Other materials have also been proposed as carriers of the "30'' $\mu$m feature. Duley (2000) suggests that the "30'' $\mu$m feature may be indicative of carbon-based linear molecules with specific side groups. Such molecules have strong absorption bands throughout the 15-30 $\mu$m range. Papoular (2000) discusses the possible contribution of carbonaceous dust grains with oxygen in the structure. Some of these materials may show IR emission in the 20-30 $\mu$m range. Since the optical properties of such grains are sensitive to the exact composition they might be able to explain the range of features found in the C-rich evolved stars. Recently, Grishko et al. (2001) have proposed hydrogenated amorphous carbon (HAC) as a possible carrier of the "30'' $\mu$m feature.

The ISO mission has provided an excellent database of observations to study the properties of the "30'' $\mu$m feature in detail and test the suggested identifications systematically. The wavelength coverage of the SWS instrument (2-45 $\mu$m) is sufficient to determine a reliable continuum. The sensitivity of the ISO spectrograph allows detection of relatively weak features. The resolving power of the instrument ($\lambda$/ $\Delta\lambda = 500$-1500) makes it feasible to study possible substructure in the "30'' $\mu$m feature. Thus these observations allow a study of the "30'' $\mu$m feature in unprecedented detail in a large sample of sources.

In this paper, we investigate the shape and strength of the "30 $\mu$m'' in a wide range of objects from visual visible C-stars, extreme C-stars, post-AGBs to PNe in order to further test the MgS or other identifications and map systematic differences between the feature in different classes of sources.

Our paper is organised as follows. In Sect. 2, we describe the sample and the data reduction. In Sect. 3, we present the way in which we modelled the continuum in order to extract the feature properties. In Sect. 4, we present the full range of extracted profile shapes and peak positions of the "30'' $\mu$m feature and we discuss the possible ways of interpreting the observed profiles. In Sect. 5, we develop a simple model using MgS for the "30'' $\mu$m feature. In Sect. 6, we present the model results and compare them to the astronomical spectra. In Sect. 7, we present a correlation study between several feature properties and stellar parameters. Finally, in Sect. 8, we discuss the implications of our model results and the consequences for the MgS identification. In particular, we discuss possible causes for the deviating profiles and the possibility that MgS produced in carbon-rich evolved stars will be incorporated in the interstellar medium (ISM).