The circumstellar envelope of the C-rich post-AGB star HD 56126

Abstract

Abstract. We present a detailed study of the circumstellar envelope of the post-asymptotic giant branch "21 µm object" HD 56126. We build a detailed dust radiative transfer model of the circumstellar envelope in order to derive the dust composition and mass, and the mass-loss history of the star. To model the emission of the dust we use amorphous carbon, hydrogenated amorphous carbon, magnesium sulfide and titanium carbide. We present a detailed parametrisation of the optical properties of hydrogenated amorphous carbon as a function of H/C content. The mid-infrared imaging and spectroscopy is best reproduced by a single dust shell from 1.2 to 2.6 radius around the central star. This shell originates from a short period during which the mass-loss rate exceeded 10 −4 M /yr. We find that the strength of the "21" µm feature poses a problem for the TiC identification. The low abundance of Ti requires very high absorption cross-sections in the ultraviolet and visible wavelength range to explain the strength of the feature. Other nano-crystalline metal carbides should be considered as well. We find that hydrogenated amorphous carbon in radiative equilibrium with the local radiation field does not reach a high enough temperature to explain the strength of the 3.3-3.4 and 6-9 µm hydrocarbon features relative to the 11-17 µm hydrocarbon features. We propose that the carriers of these hydrocarbon features are not in radiative equilibrium but are transiently heated to high temperature. We find that 2 per cent of the dust mass is required to explain the strength of the "30" µm feature, which fits well within the measured atmospheric abundance of Mg and S. This further strengthens the MgS identification of the "30" µm feature.Key words. stars: individual: HD 56126 -stars: AGB and post-AGB -stars: carbon -circumstellar matterstars: mass-loss -infrared: stars IntroductionPost-asymptotic giant branch stars (post-AGBs) are objects in which the strong AGB mass-loss has ceased while the remaining star is not (yet) hot enough to ionise the surrounding material. These objects are characterised by a double-peaked spectral energy distribution (SED). In the ultraviolet (UV) and visible range the central star is visible. In the mid-infrared (IR) range the light is dominated by emission from the dusty circumstellar envelope (CSE). Post-AGB stars are the prime objects to study stellar evolution near the very end of the AGB because they are the most recent evolutionary descendants of the AGB stars. Mass loss near the tip of the AGB is not well understood. There are indications that the mass-loss rates near the end of the AGB exceed the values predicted by Send offprint requests to: S. Hony, e-mail: shony@rssd.esa.int Based on observations taken at the European Southern Observatory, La Silla, Chile and observation obtained with ISO, an ESA project with instruments funded by ESA Member states (especially the PI countries: France, Germany, The Netherlands and the UK) with the participation of ISAS and NASA. the existing dust driven wind models...

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“…While MgS has been proposed and often used as the carrier of this feature (Hony et al 2003), recent work has cast Grishko et al 2001;Hony et al 2003), should also be considered. In the SED model for PPN HD 56126, Hony et al (2003) showed that broad emission features around 7-9 μm and 10-13 μm are partly due to HACs, making the identification of the 30 μm band with HACs (Grishko et al 2001) a possibility. We examined whether HAC can be the main contributor to the 10-13 and 30 μm broad features by applying a modified blackbody model to the IRS spectrum, including both MgS and HACs.…”

“…For MgS, we used the optical constants of nearly pure MgS, e.g., Mg 0.9 Fe 0.1 S, from Begemann et al (1994). For the HACs, we adopt the Q λ of HAC in the case of H/(H+C) = 0.3 calculated by Hony et al (2003). In two models, we consider different compositions, consisting of combinations of PAHs, amorphous carbon (AC), SiC, MgS, and HAC ( Figure 9).…”