Mid-infrared imaging and modelling of the dust shell around post-AGB star HD 187885 (IRAS 19500–1709)

Abstract

We present 10‐ and 20‐μm images of IRAS 19500−1709 taken with the mid‐infrared camera, OSCIR, mounted on the Gemini North Telescope. An extended circumstellar envelope is detected, with the N‐band image indicating an elongation in a north‐east–south‐west direction. We use a dust radiation transport code to fit the spectral energy distribution from ultraviolet to submillimetre wavelengths, with a detached dust shell model. A good fit is achieved using dust composed of amorphous carbon, silicon carbide and magnesium sulphide. We derive estimates for the inner and outer radius, density and mass of the dust in the shell. The inner radius is not resolved in our OSCIR imaging, giving an upper limit of 0.4 arcsec. With this constraint, we conclude that IRAS 19500−1709 must be at least 4 kpc away in order to have the minimum luminosity consistent with a post‐asymptotic giant branch status.

A mid-infrared imaging catalogue of post-asymptotic giant branch stars★

“…Our observations (Fig. B24) show that the envelope around 19500 is extended and elongated along two preferential direction, north‐east/south‐west as mentioned by Clube & Gledhill (2004) but also east/west. A detached shell is clearly resolved at all three wavelengths, with an inner radius of ∼0.4 arcsec.…”

“…IRAS 19500−1709 is a carbon‐rich PPN, with 21 and 30 μm features in its mid‐infrared spectrum (Justtanont et al 1996). It has been imaged in the mid‐infrared with OSCIR/ Gemini ‐North (Clube & Gledhill 2004). An extended circumstellar envelope is detected, and there are some indications that this envelope is elongated along the north‐east/south‐west direction.…”

“…An extended circumstellar envelope is detected, and there are some indications that this envelope is elongated along the north‐east/south‐west direction. Using radiative transfer modelling, Clube & Gledhill (2004) estimated the inner and outer radii of a detached dust shell around this object. They obtained an inner radius of 0.4 arcsec, not resolved with their OSCIR observations.…”

A mid-infrared imaging survey of post-AGB stars

“…IRAS 19500−1709 is a carbon-rich PPN, with 21 microns and 30 microns features in its mid-infrared spectrum (Justtanont et al, 1996). It has been imaged in the mid-infrared with OSCIR/Gemini North (Clube & Gledhill, 2004). An extended circumstellar envelope is detected, and there are some indications that this envelope is elongated along the North-East/South-West direction.…”

Optical Properties and Applications of Silicon Carbide in Astrophysics

“…The optical functions mentioned in Section 3 have been tested in a variety of radiative transfer applications. The optical functions of Bohren & Huffman (1983), Pégourié (1988), and Laor & Draine (1993) were used to place limits on the abundance of SiC dust in carbon stars (e.g., Martin & Rogers 1987;Lorenz-Martins & Lefevre 1993Lorenz-Martins et al 2001;Groenewegen 1995;Groenewegen et al 1998Groenewegen et al , 2009Griffin 1990Griffin , 1993Bagnulo et al 1995Bagnulo et al , 1997Bagnulo et al , 1998, Large Magellanic Cloud stars Srinivasan et al 2010), and (proto-)planetary nebulae (Clube & Gledhill 2004;Hoare 1990;Jiang et al 2005). Those optical functions have also been used in studies of dust formation (e.g., Kozasa et al 1996), hydrodynamics of circumstellar shells (e.g., Windsteig et al 1997;Steffen et al 1997), and mean opacities (Ferguson et al 2005;Alexander & Ferguson 1994).…”