The evolved circumbinary disk of AC Her: a radiative transfer, interferometric and mineralogical study

Abstract

We aim to constrain the structure of the circumstellar material around the post-AGB binary and RV Tauri pulsator AC Her. We want to constrain the spatial distribution of the amorphous as well as of the crystalline dust. We present very high-quality mid-IR interferometric data that were obtained with MIDI/VLTI. We analyse the MIDI data and the full SED, using the MCMax radiative transfer code, to find a good structure model of AC Her's circumbinary disk. We include a grain size distribution and midplane settling of dust self-consistently. The spatial distribution of crystalline forsterite in the disk is investigated with the mid-IR features, the 69~$\mu$m band and the 11.3~$\mu$m signatures in the interferometric data. All the data are well fitted. The inclination and position angle of the disk are well determined at i=50+-8 and PA=305+-10. We firmly establish that the inner disk radius is about an order of magnitude larger than the dust sublimation radius. Significant grain growth has occurred, with mm-sized grains being settled to the midplane of the disk. A large dust mass is needed to fit the sub-mm fluxes. By assuming α=0.01, a good fit is obtained with a small grain size power law index of 3.25, combined with a small gas/dust ratio <10. The resulting gas mass is compatible with recent estimates employing direct gas diagnostics. The spatial distribution of the forsterite is different from the amorphous dust, as more warm forsterite is needed in the surface layers of the inner disk. The disk in AC Her is very evolved, with its small gas/dust ratio and large inner hole. Mid-IR interferometry offers unique constraints, complementary to mid-IR features, for studying the mineralogy in disks. A better uv coverage is needed to constrain in detail the distribution of the crystalline forsterite in AC Her, but we find strong similarities with the protoplanetary disk HD100546.

update with final version published in A&A

References in corpus (11)

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Cited by in corpus (13)

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• A mid-IR interferometric survey with MIDI/VLTI: resolving the second-generation protoplanetary disks around post-AGB binaries
• The structure of jets launched from post-AGB binary systems
• Keplerian disks and outflows in post-AGB stars: AC Herculis, 89 Herculis, IRAS 19125+0343, and R Scuti
• Resolved imaging of the AR Puppis circumbinary disk
• What does a typical full-disc around a post-AGB binary look like? -- Radiative transfer models reproducing PIONIER, GRAVITY, and MATISSE data
• Spitzer IRAC observations of IR excess in Holmberg IX X-1: A circumbinary disk or a variable jet?
• Second-generation protoplanetary discs around evolved binaries: a high-resolution polarimetric view with SPHERE/IRDIS
• Multiwavelength Observations of the RV Tauri Variable System U Monocerotis: Long-Term Variability Phenomena That Can Be Explained by Binary Interactions with a Circumbinary Disk
• A SOFIA FORCAST Grism Study of the Mineralogy of Dust in the Winds of Proto-planetary Nebulae: RV Tauri Stars and SRd Variables