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

Abstract

Context. Many post-asymptotic giant branch (post-AGB) stars in binary systems have an infrared (IR) excess arising from a dusty circumbinary disk. The disk formation, current structure, and further evolution are, however, poorly understood. Aims. 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 and of the crystalline dust. Methods. We present very high-quality mid-IR interferometric data that were obtained with the MIDI/VLTI instrument. We analyze the MIDI visibilities and differential phases in combination with the full spectral energy distribution, 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 in our models. The spatial distribution of crystalline forsterite in the disk is investigated with the mid-IR features, the 69 µm band and the 11.3 µm signatures in the interferometric data. Results. All the data are well fitted by our best model. The inclination and position angle of the disk are precisely 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. The best-fit dust grain size distribution shows that significant grain growth has occurred, with a significant amount of mm-sized grains now being settled to the midplane of the disk. A large total dust mass ≥10 −3 M is needed to fit the mm fluxes. By assuming α turb = 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. Conclusions. The disk in the AC Her system is in a very evolved state, as shown by 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 the disk of AC Her, but we find strong similarities with the protoplanetary disk HD 100546.

TheWISEview of RV Tauri stars

“…Another good recently studied example of an evolved disc is AC Her (Hillen et al 2015) whose interfermetric measurements show that the inner radius of the disc is large, the large grains have been settled to the midplane and the gas/dust ratio is small. Interestingly also BD+33 • 2642, the central star of the PN G052.7+50.7 was found to be a depleted (Napiwotzki et al 1994) and recently found to be a binary as well .…”

“…Also the dust component in these two objects was presumed to be C-rich. Both assertions are, however, no longer supported by more recent works, like the photospheric abundance analysis of Van Winckel et al (1998) and the dust studies of , , , Gielen et al (2007Gielen et al ( , 2009 ;Hillen et al (2015). One exception may be V453 Oph, which shows a mild s-process overabundance but it is not accompanied by C enhancement (Deroo et al 2005).…”

“…de Ruyter et al 2006;Deroo et al 2006Deroo et al , 2007Gielen et al 2011;Hillen et al 2015). The dust grain processing both in dust grain growth and crystallinity is very strong, which are indicators for the longevity (e.g.…”

Spitzer Irac Observations of Ir Excess in Holmberg Ix X-1: A Circumbinary Disk or a Variable Jet?

“…In this case the blackbody model that we fit to the infrared data would represent emission from the inner circumbinary disk, while the redder 5.8 and 8.0 μm excess is likely emanating from the outer portion of the circumbinary disk as lowertemperature blackbody emission. The four IRAC data points that compose the IR excess in HoIX X-1 are not sufficient to fit a complicated circumbinary disk model (e.g., Akeson et al 2007;Hillen et al 2015); however, in its simplest form, the circumbinary disk model is very similar to an irradiated disk model, only the source of illumination in the former case is the accretion disk and the star. We therefore fit a very simple p-free disk model to the four IRAC points.…”

An N Band Interferometric Survey of The Disks Around Post-AGB Binary Stars

“…Disks with gaps or large inner holes have depressed near-to mid-IR fluxes (as is the case for AC Her, Hillen et al (2015)). …”