Amorphous carbon in the disk around the post-AGB binary HR 4049

Winckel

et al. 2017

A&A

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Aims. We present a mid-IR interferometric survey of the circumstellar environment of a specific class of post-asymptotic giant branch (post-AGB) binaries. For this class the presence of a compact dusty disk has been postulated on the basis of various spatially unresolved measurements. The aim is to determine the angular extent of the N-band emission directly and to resolve the compact circumstellar structures. Methods. Our interferometric survey was performed with the MIDI instrument on the VLTI. In total 19 different systems were observed using variable baseline configurations. Combining all the visibilities at a single wavelength at 10.7 µm, we fitted two parametric models to the data: a uniform disk and a ring model mimicking a temperature gradient. We compared our observables of the whole sample, with synthetic data computed from a grid of radiative transfer models of passively irradiated disks in hydrostatic equilibrium. These models are computed with a Monte Carlo code that has been widely applied to describe the structure of protoplanetary disks around young stellar objects (YSO). Results. The spatially resolved observations show that the majority of our targets cluster closely together in the distance-independent size-colour diagram, and have extremely compact N-band emission regions. The typical uniform disk diameter of the N-band emission region is ∼40 mas, which corresponds to a typical brightness temperature of 400-600 K. The resolved objects display very similar characteristics in the interferometric observables and in the spectral energy distributions. Therefore, the physical properties of the disks around our targets must be similar. Our results are discussed in the light of recently published sample studies of YSOs to compare quantitatively the secondary discs around post-AGB stars to the ones around YSOs. Conclusions. Our high-angular-resolution survey further confirms the disk nature of the circumstellar structures present around wide post-AGB binaries. The grid of protoplanetary disk models covers very well the observed objects. Much like for young stars, the spatially resolved N-band emission region is determined by the hot inner rim of the disk. Continued comparisons between post-AGB and protoplanetary disks will help to understand grain growth and disk evolution processes, and to constrain planet formation theories. These second-generation disks are an important missing ingredient in binary evolution theory of intermediate-mass stars.

Section: Hr4049 (Nr 7)mentioning

confidence: 99%

A mid-IR interferometric survey with MIDI/VLTI: resolving the second-generation protoplanetary disks around post-AGB binaries

Winckel

et al. 2017

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“…The MCMax code (Min et al 2009) has been widely and successfully applied to model observables obtained for gas-rich disks in a variety of environments (Mulders et al 2011;Verhoeff et al 2011;Acke et al 2013). We refer to these papers for a full description of the basic features of the MCMax code.…”

Section: The Codementioning

confidence: 99%

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

Vries

et al. 2015

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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.

“…Our choice to explore models with amorphous C was inspired by the recent result of Acke et al (2013), and by the need to investigate whether a change in dust composition can significantly affect the predicted amount of scattered light by the disk (see Sect. 8.5).…”

Section: The Dust Compositionmentioning

confidence: 99%

“…8.5). Acke et al (2013) claim that the continuum opacity source in the circumbinary disk of HR4049 can be identified as amorphous carbon, by confronting radiative transfer models with near-and mid-IR interferometric data and the IR ISO spectrum. They state that the identification can be made in essence because the temperatures reached by various dust species at a given physical distance to the star are very different.…”

Section: The Dust Compositionmentioning

confidence: 99%

“…model SEDs and visibilities. The MCMax code has been successfully used to model both spatially and spectrally resolved observations of different kinds of gas-rich disks (Mulders et al 2011;Verhoeff et al 2011;Acke et al 2013). Our modeling approach was basically the same as presented in Mulders & Dominik (2012) and we refer to this paper and Min et al (2009) for a full account of the procedures and physics implemented in MCMax.…”

Section: Introducing Mcmaxmentioning

confidence: 99%

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An interferometric study of the post-AGB binary 89 Herculis

Winckel

et al. 2014

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Context. The presence of stable disks around post-asymptotic giant branch (post-AGB) binaries is a widespread phenomenon. Also, the presence of (molecular) outflows is now commonly inferred in these systems. Aims. In the first paper of this series, a surprisingly large fraction of optical light was found to be resolved in the 89 Her post-AGB binary system. The data showed that this flux arises from close to the central binary. Scattering off the inner rim of the circumbinary disk, or scattering in a dusty outflow were suggested as two possible origins. With detailed dust radiative transfer models of the circumbinary disk, we aim to discriminate between the two proposed configurations. Methods. By including Herschel/SPIRE photometry, we extend the spectral energy distribution (SED) such that it now fully covers UV to sub-mm wavelengths. The MCMax Monte Carlo radiative transfer code is used to create a large grid of disk models. Our models include a self-consistent treatment of dust settling as well as of scattering. A Si-rich composition with two additional opacity sources, metallic Fe or amorphous C, are tested. The SED is fit together with archival mid-IR (MIDI) visibilities, and the optical and near-IR visibilities of Paper I. In this way we constrain the structure of the disk, with a focus on its inner rim. Results. The near-IR visibility data require a smooth inner rim, here obtained with a double power-law parameterization of the radial surface density distribution. A model can be found that fits all of the IR photometric and interferometric data well, with either of the two continuum opacity sources. Our best-fit passive models are characterized by a significant amount of ∼mm-sized grains, which are settled to the midplane of the disk. Not a single disk model fits our data at optical wavelengths because of the opposing constraints imposed by the optical and near-IR interferometric data. Conclusions. A geometry in which a passive, dusty, and puffed-up circumbinary disk is present, can reproduce all of the IR, but not the optical observations of 89 Her. Another dusty component (an outflow or halo) therefore needs to be added to the system.