Abstract
The presence of disks and outflows is widespread among post-AGB binaries. 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 system. The data showed this flux to arise from close to the central binary. Scattering off the inner rim of the circumbinary disk, or in a dusty outflow were suggested as two possible origins. With detailed dust radiative transfer models of the disk we aim to discriminate between these two configurations. By including Herschel/SPIRE photometry, we extend the SED such that it now fully covers UV to sub-mm wavelengths. The MCMax 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 mid-IR (MIDI) visibilities as well as the optical and near-IR visibilities of Paper I, to constrain the structure of the disk and in particular of its inner rim. The near-IR visibility data require a smooth inner rim, here obtained with a two-power-law parameterization of the radial surface density distribution. A model can be found that fits all 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 though, the reason being the opposing constraints imposed by the optical and near-IR interferometric data. A geometry in which a passive, dusty, and puffed-up circumbinary disk is present, can reproduce all the IR but not the optical observations of 89 Her. Another dusty, outflow or halo, component therefore needs to be added to the system.
Highlights
In Hillen et al (2013, hereafter Paper I), we reported the first detection of a large amount (35% of the total) of optical scattered light emerging from the close circumstellar environment of the post-asymptotic giant branch binary 89 Herculis
In this paper we have computed an extensive grid of radiative transfer models of passive disks in hydrostatic equilibrium, using a grain size distribution that includes large grains and a consistent treatment of grain-size-dependent dust settling
We showed that no firm conclusions can be drawn on the composition of the dust, based on the observables used in this paper, without taking the full disk structure into account
Summary
In Hillen et al (2013, hereafter Paper I), we reported the first detection of a large amount (35% of the total) of optical scattered light emerging from the close circumstellar environment of the post-asymptotic giant branch (post-AGB) binary 89 Herculis. Many post-AGB objects display large-scale structures in scattered light (Sánchez Contreras et al 2010; Siódmiak et al 2008; Bujarrabal et al 2002; Ueta et al 2000), but generally the postAGB stars in binary systems do not (and are classified as “stellar” in e.g., Siódmiak et al 2008). These post-AGB binaries (with P ∼ 100–3000 d) do have a circumstellar dust reservoir, but of relatively low mass, in the form of a stable Keplerian circumbinary disk (see, e.g., the census of Galactic sources by de Ruyter et al 2006). If at all, are found at the heart of PPNe (Hrivnak et al 2011)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
