Abstract
Forsterite is one of the crystalline dust species that is often observed in protoplanetary disks and solar system comets. Being absent in the interstellar medium, it must be produced during the disk lifetime. It can therefore serve as a tracer of dust processing and disk evolution, which can lead to a better understanding of the physical processes occurring in the disk, and possibly planet formation. However, the connection of these processes with the overall disk crystallinity remains unclear. We aim to characterize the forsterite abundance and spatial distribution in the disk of the Herbig Be star HD 100546, to investigate if a connection exists with the large disk gap. We use a 2D radiative transfer code, MCMax, to model the circumstellar dust around HD 100546. We use VISIR Q-band imaging to probe the outer disk geometry and mid-infrared features to model the spatial distribution of forsterite. The temperature-dependent shape of the 69 micron feature observed with Herschel PACS is used as a critical tool to constrain this distribution. We find a crystalline mass fraction of 40 - 60 %, located close to the disk wall between 13 and 20 AU, and possibly farther out at the disk surface. The forsterite is in thermal contact with the other dust species. We put an upper limit on the iron content of forsterite of 0.3 %. Optical depth effects play a key role in explaining the observed forsterite features, hiding warm forsterite from view at short wavelengths. The disk wall acts as a showcase: it displays a localized high abundance of forsterite, which gives rise to a high observed crystallinity, while the overall mass fraction of forsterite is a factor of ten lower.
Highlights
Herbig Ae/Be stars are intermediate-mass pre-main-sequence stars first described as a group by Herbig (1960)
We have studied the spatial distribution of crystalline silicate dust in the gapped disk of Herbig Be star HD 100546
We have used a 2D radiative transfer code to model the dust geometry, and study the effects of optical depth on the forsterite feature strengths in the mid and far-infrared, as well as the shape of the temperature-dependent 69 μm feature observed with Herschel/PACS
Summary
Herbig Ae/Be stars are intermediate-mass pre-main-sequence stars first described as a group by Herbig (1960). They are characterized by the presence of a circumstellar gas and dust disk Waters & Waelkens 1998), which is the remnant of the star formation process. There is a wealth of observational evidence suggesting that these disks are the site of planet formation. Imaging at optical, infrared and millimeter wavelengths shows the presence of disk gaps and/or inner holes Grady et al 2005; Verhoeff et al 2011), pointing to clearing Imaging at optical, infrared and millimeter wavelengths shows the presence of disk gaps and/or inner holes (e.g. Grady et al 2005; Verhoeff et al 2011), pointing to clearing
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