- Location and sizes of forsterite grains in protoplanetary disks: Interpretation from the Herschel DIGIT programme
- Astronomy & Astrophysics
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- Faculty of Science (FNWI)
- Anton Pannekoek Institute for Astronomy (API)
Context. The spectra of protoplanetary disks contain mid- and far- infrared emission features produced by forsterite dust grains. The spectral features contain information about the forsterite temperature, chemical composition and grain size.
Aims. We aim to characterize how the 23 and 69 μm features can be used to constrain the physical locations of forsterite in disks. We check for consistency between two independent forsterite Temperature measurements: the I23/I69 feature strength ratio and the shape of the 69 μm band.
Methods. We performed radiative transfer modeling to study the effect of disk properties to the forsterite spectral features. Temperature-dependent forsterite opacities were considered in self-consistent models to compute forsterite emission from protoplanetary disks.
Results. Modelling grids are presented to study the effects of grain size, disk gaps, radial mixing and optical depth to the forsterite features. Independent temperature estimates derived from the I23/I69 feature strength ratio and the 69 μm band shape are most inconsistent for HD 141569 and Oph IRS 48. A case study of the disk of HD 141569 shows two solutions to fit the forsterite spectrum. A model with T ~ 40 K, iron-rich (~ 0−1% Fe) and 1 μm forsterite grains, and a model with warmer (T ~ 100 K), iron-free, and larger (10 μm) grains.
Conclusions. We find that for disks with low upper limits of the 69 μm feature (most notably in flat, self-shadowed disks), the forsterite must be hot, and thus close to the star. We find no correlation between disk gaps and the presence or absence of forsterite features. We argue that the 69 μm feature of the evolved transitional disks HD 141569 and Oph IRS 48 is most likely a tracer of larger (i.e. ≳10 μm) forsterite grains.
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