Location and sizes of forsterite grains in protoplanetary disks

Interpretation from the Herschel DIGIT programme

¹ Leiden Observatory, Leiden University, PO Box 9513 2300 RA, Leiden, The Netherlands
e-mail: info@koenmaaskant.nl
² AlbaNova University Centre, Stockholm University, Department of Astronomy, 106 91 Stockholm, Sweden
³ Stockholm University Astrobiology Centre, 106 91 Stockholm, Sweden
⁴ Anton Pannekoek Astronomical Institute, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
⁵ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁶ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010 6500 GL Nijmegen, The Netherlands
⁷ NOVA, PO Box 9513, 2300 RA Leiden, The Netherlands
⁸ Dutch Space BV, PO Box 32070, 2303 DB Leiden, The Netherlands

Received: 7 March 2014
Accepted: 31 May 2014

Abstract

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.