Full 2D radiative transfer modelling of transitional disk LkCa 15

TL;DR

This study demonstrates the importance of 2D radiative transfer modeling in understanding the complex geometries of transitional disks like LkCa 15, revealing how inner disk structures influence outer disk illumination and observed spectral energy distributions.

Contribution

The paper applies 2D radiative transfer modeling to the LkCa 15 disk, showing how different inner disk geometries affect the outer disk and SED, which was not captured by previous 1+1D models.

Findings

Inner disk in hydrostatic equilibrium fits near-infrared flux but shadows outer disk.

Optically thin inner disk allows smaller outer disk scale height.

Large scale height needed for optically thin inner disk may be physically implausible.

Abstract

With the legacy of Spitzer and current advances in (sub)mm astronomy, a large number of 'transitional' disks has been identified which are believed to contain gaps or have developped large inner holes, some filled with dust. This may indicate that complex geometries may be a key feature in disk evolution that has to be understood and modeled correctly. The disk around LkCa 15 is such a disk, with a gap ranging from ~5 - 50 AU, as identified by Espaillat et al. (2007) using 1+1D radiative transfer modelling. To fit the SED, they propose 2 possible scenarios for the inner (<5 AU) disk - optically thick or optically thin - and one scenario for the outer disk. We use the gapped disk of LkCa 15 as a showcase to illustrate the importance of 2D radiative transfer in transitional disks, by showing how the vertical dust distribution in dust-filled inner holes determines not only the radial optical depth but also the outer disk geometry. We use MCMax, a 2D radiative transfer code with a self-consistent vertical structure, to model the SED. We identify two possible geometries for the inner and outer disk, that are both different from those in Espaillat et al. (2007). An inner disk in hydrostatic equilibrium reprocesses enough starlight to fit the near infrared flux, but also casts a shadow on the inner rim of the outer disk. This requires the outer disk scale height to be large enough to rise out of the shadow. An optically thin inner disk does not cast such a shadow, and the SED can be fitted with a smaller outer disk scale height. For the dust in the inner regions to become optically thin however, the scale height would have to be so much larger than its hydrostatic equilibrium value that it effectively becomes a dust shell. It is currently unclear if a physical mechanism exists which could provide for such a configuration.