Abstract

Context. With the legacy of Spitzer and current advances in (sub) mm astronomy, a considerable number of so-called "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 modelled correctly. The disk around LkCa 15 is such a disk, with a large gap ranging from similar to 5-46 AU, as identified by Espaillat et al. (2007, ApJ, 670, L135) using 1+1D radiative transfer modelling. To fit the spectral energy distribution (SED), they propose two possible scenarios for the inner (5 AU) disk-optically thick or optically thin-and one scenario for the outer disk. Aims. We use the gapped disk of LkCa 15 as a case in point 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. Methods. We use MCMax, a 2D radiative transfer code with a self-consistent vertical density and temperature structure, to model the SED of LkCa 15. Results. We identify two possible geometries for the inner and outer disk that are both different from those in Espaillat et al. (2007, ApJ, 670, L135). 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 high 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 higher 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. Conclusions. We find that the radial optical depth of dust within the inner hole of LkCa 15 is controlled by its vertical distribution. If it turns optically thick, the outer disk scale height must be increased to raise the outer disk out of the inner disk's shadow.