We present four new epochs of Ks-band images of the young pre-transitional disk around LkCa 15, and perform extensive forward modeling to derive the physical parameters of the disk. We find indications of strongly anisotropic scattering (Henyey-Green
stein g = 0.67 [-0.11,+0.18]) and a significantly tapered gap edge (round wall), but see no evidence that the inner disk, whose existence is predicted by the spectral energy distribution, shadows the outer regions of the disk visible in our images. We marginally confirm the existence of an offset between the disk center and the star along the line of nodes; however, the magnitude of this offset (x = 27 [-20,+19] mas) is notably lower than that found in our earlier H-band images (Thalmann et al. 2010). Intriguingly, we also find, at high significance, an offset of y = 69 [-25, +49] mas perpendicular to the line of nodes. If confirmed by future observations, this would imply a highly elliptical -- or otherwise asymmetric -- disk gap with an effective eccentricity of e = ~0.3. Such asymmetry would most likely be the result of dynamical sculpting by one or more unseen planets in the system. Finally, we find that the bright arc of scattered light we see in direct imaging observations originates from the near side of the disk, and appears brighter than the far side because of strong forward scattering.
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 dus
t 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.
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 comp
lex 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.
