One of the striking discoveries of protoplanetary disc research in recent years are the spiral arms seen in several transitional discs in polarised scattered light. An interesting interpretation of the observed spiral features is that they are densit
y waves launched by one or more embedded (proto-)planets in the disc. In this paper we investigate whether planets can be held responsible for the excitation mechanism of the observed spirals. We use locally isothermal hydrodynamic simulations as well as analytic formulae to model the spiral waves launched by planets. Then H-band scattered light images are calculated using a 3D continuum radiative transfer code to study the effect of surface density and pressure scale height perturbation on the detectability of the spirals. We find that a relative change of about 3.5 in the surface density is required for the spirals to be detected with current telescopes in the near-infrared for sources at the distance of typical star-forming regions (140pc). This value is a factor of eight higher than what is seen in hydrodynamic simulations. We also find that a relative change of only 0.2 in pressure scale height is sufficient to create detectable signatures under the same conditions. Therefore, we suggest that the spiral arms observed to date in protoplanetary discs are the results of changes in the vertical structure of the disc (e.g. pressure scale height perturbation) instead of surface density perturbations.
EX Lup is the prototype of the EXor class of eruptive young stars. These objects show optical outbursts which are thought to be related to runaway accretion onto the star. In a previous study we observed in-situ crystal formation in the disk of EX Lu
p during its latest outburst in 2008, making the object an ideal laboratory to investigate circumstellar crystal formation and transport. This outburst was monitored by a campaign of ground-based and Spitzer Space Telescope observations. Here we modeled the spectral energy distribution of EX Lup in the outburst from optical to millimeter wavelengths with a 2D radiative transfer code. Our results showed that the shape of the SED at optical wavelengths was more consistent with a single temperature blackbody than a temperature distribution. We also found that this single temperature component emitted 80-100 % of the total accretion luminosity. We concluded that a thermal instability, the most widely accepted model of EXor outbursts, was likely not the triggering mechanism of the 2008 outburst of EX Lup. Our mid-infrared Spitzer spectra revealed that the strength of all crystalline bands between 8 and 30 um increased right after the end of the outburst. Six months later, however, the crystallinity in the 10 um silicate feature complex decreased. Our modeling of the mid-infrared spectral evolution of EXLup showed that, although vertical mixing should be stronger during the outburst than in the quiescent phase, fast radial transport of crystals (e.g., by stellar/disk wind) was required to reproduce the observed mid-infrared spectra.
