Modeling the spectral energy distribution of VY CMa

In this chapter we study the spectral energy distribution of VY CMa. Dust particles located in the circumstellar envelope absorb the stellar photons and reemit them at longer IR wavelengths. Therefore, the SED is dominated by a huge infrared excess. In order to understand the absorption and emission processes taking place in the CSE, and their effects on the observed continuum spectrum, we will first take a look at the theory of radiative transfer in Section 4.2. Next, we will present three different methods to model the spectral energy distribution. In Section 4.4 we start with a simple spherically symmetric geometry and a basic Monte Carlo program. However, as discussed in the previous chapter, VY CMa has a rather complex envelope and spherical symmetry is not a realistic assumption. Therefore, in Sections 4.5 and 4.6 we introduce a dusty disk around VY CMa. Two different approaches to solving the radiative transfer problem are described: LELUYA and MCMax. We give a short overview of the theory behind these two methods, and present the results of the model fitting. Finally, in Section 4.7 we summarize our findings.