Contribution Contributed Talk
Dust growth towards disks: self-consistent modelling of dust evolution due to accretion and coagulation
What are the properties of dust in the interstellar medium (ISM) and how do these properties change in the transition to denser regions? This is one of the important questions when studying the denser ISM, since dust properties influence, for example, the formation and temperature of the major molecules in molecular clouds and the grain dynamical behaviour when forming protoplanetary disks. It is therefore important to characterise the grain size, structure, shape and material composition in all phases of the ISM.
The observed SEDs of the dense ISM show a decrease in colour temperature, an increase in the spectral index and an increase in emissivity in the far-IR and sub-mm (Juvela et al., 2011, Roy et al., 2013). These variations cannot be explained with environmental differences alone, but are assumed to occur due to changes in the dust properties. We show that these variations can be explained by dust evolution due to accretion and coagulations processes. Our modelling is based on the diffuse-ISM dust model of Jones et al. (2013) and Köhler et al. (2014), for which we allow for the accretion of carbonaceous and ice mantles as well as coagulation into aggregates. We have carried out detailed model calculations using DDA (Draine & Flatau, 2010) in order to derive the optical properties of these evolved grains. Subsequently, we use DustEM (Compiègne et al., 2011) in combination with the CRT radiative transfer code (Juvela et al., 2005) to determine the SEDs which we compare to observations.
The variations in the optical properties due to dust evolution are able to describe the observed changes in the SED from the diffuse to dense ISM with Av<16 (Köhler et al., 2015). With the dust evolution we are able to explain self-consistently the observed variations in colour temperature, spectral index, 250 $\mu$m emissivity and mid-infrared emission. We therefore conclude that evolutionary processes and especially grain growth begins in the low-density (diffuse) ISM, which can explain the observed changes in the SED.
Compiègne et al., A&A 525, 103 (2011), Draine & Flatau, ArXiv e-prints 1002.1505 (2010), Jones et al., A&A 558, 62 (2013), Juvela et al., A&A 527, 111 (2011), Juvela, A&A 440, 531, (2005), Köhler et al., A&A 565, 9 (2014), Köhler et al., A&A 579, A10 (2015), Roy et al., ApJ 763, 55 (2013)