Spectroscopic evidence of grain ice mantle growth in YSOsI. CO ice modeling and limiting cases

Using the Discrete Dipole Approximation for Scattering and Absorption of Light the volume attenuation coefficient of spherical and ellipsoidal coated grains is calculated, to simulate CO ice mantles on top of refractory grain cores in dense clouds. After estimating from observations and cosmic abundance constraints the expected mantle to core volume ratios, the simple analytical calculations that are generally used to model grain shape effects are discussed. It follows that the simple pure ice grain models are not applicable to the overall grain size distribution, but fairly accurately represent the smallest grains. Grain growth to micron sizes can reproduce the observed large red component in the CO absorption profiles toward many lines of sight, and traces the degree of scattering affecting the profile. Extreme case profiles such as the ones in the line-of-sight of CRBR 2422.8-3423 or Reipurth 50 are reproduced with such an approach. The continuum extraction made prior to analysing the ice profiles might be in some cases inadequate, as for micron-sized grains the influence of the extinction on the profile extends to several times the apparent pure absorption profile, modifying as a consequence their analysis.