The influence of particle shapes and sizes in the CO ice stretching mode

Ices are major components of dust grains in the interstellar medium, with H2O, CO, and CO2 ices being the most ubiquitous. We have studied the influence of particle shapes and sizes on the profile of the CO ice stretching mode by means of computational calculations. The extinction factors were calculated using Mie and DDA codes for a number of shapes and size parameters, ranging from the Rayleigh regime up to 3. A scattering peak displaced towards the reddish wavelength for size parameters larger than 1 was observed in all cases considered, independently of the ice (CO and CO2), the shapes (sphere, spheroid with axial proportion 5, DW1999, and aggregates), and the inclusion of other constituents, such as silicate. We have attributed this fact to the grain growth. Certain shapes also produced a double-peaked profile but in the wavelength range in which the surface modes can be observed. The calculations, including those for silicate, show a decrease in the band strength but the changes in the stretching mode profile of CO ice are not significant.