Spectroscopic measurements of CH$_3$OH in layered and mixed interstellar
  ice analogues
  Context. The molecular composition of interstellar ice mantles is defined by
gas-grain processes in molecular clouds, with the main components being $H_2O$,
$CO$, and $CO_2$. $CH_3OH$ ice is detected towards the denser regions, where
large amounts of $CO$ freeze out and get hydrogenated. Heating from nearby
protostars can further change the ice structure and composition. Despite the
several observations of icy features towards molecular clouds and along the
line of site of protostars, it is not yet clear if interstellar ices are mixed
or if they have a layered structure. Aims. We aim to examine the effect of
mixed and layered ice growth in ice mantle analogues, with focus on the
position and shape of methanol infrared bands, so future observations could
shed light on the structure of interstellar ices in different environments.
Methods. Mixed and layered ice samples were deposited on a cold substrate kept
at T = 10 K using a closed-cycle cryostat placed in a vacuum chamber. The
spectroscopic features were analysed by FTIR spectroscopy. Different
proportions of the most abundant four molecules in ice mantles, namely $H_2O$,
$CO$, $CO_2$, and $CH_3OH$, were investigated, with special attention on the
analysis of the $CH_3OH$ bands. Results. We measure changes in the position and
shape of the CH and CO stretching bands of $CH_3OH$ depending on the mixed or
layered nature of the ice sample. Spectroscopic features of methanol are also
found to change due to heating. Conclusions. A layered ice structure best
reproduces the $CH_3OH$ band position recently observed towards a pre-stellar
core and in star-forming regions. Based on our experimental results, we
conclude that observations of $CH_3OH$ ices can provide information about the
structure of interstellar ices, and we expect JWST to put stringent constraints
on the layered or mixed nature of ices in different interstellar environments.
