Modeling CO, CO$_2$ and H$_2$O ice abundances in the envelopes of young
  stellar objects in the Magellanic Clouds
  Massive young stellar objects in the Magellanic Clouds show infrared
absorption features corresponding to significant abundances of CO, CO$_2$ and
H$_2$O ice along the line of sight, with the relative abundances of these ices
differing between the Magellanic Clouds and the Milky Way. CO ice is not
detected towards sources in the Small Magellanic Cloud, and upper limits put
its relative abundance well below sources in the Large Magellanic Cloud and the
Milky Way. We use our gas-grain chemical code MAGICKAL, with multiple grain
sizes and grain temperatures, and further expand it with a treatment for
increased interstellar radiation field intensity to model the elevated dust
temperatures observed in the MCs. We also adjust the elemental abundances used
in the chemical models, guided by observations of HII regions in these
metal-poor satellite galaxies. With a grid of models, we are able to reproduce
the relative ice fractions observed in MC massive young stellar objects
(MYSOs), indicating that metal depletion and elevated grain temperature are
important drivers of the MYSO envelope ice composition. Magellanic Cloud
elemental abundances have a sub-galactic C/O ratio, increasing H$_2$O ice
abundances relative to the other ices; elevated grain temperatures favor CO$_2$
production over H$_2$O and CO. The observed shortfall in CO in the Small
Magellanic Cloud can be explained by a combination of reduced carbon abundance
and increased grain temperatures. The models indicate that a large variation in
radiation field strength is required to match the range of observed LMC
abundances. CH$_3$OH abundance is found to be enhanced in low-metallicity
models, providing seed material for complex organic molecule formation in the
Magellanic Clouds.
