Studies of solid carbon dioxide in interstellar ice analogs subject to thermal processing

Abstract

Solid CO₂ has been detected in many lines of sight in the interstellar medium from infrared observatories. Spectral profiles from space-based observatories have suggested that CO₂ on icy grain mantles is mixed with other common molecules such as H₂O and CH ₃OH in interstellar regions and that thermal annealing has occurred. The vibrational mode at 658 cm^-1 (15.2 μm) is suspected to be a powerful diagnostic tool as to the composition of species on icy grain mantles as well as thermal histories. However, previous studies have not systematically investigated ice composition and temperature. Laboratory spectra of interstellar ice analogs have been created in this study order to better understand the physical properties of solid CO₂ in these interstellar environments. Existing databases of ice composition studies and effects of ice thermal history were updated in this study to include a more systematic approach. The 658 cm^-1 (15.2 μm) bending mode feature of CO₂ is examined here and the subsequent astrophysical implications stated. In the first set of experiments, 47 mixtures of H₂O,CH₃OH, andCO₂ were slowly warmed and mid-infrared absorption spectra were recorded at 5K intervals. The second set of experiments involved examining the CO₂ bending mode feature of 10 different CO₂-containing ice mixtures at different temperatures where ice segregation was suspected. In these experiments, the ice mixtures were slowly heated to the desired temperature for increasing time intervals before cooling down and recording mid-IR absorption spectra. These studies may be used to analyze IR data from space-based observatories such as the Spitzer Space Telescope Infrared Spectrograph as well other future IR observations of the interstellar medium. Finally, mass spectroscopy measurements were taken from temperature programmed desorption (TPD) experiments performed on several binary mixtures of H₂O + CO₂ and CH ₃OH + CO₂. Physical properties such as desorption energy of CO2 can be determined from the TPD traces of these experiments. The work provided here addresses the physical properties of solid CO ₂ thermally processed in ice mixtures in interstellar environments by laboratory simulations spectroscopically analyzed by mid-infrared absorption profiles and TPD.