Dust evolution, a global view: III. Core/mantle grains, organic nano-globules, comets and surface chemistry

Area of Science:

• Astrochemistry
• Cosmic Dust Science
• Interstellar Medium Physics

Background:

• Interstellar and cometary grains possess core/mantle structures influencing their properties.
• Existing models struggle to explain various observed phenomena in the interstellar medium and comets.

Purpose of the Study:

• To explore surface processes and chemistry of interstellar and cometary dust grains within the THEMIS framework.
• To investigate the role of grain surface reactivity in explaining anomalous astronomical observations.

Main Methods:

• Utilized The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS).
• Investigated the chemical reactivity of carbonaceous amorphous hydrogenated carbon (a-C(:H)) grain surfaces.
• Focused on the incorporation of atomic oxygen into epoxide functional groups on grain surfaces.

Main Results:

• A realistic consideration of grain surface chemistry self-consistently explains anomalous oxygen depletion and CO dark gas.
• The model accounts for polar ice mantle formation, the 3 μm water ice band's red wing, and O-rich hot core chemistry.
• Explains the origin of organic nano-globules and the 3.2 μm carbonyl band in cometary spectra.

Conclusions:

• The reaction of gas-phase species with carbonaceous grain surfaces, especially atomic oxygen incorporation, is key to observed phenomena.
• Cosmic dust chemistry is intrinsically linked to interstellar gas chemistry.
• Current interstellar gas and dust chemistry models require fundamental modification to include these grain surface processes.