English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

We investigate the evolution of the ortho-to-para ratio of overall (gas +ice) H$_2$ via the nuclear spin conversion on grain surfaces coated with waterice under physical conditions that are relevant to star- and planet-formingregions. We utilize the rate equation model that considers adsorption ofgaseous H$_2$ on grain surfaces which have a variety of binding sites with adifferent potential energy depth, thermal hopping, desorption, and the nuclearspin conversion of adsorbed H$_2$. It is found that the spin conversionefficiency depends on the H$_2$ gas density and the surface temperature. As ageneral trend, enhanced H$_2$ gas density reduces the efficiency, while thetemperature dependence is not monotonic; there is a critical surfacetemperature at which the efficiency is the maximum. At low temperatures, theexchange of gaseous and icy H$_2$ is inefficient (i.e., adsorbed H$_2$ does notdesorb and hinders another gaseous H$_2$ to be adsorbed), while at warmtemperatures, the residence time of H$_2$ on surfaces is too short for the spinconversion. Additionally, the spin conversion becomes more efficient withlowering the activation barriers for thermal hopping. We discuss whether thespin conversion on surfaces can dominate over that in the gas-phase in star-and planet-forming regions. Finally, we establish a simple but accurate way toimplement the H$_2$ spin conversion on grain surfaces in existing astrochemicalmodels.

H$_2$ ortho-para spin conversion on inhomogeneous grain surfaces