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 Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

Carbon monoxide (CO) is a poor tracer of H$_{2}$ in the diffuse interstellarmedium (ISM), where most of the carbon is not incorporated into CO moleculesunlike the situation at higher extinctions. We present a novel, indirect methodto constrain H$_{2}$ column densities ($N_{H_{2}}$) without employing COobservations. We show that previously-recognized nonlinearities in the relationbetween the extinction, $A_{V} ({H}_{2})$, derived from dust emission and theHI column density ($N_{HI}$) are due to the presence of molecular gas. Weemploy archival $N_{H_{2}}$ data, obtained from the UV spectra of stars, andcalculate $A_{V} ({H}_{2})$ towards these sight lines using 3D extinction maps.We derive an empirical relation between $A_{V} ({H}_{2})$ and $N_{H_{2}}$,which we use to constrain $N_{H_{2}}$ in the diffuse ISM. We construct a$N_{H_{2}}$ map of our Galaxy and compare it to the CO integrated intensity($W_{CO}$) distribution. We find that the average ratio ($X_{CO}$) between$N_{H_{2}}$ and $W_{CO}$ is approximately equal to $2 \times 10^{20}$ cm$^{-2}$(K km s$^{-1}$)$^{-1}$, consistent with previous estimates. However, we findthat the $X_{CO}$ factor varies by orders of magnitude on arcminute scalesbetween the outer and the central portions of molecular clouds. For regionswith $N_{H_{2}} \gtrsim 10^{20}$ cm$^{-2}$, we estimate that the averageH$_{2}$ fractional abundance, $f_{{H}_{2}}$ = $2N_{H_{2}}$/(2$N_{H_{2}}$ +$N_{{HI}}$), is 0.25. Multiple (distinct) largely atomic clouds are likelyfound along high-extinction sightlines ($A_{V} \geq 1$ mag), hence limiting$f_{{H}_{2}}$ in these directions. More than $50 \%$ of the lines of sight with$N_{H_{2}} \geq 10^{20}$ cm$^{-2}$ are untraceable by CO with a $J$ = 1-0sensitivity limit $W_{CO} = 1$ K km s$^{-1}$.

Constraining the H2 column densities in the diffuse interstellar medium using dust extinction and HI data