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

Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) inGuizhou, China, we detect the 21cm neutral atomic hydrogen absorption in theyoung planetary nebula IC 4997. The absorption arises from a shell alsoassociated with Na I D lines. The H I shell has a mass of $1.46\times10^{-2}$M$_\odot$ and a dynamic age of 990yr. The column density of H I is estimated tobe $7.1\times10^{20}$ cm$^{-2}$, which can be well explained in terms of aphotodissociation region around the ionized nebula, limited by self shieldingof H$_2$. We find that the atomic-to-ionized hydrogen ratio is 0.6, suggestingthat H I substantially contributes to overall nebular mass.

FAST search for circumstellar atomic hydrogen--I: the young planetary nebula IC 4997