U Aquarii

Lore & Background

Christian Heinrich Friedrich Peters discovered the variability of U Aquarii based on observations made from 1875 to 1878. Its variable star designation was published in Annie Jump Cannon's Second catalogue of variable stars in 1907, at which time its class was still unknown. The star lies several kiloparsecs below the galactic plane, suggesting it may belong to an old stellar population.

The stellar classification of U Aquarii is C-Hd, and it is classified as an R Coronae Borealis variable. It is a carbon star with a hydrogen-deficient spectrum that also shows evidence of s-process elements, including overabundances of strontium and yttrium, but no barium. This combination of properties is exceptionally rare; only one other example had been found as of 2012.

The elemental abundances are explained as the result of a single neutron exposure event, which is difficult to reconcile with a conjecture that this may be a post-AGB-type star. In 1999, U Aqr was proposed to be a Thorne-Zytkow object, instead of being a simple R Coronae Borealis variable.

Reader's Guide

U Aquarii holds significance as an exceptionally rare variable star, with only one other known example as of 2012 sharing its combination of hydrogen deficiency, carbon-rich spectrum, and specific s-process element overabundances (strontium and yttrium, but no barium). Its discovery by Christian Heinrich Friedrich Peters in the late 19th century and its designation in Annie Jump Cannon's 1907 catalogue mark early milestones in variable star astronomy. The star's distance estimates—38 kly from parallax and 43 kly from a bolometric assumption—and its location several kiloparsecs below the galactic plane hint at an old stellar population. The unusual elemental abundances, attributed to a single neutron exposure event, challenge the post-AGB conjecture. The 1999 proposal that U Aqr may be a Thorne-Zytkow object rather than a simple R Coronae Borealis variable underscores the ongoing uncertainty about its true nature. Its legacy lies in its role as a test case for stellar evolution models, particularly for hydrogen-deficient stars and exotic objects like Thorne-Zytkow candidates.

Did You Know?

• U Aquarii was discovered as a variable star by Christian Heinrich Friedrich Peters based on observations from 1875 to 1878.
• As of 2012, only one other star with the same combination of hydrogen deficiency and s-process element abundances had been found.