The $^{87}$Rubidium Atomic Clock Maser in Giant Stars

TL;DR

This study searched for the 6.8 GHz hyperfine transition of rubidium in giant stars using the Green Bank Telescope, aiming to detect potential natural masers analogous to atomic clocks, but found no signals.

Contribution

First observational search for rubidium hyperfine masers in giant stars, linking atomic clock physics with stellar atmospheres.

Findings

No 6.8 GHz $^{87}$Rb lines detected above 3.8$\sigma$

Provides upper limits on rubidium maser emission in giant stars

Suggests optical pumping of rubidium in stellar atmospheres is weak or absent

Abstract

We conducted a Green Bank Telescope search for the ground state 6.8 GHz hyperfine transition of rubidium ($^{87}$Rb) toward giant stars detected in Rb I optical resonance lines. The spin-flip transition of $^{87}$Rb is one of the principal transitions used in atomic clocks, in addition to the hydrogen 21 cm maser and the $^{133}$Cs hyperfine transition (which defines the second). The optical lines of $^{87}$Rb and $^{85}$Rb can together pump the 6.8 GHz transition to form a maser, and the same optical pumping used in atomic clocks may occur in the atmospheres of evolved stars. No 6.8 GHz $^{87}$Rb lines were detected above 3.8$\sigma$.