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

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$\sigma$.