The ⁸⁷Rubidium Atomic Clock Maser in Giant Stars

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We conducted a Green Bank Telescope search for the ground state 6.8 GHz hyperfine transition of rubidium (⁸⁷Rb) toward giant stars detected in Rb I optical resonance lines. The spin-flip transition of ⁸⁷Rb is one of the principal transitions used in atomic clocks, in addition to the hydrogen 21 cm maser and the ¹³³Cs hyperfine transition (which defines the second). The optical lines of ⁸⁷Rb and ⁸⁵Rb 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 ⁸⁷Rb lines were detected above 3.8σ.