  Calcium monohydroxide radical (CaOH) is receiving an increasing amount of
attention from the astrophysics community as it is expected to be present in
the atmospheres of hot rocky super-Earth exoplanets as well as interstellar and
circumstellar environments. Here, we report the high-resolution laboratory
absorption spectroscopy on low-$J$ transitions in
$\tilde{A}^2\Pi(0,0,0)-\tilde{X}^2\Sigma^+(0,0,0)$ band of buffer-gas-cooled
CaOH. In total, 40 transitions out of the low-$J$ states were assigned,
including 27 transitions which have not been reported in previous literature.
The determined rotational constants for both ground and excited states are in
excellent agreement with previous literature, and the measurement uncertainty
for the absolute transition frequencies was improved by more than a factor of
three. This will aid future interstellar, circumstellar, and atmospheric
identifications of CaOH. The buffer-gas-cooling method employed here is a
particularly powerful method to probe low-$J$ transitions and is easily
applicable to other astrophysical molecules.
