A Network Approach for the Accurate Characterization of Water Lines Observable in Astronomical Masers and Extragalactic Environments

The water molecule, crucial to the chemical composition
and dynamics
of the universe, is typically identified in its gas phase via radio
and submillimeter transitions, with frequencies up to a few THz. To
understand the physicochemical behavior of astronomical objects, accurate
transition frequencies are required for these lines. From a set of
26 new and 564 previous Lamb dip measurements, utilizing our ultrasensitive
laser-based spectrometers in the near-infrared region, ultrahigh-precision
spectroscopic networks were set up for H216O and H218O, augmented with 40 extremely accurate
frequencies taken from the literature. Based on kHz-accuracy paths
of these networks, considerably improved line-center frequencies have
been obtained for 35 observed or predicted maser lines of H216O, as well
as for 14 transitions of astronomical significance of H218O. These reference
frequencies, attached with 5–25 kHz uncertainties, may help
future studies in various fields of astrochemistry and astrophysics,
in particular when precise information is demanded about Doppler-velocity
components, including the gas flows of galactic cores, the kinematics
of planetary nebulae, or the motion in exoplanetary atmospheres.