  Zirconium oxide(ZrO) is an important astrophysical molecule that defines the
S-star classification class for cool giant stars. Accurate, empirical
rovibronic energy levels, with associated labels and uncertainties, are
reported for 9 low-lying electronic states of the diatomic 90Zr16O molecule.
These 8088 empirical energy levels are determined using the Marvel (Measured
Active Rotational-Vibrational Energy Levels) algorithm with 23 317 input
assigned transition frequencies, 22 549 of which were validated. A
temperature-dependent partition function is presented alongside updated
spectroscopic constants for the 9 low-lying electronic states.
