Near-infrared spectroscopy of RS Ophiuchi in 2021: the calm, the storm, and the abatement

ABSTRACT We present near-infrared (NIR) observations of the 2021 eruption of the recurrent nova RS Ophiuchi. The data set provides both pre- and post-eruption perspectives on the eruption, as well as NIR spectra at high cadence. The spectrum obtained in 2020 June (14.3 yr after the 2006 eruption, and 428.1 d before the 2021 eruption) is that of the red giant secondary, on which are superimposed several emission lines which most likely arise in the red giant wind. Spectra obtained during the eruption consist of emission (including coronal) lines, superimposed on a bremsstrahlung continuum at 8900 K. The temperature of the coronal gas is estimated to be $10^{6.0}$ K on day 11.7, and $10^{5.9}$ K on day 31.7. The high-cadence observations, obtained on day 31.7 of the eruption, provide no conclusive evidence for rapid ($\lesssim 1$ min) variations in the He i1.0833$\, \mu$m line. Data obtained about one year after the eruption show that there may have been changes in the spectral type of the secondary after the 2021 eruption.