Near-infrared spectroscopy of the recurrent nova M31N 2008-12a

ABSTRACT Near-infrared (NIR) 0.9–2.5 $\mu$m spectra of the remarkable recurrent nova (RN) M31N 2008-12a were obtained on days 6.3 and 10.3 after discovery of its 2024 outburst, and are the first NIR spectra of this object. The only prominent line seen in the spectra is that of He i 1.083 $\mu$m, on day 6.3. Apart from this He i line, there are only two other weak emission features: one at 1.0786 $\mu$m, suggested to be the [Fe xiii] 1.075 $\mu$m coronal line, and one unidentified feature at 1.0969 $\mu$m. The observed full width at half-maximum of the He i line on day 6.3 (1350 km s$^{-1}$) is consistent with the behaviour of optical He i lines during earlier eruptions of this RN, which show that the nova ejecta decelerate as they interact with the secondary’s wind. The He i 1.083 $\mu$m line faded rapidly, and was absent in the day 10.3 spectrum, along with any other emission lines. We use the relative strengths of optical He and H lines in previous eruptions to estimate the expected strengths of the He i 1.083 $\mu$m line and of other infrared (including coronal) lines at 6.3 d after eruption. Our findings are consistent with the infrared spectra we observed during the 2024 eruption. We apply our analysis to account for the relative weakness of NIR coronal emission in known Galactic RNe with giant secondaries.