Formation of CN and CO Molecules in the Envelope of Nova V1391 Cas During the Near-maximum Phase

Moderate-resolution spectra ( R ≈ 2700) of the classical nova V1391 Cas were obtained in the near-infrared wavelength range of 0.95–2.45 μ m on UT 2020 August 14.41 using the Keck II telescope and the NIRES spectrograph. The spectral analysis revealed simultaneous detection of CO and CN molecular features. The CN red-system band was observed solely in absorption, while the CO first overtone bands, encompassing both 12 CO and 13 CO, displayed a combination of emission and absorption features at different velocities, resembling a P-Cygni profile. This specific CO profile is unprecedented in classical novae observations. Additionally, the negative velocity shifts in the absorption bands of both CN and CO were found to be consistent with each other.