Late epoch near-infrared spectroscopy of V1405 Cas: The emergence of coronal line emission

ATel #15540; C. E. Woodward (U. Minnesota, USA), A. Evans (Keele U., UK), D. P.K. Banerjee (Physical Research Laboratory, India)
on 4 Aug 2022; 13:39 UT
Credential Certification: C.E. Woodward (chickw024@gmail.com)

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ATel #15518 reported the resolving of the nova shell of V1405 Cas (= Nova Cas 2021) at GHz frequencies with the VLA on 2022 July 04. As part of an on-going near infrared spectroscopic study of this very slow nova (ATel #14665, #14794), we report late epoch (+514.80 days post-outburst) 0.7 to 4.2 micron spectra obtained on 2021 July 31.54 UT on the 3.2m IRTF telescope using SpeX in cross-dispersed mode with an 0.8 arcsecond slit (R=750) under photometric conditions and moderate seeing (1.1 arcsec at K-band). The NIR spectra at this epoch is dominated by an exceptionally strong He I 1.0830 emission, total integrated observed flux of 1.06E-10 erg/s/cm^2. The emission feature is comprised of three Gaussian components, a strong central core with a Gaussian FWHM of order 862 km/s, and two narrower components with average widths of 422 km/s off set by +/- 850 km/s. Similar structure is evident in the Brackett-alpha, 4.40E-12 ergs/s/cm^2, and Pfund-gamma, 7.27E-13 ergs/s/cm^2, and Paschen-alpha, Paschen-beta, 8.70E-12 ergs/s/cm^2 and the He I 2.058, 9.71e-13 ergs/s/cm^2 lines. The P-Cygni absorptions in the H lines (ATel #14665) and He I lines are no longer detected. The spectral energy distribution shows no evidence for dust formation, and the overall infrared continuum has declined a factor of approximately 10 over the last 363 days. However, now present are broad coronal line emission features from [Ca V] 4.157, [Ca IV] 3.206, [Si VI] 1.966, and [Al IX] 2.040, and [P VII] 1.375 micron. The strength of the emission features over 500 days-post outburst, similar to that seen in other old classical novae such as V1974 Cyg, V382 Vel, V1494 Aql more that 4000 days post eruption (e.g., Helton et al. 2012, ApJ 755, 37), indicate that further synoptic spectroscopic observations of the very late evolutionary stage of V1405 Cas in the infrared is possible which can provide accurate elemental abundance determinations in the ejecta. With future JWST narrow-band imagery and IFU spectroscopy, spatially resolving the shell may also be feasible. These observations were conducted under the NASA IRTF program 2022A004, and we thank the IRTF staff for their assistance.