Near Infrared Spectroscopy of V1674 Herculis

ATel #14728; C. E. Woodward (U. Minnesota), D. P.K. Banerjee (PRL, India), R. M. Wagner (Ohio State U.), S. Starrfield (Arizona State U.), A. Evans (Keele U., UK)
on 20 Jun 2021; 13:24 UT
Credential Certification: C.E. Woodward (chickw024@gmail.com)

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We report 0.7 to 2.5 micron spectra of the very fast nova V1674 Her obtained on 2021 June 18.544 UT on the 3.2m IRTF telescope using SpeX in cross-dispersed mode (SXD) with a 0.5 arcsec slit (R = 1200) in broken-cloud conditions and moderate seeing (1.0 arcsec in the K-band). The significant finding is that the nova which initially showed signatures of belonging to the FeII class ( ATel #14723; Fe II lines were also reported in ATels #14710, #14718) now displays a spectrum more representative of the He/N class. Either ionization conditions have increased unusually fast, within 5.5 days of outburst, or V1674 Her is a hybrid nova (or equivalently Fe IIb class as per the classification of Williams 1992, AJ, 104, 725) which is making a very quick transition from the FeII to He/N class. Other hybrid novae (defined in Williams 2012, AJ, 144, 98) include V458 Vul (Poggiani 2008, Ap&SS, 315, 79), M31N 2006-10b (Shafter et al. 2011, ApJ, 734, 12), and V906 Car (Pavana et al. 2002, MNRAS, 495, 2075). A deeper analysis of the optical and NIR data that has been collected by us should throw more light on the observed developments.

The near-infrared spectrum reported here is dominated by H and He lines. The He lines in particular are strong with He 1.083 micron being the strongest line in the spectrum and the HeI 2.0581 micron line being as strong as Brackett gamma at 2.1656 micron. A single Gaussian fit to the broad emission line profiles yields line fluxes: HeI 2.0581 = 7.22e-11 erg/s/cm2, line flux of Br gamma = 7.14e-11 erg/s/cm2. Because of the large line widths (FWHMs of all lines are close to 5000 km/s) He 1.083 micron and H1.094 micron are merged into one single profile. On the blue wing of this combined profile, CI 1.0689 micron, which is the strongest Carbon line expected in the 1 to 2.5 micron NIR region, may be very weakly present. But otherwise, there are no signs of the several strong Carbon lines which are expected to be present in the NIR spectra of all FeII nova during early decline from maximum. Lines of oxygen are prominently present viz. 0.7773, 0.8446, 1.1287, 1.3164 micron etc. The H, He, and O lines show considerable structure (sub-peaks) in the profile peak as reported in ATel #14723, indicating many velocity components are being observed and suggests the presence of jets and knots in the ejecta. The measured FWHMs of OI (0.8446 micron) and Paschen beta (1.2818 micron) from Gaussian fitting are 4319 km/s and 4579 km/s and their associated fluxes are 4.43e-10 erg/s/cm2 and 3.52e-10 ergs/s/cm2, respectively. All the spectra taken of V1674 Her show that material has been ejected non-spherically and, thus, we are observing gas with very different densities and opacities expanding with extremely high velocities compared to the typical classical nova. Therefore, we expect to see lines arising from different regions in the ejecta making this nova difficult to classify now that it is a week old. The complexity of the evolution could explain why VHE gamma-rays (ATel#14707, #14705) were observed so early in the evolution. There is yet no evidence for carbon-monoxide emission, coronal line emission or dust formation. Further temporal optical and infrared observations are encouraged.

The observations were conducted in part under IRTF program 2021A012, and the authors thank the NASA IRTF staff for their assistance.