Infrared Spectroscopy of RS Oph on day +12 after the 2021 August Outburst

ATel #14866; C. E. Woodward (U. Minnesota), A. Evans (Keele U., UK), D. P.K. Banerjee (PRL, India), S. Starrfield (Arizona State U), R. M. Wagner (Ohio State U.), R. D. Gehrz (U. Minnesota)
on 22 Aug 2021; 15:11 UT
Credential Certification: C.E. Woodward (chickw024@gmail.com)

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We report low resolution (R = 1200) 0.69-to-4.19-micron spectroscopy of RS Oph obtained on 2021 August 20.203 UT on the NASA IRTF 3.2 m telescope under excellent seeing conditions (0.6 arcsec in the K-band) 12 days after the 2021 August 8.083 UT eruption, complementing the on-going multi-wavelength study of this recurrent nova (ATEL#14864, #14860, #14857, #14852, #14848, #14834 among others). At this epoch (+12d), the infrared spectrum is dominated by hydrogen and helium emission lines, including Brackett 19-4 through 5-4, Humphreys 24-6 through 13-6, Pfund 23-5 through 8-5 series, and some of the Paschen series (both Pa-α, and Pa-β are saturated) Pa 12-3 through Pa 8-3, with HeI emission lines at 1.083, 1.197, 1.252, 1.700, 2.058, and 2.113 micron. Lines of carbon (e.g., CI 1.175 micron), nitrogen (e.g., NI 1.247 micron) and oxygen (e.g., OI 0.8446, 1.129, and 1.3171 micron) are present. An emission line tentatively identified as FeII 1.687 is on the red-wing of the Brackett-11 emission line profile. The FWHM of the H lines correspond to expansion velocities of the order 1250 km/s, and no P-Cygni profiles are evident. Representative observed integrated line fluxes (erg/s/cm2) for various lines include: Br-α 4.7e-10, Br-γ 3.2e-10, Br-δ 2.9e-10, Pf-ε 1.3e-10, HeI (2.058 micron) 4.5e-10, OI (0.8446 micron) 1.2e-9, and OI (1.3171 micron) 4.6e-11. The width of the emission lines is similar to those seen on day 12+ after the 2006 event (Evans et al. 2007, MNRAS 374, L1; Das, Banerjee and Ashok 2006, ApJL 653, L141). However, the peaks of the +12d 2021 hydrogen line profiles (as also are the peaks of the He profiles) clearly are asymmetric and saddle shaped with the blue wing being weaker than the peak intensity of the red wing; two-component Gaussian fits suggest two velocity components at -329.8 km/s and +227.0 km/s. The Paschen and Brackett limits at 0.85 micron and 1.5 micron are also evident as they were in the same evolutionary phase in 2006, suggesting flash ionization of the wind (Evans et al. 2008, MNRAS 374, L1). The spectral energy distribution shows no definitive evidence for dust or coronal line emission (see Banerjee et al. 2009, MNRAS 399, 357). Likely the onset of the coronal line emission phase is imminent as the evolution of the system proceeds, and follow-up spectroscopic observations are warranted.

The investigators appreciated the NASA IRTF for providing the quick-access DDT opportunity (2021B996), and we thank the observatory staff for their assistance.