Near-Infrared spectroscopy of the slow nova V2891 Cygni

ATel #13759; C. E. Woodward (U. Minnesota), D. P.K. Banerjee (Physical Research Laboratory, India), A. Evans (Keele U.)
on 24 May 2020; 11:19 UT
Credential Certification: Dipankar P.K. Banerjee (dpkb12345@gmail.com)

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We report near-infrared spectroscopy of the slow FeII Nova V2891 Cygni (AT 2019qwf) discovered in 2019 mid September (ATel #13130, ATel #13149). After the initial phase of brightening, the object's brightness has remained fairly constant during the last 250 days (between 1 October 2019 and 18 May 2020), varying irregularly over a small range between R = 15.3 to 13 during this period. A decline towards quiescence may have lately begun as indicated by the lightcurve at https://antares.noao.edu/alerts/locus/5430018.

A 0.7-2.5 micron spectrum was obtained on 2020 May 20.583 UT on the 3.2m IRTF telescope using SpeX in the cross-dispersed mode with a 0.5 arcsec slit (R = 1200) under sub-arcsec seeing (< 0.6 arcsec) conditions. The nova displays a rich emission line spectrum comprising mostly of H, He, O, N and Fe lines along with numerous weaker lines whose identification is pending. The Paschen and Brackett Hydrogen lines are prominent, all of which show a double peaked structure, the peaks separated by ~ 480-540 km/s, with the red peak stronger than the blue. The FWHMs of the Paschen beta and Brackett gamma lines are in the range 1200-1250 km/s. The He lines that are generally seen in novae spectra are now prominent viz. lines at 1.0831, 1.7002, 2.1120, 2.1132, 2.0581 micron. HeI 1.0831 displays a slight but noticeable P-Cygni absorption even at this late a stage with the absorption minimum separated from the line center by 1440 km/s. Another line to show a P Cygni feature is OI 7772 with a similar separation of ~1275 km/s between the emission peak and absorption minimum. Numerous NI lines are seen, but all the carbon lines (the NIR hallmarks of the FeII class of novae) which were seen early during the nova's development (ATel #13301) have now disappeared.

The OI lines show interesting behaviour. The OI 0.8884 and 1.1287 micron lines, presumably excited predominantly by Lyman beta fluorescence, show similar profiles as the H lines with double peaked structures, with the red peak stronger than the blue and peak-to-peak separations in the range 500-600 km/s. The FWHM's of both lines are in the range 1025-1075 km/s. On the other hand the OI 1.3164 micron line, expected to be excited by collisions or continuum fluorescence, shows a reversal in the peak strengths with the blue peak being stronger than the red (the peak-to-peak separation is 600 km/s). The difference in profile structure suggests these OI lines emanate from different regions of the ejecta as is expected if their excitation mechanisms are different. The largely diminished strength of the OI 0.8440 micron line relative to the OI 1.1287 micron line [I(8440)/I(11287) = 0.55] implies substantial reddening towards the nova. The large (B-V) color during the early stages of the outburst (see the AAVSO lightcurve) also supports this.

The 1 micron Fe II lines (expected at 0.9997, 1.0501, 1.0863, and 1.1126 microns respectively, Rudy et al 2000, ApJ, 539, 166) are seen. The nova in addition also shows prominent emission in the uncommonly seen Fe II lines at 1.6872 and 1.7414 micron that were suggested to be excited by Lyman alpha and Lyman continuum fluorescence (Banerjee et al. 2009, MNRAS, 399, 357). No evidence for coronal lines, first overtone CO emission, or dust formation is seen as yet.

We acknowledge partial support from NASA grant 80NSSC19K0868 and thank the IRTF staff for their assistance with our program 2020A010.