Near-infrared spectroscopy of RS Ophiuchi in 2021: the calm, the storm, and the abatement

Abstract

We present near-infrared (NIR) observations of the 2021 eruption of the recurrent nova RS Ophiuchi. The dataset provides both pre- and post-eruption perspectives on the eruption, as well as NIR spectra at high cadence. The spectrum obtained in 2020 June (14.3 years after the 2006 eruption, and 428.1 days before the 2021 eruption), is that of the red giant secondary, on which are superimposed several emission lines which most likely arise in the red giant wind. Spectra obtained during the eruption consist of emission (including coronal) lines, superimposed on a bremsstrahlung continuum at 8900K. The temperature of the coronal gas is estimated to be $10^{6.0}$K on day 11.7, and $10^{5.9}$K on day 31.7. The high cadence observations, obtained on day 31.7 of the eruption, provide no conclusive evidence for rapid ($<\sim1$~minute) variations in the HeI 1.0833$\,μ$m line. Data obtained about one year after the eruption show that there may have been changes in the spectral type of the secondary after the 2021 eruption.

Figures (11)

• Figure 1: AAVSO $V$ band light curve with the dates (see Table \ref{['obs']}) of the IRTF spectroscopy marked (except for the days $-428.1$ pre- and 624.1 post- outburst). In addition, spectra at high cadence were obtained on day 31.7 (see Section \ref{['scadence']}). The horizontal dotted line is the mean value of the pre-eruption (MJD$<591200$) $V$ magnitude.
• Figure 2: Quiescent (i.e., pre-2021 eruption) spectrum of RS Oph, obtained on 2020 June 10 (black). Grey block indicates wavelength region over which the atmospheric transmission is poor. Spectrum of the M2III star HD120052, scaled to that of RS Oph, is shown in red. Locations of the $\gamma(0,0)$ TiO band (in RS Oph), the H$^-$ continuum, and of the first overtone CO bands are indicated. (cf. Fig. \ref{['post1']}).
• Figure 3: Observed near-infrared spectra of RS Oph at different epochs (Table \ref{['obs']}). (A): Evolution of the SpeX SXD 0.7--2.5$\,\mu$m range; data are offset by values stated in the figure for clarity. (B) SpeX SXD 0.7--2.5$\,\mu$m spectra obtained on day +31.7. Indicated in the panel are the wavelength of various hydrogen emission line series (Brackett, Paschen, Pfund). Representative He and O lines are also identified, together with various infrared coronal lines. (C): Evolution of SpeX LXD mode (thermal wavelengths) 2.7--4.3$\,\mu$m range. Days past outburst are indicated, together with the wavelengths of emission lines in the hydrogen Brackett, Humphries, and Pfund series. The absorption features around 3.4$\,\mu$m on day +11.7 are telluric.
• Figure 4: Top: IRTF spectrum for day 11.7; bottom: spectrum for day 31.7. Coronal lines are indicated by the vertical dotted lines (see Table \ref{['all-data-tab']}). Data are shown in black. Full red curves are nebular continua for temperature 8900 K, for which the scale is set by H line fluxes. Broken line in top frame is the free-free continuum for temperature 8900 K, for which the scale is set by fitting the long ($\lambda>3.6$$\,\mu$m) wavelength continuum. Broken line in bottom frame is the 8900~K free-free day~31.7 data as described in text. Blue points are AAVSO $BV\!R$ data, for which the wavelength "error bars" are the FWHM of the $BV\!R$ filters. See text for details. The plots are extended to 0.3$\,\mu$m to show the expected extent of the Balmer discontinuity.
• Figure 5: Dependence of the Paschen and Brackett discontinuities on temperature for a pure hydrogen plasma. Black points: theoretical values from baker38; blue lines are polynomial fits.