2021 outburst of RS Oph 2 March 2022

The 2021 outburst of RS Oph: a pictorial atlas of the spectroscopic evolution.
II.

From day 19 to 102 (solar conjunction)

Ulisse Munari1 and Paolo Valisa2

1: INAF National Institute of Astrophysics, 36012 Asiago, Italy
2: ANS Collaboration, c/o Astronomical Observatory, 36012 Asiago, Italy

Abstract. We present the second part of our atlas of the spectral evolution of
RS Oph during the 2021 nova outburst, characterized by a tight-monitoring in
high-resolution with Echelle spectrographs mounted on the Varese 0.84m and
Asiago 1.82m telescopes. In this Paper II we cover from day 19 to 102 (Aug 28
to Nov 19), the interval going from appearance of high ionization lines to the
stop imposed on observations by the Solar conjunction. A third and final paper
will map the spectral evolution after emersion from Solar conjunction (Jan 16,
2022) and up to the re-establishment of pre-outburst appearance, marking the
spectroscopic end of the outburst. Some quick conclusions can be drawn from
the data presented in this second part of our atlas:
(1) the spectra evolution has progressed in a smooth and gradual way,
suggesting that both the expansion of the ejecta through the pre-existing RG
wind and the photo-ionization from the central source did not encountered
sudden discontinuities prior to switch-off of the nuclear burning;
(2) the 2021 spectral evolution is similar to that of the 2006 eruption (the
latter much less accurately mapped), with a mirror-appearance of the triple-
peaked line profiles (blue peak brighter than red in 2006, the opposite in
2021) probably related to the two outbursts occurring at opposite sides of the
binary orbit followed by the red giant and the white dwarf companion;
(3) the evolution of the triple-peaked profiles progressed similarly through
different excitation and ionization degrees, arguing in favor of a geometrical
origin, as in a bipolar outflow nested to an equatorial torus; a discontinuity
occurred around day 30 (at the time when the coronal lines were rising to
their maximum), when the rate of shrinking in the velocity separation of the
red and blue peaks dropped from $-$10 to $-1$ km s-1 per day;
(4) the time-behavior of coronal emission lines appears quite smooth, both in
terms of radiated flux as well as evolution of the triple-peaked profiles,
with a plateau at maximum lasting up to about day 87 (counted from maximum
brightness), and the rise/decline times in agreement with progressive clearing
of the ejecta and then cooling of the WD following switch-off of the nuclear
burning;
(5) there seems to exist within RS Oph two distinct types of ejecta: (1) fast
moving ones producing Gaussian-like line profiles (primarely [NII] and
[OIII]), that keep expanding at FWHM=1000 km s-1 with no sign of ongoing
deceleration during the two months leading up to Solar conjunction, and (2)
slower moving ejecta giving rise to the triple-peaked line profiles (HeI,
HeII, coronal lines, and also Balmer lines at later times), for which the
separation in velocity of blue and red peaks is still shrinking at the time of
Solar conjunction when it reaches an average 330 km s-1;
(6) the ”symbiotic band” at 6825 Å, due to Raman scattering of OVI 1032 Å by
neutral hydrogen, appears simultaneously with the coronal lines and evolves in
parallel with them in terms of integrated flux: its very presence indicates
that a sizeable part of the wind of the red giant was neutral while coronal
lines were developing. In symbiotic stars in quiescence the 6825 Å band is
normally observed only in systems undergoing stable nuclear burning, a
condition that appears holding for RS Oph up to around day +87.

1\. INTRODUCTION

This is the second of a three-parts atlas of the spectral evolution of the
symbiotic and recurrent nova RS Oph during the 2021 outburst. The first part
(Munari and Valisa 2021c, hereafter Paper I) covered the initial phase of the
evolution up to emersion of high ionization emission lines, corresponding to
interval 0 to 18 days past optical maximum (Aug 9 to 27, 2021). The present
Paper II goes from day 19 to 102 (Aug 28 to Nov 19, 2021), when a stop was
imposed on observations by Solar conjunction. A third and final paper will
deal with the period going from emersion from Solar conjunction (we resumed
observations of RS Oph in the pre-dawn sky on January 16, 2022; Munari and
Valisa 2022) up to the re-establishment of pre-outburst brightness (currently
dropped below that level as in all previous outbursts at similar phases) and
of pre-outburst spectral appearance.

The amount of information stored in a $\sim$daily spectral monitoring in high-
resolution/high-SN/absolute-fluxes of the current outburst of RS Oph is just
immense, and will take a long time and a great effort to be (at least in part)
digested. Nonetheless, there is a certain urgency to present at least a quick
overlook (even if without much discussion) of some basic characteristics of
such spectral evolution, in support of investigations at wavelengths other
then optical.

A relevant feature of our atlas is in the great homogeneity maintained through
the whole observing campaign: just three and the same spectrographs on meter-
class telescopes (Asiago 1.22m and 1.82m, Varese 0.84m; all located on the
Italian Alps), identical IRAF-based data reduction and calibration procedures
(as described in the book by Zwitter and Munari 2000), same set of
spectrophotometric standards adopted at all three telescopes, etc.

Following on the style of Paper I, a series of 3200-8000 Å spectra at selected
dates are presented to document the overall appearance and evolution at
optical wavelengths, and then the focus shifts to selected emission lines for
which the absolutely-fluxed profiles are presented at 42 distinct dates
distributed over the day 19-to-102 time interval. A few key plots than follow,
presenting: a comparison of lines profiles for the 2006 and 2021 outbursts;
the shrinking with time of the separation in velocity of triple-peaked lines;
the evolution in the integrated flux of some lines and of the underlying true
continuum away from lines; the behavior with time of the FWHM of emission
lines; and the evolution of the integrated flux of iron coronal lines [FeX],
[FeXI], [FeXIV] and for reference of [FeVII], as well as the Raman scattering
at 6825 Å of OVI 1032 Å by neutral hydrogen.

2\. RS OPH

The current outburst has triggered observations of RS Oph over the whole
electromagnetic spectrum. As noted in Paper I, preliminary reports have been
presented by Cheung et al. (2021a,b) and Wagner & HESS Collaboration (2021a,b)
for $\gamma$-rays; Enoto et al. (2021a,b), Ferrigno et al. (2021), Luna et al.
(2021), Page (2021a), Page et al. (2021), Rout et al. (2021) and Shidatsu et
al. (2021) for the X-rays; Mikolajewska et al. (2021), Munari & Valisa P
(2021a,b), Shore et al. (2021a,b,c), and Taguchi et al. (2021a,b) for the
optical; Woodward et al. (2021a) for the infrared; and Sokolovsky et al.
(2021) and Williams et al. (2021) for the radio. The results of the search for
neutrino emission has been given by Pizzuto et al. (2021), and on the
polarization of optical light by Nikolov & Luna (2021).

Following the release of Paper I, Page (2021b) announced RS Oph entering the
super-soft X-ray emission phase as later confirmed by Pei et al. (2021) and
Orio et al. (2021b,c), with description of X-ray grating spectra given by Orio
et al. (2021a). Montez et al. (2021) reported on the detection and analysis of
extended bipolar X-ray emission stemming from the 2006 eruption. Detection of
very-high-energy gamma rays from RS Oph have been reported and analyzed in
conjunction with Fermi-LAT data by the H.E.S.S. Collaboration (2022) and MAGIC
Collaboration (2022). Zamanov et al. (2021a) noted the disappearance of
flickering and some optical photometry was provided by Ricra et al. (2021),
while Fajrin et al. (2021) reported about their spectroscopic monitoring in
H$\alpha$ as also done by Zamanov et al. (2021b). Emersion of coronal emission
lines in optical spectra was reported by Munari et al. (2021), followed by a
similar detection at infrared wavelengths by Woodward et al. (2021). Finally,
the photometric and spectroscopic status of RS Oph after emersion in mid
January 2022 from Solar conjunction has been described by Munari et al.
(2022).

3\. OBSERVATIONS

We have used the same telescopes/spectrographs and data acquisition/reduction
procedures as in Paper I.

Echelle observations have been recorded with (1) the 1.82m telescope + REOSC
Echelle spectrograph operated in Asiago by the National Institute of
Astrophysics INAF, that covers in 32 orders and at 22,000 resolving power (for
the usual 1.8-arcsec slit width) the wavelength range 3500-7350 Å, without
inter-order gaps; and (2) the Varese 0.84m telescope operated by ANS
Collaboration and equipped with an Astrolight Inst. mk.III Multi-Mode
spectrograph, that in the Echelle configuration covers the 4250-8900 Å range
at a 18,000 resolving power (for a 2-arcsec slit) and without inter-order
gaps.

Low resolution observations have been acquired with the B&C spectrograph on
the 1.22m telescope operated in Asiago by the University of Padova. It offers
a great sensitivity at near-UV wavelengths, and spectra obtained with the 300
ln/mm grating blazed at 5000 Å allows to cover the 3200-8000 Å range at 2.3
Å/pix dispersion. The observations here presented at been recorded at
FWHM(PSF)=2 pix sampling.

Data reduction at all three telescopes has been performed in IRAF and has
included all usual steps of correction for bias, dark and flat, sky
subtraction, wavelength calibration, and heliocentric correction.
Spectrophotometric standards, located close on the sky to RS Oph, have been
observed each night soon before and/or after the RS Oph to achieve an optimal
flux calibration, which was also essential to accurately merge the individual
30 Echelle orders into a single 1D fluxed spectrum without the usual dents at
the points of junction.

Journals of observations for Echelle and B&C spectra are provided in Tables 1
and 2, where the elapsed time $t-t_{\rm max}$ is computed from maximum optical
brightness taken from Paper 1 to have occurred on

$t^{V}_{\rm max}=2459436.18~{}{\rm JD}~{}~{}~{}~{}~{}~{}2021~{}{\rm
Aug}~{}09.68~{}~{}~{}~{}(\pm 0.05)$ (1)

Table 1: Journal of Echelle spectroscopic observations obtained with the Varese 0.84m and Asiago 1.82m telescopes. date | HJD |  t-tmax | expt | tel. |  |  | date | HJD |  t-tmax | expt | tel.
---|---|---|---|---|---|---|---|---|---|---|---
|  |  | (sec) |  |  |  |  |  |  | (sec) |
2021-08-09 | 2459436.300 | 0.12 | 540 | 0.84m |  |  | 2021-09-07 | 2459465.312 | 29.13 | 5400 | 0.84m
2021-08-09 | 2459436.337 | 0.16 | 540 | 0.84m |  |  | 2021-09-08 | 2459466.281 | 30.10 | 3000 | 0.84m
2021-08-09 | 2459436.373 | 0.19 | 540 | 0.84m |  |  | 2021-09-09 | 2459467.322 | 31.14 | 2700 | 0.84m
2021-08-09 | 2459436.404 | 0.22 | 720 | 0.84m |  |  | 2021-09-10 | 2459468.308 | 32.13 | 4200 | 0.84m
2021-08-10 | 2459437.320 | 1.14 | 720 | 0.84m |  |  | 2021-09-11 | 2459469.285 | 33.11 | 2430 | 0.84m
2021-08-10 | 2459437.389 | 1.21 | 720 | 0.84m |  |  | 2021-09-12 | 2459470.268 | 34.09 | 2700 | 0.84m
2021-08-11 | 2459438.342 | 2.16 | 1440 | 0.84m |  |  | 2021-09-13 | 2459471.271 | 35.09 | 3000 | 0.84m
2021-08-12 | 2459439.332 | 3.15 | 1200 | 0.84m |  |  | 2021-09-14 | 2459472.290 | 36.11 | 3600 | 0.84m
2021-08-13 | 2459440.308 | 4.13 | 900 | 0.84m |  |  | 2021-09-17 | 2459475.277 | 39.10 | 4200 | 0.84m
2021-08-14 | 2459441.367 | 5.19 | 1440 | 0.84m |  |  | 2021-09-21 | 2459479.297 | 43.12 | 3000 | 0.84m
2021-08-15 | 2459442.310 | 6.13 | 1200 | 0.84m |  |  | 2021-09-24 | 2459482.269 | 46.09 | 3000 | 0.84m
2021-08-16 | 2459443.324 | 7.14 | 3000 | 0.84m |  |  | 2021-09-27 | 2459485.282 | 49.10 | 4050 | 0.84m
2021-08-17 | 2459444.306 | 8.13 | 900 | 0.84m |  |  | 2021-09-29 | 2459487.246 | 51.07 | 4050 | 0.84m
2021-08-18 | 2459445.310 | 9.13 | 1500 | 0.84m |  |  | 2021-10-01 | 2459489.242 | 53.06 | 4050 | 0.84m
2021-08-19 | 2459446.299 | 10.12 | 240 | 1.82m |  |  | 2021-10-06 | 2459494.240 | 58.06 | 4800 | 0.84m
2021-08-19 | 2459446.353 | 10.17 | 1500 | 0.84m |  |  | 2021-10-08 | 2459496.235 | 60.05 | 4800 | 0.84m
2021-08-20 | 2459447.308 | 11.13 | 1500 | 0.84m |  |  | 2021-10-10 | 2459498.230 | 62.05 | 3150 | 0.84m
2021-08-21 | 2459448.301 | 12.12 | 1500 | 0.84m |  |  | 2021-10-12 | 2459500.231 | 64.05 | 4800 | 0.84m
2021-08-22 | 2459449.296 | 13.12 | 1500 | 0.84m |  |  | 2021-10-15 | 2459503.235 | 67.06 | 3600 | 0.84m
2021-08-22 | 2459449.297 | 13.12 | 540 | 1.82m |  |  | 2021-10-17 | 2459505.225 | 69.04 | 3600 | 0.84m
2021-08-23 | 2459450.294 | 14.11 | 2400 | 0.84m |  |  | 2021-10-19 | 2459507.222 | 71.04 | 4050 | 0.84m
2021-08-24 | 2459451.314 | 15.13 | 1800 | 0.84m |  |  | 2021-10-22 | 2459510.217 | 74.03 | 900 | 1.82m
2021-08-25 | 2459452.296 | 16.12 | 1800 | 0.84m |  |  | 2021-10-23 | 2459511.218 | 75.04 | 4050 | 0.84m
2021-08-25 | 2459452.312 | 16.13 | 540 | 1.82m |  |  | 2021-10-26 | 2459514.223 | 78.04 | 4800 | 0.84m
2021-08-26 | 2459453.285 | 17.11 | 2400 | 0.84m |  |  | 2021-10-28 | 2459516.218 | 80.04 | 4050 | 0.84m
2021-08-27 | 2459454.309 | 18.13 | 600 | 0.84m |  |  | 2021-11-01 | 2459520.234 | 84.05 | 4800 | 0.84m
2021-08-28 | 2459455.302 | 19.12 | 2100 | 0.84m |  |  | 2021-11-04 | 2459523.209 | 87.03 | 4200 | 0.84m
2021-08-29 | 2459456.292 | 20.11 | 2400 | 0.84m |  |  | 2021-11-05 | 2459524.210 | 88.03 | 4200 | 0.84m
2021-08-30 | 2459457.366 | 21.19 | 3600 | 0.84m |  |  | 2021-11-07 | 2459526.210 | 90.03 | 4500 | 0.84m
2021-08-31 | 2459458.280 | 22.10 | 2400 | 0.84m |  |  | 2021-11-11 | 2459530.209 | 94.03 | 3600 | 0.84m
2021-09-01 | 2459459.287 | 23.11 | 2100 | 0.84m |  |  | 2021-11-12 | 2459531.209 | 95.03 | 2700 | 0.84m
2021-09-02 | 2459460.274 | 24.09 | 1500 | 0.84m |  |  | 2021-11-18 | 2459537.214 | 101.03 | 2700 | 0.84m
2021-09-03 | 2459461.289 | 25.11 | 3000 | 0.84m |  |  | 2021-11-19 | 2459538.204 | 102.02 | 2700 | 0.84m
2021-09-06 | 2459464.314 | 28.13 | 4200 | 0.84m |  |  |  |  |  |  |
Table 2: Journal of B&C spectroscopic observations obtained with the Asiago 1.22m telescope. date | HJD |  t-tmax | expt | tel. |  |  | date | HJD |  t-tmax | expt | tel.
---|---|---|---|---|---|---|---|---|---|---|---
|  |  | (sec) |  |  |  |  |  |  | (sec) |
2021-08-09 | 2459436.284 | 0.10 | 100 | 1.22m |  |  | 2021-08-19 | 2459446.281 | 10.10 | 110 | 1.22m
2021-08-09 | 2459436.331 | 0.15 | 150 | 1.22m |  |  | 2021-08-19 | 2459446.330 | 10.15 | 90 | 1.22m
2021-08-10 | 2459437.300 | 1.12 | 120 | 1.22m |  |  | 2021-08-20 | 2459447.307 | 11.13 | 120 | 1.22m
2021-08-10 | 2459437.383 | 1.20 | 150 | 1.22m |  |  | 2021-08-20 | 2459447.369 | 11.19 | 90 | 1.22m
2021-08-11 | 2459438.298 | 2.12 | 90 | 1.22m |  |  | 2021-08-22 | 2459449.321 | 13.14 | 150 | 1.22m
2021-08-11 | 2459438.419 | 2.24 | 130 | 1.22m |  |  | 2021-08-23 | 2459450.279 | 14.10 | 150 | 1.22m
2021-08-12 | 2459439.281 | 3.10 | 90 | 1.22m |  |  | 2021-08-31 | 2459458.297 | 22.12 | 95 | 1.22m
2021-08-12 | 2459439.295 | 3.12 | 170 | 1.22m |  |  | 2021-09-01 | 2459459.282 | 23.10 | 85 | 1.22m
2021-08-12 | 2459439.365 | 3.19 | 120 | 1.22m |  |  | 2021-09-02 | 2459460.267 | 24.09 | 135 | 1.22m
2021-08-13 | 2459440.319 | 4.14 | 120 | 1.22m |  |  | 2021-09-02 | 2459460.354 | 24.17 | 90 | 1.22m
2021-08-13 | 2459440.378 | 4.20 | 120 | 1.22m |  |  | 2021-09-03 | 2459460.349 | 24.17 | 90 | 1.22m
2021-08-14 | 2459441.351 | 5.17 | 100 | 1.22m |  |  | 2021-09-05 | 2459463.267 | 27.09 | 110 | 1.22m
2021-08-15 | 2459442.342 | 6.16 | 120 | 1.22m |  |  | 2021-09-12 | 2459470.284 | 34.10 | 130 | 1.22m
2021-08-15 | 2459442.386 | 6.21 | 90 | 1.22m |  |  | 2021-09-27 | 2459485.264 | 49.08 | 120 | 1.22m
2021-08-17 | 2459444.301 | 8.12 | 120 | 1.22m |  |  | 2021-10-16 | 2459504.233 | 68.05 | 900 | 1.22m
2021-08-18 | 2459445.309 | 9.13 | 120 | 1.22m |  |  | 2021-11-11 | 2459530.201 | 94.02 | 1200 | 1.22m

4\. OVERALL SPECTRAL APPEARANCE

The low-resolution spectra obtained with the Asiago 1.22m + B&C telescope
allow to grasp an overall picture of the evolution of RS Oph during the 19 to
102 day interval under consideration in this Paper II. A selection of them is
presented in Figure 1.

The brightness of the underlying continuum and the integrated flux of emission
lines have both declined steadily, the latter at a lower pace so that - as in
most novae - the contrast of emission lines appear to increase with time,
expecialy for the nebular ones like [OIII], [OII], [OI], [NeIII], [NII],
[SII], etc. Coronal emission lines started to emerge between days 25-30, and
rapidly increased in intensity with [FeX] remaining always the strongest of
all. The increase in ionization is well illustrated by the steady progress
with time of the [NeV] 3427 / OIII 3440 ratio.

It is evident in Figure 1 how the region around 4770 Å has remained rather
free from the presence of emission lines, and consequently it offers the
possibility to evaluate there the flux level of the background continuum, at
blue wavelengths where the direct emission from the stable red giant is
expected to emerge at later times than in the red.

Figure 1: Sequence of Asiago 1.22m + B&C spectra highlighting the evolution of
RS Oph during the 19 to 102 day interval. The most relevant emission lines are
identified at the top, and the comb-markings at the bottom point to FeII
multiplets. The observing epochs are marked as days passed since maximum
brightness $t-t_{\rm max}$ (cf. Table 2).

5\. COMPARISON OF 2006 AND 2021 LINE PROFILES

As the photometric evolution of the current outburst is a close replica of
what seen in 2006 and previous events, a marked similarity affects also the
spectra, passing through similar phases at similar times. There is however a
striking difference: the mirror appearance of tripled-peaked profiles, as
illustrated in Figure 2. Throughout the 2006 event and for all lines
presenting a triple-peaked profile, the blue peak has always been brighter
than the red one: the exact contrary is observed for the 2021 outburst, with
the red peak stronger than the blue one at all phases and for all lines as
illustrated by the time-sequences of profiles presented in Figures 6 to 11.
With all probability this difference relates to the fact the 2006 and 2021
outbursts occurred on opposite sides of the binary orbit.

The triple-peaked profile is present or at least recognizable in nearly all
lines observed in 2021, with the exception of [NII], [OIII] and a few others.
Also Balmer lines evolved to a triple-peaked profile by the time the Solar
conjunction was approaching.

For all triple-peaked line profiles (cf HeI, HeII, or coronal lines in Figures
6 to 10), the side peaks (i.e. the red and the blue peak) decline faster than
the central peak. At late times, for some of the riple-peaked lines (eg.
[FeXIV] 5303) only the central peak remained visible.

Balmer lines presented a lower degree of ”mirror symmetry” when comparing 2006
and 2021 profiles. From Figure 2, a triple-peaked profile seems present in
H$\beta$ in 2006 already at day 30, but with a reversed blue/red peak ratio
compared to the other 2006 lines and with a prominent central peak, while
H$\alpha$ looks rather symmetrical. With a certain degree of imagination, a
triple-peaked profile can be recognized also in the 2021 H$\beta$ profile for
day 30 in Figure 2 but, differently from 2006, with a insignificant central
peak and a blue/red peak ratio similar to that of the other lines and not
reversed.

When comparing the relative displacement of the narrow (from the ionized wind)
and very broad (from expanding ejecta) components in the [NII] 5755 profile, a
”mirror symmetry” is present comparing 2006 to 2021 profiles in Figure 2.

Figure 2: A comparison of the profile of a few representative lines, at day
+30 from optical maximum, for the 2006 (from Munari et al. 2007) and 2021
outbursts.

6\. VELOCITY SEPARATION OF TRIPLE-PEAKED LINE PROFILES

The velocity separation ($\Delta$V) of red and blue peaks has been measured on
the Echelle spectra of Figures 6 to 11 for a number of emission lines and its
progression with time is plotted in Figure 3.

Higher ionization lines as HeII, [FeX] and [FeXIV] behave all rather
similarly, with a separation $\sim$60 km s-1 larger than for lower excitation
lines like FeII and OI. The HeI lines locate neatly in between, with the green
line in Figure 3 being a spline fitting to all measurable HeI lines, in
particular to 6678 and 5015 singlet lines, and 5876 and 7065 triplet lines.

The behavior of $\Delta$V for HeI lines (copied by HeII) is characterized by a
marked change in slope taking place during the day 30 to 50 interval (this is
the same time interval over which coronal lines reach in turn their respective
plateau maxima, from [FeX] around day 30 to [FeXIV] about day 50; see Figure
sect. 7 and Figure 5 below): namely, before day 30 $\Delta$V steadily reduces
by $-$10 km s-1 each day, while after day 50 such shrinking is drastically
slowed to a mere $-$1 km s-1 per day.

Figure 3: Velocity separation of the blue and red peaks of the triple-peaked
profiles for selected lines on the 2021 Echelle spectra of RS Oph presented in
Figures 6 to 11. By ”HeI various” we mean a spline fitting to all measurable
HeI lines, in particular the 6678 and 5015 singlet lines, and 5876 and 7065
triplet lines.

7\. EVOLUTION OF INTEGRATED FLUX AND WIDTH OF BALMER LINES

The evolution in terms of integrated flux of H$\alpha$ and H$\beta$ emission
lines is presented in the top panel of Figure 4, compared to the background
brightness measured at 4770 Å. The decline in flux of H$\alpha$ and H$\beta$
has been very smooth and very similar, with measurement of initial H$\alpha$
values being probably affected by P-Cyg absorptions, overlapping nearby lines
and the blackground glare of the flash-ionized wind of the red-giant. A marked
acceleration of the decline in flux sets around day +85, contemporaneous with
the coronal lines abandoning the plateau maximum and beginning their rapid
descent (cf. next section and Figure 5). The change in slope of H$\alpha$ and
H$\beta$ has been somewhat mirrored by a similar change visible in the
intensity of the background measured at 4770 Å. At early times the flux in the
continuum declines must faster that flux in the Balmer lines, and only after
day 30 they behave in parallel. The fluxes for 4770 Å and H$\beta$ are
tabulated in Table 3.

The evolution of the width of H$\alpha$ and H$\beta$ emission lines is
illustrated in the bottom panel of Figure 4. Both behave very smoothly and
rather similarly, with differences at latest epochs being probably spurious
and induced by the appearance of the triple-peaked profile occuring first in
H$\gamma$, then H$\beta$, and only last in H$\alpha$.

From the bottom panel of Figure 4 it is evident how the width of all lines
displaying, sooner or later, a triple-peaked profile ended converging toward
FWHM$\approx$330 km s-1, a value than was still shrinking at the time when
Solar conjunction put a halt to the observations. On the contrary, emission
lines like [NII] and [OIII] showing a profile characterized by a single broad
Gaussian (ignoring the superimposed sharp peak originating in the
kinematically quiet and flashed wind of the red giant), had already reached
their terminal width at much earlier times (day $\sim$45), with no further
significantive shrinking afterward. Their terminal width, FWHM$\approx$1000 km
s-1, is three times broader than for triple-peaked profiles.

It therefore seems that within RS Oph co-existed two distinct types of ejecta:
($i$) a faster moving one, giving rise to the Gaussian-like broad profiles for
[NII] and [OIII], leaving the central binary in a sort of symmetrical
arrangement at a terminal $\sim$425 km s-1 velocity (taking it equal to
FWHM/$2\sqrt{2\,\ln 2}$); and ($ii$) a slower moving one, giving rise to the
triple-peaked profiles, with the material confined in the bipolar regions
moving at $\sim$$|$165$|$ km s-1 away from the material that originates the
central peak and which is at rest with the binary system.

Figure 4: Top panel: evolution of the integrated flux of H$\alpha$ and
H$\beta$ emission lines (log scale), measured on the 2021 Echelle spectra of
RS Oph presented in Figures 6 to 11. Note the ”$-$1” offset for H$\alpha$ in
the top panel, for an easier comparison with H$\beta$. The dot-dashed line is
the flux in the background continuum measured at 4770 Å (and expressed in erg
cm-2 s-1 Å-1). Bottom panel: evolution of the FWHM of the H$\alpha$ and
H$\beta$ emission lines (log-log plot), and for comparison also of other
lines. The dot-dashed line is the ”HeI various” from previous Figure 3 (cf.
sect.5).

8\. EVOLUTION OF CORONAL AND OVI EMISSION LINES

A hallmark of the 2021 and all previous outbursts of RS Oph is the development
of strong coronal lines. We have measured the integrated flux of the iron
coronal lines in the Echelle spectra of Figures 6 and 7, and tabulated them in
Table 3 with a graphical presentation given in Figure 5. In the same figure,
we have plotted for comparison the behavior of the continuum at 4770 Å and of
the Raman scattering by neutral hydrogen at 6825 Å of OVI 1032.

The coronal lines started to appear with day 23, when the optical thickness of
the expanding ejecta lowered sufficiently to allow ionizing photon to spread
through. After a rapid rise in intensity, they all levelled off at a plateau
maximum which was very smooth and that mildly declined in pace with the
continuum background emission. Even if their appearance and their reaching
maximum brightness happened in a succession paralleling their ionization
degree, all the coronal lines left the plateau maximum at basically the same
time, around day 87, to begin a rapid decline consistent with a switching-off
of the nuclear burning on the white dwarf of RS Oph.

Of special interest is the presence and behavior of the ”symbiotic band” at
6825 Å. This band is only observed in the symbiotic stars that in quiescence
are powered by nuclear burning of the material accreted by their WDs. For a
long time its origin has remained unidentified, until the mystery was solved
by Schmid (1989) is terms of Raman scattering at 6825 and 7088 Å of the
ultraviolet resonance doublet OVI 1032, 1038 Å by neutral hydrogen present
within the binary. During the quiescence in between the outbursts, RS Oph does
not burn on its WD and does not display the 6825 and 7088 Å bands (Munari and
Zwitter 2002), which are observable only during nova outbursts (the 7088 Å
band is much weaker than 6825 Å and essentially lost in the bright wings of
HeI 7065). The behavior of 6825 Å in Figure 5 is rather similar to the coronal
lines in terms of time of appearance, rising rate, plateau maximum, and onset
of the decline. The very presence of 6825 Å proves that a sizeable fraction of
the wind of the red giant remained neutral during the development of the
coronal lines, and reinforces the notion that nuclear burning was probably
active through all that period.

Figure 5: Evolution along the 2021 outburst of the integrated flux of the coronal lines [FeX] 6375, [FeXI] 7892, and [FeXIV] 5303 Å, and for comparison of [FeVII] 6087 Å, the ”symbiotic band” at 6825 Å (Raman scattering by neutral hydrogen of OVI 1032), and of the continuum at 4770 Å (the latter expressed in erg cm-2 s-1 Å-1). Table 3: Logarithm of the flux in the continuum at 4770 Å (in erg cm-2 s-1 Å-1) and the logarithm of the integrated flux (in erg cm-2 s-1) of H$\beta$, [FeVII] 6087, [FeX] 6375, [FeXI] 7892, and [FeXIV] 5303 emission lines from the Echelle spectra listed in Table 1 and displayed in Figure 6 to 11. t-tmax | 4770 Å |  H$\beta$ | [FeVII] | [FeX] | [FeXI] | [FeXIV]
---|---|---|---|---|---|---
1.14 |  $-$9.731 |  $-$8.756 |  |  |  |
1.21 |  $-$9.747 |  $-$8.770 |  |  |  |
2.16 |  $-$9.951 |  $-$8.778 |  |  |  |
3.15 |  $-$10.129 |  $-$8.878 |  |  |  |
4.13 |  $-$10.280 |  $-$8.853 |  |  |  |
5.19 |  $-$10.415 |  $-$8.930 |  |  |  |
6.13 |  $-$10.514 |  $-$9.017 |  |  |  |
7.14 |  $-$10.602 |  $-$9.193 |  |  |  |
8.13 |  $-$10.671 |  $-$9.197 |  |  |  |
9.13 |  $-$10.728 |  $-$9.160 |  |  |  |
10.12 |  $-$10.773 |  $-$9.340 |  |  |  |
10.17 |  $-$10.775 |  $-$9.196 |  |  |  |
11.13 |  $-$10.812 |  $-$9.264 |  |  |  |
12.12 |  $-$10.846 |  $-$9.283 |  |  |  |
13.12 |  $-$10.879 |  $-$9.402 |  |  |  |
14.11 |  $-$10.912 |  $-$9.318 |  |  |  |
15.13 |  $-$10.952 |  $-$9.405 |  |  |  |
16.13 |  $-$10.995 |  $-$9.443 |  $-$12.734 |  |  |
17.11 |  $-$11.041 |  $-$9.461 |  $-$12.710 |  |  |
19.12 |  $-$11.147 |  $-$9.537 |  $-$12.588 |  |  |
20.11 |  $-$11.203 |  $-$9.535 |  $-$12.515 |  |  |
21.19 |  $-$11.265 |  $-$9.556 |  $-$12.396 |  |  |
22.10 |  $-$11.319 |  $-$9.586 |  $-$12.282 |  |  |
23.11 |  $-$11.378 |  $-$9.601 |  $-$12.305 |  $-$11.824 |  |
24.09 |  $-$11.435 |  $-$9.656 |  $-$12.234 |  $-$11.154 |  |
25.11 |  $-$11.493 |  $-$9.628 |  $-$12.285 |  $-$11.005 |  |
28.13 |  $-$11.649 |  $-$9.772 |  $-$12.400 |  $-$10.612 |  $-$12.056 |
29.13 |  $-$11.693 |  $-$9.795 |  $-$12.465 |  $-$10.632 |  $-$11.789 |
30.10 |  $-$11.731 |  $-$9.850 |  $-$12.513 |  $-$10.559 |  $-$11.402 |
31.14 |  $-$11.768 |  $-$9.879 |  $-$12.532 |  $-$10.501 |  $-$11.339 |  $-$12.113
32.13 |  $-$11.798 |  $-$9.891 |  $-$12.493 |  $-$10.519 |  $-$11.262 |  $-$12.089
33.11 |  $-$11.825 |  $-$9.871 |  $-$12.482 |  $-$10.448 |  $-$11.070 |  $-$11.886
34.09 |  $-$11.849 |  $-$9.939 |  $-$12.574 |  $-$10.415 |  $-$10.980 |  $-$11.738
35.09 |  $-$11.870 |  $-$9.956 |  $-$12.601 |  $-$10.372 |  $-$10.919 |  $-$11.621
36.11 |  $-$11.889 |  $-$10.028 |  $-$12.508 |  $-$10.455 |  $-$10.936 |  $-$11.608
39.10 |  $-$11.933 |  $-$10.013 |  $-$12.683 |  $-$10.360 |  $-$10.763 |  $-$11.337
43.12 |  $-$11.981 |  $-$10.078 |  $-$12.690 |  $-$10.380 |  $-$10.739 |  $-$11.202
46.09 |  $-$12.020 |  $-$10.111 |  $-$12.676 |  $-$10.414 |  $-$10.685 |  $-$11.144
49.10 |  $-$12.064 |  $-$10.143 |  $-$12.760 |  $-$10.407 |  $-$10.656 |  $-$11.074
51.07 |  $-$12.093 |  $-$10.193 |  $-$12.751 |  $-$10.362 |  $-$10.553 |  $-$11.068
53.06 |  $-$12.121 |  $-$10.253 |  $-$12.813 |  $-$10.405 |  $-$10.600 |  $-$11.087
58.06 |  $-$12.180 |  $-$10.355 |  $-$12.729 |  $-$10.421 |  $-$10.597 |  $-$11.097
60.05 |  $-$12.196 |  $-$10.351 |  $-$12.762 |  $-$10.426 |  $-$10.644 |  $-$11.079
62.05 |  $-$12.208 |  $-$10.390 |  $-$12.742 |  $-$10.433 |  $-$10.590 |  $-$11.069
64.05 |  $-$12.218 |  $-$10.407 |  $-$12.789 |  $-$10.431 |  $-$10.551 |  $-$11.079
67.06 |  $-$12.232 |  $-$10.433 |  $-$12.861 |  $-$10.485 |  $-$10.648 |  $-$11.089
69.04 |  $-$12.243 |  $-$10.414 |  $-$12.840 |  $-$10.517 |  $-$10.690 |  $-$11.070
71.04 |  $-$12.258 |  $-$10.442 |  $-$12.889 |  $-$10.527 |  $-$10.745 |  $-$11.100
74.04 |  $-$12.290 |  $-$10.429 |  $-$12.906 |  $-$10.601 |  $-$10.711 |  $-$11.137
75.04 |  $-$12.303 |  $-$10.553 |  $-$12.896 |  $-$10.545 |  $-$10.697 |  $-$11.178
78.04 |  $-$12.353 |  $-$10.614 |  $-$12.939 |  $-$10.570 |  $-$10.708 |  $-$11.199
80.04 |  $-$12.392 |  $-$10.570 |  $-$13.040 |  $-$10.655 |  $-$10.837 |  $-$11.202
84.05 |  $-$12.482 |  $-$10.719 |  $-$13.196 |  $-$10.648 |  $-$10.831 |  $-$11.273
87.03 |  $-$12.556 |  $-$10.763 |  $-$13.291 |  $-$10.786 |  $-$11.037 |  $-$11.333
88.03 |  $-$12.582 |  $-$10.779 |  $-$13.288 |  $-$10.779 |  $-$10.972 |  $-$11.364
90.03 |  $-$12.635 |  $-$10.845 |  $-$13.398 |  $-$10.861 |  $-$11.091 |  $-$11.449
94.03 |  $-$12.741 |  $-$11.038 |  |  $-$11.074 |  $-$11.234 |  $-$11.709
95.03 |  $-$12.767 |  $-$11.016 |  |  $-$11.193 |  $-$11.387 |  $-$11.788
101.03 |  $-$12.913 |  $-$11.342 |  |  $-$11.417 |  $-$11.681 |  $-$12.423
102.02 |  $-$12.935 |  $-$11.349 |  |  $-$11.459 |  $-$11.736 |  $-$12.567

9\. PICTORIAL EVOLUTION OF SELECTED EMISSION LINES

In Figures 6 to 11 we present the temporal evolution of a sample of emission
lines from our Echelle spectra. The $t-t_{\rm max}$ time from maximum (in
days) is marked in red next to each spectrum. The spectra are plotted in a
log(flux) + offset scale, which makes visible both faint features on the wings
as well as the vastly brighter core. Earlier epochs are covered in Paper I,
from which for continuity and overlap we take the first two spectra for day 16
and 17.

Figure 6: Evolution from day 16 to 102 of selected emission lines in our
Echelle spectra of the 2021 outburst of RS Oph (cf Table 1). The $t-t_{\rm
max}$ time from maximum (in days) is marked in red next to each spectrum.
Identification of OIV 6106 following R.M. Wagner (private comm.). Figure 7:
continues from Figure 6. Figure 8: continues from Figure 6. Figure 9:
continues from Figure 6. Figure 10: continues from Figure 6. Figure 11:
continues from Figure 6.

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