Optical observations of RS Oph after its 2021 outburst

. RS Oph is a recurrent symbiotic novae which underwent its last optical eruption in August 2021. The early H α spectroscopy of the system (at days 11–15 of the outburst) reveals satellite line components at the velocity position of about ± 2400 km s − 1 which are an indication of bipolar collimated outﬂow. We derived some parameters of the outﬂows and system’s components and their evolution during our observation.


Introduction
RS Oph consists of a red giant and a massive white dwarf with a heavy mass loss during activity.Its last 2021 outburst began on August 8.931 and was observed over the whole electromagnetic domain.Here we report high-resolution Hα spectroscopy of the system, obtained at days [11][12][13][14][15][19][20][21][22][23] of the outburst with the Coudé spectrograph of the 2m RCC telescope at Rozhen NAO, Bulgaria.We present low-resolution observations (the resolving power is R ∼ 1100) secured with the 2-Channel-Focal-Reducer Rozhen, attached to the Cassegrain focus of the 2m RCC telescope as well.

Spectral Energy Distribution (SED)
To examine the outflow structure of the outbursting compact object, we built the SED of the system for the first and last day of the observations.We used average U , B, V , R C , and I C photometric estimates from the light curves of the AAVSO database 2 taken during our observations from August 19 to 23.The approximation of the U BV R C I C fluxes showed that for this period T eff of the pseudophotosphere and T e of the nebula have not changed: T eff = 15 000±1 000 K and T e = 17 000 ± 3000 K.We obtained R eff = (13.3± 2.0)(d/1.6kpc)R and EM = (9.50 ± 0.59) 10 61 (d/1.6kpc) 2 cm −3 for August 19 and R eff = (10.3 ±  1.6)(d/1.6kpc) R and EM = (5.60 ± 0.35) 10 61 (d/1.6kpc) 2 cm −3 for August  23.The SED is presented in Fig. 1 (Tomov et al., 2023).

Hα profile, spectral evolution and mass-loss rate
Our spectral observations reveal a typical behaviour of a nova with a complex structure dominated by broad Balmer, He, O and Fe lines with some P Cyg profiles, evolved towards a supersoft source phase (see Fig. 1).The most prominent feature on the spectra is the Hα emission line.A P Cyg profile with velocity up to ∼ −4260 km s −1 (Nikolov, 2023) is detected on day 2 (Aug.10).
The complex structure of Hα line is better seen in our high-resolution spectra, obtained at days 11-15 of the outburst (Tomov et al., 2023).The Hα profile has strongly changed -the very sharp emission and absorption spikes, which were observed at day 2.3 (Munari et al., 2022) on top of the much wider and stronger emission line, were very weak.The very intensive broad component had an appreciable asymmetry and very broad low-intensity wings reaching ±3500 km s −1 (Fig. 2).We assume this velocity to be related to nebular material ejected by the outbursting component.Hα had very weak satellite components at a velocity position of about ±2400 km s −1 as well.We suppose that the satellite components are an indication of bipolar outflow from the outbursting component as during the 2006 eruption.It is worth noting that satellite components were also present in 2006 (Skopal et al., 2008) and they were much more intense relative to the central emission of the line than during the 2021 eruption.To obtain the parameters of the stellar wind and bipolar outflow, we analysed the Hα profile by means of approximation with different functions, which are shown in Fig. 2. The parameters of the satellite components determined by this approximation are listed in Tab. 1 (Tomov et al., 2023).Table 1.Parameters of the satellite components.The flux is in units of 10 −10 erg cm −2 s −1 , velocity and FWHM in km s −1 , linear angle θ in deg, and mass-loss rate in 10 −6 (d/1.6kpc)

Conclusions
We analysed the Hα profile with the aim being to study the structure of the outflowing material.We observed a disc-shaped warm shell occulting the central hot object and bipolar outflow.We find that during the 2021 eruption, about 30%-50% of the nebular emission belongs to the high-velocity wind, the Hα luminosity of which was less than 2700 L .The mass-loss rate of the outbursting object through its wind is much greater than through its streams.The total rate (from wind + streams) was less than (4 − 5) 10 −5 (d/1.6kpc) 3/2 M yr −1 .

Figure 2 .
Figure 2. Hα line.The spectrum of lowest intensity relates to August 19.