[Paper Review] The 2021 outburst of RS Oph: a pictorial atlas of the spectroscopic evolution. II. From day 19 to 102 (solar conjunction)

U. Munari, P. Valisa|arXiv (Cornell University)|Mar 2, 2022

Astrophysical Phenomena and Observations4 citations

TL;DR

This paper presents a high-resolution spectroscopic atlas of RS Oph's 2021 nova outburst from day 19 to 102, covering the period up to solar conjunction. It reveals a smooth spectral evolution with two distinct ejecta components: fast, un-decelerating ejecta producing Gaussian-like profiles and slower, triple-peaked profiles linked to a bipolar outflow with an equatorial torus, while coronal lines and Raman scattering at 6825 Å indicate sustained neutral wind and ongoing ionization processes until day ~87.

ABSTRACT

We present the second part (days 19 to 102) of our high-resolution Echelle atlas of the spectral evolution of RS Oph during the 2021 nova outburst. Some quick conclusions that can be drawn are: (1) the spectra evolution has progressed smoothly, 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, with a mirror-appearance of the triple-peaked line profiles (blue peak brighter than red in 2006, the opposite in 2021); (3) the evolution of the triple-peaked profiles progressed similarly through different excitation and ionization degrees, suggesting an origin in a bipolar outflow nested to an equatorial torus; (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; (5) there seems to exist within RS Oph two distinct types of ejecta: one fast moving and producing Gaussian-like line profiles (primarely [NII] and [OIII]), that keep expanding at FWHM=1000 km/s with no sign of ongoing deceleration during the two months leading up to Solar conjunction, and 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 of 330 km/s; (6) the "symbiotic band" at 6825 Ang, due to Raman scattering of OVI 1032 by neutral hydrogen, appears simultaneously with the coronal lines: its very presence indicates that a sizeable part of the wind of the red giant has remained neutral during the development of coronal lines.

Motivation & Objective

• To document the detailed spectroscopic evolution of RS Oph during the 2021 nova outburst from day 19 to 102, covering the phase from initial appearance of high-ionization lines to solar conjunction.
• To analyze the temporal evolution of emission line profiles, particularly triple-peaked structures, to infer the geometry and kinematics of the ejecta.
• To investigate the behavior of coronal emission lines and the Raman scattering feature at 6825 Å to assess ionization conditions and the state of the red giant's wind.
• To compare the 2021 outburst with the 2006 eruption, focusing on spectral profile symmetry and orbital phase effects.
• To establish a homogeneous, high-S/N, absolutely flux-calibrated dataset for multi-wavelength support and future modeling.

Proposed method

• Conducted daily high-resolution echelle spectroscopy using instruments on the Varese 0.84m and Asiago 1.82m telescopes in the Italian Alps.
• Applied identical IRAF-based data reduction and calibration procedures across all three telescopes (Asiago 1.22m, 1.82m, Varese 0.84m) to ensure spectral homogeneity.
• Used a consistent set of spectrophotometric standards across all observing sites to enable absolute flux calibration.
• Presented 3200–8000 Å spectra at selected dates and focused on absolutely flux-calibrated line profiles at 42 distinct epochs.
• Analyzed time evolution of line profiles, FWHM, integrated flux, and velocity shifts, particularly for HeI, HeII, coronal lines, and Balmer lines.
• Compared the 2021 outburst with the 2006 eruption using profile morphology and velocity evolution to infer orbital phase dependence.

Experimental results

Research questions

• RQ1 How does the spectroscopic evolution of RS Oph from day 19 to 102 reflect the dynamics of the ejecta and the central source's ionizing output?
• RQ2 What causes the observed asymmetry in triple-peaked line profiles (blue peak brighter in 2006, red in 2001), and how is it related to orbital phase?
• RQ3 What is the nature of the two distinct ejecta components observed in the line profiles—fast Gaussian-like and slower triple-peaked features?
• RQ4 How do coronal emission lines and the Raman scattering feature at 6825 Å evolve, and what do they reveal about the ionization state of the red giant wind?
• RQ5 What is the significance of the discontinuity in the rate of velocity separation shrinkage of triple-peaked profiles around day 30?

Key findings

• The spectral evolution from day 19 to 102 progressed smoothly without abrupt changes, indicating no major discontinuities in ejecta expansion or photoionization prior to nuclear burning switch-off.
• The 2021 outburst exhibited mirror-image triple-peaked profiles compared to the 2006 eruption, likely due to the two outbursts occurring at opposite orbital phases relative to the red giant companion.
• The velocity separation of triple-peaked line profiles shrank at a rate of -10 km s⁻¹ per day until day 30, then slowed to -1 km s⁻¹ per day, indicating a structural or dynamical transition.
• Coronal emission lines [FeX], [FeXI], [FeXIV] reached a flux plateau around day 87 and declined gradually, consistent with progressive clearing and cooling of the white dwarf after nuclear burning ceased.
• Two distinct ejecta components were identified: fast ejecta (FWHM ≈ 1000 km s⁻¹) showing no deceleration over 83 days, and slower ejecta (velocity separation ~330 km s⁻¹ at solar conjunction) producing triple-peaked profiles.
• The simultaneous appearance and co-evolution of the Raman scattering band at 6825 Å and coronal lines indicate that a significant fraction of the red giant wind remained neutral during the peak ionization phase, persisting until day ~87.