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Resolving the 3D structure of RS Oph, the fastest repeater among symbiotic recurrent-novae

by Ulisse Munari (INAF Padova, Italy), Marcello Giroletti (INAF Bologna, Italy), Benito Marcote (JIVE, the Netherlands), Tim J. O’Brien (Jodrell Bank, UK), Péter Veres (CSPAR, USA), Jun Yang (Chalmers Univ. Onsala, Sweden), David R. A. Williams (Jodrell Bank, UK), and Patrick Woudt (Univ. Cape Town, RSA)

Munari 1
Figure 1: Top: EVN image at 5 GHz of RS Oph on day +34 of its 2021 nova
eruption. The radio emission is elongated over the east–west direction for a total
extension of about 90 mas (or about 240 AU at the Gaia distance
d = 2.68 [+0.17/-0.15] kpc). A compact central component (CC) lies within a few mas
of the Gaia position (white cross) extrapolated to the observing epoch. Emission is
present on either side of the CC. This emission is brighter on the western side,
forming a circular lobe (WL), similarly to what was observed in the
2006 explosion (O’Brien et al. 2006). Fainter emission is present also to the east
of CC, in the shape of an arc (EA). Bottom: simple sketch of the expanding and
bipolar arrangement of RS Oph ejecta at the time of the EVN observation above.
The orbital plane is vertical and the diffuse and reddish component represents
the density enhancement on the orbital plane (DEOP). Comparing with the profile
of emission lines in optical spectra, the inclination of the orbital plane
is constrained to 54° +/- 1 and the velocity of the leading edge of both
WL and EA - averaged over the first 34 days of the outburst - to 7550 +/- 150 km/s.
At the time of this EVN observation, the leading edge of the receding lobe (EA)
has moved past the projected distance from the center where
the free–free opacity of the DEOP in the foreground drops to τν = 1.

An EVN monitoring campaign of the 2021 outburst of the symbiotic recurrent nova RS Oph, in combination with high-resolution ground-based spectroscopy and the accurate astrometric position by Gaia, has allowed us to derive an unprecedented 3D view of the pre-existing circumstellar environment and of the nova ejecta expanding through it: launched at an initial velocity >8000 km/s, the ejecta have been rapidly decelerated into a bipolar shape by the density-enhancement laying on the orbital plane, with strong non-thermal emission originating from the shock interface with the slow and thick wind of the red-giant (RG) companion to the erupting white dwarf (WD).

A nova eruption is rather violent, with 10-6 to 10-4 M⊙ of material being expelled at high velocity (103 km/s), as consequence of a thermonuclear runaway (TNR) igniting in the accreted and electron-degenerate shell of a white dwarf (WD).  The binary system survives unscathed the eruption, and shortly after it the donor companion resumes transferring material to the WD.  The refuelling time to the next eruption stretches over various orders of magnitudes, reaching the shortest recurrence times for the most massive WD accreting from cool giant companions (which are powerful mass-loser).  Such kind of binary is known as "symbiotic", and RS Oph is undoubtedly the most famous and best studied example of a symbiotic recurrent-nova, with eruptions recorded in 1898, 1933, 1958, 1967, 1985, 2006, and 2021, and possibly many other passed unnoticed during Solar conjunctions and the early years of only sparse monitoring.

In a "classical" nova, the donor is a tiny lower-main sequence star, orbiting the WD in a few hours at a separation of about one Solar radius, and the recurrence time is usually vastly longer than historical records so that practically none of these novae has been observed to recur.  The rapidly advancing ejecta quickly overtake the orbital separation (≤ an hour), after which they keep expanding in the surrounding void for the time being, freezing the imparted geometry.  A nova outburst developing in a symbiotic binary is a much more spectacular and dynamic show, thanks to the presence of the pre-existing, slow, thick and massive wind of the cool giant companion that permeates the circum-binary space for hundreds of AUs.  Such wind plays two main characters during a symbiotic nova eruption: (1) it offers an absorbing medium for the initial UV-flash of the TNR, get ionized glowing brightly, and recombines over time-scales of a few days, and (2) decelerates, shocks, and reshapes the ejecta that try expanding through it, which is the phenomenon we targeted with our EVN monitoring campaign of the 2021 eruption of RS Oph.

Our 2021 EVN image at 5 GHz for RS Oph on day +34 (counted from maximum optical brightness) is shown in the top panel of Figure 1, with the sketch in the bottom panel illustrating our interpretation of the observed structure.  In addition to a massive accretion disk (AD) around the WD, 3D hydro-dynamical simulations of symbiotic stars show that much of the mass loss from the RG is gravitationally focused by the companion towards the orbital plane, creating a strong and disk-like density enhancement with the binary at its center.  In Figure 1, the density enhancement on the orbital plane (DEOP) is represented by the diffuse and reddish component.  During a nova eruption, the combined effect of the AD and the DEOP is to confine the ejecta primarily within a bipolar structure, which expands perpendicular to the orbital plane.  The non-thermal radio emission originates at the shock interface between the fast-expanding lobes and the pre-existing slow wind of the RG.  The WL component in Figure 1 is the projection on the plane of the sky of the lobe moving towards us, in the foreground to the DEOP.  EA is the visible outer portion of the lobe expanding opposite the WL, moving away from us, and located behind the DEOP.  The projected surface density of the DEOP, and probably also its ionization degree, radially declines moving away from the central binary, which implies that the free-free opacity exerted by the DEOP also reduces as a function of the increasing projected distance from its centre.  For its synchrotron radiation (of frequency ν) to be able to reach us, the EA component has to move to a projected distance from the central binary that is large enough to cross the DEOP where the optical depth turns τν < 1, a condition occurring three weeks past optical maximum.  We interpret the CC as the interface between the DEOP and the impacting fast ejecta.  Its much higher density turns the DEOP into a decelerating medium that is far more efficient than the diffuse wind of RG: a lot of kinetic energy is transformed within a small volume of space, leading to the bright non-thermal central emission spike.

The 2021 EVN observations of RS Oph, coupled with high-resolution optical spectroscopy and Gaia astrometric position, has allowed us to obtain the most detailed 3D-view to date of a symbiotic recurrent-nova and its dynamical evolution.  It offers a complementary view to the one we obtained - again with EVN - of the symbiotic Mira V407 Cyg during its nova eruption of 2010, and provides a stimulating term of comparison for the imminent and much anticipated eruption of T CrB, which we hope to similarly image with EVN.

Published in Munari et al 2022 A&A 666, L6