Discovery of a nova super-remnant surrounding the recurrent nova LMCN 1971-08a in the Large Magellanic Cloud

TL;DR

Nova super-remnants (NSRs) are large shell-like structures produced by the cumulative ejecta of recurrent nova eruptions. The authors combine optical narrowband imaging (Hα, [S II], [O III]), HI kinematics, and deep radio data with Morpheus hydrodynamic modeling to study a newly identified NSR surrounding the recurrent nova LMCN 1971-08a in the LMC. They find a coherent circular shell of ~$200$ pc in diameter, bright in Hα and [S II], faint in [O III], with a coincident HI shell indicating shock-ionized gas expanding at about $10-20$ km s$^{-1}$ into a diffuse ISM, and a modeled shell mass around $4.13 imes 10^{3}$ M⊙ expanding at ~ $20$ km s$^{-1}$ into an ambient density of ≈$0.04$ cm$^{-3}$, implying an age of about $2.4$ Myr. The discovery supports the NSR hypothesis that all novae form such remnants, suggests a shorter recurrence time for LMCN 1971-08a than previously thought, and expands the NSR census to the LMC and extragalactic environments.

Abstract

A nova super-remnant (NSR) is a greatly-extended structure grown by repeated nova eruptions sweeping the surrounding material away from a nova into a dense outer shell and are predicted to form around all novae. To date, four NSRs are known, with three located in the Galaxy and one residing in M31. Here we present the discovery of the first NSR in the Large Magellanic Cloud and only the second extragalactic nova shell to be identified, hosted by the recurrent nova LMCN 1971-08a. The structure is coincident with the nova, has a circular morphology and is visible in narrowband H$α$ and [S II] filters but very faint in [O III], as expected. HI data also potentially reveal the existence of a coincident structure. Further, with a diameter of ${\sim}200$ pc, this NSR is the largest example yet found, with models indicating an ${\sim}4130 \ \text{M}_{\odot}$ shell expanding at ${\sim}20 \ \text{km} \ \text{s}^{-1}$ into the surrounding medium and an age of $\sim$2.4 Myr. The existence of the NSR also suggests that LMCN 1971-08a may have a much shorter recurrence period than currently presumed.

Figures (9)

• Figure 1: Approximate $40 \times 40$ arcminute field of view showing the nova super-remnant around LMCN 1971-08a seen across different surveys and wavebands. The location of the nova is indicated with the cyan cross in each panel. The top row shows the surroundings of the nova in MCELS continuum-subtracted H$\alpha$, $[\ion{\text{S}}{ii}]$ and $[\ion{\text{O}}{iii}]$ data, with the NSR apparent in H$\alpha$ and $[\ion{\text{S}}{ii}]$ but negligible in $[\ion{\text{O}}{iii}]$. The bright "stars" in the MCELS images are points of oversubtraction from saturated stars. The left panel on the bottom row showing MeerKAT data does not clearly reveal a corresponding structure. The middle and right panels in the bottom row show data cube slices at ${\sim}294 \ \text{km} \ \text{s}^{-1}$ and ${\sim}314 \ \text{km} \ \text{s}^{-1}$, respectively, from the HI4PI survey, with potential evidence for the same northeast (in the $314 \ \text{km} \ \text{s}^{-1}$ panel) and southwest (in the $294 \ \text{km} \ \text{s}^{-1}$ panel) components of the NSR seen in the optical.
• Figure 2: Left: DeMCELS continuum-subtracted H$\alpha$ image of the nova super-remnant surrounding the recurrent nova LMCN 1971-08a. The dashed ellipse is provided to approximately indicate the location of the NSR and its extent. The location of the nova is indicated by the cyan cross. Right: DeMCELS continuum-subtracted $[\ion{S}{ii}]$ image of the same region. As with Fig. \ref{['observations']}, the bright "stars" in the DeMCELS images are points of oversubtraction resulting from saturated stars in the R-band data used for continuum subtraction.
• Figure 3: DeMCELS continuum-subtracted H$\alpha$ image with an approximately $39 \times 39$ arcminute field of view, showing the nova super-remnant surrounding LMCN 1971-08a and the extended environment including the multitude of $\ion{\text{H}}{ii}$ regions to the west. The location of the nova is indicated by the cyan cross near the centre of the NSR shell. Other catalogued structures in the vicinity are labelled as follows: $\ion{\text{H}}{ii}$ regions (green square) from 2012ApJ...755...40P and SNRs (red triangles) from 2024AA...692A.237Z.
• Figure 4: HI4PI survey image, as shown in Fig. \ref{['observations']} (at ${\sim}294 \ \text{km} \ \text{s}^{-1}$), with the two slits used for the position-velocity analysis indicated with orange lines (one at constant right ascension and the other at constant declination). The location of the nova is marked with a cyan cross.
• Figure 5: Parkes $\ion{\text{H}}{i}$ position-velocity diagrams around the position of the nova super-remnant. The location of LMCN 1971-08a is indicated by the dashed line. The arc ranging from ${\sim}290-310 \ \text{km} \ \text{s}^{-1}$ is approximately outlined with a dashed line.