There is a new paper out called a Recurrent Nova Super-Remnant in the Andromeda Galaxy which is about M31N 2008-12a. You can find the article at this LINK. The article summarizes more of what astronomers have discovered about M31n 2008-12a or 12a as they are calling it. I'll follow suit here. The paper introduces the notion based on observations from the Liverpool Telescope at La Palma and the Hubble Space Telescope of the system. 12a as it is called in the paper, is now shown to be surrounded by a nova super-remnant that is at least 134 parsecs by 90 parsecs, which is larger than all known remnants of supernova explosions we have documented. The Veil Nebula in Cygnus, is about 110 light years in length and 50 light years across. 12a is 436.84 light years in length, and 293.40 light years across. You can see how much more massive the nova super remnant is when compared to the Veil Nebula complex.
To put this into astronomical terms, 1 parsec is equal to about 3.26 light years or 31 trillion kilometers or 19 trillion miles in length. The 134 parces is about the distance from our Sun to the Messier 45, the Pleiades. Look up this winter at M45 and think that is how long the super remnant of 12a is in length! These images from the paper will show that the super remnant is elliptical in shape and brighter on the southwest than on the northeast edge.
Here you can see the location of 12a in the first two images going left to right, and in the first or green image, the blue sources are field stars. The view of the elliptical and closed ring nebula is seen within that white dashed ellipse. The midbdle image is from the HST and is of the same region though all stellar sources have been removed. Here you can clearly see that the nebulosity is not smooth, but that there are filaments and knots interwoven here. It is quite similar to the image below of the Galactic recurrent nova T Pyxidis. The red squares discuss the two regions discussed in the paper linked above. The third image on the far right, in black and white is from the Hubble Space Telescope and is a zoomed in image showing the region withing the large red box in the center image. Here the filaments, knots and viewable separated by 12pc and 5pc.
To highlight the paper, and I am sure to massacre it, but the paper covers 7 major points about M31N 2008-12a. First is that this recurring nova erupts about every 347 +/- 10 days though there is the possibility that there is a frequency of 174 +/- 10 days. The reason for the second figure is that it is quite possible when this event occurs when M31N 2008-12a is near the Sun in March or so and that we cannot observe it as a result.
Second, it is the "fastest optical evolution and the highest ejection velocities, the hottest X-ray source, and the most rapid recurrence cycle of any known thermonuclear nova." When you add these together it equals the most massive white dwarf ever discovered. That covers three through six. The seventh and last point is that based on ground based imaging the images show a ring like structure that is spatially coincident with the nova. This is see above in the images. As the images show, especially in the high spatial resolution from the HST, the out shell is not smooth, but fragmented into filaments and knots as mentioned above.
A spectrum taken of the super-remnant shell shows that here are no bands or findings of neon in the super remnant nova and that may indicate that the white dwarf, that is approaching the Chandrasekhar limit of 1.4 solar masses, is likely composed of carbon and oxygen, indicating that in 20,000 to 40,000 years the white dwarf is likely to go off as a Type Ia Supernova, though we may find that there is a Neon-Oxygen core and that the white dwarf may collapse into a neutron star directly.
The paper then discusses how they developed a model to run to show how the nova super-remnant developed. Actual observations of this nova has occurred over the last decade, but the model shows that M31N 2008-12a super-remnant has been erupting for well over a million years. That is the only way the nova super-remnant could have obtain so much mass and size was for eruptions occurring over a million years.
The model shows that the super-remnant contains three distinct regions. The inner cavity where the recent ejecta effectively undergo free, high velocity expansion while cooling." Next the ejecta pile up where the ejecta from previous eruptions collide, then slow by interaction with the Interstellar Medium or ISM. The third is super-remnant shell, "which consists almost entirely of swept-up ISM that is slowly driven outward by the multiple-ejecta pile up, occurring in its inner edge."
A few other takeaways from the paper. The binary pair is most likely a red giant and it is accreting most of its mass onto the white dwarf via a wind that transfers the mass. Some mass transfer may be occurring via an accretion disk, but most is being transferred via the wind. That is the other finding, that the binary companion is indeed a red giant.
To summarize, the paper reveals that there is an extremely large super-remnant nova around M31N 2008-12a, a white dwarf that is accreting mass from a red giant companion. The red giant is transferring its mass via a wind, and perhaps some mass in an accretion disk. Also the super-remnant nova is massive in size, is made of 3 components where material ejected is speeding out, then impacting with previous ejecta to create the other the super-remnant nova. The nova is not smooth but is comprised of knots and filaments. If M31N 2008-12a ends it cycle as a Type Ia Supernova, then the super-remnant will be destroyed from the resulting supernova. If 12a ends up as a neutron star, the remnant will eventually over time dissipate and blend into the ISM.