The accretion of hydrogen onto a white dwarf star ignites a classical nova eruption1,2—a thermonuclear runaway in the accumulated envelope of gas, leading to luminosities up to a million times that of the Sun and a high-velocity mass ejection that produces a remnant shell (mainly consisting of insterstellar medium). Close to the upper mass limit of a white dwarf3 (1.4 solar masses), rapid accretion of hydrogen (about 10−7 solar masses per year) from a stellar companion leads to frequent eruptions on timescales of years4,5 to decades6. Such binary systems are known as recurrent novae. The ejecta of recurrent novae, initially moving at velocities of up to 10,000 kilometres per second7, must ‘sweep up’ the surrounding interstellar medium, creating cavities in space around the nova binary. No remnant larger than one parsec across from any single classical or recurrent nova eruption is known8,9,10, but thousands of successive recurrent nova eruptions should be capable of generating shells hundreds of parsecs across. Here we report that the most frequently recurring nova, M31N 2008-12a in the Andromeda galaxy (Messier 31 or NGC 224), which erupts annually11, is indeed surrounded by such a super-remnant with a projected size of at least 134 by 90 parsecs. Larger than almost all known remnants of even supernova explosions12, the existence of this shell demonstrates that the nova M31N 2008-12a has erupted with high frequency for millions of years.
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The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias with financial support from the Science and Technology Facilities Council (STFC). This work is based on observations made with the NASA/ESA HST, obtained from the Data Archive at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy (AURA) under NASA contract NAS 5-26555. These observations are associated with programmes GO:14125 and GO:14651 for which financial support for R.H., M.H., M.M.S. and A.W.S. was provided by NASA through grants from STScI. This work is based on observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma. The Hobby–Eberly Telescope (HET) is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen. HET is named in honour of its principal benefactors, William P. Hobby and Robert E. Eberly. We thank Z. Levay for creating a colour composite image of the nova super-remnant, I. A. Steele for assistance with the Liverpool Telescope spectra, K. L. Page for assistance with XSPEC, K. A. Misselt and D. Baer for assistance with the Steward 2.3-m observations, also M. Link and C. Proffitt, and W. Eck and K. Long, the programme coordinators and contact scientists for HST GO:14125 and GO:14651, respectively. M.J.D. and M.W.H. acknowledge financial support and a PhD studentship, respectively, from the STFC. N.M.H.V. acknowledges support from the European Commission through the Horizon 2020 Marie Sklodowska-Curie Actions Individual Fellowship 2014 programme (grant agreement number 659706). V.A.R.M.R. acknowledges financial support from the Fundação para a Ciência e a Tecnologia in the form of an exploratory project (reference IF/00498/2015), from the Center for Research and Development in Mathematics and Applications (strategic project UID/MAT/04106/2013), and from Enabling Green E-science for the Square Kilometer Array Research Infrastructure (ENGAGE SKA), POCI-01-0145-FEDER- 022217, funded by Programa Operacional Competitividade e Internacionalização (COMPETE 2020) and the Fundação para a Ciência e a Tecnologia, Portugal.
Nature thanks S. Shore and J. Sokoloski for their contribution to the peer review of this work.
Extended data figures and tables
a, The Steward 2.3-m Bok Telescope Hα image that allowed the association between the nebulosity and 12a to be made. Image orientation is as in Fig. 1 but the image is 80″ × 80″. b–d, HST Wide Field Camera 3 broad-band filter images of the region around 12a. Image sizes are 40″ × 40″. These three panels show the F275W (ultraviolet) (b), F475W (optical) (c), and F814W (optical) (d) filters. The white contours in c show iso-flux regions as derived from the ground-based Hα + [N ii] image. As b–d were taken towards the end of the 2015 eruption, the nova can be seen in the images. The F275W image clearly illustrates the lack of bright ultraviolet sources within the super-remnant. The white dashed ellipse indicates the extent of the super-remnant; the red dotted lines indicate the position of 12a.
As with the GTC spectrum of the same region (Fig. 2), there is negligible continuum and hydrogen Balmer emission lines and nebular lines of [N ii], [O ii] and [S ii]. The mean spectral resolution for the ‘blue arm’ is 1.68 Å and for the ‘orange arm’ is 4.04 Å (see Methods). Gaps in the spectrum indicate areas where skyline subtraction residuals remained.
Extended Data Fig. 3 Comparison of results from the hydrodynamic modelling using a range of spatial resolutions.
The blue and green lines indicate simulations of 20 eruptions with spatial resolutions of 0.02 au and 0.2 au, respectively, while the red and black lines indicate simulations of 100 eruptions with resolution 0.2 au and 0.4 au, respectively. a, Gas density radial distribution; the lower black dotted horizontal line indicates the ISM density, with the upper dotted line showing the consistent peak density of the super-remnant shell. b, Gas pressure radial distribution. c, Gas velocity radial distribution. d, Gas temperature radial distribution.
a–d, Panels as in Extended Data Fig. 3. The close match between the results of simulations of 1,000 eruptions without radiative cooling (black) and with radiative cooling (blue).
Extended Data Fig. 5 Additional results of the hydrodynamic simulations of the interacting ejecta of multiple recurrent nova eruptions.
a, The mass growth of the super-remnant outer shell for up to 100,000 eruptions (see key). The diagonal dotted line illustrates a power-law extrapolation of the outer-shell mass to further eruptions. The upper and lower solid grey lines indicate the growth of the outer-shell mass for higher and lower ISM densities, respectively. The horizontal dotted line marks the predicted outer-shell mass at the current extent of the super-remnant. b, The evolution of the expansion velocity of the outer shell (black) compared to the mean velocity within the ejecta pile-up region (red). The diagonal dotted lines indicate power-law extrapolations, the horizontal line the initial injection velocity, and the vertical line the predicted current epoch. c, The evolution of the X-ray (0.3–10 keV) luminosity of the super-remnant (black), the hard (1–10 keV; red) and soft (0.3–1 keV; blue) components are shown for information along with the hardness ratio HR (hard/soft; right-hand axis) evolution; for each, the dotted line indicates a power-law extrapolation to later times. The horizontal dotted black line indicates the 3σ upper limit from the XMM-Newton observations.
a–d, Panels as in Extended Data Fig. 4.
In both panels the cyan, blue, green, red and black lines indicate simulations of 10, 100, 1,000, 10,000 and 100,000 eruptions, respectively. The top panel shows the contribution to the super-remnant emission as a function of photon energy (in units of kT). The vertical dotted line indicates the lower-limit (0.08 keV) cut-off for input into XSPEC. The bottom panel shows the resultant synthetic X-ray spectra of the super-remnant (0.3–10 keV).