PoS(GOLDEN2019)044 https://pos.sissa.it/ * [email protected] Speaker. All novae recur, but only atems, handful the recurrent have novae been (RNe), observed are in amongbeen eruption the thought most more of extreme than as examples once. potential of novae. type Theserecently RNe Ia been sys- have long strengthened. progenitors, and In their this claimthe short to maximum review this RNe magnitude—rate ‘accolade’ will of has be decline presentedflashes (MMRD) within into phase-space. the framework models, Recent of and work thereview integrating also subsequent He- presents growth an of overview the of the whitethe Galactic dwarf, newly and will identified extragalactic be ‘rapid populations explored. of recurrent RNe, nova’or This including subset less. — The those most with exciting recurrence novations periods system and vast of yet nova discovered ten super-remnant, is — , introduced. M31N 2008-12a, Throughout,some open with of questions its the regarding annual expected RNe, challenges and erup- and opportunities that the near future will bring are addressed. * © Copyright owned by the author(s)Attribution-NonCommercial-NoDerivatives 4.0 under International the License terms (CC of BY-NC-ND the 4.0). Creative Commons The Golden Age of Cataclysmic Variables2-7 and September Related 2019 Objects V (GOLDEN2019) Palermo, Italy Matthew J. Darnley Astrophysics Research Institute, Liverpool John Moores146 University, IC2 Brownlow Liverpool Hill, Science Liverpool, Park, L3 5RF, UK E-mail: Accrete, Accrete, Accrete. . . Bang!The (and Remarkable repeat): Recurrent Novae PoS(GOLDEN2019)044 , ]; 5 5 yrs , , 6 4 4 10 . ], rec 20 P < Matthew J. Darnley ] for an overview. http://www.ljmu. 17 ]). ] for compendia of recent 9 yet discovered. They are , 12 8 , , 7 11 , 6 explosions eruptions every in the Milky Way 31 23 + − O (e.g., [ 100 yrs). 17 < ], and as many as 300 annually in M 87 [ 3 rec P 1 N, and 15 C, ]. 13 15 Li, 7 ]). Post-eruption, the disk will recover or reform [ ]. Once the temperature has increased sufficiently to lift that -rays bursts and the full gamut of supernovae (SNe; see, for e.g., RNe are explored in the context of the ‘MMRD’ relationship. In γ 19 14 2 , 13 ]. In a typical system, material lost from the donor accumulates in an , 10 9 ] versus [ 18 . In Section 1 ], and a second eruption will occur once the WD has again accreted a critical mass in the Andromeda (M 31) [ 1 SN per century per host. Like all explosive transients, novae enrich the ISM; in 1 100 yrs) and recurrent novae (RNe; ∼ 21 − ]. A value of a century arbitrarily delimitates the nova population into classical novae > yr , the reinvigorated link between novae and type Ia SNe is discussed. In Sections 24 3 16 15 rec , P + − the Galactic, extragalactic, and the new population of ‘rapid recurrent novae’ are presented. 23 A recording of the review talk, and the accompanying slides, may be downloaded from Novae are binary systems. At their heart is a (WD) that accretes material from The gross observable features of novae are powered by the nuclear burning. Until recently (see Novae are inherently recurrent in nature. A nova eruption does not irreversibly impact the This article provides a summary of an invited review talk on the topic of ‘Recurrent Novae’, Nova eruptions rank among the most luminous stellar Once a critical quantity of material has accreted on to the WD surface, the conditions for 6 1 , ]), it was understood that the ejected and expanding envelope acts solely as a photon convertor. ]). Yet novae are substantially more numerous,], with 65 50 22 16 1 2 Section [ ac.uk/~mjd/talks.html of material. Theoretical work suggests that recurrence periods range from 50d (CNe; a nearby companion —via the Roche-lobe donor. overflow, and For main theis sequence system a or is red sub-giant considered giant, donors, a mass-loss mass-lossas cataclysmic is occurs a variable generally symbiotic (CV). driven variable If by [ the that donor star’s stellar winds, and the system is classed [ The rapid rise in optical luminosityslower optical is decline driven from by peak the is expansionenvelope. linked of Subtleties to an of the the optically decreasing observed thick features optical envelope. relate depthand to The of recombination, factors the geometry, such inclination, then as and shocks, optically ejecta molecule/dust thin photoionisation formation. See [ system as a whole — although thecases accretion (see, disk for is e.g., understood [ to be disrupted or destroyed in most presented at The Golden Age ofItaly Cataclysmic Variables (Sep. and 2019) Related Objects V meeting in Palermo, [ degeneracy, the rapid expansion ofenvelope. the The accreted system envelope then leads enters to athe period the available of fuel ejection quasi-stable source of nuclear is a burning exhausted on portion [ the of WD the surface until will resume [ and [ accretion disk around the WDpresence before of it a is strong transported WD to(polar) magnetic channeled the field via surface accretion that of can magnetic field. the be The partially later. reader (intermediate is However, polar) in referred or to the [ entirely this case through the production of nuclear burning are met, which ultimatelyconditions leads of to a the thermonuclear WD runaway under [ the degenerate The Remarkable Recurrent Novae 1. Introduction surpassed in brightness by only compared to detailed and comprehensive reviews of the nova phenomenon. PoS(GOLDEN2019)044 ). ]. ˙ M ]). 26 , ], and ]) that 42 25 43 10 reaching ]) has yet ˙ M 40 ] versus [ 41 Matthew J. Darnley ], with (DR2; [ of two fundamental 10 ]. Using M 31 and the ]; or the zero-age WD 28 31 Gaia , 27 ). The lower ignition masses 7 combination ]. For example, the RN population 98 yrs [ 26 ≤ is achieved via Roche-lobe overflow of rec ˙ M P ≤ ]). Although, in the most extreme case, mass loss 2 10 the more rapidly that critical mass is acquired. ˙ ]. Yet two systems, and IM Normae — the ). The high M 3 10 presents a colour-magnitude diagram (based on [ 1 ]. 30 ]. Simply, the MMRD states that the brighter the nova (at peak) the ]). Figure 39 , 34 a few hours for CNe [ , 38 ∼ 33 the most remarkable and extreme nova yet discovered is featured. It is hoped , orb 7 32 P . Such systems therefore have orbital periods greater than those of typical CNe 1 ], a new population of ‘faint–fast’ novae has been proposed — novae that populate − yr 44 ]. However, there are clear and substantial selection effects at play with regard to both

29 M 1 d versus 7 & − The majority of Galactic RNe follow the picture painted above, with 30% hosting a sub-giant Further afield evidence slowly mounts against the MMRD relationship. First in M 31 [ Any nova that has been observed in eruption more than once is referred to as recurrent. Ob- A high mass WD in a nova system can be acquired by two routes: the zero-age WD mass The Maximum Magnitude—Rate of Decline (MMRD) relationship for novae has been with The short recurrence periods of the RNe are driven by a 10 orb P a sub-giant or through the wind of a donor (the symbiotic case) [ ( donor and 50% with a red giant donor [ The increased surface gravityignition of criteria for a H-burning. high The higher mass WD means less material is required to meet the ∼ required by RNe leads to substantiallyfrom less CNe. massive, and hence faster evolving, ejecta than are seen more rapidly it fades (optically),luminosity and and typically decay-rate. takes the Forgalactic form a distance of time, indicators a the — power-law MMRD a between enabled potentialand maximum the rival the to popularisation high Cepheid of observational variables. novae overhead ButMMRD as hindered the relationship extra- large its has MMRD uptake. been scatter questioned.been More unable But recently, to even the provide the very compelling much evidence concept heralded either of way the for Galactic novae (seethen [ M 87 [ a previously devoid area of the MMRD phase space. Well established nova eruption models (e.g., may occur through Roche-lobe overflow of a red giant (see Section “recurrent unusual novae” — seeminglyorbital with periods, main may sequence buck this donors, trend lower (forunusual mass a RNe WDs, non-complete see selection and [ of short discussion articles about these served recurrence periods currently range between 1 was high, the WD is therefore most likely to be ONe in composition [ us for almost a century [ 2. Recurrent Novae and the MMRD Milky Way as examples, the RN population onlyulation accounts [ for a few percent of the known nova pop- of the extensively observed Large Magellanicthe Cloud host’s (LMC) overall makes nova up population a [ much larger proportion of system parameters. (Most) RNe contain high mass WDs and have high mass accretion rates ( illustrates how near-IR quiescent photometry is used to determine the donor type in a nova system. the RN population size and the range of observed periods [ The Remarkable Recurrent Novae Finally, in Section that this article compliments a pair of recent comprehensive reviews of extragalactic novae [ mass was low(er) and theWD WD is mass more has likely increased to over be the CO (see lifetime Section of the nova system; here the PoS(GOLDEN2019)044 ] 35 catalogue [ Solar composi-

Matthew J. Darnley Hipparcos ]. 10 10% extracted from the 3 < ], and are included for informative purposes only. The dashed 36 ]. An updated version of Figure 1 from [ 37 ), with the arrow indicating the evolution during quiescence. The grey data points indi- 7 black line and the solid black lines represent evolutionary tracks of 1 and 1.4 M cate with parallax and photometric errors cross-correlated with 2MASS [ tion stars, respectively [ Figure 1: Near-infraredGalactic colour–magnitude novae at diagram quiescence. Red showing datawith points the indicate sub-giant those position donors, with red of and giantindicate donors, selected green known dark-blue those recurrent those (mostly) novae. with The main blackSection data sequence point donors. denotes the position Points of additionally M31N 2008-12a circled (see The Remarkable Recurrent Novae PoS(GOLDEN2019)044 ) of their 7 ] proposed that 26 Matthew J. Darnley process) will ensue α over the course of an η . The faint–fast novae, therefore, 2 ]), i.e. the WD mass increases from ] 58 26 ]). But two barriers have traditionally [ , 49 ˙ 57 M , , 100%) of the accreted envelope. Some of the 48 56 , < , 47 55 , 4 , 46 54 — in this case He — onto the WD core (see, for e.g., , ]) the ejecta mass can, in principle, be larger than the . Many of the eight long orbital period Galactic RNe 24 53 ˙ , M 23 , 8 waste products ]. Whether H-flashes allowed WD growth or not, it seemed the He- 61 , ]), which populate the infrared luminosity space between novae and SNe. 60 45 0 (see, for e.g., [ — might be occupied by ‘SPRITES’ (eSPecially Red Intermediate-luminosity > ˙ M ]). Yet novae are probably the most luminous of all pre-SN Ia systems, possibly ) can be classified as faint–fast novae, but to-date, only one (see Section 4 52 ] to propose that exploration of a ‘Bolometric MMRD’ could have the potential to , 26 51 , 50 We note that reliable direct measurements or estimates of the WD mass are only available for a small proportion of The second barrier relates to the WD mass and how it changes with a nova eruption. For Each H-flash deposits its Nova have long been proposed as a potential progenitor pathway along the single-degenerate The first is simply whether there are enough novae to make a significant enough contribution 2 ]). As time progresses, this leads to a growing He layer on the WD. Just as for the accreted H , ]) allow us to interpret the positioning of a nova in the MMRD phase space. The traditional 59 29 24 all novae. In general thefrom WD the mass X-ray is properties. inferred from observational properties such as the speed class and independently layer, once the He layer attains enough mass, nuclear burning (via the triple- flashes ultimately whittled away the WD. [ an accreting WD to reachEach the nova Chandrasekhar eruption can mass only it ever eject must, a of proportion ( course, increase in mass with time. even during quiescence. Thus nova populations canstraints be of studied the well Milky beyond Way, the and heavilyupon the biased host size con- morphology of or their star contribution formation to history, can the in SN Ia principle rate, be and determined observationally. any dependance critical (ignition) mass — causingof the nova WD eruptions mass — to or decrease.eruption cycle H-flashes The is — most agree recent that theoreticalone H-flash the studies to accretion the efficiency next. within the He layer.all Initial accreted simulations material from ofmaterial the such (see, WD He-flashes usually e.g., indicated along that [ with they a violently substantial ejected proportion of mixed in core route toward SN Ia explosions (see, for e.g., [ (see Section This led [ restore the usefulness of the MMRD. 3. Helium Flashes and SN Ia Progenitors accreted hydrogen must remain on the surfaceas to the fuel super-soft the X-ray on-going source nuclear burningwith (SSS). that the However, is if accreted observed a material large (see, quantity for of e.g., the [ WD core material mixes to the menagerie of potential SN[ Ia progenitors — and this remains an open question (see, for e.g., The Remarkable Recurrent Novae [ MMRD relationship spans from bright–fastrate, novae that to contain faint–slow high novae mass witharise WDs low accreting mass from at WDs high a and mass low high WDs with high faint–fast M 31 and M 87the counterparts apparently has empty been bright–slow MMRD observedWDs phase to with space recur. low — Recently, expectedTransient [ to Events; be [ occupied by low mass stood in their way. PoS(GOLDEN2019)044 ]. ). At the ]. All of 64 6 - , 72 63 , and , 100 H-flashes) 71 5 ∼ 50 , , Matthew J. Darnley ] compiled a set of 70 26 accretion disk as the , 50 , ˙ 69 M 10 , 50 as CNe, [ ]. After a number of He-flashes 62 , 23 masquerade ; the three with sub-giant donors, or the 5 — therefore bright — accretion disk); long orbital ˙ M ]. The Galactic RNe naturally separate into three distinct 63 ]. Ultimately, the WD mass does grow with time — although , which draws from data published in [ 1 62 ]. When the WD core (and He layer) temperature was tracked , 23 RS Ophiuchi-class 23 ] — an upper limit that must surely only be related to the availability ], to be due to unveiling of the surviving high 28 73 , -class, those with short CN-like orbital periods. We summarise some of the 19 Galactic RNe are by far (with the exception of perhaps M31N 2008-12a; see 200 yrs. ]; and more ‘conflicted’ cases, such as V2491 Cygni [ ∼ T Pyxidis known 68 , ) the best studied. An extremely comprehensive compilation of their observational prop- 67 7 ] for a compilation). In all cases these are CNe (a single eruption) that share many obser- , 50 ; and the 66 The population of known Galactic RNe must clearly suffer from (in many cases) severe selec- In an attempt to quantify the number of RNe that While there are only ten confirmed Galactic RNe, there are almost as many strong candidates On this subject, we also refer the reader to the contribution in these proceedings from Sumner The ten But, the first study to simulate a long series of He-flashes (each punctuated by , 65 periods (required to physically accommodate theat enlarged quiescent radius of (again an a evolved property donor);and of NIR hence the bright high donor); escape high velocity, velocity ofthe ejecta the eruption (due WD); and the high to low excitation the optical linesby high depth (visible many, surface of due including gravity, the to [ low the mass energetics ejecta); of and a light curve plateau (proposed tion effects and biases. At the timeyears of that writing, elapsed the between shortest inter-eruption the period 1979of yet and much seen 1987 shorter is the recurrence eruptions eight periods of have U Sco been — seen whereas extragalactically (see numerous Sections examples class properties of these novae in Table other end of theeruptions scale, of the V2487 Oph longest [ inter-eruption period is the 98 years between the two known these systems deserve continued study andproposed, warrant second long-term eruptions. monitoring to catch the expected, or observational properties of RNe. These include: lowluminosity amplitude of eruptions (due an to evolved the donor high quiescent and a high of historical observations. In a century’s time,period I will firmly be predict that the longest known inter-eruption (see [ vational properties with an eruption of[ a RN. There are very strong contenders, such as KT Eridani made an important discovery [ the core temperature had increasedand sufficiently ultimately to, remove at it. first,eject lessen As material the such, from degeneracy the these of system. later thethat Instead, He-flashes He the directly did later on layer, He-flashes the not simply WD result convert core He in [ to TNRs C/O and and would deposit not erties and history was publishedclasses: in the [ symbiotic RNe,donors, the often referred five to (four as the confirmed; V2487 Ophiuchi suspected) with red giant Starrfield and collaborators. 4. Galactic Recurrent Novae Section through the evolution of these flashes it was seen to increase [ initially it may decrease until the WDthe is RNe, sufficiently heated. as SN The Ia potential progenitors of novae, was and re-ignited. particularly The Remarkable Recurrent Novae The Remarkable Recurrent Novae Matthew J. Darnley c 1 1 4 12 ± ± ± ± 2022 2096 2084 2025 ) are relatively ˙ M ∼ ∼ ∼ ∼ 3 2048 . 0 20 2040 ± 7 [days] Next eruption ± PoS(GOLDEN2019)044. orb P b ] identified the donor as a low-luminosity 10 61 455.721 519 up to4 2027 510 0.62 2027 12 0.08 2035 1 1.23 up to 2021 [years] ± ± ± ± ± ± 8098 227.57 . . . 82 0.10 38 1.52 rec P ∼ ∼ ∼ ∼ 10 . rec 1927), ∼ observed P ] and references therein. for each system. At the time of writing, RS Oph and U Sco are already within 64 rec , P 63 , 1958, 1967, 1985, 2006 15 1894), 1906, 1917, ( , ? ∼ 50 , 26 , a 1884, 10 1968), 2000 27 1955), 1969, 1979, 1987, 1999, 2010 ∼ ∼ ∼ The U Sco-class inter-eruption times are presented based on the assumption that eruptions (i.e. The RS Oph-class , 1933, 1945 ? 1963), 1989, 2014 26 1873, ∼ ∼ ; assumed eruptions (by this author) indicated by parentheses. A 1945 eruption of RS Oph was long The short orbital period systems ? average Table 1: Selected properties of the ten known Galactic recurrent novae. ). For T Pyx we simply report the mean 1900, 1998 1936, 1945, ( 7 d ] presented evidence in support that eruption occurring during a seasonal gap. 74 With the exception of T Pyx, the There are no orbital period data for V2487 Oph available. Based on quiescent NIR photometry, [ NameT Coronae BorealisV2487 Ophiuchi 1866, 1946 V745 Known Scorpii eruptions CI Aquillae 1937, ( 1917, 1941, ( IM Normae 1920, 2002 RS OphiuchiV3890 Sagittarii 1898, 1907 1962, 1990, 2019T Pyxidis 1890, 1902, 1920, 1944, 29 1967, 2011 24 V394 Coronae Australis 1949, 1987 U Scorpii 1863, ( Suspected eruptions indicated by Predicted dates of next eruptions are based on the assumed This table is based upon data compiled within [ a red giant (possibly red-clump) star and hence placed V2487 Oph in the RS Oph class. suspected, [ b uniformly spaced (see Section c their predicted windows. d

6 PoS(GOLDEN2019)044 ], 76 ]; see 10 candidate ], utilising 78 ] demonstrated ]. That 91 Matthew J. Darnley ] are the next best ] indicated that the ] found significant 4 90 3 29 RNe was strongly asso- ] reported that 30% of M 31 ] is that around a third of all ]) to have yet reached a good ]. Indeed there are more nova 52 50 88 ] and M 87 [ 75 potential [ ] and that’s it — 32 known RNe , 89 ]. However, [ 3 30 30 3 ] and severely challenges efforts to ACS/WFC data [ , 77 ]. But M 31 is far from ideal, its large 26 , 3 ] result was indeed statistically consistent 26 100 yrs; A. Pagnotta, priv. comm.). 52 ≤ http://www.mpe.mpg.de/~m31novae/index. rec ]). P 7 79 ≤ ] combined with its closeness, M 31 has become the 3 ] who produced a catalogue of eleven progenitor systems, ]. This is far from surprising, with the nova population of a 51 26 ]. A completeness analysis (of the 16) [ 86 within the Local Group. The first extragalactic survey for quiescent Hubble Space Telescope (HST) ], who found 16. An additional three M 31 RNe have been added 29 100 yrs). HST ≤ ]) or the nova rate too low (e.g., SMC; [ rec P 87 ≤ ], and one recanted [ ]. Within that subset lie eighteen known RNe. While a small number of the M 31 85 81 , , 84 ) are accessible to 80 , 1 For the most complete, and up to date, catalogue, see: The first extragalactic nova progenitor, or quiescent system, that of M31N 2007-12b, was re- Of those 1100 M 31 nova candidates, in excess of 200 have now been spectroscopically con- Add in to the mix the four known RNe in the LMC [ There are currently over 1100 nova candidates within M 31 83 3 , 82 php spanning just three .sparse (e.g., In M 33; most [ otherprobability Local of detecting Group a galaxies RN. the Beyond the observations Local are Group, just M 81 too [ that all novae with red giant donors (which Galactically arenovae was dominated conducted by within known M 31 RNe; byeach [ [ hosting a red giant donor.novae may The contain subsequent a red statistical giant analysis donor. [ Moreover,ciated that with population the of (young) disk population of M 31. The [ covered in M 31 using archival preferred laboratory for the study of nova populations [ which results in largedisentangle internal the extinction bulge uncertainties and [ disk populations within that host [ RNe have been knownsystems or was suspected published for by some [ [ time, the firstdetection comprehensive efficiency catalogue of M of 31 RNe these may — be i.e. as the low probability as(here, of 10%. formally, detecting 1 Subsequently, at as least many a as second 1 eruption in — 3 M 31 nova eruptions may be from a RN sampled hosts, but they too are yet to produce a published RN. (still unconfirmed) RN was shown to harbour a red giantFigure donor. Subsequently, [ firmed [ galaxy broadly following the light, additional fieldsreturns. away from Of the course centre only this provideusing places diminishing substantial those spatial novae. (and temporal) Viewed biases from on the any Milky population Way, studies M 31 is relative close to being edge-on [ The Remarkable Recurrent Novae photosphere recedes back toward the WD).Galactic A novae key may prediction be from recurrent [ (formally 10 candidates in M 31 thanBy known virtue novae in of all its other high galaxies nova (the eruption Milky rate Wayangular [ included) size combined. has led to thethe bulge majority or of the brighter surveys inner (particularly regions historic [ campaigns) focussing only on evidence for separate bulge andwide-field disk high-cadence nova surveys populations of within M M 31,Giant 31. reported Stellar the More Stream discovery recently to of the [ two south novae of associated M 31 with (see the [ 5. Extragalactic Recurrent Novae PoS(GOLDEN2019)044 10 yrs ≤ rec P Matthew J. Darnley may be as low as ], have permitted the as U Sco. But of the rec should decrease (see, P 96 , rec fast P 95 follow-up facilities, such as [ 10 yrs — close to two orders of ]. Thus RRNe should be faint–fast ‘fingerprint’. The recently launched 24 only ] to dub all systems with 26 Rapid response unique ] — almost twice as , they are most likely hiding in plain sight. RRNe . 92 4 8 2 yrs [ . have recurrence periods similar to or shorter than U Sco! 1 If they exist ± Neil Gehrels Swift Observatory 2 half . illustrates the distribution of RN recurrence periods within the 6 ) 5 2 population of RNe led [ = ] for an authoritative overview). Unfortunately, most DNe are not ]), because M 31 (and the LMC) cover relatively small regions of rec 93 ] and the 27 P ]) could be vital for detecting new RRNe in the Milky Way. Optical 94 97 quick-fire ). RRNe are therefore particularly challenging to detect, let alone classify. 2 ]. But when we look at the Galactic RN population the most rapidly recurring 23 , 22 ]). Theory shows us that on the cusp of breaching that limit (see, for e.g., [ 24 the red giant novae being disk novae. While far from proven, a potential consequence all We stand on the precipice of a new era of all-sky high cadence surveys. TheIt SSS-phase may also of be worthwhile noting that (M 31) DNe are not visible above from unresolved surface brightness of So where are the Galactic RRNe? But we have been able to successfully detect and follow-up RRNe beyond the Milky Way, If one examines the RN populations of the LMC and M 31 there is a striking difference be- As a WD in a nova system approaches the Chandrasekhar mass, 4 eROSITA RRNe (and novae in general) is perhaps their most M 31 with the typical seeing experienced by ground-based observations. study and confirmation of those RRN candidates particularly in M 31 (e.g., [ The existence of this Milky Way, M 31, and thefore LMC. nova) Is populations this of difference due thesemetallicities, to galaxies, or variation related are between possibly observational the to select stellar effects differing (and at star there- play? formation histories and Galactically, perhaps somewhat ironically, the largest backgroundthe signal dwarf to novae RRNe (DNe; may see actuallyfollowed-up be [ sufficiently (i.e. optical spectroscopyeven and photometrically) X-ray to observations, be and able innova to many eruption cases — distinguish a not them RRN from eruption. a low amplitude, quasi-short-periodic, the sky. The adventvaluable of input large from area the detectors amateurunprecedented coupled astronomy depth community, with have and high enabled cadence cadence M over 31 observations,and the to capability and be last led the covered directly decade, in- in to or both the so. discovery of This RRNe. change in observing strategy tween the RNe from thoseone hosts (LMCN 1968-12a) and has those from the Milky Way. Of the four known LMC RNe, two months [ magnitude slower than the predictedChandrasekhar mass, extreme. where are So the if more a rapidly recurring nova systems? WD really can be grown toward the eighteen M 31 RNe (see Section ‘rapid recurrent novae’ (RRNe). In factare all new seven M — 31 RNe are discovered RRNe. since 1984 Figure — of which there must contain high mass WDsnovae accreting (see at Section an elevated rate [ the Liverpool Telescope [ system is U Sco with a mean inter-eruption time scale of The Remarkable Recurrent Novae with might be that novae (with red giant donors) only contribute to the SN Ia rate6. in late-type Rapid galaxies. Recurrent Novae for e.g., [ PoS(GOLDEN2019)044 38 36 34 Matthew J. Darnley 32 ]. Any denizen of the 30 100 28 10 yrs). Adapted and updated ]) should dramatically alter the ≤ 26 98 ] s rec r P 24 a e y [

22 c e r P

20 d e v r Milky Way M31 LMC 9 18 e s b o 16

n a e 14 M 40 yrs; 9 RNe with longer periods are not shown). The dotted 12 ≤ rec 10 P ]) there are unlikely to be anywhere near enough to classify all new 8 99 6 ]. 4 26 2 0

“In all your travels, have you ever seen a star go supernova?” [ 1 2 0 Known recurrent novae recurrent Known landscape for transient object astronomy.in LSST, the for Milky example, Way will and be Magellanicultimately sensitive Clouds. lie to with But RRN the for eruptions availability opticalmany of discoveries follow-up fast of facilities enough RRNe, spectroscopic the complementing (orplanned problem LSST soft will and and X-ray) built its follow-up. (e.g., contemporaries While [ have been or are being Andromeda Galaxy, particularly the regionRX J0045.4+4154, surrounding may the have unassumingly borne named binary witness system to a nearby star undergoing a fantastic increase in transient detections. So for any novae,the not value specifically of RRNe, nova to science reap to anysystem benefit all in from of particular LSST astronomy may et requires be al., substantial key. promotion. Right now, one RRN 7. M31N 2008-12a Figure 2: The distributionplot of only the shows systems mean with inter-eruption periods of the known recurrent novae (the facilities, such as the Large Synoptic Survey Telescope (LSST; [ vertical line demarks the realmfrom of Figure the 3 rapid of recurrent [ novae ( The Remarkable Recurrent Novae PoS(GOLDEN2019)044 , , ]. ]; ± first 109 108 99 101 109 . , , 0 models 26 = ]. Hydro- rec 105 P rec 113 , outflows [ Matthew J. Darnley mean P ]. (jet-like) ]); detection of significant 113 ]. ]. This ‘nova super-remnant’ 109 105 of swept up ISM [ 113 , — that the 12a WD is CO and that , , most recently in November 2019

27 M 112 120 eV; [ 6 − ∼ 5 data [ ]. eff 10 T , high mass WD, and ultra-short 107 ∼ ˙ , HST M 106 10 every single year but does not confirm — ]. Together, this drives eruptions with a ]. At present, the composition of the 12a WD has not been 105 ]; remarkable similarity between the 2013–2015 and 2017–2019 [ 105 ]. 1 19 implies − ]) accreting, stably, at an exceptionally high rate in the region of 105 yr 58

;[ M ]. This

6 is a signature of an ONe WD [ ]. At the time, annual nova eruptions were completely unprecedented. Al- − 20 kyr [ by the combination of the most massive WD known (predicted via some STIS FUV spectra taken just 3 d after the 2015 eruption showed no indication 108 ]; detection of short-lived high velocity, collimated [ 10 < 38M 104 5 . , luminosity, consistent with high × 1 ]; ejecta deceleration as they shock the pre-existing disk/donor wind [ HST 111  , 4 ' 58 . 103 , 1 , ]. The high accretion rate has been proposed to be due to a Roche-lobe overflowing powered 109 24 . 27 optical over-abundance 26 ˙ M . A Ne But the most remarkable aspect of 12a is the effect it has had upon the local environment. The Any nova system, as it grows toward the Chandrasekhar mass, should therefore be surrounded Comprehensive overviews and summaries of 12a and its eruptions are presented in [ 12a is RX J0045.4+4154 itself usually goes by its more familiar (yet equally cumbersome) name The first observed eruption of ‘12a’ (by humans) took place on Christmas Day 2008 [ ]. Although suspected beforehand, the true nature of 12a was not confirmed until the 2013 ]; and recovery of the progenitor from archival ]. Selected highlights, that illustrate the extreme nature of the system include: low mass ejecta 5 6 . 02 yrs [ . 0 102 system is surrounded by aenabled vast the shell-like WD to that grow is toward the the remnant Chandrasekhar of mass all [ the eruptions that have by a similar NSR.the It zero-age is and clear current that WD the mass, size the of accretion these history, and objects surrounding will ISM depend density. upon But parameters fac- such as high-amplitude SSS variability [ (NSR) has a major axis of 134 pc and consists of with low eruptions [ 109 of Ne in the ejecta 110 Since then, it has been[ observed in eruption models to be 0 red giant (or red clump)confirmed. But, donor [ the system is building toward aevent SN will Ia occur explosion. in A very conservative prediction suggested such an of TNRs [ the highest SSS effective temperature seen in a nova ( dynamical simulations of 100,000 periodicrepeated (annual) RN eruptions 12a-like are eruptions capable clearly of producing demonstrated such that a phenomenon [ eruption [ though a small number ofdetail. RRNe Until had then the already most been rapidlybe found recurring U Sco. (and in therefore M extreme) 31, RN none was had generally accepted been to studied in any M31N 2008-12a — it is theered. prototype Of RRN, all and known novae, the probably most of extreme all example known of stellar a systems, is nova yet the most discov- likely to explode The Remarkable Recurrent Novae luminosity around once a (Terran) year.those Perhaps nearby this ‘Andromedans’. system, Little may might even theycomposition, be know RX central that, J0045.4+4154 to given may the the one right culture day conditions, of — explode and wipe as the out a right all SN in Ia, the and immediate potentially vicinity. — unfortunately as a SN Ia. And, given the distance to M 31, it probably already has. PoS(GOLDEN2019)044 signpost to the Matthew J. Darnley and persistent 11 of (predominantly) hydrogen away from the site of

M 6 Of course, the big unanswered question is whether M31N 2008-12a and its super-remnant is a The study of novae, Galactically and extragalactically, is heavily indebted to the continuing As a sub-set of all novae, RNe present generally the more extreme system and eruption pa- Recurrent nova super-remnants present a new opportunity to not only identify previously un- rare, or even unique, phenomenon; or is it just the tip of the iceberg? Acknowledgments support provided by the amateur astronomytunity community. to I pass would on therefore my likeastronomy thanks to in take to general. this all That oppor- those includesfrom who which those contribute the affiliated 12a to with work the the inOrganising work particular AAVSO, the Committees draws of heavily. BAA, of the I and The would nova the Golden like community, VSOLJ, Ageticularly to and of Franco thank the Giovannelli Cataclysmic Scientific and Variables and and Francesco Local Related Reale,on Objects for Recurrent their V, par- Novae. kind invitation I toto present would a accommodate also review me like talk (remotely) to once extendI it my would became thanks also clear to like that to the Imanuscript. thank organisers was my for I unable friend must still to acknowledge and being travelCouncil partial collaborator to able (STFC), funding Martin the who supported from Henze workshop. much the for of my UK usefulto own Science comments pass work over my and on the thanks Technology past this to Facilities six the years.this referee Finally, work. I of would this like review, Sumner Starrfield, whose comments help improved the imminent SN Ia, and itmaterial. will RNe take and centuries NSRs may before therefore an providewhere a expanding is solution SN the to Ia the hydrogen? shock single-degenerate SN interacts Ia with ‘paradox’; that rameters. Their short inter-eruptionsame periods system. permit detailed With growing study WDs, ofway. RNe repeated are eruptions In strong from general, candidates the forChandrasekhar the mass. a shorter Currently, single-degenerate the the SN known Ia recurrence RN path- particularly population period Galactically. is the beset But by closer those substantial a challenges selectioncontribution effects, can of given novae be, system to and the is are SN to Ia being, rate overcome. reaching can the By be quantified. doing soknown the RNe and RRNe, butdonor potentially mass to reservoir locate has any ‘extinct’ beena systems depleted. SN — Ia But former — perhaps RNe, due more where intriguingly, the to should its a vast RN scale explode — as the NSR will provide a clear The Remarkable Recurrent Novae tors such as metalicity, donor type,properties. and NSRs even clearly line-of-sight deserve inclination substantial mayis further affect already theoretical the on and observational to observational find study. further The examples. hunt 8. Summary progenitor-type of that SN Ia.local By environment its in very which nature, the SNRN NSR Ia eruptions formation will will will explode. have have moved Importantly, also around if pre-prepared 10 the the 12a NSR is common, such PoS(GOLDEN2019)044 arXiv , ]. ]. Beryllium On the , vol. 490 of Carbon-Oxygen Matthew J. Darnley A Hubble Space Classical novae from , Ph.D. thesis, California A nova outburst powered 1502.05598 Cambridge Astrophysics [ 1606.02358 (2018) 1601 [ Research Notes of the American 478 , ]. . CRC/Taylor and Francis, Boca ]. , vol. 43 of (2015) 381 ]. 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