PoS(Golden2015)001 http://pos.sissa.it/ † ∗ [email protected] [email protected] Speaker. A footnote may follow. This paper is an extended andnelli updated & version Sabau-Graziati of (2015a). the reviews Inseveral by this hot Giovannelli paper (2008) points we and Giovan- review aboutwill cataclysmic the briefly variables discuss renewing (CVs) also discussing interest about ofprogenitors classical of and today the recurrent astrophysics Type novae, Ia as about supernovae. However, well the these as bulk sources. the of this intriguing paper problem We consiststhe of in magnetic our field attempt intensity of (B) studying at CVsBecause the from surface of the of point limited their of white length view dwarfs. of ofcomplete. the However, paper we and would ourphysics like knowledge, of to this our demonstrate Universe review is that does strictly the notapparently related improvement in pretend also the on with to recent knowledge the past be multifrequency of lost behaviour to the of have a CVs, leading which position in modern astrophysics. ∗ † Copyright owned by the author(s) under the terms of the Creative Commons c ⃝ Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). The Golden Age of Cataclysmic Variables7-12 and September Related 2015 Objects - III, Golden2015 Palermo, Italy Lola Sabau-Graziati INTA- Dpt. Cargas Utiles y CienciasArdoz, del Madrid, Espacio, Spain C/ra de Ajalvir, KmE-mail: 4 - E28850 Torrejón de Franco Giovannelli INAF - Istituto di Astrofisica eRoma, Planetologia Italy Spaziali, Via del Fosso delE-mail: Cavaliere, 100, 00133 The Golden Age of Cataclysmic VariablesRelated and Objects: The State of Art PoS(Golden2015)001 1 ∼ wd M -rays. The orbital Franco Giovannelli γ G). A few dozen CVs 8 –10 7 ) (e.g., Smak, 1985a). ⊙ 10 ≈ 1 M ≤ (from the Greek word kataklysmos = s M 2 cataclysmic 12 h with a distribution showing a gap between 2 ∼ 80 m to ∼ Historically, because CVs were observed photometrically and without seeming to follow any The first CV detected in the X-ray range, with rocket experiments, was the dwarf nova SS The catalog of Downes et al. (2001, 2006) reports 1830 CVs. Adding the new discoveries HEASARC database (https://heasarc.gsfc.nasa.gov/W3Browse/all/rittercv.html) table contains Mass transfer is strongly depending, besides the orbital parameter of the system, on the mag- In the 1950s it was recognized that the various phenomena displayed by the CVs are all the ) and the secondary is a late type Main Sequence star ( ⊙ were detected in X-rays with HEAO-1 satellite,reviews with of EXOSAT, and Cordova with & the Einstein Mason, satelliteCVs 1983; (e.g., from Cordova, a 1995). sample Later ofROSAT XRT-PSPC Verbunt 162 All et Sky systems al. Survey. with known (1997) or recognized suspected 91 binary periods by using data of the flood, storm; Hack & la Dous,parent 1993). that As these collecting objects were of regular observational binarysome data systems progressed of which it for them became some ap- reason also changed(Classical regularly in Novae). brightness; (Recurrent Novae Therefore and theties, classification Dwarf by of Novae) CVs which while was one some based may(iii) others on distinguish dwarf only the four novae; once optical groups (iv) outburst of nova-liketherein; proper- CVs: objects Ritter, (e.g., (i) 1992; Giovannelli classical Giovannelli, & novae; 2008). Martinez-Pais, (ii) 1991 recurrent This and novae; classification, references however, is neither self-consistent regular pattern, they were named with the term and 3 hours, in which fewthe systems reason have because been was detected. nicknamed In ’period the gap’. past this gap was empty and this was Cyg (Rappaport et al., 1974;were Heise not et recognized al., as 1978). such.Hya, The and Warner UHURU the (1976) satellite variable proposed detected AM the twoet Her, identification CVs, which al., of which on 1978). 4U further The 1249-28 optical magnetic studies with field was EX in recognized these as two a systems CV is (Forman strong ( (530 new CVs) made by the MASTER-NETknown experiment is (Buckley greater et than al., 2000. 2015), the number of CVs periods of CVs are ranging from information on cataclysmic binaries only, as takenMass from X-ray the Catalog Binaries, of Cataclysmic and Binaries,Kolb. Related Low- The Objects complete catalog (7th (Ritter Edition, &nitudes, Kolb, Release 2003, orbital 7.21, A&A 404, parameters, March 301) stellar 2014) listserties coordinates, parameters of apparent of of Ritter mag- 1166 the & cataclysmic components,known binaries, and or 105 other suspected characteristic low-mass orbital X-ray prop- periods.and binaries, some, Most and especially of 500 those CVs in relatedX-ray were observations, which discovered objects but the through with with optical magnetic the observations, field detectorsgeneral of of not very the the bright second WD in and is X-ray further energy strong, generations, range. discovered since CVs through are in netic field intensity at thethat surface are of detectable the in primary. different energy Such ranges: process from produces radio a to X-rays, large and fan even of in behaviour consequence of accretion ofWarner, matter 1976; 1995a). onto CVs a are white binary systems dwarf in (WD) which the from primary a component is low a mass WD ( donor star (e.g., Golden Age of CVs 1. Introduction M PoS(Golden2015)001 Franco Giovannelli (Patterson, 1984); the time scales 1 − yr ´ 65 minutes. Their spectra do not show nski (1967), after the discovery of the ⊙ − M 8 10 − ∼ 3 to 10 11 − 17 minutes (Smak, 1967). ∼ The recurrence time-scale of outbursts in dwarf novae is correlatedA with recent their paper amplitude by and Otulakowska-Hypka, Olech & Patterson (2016) present a statistical study CVs in a time scale of order between weeks and years flare up almost periodically, about few Depending on the magnetic field intensity at the WD, the accretion of matter from the sec- There is another class of CVs, the rare AM Canum Venaticorum (AM CVn) star systems. However, in the class of nova-like objects there are two sub-classes: the DQ Her stars and the outburst duration is depending on the orbital period (Warner, 1987). of all measurable photometricand features superoutbursts. of a They large usedstudy sample as all complete of accessible and dwarf photometric up novaephotometric data to during features for date their in as all possible. outbursts order theirdwarf to Their objects novae aim constrain outbursts. to was They theoretical make managed to models the to checkand confirm which correlations Schoembs a try between few relation, these of to the the explain Bailey knownbetween the correlations, relation the that for nature is cycle long of the and outbursts Stolz and above supercycle duration the lengths, of period amplitudes superoutbursts, gap, of the outburst normal relations duration and and orbital superoutbursts, period, amplitude outburst duration and mass magnitudes in optical wavelengths; the duration oftime. the outbursts Typical is light much curves shorter for than classical thetypes novae recurrence and can dwarf be novae seen of in the Ritter U (1992) Gem, and Z e.g. Cam, in and SU Giovannelli (2008). UMa ondary star onto the primary canCVs: occur NMCVs) either or via a an channelling accretionintermediate through disc way the (in (in magnetic the the poles so-called case (in Non-Magnetic of the Intermediate case Polars: of IPCVs). Polars: PCVs) or in an They have extremely short orbital periods between 2. The role of magnetic field in cataclysmic variables the AM Her stars.enough high In for these dominating the sub-classes accretion of diskmagnetic and CVs, CVs all whose the the names phenomena WDs related are tonames possess coming it. of from magnetic These Intermediate the fields classes Polars prototypes of with and DQ Polars,discussed intensity Her respectively. by and A Warner AM (1995b). short Her history Fundamental took(1994), of papers later Warner their about (1996). the discovery these The has sub-classes class been of arerelatively IPs those has weak by been magnetic Patterson split field into two (Nortonlatter subclasses subclass et with is relatively al., large DO 1999). and Dra (previously registered One as example YY of Dra) (Andronov a et system al., belonging 2008). to the Golden Age of CVs nor adequate and it is1985b). much One better obvious to advantage consider ofaccretion primarily such phenomena, the an which observed approach are accretion is sufficiently behaviour short connectedaccretion (Smak to rates with avoid in the any CVs time usually major range scales observational from bias: of 10 various the mass evidence for hydrogen.Nelemans, They 2005). appear to There beby is helium–rich gravitational an wave versions old radiation of suggestion, losses,prototype CVs that with proposed (e.g. in an by orbital these Paczy period Warner, systems of 1995c; the mass transfer is driven are from tens of seconds (oscillationsUMa in stars or dwarf long novae at term variations outbursts) in to VY years Scl (super-outbursts stars). of SU PoS(Golden2015)001 , not necessarily Franco Giovannelli µ ⃗ ˙ M, rotational velocity ˙ M from the optical companion and having a magnetic moment 4 ⃗ ω differ by around 2% or less. These are assumed to be polars ). However, there are few systems that may be best described as orb orb P . In the plane spin period of the WD (and in general of the compact and P ≪ µ ⃗ spin spin 10–200 MG), the WD rotation is synchronized with the binary orbital period ∼ The last group defined by the accretion structure criterion, NMCVs, includes those systems IPCV WDs have moderate magnetic fields (order of a few MG); the Alfvén radius is smaller Boyd et al. (2014) reported the results of a 15-year campaign by the globally distributed Center In PCVs the WD magnetic field is strong enough to make the Alfvén radius greater than the , and magnetic moment ⃗ (the secondary star) — rotating with a velocity ω whose WD magnetic fields arethe not accretion relevant in disk governing extends the downshows accretion to a structure. the great In diversity WD of these surface observational systems andfication behaviour; is, for a in this boundary this reason layer case, the more is historicalan appropriate formed. criterion for attempt of distinguishing This classi- of their family sub-classes. classification However,1991 for it and is lack references simply of therein). a Indeed,netic in more rotator, general, characterized general we by can a physical mass consider classification M the — (e.g., WD accreting of matter Giovannelli, at a a CV rate as a gravimag- than the circularization radius butformed it in is these greater systems than but the beingmagnetic WD disrupted field radius. at lines its Therefore but inner an just region. accretionbinary inside In orbital disk the IPCVs period is Alfvén matter (P follows radius. again the The rotating WD is asynchronous with the that have been disturbed from synchronismWynn, 2004, by and a the recent references nova therein). explosion (Norton, Sommerscales & nearly synchronous intermediate polars (V381(Norton, Vel, RXJ0524+42, Sommerscales HS0922+1333, & and Wynn, V697 2004,the Sco) and ‘period the gap‘. references therein). Probablyevolving into all Two polars. of four these systems systems are lie IPs in in the process of attaining synchronism and coaxial with the rotational axischaracterized (Lipunov, by 1987; the following 1991). physical Then parameters: the mass accreting M, accretion system rate is completely for Backyard Astrophysics to observe V1432 Aqland – investigate its the return only to known synchronism. eclipsing asynchronous Originallybefore polar knocked observing – out records began, of the synchrony magnetic by white athan dwarf nova in explosion the V1432 Aql orbital is currently period rotating butachieved slower is around 2100. gradually catching The up.the continually magnetic changing At field trajectory the lines of present ofthat the rate, the Boyd accretion rotating synchronism et stream white should al. as dwarf be it producesevidence (2014) against a follows have which complex measured physical pattern models and of of documented, the light providing system emission can comprehensive be observational tested. circularization radius, so no accretion disk isthe formed and magnetic the field, accretion which structure is canalize fully themagnetic governed by accreting field matter ( across the field(a lines. few Owing hours). to the However, intense and there CD are Ind) few in systems which (V1432 P Aql, BY Cam, V1500 Cyg, V4633 Sgr, Golden Age of CVs ratio for short and normalsuperoutbursts. outbursts, However, as they well question as thefound the existence an relation of analogous between the the Kukarkin-Parenago relation relationbetween rise for but and outburst superoutbursts. they decline duration rates They and of theoretical also mass work failed dedicated ratio to to for dwarf find novae. superoutbursts. one presumed This relation study should help to direct PoS(Golden2015)001 g 42 − white dwarf (after Franco Giovannelli ⊙ 5 it is possible to find any sort of physical conditions of 2 µ ˙ M/ white dwarf. The polars AM Her and AE Aqr lie in the zone of ˙ M) from the secondary to the white dwarf is driven by magnetic ⊙ . This diagram shows the positions of few well known CVs: AM Her (Terada et 6 − The positions of several CVs in Lipunov’s diagram calculated for 1 M cm 2 − is expressed in seconds and the gravimagnetic parameter is expressed in unit of 10 As recalled by Giovannelli & Sabau-Graziati (1999), it is evident that the properties of an Figure 1 shows the diagram spin period of the WD versus gravimagnetic parameter, where G 1 spin − s al., 2010 and references therein),Horne, AE 1997), Aqr DQ (Patterson, Her 1979; (Patterson,Cyg de (Giovannelli 1994; Jager & et Zhang Sabau-Graziati al., 2012a). et 1994; al.,by It appears Wynn, Lipunov 1995), King evident (1987) EI the & using UMa power 1 of (Reimer this M et diagram al., obtained 2008), SS outburst in CVs dependcomposition crucially of on its the hydrogen accretion richaccretion envelope rate, process in the onto which the mass the WDthree of thermonuclear is kind runaway the strongly occurs. of WD, influenced CVs And and by (non-magnetic,the the its the polars, orbital magnetic period chemical and of field intermediate the intensity. polars) systemwhere and obey Indeed, the the to the spin relationships magnetic period between of field thehence intensity WD the plays (Warner & a mass-transfer Wickramasinghe, 1991), fundamental rate role. ( The orbital evolution of CVs, and propeller, how they must stay,very while well known the IPCV), IPCV and DQ SSthe Cyg Her base (whose (the of nature prototype as many IPCV circumstantial of isGiovannelli proofs this & claimed and class), Sabau-Graziati by also 2012a,c) Giovannelli’s EI group because lie UMa on of just (a in a the cogent zone similarity predicted with by EI Lipunov UMa, for e.g. such objects. P Figure 1: Lipunov, 1987). Golden Age of CVs object) – gravimagnetic parameter y = gravimagnetic rotators, as discussed by e.g. Giovannelli (1991). PoS(Golden2015)001 vs log y) appears Franco Giovannelli spin 3 hr) and gravitational radiation for short- > orb 6 2 hr). < orb Number of CVs versus orbital period. Light blue rectangle shows the so-called "period gap"; light The shorter period AM Canum Venaticorun (AM CVn) systems lie well below the "period Accreting binaries with white dwarf primaries and main sequence secondaries have binary Therefore, the role of magnetic field intensity plays a fundamental role in the processIt of is ac- evident that a fundamental parameter characterizing the whole sample of CVs is the orbital However, such a gap – which was believed true for long time – is now partially filled by the 39% and 50% of CVs lie, respectively; about 11% of CVs lie within the "period gap" which is as the best way for localizing the position of MWDs, both polars and IPs, as shown in Fig. 1. red rectangle shows the range ofintersection periods between where the "period SW gap" Sextantis and systems SW lie. Sex Cyan-50 periods rectangle (after represents Gänsicke, the 2005 and Rodriguez-Gil, 2003). orbital periods greater than 80or min. semi-degenerate: For e.g. shorter white period dwarf systems - the white dwarf secondary binaries. must be degenerate Figure 2: cretion of matter onto the compact star. Therefore Lipunov’s diagram (log P period, which is strictly connectedCVs with versus the the evolution orbital of period the (after systems.∼ Gänsicke, Fig. 2005). 2 Below shows and the abovepartially filled number the by so-called of the "period zone gap" where theNelson SW & Sextantis systems Rappaport lie (e.g. (2001) Rodriguez-Gil,"period discussed 2003). gap" critically in Howell, CVs. the basic paradigm for the origin of the 2-3 hr SW Sex systems (e.g. Rodriguez-Gil, 2003;was Rodriguez-Gil et due al., to 2007). a The apparentescaping smaller ’period from gap’ number the of observations. systems having orbital periods in such an interval, which were Golden Age of CVs braking of the secondary for long-period systemsperiod (P systems (P PoS(Golden2015)001 2 36 the . 10 0 > ≤ donor ⊙ M –M / s orb M " was published Franco Giovannelli ≤ 17 . The distribution of . | µ ⃗ | and . This functional form obtained 2 . orb 5 − and P 5 min, was discovered by Smak (1967) . 17 spin /1000 s) ∼ orb (P orb 7 × 1 , where a priori there are not restricted ranges of mag- − AM CVn Stars: Status and Challenges yr orb ⊙ M 9 –logP − spin 10 × 5 ∼ , or special correlations between P | µ ⃗ " and later a review about " | Davis et al. (2008) applied population synthesis techniques to calculate the present day num- Cannizzo & Nelemans (2015) use the observed range of outbursting behavior for AM CVn Therefore the investigation on the magnetic field intensities in WDs is crucial in understand- And the merging process of two compact stars has recently become extremely important for Kalomeni et al. (2016) present a binary evolution study of CVs and related systems with AM CVn Stars objects in that diagram is owed tosuperposition the of interaction the of observed braking or torques implied andyr), variations accretion acting of torques, on the with a the accretion continuum rate ofby on magnetic those long moments. physical time In parameters. scale this ( way each system is completely described ber of two types ofare WD-main post-common sequence envelope binaries star with (WDMS) secondary(gPCEBs), binaries stars such within that that the they have will masses ’period commence 0 gap’. mass transfer The within first the period gap. The second type are systems as a functionperiod. of orbital They infer period a to rate place a constraint on mass transfer rate versus orbital ing the evolution of CVsmoment, systems. the mass The accretion fundamental ratepossible parameters and to to the fulfill be orbital the searched plane parameters logP are ofnetic the the moment systems. magnetic In this way it will be the gravitational wave (GW) astronomy.signal coming It from is GW necessary detectors, toranges, possibly explore triggered with by with the multifrequency particular GW observations, attention detection. in each all the possible white dwarf accretors, including forsources, example, and AM systems CVn with giant systems, donor classical stars.accretion novae, They disks indicate supersoft will where tend X-ray in to the be relationship stable P radiation against signatures the thermal-viscous may instability, be and found where with gravitational LISA. is consistent with the recurrence timea – simple orbital theory period for relation the found recurrence by time. Levitan et al. (2015) using and was recognized as a CV" by Patterson (1992). Nelemans (2005) published a short review about Golden Age of CVs gap". The prototype HZ 29 = AM CVn, with P by Solheim (2010). The importance ofwhite such dwarf systems binaries is remarked that in canfaint the give study thermonuclear rise of and interacting to Type–Ia double supernovae a (SNeaccreting wide Ia) AM variety to CVn of the systems. formation astrophysical of outcomesremains One neutron ranging dynamically key stars stable from factor and or affecting stably instead the diverges,merger leading final of to outcome the the is tidal binary. whether disruption mass ofmass, It the transfer mass is transfer donor typically remains and stable, thought the especially thatlow if for mass accretion occurs ratio low via double ratios a WD of disk. binariesleads the Shen to and (2015) a donor finds classical examines mass nova-like that outburst to the on the theshell initial accretor. shrinks phase accretor Dynamical the friction of orbit within and hydrogen-rich the causes mass expanding the nova Eddington transfer mass limit, transfer possibly rate to resulting increase in dramatically a above binarynova the merger. outbursts, accretor’s If dynamical the binary friction survives within thesystem first the even hydrogen-rich more subsequent strongly helium-powered toward nova merger. shells pushes the PoS(Golden2015)001 Franco Giovannelli 75 h, respectively. . 2 he divided the MCVs in three > m. The theoretical orbital period µ orb P / 10 G) differs from that of single MWDs ≥ spin 9 G still needs to be established by sensitive 3 75 h, and 2) –10 . 5 2 10 < 8 ∼ of the mass transfer rate, the model systems exhibit a 4 G appears to be real, the lower limit is more difficult to − 9 10 × 13. This excess is revealed as a prominent peak at the location of the ’period ∼ 4 to G) (Ferrario, de Martino & Gänsicke, 2015), although both cluster around 30 MG. ∼ 3 h as a consequence of disrupted magnetic braking, and are crossing the ’period gap’ 9 ≈ –10 3 In accreting WDs of CVs, as far as is presently known, there is a lack of systems at both high The field strength distribution of MCVs ( 2) The population of NMCVs with unevolved main–sequence–like donors at orbital periods Willems et al. (2005) and Willems et al. (2007)1) A by grid using of population detailed synthesis binary tools evolutionary studied sequences was calculated and included in the simu- 10 ∼ distribution is in reasonable accord with thetems combined above population the of period dwarf gap, novae suggesting anddetach the novalike and sys- possibility evolve that below systems the with period unevolveddata gap donors are as need necessary in not for the checking disrupted the magnetic validity braking of model. theoretical Experimental predictions. ( spectropolarimetric surveys conducted on 8 m class telescopes. and low field strengths. However, thethe apparent IPs, absence which of generally low have fieldpossible, unknown MCVs field is might strengths, be still and explained the not by of lack understood of accretion (e.g., high flows field Beuermann, in systems, 1998). MCVs. though On Wynn (2000) the base discussed of the the problem ratio P While the upper limitinvestigate. cutoff The incidence near of magnetism 10 below a few 10 greater than 2.75 h wasgravitational investigated. radiation, In addition magnetic to braking, the andalso angular included mass momentum the loss losses effects associated from of with theinto circumbinary system, the disks Willems disk, on et the corresponding al. evolution. to For (2007) 3 a fractional mass input rate the population of NMCVs with orbital periods 1) lations to take account ofitational additional radiation angular and momentum mass losses loss, beyondWillems due that et to associated al. nova with outbursts, (2005) grav- fromdients considered the necessary the system. to effect reproduce of As the a ashowed observed circumbinary specific that orbital disk example, period the to distribution. gain periodtonically insight The minimum resulting into increasing lies distributions the with at ingre- increasing aboutevidence orbital 80 for period minutes, an to accumulation with a ofsimulations the maximum systems in number near at which 90 of the only minutes. systems period gravitationalabout mono- minimum, 90 radiation There minutes which losses is is is are a no a direct considered. result common of The feature the shift of inclusion of of systems the formed peak within to the period gap. gap’ in the orbital period distributionthat of if the combined such gPCEB a and feature dCVorbital is populations. period observed WDMS They binaries, suggest in this the would orbital strongly corroborate period the distribution disruption of of magnetic an braking. observed sample of short Golden Age of CVs systems that were CVs at someperiod point to in their past, butvia detached gravitational once radiation they (dCVs). evolved They down predicted inriod an orbital gap’ excess of of dCVs over gPCEBs within the ’pe- bounce at orbital periods greater thanexist 2.75 as hr. dwarf The novae simulations throughoutand revealed their that: iii) lifetime, i) novalike ii) some systems dwarf systems canlonger novae can evolve orbital can back periods. evolve into into Among dwarf novalike thesecandidates novae systems, subclasses, to during search novalike their for cataclysmic circumbinary variables postbounce disks would evolution at be wavelengths to the best PoS(Golden2015)001 ) than ⊙ yrs. The and mass µ 11 047 M 10 . 0 ∼ Franco Giovannelli 2 hr. DD Cir and ± 1, respectively. For < . ˙ 0 M, as shown in Fig. 3 784 . orb ≫ 0 Gyr. In the sub–sample of vs log . 1 Gyr. In the sample, 134 of 1 X 1 and P ± 1, and . ± 5 Gyr, whilst 81 of 134 systems . . 0 0 0 1 . = 5 4 > ∼ ± , logL 0 = 1 and may be included in the class 2 17 . 159 1, orb . 3 hr will evolve into PCVs, whilst those . ˙ 0 P M 0 / > ∼ ≪ NMCV spin orb orb vs log P / 33 9 and P µ spin 3 Gyr. This means that CVs below the ’period gap’ 3 . 1 , log ± G cm 6 orb . 3 hr will either evolve into low field strength polars that are 33 > 10 62 h and their MKA is 5 . × 2 orb 5 < ˙ M vs logP ≥ orb µ and P 100 yrs) inferred for a large number of isolated MWDs in comparison to 3 ∼ G cm 33 62 h and their MKA is 3 10 . 2 × 5 > ≤ orb µ Following the interesting review by Ferrario, de Martino & Gänsicke (2015), highly magnetic Ak et al. (2010), using available astrometric and radial velocity data, computed the space Warner (1996) deeply discussed torques and instabilities in IPs on the base of measured spin those of non-magnetic WDs (a fewof days) the suggest stellar that core. strong magnetic MWDs, fields as augment a the braking class, also appear to be more massive (0 in the left, central, and right panels, respectively. There is a range of magnetic moments WDs (HMWDs) tend to exhibit aing complex and the non-dipolar presence field structure of with higherover some objects time, order show- multipoles. which is There consistentslow is rotation with periods no the ( evidence estimated that Ohmic fields decay of times HMWDs scales decay of velocities of CVs with respectgroups to of the CVs. Sun The orbital and periodbles investigated distribution that kinematical of of properties CVs the in of the whole various refinedmean sample sub- sample kinematical of of CVs age 159 (e.g. (MKA) systems resem- of Connon the Smith, 159 2007). systems Ak is et MKA al. (2010) found that the periods of the primaries and found severalof important relationships these between systems, fundamental such parameters as log transfer rates in which synchronized rotationaccretion of disk. the primary can occur even though it possesses an 159 systems are non magnetic (NMCV)NMCVs, having 53 MKA of 134 have P Golden Age of CVs classes: class 1, class 2 and class 3 if such a ratio is with presumably unobservable, and possibly EUVhigh emitters, accretion rate, or revive into when PCVs the mass when accretion their rate fields, reduces. buried by have P are older than systems above theof gap. CVs. This results The in selection agreement ofthe with middle 2.62 the of standard h the evolution as ’period theory gap’, the wheredoes border systems not between have exist been the anymore detected. two and This groups theexperiencing means of systems that gravitational the radiation inside systems ’period and this lies gap’ those ’gap’ roughly experiencing arethey in magnetic just are braking. frontier not The objects so reason between numerous because as systems in those the placed ’gap’, at and sides then could difficult to be be the detected. relative shorter time of permanence the systems in class 1unlikely the disk to equilibrium possess condition accretion isbelow clearly disks. the satisfied. ’period Those The gap’ in systems and class cannotRXJ1039.7-0507, in 2 possibly and are class contain V1025 very 3 accretion Cen. disks. are These EX These all are Hya-like EX have systems Hya, P which HT Cam, lie V795 Her lie within the ’period gap’ with P (Norton, Somerscales & Wynn, 2004, andthe the MCVs references according therein). to Wynn the (2000)moment magnetic crudely moment similar classified and to orbital IPs period. abovesystems. EX the Hya This ’period systems indicates gap’ have that magnetic and MCVsit. above comparable the to Norton, ’period the gap’ Wynn weakest willpredicted & field evolve that Somerscales to AM IPCVs long with (2004) Her-like spin investigated periods the below rotational equilibria of MCVs. They PoS(Golden2015)001 a, 600 × ∼ = f out . Boundaries for 1 − Franco Giovannelli g s 600 if we also count 17 ∼ 4 h (by courtesy of Warner, = orb ). . The two lines are lines of disc stability: ⊙ orb ˙ M-P 136 M . 0 ± 7–230 MG. Complex field geometries have been 10 663 ∼ . have been computed for P ⊙ 60 systems (see Table 3 of the review paper by Ferrario, 6 M versus mass accretion rate in units of 10 . 0 3 ∼ = 1 G cm 250 MWDs with well determined fields (as reported in the Table 1 33 ∼ and M ⊙ 1 M = 1 20–25 % of all known CVs. Zeeman and cyclotron spectroscopy of MCVs have ∼ Left upper panel: Intermediate polars in the plane To date there are about MWDs are also found in CVs, and are called magnetic Cataclysmic Variables (MCVs). They white dwarf mass M being a the separationMagnetic of moment the in two units of stars 10 in the system (by1996). courtesy Lower of panel: Warner, Mass 1996). transferof Warner, rate 1996). onto Right white upper dwarf panel: versus (2-10 keV) X-ray luminosity (by courtesy of the review paper byobjects Ferrario, with de no Martino or & uncertain Gänsicke,identified 2015) field IPs and determination has over (see now Kepler increased et to al. 2013, 2015). The number of comprise revealed the presence of fields in the range their weakly or non-magnetic counterparts (0 inferred in the highlower field field MCVs group (intermediate (the polars, polars) IPs) are whilst much magnetic harder to field establish. strength and structure in the de Martino & Gänsicke, 2015 and updated results in Bernardini et al. (2015). The other Figure 3: stable above and dwarf nova outburst below. They were computed for f=0.2 and 0.3, with R Golden Age of CVs PoS(Golden2015)001 2% of the . The exact . ⊙ Franco Giovannelli G on the internal field. Therefore 16 10 . 11 caused by the dependence of the cooling rate on the ⊙ 1 M ∼ However, we can say that CVs form a broad stellar family of highly variable and dynamical It is interesting to mention the review by Beskin et al. (2016a) about the fundamental role that Ferrario, Melatos & Zrake (2015) review recent work in this area of research. In particular, HMWDs have been long suspected to be the result of stellar mergers. However, the nature of Enormous progress has been made on observing stellar magnetism in stars from the main se- members. When it comes to explainingtransferred particulars matter about, and e.g., the the detailed WD’sturbulent interaction atmosphere; between irregularities transport the within in regular the photometricknown, behaviour; disk; rendering their or study the everlaboratories final more offering challenging. fate us of the At possibility least, theseunder of CVs objects, studying extreme are in more physical natural detail multi–wavelength is conditions the (e.g. behaviour missing of than plasma review what and by radiation is Giovannelli & Sabau-Graziati, 2015a). The strong magnetic fields play in the universe. mass of the WD. This bumpthere is is absent a in hint the of mass it function in obtained the from mass the function SDSS of catalogues the but local sample. they look at the fossilmain field sequence hypothesis and which pre-main linksquestioned sequence magnetism particularly stars in in the and compact context they stars ofversus highly consider to dynamo magnetic debate why magnetism white in its in dwarfs. the context They feasibility of alsosuch neutron has review as stars the buoyancy, now and helicity, fossil the and been roles superfluid played turbulence,fields. by in key the physical generation processes Independent and stability information offuture on neutron gravitational star the wave internal detections. magnetic(LIGO) field have The already of Laser constrained neutron the Interferometer stars Crab Gravitational pulsar will Wave gravitational Observatory come wave luminosity from to be location and shape of this bumpand depends He on and the of details two ofof CO the WDs hot merging only process. WDs introduces The they small merger found variations of a in CO the bump mass at distribution. In the case observed spin-down luminosity, thus placingwe a are limit witnessing of the dawn ofby a the new opening era of of a exciting new, discoveries non-electromagnetic in observational compact window. star magnetism driven the coalescing stars and the preciseGarcía-Berro mechanism et that al produces (2012, the 2013) show magnetic thatin field the the are hot, outer still convective, layers differentially unknown. of rotating the corona remnantfields present of of the the merger required of two strength degenerate thatof-the-art cores do is Monte not able decay Carlo to for produce simulator, magnetic long thatconsistent timescales. with the They that expected found also in show, number the using solar of a neighborhood.of state- HMWDs The WDs impact produced has of in mergers been in this discussed theinteraction mass way by of distribution stars is Isern in et close enough al. binarymass systems (2013). distribution can of introduce Their WDs important changes toy resulting in from modelWDs, the the indicates expected the evolution that merging of effectively of single the stars. such stars Besides the can existence produce of a He pronounced bump around 0.7-0.8 M quence through to compact objects. Recentof data the have thrown origin into of sharper relief stellarastrophysics. the magnetic vexed question fields, which remains one of the main unanswered questions in Golden Age of CVs candidates still awaiting confirmationXMM-Newton through and X-ray NuSTAR (see follow-ups http://asd.gsfc.nasa.gov/Koji.Mukai/iphome/iphome.html). with sensitive facilities such as PoS(Golden2015)001 Franco Giovannelli 3 hr (e.g. Šimon et 78 min and have in- for detected systems (Warner, 2001). This 3 < ∼ 3 − 2 hr (Woudt, Warner & − orb at pc < pc 5 6 − ˙ orb M systems detected by X-ray − 10 10 × × 1 3 ∼ ∼ 587 hr (Woudt, Warner & Spark, 2005); iv) 3 . 1 orbital period minimum = 12 3 hr, and only one with P orb > orb The Impact of Space Experiments on our Knowledge of the by Giovannelli & Sabau-Graziati (2004) contains also a part devoted to , X-ray All-Sky surveys give densities of 3 3 hr (Witham et al., 2007). 509 hr (Woudt & Warner, 2003); ii) 5 of 13 have P − . 1 > pc 6 = orb − 100 min that have passed through the orb 10 ˙ − M in hard X-rays (Patterson, 1998). Then from observational point of view, it is neces- × ) 80 6 For reviews about CVs see the fundamental papers by Robinson (1976), Patterson (1984, The current estimate of the space density of CVs is of Variability, from milliseconds to hundreds of years, follows from different physical processes − ∼ 3 Pretorius, 2004); iii) 1 (CAL 86) of 12 has P of 11 have P may be a significant underestimatethough densities of from CVs the most space comprehensive density, optical( Palomar–Green as survey discussed raises the by estimate Patterson at of (1984). low Al- surveys. Thanks to its highdiscovered sensitivity, several INTEGRAL new is faint very CVs, useful withal., for 2006; P Hudec this et purpose. al., 2008). UpCrv High to has speed now, P photometry it of faint CVs have shown that: i) 1 of 10, TV 1994), Hack & la Dous (1993),reviews and are the those books of by Warner(2012c, Connon (1995a) and 2015a). Smith Hellier (2007), (2001). The More GiovannelliPhysics long recent of (2008), review the Giovannelli Universe & Sabau-Graziati sary an intensive search forat the faint CVs predicted by population synthesis with orbital periods CVs. creasing orbital periods. This research must be done among the low taking place in these systems andour can skills be in studied developing by further meansgrows; these of and techniques several the grow astronomical more our techniques. we understanding As hand, learn of about it the CVs CVs is the insights well further also knownwards techniques or that and downwards conclusions in theory scale, obtained develop. to in On otherexchanges fields the the of such information other field as and of results AGNs or astrophysical CVs research LMXRBs,lations in and have general in vice been always CVs versa. benefits. extrapolated, are From Rapid particularly up- such oscil- interesting.of As dwarf reviewed by nova oscillations Warner (2004), (DNOs) the and rich quasi–periodicthe phenomenology oscillations interpretation (QPOs) that observed these in CVs rapidin favour brightness nature modulations – magnetically (3 channelled to accretion 11,000magnetically from s the excited timescales) inner traveling accretion are waves disk magnetic in formagnetic DNOs the aspects, and disk which possible for extend QPOs. toand lower intermediate There fields strength is the field increasing well–known (IPCVs) evidence propertiesof CVs. for of magnetic The fields strong the result on field is their (PCVs) that WDhanced almost primaries, by all although the CVs for accretion show many the process theX-ray presence intrinsic itself. binaries, field with There may be high– are locally many andand en- behaviour low–frequency QPOs that X–ray in parallel QPOs CVs. the resembling,(2005) QPOs Other and respectively, seen Pretorius, the papers Warner in DNOs & about Woudt rapid (2006). oscillations in CVs are those by Warner & Woudt Golden Age of CVs understanding of stellar evolution, electromagnetism andor polarization, 3-D mass geometrical and effects, radiation in transfer improving a the broad knowledge of spectral the range nature from of hard CVs. X-rays to radio, is mandatory for PoS(Golden2015)001 CVs orb ), it is possible to Franco Giovannelli orb decreases. All long P orb 13 Distribution of magnetic field strength in polars (blue line), and IPs (red line) compared to that of Taking into account the average values of magnetic field intensity and orbital periods for polars In MCVs there is an interesting relationship between the magnetic field strength and orbital Figure 4 shows the distribution of B for MCVs and MWDs (adopted from Ferrario, de Martino It is convenient to remind how the magnetic field intensity can be measured. A detailed dis- and IPs, and the minimumconstruct and a maximum very value interesting for plot both (Fig.of parameters 6) MCVs. (B that and shows the P Such evident aevolution continuity is between continuity driven the has by two angular been classes momentumcross noted loss; SW by as Sex consequence Schmidtobreick regime P & beforeevolutionary Tappert entering stage (2014, in in the 2015): the life "period of CVs gap". CVs (e.g. Therefore Rodriguez-Gil, SW 2003). Sex phenomenon is an period of the systems, as reportedthat in marks Fig. the 5 synchronization where between theFerrario, orbital polars de and and Martino spin IPs & periods are Gänsicke, of separated 2015). the by cataclysmic the blue systems line (after Figure 4: single magnetic WDs (black line) (adapted from Ferrario, de Martino & Gänsicke, 2015). & Gänsicke, 2015). cussion about the field determinationreported in in isolated the magnetic review WDs paper and bythe WD Ferrario, in magnetic de WDs field Martino in strength in & binary the Gänsickethrough systems high Zeeman (2015). field is splitting magnetic Direct of CVs, measurements the the polars, photospheric of low can hydrogen be absorptions accretion obtained lines states either when or (i) these (ii) systemsthe through enter optical the to modeling IR of spectra during cyclotronrario, intermediate emission and 2000) features high or that accretion (iii) states characterizes (see viathe Wickramasinghe the & accretion study Fer- shock. of Zeeman features arising from the halo of matter surrounding Golden Age of CVs 2.1 Magnetic field intensity measures PoS(Golden2015)001 Franco Giovannelli 14 Magnetic field strength versus orbital period in MCVs. Polars and IPs are separated by the blue Magnetic field intensity versus orbital period for MCVs. Polars and IPs are contained in the light Figure 6: blue and light green rectangles, respectively.rectangle Violet represents rectangle the indicates intersection the between so-called the "period Polars gap". and IPs. Cyan-50 An interesting indirect method forcussed evaluating by the Giovannelli & magnetic Sabau-Graziati field (2012a) in intensity the in case of MCVs SS has Cyg whose been nature dis- (non magnetic Figure 5: line that marks the borderand those between without systems synchronization (IPs). with Light orbitalde red period rectangle Martino marks synchronized & the Gänsicke, with so-called 2015). "period the gap" spin (after Ferrario, period (polars) Golden Age of CVs PoS(Golden2015)001 2 9 . 1 ≈ MG, ≤ 7 MG 3 6 . . . 0 0 0 + − 1 ± . 6 1 . 1 = = Franco Giovannelli CIV MG, and B 5 4 . . 0 0 + − 0 . 2 = . Using these values of luminosity and the CII 1 − erg s 15 31 10 × 2 . 6 ≃ CIV and L 7%, respectively. . 1 2 − ± 2 . erg s 3 − 30 10 × Observed emission fluxes converted to luminosity for magnetic CVs (after Howell et al., 1999) 8 7 pc (Harrison et al., 1999), Giovannelli & Sabau-Graziati (2012a) – by using the IUE . 7% and 7 . ± 2 INTEGRAL/IBIS and SWIFT/XRT observations have shown that a conspicuous number of It is hard to explain these results without invoking the presence of a magnetic field (B Many other circumstantial proofs in favor of the IP nature of SS Cyg have been discussed Then a reasonable value of the white dwarf magnetic field in SS Cyg is B ≃ ± 2 CII . MG) in the white dwarf of SS Cyg system. CVs have a strong hardlished X-ray sample emission of (Landi 23 etmeanwhile CVs, al., all 2009; 22 its Scaringi are characteristics et classified arestrong al., circumstantial as practically 2010). proof equal magnetic in to In favor IPs of their thosethat and the SS pub- of magnetic only Cyg the nature emits one of other in (SS SS the 22its Cyg. hard Cyg) magnetic objects. X-ray The nature. as energy experimental range This evidence NMCV, is, is in a our opinion, the conclusive evidence about by Giovannelli & Sabau-Graziati (2012a).paper by One Körding of et the al.diagram most shows (2008). important an proof analogy They is between detectedCyg. coming the a from XRBs radio Moreover the there jet (the from BH isthe SS GX a 1.1-mJy Cyg. 339-4, radio "flare" and they flare The found the simultaneous hardness3 upper NS with intensity limits Aql the for X-1) optical the and outburst linear SS of polarization and SS circular Cyg. polarization of During Figure 7: are indicated with black square.B. SS Left Cyg panel: positions C are II indicated with luminosity red versus areas B; (Adopted right from panel: Giovannelli & C Sabau-Graziati, IV 2012a). luminosity versus Golden Age of CVs or IP) is largely disputed.= From 166 the fluxes ofmeasurements UV obtained emission by lines of GaudenziL SS et Cyg, al. placed at (1986) distanceextrapolation of d – the derived line best the fitting theet luminosity emission al., of line 1999), luminosity the C of magnetic C II field II intensity and and of C SS C IV Cyg versus IV: is B B (Howell This value is in complete agreementMG) with by using the simultaneous evaluation X-ray, made UV, by and Fabbiano optical data. et al. (1981) (B as shown in Fig. 7, left and right panels, respectively. PoS(Golden2015)001 from AE Aqr γ Franco Giovannelli -ray telescope at Narrabri, Australia. γ 16 4200 s at phase 0.62-0.74 of the 9.88 h orbital period. Lang et al. (1998) ∼ Indeed, many measurements were later performed after the news about the possibility of VHE At the beginning of the nineties of the last century, acceleration of particles by the rotating Before the advent of ROSAT X-ray satellite, MCVs were relegated to a subsection of confer- Our opinion is that a more appropriate investigationHowever, of all the the discussion class about of CVs would the teach so-called a IPCVs lesson: is it is mandatory to observe CVs Therefore, our suggestion is to reconsider the problem about the nature of SS Cyg without any Probably, the "mistake" about the nature of SS Cyg born after the publication of a paper in Moreover, a simple question arises: "why all the CVs detected by the INTEGRAL observatory The hard X-ray emission detected in those CVs is possible only if a sufficiently high magnetic emission from AE Aqr. Chadwick eton al. 1993 (1995) October reported 11, an with excess the of pulsed University VHE of Durham VHE magnetic field of theby WD ground-based Cherenkov in telescopes intermediate inemission polars the from TeV the in passband polar AM the (e.g. Her detected propeller Meintjes– by et regime ground-based measurements Cherenkov al. never telescopes – confirmed (Bhat 1992), AE et – and al. wereenergy Aqr TeV 1991) the astrophysicists – community. main reasons detected of renewed interest for CVs in the high ences about CVs that were mainly concentratedMCVs on that NMCVs. even The menaced ROSAT to satellite discovered overthrow many ourby understanding appearing of – the apparently secular inexplicable evolution – ofCVs in ‘normal‘ the (e.g. CVs so–called Vrielmann ‘period & gap‘ Cropper, infor 2004). the many orbital–distribution But years of in as spite principal of targets this, of CVs high were energy X-ray not experiments. considered, in general, 3. Renewed Interest for Cataclysmic Variables necessary. for long time and possiblyfrom radio with to simultaneous gamma observations rays acrosstwo in the successive order outbursts. to electromagnetic follow This spectrum, at is, leastalmost of a periodical course, whole outbursts possible occur period only in for of time systems thecan scales like help binary of in dwarf system weeks-months. this novae between where matter Networks (Giovannelli the of & robotic Sabau-Graziati, telescopes 2012b) The burst lasted observed AE Aqr for a total of 68.7 hours over the epoch 1991-95 using the Whipple Observatory a priori bias. Nature by Bath &tical van behaviour, Paradijs typical (1983) of where DNe.Michael SS Friedjung’s Cyg This comment was in paper classified the originatedthat as first a "obliged" historical DN bandwagon almost Frascati on effect all Workshop the in(NMCVs), 1984: the basis the without subsequent Giovannelli, paying literature of authors 1985) attention (see op- to to otherclass start possibilities literature. the well documented papers in saying the that so-called second SS Cyg is a DN are magnetic (IPs) and SSnot Cyg detect non-magnetic? any other If DN?". so, why the same INTEGRALfield observatory is present did in those systems. Onlyin accretion quiescence. onto magnetic poles justify the hard X-ray emission Golden Age of CVs PoS(Golden2015)001 . This may put 1 − Franco Giovannelli -ray emission from γ erg s -ray emission from AE γ 34 10 ∼ -rays is a manifestation of the γ prop 17 -ray emission. It is remarkable to notice that AE Aqr γ -ray emission. γ -ray ranges. They did not find any significant γ with accreting WDs, neutron stars, and black holes (Knigge, 2010, 2011). Knigge, 5% of the total sources. ∼ -ray telescope. No evidence for any steady, pulsed or episodic TeV emission. Sidro et γ In the standard model of CV evolution, angular–momentum–loss (AML) below the period Moreover, since the CVs measured by the INTEGRAL observatory are magnetic in nature,The the long–standing fundamental predictions of evolution theory are finally being tested obser- The INTEGRAL observatory, until the beginning of 2007, had observed over 70 percent of Aleksic et al. (2014) with the MAGIC experiment searched for steady Meintjes, Oruru & Odendaal (2012) discussed the multifrequency properties of AE Aqr within universal interest for such class of objects has been addressed to evolutionary problems. vationally. All facets ofare the accretion process in CVs, including variability, disk winds and jets, Baraffe & Patterson (2011) extensively discussedpath the followed reconstruction by of CVs, the basedlowing complete on evolutionary Knigge the (2006) observed that mass–radiusproperties discussed of relationship the secondary of observational stars their in and CVs donorthat theoretical using stars, most the constraints CVs fol- semi–empirical follow on CV a donor the unique sequence, evolutionary global and track. concluded Aqr during 12 h ascovering part the of optical, X-ray, a and multi-wavelength campaign carried out between May and June 2012 the sky, with a totalINTEGRAL exposure catalogue time of of gamma-ray sources. 40have It million been contains seconds. a identified total Bird as of et eitherholes 421 al. binary gamma-ray and objects. stars neutron (2007) Most in stars, published our or theremain Galaxy unidentified active third containing so galaxies, far. exotic far They objects awayIntegral could such in be observes as space. either in black star But the systems ademonstrated enshrouded gamma-ray that puzzling in band it dust quarter can so and discover of sources gas, it sources obscuredefficiency or can at with CVs. other which see wavelengths. INTEGRAL through One has surprise detected the hasInitially just been intervening one astronomers the minor material. were subclass CVs, not the It surealready so-called shown has that IPCVs. that only CVs about would one emit percentfor of CVs, gamma them apparently rays. do. fallen This into fact Indeed, disgraceor overbearingly in INTEGRAL black renewed favour holes. the has of interest The binary fourth systems IBIS/ISGRI containing catalogthird reports either catalog. 331 neutron additional Of stars sources these, when 120 compared to arewith the associated known with Galactic extragalactic sources, sources, whileconstitute and only 25 the are remainder associated are so far unidentied (Bird et al. 2010). CVs the framework of its secular evolutionvery and peculiar a very broad effective band magnetospheric propeller emission process. from The radio to possibly TeV Golden Age of CVs 10-m al. (2008) reported the observations2005 of during AE four Aqr consecutive with nightscampaign the within covering MAGIC the the telescope, context radio, performed of optical, UV, in a andevidence August quasi-simultaneous X-ray for ranges. multi-wavelength any The steady analysis or of pulsed these data revealed no AE Aqr in the classthe earlier of reports rotation-powered of pulsars, the transient andlies VHE-TeV provide just an in the attractive propeller framework area to in explain the Lipunov’s diagram reported in Fig. 1. propeller driven spin-down power of the white dwarf, which is L AE Aqr in any of the searches performed. PoS(Golden2015)001 WD. = 2.47 ⊙ GR = 1. This revised Franco Giovannelli MB = f GR WD to 100,000 years for a 0.6 M ⊙ 18 20 yrs) suggests that RS Oph hosts a massive WD ∼ = 0.66 above, whilst the standard model gives f MB In the latter paper Schaefer discussed not only RS Oph,Classical but and also the recurrent photometric nova histories outbursts of have been discussed by Bode (2011a,b) and Evans One of the most interesting RNe is RS Ophiuchi (RS Oph). It is an amazingly prolific recurrent Recurrent novae are a rare sub-class of CVs; WDs accreting material from a binary companion The long term behaviour of classical old novae, and the optical behaviour of CNe in outburst Classical novae are expected to recur on timescales from 100,000 years to just a few decades. The sub-class of CVs, named Classical Novae (CNe), which are the third more powerful stellar all known galactic RNe. (2011). The proceedings ofuseful a for conference details about (Evans et RSby al., Oph Warner 2008). and (2002). General recurrent The properties very phenomenon of useful can quiescent book be novae of have very Bode been & discussed Evans (2008) about classical novae examines nova, with recorded outbursts in2010). 1898, 1907, The 1933, short 1945, time between 1958, outbursts 1967, ( 1985 and 2006 (Schaefer in which more than one2001 classical for a nova-type comprehensive outburst review has ofclear been CVs). runaway observed Nova on (see outbursts the are the surface suspected bookcritical to of pressure of be the is Hellier, due reached WD, to at which a the releases thermonu- base huge of the amounts shell of of thermal accreted energy material. once a accreting material at a significantly high rate. were discussed by Bianchini (1990),Viotti and (1990) Seitter and (1990), by respectively. Bode Thenovae. books & by Evans Cassatella (2008) & are very useful for studying the physics of classical The most important physical parametersand the controlling mass this accretion recurrence rate from timescaleis the recorded are secondary more the (e.g. than WD once, Yaron et it mass, system al. can remain be 2005). intact designated after Once as an classical outburst, "recurrent" it nova (RN).recurrent is (CN) Since novae possible if the that observed WD classical over novae a and may long the actually enoughrecurrent binary be time novae the period. range same from While as 10 the interval to between 100novae outbursts years, would of it range has from been about estimated 30,000 thatGiven years the long for time enough a interval - for 1.3 it classical M is expected that all classical novae will be observed as recurrent novae. 4. Classical and Recurrent Novae explosions in a galaxy, have been observedThe as close time as to a report kpc and on as theclass far recent as telescopes, renaissance galaxies in high in Fornax studies resolution cluster. onout spectroscopy, CNe with in thanks Swift, to synergy XMM, observations Chandra, with withthe HST, 8-10m observations outburst, and from seems Spitzer, coupled space now with in carried SNe-Ia recent order. will advances definitively Moreover, in justify the the the possible renewed theory interest connection of about among CVs. some CV-types and Golden Age of CVs gap are assumed to beare driven usually solely described by by gravitational aBaraffe radiation magnetic & (GR), bracking Patterson (MB) while (2011) (Rappaport, AMLs with Verbuntbelow above & their the the Joss gap revised gap (1983). and model, f found Knigge, the optimal scale factors f model describes the mass–radius data much better than the standard model. PoS(Golden2015)001 Franco Giovannelli 4% of the nova eruptions seen in M31 (SD) model (Whelan & Iben, 1973), in ∼ 19 single degenerate 400 known Galactic classical novae, only 10 of them are recurrent. Eight of them har- ∼ It is well accepted by the community that Type–Ia SNe are the result of the explosion of a In order to brave this important problem the use of archival data is the only way to answer the Important works have been developed about extragalactic nova populations (Shafter et al., On the contrary, Shafter et al. (2015) estimated that RNe play an important role in the studies of SN Ia progenitors (Surina, Bode & Darnley, 2011). Of the An important not yet resolved problem is connected with the evolution and fate of Classical 100 yr. For plausible system parameters, it appears unlikely that RNe can provide a significant which the WD accretes and burns hydrogen–rich material from the companion. The second family carbon–oxygen WD that grows toFowler, near 1960). Chandrasekhar’s But limit the in debate is apast, focussed two close around families binary the of system different progenitor (Hoyle kinds modelsincrease. of & have The progenitors. been first proposed. Indeed, family in They is the the differ so–called in the mode of WD mass big question. Now, huge and comprehensive set of archival RN data go back5. to 1890. Progenitors of SN Ia 2014). Nova rates have beenHubble types. measured They for found more that than thethat a recurrent seen nova dozen in population galaxies M31 in spanning and the the LMC a Galaxy. appears wide to range be of higher than RNe are likely progenitors of Type–Ia supernovae. bour evolved secondary stars, contrary to classicalet novae that al., contain 2011). main sequence They starsondary (Darnley propose star, a contrary new the current nova schemes classificationcontains based based three on on groups the the of properties evolutionary ofNova); novae: state and outbursts. iii) i) of Such Red the Main classification Giant sec- Sequence branch Nova Nova (RG–Nova). (MS–Nova); ii) Sub–Giant Nova (SG– Novae. Patterson (2014) discussedshine this among crucial the brightest problem. starsonly Classical in dimly the novae known. Galaxy. rise Vast from The amountslight story obscurity of curves of of to energy post-novae how are suggest they that loosed they returnto upon take to not quiescence. the quiescence a WD few is years, In and still but its the aradiation few companion, meantime, - thousand and years, enough the to the to secondary return overwhelmthe may its case experience intrinsic of a BK nuclear lot Lyncis luminosity. –2013). of the For He heating oldest discussed this from stellar old purpose the physics nova he behind andtime-series WD’s this mentioned photometry a suggestion in bell-wether and the for proposed months CVs how and evolution it years (Patterson might (and et be if al., tested possible, by centuries) after outburst. Golden Age of CVs thermonuclear processes, the evolution of nova systems, novaof atmospheres dust and winds, and the molecules evolution in novae,cludes nova observations remnants, across and the electromagnetic observations spectrum, ofsome from novae of radio in the to other most gamma galaxies. important rays, It and outstanding in- discusses problems in classical nova research. over the past century areefficiency associated for with RNe may RNe. be A asin low Monte as three Carlo 10% nova analysis that eruptions shows for novae that observed. in the in general, discovery M31 suggesting that arise as from manychannel as progenitor for one the systems production having of recurrence Type–Ia supernovae. times PoS(Golden2015)001 ), km 3 10 Chandra × M erg. ). ˙ M). < ⊙ rec 51 τ WD 10 ˙ Franco Giovannelli M M ∼ (0.2M < × ˙ << M (mass accretion rate ⊙ RN N ejected = ); ii) the rate of death RNe must be RNdeath rec τ ˙ M (recurrence time scale) from archive plates, (mass ejected in eruption) from pre–eruption < rec τ ), being R erg, and Type–Ia supernovae ejected 20 SNIa ejected 44 R . Then there are enough RNe to supply the Type–Ia 10 ). SN Ia occurs if: i) the mass ejected for each eruption = RN 1 ∼ − N yr ∼ ⊙ (DD) model, in which the merging of two WDs in a close RNdeath M 7 − RNdeath 10 from a Mira–type companion (Mikołajewska, 2011). 1 ∼ − are just a subset of ordinary novae that happen to go off more than once contain WDs efficiently accreting material from the secondary star. In 100 years, RNe must have: high WD mass (1.2M ˙ M yr < ⊙ rec M τ 7 double degenerate − 10 ∼ (number of RNe in the Milky Way) from archive plates and AAVSO, Thus, WDs are gaining mass and the latter RNe will collapse asii) SN Symbiotic Ia. Stars Moreover, for the Some results have been obtained for becoming optimists in solving the problem of SN Ia To recur with In order to solve the problems we need to know Only 10 RNe are known in our Milky Way galaxy, including: U Sco (1863, 1907, 1917, 1936, The possible progenitors of SN Ia are: i) Recurrent Novae; ii)i) Symbiotic Recurrent stars; Novae iii) Super-soft As such, they are binary systems with matter flowing off a companion star onto a WD, accu- The energy released through runaway thermonuclear process ejects the majority of the unburnt Although Type–Ia supernovae appear to have similar origin to classical novae, there are key . This produces a bright but short-lived burst of light - the nova. 1 RN − Milky Way, M31, and LMC R most cases they steadilysystems burn can H–rich produce material high mass allowingclose to WDs. them Chandrasekhar’s limit. to In For grow symbiotic instance inat RNe in V a (SyRNe) mass. 407 rate Cyg the of a WD very Some mass massive WD of is is these already accreting material very production. Indeed Schaefer (2011) obtained for CI Aql and U Sco M SN events. 1945, 1969, 1979, 1987, 1999);Oph T (1898, Pyx 1907, 1933, (1890, 1945, 1902, 1958, 1920, 1967, 1944, 1985, 1967); 2006). T CrB (1866, 1946); RS sources; iv) Double WD Binaries; and v) WDs accreting material from red–giant companions. mulating on its surface until theis pressure the gets nova. high enough to trigger a thermonuclear runaway that enough to produce the SN Ia rate (R N onto WD) from the average in the last century, M per century. hydrogen from the surface of the star in a shell of material moving at speeds of updifferences. to The 1.5 most important is thaton in the a surface classical of nova, the the thermonuclear star,& runaway allowing Bildsten, occurs the 2005). only WD and In the acompletely binary Type–Ia disrupting system supernova, to the the remain thermonuclear progenitor. intact runawayexplosions, (e.g. with occurs This classical within Townsley novae is WD releasing reflected itself, in the amount of energy released in the eclipse timing. Golden Age of CVs is the so–called binary triggers the explosion (Webbing, 1984;different Iben delay & times for Tutukov, 1984). the birth The ofdiscover two the the scenarios binary progenitors produce of system Type–Ia to SNe explosion. by Thus studyingcan it their be is delay time hopefully determined distribution possible empirically (DDT). to The fromSN DDT birthrate. the lag between the cosmic star formation rate and Type–Ia s and high accretion rate ( is less than the mass accreted onto the WD (M PoS(Golden2015)001 max worth –V ⊙ max 2 M . 0 ∼ Franco Giovannelli level is ruled out. σ . Observations carried out by Patat ) WDs. Milky Way disc ELM WDs ⊙ (B) which can, in turn, be used to sep- 15 m 25 M . ∆ 0 ≤ , and than 20% at 3 21 σ due to gravitational wave radiation. The ELM WD systems 1 − yr 5 are systems containing two WDs that can merge and giving rise to − are probably WDs that accrete material and burn hydrogen. Voss 10 × 4 ≈ colours on the light curve parameter max –I max Type–Ia SNe are used as primary distance indicators in cosmology (e.g. Phillips, 2005). Bianco et al. (2011) searched for a signature of a non–degenerate companion in three years of v) WDs accreting material from red–giant companions iv) Double WDs Binaries Greggio, Renzini & Daddi (2008) starting from the fact that Type–Ia SN events occur over an iii) Super–soft Sources Phillips (2011) reviewed the near–infraredessentially perfect (NIR) standard of candles Type–Ia in SNe thenosity concluding NIR, on decline displaying that rate only such and a colour. SNe slight30 Lira dependence are to (1995) 90 of first days noted peak after that lumi- V B–V maximumshape. evolution is during remarkably the This similar period fact for from all was SN usedand Ia by events, V regardless Phillips of et light–curve al. (1999) to calibrate the dependence of the B Supernova Legacy Survey data. They foundto that Type–Ia SN a explosions contribution greater from than WD/red–giant 10% binary at system 2 et al. (2008) withThe VLT–UVES allowed expansion velocities, to densities detect and circumstellar dimensionsmaterial material of was ejected in the from circumstellar a the envelope system normal indicate priorfavour Type–Ia to that a SN. the this explosion. progenitor The system relatively where low expansionred–giant a velocities phase WD at accretes the material time from of explosion. a companion star, which is in the extended period of time, following afunctions distribution of that delay accommodate times (DDT), both discussedDDT ‘prompt‘ theoretical functions. DDT and ‘tardy‘ Moreover such SNtional theoretical events DDT constraints. derived functions by can The empirically–based account resultfashion for as is function all that of available cosmic SD/DD observa- time mix (redshift). of SNIa’s isSN predicted explosion. to vary Yoon, in Podsiadlowskitwo & a carbon–oxygen Rosswog systematic (CO) (2007) WDs. explored Their results themerger imply may evolution that be of at considered least good the candidates some for merger productsand the of of progenitors double Kilic of CO Type–Ia et SNe. WDs Brown al. etsystems al. (2011) containing (2011) extremely studied low a mass complete (ELM) colour–selected ( sample of double–degenerate binary Golden Age of CVs & Nelemans (2008) discovered an objectgalaxy at NGC1404 on the pre–supernova position archival X–ray of images. the(SD) This Type–Ia for SN2007on result favours this in the supernova, the accretion although elliptical model thepredicted host in galaxy SD is models. older than However,accretion the the age disc DD at model is which cannot probably the beRoche–lobe explosions the ruled contact are out intermediate and by explosion configuration (Yoon, this Podsiadlowski of event & because the Rosswog, a 2007). system, hot between first WD–WD have a merger rate of that undergo stable mass transfer canfact account is for that about the 3% ELM of WDof systems AM underluminous that CVn may SNe. stars. detonate These The merge SNe most at are important a rare rate explosions comparable estimated to to the estimate produce rate only of ejecta. At least 25% ofGalaxy. ELM WD Thus, sample if belong merging to ELM thesurveys WD old must tick systems find disc are them and the in halo both progenitors components elliptical of of and our underluminous spiral SNe, galaxies. transient PoS(Golden2015)001 069 (sys) using . 0 . Franco Giovannelli 1 ± (sys) using a set of − 036 039 . . 0 0 Mpc + − 5500 ) spectra of 32 low- 1 − < 034 (stat) . λ 0 (stat) ± < 3 km s . 027 029 . . 3 0 0 211 + − . ± 2 . 2 ± 3 . 63 22 = 0 CDM models with values of 0 Λ 08) SNe Ia, obtained with the Hubble Space Telescope (HST) using the . 0 < that fit a z Λ Ω < and 001 . M Ω In order to solve the problem of determining the SN Ia progenitors, it is also important to look Darnley et al. (2014) discussed on the galactic nova progenitor population. They presented The research about the progenotors of SN Ia is of course one of the most important problems, In order to explore the difficult topic of the expansion of the Universe it is necessary to know The use of Type–Ia SNe is also fundamental for determining some cosmological constraints, light curves obtained with robotic telescopes to measure SN Ia photometric parameters, such at the RNe thatRNe show play many an similarities important role to asused CNe, one as but of primary the have distance suspected indicators had progenitor intheir more systems cosmology. central of than Thus, binary Type–Ia it one systems SNe, is to which recorded important determine are and outburst. to the the investigate relation outburst the between type, nature the of and parameters finally ofrise the ascertain to central the the system population progenitors of novae ofamplitude that as Type–Ia might a SNe. be criterion available that Surina, to maytargets Bode give help for & distinguish observations RNe Darnley from from (2015) CNe ground-basedthe adopted and observatories Southern a was including therefore low African used the outburst Large to LiverpoolMass select Telescope Telescope Ejection Imager and as (SMEI). well They found as that at the least full-sky four objects space-based currently classified archive as of CNe are the Solar Space Telescope Imaging Spectrograph. They combine thisgri spectroscopic sample with high-quality as stretch (light-curve width), opticalconclusions colour regarding and the brightness. complex They behaviourthat confirm of the and SNe correlations strengthen found Ia are earlier in moreof the useful SNe NUV Ia in and spectral putting how tighter region, their constraints progenitor butKistler on channels et suggest may the al., vary progenitor 2013) with systems rather host galaxy than properties improving (e.g. the use metallicity: of SNea Ia as selection cosmological of probes. the workbased and not rationale on that the led outburst to propertiesthe the but results on proposal the of of nature a a ofevolutionary new state photometric the nova of quiescent the classification survey secondary system. scheme can of be They easilyincluding a also determined their outlined and sample relevance leading to to of extragalactic a number work quiescent of and predictions, Galactic the proposed novae, link showing to type–Ia that SNe. the since it is strictly connected withan the analysis evolution of of CVs. the For maximum instance,redshift light, Maguire (0 et near-ultraviolet al. (NUV; (2012) 2900 present the evolution of metallicity in oldspace Universe that observatory changes Super the Nova Hubble Acceleration Diagramof shape. Probe the The (SNAP) Universe proposed is and to designed determine toexpansion the measure (Aldering, nature the of 2005). the expansion mysterious SNAP Dark(JDEM) is Energy (Stril, that Cahn being is & proposed accelerating Linder, this as 2010),Department which part of is of Energy. a If cooperative the venture selected Joint between itgoal Dark NASA will without and Energy progenitor/evolution be the solution. launched Mission U.S. before 2020. SNAP cannot achieve its main such as Golden Age of CVs arately evaluate the host galaxyreddening, extinction. they Using reanalyzed these the methods functionaland for form gave eliminating a of the value the of effect decline the of Hubble rate the constant versus of luminosity H relationship low–redshift nearby–Hubble–flow SNe (Kowalski et al., 2008), respectively. a set of 252 high–redshift SNe (Guy et al., 2010) and 0.713 PoS(Golden2015)001 51 500 10 ± × yr since 4 . 8 1 Co lines at 2100 10 ∼ 56 ∼ k ≤ ). The expected e 20 Franco Giovannelli − 10 ejecta composed from − ⊙ 19 − Ni were synthesized during 4 M . 56 10 1 ≃ ∼ H (Scalzo, Ruiter & Sim, 2014); and ⊙ of radioactive ⊙ 23 13 M . 0 ± 62 . ) and sufficiently rich in residual hydrogen (X ⊙ 0 M . 1 > . The flux at lower energies (200-400 keV) is consistent with the three-photon positron- 1 − An important paper by Churazov et al. (2014) reports the first ever detection of Williams et al. (2014, 2016) report the results of a survey of M31 novae in quiescence. The de- Another possible channel for triggering the explosion of SN Ia is that discussed by Chiosi the explosion. Line broadeningkm gives a s characteristic ejecta expansionium velocity annihilation, V Compton downscattering and absorption in the time delay after formation can be as low as about a few ten of thousand years. 847 and 1237 keV andwith a INTEGRAL continuum observatory. in The data the were 200-400 takenoutburst. keV between The 50th band and line from 100th fluxes the day suggest since Type–Ia that the SN2014J SN2014J 0 in M82 equal fractions of iron-group and intermediate-masserg. elements and a All kinetic energy these E a parameters Chandrasekhar-mass are WD, in providing broad anthermonuclear agreement unambiguous explosion with proof of a of a "canonical" the solarexplosion) model nature mass show rather of compact of symmetric Type–Ia object. an Co SNe explosion and Late as Fe of optical a line spectra profiles, (day suggesting 136 that, after unless the the viewing an- the WD formation (see Tsebrenko &al. Soker, (2015), 2015, a for single more CO–WD details).massive may ( With reach the the models explosion of stage soon Chiosi after et the formation if sufficiently rived catalog contains data for 38 spectroscopicallyLiverpool confirmed Telescope novae from images 2006 of to 2012. eachsystem. They nova used These during positions eruption were to then matched definethey to an performed archival accurate Hubble photometry Space position on Telescope for (HST) anytions. images each resolved and This objects in order that to were facilitateTelescope coincident a (HST) data, search with with for the the their eruption aim progenitorluminous of systems posi- accretion detecting within systems disks. archival Hubble with They red Space found giant andaries secondaries elevated that (RG-novae) proportion may or of imply nova the systems presencethought. with of evolved This a secon- would much have larger considerable population impact,Ia of particularly SN with recurrent progenitors. regards novae to than Their their previously results potential alsowith as imply the Type– that M31 RG-novae disk in M31 populationresiding are than in more the the likely bulge, to disk. indeed be If associated theoriginate this results from result are RG-nova is consistent systems confirmed with are inmay all other more be RG-novae galaxies, rare likely in it to old suggests be stellar any associated populations, Type–Ia such SNe with as that younger early-type populations, galaxies. and et al. (2015).Chandrasekhar limit They may explore undergo theconfirmed nuclear possibility runaway it that and should isolated SN be CO–WDsrate explosion. possible (e.g. with (i) If mass Mannucci, to this Della smaller channel explainobservational Valle than & data could the the Panagia, on be star 2006); the formation (ii) ejectednon-Chandrasekhar-mass rate to progenitors mass of provide dependence distribution mass some of as of clues low type–Ia the to as SNe 0.8 SN interpreting M showing the Ia a significant rate of Golden Age of CVs possibly RNe candidates based onfor their additional quiescent outbursts spectra. of bright Theyconclusive example. also CNe searched that the might SMEI otherwise archive have been missed but did not find a (iii) to account for the SNe explodingscenario inside in Planetary which Nebulae a in alternative WD to merges the with core-degenerate the hot core of an AGB star on a time interval PoS(Golden2015)001 Franco Giovannelli 24 Gaudenzi et al. (1990), analyzing IUE spectra of SS Cygni,Alternatively, discussed could about nuclear the burning outburst be responsible of the production of outbursts in CVs? The rare AM CVn stars have extremely short orbital periods, between 10 and 65 minutes, and Despite all the work developed during the last decades, the problem of modeling accretion Moreover, the detection of several SW Sex systems having orbital periods inside the so-called Could CVs be considered simply gravimagnetic rotators? ThisStudies should of be rotational the equilibria most of MCVs suitable predict that IPCVs will evolve either into PCVs or Several fundamental questions concerning CVs still remain waiting for a proper answer.One Here of them is the lack of a coherent classification, especially for NLs. On the other hand, For comments and prospects about Type–Ia SN science in the decade 2010–2020 see the paper production as due to the destruction ofLong the and accretion short disk. outbursts correspond The matter to slowly total accretes or onto partial the destruction WD. of theIndeed, disk, nuclear respectively. burning onto white dwarf’ surface was proposed by Mitrofanov (1978, 1980) as a their spectra show no evidence for hydrogen.are They still appear waiting to be for helium-rich a versionseven general of this CVs. model. is They still They controversial. are probably binary systems of twodisks white in CVs dwarfs, is but by nocause means of closed, outbursts. especially We in really quiescence. do not Closelybility know related Models which is or of the the Secondary problem present of Instability twoor the Models) families in of is which models responsible (Disk system for Insta- isin the each solving CVs model this outburst valid, problem phenomenon, although at Martinez-Pais leastof et in the the al. mass case transfer (1996) of rate gave SS frombursts. a the Cygni; Something secondary contribution they similar star found can as some be the evidence said mechanism for about responsible an the for increase super-outburst symmetric phenomenon out- in SU UMa systems. ’period gap’ opens a new interesting problem about the continuity in the evolution of CVs. approach for studying them from a physical point of view. into low field strength polars –on presumably their unobservable, and magnetic possibly moments EUVhaving emitters and magnetic – moment orbital depending similar periods. to IPCVsfield AM above Indeed, Her-like the systems. there ’period gap’ are and systems, comparable to like the EX weakest Hya-type, we will present briefly only some of them. in gross features and intinguishable, most although, respects, in DN addition and to NLs,further their as minor different well differences outbursts’ as which behaviour, quiescent there are novae, appeararises not are to yet of almost be understood indis- whether some (see the Hack outburstsuitable & criterion la behaviour, for Dous the sorting 1993). CVs current in The basiseither physically question related of systems groups. that almost There do all are not also classificationbe fit too able is in many to exceptions, any really render particular a the group observational or behaviour, at that least can as be it included is in used several at of the them, present, to suitable. by Howell et al. (2009). 6. Some Open Questions Golden Age of CVs gle is special, the distribution2015). of radioactive elements is symmetric in the ejecta (Churazov et al., PoS(Golden2015)001 100 ∼ Franco Giovannelli 25 CVs. One of the big questions that arises is: "can all of the observed CVs and 6 plane is very close to the line where IPs lie. 10 orb ≈ m emission from MCVs, due to dust (Howell et al., 2006; Brinkworth et al., 2007). µ –logP spin Further information can be considered in order to better synthetizeSion (http://astronomy.villanova.edu/faculty/sion/CV/index.html) the states open that in problems the in Galaxy the we The SPITZER space telescope detected the presence of fullerenes in a young planetary nebula Important results are coming from the SPITZER space telescope with the detection of an ex- Could some systems behave in different ways depending on their instantaneous physical condi- An example very clear is that of SS Cygni, usually classified as a non-magnetic dwarf nova. Are the IPCVs and PCVs smoothly connected via the SW Sex-like systems placed just in Accretional heating by periodic DN events increases substantially the surface temperature Another problem still open is connected with the classification of CVs in three kinds, namely knowledge of CVs and related objects. could expect the phenomena associated with them be understood in terms of a single unified picture?" Other (Cami et al., 2010). Fullerenes aresible the presence first of bricks fullerenes for in the CVs emergence openssurprises. of a the new line life. of Therefore, investigation, the foreboding pos- of new interesting cess (3-8) Gaudenzi et al. (2011) discussed aboutstrated the reasons that of this the variable reddening reddening is insecond formed SS (intrinsic Cyg by and to two demon- the components: system thephase. itself) first Moreover, is is an variable interstellar orbital and in modulationsystem origin, changes also are and during exists. consistent the the with The evolution the physical of possibility and of a chemical formation quiescent parameters of fullerenes. of the tions? For this reason they could apparently behave sometimes as PCVs and sometimesIt as has NPCVs. been detectedkeV). by This the emission is INTEGRAL very observatory hardthe in to system. be a Several explained proofs region without have theorder of been presence to shown the of and demonstrate spectrum polar discussed the caps (up many Intermediatetherein; in times to the Polar Giovannelli by WD & nature Giovannelli’s of group Sabau-Graziati, of in 1998; ita 2012a); NMCV, (e.g., as indeed, Giovannelli, well SS 1996, as Cygni andlogP those shows references of characteristics IP of and sometimes even those of polars, although its position in the between? SW Sex systemsthen have their indeed presence orbital there periods sure cancel belong that to gap. the so-called ’period gap’, and of the WD incompression CVs and (Godon irradiation should & be Sion, a crucialof 2002). component a in pre–nova Then, understanding WD. the the envelope structure envelope thermal structure resulting from NMCVs, PCVs and IPCVs.artificial, probably This not necessary is, if in CVs areevolution our studied of opinion, as the gravimagnetic systems another rotators. could In convenient be this classification, responsible way of a although smooth the variations of the gravimagnetic parameters. Golden Age of CVs mechanism suitable to generate X-rays inof CVs. theoreticians In did spite not of consider thispart shrewd such of suggestion, a their the mechanism time. community –of However, certainly we outbursts possible believe in that – CVs this worthy wouldsurface alternative of deserve interested solution taking theoretician in in community’s up the explaining care. a accretion the(Gaudenzi in For generation et the instance, al., system the 2002). SS white There, Cygni nuclear dwarf burning has could been occur. evaluated as 24% of the total PoS(Golden2015)001 It seems that ". Franco Giovannelli ." 26 What causes the period gap? How do Polars form and whyLARPS are (Low no Accretion magnetic Rate white Polars) dwarfsemergence the in of progenitors wide systems binaries of containing observed? polars? magnetic white Are Is dwarfs there versus non-magnetic? a difference in the What happens to CVs once they reach the period minimum? What are the detailed physics occurring in the common envelope? What is the correct physics to describe viscosity in accretion disks? What is the correctradiation) angular that momentum can prescription account belowdistribution? for the the gap observed (besides period gravitational minimum spike and the exact period What is the actual number density and distribution of CVs in the Galaxy? Szkody & Gänsicke (2012) provided a list of unanswered problems and questions and refer- What was surprising was the large number of short-period "ultracompact" binaries (AM CVn While many of the results confirmed what had already been inferred about the properties of 5. 6. 7. 3. 4. 1. 2. This is the most ambitious analysis of the properties of an entire CV population that has ever been ences for seeking additional information.characteristics of Indeed, the while types the ofwhich general remain. CVs evolutionary These are picture include: known and at the some level, there are major unsolved questions stars) that were produced and, especially, theoxygen enormous that depletion is of predicted carbon at relative certain to epochsnature nitrogen for and has evolved systems. provided As us Nelson with pointsstate out, a based " on unique their way carbon to abundances.that identify There there CVs is is already that a some descended significant observationalthat from depletion evidence will a to of allow suggest us highly carbon to evolved in infer certain the lineage CVs. of some This CVs could and be predict what a their really fate critical will test be CVs, there were a numberexplored of evolutionary surprises pathways. including But, the as identification expected,CVs a of that sharp a evolved bifurcation number to was of produce foundhybrid previously double between white un- nascent dwarfs), white-dwarf and binaries ones (including that continuouslyIn ones transferred addition, containing mass the helium over predictions the and of lifetime the ofobservations theoretical the of universe. simulations CVs were with in reasonably good well-measured general properties. agreement with the Golden Age of CVs questions relate to the relativeevolution, probabilities and that CVs how will the be observationsultimate observed fate. of at To CVs address particular these at stages questions the in Nelsontaken (2012) their current a and massive epoch Goliasch computational & can effort Nelson to be (2015) theoreticallyCVs have used simulate that under- the to could evolution determine of be most their producedlowed of over the by an possible nature. age of 10 The billion" temporal years using evolution the of MESA stellar 56,000 evolutionundertaken. nascent code. The CVs According whole to was project Nelson, required fol- several core-years of CPU time PoS(Golden2015)001 Franco Giovannelli " has been organized since 27 This research has made use of NASA’s Astrophysics Data System. The Golden Age of Cataclysmic Variables and Related Objects Do CVs contain exoplanets? What causes Polars, asand well 4 as hours, the to cease novalikevariations mass of disk the transfer systems companion and stars with enter a orbital general low phenomenon periods states? among between all Are CVs? 3 theCan the associated white mass dwarfs transfer in CVs grow in mass? A general data base for Experimental Nuclear Reaction Data (EXFOR) can be found in: The LUNA (Laboratory for Underground Nuclear Astrophysics) is devoted to measure nuclear In those catastrophic processes the production of light and heavy elements, and then the knowl- We want to conclude with a general warning, apparently underestimated, like remarked by At the end of this review it appears evident that theThe most detection suitable of approach several for SW studying Sex CVs systems having orbital periods inside the so-called ’period In order to fully understand the emission properties and evolution of CVs, the mass–transfer In order to answer to these not yet solved problems, a series of biennial Palermo Workshops 9. 8. 10. https://www-nds.iaea.org/exfor/exfor.htm. Acknowledgments cross sections relevant in astrophysics andrunning astroparticle physics. underground It in is the the Gran most2010). valuable Sasso experiment LUNA Laboratory experiment of provided the theoccurring INFN in measures the (e.g. of stars the for review a cross-sections by better2015b, of knowledge Broggini and many of references et stellar nuclear therein). evolution al., reactions (see Giovannelli & Sabau-Graziati, edge of their abundances providesgeneration problems. strong direct inputs for cosmological models and cosmicGiovannelli ray & Sabau-Graziati (2015b):sections if of we nuclear have reactions not occurring in experimental the information stars about it is the hard cross to describe the correct star evolution. from a physical point of view is to consider them as gravimagneticgap’ rotators. opens a new interesting problemand about PCVs the smoothly continuity connected in via the the evolution SW of Sex-like CVs. systems Are placed the just IPCVs inprocess between? needs to be clearly understood,of especially magnetic magnetic mass viscosity transfer, in as the welltion accretion as on disks the the properties around magnetic compact field intensities objects. insystems, Consequently, WDs by the appears crucial investiga- which in it understanding the issupernovae evolution (e.g., possible of Isern CVs to et al., generate 1993). classical novae (e.g., Isern et al., 1997) and type-Ia 7. 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