PoS(GOLDEN 2017)001

45 Pages·2017·4.93 MB·English

The Golden Age of Cataclysmic Variables
and Related Objects (A Very Personal Review)
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Franco Giovannelli∗† G
INAF-IstitutodiAstrofisicaePlanetologiaSpaziali,ViadelFossodelCavaliere,100,00133
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Roma,Italy
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E-mail: [email protected]
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InthelasttwoyearsweassistedtoasubstantialimprovementonthenumberofCVsdiscovered, E
but only few exciting results have been obtained. Thus, in this paper I cannot give astonishing N
news with respect to those discussed in the review paper published by Giovannelli & Sabau-

Graziati (2015a), but simply I will present a personal view about the route to be followed in
the investigations on CVs. The exception has been the very exciting news about the discovery 2
of a white dwarf pulsar in AR Sco (Buckley et al., 2017) presented by David Buckley in his
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talk (Bukley, 2017a, this workshop) and followed by a series of related talks by Beskrovnaya,
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Meintjes,Isakova,andIkhsanov(2017,thisworkshop).
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TheGoldenAgeofCataclysmicVariablesandRelatedObjectsIV
11-16September,2017
Palermo,Italy
∗
Speaker.
†Afootnotemayfollow.
⃝c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommons
Attribution-NonCommercial-NoDerivatives4.0InternationalLicense(CCBY-NC-ND4.0). https://pos.sissa.it/
TheGoldenAgeofCVs FrancoGiovannelli
1. Introduction
The birth of the universe and its present status constitute the two banks of a river in which
the life of the universe is slowly flowing. Undoubtedly the two banks are joined by a bridge that
Giovannelli (2001a) nicknamed "The Bridge between the Big Bang and Biology" that constituted
the title of the workshop held in Stromboli (Aeolian Archipelago, Sicily, Italy) in 1999. The big
problemishowtocrossthisbridge, andthemainquestionis: whataretheexperimentaltoolsfor
understandingthepillarsofthisBridge?
Inordertocrossthisbridge,asalwayswhenwecrossabridge,weMUSTadvanceslowly,step
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by step, with continuity, because everything is smoothly linked in the "magma" of the Universe,
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fromtheinfinitelysmalltoinfinitelybig,assketchedinFig. 1(adoptedfromGiovannelli&Sabau-
Graziati,2017afterRees, 1988inOrigins. TheDarwinCollegelectures(1986)editedbyFabian, S
1988). (
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Figure1: Fromtheinfinitelysmalltoinfinitelybig(adoptedfromGiovannelli&Sabau-Graziati,2017after
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Rees,1988).
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Indeed, if we look at the Fig. 2 (left panel) – where a section of the metabolic network of a
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"simple"bacteriumisshown–wecannotethateachpoint(eachchemicalcompound)isconnected
toanyotherpointthroughthecomplexityofthenetwork(Luisi&Capra,2014)exactlythesameoc-
curringinthe"cosmicnetwork"whereeachpointisconnectedtoanyotherpointthroughthecom-
plexity of the network as shown in Fig. 2 (right panel) (https://it.wikipedia.org/wiki/Cosmologia
del plasma). The large-scale structure of the Universe, as traced by the distribution of galaxies,
is now being revealed by large-volume cosmological surveys. The structure is characterized by
galaxiesdistributedalongfilaments,thefilamentsconnectinginturntoformapercolatingnetwork.
Shandarin,Habib&Heitmann(2010)objectivewastoquantitativelyspecifytheunderlyingmech-
anisms that drive the formation of the cosmic network. By combining percolation-based analyses
with N-body simulations of gravitational structure formation, they elucidate how the network has
itsorigininthepropertiesoftheinitialdensityfield(nature)andhowitscontrastisthenamplified
bythenonlinearmappinginducedbythegravitationalinstability(nurture).
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TheGoldenAgeofCVs FrancoGiovannelli
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Figure 2: Left panel: Section of the metabolic network of a "simple" bacterium (Luisi & Capra, 2014).
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Right panel: the "cosmic network" (https://it.wikipedia.org/wiki/Cosmologia del plasma) (adopted from
Giovannelli&Sabau-Graziati,2017). L
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Figure3: Thecosmicbudget. )
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As Albert Einstein affirmed, we can’t solve problems by using the same kind of thinking we
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usedwhenwecreatedthem. Icanaddsomethingmore,byusingthewisdom: wecanattacheach
kind of problem in a way as general as possible, and in any case it is necessary to go on without
blinkers.
Afundamentalquestionnaturallyarises: whatisnowthesituationaboutourknowledgeofthe
Universe? Theanswerisdiscouraging: weknowaverysmallpartofit,andnotverywell.
Indeed, Fig. 3 shows schematically such a situation. We can really discuss only on ∼ 5%
of the content of the Universe. The remnant of ∼ 95% is almost completely unknown. However,
the recent fundamental progress in gravitational astronomy could open an incredible source of
informationaboutthis"unknown"content.
InthispaperIwilldiscusstheproblemofCataclysmicVariablesandRelatedObjects, which
constituteasmallpartofthe"known"∼5%ofthecontentoftheUniverse.
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TheGoldenAgeofCVs FrancoGiovannelli
2. AccretionProcessesinCosmicSources
Accretion is a universal phenomenon that takes place in the vast majority of astrophysical
objects. Theprogressof ground-basedand space-borneobservationalfacilitieshasresulted inthe
great amount of information on various accreting astrophysical objects, collected within the last
decades. Theaccretionisaccompaniedbytheprocessofextensiveenergyreleasethattakesplace
on the surface of an accreting object and in various gaseous envelopes, accretion disk, jets and
otherelementsoftheflowpattern. Theresultsofobservationsinspiredtheintensivedevelopment
of accretion theory, which, in turn, enabled us to study unique properties of accreting objects and
physicalconditions in thesurrounding environment. Oneof the most interestingoutcomes of this P
intensive study is the fact that accretion processes are, in a sense, self-similar on various spatial o
scalesfromplanetarysystemstogalaxies.
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Figure 4: Accretion processes in different cosmic sources (adopted from Giovannelli & Sabau-Graziati,
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2016a,afterScaringi,2015).
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This fact gives us new opportunities to investigate objects that, by various reasons, are not
availablefordirectstudy.
Cataclysmicvariablestarsareuniquenaturallaboratorieswhereonecanconductthedetailed
observationalstudyofaccretionprocessesandaccretiondisks.
Figure 4 shows a sketch of cosmic systems where accretion processes occur (Giovannelli &
Sabau-Graziati,2016a,afterScaringi,2015).
AninternationalworkshoponAccretionProcessesinCosmicSources: YoungStellarObjects,
Cataclysmic Variables (CVs) and Related Objects, X-ray Binary Systems, Active Galactic Nuclei
wasorganizedbymeincollaborationwithseveralcolleaguesofdifferentinternationalinstitutions,
and took place in Saint Petersburg (Russian Federation) on September 2016. The proceedings
discuss in details the physics of accretion processes in all the cosmic sources shown in Fig. 4
(Giovannelli&Sabau-Graziati,2016b).
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TheGoldenAgeofCVs FrancoGiovannelli
3. AboutCataclysmicVariablesandRelatedObjects
Historically, the classification of CVs was based on the optical outburst properties, by which
one may distinguish four groups: (i) classical novae; (ii) recurrent novae; (iii) dwarf novae; (iv)
nova-likeobjects.
This classification, however, is neither self-consistent nor adequate and it is much better to
considerprimarilytheobservedaccretionbehaviour(Smak,J.: 1985).
Figure5(leftpanel)showsthemaincharacteristicsofthevisuallightcurvesofclassicalnovae
(top panel) and of dwarf novae of the U Gem, Z Cam, and SU UMa types (lower three panels)
(Ritter, 1992). In the right panel of Fig. 5 different kind of humans are reported. They show P
similarities with the light curves of CVs. Apparently they are different, but all of them belong to o
thesameHumanSpecies,likealltheCVsareCVs.
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Figure5: Leftpanel: themaincharacteristicsofthevisuallightcurvesofCVs(Ritter,1992). Rightpanel:
differentcharacteristicsofhumans.
Therefore, it is necessary to find a method as general as possible to describe the behavior of
CVs. ThiscanbeobtainedlookingattheAccretionBehaviourandMagneticField.
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TheGoldenAgeofCVs FrancoGiovannelli
3.1 Classificationonthebaseofthemagneticfieldintensity
Theaccretionstructuredependsonthemagneticfieldofwhitedwarf(B )andonthetransfer
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massrate.
Following the popular classification, depending on B it is possible to classify CVs in three
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groups:
• NonMagneticCVs(NMCVs): B ∼104 -106 G;
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• IntermediatePolars(IPs): B ∼106 -107 G;
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• Polars(MCVs): B ∼107 -108 G.
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Figure6showssuchaclassification. Howeverwehaveasmoothcontinuityamongtheclasses, S
asshowninfig. 7(Giovannelli&Sabau-Graziati,2015a). (
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Figure6: Fromlefttoright: sketchofNMCVs,IPs,andPolars.

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Figure7: MagneticfieldintensityversusorbitalperiodforMCVs. PolarsandIPsarecontainedinthelight
blueandlightgreenrectangles,respectively. Violetrectangleindicatestheso-called"periodgap". Cyan-50
rectanglerepresentstheintersectionbetweenthePolarsandIPs(adoptedfromGiovannelli&Sabau-Graziati,
2015a).
Indeed, taking into account the average values of magnetic field intensity and orbital periods
for polars and IPs, and the minimum and maximum value for both parameters (B and P ), it is
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possibletoconstructaveryinterestingplot(Fig. 7)thatshowstheevidentcontinuitybetweenthe
twoclassesofMCVs. SuchacontinuityhasbeennotedbySchmidtobreick&Tappert(2014,2015):
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TheGoldenAgeofCVs FrancoGiovannelli
CVsevolutionisdrivenby angularmomentumloss; asconsequenceP decreases. Alllong P
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CVscrossSWSexregimebeforeenteringinthe"periodgap". ThereforeSWSexphenomenonis
anevolutionarystageinthelifeofCVs(e.g. Rodriguez-Gil,2003).
Thenatureinallitsmanifestationsshowscontinuity. Thenwehavetoabandonthe"convenient
method"ofthinkingeverythinginwatertightcompartmentsandtogotowardageneralmodelfor
compactaccretingstars,likewasdonebyVladimirLipunovandcollaboratorswhentheydeveloped
the"ScenarioMachine".
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Figure8:Gravimagneticrotator:abodywithmassM,havingamagneticmoment⃗µ,rotatingwithrotational
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velocityω⃗ (adoptedfromGiovannelli, 2016). Theparametery=M˙/µ2 iscalledgravimagneticparameter
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(Lipunov,1987;Lipunov&Postnov,1988).

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StartingfromthetrivialdefinitionofX-rayBinarySystems(XRBs): theyarebinarysystems
emittingX-rays,anaturalquestionarises. Arethesesystemsgovernedbyfewphysicalparameters 0
independent of their nature? The answer is positive. Indeed, High Mass XRBs (HMXRBs), Low 1
MassXRBs(LMXRBs),AnomalousX-rayPulsars(AXPs),andCataclysmicVariables(CVs)can
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be considered as gravimagnetic rotators: a body with mass M, having a magnetic moment ⃗µ,
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rotating with rotational velocity ω⃗, being the two axis not necessarily coincident, as sketched in
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Fig. 8 (Giovannelli, 2016). Introducing a physical parameter, y = M˙ /µ2, named gravimagnetic
parameter, all the gravimagnetic rotators are contained in a plane Log P vs Log y (Lipunov, 0
spin
1987;Lipunov&Postnov,1988). 1
TheScenarioMachine(MonteCarlosimulationsofbinaryevolution)permitstobuildupthe
completepictureofallpossibleevolutionarystagesofbinariesintheGalaxy. Thebasicevolution
equation(3.1)usedfor500,000systemscontainingmagnetizedstarsprovidedtheresultscontained
in the plane Log P –Log y, reported in the upper panel of Fig. 9. P is expressed in seconds
spin spin
andthegravimagneticparameterisexpressedinunitof10−42 gs−1 G−2 cm−6. Thesymbolsused
for the different types of binaries are explained in the lower panel of Fig. 9. The definition of
the characteristic radii can be found in the paper by Lipunov (1987). Observational examples of
varioustypesofrotatorsarereportedinFig. 10(Lipunov,1987).
dIω κµ2
=M˙ K − t (3.1)
dt su R3
t
where:
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TheGoldenAgeofCVs FrancoGiovannelli
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Figure9: Upperpanel: distributionofmagneticrotatorsintheplane"SpinPeriod"versus"Gavimagnetic 0
Parameter" (adopted from Giovannelli, 2016 after Lipunov, 1995); lower panel: classification of rotators
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(Lipunov,1987).
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K =specificangularmomentumappliedbytheaccretionmattertotherotator;
su √
K = GM R forKepleriandiskaccretion;
su x d
K =ηΩR2 forwindaccretioninabinary;
su t g
K ∼0 forasinglemagneticrotator;
su
R =radiusoftheinnerdiskedge;
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Ω=rotationalfrequencyofthebinarysystem;
η =1/4 (Illarionov&Sunyaev,1975);
t
κ =dimensionlessfactor;
t
R =characteristicradius;
t
M˙ =accretionrateindifferentregimes.
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TheGoldenAgeofCVs FrancoGiovannelli
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Figure10: Observationalexamplesofrotators(Lipunov,1987).
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3.2 HighMagneticFieldWhiteDwarfs(HMFWDs)
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Magnetic fields are observed in main sequence stars and their white dwarf and neutron star D
progeny. The fields in these three groups of stars are likely to be linked via stellar evolution.
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However,stellarevolutioncalculationsthatincludemagneticfieldsandrotationinaselfconsistent
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manner are only now beginning to be carried out, and then only with simplifying assumptions.

Some insights into the nature of magnetism of stars in these three groups can be obtained by a
comparativestudyoftheirgroupproperties(e.g. Ferrario&Wickramasinghe,2007). 2
Whilethereisstrongevidencethatthemagneticfieldsinlatetypestarsaredynamogenerated, 0
itislikelythatthemagneticfieldsofstarsontheuppermainsequenceareoffossilorigin,perhaps
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datingbacktothetimeofstarformation.
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Wehavethefollowingapproximatecorrespondencesunderfluxconservation:
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BB−F ∼300−30000G=⇒BHFMWD ∼106−109 G
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BO−B ∼??−2000G=⇒BNS ∼1011−1015 G
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ThelowerfieldlimitisstillunknownforearlyBtoO-typestars. Astrongcasecantherefore
bemadeforafossiloriginoffieldsatleastatthehighfieldendofthefielddistribution(Ferrario& 1
Wickramasinghe,2007).
TherotationalperiodsofWDsandNSsarescalingasP /P ≈(R /R )2.
NS WD NS WD
TherearetwopossibleoriginofstrongmagneticfieldsinWDs:
• Thefossilfieldmodel(nostellarevolutionmodelsshowhowstrongfossilmagneticfluxcan
survivethroughthevariousstagesofstellarevolutionthroughtotheWDphase).
• Themergingstarmodel(dynamomodelofWickramasinghe,Tout&Ferrario,2014): similar
maximum magnetic fluxes may be expected for physical reasons if the fields are generated
fromdifferentialrotationcausedbymerging).
In the fossil scenario, the magnetic field strength B scales with the radius of the star R as B ∝
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R −2 (Ferrario,Melatos&Zrake,2015).
⋆
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TheGoldenAgeofCVs FrancoGiovannelli
Figure 11 shows the distribution of rotation periods for magnetic white dwarfs (MWDs) (left
panel) and neutron stars (NSs) (right panel) (adapted from Ferrario & Wickramasinghe, 2005).
Bothdiagramshavethesametrend,exceptthescalefactorsofrotationalperiodsP (minutesfor
rot
WDs,secondsforNSs)andmagneticfieldintensity(B∼107−109 forWDsand∼1013−1015 G
forNSs).
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Figure11: Thedistributionofrotationperiodsformagneticwhitedwarfs(MWDs)(leftpanel)andneutron
stars(NSs)(rightpanel)(adaptedfromFerrario&Wickramasinghe,2005). L
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Westillneedtoconstruct: (i)morerealisticmodelatmospheresthatallowforthepresenceof
magnetic fields; (ii) stellar evolution tracks of intermediate mass stars that take into consideration N
bothfossilanddynamogeneratedfields. SuchcalculationsmaybeabletotelluswhetherallWDs
aremagneticatsomelevel.
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The origin of fields in highly magnetic WDs is currently being debated. Although the newly
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proposed scenario that all high field MWDs (single and in binaries) are the result of close binary
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evolutionandmergersisgainingmomentum,thefossilfieldhypothesiscannotbetotallydismissed
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(Ferrario,deMartino&Gänsicke,2015).
It is interesting to note in Fig. 12 the distribution of B for MCVs and MWDs (adapted from )
Ferrario,deMartino&Gänsicke,2015). 0
In MCVs there is an interesting relationship between the magnetic field strength and orbital
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period of the systems, as reported in Fig. 13 where the polars and IPs are separated by the blue
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line that marks the synchronization between orbital and spin periods of the cataclysmic systems
(Giovannelli&Sabau-Graziati,2015aafterFerrario,deMartino&Gänsicke,2015).
AlsothesystemAMCVn≡HZ29withorbitalperiodof17.5minutes, discoveredbySmak
(1967) was later recognized as a CV by Patterson (1992) and it constitutes the prototype of AM
CVnstars–ultra-compactbinaries–formedbyaprimarystarWD,andasecondarydegenerateor
semi-degenerate star: e.g. white dwarf - white dwarf binaries (P <80 minutes). White dwarf
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primariesandmainsequencesecondarieshavebinaryorbitalperiodsgreaterthan80minutes.
Levitan (2013) in his PhD thesis about "AM CVn Systems with Palomar Transient Factory"
updates the number of AM CVn stars. Figure 14 shows the number of CVs versus orbital period.
It is clearly evident that 50% of CVs lie over the "period gap", 39% of CVs lie below the "period
gap", and 11% within the "period gap" (indicated with the light blue rectangle). The place where
SW Sex systems lie (indicated with the light red rectangle) is partially in the "period gap". This
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