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The Golden Age of Cataclysmic Variables { 50 Years of CV Research { a Workshop Summary Mem. S.A.It. Vol. 83, 876 c SAIt 2012 Memorie della The Golden Age of Cataclysmic Variables { 50 Years of CV Research { A Workshop Summary Edward M. Sion Department of Astronomy & Astrophysics, Villanova University, 800 Lancaster Ave., Villanova, PA 19085, USA Abstract. When Franco asked me to present a summary of this conference, I answered yes, almost without hesitation, not realizing the daunting challenge posed by the emergence of so many exciting new results. Obviously, this summary contains impressions of the workshop viewed through my own prism. Let me begin on a personal note, by stating that I cannot imagine any venue for a conference on Cataclysmic Variables more striking than Mondello. The overall description of the conference that comes to my mind is: masterfully organized, comprehensive, up to the minute, new science in a wonderful and historic setting of nearly unmatched beauty. The stage was set for the conference by simulations presented by Dmitry Bisikalo and outstanding, comprehensive reviews of cata- Irit Idan. The 3D magnetohydrodynamic sim- clysmic variables, the first one covering the ulations by Bisikalo offered detailed compar- past ”Golden Age” of fifty years by Franco isons with observations of the accretion en- Giovannelli. This provided the participants vironment and complex flows in cataclysmic with a thorough historical perspective on how variables with magnetic WD accretors, includ- the field evolved to its present state of knowl- ing the physical interpretation of phase depen- edge. This was followed by another excel- dent structures and asymmetries in gas flows lent review by Andrew King on Accretion around the white dwarf primary and its appli- Disk theory since Shakura-Sunyaev , deliv- cation to both Roche-lobe overflow systems. ered with the clarity we have come to ex- The 1D evolutionary hydrodynamic/quasi- pect. Hans Ritter’s talk on the Structure and static simulations of accreting white dwarfs Evolution of Cataclysmic Variables enriched in cataclysmic variables undergoing nova out- the participants with still another outstanding bursts, presented by Irit Idan, cover the re- overall review. Among the many highlights in sponse of the white dwarf to thousands of these both theory and observation that I found par- thermonuclear events over timescales up to a ticularly outstanding were two breakthrough billion years or longer. While these long term numerical simulations. I was blown away by evolutionary sequences are being carried out with a constant accretion rate, her code can be Send offprint requests to: Edward M. Sion easily modified to handle variations in the ac- ([email protected]) the ever more physically realistic theoretical cretion rates due to changes in the rate of an- Sion: Concluding Remarks 877 gular momentum loss versus time. It appears erates to weak multi-kilogauss field regime at that such sequences will ultimately provide the threshold of detection, it would not be sur- strong constraints on scenarios of CV evolu- prising that, at some level, every white dwarf tion, above and below the period gap. in a CV is magnetic. Irina Voloshina further The ongoing debate revolving around the underscored the complexity of SS Cygni by disk instability model (DIM) versus mass discussing its three types of outbursts; nor- transfer instability (MTI) as the mechanism mal, long, and anomalous, as well as its QPOs, for dwarf nova outbursts found center stage at DNOs and flickering. She also discussed a this workshop. Jean-Pierre Lasota delivered a newly derived white dwarf mass estimate of stimulating talk on the present state of accre- 0.66 M and an orbital inclination of 50 de- tion disk theory but with a dose of skepticism grees. The mass quoted above is lower than concerning the mounting evidence which chal- the white dwarf mass of 0.81 M derived by lenges the validity of the disk instability model Bitner, Robinson and Behr (2007) and much (DIM) and the use of the alpha viscosity. On lower than the widely used value of 1.2 M the other hand, Oded Regev reminded the par- derived by A. Shafter from disk emission line ticipants that it is okay to use the alpha viscos- velocities. Irina Voloshina’s determinations are ity, that there is no need for apologies in do- also in disagreement with the values of the ing so since magnetohydrodynamic turbulence white dwarf mass of 0.97 M and orbital in- is too complex to understand and currently in- clination angle of 40 degrees, obtained by soluble. Adding fuel to the fire, Raymundo Giovannelli et al. (1983) using physical con- Baptista reported eclipse mapping analyses of straints derived from multifrequency observa- light curves of the eclipsing dwarf nova HT tions. Interesting new results on SS Cygni from Cas which revealed a very fast response of the SUZAKU were presented by Dai Takei, in- underlying white dwarf (which brightened by cluding X-ray observations of SS Cygni in a factor of 2) to the increase in mass transfer both outburst (multi-temperature plasma) and rate, a simultaneous increase in the expansion quiescence (Fe emission lines at 6.4, 6.7 and 10 rate of the accretion disk (which brightened by keV) and a derived Fe abundance of 0.37 x so- a factor of 3) and a relative amplitude of the lar. Kononov obtained tomograms of SS Cygni high-frequency flickering which implies a high that he directly compared with 3D gas dynamic viscosity (alpha 0.3-0.7) for the quiescent disk simulations including the hot line, tidal shock and hence disagrees strongly with the key pre- spirals and a bow shock where trailing matter diction of DIM. His work suggests that the out- accumulates behind the shock, which accounts bursts of HT Cas may be due to bursts of en- for an observed asymmetry. hanced mass transfer from the donor star, i.e. Still another complex CV, the unique IP, the mass transfer instability. AE Aquarii, was the focus of three speak- Speaking of dwarf novae, no less than four ers, Pieter Meintjes, Chris Mauche and Robert talks, and portions of two other talks (the Smith. Meintjes reviewed the physical and equivalent of one full session), were devoted orbital parameters of AE Aqr, as well as to the prototypical dwarf nova SS Cygni and multiwavelength observations spanning the its associated puzzles by Franco Giovannelli, Gamma-ray and X-ray region out to radio Dmitry Kononov, Manabu Ishida and Irina wavelengths. Due to the operation of a mag- Voloshina. Franco presented a list of pros and netic propeller, many argue that the white cons on the issue of whether SS Cygni is an dwarf is accreting very little matter but the intermediate polar. He concluded with com- mass leaving the system is significant. The sys- pelling evidence that SS Cygni is indeed an tem exhibits the N/C abundance anomaly in- IP with a likely 2,000,000 Gauss field, and dicative of CNO processing. It is clear that the hence should have a truncated accretion disk. white dwarf is spinning down but what is ex- Since among single white dwarfs, there is a tracting its rotational kinetic energy? It may continuum of field strengths from multi Mega- have evolved to its present state from a violent gauss fields of the class of magnetic degen- accretion history which may link AE Aquarii 878 Sion: Concluding Remarks to the supersoft X-ray binaries. Chris Mauche fication of nova light curves by speed class ver- delivered an interesting review of multiwave- sus nova progenitor properties and population length observations of AE Aquarii and pre- membership as well as reviewing the charac- sented evidence refuting that it is a Gamma ray teristics of two broad composition classes, the source. Fe II class and the He/N, the latter being faster S. Scaringi presented breathtakingly pre- novae with more massive white dwarfs while cise Kepler data and light curves of CVs in 82 the Kepler field, including V344 Lyra with evi- There were outstanding comprehensive re- dence of positive and negative superhumps and views by Klaus Reinsch on multi-frequency an outside-in outburst. For the VY Scl-type observations of polars, by Solen Balman on nova-like variable MV Lyr, Kepler recorded the X-ray properties of cataclysmic variables flickering intrinsic to MV Lyra’s accretion in general and by Martine Mouchet of XMM- disk. Scaringi reminded us of the remarkable Newton X-ray studies of magnetic CVs. I was similarities between the flickering behavior in also intrigued by Takayuki Yuasa’s discus- CVs like MV Lyra and X-ray binaries con- sion of the determination of the masses of taining neutron stars and black holes, which white dwarfs in intermediate polars based on show similar behavior. The common thread SUZAKU observations. A mean mass of the among this flickering may be a fluctuating ac- white dwarfs in IPs was derived < Mwd >= cretion disk. In a stunning demonstration of 0:88 ± 0:2 M . Kepler’s photometric precision, he displayed Paula Szkody described her work on iden- accretion disk fluctuations superposed on each tifying the instability strip for accreting white other, from different accretion disk radii; i.e. dwarfs in CVs which undergo non-radial g- Kepler is probing dynamics within the accre- mode oscillations. She showed evidence that tion disk. an accretion event like an outburst or super- Classical Novae and the supersoft X-ray outburst can heat the white dwarf, moving it phase of their outbursts were covered exten- out of the instability strip but then as the white sively in both theory and observation via su- dwarf cools, it moves back into the strip. Her perb talks by Massimo Della Valle, Mariko observations make it possible to apply the tech- Kato, Margarita Hernanz, Nir Shaviv and niques of asteroseismology to the probe the in- Elena Mason.
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