The White Dwarf in SS Cygni: FUSE + HST Spectral Analysis Edward M. Sion, Patrick Godon, Janine Myszka Edward M. Sion, Patrick Godon, Janine Myszka Department of Astronomy and Astrophysics Villanova University

Outline of Talk 1.CV White Dwarf Overview 2. The White Dwarf in the SS Cygni 3. Comments on Accretion Heating of CV White Dwarfs 4.The Big Picture of CV White Dwarf Surface Temperatures 5. Conclusion

White Dwarfs undergoing extreme accretion Debris Disk accretion: ~ 10^8 g/s Cataclysmic Disk accretion: ~ 10^18 g/s CATACLYSMIC VARIABLE WHITE DWARFS: Extreme Accretion Laboratories

Figure 6 Sion, E. M. et al. (2008), ApJ, 681, 543

SS Aur Godon, P. et al. (2008), ApJ, in press

Brief Summary of CV WD Properties CV white dwarfs have temperatures 8000K < Teff < 70,000K CV white dwarfs have rotation velocities 100 < Vsini < 1200 km/s CV white dwarf metal abundances tend to be subsolar. Several have N/C ~ 5 to 10 suprasolar P, Al = 15,000K below gap, = 35,000K above gap

OPEN QUESTIONS Do CVs really evolve across the period gap? Do CV white dwarf masses increase, stay the same, or decrease with time? Are AM CVn helium transfer binaries Type Ia SN progenitors? What is the evolutionary status of the Nova-like Variables? Do we really understand accretion disk structure? Critical need for masses and parallaxes

Far Ultraviolet Spectra  IUE Archival  HST FOS, GHRS, STIS, COS  FUSE  EUVE

Synthetic Spectra  High Gravity LTE and NLTE Model Atmospheres (TLUSTY200, SYNSPEC98)  Optically Thick, Steady State, Accretion Disk Models (TLUSDISK200)  Accretion Belt Models  Accretion Rings  Accretion Curtain Models

Evolutionary Model Simulations of Accretion  1D Quasi-Static Evolutionary Code,  2D Hydrodynamic Code  OPAL Opacities  Time-variable accretion with compressional heating and boundary layer irradiation with stellar rotation (Sion, E.1995, ApJ,438,876)  Equations of State (ideal gas to relativistic degeneracy)

SS Cygni The Brightest Dwarf Nova One of the first CVs Shown to be a Binary White Dwarf + K4-5V Roche-lobe filling companion P_orb = days (6.6 hours) Distance = 166 pc +/- 12pc(Trig.parallax; Harrison et al.1999) (but see Schreiber&Lasota,2007,A\&A,473,897; Schreiber&Gaensicke,2002,A\&A,382,124) = 50 days = days = days

Best optically thick accretion disk model E(B-V) = 0.04

Best optically thick accretion disk model E(B-V) = 0.07

Best white dwarf photosphere model Teff = 47000K, Log g =8.3, Vsini =200 km/s

Best WD + Disk Combination Fit

-8.0 & 41 & --- & & 862 & --- & 100 & & 60 & --- & & 629 & --- & 100 & & 41 & --- & & 308 & --- & 100 & & 60 & --- & & 216 & --- & 100 & & 41 & --- & & 157 & --- & 100 & & 50 & --- & & 142 & --- & 100 & & 60 & --- & & 106 & --- & 100 & & 41 & --- & & 741 & --- & 100 & & 60 & --- & & 541 & --- & 100 & & 41 & --- & & 265 & --- & 100 & & 60 & --- & & 186 & --- & 100 & & --- & & & 138 & 100& --- & & --- & & & 167 & 100& --- & & --- & & & 139 & 100& --- & & --- & & & 164 & 100& --- & & 50 & & & 143 & 98& 2 & & 50 & 46,000 & & 173 & 88& 12 & & 50 & 55,000 & & 233 & 66& 34 & & 50 & 49,000 & & 149 & 98& 2 & & 50 & 55,000 & & 172 & 91& 9 & & 50 & 70,000 & & 222 & 72& 28 & 0.07 M i T_wd Chi^2 d %WD %disk E(B-V)

Cooling Curve WZ Sge

WD mass from Steeghs et al. 2007,ApJ, 667, 442

Temperatures from Long,K.et al. 2009, ApJ, 697, 1512

Summary 1.The white dwarf in SS Cygni dominates the quiescent FUV flux from the Lyman Limit to 2000A. 2. With M_wd = 0.81 Msun(Bitner, Robinson & Behr 2007, ApJ, 662, 564) the WD surface temperature is in the range of 46000K < T_eff < 55000K depending upon whether E(B-V) =0.04 or Compressional heating ALONE may not explain the cooling of the superoutburst accretion-heated white dwarfs in two of the best studied dwarf novae, WZ Sge and VW Hydri. 4.The very broad distribution of CV white dwarf temperatures, versus, above the period gap poses a severe challenge to our understanding of CV evolution.