35 Acta Polytechnica CTU Proceedings 2(1): 35–40, 2015 35 doi: 10.14311/APP.2015.02.0035 White Dwarfs in Cataclysmic Variables: An Update E. M. Sion1, P. Godon1 1Department of Astrophysics & Space Science, Villanova University, 800 Lancaster Ave., Villanova, PA 19085, USA Corresponding author: edward.sion@villanova.edu Abstract In this review, we summarize what is currently known about the surface temperatures of accreting white dwarfs in non- magnetic and magnetic cataclysmic variables (CVs) based upon synthetic spectral analyses of far ultraviolet data. We focus only on white dwarf surface temperatures, since in the area of chemical abundances, rotation rates, WD masses and accretion rates, relatively little has changed since our last review, pending the results of a large HST GO program involving 48 CVs of different CV types. The surface temperature of the white dwarf in SS Cygni is re-examined in the light of its revised distance. We also discuss new HST spectra of the recurrent nova T Pyxidis as it transitioned into quiescence following its April 2011 nova outburst. Keywords: cataclysmic variables - dwarf novae - intermediate polars - spectroscopy - UV - individuals: T Pyx, CC Syg. 1 Introductory Overview The white dwarfs in cataclysmic variables (hereafter CVWD) are the central engines of the observed out- bursts, either as potential wells for the release of gravi- tational energy during accretion (dwarf nova - DN), or as the sites of explosive thermonuclear runaway (TNR) shell burning (classical novae), steady shell burning (su- persoft X-ray binaries) or instantaneous collapse and total thermonuclear detonation if the WD reaches the Chandrasekhar limit (Type Ia supernova? SN Ia). Therefore, the accreting WDs serve as probes of ex- plosive evolution and accretion physics and diffusion, as they bear the thermal, chemical and rotational im- print of their long term accretion and thermonuclear history (Sion 1991 & 1995; Townsley & Bildsten 2003; Townsley & Gänsicke 2009). Deeper physical insights however require a larger number of chemical abundances, rotation rates, sur- face temperatures, mass accretion rates, and masses for each spectroscopic subclass of CVs. Only then can any definitive conclusions be drawn. In order to adequately sample the parameter space (Mwd, i, Ṁ, Teff , Porb) of the DNs, Nova-like variables, and magnetic CVs, a large GO program was approved in Cycle 20 (B.Gänsicke, Principal Investigator) to secure high quality COS spec- tra for CV classes underrepresented in the current over- all CV sample. The data is in hand and undergoing analysis at the time of this writing. Ultimately, we hope to obtain data for > 30 CVs per class. Therefore, in this review our focus is restricted to white dwarf sur- face temperatures, since in the area of chemical abun- dances, rotation rates, WD masses and accretion rates, relatively little has changed since our last review in the 2011 Palermo meeting proceedings. In section 2, our FUV analysis techniques are briefly summarized, sec- tion 3 we address how the revised (shorter) distance to SS Cygni affects the results of our analysis of the FUSE + HST STIS spectra of SS Cygni by Sion et al. (2010), in section 4 we tabulate and display the current distribution of CVWD surface temperatures versus or- bital period and in the final section, we include some remarks on new HST spectra of the recurrent nova T Pyxidis. 2 Synthetic Spectral Analysis of FUV Spectra of CVWDs We have modeled the FUV spectra of WDs in CVs dur- ing DN quiescence and Nova-like low states from IUE, FUSE and HST (FOS, GHRS, STIS, COS) archival data, and through our ongoing collaboration with past HST surveys led by B.Gänsicke, and P. Szkody. The IUE archival spectra and high quality FUSE and HST FUV data are fit with the latest versions of the TLUSTY/SYNSPEC model photosphere code and model accretion disk codes (Hubeny 1988; Hubeny & Lanz 1995). We are taking into account the BL explic- itly in our modeling of the FUV spectra of disk accret- ing systems, by replacing the very inner rings of the standard accretion disk model with high temperature rings in agreement with the temperature and density in the boundary layer. This improves the model fitting at the shorter wavelengths. As an example of our CVWD 35 http://dx.doi.org/10.14311/APP.2015.02.0035 E. M. Sion, P. Godon photosphere fitting, in Fig.1, we display a WD solar composition fit to the HST STIS spectrum of V442 Cen with E(B-V) = 0.10. The WD model has T=47,000K ±2000K and Log(g) = 8.3±0.2, Vsin(i) = 300km/s ±50 km/s (Sion et al. 2008). Figure 1: The best-fitting single temperature WD fit to the HST/STIS spectrum of V442 Cen. The model consists of a 47,000K WD with Log(g)=8.3 and scaled to a distance of 328pc. 3 The White Dwarf in SS Cygni:The VLBI and Corrected Hubble FGS Distance Schreiber & Lasota (2007), and references therein, pointed out that the previously published Hubble FGS parallax of SS Cygni, with a distance of 166 +/- 12 pc, posed serious problems for the disc instability model (DIM) of dwarf nova outbursts because it would fail to explain the absolute magnitude during outburst. With the new VLBI-derived distance (Miller-Jones et al. 2013) to SS Cygni (114 pc), and the corrected HST distance (Nelan & Bond 2013) of 120 pc instead of 166 pc, the concern for the validity of DIM is alleviated. An obvious question is: How does the new, shorter dis- tance affect the analysis of Sion et al. (2010) regarding their detection of the WD during quiescence and their derivation of the WD’s surface temperature from the modeling of FUSE + HST STIS spectra? To answer this question, we have carried out model fits using the new distance in the range of 114 pc to 120 pc. When we used single, steady state disk models, we obtained the same results as in Sion et al. (2010). The disk does not fit the flux in the shorter wavelengths un- less the accretion rate is very large, which is inconsistent with dwarf nova quiescence, and the model-derived dis- tance is far too large. When we combined a model disk with a model white dwarf photosphere (using the Bitner et al. WD mass as in Sion et al. (2010), the correct dis- tance to SS Cygni was obtained for a low accretion rate and a lower temperature white dwarf. However, the fits with the WD + disk are inferior to the fits where the WD dominates the FUV flux. In order to match the best fit solution to the FUSE + HST STIS wavelength range in Sion et al. (2010), the white dwarf must have Teff = 45000K - 50000K for a distance of 112 pc to 120 pc and a white dwarf mass Mwd = 0.95M�. This value of the WD mass is within the error range of the Bitner et al. mass. Our conclusion about the Teff of the WD in SS Cygni during quiescence is essentially unchanged from Sion et al. (2010). 4 The Current Distribution of CV White Dwarf Surface Temperatures Versus Orbital Period The effective temperatures Teff of the WDs (obtained during dwarf nova quiescence and nova-like low states when the WD is exposed), are the critical key to re- vealing the thermal response of the WD to mass accre- tion and the long term accretion rate < Ṁ > which, through compressional heating by the accreted mate- rial, is linked to the WD surface temperature (Sion 1995; Townsley & Bildsten 2003; Townsley & Gänsicke 2009). In Table 1, we have tabulated what we regard as the most reliably secured WD surface temperatures. The first column gives the name of the CV, the second col- umn the CV subtype, the third column, the orbital pe- riod in minutes, the fifth column the WD Teff and the sixth column, the reference for the temperature. These temperatures are derived from a variety FUV spectra and all are known with a precision of at least 3000K and in the majority of cases, better that +/- 2000K. With the paucity of FUV-derived temperatures per CV subclass, one should not ignore usable FUV data from IUE, FUSE and HUT because the ”quality” of the data is deemed inferior to HST STIS and COS. The current distribution of WD Teff against the or- bital period Porb is displayed in Fig.2. (see also Fig.4 in Townsley & Gänsicke 2009). Note the continued rel- atively sparse coverage in temperature of the WDs in CVs above the period gap (Porb > 3hr), compared with the coverage below the period gap. It is possible that the apparent trend toward higher temperatures with increasing Porb (i.e. higher long term average < Ṁ >) could be due to observational selection since the WD Teff ’s were derived primarily in the FUV where the Planckian peak occurs for hotter accreting WDs. For example, Copperwheat et al. (2010) derive a Teff = 10 − 15, 000K in the optical for the WD in the eclipsing DN IP Peg which has Porb = 3.8 h while the eclipsing system SDSS1006 with Porb = 4.46 h, has Teff = 16, 500 ± 2000K (Southworth et al. 2009) in the optical. Both of these objects should contain much hotter WDs (higher < Ṁ >) for their orbital periods. 36 White Dwarfs in Cataclysmic Variables: An Update Table 1: The temperature of CV white dwarfs SYSTEM CV TYPE Porb (h) Teff (K) REF SDSS1507 DN/SU 1.11 11000 Szkody et al. (2010a) GW Lib DN/WZ 1.280 14700 Szkody et al. (2002a) BW Scl DN? 1.304 14800 Gänsicke et al. (2005) LL And DN/WZ 1.321 14300 Howell et al. (2002) PQ And DN/SU 1.34 12000 Szkody et al. (2010a) EF Eri AM 1.350 9500 Szkody et al. (2010b) SDSS J1610-0102 DN? 1.34 14500 Szkody et al. (2007) V455 And DN/SU 1.35 10500 Szkody et al. (2013) HS2331+3905 DN 1.351 10500 Araujob-Betancor et al. (2005a) AL Com DN/WZ 1.361 16300 Szkody et al. (2003) WZ Sge DN/WZ 1.361 14900 Sion et al. (1995) SW UMa DN/SU 1.364 13900 Gänsicke et al. (2005) SDSS0919 DN 1.36 13500 Szkody (2014) SDSS1035 DN? 1.37 10500 Southworth et al. (2006); Littlefair et al. (2006b) HV Vir DN/WZ 1.370 13300 Szkody et al. (2002b) SDSS1339 DN 1.38 12500 Szkody et al. (2010a) SDSS2205 DN 1.38 15000 Szkody et al. (2010a) WX Cet DN/WZ 1.399 13500 Sion et al. (2003) T Leo DN/SU 1.41 16000 Hamilton & Sion (2004) EG Cnc DN/WZ 1.44 12300 Southworth et al. (2006) XZ Eri DN/SU 1.468 15000 Szkody et al. (2010a) SDSS1514 DN 1.48 10000 Szkody (2014) DP Leo AM 1.497 13500 Schwope et al. (2002) V347 Pav AM 1.501 11800 Araujob-Betancor et al. (2005b) BC UMa DN/SU 1.503 15200 Gänsicke et al. (2005) EK TrA DN/SU 1.509 18000 Godon et al. (2008) VY Aqr DN/WZ 1.514 14500 Sion et al. (2003) OY Car DN/SU 1.515 15000 Cheng et al. (2000) SDSS0131 DN/SU 1.63 14500 Szkody et al. (2010a) VV Pup AM 1.674 11900 Araujob-Betancor et al. (2005b) V834 Cen AM 1.692 14300 Araujob-Betancor et al. (2005b) HT Cas DN/SU 1.768 14000 Wood et al. (1992) VW Hyi DN/SU 1.783 22000 Godon et al. (2008) CU Vel DN/SU 1.88 18500 Gänsicke & Koester (1999) MR Ser AM 1.891 14200 Araujob-Betancor et al. (2005b) BL Hyi AM 1.894 13300 Araujob-Betancor et al. (2005b) ST LMi AM 1.898 10800 Araujob-Betancor et al. (2005b) AR UMa AM 1.93 20000 Schmidt et al. (2005) REJ1225 DN/SU 1.99 12000 Szkody et al. (2010a) EF Peg DN/WZ 2.01 16600 Howell et al. (2002) DV UMa DN/SU 2.138 20000 Feline et al. (2004) HU Aqr AM 2.084 14000 Gänsicke (1999) QS Tel AM 2.332 17500 Rosen et al. (2001) SDSS J1702+3229 DN/SU 2.402 17000 Littlefair et al. (2006a) TU Men DN 2.813 28000 Sion et al. (2008) AM Her AM 3.094 19800 Gänsicke et al. (1995) MV Lyr NL/VY 3.176 45000 Godon et al. (2012) DW UMa NL/VY 3.279 50000 Araujob-Betancor et al. (2003) TT Ari NL/VY 3.301 39000 Gänsicke et al. (1999) IP Peg DN 3.80 15000 Southworth et al. (2009) VY Scl NL 3.99 45000 Hamilton & Sion (2008) V1043 Cen AM 4.190 15000 Araujob-Betancor et al. (2005a) WW Cet DN 4.220 26000 Godon et al. (2008) UGem DN/UG 4.246 30000 Sion et al. (2001) SSAur DN/UG 4.391 34000 Godon et al. (2012) SDSS1006 DN 4.46 16500 Southworth et al. (2009) V895 Cen AM 4.765 14000 Araujob-Betancor et al. (2005b) RX And DN/ZC 5.037 34000 Sion et al. (2001) SS Cyg DN/UG 6.60 47000 Sion et al. (2010) VY Scl NL 3.99 45000 Hamilton & Sion (2008) EM Cyg DN/ZC 6.98 40000 Godon et al. (2012) TT Crt DN 7.30 29000 Sion et al. (2008) RU Peg DN 8.99 70000 Godon et al. (2012) V442 Cen DN 11.04 47000 Sion et al. (2008) 37 E. M. Sion, P. Godon Figure 2: Effective White Dwarf Temperature as a function of the orbital period. The references for the indivdual temperatures can be found in Sion et al. 2008, Townsley & Gänsicke 2009, Araujo-Betancor 2005a & b and references therein). The traditional magnetic braking above the period gap is shown between the parallel diagonal solid lines. On the right hand side are the time-averaged accretion rates correpsonding to the temperature scale on the left hand side of the diagram. Shown for comparison between the dotted lines is the long term evolutionary path of a 0.8 solar mass white dwarf (with an initial core temperature of 30 million degrees K) which has undergone 1000 nova outburst cycles accreting at the long term rate of 10−8M�/yr 5 The HST COS + STIS Spectra of the Recurrent Nova T Pyxidis:A Progress Report The interstellar reddening E(B-V) of the recurrent nova T Pyxidis is a critical parameter in the determination of the best-fitting model parameters. The UV spectra exhibit a minimum near 2175 A which is due to the interstellar extinction. Since the GALEX spectrum is the most reliable (i.e. highest S/N ratio) in that wavelength region, we used the GALEX spectrum to determine E(B-V). The value of E(B-V) for which the 2175 A feature disappears from the dered- dened spectrum, E(B-V)=0.35, is taken as the E(B- V) value towards T Pyx (see Fig.3). We use this value to deredden the IUE, GALEX and HST spectra, and we also consider the effects of different reddening values on our results. The HST STIS and COS spectra obtained in De- cember 2012 and July 2013 are identical. Thus, we co-added them to improve the signal-to-noise (S/N). We have found that the pre-outburst IUE and GALEX spectra together with the HST post-outburst spectrum. We note that the UV flux has remained constant not only before the outburst, but it has now come back pre- cisely to the same value. This is an indication that 1600 1800 2000 2200 2400 2600 2800 1e-14 1e-13 1e-12 E(B-V) - 0.00 - 0.10 - 0.20 - 0.25 - 0.30 - 0.35 - 0.40 - 0.45 - 0.50 A Log (Flux) er g/ s/ cm ^2 /Ao o Figure 3: The merged Galex-IUE spectrum of T Pyx has been dereddened for different values of E(B-V) as indicated on the right. The 2175Å feature associated with the reddening is clearly seen in absoprtion for low balues of E(B-V), and it appears as extra flux for large values of E(B-V). We deduce that the reddening to- wards T Pyx must be E(B-V)=0.35, the value for which the 2175Å feature vanishes. 38 White Dwarfs in Cataclysmic Variables: An Update the mass accretion rate remained constant before and after the outburst (see Godon et al. 2014). 0.4 0.6 0.8 1 1.2 1.4 1.6 1e-06 1e-05 0.0001 0.001 Mwd/Msun Envelope Mass (Msun) E(B-V)=0.45 Luv E(B-V)=0.35 Luv E(B-V)=0.25 Luv E(B-V)=0.50 Disk E(B-V)=0.35 Disk [1] E(B-V)=0.50 E(B-V)=0.25 Disk [2] Accreted Accreted Accreted Accreted Accreted Accreted Ejected Ejected extrapolation Figure 4: The mass of the accreted envelope and ejected envelope are shown as a function of the WD mass for different values of the reddening for T Pyx. Our disk model results are drawn with a solid line (“Disk”). The lower limit of the accreted envelope in- ferred from the UV flux is shown (dotted line, “Luv”). In comparison we show the ejected envelope [1] as esti- mated by Nelson et al. (2012) (square symbol), as well as [2] computed in Patterson et al. (2013) (dashed line). The accreted envelope is larger than the ejected enve- lope, except for a value of E(B-V)=0.25 combined with a WD mass ∼ 0.9M� and larger. However, the best re- sults were obtained for a larger value of the reddening E(B − V ) > 0.30. Using the value of the reddening we derived, to- gether with the new distance estimate of 4.8 kpc (Sokoloski et al. 2013), we fit the observed (back-to- quiescence) HST spectra with disk models for different WD masses. We then computed the accreted mass over a period of 44yrs, which we then compared to the es- timates of the ejected envelope mass (during the 2011 outburst). We recapitulate our results in Fig.4, where we also consider different reddening values for the sake of completness. 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H., Horne, K., & Vennes, S.: 1992, ApJ, 385, 294 40 http://dx.doi.org/10.1046/j.1365-8711.2001.04141.x http://dx.doi.org/10.1086/431969 http://dx.doi.org/10.1086/586699 http://dx.doi.org/10.1088/2041-8205/716/2/L157 http://dx.doi.org/10.1088/2041-8205/770/2/L33 http://dx.doi.org/10.1111/j.1365-2966.2006.11042.x http://dx.doi.org/10.1086/511854 http://dx.doi.org/10.1088/0004-637X/716/2/1531 http://dx.doi.org/10.1088/0004-637X/775/1/66 http://dx.doi.org/10.1088/0004-637X/693/1/1007 Introductory Overview Synthetic Spectral Analysis of FUV Spectra of CVWDs The White Dwarf in SS Cygni:The VLBI and Corrected Hubble FGS Distance The Current Distribution of CV White Dwarf Surface Temperatures Versus Orbital Period The HST COS + STIS Spectra of the Recurrent Nova T Pyxidis:A Progress Report