The Astrophysical Journal, 582:352–357, 2003 January 1 # 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A.
COMPOSITE ACCRETION DISK AND WHITE DWARF MODEL ANALYSES OF THE QUIESCENCE OF DWARF NOVAE: EM CYGNI, CZ ORIONIS, AND WW CETI Lisa Winter and Edward M. Sion Department of Astronomy and Astrophysics, Villanova University, Villanova, PA 19085;
[email protected],
[email protected] Received 2002 March 2; accepted 2002 August 29
ABSTRACT We explore the origin of the far-UV (FUV) spectra of three dwarf novae using combined high-gravity photosphere and accretion disk models. We have carried out an IUE archival comparative study of the three U Gem–type dwarf novae EM Cyg, CZ Ori, and WW Cet. For EM Cyg, the FUV spectrum during quiescence _ ¼ 5 1011 M yr1 contributing 92% of the FUV light and a is dominated by an accretion disk with M white dwarf with upper limit Teff < 24; 000 K contributing d8% of the light. For CZ Ori, the accretion disk _ ¼ 3:5 1010 M yr1 contributes 99% of the FUV light while the white dwarf has an upper limit with M Teff < 21; 000 K and contributes d1% of the light. For WW Cet, we find that best-fitting disk models and disk plus white dwarf models yield a distance that appears far too large. A single-temperature white dwarf fit with Teff ¼ 22; 000 K implies a distance of 150 pc. CZ Ori and EM Cyg are dominated by the accretion disk during quiescence but with accretion rates that differ by over a factor of 10. In the case of EM Cyg, which has a higher inclination, it is possible that the nearly edge-on aspect of the disk may hide a much hotter white dwarf. There are now 17 analyzed dwarf nova systems with Porb < 120 mintues and 11 systems with Porb > 180 m. The average Teff below the lower boundary of the period gap is Teff ¼ 15; 547 K, while the average Teff above the upper boundary of the period gap is Teff ¼ 31; 182 K. The Teff of the white dwarfs strengthens the overall conclusion that the white dwarfs in cataclysmic variables above the period gap appear to be a factor of 2 times hotter than the accreting white dwarfs in dwarf novae below the period gap. Subject headings: accretion, accretion disks — binaries: close — novae, cataclysmic variables — stars: individual (WW Ceti, EM Cygni, CZ Orionis) — white dwarfs
LaDous used the FUV spectra of observed single white dwarfs as ‘‘ templates ’’ to assess the contribution of the underlying, accreting white dwarf to the spectra of dwarf novae during quiescence. Since the FUV spectral region is where both the accretion disk and white dwarf should have their peak energy output, model analyses of this region should reveal their properties. In response to the relative absence of detailed synthetic spectral analyses of dwarf novae in the FUV, we have begun a series of studies utilizing newly available models of both solar composition photospheres and accretion disks. These initial studies (Nadalin & Sion 2001; Henry & Sion 2001; Stump & Sion 2001; Lake & Sion 2001; Urban et al. 2000) applied grids of model disks and model white dwarf atmospheres separately in attempts to explain the FUV spectral energy distribution and absorption lines. In this paper, our approach is to utilize grids of composite models (accretion disk plus white dwarf) for the first time on the IUE spectra of dwarf novae in quiescence. This approach is most useful for dwarf novae in which we cannot easily identify the dominant light source during quiescence in the FUV. Of course, this is not the case for dwarf novae in which the white dwarf photosphere is clearly exposed spectroscopically and the quiescent accretion rate is known to be extremely low (e.g., U Gem above the period gap or VW Hyi, WZ Sge, and the SU UMa systems near the period minimum). However, for the vast majority of dwarf novae, we do not know the accretion rate during quiescence, nor is the spectrum of the underlying white dwarf easily identifiable. Among the key questions we seek to answer are: What is the accretion rate during quiescence? Is this rate of accretion consistent with the disk instability model (Shafter,
1. INTRODUCTION
Dwarf novae are a subset of cataclysmic variables (CVs) consisting of a white dwarf accreting matter from a Roche lobe–filling, low-mass secondary star. They undergo outbursts (brightness increases of 3–7 mag) lasting for days to weeks, separated by intervals of quiescence lasting weeks to months. The outbursts are thought to be caused by the release of gravitational potential energy when matter in the disk periodically accretes onto the white dwarf. The trigger for the accretion episode is widely held to be the disk instability mechanism (e.g., Cannizzo 1998 and references therein). These systems were observed extensively with IUE in both outburst and quiescence until its shutdown in 1994. The long service of the IUE left a rich archive of spectroscopic observations of dwarf novae in both brightness stages. During outburst, these systems are dominated by a luminous accretion disk, a high rate of accretion, and wind outflow. The spectra during quiescence are more difficult to interpret and generally exhibit a mix of absorption and emission features with a continuum energy distribution that may or may not be consistent with an accretion disk. Analyses of this body of FUV data during dwarf nova quiescence have largely been phenomenological without the application of realistic model disk and model atmosphere synthetic spectra. For example, the seminal global studies of dwarf novae during quiescence are largely contained in the archival studies of Verbunt (1987), Szkody et al. (1991), and LaDous (1991). In the former two studies, flux ratios were used to characterize the evolution of the spectra throughout the outburst-quiescence cycle, while in the latter study, 352
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a K5 V yielded a distance of 173 pc. The most reliable method, which agrees with Bailey’s computed distance, was Warner’s Mv versus Porb relation, which yields a distance of 411 pc. In the present study, we adopt 350 pc as the distance to EM Cyg. For CZ Ori, the distance using the above three methods yielded an average distance of 220 pc. Spogli & Claudi (1994) classified the secondary as spectral class M2:5 1:0, which yields a distance to the system of 260 110 pc. In the present study, we adopt 260 pc as the distance to CZ Ori. For WW Cet, a distance range of 90–300 pc was derived by Young & Schneider (1981) on the basis of their near-IR CCD spectra of the cool companion (see also Sproats, Howell, & Mason 1996). The distance using Warner’s MvðmaxÞ versus Porb relation is 186 pc. Therefore, since this is in the midrange of previous estimates, we adopt 186 pc as the distance to WW Cet.
Wheeler, & Cannizzo 1986)? How much flux does the white dwarf contribute to the FUV? How much flux does the accretion disk contribute to the FUV? How hot is the white dwarf? For this pilot study, we have selected three systems above the period gap, EM Cyg, CZ Ori, and WW Cet. EM Cyg is classified as a Z Cam subtype, while CZ Ori and WW Cet are classified as U Gem–type dwarf novae. The U Gem–type and Z Cam–type systems lie above the period gap, which is a range of orbital period between 2 and 3 hr, where virtually no cataclysmic variables are found (Warner 1995 and references therein). Systems above the period gap should tend to have higher mass-transfer rates than those below the gap. The U Gem–type systems exhibit normal dwarf nova outbursts but no superoutbursts, while the Z Cam systems have both high, low, and intermediate brightness states, the latter known as standstills. Generally, these longer period systems above the gap tend to have somewhat earlier spectral type secondaries, accretion rates that tend to be higher even during quiescence, and accretion disks that tend to be larger. Evidence supporting a higher degree of accretion heating above the period gap is presented in x 4. The observed characteristics of all three systems are given in Table 1 where, by column, we list (1) the system name, (2) CV subtype, (3) orbital period in days, (4) recurrence time between outbursts in days, (5) orbital inclination, (6) spectral type of the secondary, (7) mass of the primary in solar masses, (8) mass of the secondary in solar masses, (9) apparent magnitude in outburst, and (10) apparent magnitude in quiescence. The references to the entries are listed below the table.
2. OBSERVATIONS
The placement of the archival spectra of EM Cyg, CZ Ori, and WW Cet was determined on the basis of the observed flux levels and IUE fine error sensor visual magnitudes of the systems. The system behavior and brightness states of EM Cyg, CZ Ori, and WW Cet at the time of the IUE observations can be further assessed by comparison with the AAVSO light-curve data (visual magnitude vs. Julian Date). All three systems were observed with the IUE telescope and short-wavelength prime camera (SWP) through the ˚. large aperture at low dispersion with a resolution of 5 A The SWP spectra covered the wavelength range 1170–2000 ˚ . The observing log is presented in Table 2 below. A These archival IUE NEWSIPS spectra were flux calibration–corrected using the algorithm of Massa & Fitzpatrick (2000). Massa & Fitzpatrick (2000) have shown that the absolute flux calibration of the NEWSIPS low-dispersion data was inconsistent with its reference model and subject to time-dependent systematic effects, which, together, amount to as much as 10%–15%. Therefore, in order to correct the data and optimize the signal-to-noise ratio, we used the IDL programs that apply Massa-Fitzpatrick corrections to the low-dispersion IUE data.
1.1. System Distances Our model analyses hinge critically upon a knowledge of the system distance, since this provides a comparison with the distance implied by the scale factor of a given fit and yields the stellar radius for a given distance. For each system in Table 1, in the absence of an accurate parallax (e.g., Hubble fine guidance sensor, Hipparcos), we calculated the system distance using three methods: (1) the recurrence time versus orbital period relation in Warner (1995), (2) the absolute magnitude at outburst versus orbital period relation of Warner (1995), and (3) the spectral type of the secondary star. For EM Cyg, the distance, as obtained by determination of the surface brightness, radius, and effective temperature of the secondary star, was found to be 350 pc (Bailey 1981). Using the recurrence time, an unreliable distance of 59 pc (far from the accepted value of 350 pc) was obtained, and using the absolute magnitude of the secondary to be that of
3. SYNTHETIC SPECTRAL FITTING
Model spectra with solar abundances were created for high-gravity stellar atmospheres using TLUSTY (Hubeny 1988) and SYNSPEC (Hubeny & Lanz 1995). We adopted model accretion disks from the optically thick disk model
TABLE 1 Dwarf Nova System Parameters
System (1)
Subtype (2)
Porb (3)
trec (4)
i (5)
Spectral Type (M2 ) (6)
M1 (7)
M2 (8)
Vmax (9)
Vmin (10)
EM Cyg ...... CZ Ori ........ WW Cet ......
Z Cam U Gem U Gem
0.290909 0.214667 0.1758
22 26 11-43
75 23 54
K5 V M2.5 V M5?
1.12 0.6 0.85
0.99 0.5 0.41
12.0 12.1 9.3
14.2 16 13.9
References.—EM Cyg: Stover, Robinson, & Nather 1981; Shafter 1983; Beuermann & Pakull 1984; North et al. 2000. CZ Ori: Ringwald, Thorstensen, & Hamwey 1994; Spogli & Claudi 1994. WW Cet: Hawkins, Smith, & Jones 1990; Ringwald et al. 1996; Tappert et al. 1997; Ritter & Kolb 1998.
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TABLE 2 Observing Log
System
SWP Camera
Aperture
Dispersion
Date
Time of Observation
texp (s)
Count Rate
Background
EM Cyg ...... CZ Ori ........ WW Cet ......
08088 16042 24866
Large Large Large
Low Low Low
1980 Feb 29 1982 Jan 14 1985 Jan 8
15:32:22 17:22:22 16:29:20
4500 5400 9899
54 81 76
25 24 34
Note.—SWP = Short-wavelength primary.
grid of Wade & Hubeny (1998). Using IUEFIT, a 2 minimization routine, both 2 values and a scale factor were computed for each model. The scale factor, normalized to a kiloparesec, can be related to the white dwarf radius through FðobsÞ ¼ 4ðR2 =d 2 ÞHðmodelÞ , where d is the distance to the source. For each system, the best-fitting accretion disk model was combined with the best-fitting photosphere model to determine the contribution of both the accretion disk and the white dwarf. While the IUE spectrum of EM Cyg is quite noisy, this system is one of the best-observed dwarf novae, and its system parameters, derived largely from optical studies, are not substantially less accurate than other noneclipsing dwarf novae. Since we know of no noneclipsing dwarf nova for which the inclination (i), the white dwarf mass, and the distance are known with a high degree of confidence, we use EM Cyg to demonstrate our composite fitting methods and the associated formal errors. The only exception to this latter statement is the dwarf nova U Gem, which now is known to have Mwd ¼ 1:12 M (Sion et al. 1998; Long & Gilliland 1998) and i ¼ 78 . Our method of composite disk plus white dwarf fitting of U Gem confirms the white dwarf to be the overwhelmingly dominant source of the FUV and yields a very low accretion rate for U Gem in quiescence. Our method is most useful for systems in which the dominant FUV light source is unknown and the disk is a large contributor in quiescence. While there are a large number of free parameters, the best we can do at present is try to reduce the number by using the most reliable published values in the refereed literature. First, we masked all of the emission-line regions in the spectrum of EM Cyg. This reduced the number of data points to 443. Second, we carried out white dwarf–only fits to the masked spectrum by fixing log g ¼ 8:5 (in effect, adopting a mass close to the published value for the white dwarf primary) and assuming solar abundances for the white dwarf atmosphere. Third, we carried out accretion disk–only fits to the masked spectrum by fixing Mwd ¼ 1 M and i ¼ 75 (in effect, adopting the published values given in x 1). To give the reader a better sense of how the separate accretion disk model and separate white dwarf model compare with the actual observed spectrum, we heavily smoothed the IUE spectrum and superposed the best-fitting disk–only model and the best-fitting white dwarf–only model. The result is displayed in Figure 1. We fitted disk models with four values of the accretion rate, 109, 109.5, _ from 1010, and 1010:5 M yr1. We proceeded to vary M 10.5 9 1 to 10 M yr in steps of 0.05 between the four 10 models by interpolating fluxes. Fourth, we carried out combined white dwarf plus accretion disk fits, varying the white dwarf Teff from 9000 to 27,000 K in steps of 1000 K. For each of the 19 values of Teff , we varied the accretion rate from 1010.5 to 109 M yr1 in steps of 0.05. The fitting
results for the masked spectrum of EM Cyg are listed in Table 3, where, in the first column, we list the fitting method, the white dwarf model (T, log g) in the second column, the disk model accretion rate in the third column, the 2 value in the fourth column, the scale factor in the fifth column, and the distance corresponding to the scale factor of the best fit in the sixth column. By reducing the number of free parameters down to two with 443 data points, the least 2 value is 2.5504. Therefore, the 3 error bar corresponding to 2 ¼ 2:5504 11:8 ¼ _ , we have obtained 30. Hence, by fixing the Teff and the M _ ¼ 10:3þ0:35 M yr1 and an upper an accretion rate log M 0:2 limit to the white dwarf surface temperature of Teff < 24; 000 K (see Table 4). In Figure 2, we present a _ plane, 1 , 2 , contour diagram displaying, in the Teff -M and 3 confidence contours. In a similar fashion, for CZ Ori, we masked the emissionline regions, which resulted in 433 data points and constructed photosphere models. The parameters varied in our fitting were as follows: TðwdÞ =1000 ¼ 10; 10:5; . . . 28:5; 29:0 K, and we adopted log g ¼ 8:0. We took V sin i ¼ 100 km s1. The best-fitting model yielded TðwdÞ ¼ 21; 000 K with a reduced 2 ¼ 5:85030 and scale factor S ¼ 1:3784 102 . Next, we tried accretion disk–only fits to CZ Ori. We adopted i ¼ 18 , which is consistent with the published estimates of i. The white dwarf mass for the accretion disk models was 0:55 M . We fitted disk models with four values of the accretion rate, 108.5, 109.0, 109.5, and 1010.0 M _ from 1010.0 to 108:5 M yr1. We proceeded to vary M yr1 in steps of 0.05 between the four models. We found that
Fig. 1.—Best-fitting accretion disk model and best-fitting white dwarf model, each shown separately, in comparison with a heavily smoothed (five-point smoothed) IUE (SWP08088) FUV spectrum (F vs. ) of the dwarf nova EM Cyg during quiescence. See the text for details.
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TABLE 3 EM Cyg Fitting Parameters
Fitting Method (1)
White Dwarf Model (Ieff ; log g) (2)
_ M (3)
2 (4)
Scale Factor (5)
Distance (pc) (6)
WD only ................ Disk only ............... WD + Disk............
18,000, 8.5 ... ...
... 10.30 10.30
3.6874 2.5693 2.5504
1.3615 102 22.9037 22.6622
8570 209 210
_ ¼ 9:45, the best-fitting accretion disk model had log M 2 ¼ 2:4670, and S ¼ 8:0939, which yielded a distance of 351 pc. This value is within the uncertainty range of the distance to CZ Ori given in x 1.1. When we combined the above best-fitting photosphere with the best-fitting accretion disk, we found that the accretion disk accounts for 99% of the FUV light. In other words, CZ Ori is overwhelmingly diskdominated during its quiescence, with the white dwarf being an insignificant contributor. Our composite disk plus white dwarf fit had 2 ¼ 2:3934 and S ¼ 8:0333 with a distance of 353 pc and is displayed in Figure 3. Our best-fitting accre-
TABLE 4 Temperatures of White Dwarfs in Dwarf Novae
Object
DN Subtype
Porb (minutes)
Teff (K)
Reference
BW Scl .......... LL And ......... HV Vir .......... WX Cet......... EG Cnc ......... VY Aqr ......... BC UMa ....... EK TrA......... AL Com........ WZ Sge ......... SW UMa....... OY Car ......... HT Cas ......... VW Hyi......... Z Cha............ CU Vel.......... EF Peg .......... UU Aql......... CM Del......... CN Ori.......... X Leo............ VW Vul......... UZ Ser .......... U Gem .......... SS Aur .......... RX And ........ CZ Ori .......... AH Her......... EM Cyg ........ RU Peg .........
WZ WZ WZ WZ SU WZ WZ SU WZ WZ SU SU SU SU SU SU SU UG UG UG ZC ZC ZC UG UG ZC UG ZC ZC UG
78 79.8 83.4 84 86.4 91.2 91.2 91.8 81.6 81.6 81.6 81.8 106.1 107.0 107.3 113.0 123 202 233.3 234.7 237 243 249 254.8 263 302.2 315 371.8 419 539
14,800 14,300 13,300 13,000 12,300 13,500 15,200 18,800 20,000 15,000 14,000 16,000 15,500 22,000 15,000 15,000 16,600 27,000 24,000 30,000 33,000 27,000 27,000 30,000 30,000 35,000
