FUSE Observations of the Dwarf Novae UU Aql, BV Cen, and CH
UMa in Quiescence1
Edward M. Sion, Patrick Godon2, Fuhua Cheng
Astronomy and Astrophysics, Villanova University,
800 Lancaster Avenue, Villanova, PA 19085, USA
edward.sion@villanova.edu, patrick.godon@villanova.edu, fcheng@ast.vill.edu
Paula Szkody
Department of Astronomy, University of Washington, Seattle, WA 98195, USA
szkody@astro.washington.edu
ABSTRACT
We report on FUSE spectra of three U Gem-type, long period, dwarf novae,
UU Aql, BV Cen and CH UMa taken during their quiescence intervals. We
discuss the line identifications in their spectra and attempt to characterize the
source(s) of their FUV flux distribution. Archival IUE spectrum of CH UMa and
BV Cen in quiescence were identified as having a matching flux level with the
FUSE spectra and these were combined with each FUSE spectrum to broaden the
wavelength coverage and further constrain model fits. Multi-component synthetic
spectral fits from our model grids, consisting of single temperature white dwarfs,
two-temperature white dwarfs, accretion disks and white dwarfs plus accretion
disks, were applied to the FUSE spectra alone and to the combined FUSE + IUE
spectra. We present the results of our model analyses and their implications.
Subject headings: accretion, accretion disks - novae, cataclysmic variables - stars:
dwarf novae - stars: individual (UU Aql, BV Cen, CH UMa) - white dwarfs
1Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE
is operated for NASA by the Johns Hopkins University under NASA contract NAS5-32985
2Visiting at the Space Telescope Science Institute, Baltimore, MD 21218, USA; godon@stsci.edu
http://arxiv.org/abs/0704.1133v1
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1. Introduction
Dwarf novae (DNe) are close interacting binaries in which a Roche-lobe filling main
sequence-like dwarf transfers matter with angular momentum through a disk onto a white
dwarf (WD). The rapid disk accretion during outburst, due to a thermal instability that
causes cyclic changes of the accretion rate, releases gravitational potential energy identified
as the DN outburst. The high accretion rate (∼ 10−8 to 10−9 M⊙/yr) outburst phase (which
lasts a few days to weeks) is preceded and followed by a low accretion rate (∼ 10−11M⊙/yr)
quiescence stage. This DN behavior is believed to be punctuated every few thousand years
or more by episodes of catastrophically unstable thermonuclear burning, the classical nova
explosion. Perhaps the least understood topic in CV/DN research (along with what drives the
wind outflow in outburst) is the state and structure of the boundary layer and accretion disk
during quiescence and the physics of how long term accretion of mass, angular momentum
and energy affects the WD. Our studies with archival IUE, and HST/STIS have found that
∼50% of the DNe in quiescence are dominated (i.e., > 60% of UV flux) by a component
of FUV flux other than the WD called the ”accretion disk”; ∼25% are dominated by the
WD and ∼25% have nearly equal contribution of WD and accretion disk (40-60% each)
(Urban & Sion 2006).
A number of studies (Araujo-Betancor et al. 2003; Sion 1991, 1999; Urban & Sion
2006) have shown that CV WDs above the gap are typically on-average 10,000K hot-
ter than CV WDs below the period gap (almost certainly a consequence of higher time-
averaged accretion rates of systems above the gap but possibly with system total age also
being a factor). Since the white dwarf surface temperature is crucial for understanding CV
evolution and whether CVs evolve across the period gap, the use of cooling ages and long
term evolutionary model sequences with accretion (including the effects of nova explosions,
Townsley & Bildsten (2003)) must rely on the empirical WD temperature of the photo-
sphere in equilibrium with long term compressional heating from accretion. The work of
Townsley & Bildsten (2002, 2003) allows measured Teff ’s of CV WDs to be converted to
the accretion rate per unit WD surface area averaged over the thermal time of the WD
envelope.
Unfortunately, there are far fewer systems with reliably known WD properties above
the period gap compared with below the gap, thus impeding detailed comparisons between
the two groups. For example, among CVs below the gap, there are now roughly 20 systems
with reliable WD temperatures but only 5 systems above the gap with reliable WD temper-
atures. The primary reason for this disparity is that in long period CVs with higher mass
transfer rates, the disks may remain optically thick even during quiescence, making the disk
contribution to the total flux typically larger in systems above the gap. Hence, it is more
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difficult to disentangle the white dwarf flux contribution from that of the accretion disk.
As part of our effort to increase the sample of CV degenerates with known properties
above the gap, we have used FUSE and IUE archival spectra to analyze three long period
dwarf novae, UU Aql, BV Cen, and CH UMa. For UU Aql, system properties were adopted
from Ritter & Kolb (2003) and from Szkody (1987). For BV Cen parameters were adopted
from Ritter & Kolb (2003). For CH UMa, we adopted values from Friend et al. (1990).
For all three systems, the distances were the same as those in Urban & Sion (2006) where
the Warner (1995) and Harrison et al. (2004) Mv(max) versus Porb relations, calibrated with
trigonometric parallaxes, were used. The reddening values were the same as those quoted in
Urban & Sion (2006) which were from Verbunt (1987), laDous (1991) and Bruch & Engel
(1994).
The dwarf nova systems analyzed in this work are UU Aql, BV Cen, and CH UMa. In
Table 1, the observed properties of these dwarf novae are summarized by column as follows:
(1) system name; (2) dwarf nova subclass with UG denoting a U Gem-type system; (3)
orbital period in days; (4) the recurrence time of dwarf nova outbursts in days; (6) the
apparent magnitude at minimum (quiescence); (7) the apparent magnitude in outburst; (8)
secondary spectral type; (9) orbital inclination in degrees; (10) white dwarf mass in solar
masses; (11) secondary star mass in solar masses; (12) adopted reddening value and ; (13)
distance in parsecs.
2. Observations and Data Reduction
The instrumental setup and exposure details of the FUSE spectra of BV Cen, CH UMa
and UU Aql in quiescence are provided in Table 2. The LWRS was used in all cases since
it is least prone to slit losses due to the misalignment of the four FUSE telescopes. All
the spectra were obtained in time tag (TTAG) mode, and each one of them consists of 7
individual exposures (corresponding to 7 FUSE orbits). It is clear that the relatively poor
FUSE spectral quality of the spectra speaks to the requirement for more observing time.
Nevertheless, we deemed that there was sufficient S/N to warrant a first attempt multi-
component FUV analysis of each system
All the data were reduced using CalFUSE version 3.0.7. In this version of CalFUSE
the data are maintained as a photon list: the intermediate data file - IDF. Bad photons are
flagged but not discarded, so the user can examine and combined data without re-running
CalFUSE. For each target, we combined the individual exposures (using the IDF files) and
channels to create a time-averaged spectrum weighted in the flux in each output datum by
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the exposure time and sensitivity of the input exposure and channel of origin. The details are
given here. The spectral regions covered by the spectral channels overlap, and these overlap
regions are then used to renormalize the spectra in the SiC1, LiF2, and SiC2 channels to the
flux in the LiF1 channel. We then produce a final spectrum that covers almost the full FUSE
wavelength range 905 − 1182Å. The low sensitivity portions of each channel are discarded.
In most channels there exists a narrow dark stripe of decreased flux in the spectra running
in the dispersion direction. This stripe has been affectionately known as the ”worm” and it
can attenuate as much as 50% of the incident light in the affected portions of the spectrum.
The worm has been observed to move as much as 2000 pixels during a single orbit in which
the target was stationary. The ”worm” appears to be present in every exposure and, at this
time, there is no explanation for it. Because of the temporal changes in the strength and
position of the worm, CalFUSE cannot correct target fluxes for its presence. Here we take
particular care to discard the portion of the spectrum where the so-called worm ’crawls’,
which deteriorates LiF1 longward of 1125Å . Because of this the 1182−1187Å region is lost.
We then rescale and combine the spectra. When we combine, we weight according to the
area and exposure time for that channel and then rebin onto a common wavelength scale
with a 0.1Å, 0.2Å, and 0.5Å resolution.
In the observing log given in Table 2, the entries are by column: (1) gives the target,
(2) FUSE spectral data ID, (3) the aperture used, (4) the date and time of observation, (5)
the (good) exposure time in seconds, (6) central wavelength, and (7) S/N.
The FUSE spectra for the three systems, UU Aql, BV Cen and CH Uma are displayed in
Figures 1, 2, and 3 respectively. A quantitative sense of the relative data quality is provided
by the signal to noise for the three FUSE spectra. We binned the data by 0.1Å for which
the S/N of UU Aql, BV Cen, and CH UMa is 5.15, 5.9, and 3.6, respectively.
For UU Aql, Fig. 1 reveals a rich line spectrum with numerous lines of molecular
hydrogen, interstellar species and possible accretion disk or photospheric absorption features.
The spectrum reveals a downturn in the continuum shortward of 1000Å. The spectrum does
not exhibit any evidence of emission lines from the source, the only emission lines are from
air glow and heliocoronal (e.g. sharp emissions lines from C iii (around 977Å) and the Ovi
doublet).
In figure 2, the FUSE spectrum of BV Cen has a variety of interstellar and stellar features
but has a continuum shape distinctly different from UU Aql. The broad C iii absorption
feature around 1175Å is definitely from the source, and the C iii (around 977Å) and the
Ovi doublet broad emission features could also possibly be associated with one of the FUV
components in BV Cen. All the other sharp emission features are either heliocoronal or
geocoronal in origin.
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In figure 3, CH UMa reveals numerous absorption lines due to highly ionized and singly
ionized metals. It has a continuum energy distribution similar to UU Aql. There are def-
initely some broad emission lines from the source itself. The most prominent one is the
Ovi doublet (the right component being strongly attenuated by molecular hydrogen sharp
absorption lines), C iii (both around 977Å and 1175Å) which also seems to be in emission
and a tentative identification of N iv emission in the short wavelengths. The source is also
contaminated with sharp emission lines due to air glow. Here too the N i & N ii are geo-
coronal in origin, and the sharp peaks on top of the broad C iii (977Å) and Ovi emissions
are heliocoronal in origin. We note that ISM molecular hydrogen absorption is affecting the
continuum.
Since the wavelength range covered by FUSE overlaps with HST/STIS or IUE in the
region of C iii between 1170Å and 1180Å, a much broader FUV wavelength coverage is
afforded by combining the spectra when the flux levels of the two spectra in the wavelength
overlap region match closely enough. We found archival IUE spectra matching the FUSE
spectra of two of the three systems (CH UMa and BV Cen) but unfortunately no HST
spectra exist for the three systems.
The FUSE + IUE combination of spectra rests on the assumptions that (1) differences
between the two spectra in orbital phase and (2) in time after the last outburst can be
ignored. Given the long exposure times of the FUSE and IUE spectra, the questionable
reliability of the orbital ephemerides, including UU Aql’s and (3) the limited S/N of the
quiescent spectra, the influence of phase-dependent variations is not considered. However,
the time since the last outburst as well as the brightness state of the system at the times of
the FUSE and IUE observations for the three systems is considered in detail using AAVSO
archival light curve data. For UU Aql, the FUSE spectrum was obtained approximately
50 days after its last major dwarf nova outburst, however, the IUE spectrum was obtained
during the transition to a brightening that appeared not to be a major outburst. Therefore,
we have excluded the IUE spectrum from use in combination with the FUSE data for UU
Aql. For BV Cen both the FUSE and IUE spectra were obtained during quiescence but
the FUSE spectrum was acquired approximately 159 days after the last outburst while the
IUE spectrum was taken roughly 50 days after the last outburst. Thus, the FUSE spectrum
probably recorded a greater degree of white dwarf cooling than the IUE spectrum obtained
closer to the last outburst. Likewise for CH UMa, both the FUSE and IUE spectra were
obtained during quiescence but the FUSE spectrum appears to have been obtained roughly
125 days after the last outburst while the IUE spectra were acquired about 83 days after
the last outburst. Since the e-folding times for white dwarf cooling in both of these systems
following the outburst heating episode is typically shorter than the above two post-outburst
intervals, it is probably acceptable to combine the FUSE and IUE spectra for BV Cen and
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CH UMa.
In Table 3 we present the observing log of IUE observations which matched the FUSE
flux level in the wavelength overlap region of these two systems. The entries by column are
(1) the target name, (2) the observation ID, (3) aperture, (4) dispersion mode, (5) date of
the observation, (6) time of mid-exposure, and (6) the exposure time in seconds.
Thus, our analysis was carried out first for the FUSE spectra of the three systems and
then separately for the combined FUSE plus IUE data of BV Cen and CH UMa.
3. Multi-Component Synthetic Spectral Fitting
Our data analysis and modeling involves the full suite of multi-component (accretion
disk, white dwarf photosphere, accretion belt) synthetic spectral codes, which we have uti-
lized in our spectral fitting of FUSE and IUE data. Based upon our expectation that the
accreting white dwarf is an important source of FUV flux in these systems during quies-
cence, we carried out a high gravity photosphere synthetic spectral analysis first. The model
atmosphere (TLUSTY200; Hubeny (1988)), and spectrum synthesis codes (SYNSPEC48
and ROTIN4 Hubeny & Lanz (1995)) and details of our χ2ν (χ
2 per degree of freedom)
minimization fitting procedures are discussed in detail in Sion et al. (1995) and will not be
repeated here. To estimate physical parameters, we generally took the white dwarf photo-
spheric temperature Teff , log g, and rotational velocity vrot and chemical abundances as free
parameters.
We normalize our fits to 1 solar radius and 1 kiloparsec such that the distance of a
source is computed from d = 1000(pc) ∗ (Rwd/R⊙)/
S, or equivalently the scale factor
, is the factor by which the theoretical flux (integrated over the entire
wavelength range) has to be multiplied to equal the observed (integrated) flux.
The grid of WD models extended over the following range of parameters: log g =
7.0, 7.5, 8.0, 8.5, 9.0; Teff/1000 (K) = 22, 23, ..., 75; Si = 0.1, 0.2, 0.5, 1.0, 2.0, 5.0; C = 0.1,
0.2, 0.5, 1.0, 2.0, 5.0; and vrot sin i (km s
−1) = 100, 200, 400, 600, 800.
For the synthetic accretion disk models, we used the latest accretion disk models from
the optically thick disk model grid of Wade & Hubeny (1998). The range of disk model
parameters varies as follows: WD mass (in solar units) value of 0.35, 0.55, 0.80, 1.03, and
1.21; orbital inclination (in degrees) of 18, 41, 60, 75 and 81. The accretion rate ranges from
10−10.5M⊙yr
−1 to 10−8.0M⊙yr
−1 by increments of 0.5 in logṀ .
For each dwarf nova, we adopted the following procedure. First, we masked out all of
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the obvious emission features and artifacts in both the FUSE and IUE spectra of each object.
Second, we carried out synthetic spectral fits using and/or combining model components in
this order: a white dwarf model alone, accretion disk model alone, combination white dwarf
plus accretion disk model, and two-temperature white dwarf model (the latter to simulate a
hotter equatorial region as well as a cooler photosphere at higher latitudes). For accretion
disk fits, we ”fine-tuned” the derived accretion rate of the best-fitting disk model by changing
the accretion rate in increments of 0.1 over the range 0.1 to 10, on the assumption that the
disk fluxes scale linearly over that range.
In Table 4, we indicate where we masked any strong emission features, artifacts, or
negative fluxes in the FUSE and IUE spectra of each object.
4. Synthetic Spectral Fitting Results
The noise level of the FUSE spectra precludes the opportunity to extract reliable pa-
rameters for the accreting white dwarfs in these three systems. This is especially true for
deriving rotational velocities which rely on well-resolved, strong absorption lines arising in
the photosphere. The rotational velocity is also affected by underlying emission filling of
absorption features and by the chemical abundances one uses. With these caveats in mind,
we proceeded to apply our grid of WD photosphere models (keeping the chemical abundance
fixed at solar) and accretion disk synthetic spectra.
For any dwarf nova in quiescence, a single temperature white dwarf model should be
a reasonable first approximation as the source of the FUV flux. For UU Aql, we adopted
two possible distances, 150 pc and 350 pc and carried out detailed fits for both values. For
a distance of 350 pc, the best-fit WD model to the FUSE spectrum gave Teff = 27, 000K,
Rwd/R⊙ = 1.13× 10−2 and a χ2 = 0.963. This best-fitting model is shown in figure 4. The
continuum of the model gives a fair representation of the observed continuum down to about
1060Å but there is a shortfall of model flux relative to the data at wavelengths shorter than
1030Å. For the same distance, an accretion disk alone yielded a best fit with χ2 = 1.14, an
accretion rate of 5 × 10−11M⊙/yr and inclination i = 41 degrees, Mwd = 0.8M⊙. This disk
fit is shown in figure 5. The model disk continuum, unlike the WD, fails to match the flux
level of the data between about 1090Å and 1180Å and the solar abundance accretion disk
model fails to provide a sharp absorption features.
A combination white dwarf plus accretion disk yielded a modest improvement with
χ2 = 0.71, Ṁ = 1.6× 10−11M⊙/yr, i = 41 degrees and Mwd = 0.8M⊙. Finally we tried two
temperature (WD + belt) fits. The best fit two temperature WD yields χ2 = 0.74 with the
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cooler white dwarf portion ( Teff = 24, 000K) giving 57% of the flux and the hotter belt
(Tbelt = 33, 000K) providing 43% (figure 6).
In general, for a distance d = 150 pc, the model fits to UU Aql’s FUSE spectrum are
worse than for 350 pc. Applying single temperature white dwarf fits, we obtained Teff =
17, 000K, Rwd/R⊙ = 1.06 × 10−2 and a χ2 = 1.13. An accretion disk alone yielded a
best fit with χ2 = 1.23, an accretion rate of 1.3 × 10−11M⊙/yr and inclination i = 41
degrees and Mwd = 0.8M⊙. A combination white dwarf plus accretion disk yielded a modest
improvement with χ2 = 1.13, Ṁ = 1.6× 10−11M⊙/yr, i = 41 degrees and Mwd = 0.8M⊙. A
two temperature white dwarf (WD + belt) gives a best-fit with a χ2 = 1.02, with a 17,000K
white dwarf providing 77% of the FUV flux and the belt giving 23% of the flux. In view of
the much better agreement of the models with the observations of UU Aql for our assumed
distance of 350 pc than for a distance of 150 pc, the closer distance can be ruled out.
For CH UMa, the FUSE spectrum is very noisy and underexposed. We took into account
our adopted distance of 300 pc in the model fitting, we fixed the WD mass at Mwd = 1.2M⊙
(Log g = 9.0, with a radius of ≈ 4, 000km), and fixed the disk inclination at the published
value of 18 degrees. A single temperature white dwarf fit to CH UMa had χ2 = 0.227,
and yielded a best fit Teff = 29, 000K, and a distance of 310pc. This model however did
not fit very well in the shorter wavelengths (<1020Å). A lowest χ2 fit for this same WD
mass was obtained for T=40,000K, with χ2 = 0.208, but it yielded a distance of 600pc,
twice the adopted estimate of the distance. Since the Ritter catalog give a mass of 1.95M⊙
(well above the Chandraskhar mass limit for a WD), we decided to try a larger mass with a
correspondingly smaller radius which lead to a smaller emitting surface area and therefore
a shorter distance. We assumed M = 1.38M⊙ (logg = 9.5, with a radius of ≈ 2, 000km)
and found that the best fit was for T = 40, 000K. This model yielded a distance of 307pc
with χ2 = 0.199. Since the absorption features around 1120Å - 1150Å are not pronounced
in the observed spectrum, we decided to increase the rotational velocity to improve the fit
of this model. However, a better result was obtained by simply reducing the abundances
of Si and C to 0.01 their solar value. This low Si and C model had a χ2 = 0.184 and a
distance of 314pc. This model is shown in Figure 7. Because of the low S/N of the spectrum
the assessment of the error on the temperature estimate is of the order of 5,000K, namely
Twd = 40, 000± 5, 000K.
Though the WD fit yielded the lowest value of χ2, we tried accretion disk fits alone to
CH UMa’s FUSE data. The best fit gave a χ2 = 0.213, an accretion rate of 5× 10−12M⊙/yr
for Mwd = 1.2M⊙ and i = 18 degrees.
Various attempts to fit the combined FUSE + IUE spectra of CH UMa met with limited
success as summarized in Table 5. The best combination fit was for an accretion disk plus
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WD with the disk contributing 79% of the FUV flux. However, these fits were less satisfactory
than the fits to the FUSE spectrum alone.
Because the FUSE spectrum of CH UMa is very noisy, and because of the broad emission
lines and the absence of strong absorption lines, it is difficult to assess which model (disk
or WD) is the best solution. However, the absence of strong absorption lines would favor
the accretion disk model because of Keplerian broadening. Also because of the poor quality
of the spectrum, composite models (WD+accretion disk, two-temperature WD) did not
improve the fit. In view of all of the above, we cannot be confident that we have determined
the temperature of the white dwarf in CH UMa.
Next we analyzed the combined FUSE + IUE spectra of CH UMa. The best white
dwarf-only fit yielded Teff = 31, 000K, log g = 9 a χ
2 = 3.02 and Rwd/R⊙ = 6.02 × 10−3.
For accretion disk models only, the best fit occurred with a χ2 = 2.02, corresponding to
Ṁ = 3× 10−9M⊙/yr. This disk fit is a modest improvement of the fit to CH UMa since the
χ2 value was lowered to 2.02. The best-fitting white dwarf + accretion disk model resulted
in only a modest, statistically insignificant improvement. The χ2 value was lowered to 1.87,
the accretion rate was 6.4× 10−11M⊙/yr with the WD Teff = 22, 000K and the scale factor
yielding a white dwarf radius of 5.84 × 10−3R⊙. In this composite disk plus white dwarf
fit, the WD contributes 21% of the FUV flux while the accretion disk contributes 79% of
the FUV flux. We also tried a two-temperature WD solutions with the best-fitting model
consisting of a 26,000K WD providing 77% of the flux and a hot accretion belt/ring with
Tbelt = 50, 000K giving 23% of the UV flux. However, this two-temperature fit was no better
than the WD + accretion disk fit.
For BV Cen we dereddened the spectrum assuming E(B-V)=0.10, and first tried single
temperature white dwarf fits with a white dwarf mass = 0.83M⊙, and used solar chemical
abundances. We found the best-fitting white dwarf model to have Teff = 40, 000± 1000K,
log g = 8.3, V sin i = 500 km/s ±100 km/s. This fit yielded χ2 = 0.2701 and a distance
d=435pc (see figure 8). We note here that the WD solution fits the following absorption
features quite accurately: C ii (1066Å), S iv (1073Å), N ii (1084Å), S iiv (1122.5Å, 1128.3Å),
and S iiii (1140Å- 1146Å).
For an optically thick, steady state accretion disk alone, we chose Mwd = 0.80M⊙,
i = 60 degrees. The best fit we obtained has a mass accretion rate of 10−8.5M⊙yr
−1, too
large for dwarf nova quiescence. Moreover, the longer wavelength part of the spectrum is
rather flat and unable to fit the absorption features around 1120Å and 1130Å. This fit, with
i=60deg, leads to a distance of 1255pc. In order to fit these absorption features, one needs
to assume an inclination of 18 degrees, inconsistent with the known inclination of the system
(62 degrees). Such a model leads to a distance of more than 2000pc, again inconsistent
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with all estimates of the system distance (500pc). Therefore, based on the parameters of
the system, the disk solution is completely inconsistent. We tried composite model fits, but
they also led to very poor results. We also combined BV Cen’s FUSE + IUE spectra but
they led to very poor fits, much worse than the fits to the FUSE spectrum alone. Since
the WD solution is consistent with the parameters of the system and fits the absorption
features of the spectra, it is clear that the favored solution of the FUSE spectrum of BV Cen
is Twd = 40, 000± 2, 000K.
5. Discussion
Our principal objective of determining the surface temperatures of the white dwarfs
in these three dwarf novae during quiescence has met with mixed results. From synthetic
spectral fits to the FUSE spectra of the three long period dwarf novae, UU Aql, CH UMa
and BV Cen, and the FUSE + IUE archival SWP spectra of CH UMa we have presented
preliminary evidence that during quiescence, their accreting white dwarfs all have surface
temperatures hotter than 20,000K. Unfortunately, all three temperatures have considerable
uncertainty due to the low S/N of the FUSE spectra and IUE spectra as well as the difficulty
of disentangling the flux contribution of the second component of FUV flux or ”accretion
disk” during quiescence. Of the three systems in this study, we regard our estimate of the
Teff of the WD in UU Aql to be the most reliable since that system appears to be dominated
in the FUV by the white dwarf flux.
For UU Aql, we used both assumed distances, 150 pc and 350 pc, and tried the composite
fits (WD + disk) to UU Aql’s FUSE spectrum. However, the quality of the fits for WD-only,
disk-only, WD + disk, and WD + ”belt” were all roughly comparable and thus it is difficult
to distinguish the best-fit case. For both distances, statistically insignificant improvements
in the fits result when a white dwarf and accretion disk are combined or a two-temperature
WD (WD + belt) is applied to the FUSE data. The results in Table 5 illustrate the difficulty.
Qualitatively, the composite fits involving combinations of white dwarf plus accretion disk
or accretion belt models look marginally more reasonable than the fits that involve single
component (WD or Disk). It appears that for both distances, the white dwarf component is
the dominant source of FUV flux and that the Teff of the WD is probably between 17,000K
and 27,000K (say 22,000±5,000K).
For BV Cen, the WD model fit to the FUSE spectrum gave the best result, both in
fitting actual features of the observed spectrum and in leading to consistent values of the
system parameters. This was not the case for the disk model and the composite models.
From the best WD model fit we obtained that the WD of BV Cen must have a temperature
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of about 40,000K.
For CH UMa, the broad and dominant emission lines together with the poor S/N (and
possible detector noise at very short wavelengths) of the FUSE and IUE spectra precluded
the opportunity to obtain conclusive results. However, the results for the FUSE spectrum
were of a much higher quality than for the FUSE + IUE combined spectrum. Therefore we
adopt the FUSE results for CH UMa with the possibility that the WD temperature could
be as high as 40,000K.
In Table 6, we list the dwarf novae above the period gap whose white dwarfs have surface
temperature determinations. In Column (1) we give the system name, column (2) the orbital
period; column (3) the surface temperature and column (4) the temperature reference. As
seen in Table 6, there is now a sample of eight long period dwarf novae of which roughly
seven have relatively secure white dwarf temperatures obtained during quiescence. In the
case of BV Cen, the inclination is expected to be high. Thus, it is plausible to expect that
the quiescent accretion disk may be blocking the direct radiation from the accreting white
dwarf.
The relatively poor FUSE spectral quality of CH UMa underscores the need for re-
observation with longer exposure times. However, analysis will remain hindered until more
reliable information on white dwarf masses and distances becomes available. Until then, the
conclusions in this work must be regarded as preliminary.
6. Acknowledgements
We thank an anonymous referee for helpful comments and corrections. PG wishes to
thank the Space Telescope Science Institute for its kind hospitality. This work was supported
by NSF grant AST05-07514 and NASA grants NAG5-12067 and NNG04GE78 to Villanova
University. This research was based on observations made with the NASA-CNES-CSA Far
Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins
University under NASA contract NAS5-32985.
– 12 –
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– 14 –
Table 1: Dwarf Nova Parameters
System Porb trec Vq Vo Sec. i Mwd EB−V Distance
Name (days) (days) (deg) (M⊙) (pc)
BV Cen 0.610108 150 12.6 10.5 G5-8V 62± 5 0.83± 0.1 0.10 500
CH UMa 0.343 204 15.3 10.7 K4-M0V 21± 4 300
UU Aql 0.14049 71 16.0 11.0 M2-4V 150-350
Table 2. FUSE Observations
Target Data ID Aperture Time of Observation texp λcentered S/N(0.1Å)
UU Aql C1100301000 LWRS 30x30 2004-05-16 13h:48m:00s 16,121s 1035Å 5.15
BV Cen D1450301000 LWRS 30x30 2003-04-13 20h:26m:00s 26,545s 1035Å 5.9
CH UMa D1450201000 LWRS 30x30 2003-04-02 22h:00m:00s 17,311s 1035Å 3.6
Table 3. IUE Observations
Target SWP No. Aperture Dispersion Date Time 0f Mid-Exposure texp(s)
BV Cen 26623 Lg Low 08/16/85 17:17:00 14,400
CH UMa 56270 Lg Low 12/06/95 12:15:49 13,800
– 15 –
Table 4. Masked Spectral Regions
Target Emission Features
UU Aql for λ < 1050Å if Fλ > 6× 10−15ergs s−1cm−2Å−1
for 1050 Å < λ < 1185Å if Fλ > 1.8× 10−14ergs s−1cm−2Å−1
975-985Å
CH UMa for λ < 957Å if Fλ > 1× 10−14ergs s−1cm−2Å−1
970-980, 987-994, 1023-1042, 1071-1090, 1107-1136, 1167-1180, 1200-1260,
1285-1315, 1380-420, 1532-1560, 1630-1650, 1846-1940Å
BV Cen for λ < 1185Å if Fλ > 2.5× 10−14ergs s−1cm−2Å−1
1190-1220, 1320-1340, 1375-1415, 1530-1570, 1625-1650, 1835-1880Å
Table 5. UU Aql, BV Cen, and CH UMa Fitting Results
System Spectrum d χ2 Twd Tbelt Ṁ % Flux Fig
(pc) (103K) (103K) (M⊙/yr) (Contribution)
UU Aql FUSE 350 0.96 27 - - 100(WD) 4
FUSE 150 1.13 17 - - 100(WD) -
FUSE 350 1.13 - - 5× 10−11 100(Disk) 5
FUSE 150 1.24 - - 1× 10−11 100(Disk) -
FUSE 350 0.714 24 - 2× 10−11 73(WD)/27(Disk) -
FUSE 150 1.13 17 - 3× 10−11 60(WD)/40(Disk) -
FUSE 350 0.738 24 33 - 57(WD)/43(Belt) 6
FUSE 150 1.021 16 29 - 77(WD)/23(Belt) -
CH UMa FUSE 314 0.184 40 - - 100(WD) 7
FUSE 313 0.213 - - 5× 10−12 100(Disk) -
FUSE+IUE 300 3.02 31 - - 100(WD) -
FUSE+IUE 300 2.02 - - 3× 10−9 100(Disk) -
FUSE+IUE 300 1.87 22 - 6.4× 10−11 21(WD)/79(Disk) -
FUSE+IUE 300 2.00 26 50 - 77(WD)/23(Belt) -
BV Cen FUSE 435 0.27 40 - - 100(WD) 8
FUSE 1255 0.27 - - 3× 10−9 100(Disk) -
– 16 –
Table 6: Surface Temperatures of White Dwarfs in Dwarf Novae above the Period Gap
System Name Period Teff References
(min) (Kelvin)
BV Cen 878.6 40,000: This paper
RU Peg 539.4 49,000 Sion et al (2004)
Z Cam 417.4 57,000 Hartley et al. (2005)
RX And 302.2 34,000 Sion et al. (2001)
SS Aur 263.2 31,000 Sion et al (2004)
U Gem 254.7 31,000 Sion et al. (1998)
WW Ceti 253.1 26,000 Godon et al. (2006)
UU Aql 202.3 27,000: This paper
– 17 –
Figure Captions
Fig. 1.— The flux Fλ (ergs/cm
2/s/Å) versus wavelength (Å) FUSE spectrum of the U Gem-
type dwarf nova UU Aql during quiescence. The identifications of the strongest neutral and
ionized line features as well as rotational-vibrational transitions of molecular hydrogen are
marked with vertical tick marks. The molecular hydrogen absorption are modestly affecting
the continuum, slicing it at almost regular wavelength intervals. We identify the most
prominent molecular hydrogen absorption lines by their band (Werner or Lyman), upper
vibrational level (1-16), and rotational transition (R,P, or Q) with lower rotational state
(J=1,2,3). Air glow line features are indicated with the earth symbol. The C iii (977Å) and
Ovi doublet sharp emissions are due to heliocoronal emission often present in some of the
FUSE channels when the target (i.e. UU Aql) is faint.
Fig. 2.— The flux Fλ (ergs/cm
2/s/Å) versus wavelength (Å) FUSE spectrum of the U Gem-
type dwarf nova BV Cen during quiescence. The identifications of the strongest neutral and
ionized line features as well as rotational-vibrational transitions of molecular hydrogen are
marked with vertical tick marks. Air glow line features are indicated with the earth symbol.
The O i emissions are also due to air glow. The broad C iii (977Å) and Ovi (doublet)
features might be real broad emissions from the source, as these correspond to the left wing
of the Lyman γ & β (respectively) and not much flux is expected there. The N i and N ii
sharp emission features are geocoronal.
Fig. 3.— The flux Fλ (ergs/cm2/s/Å) versus wavelength (Å) FUSE spectrum of the U Gem-
type dwarf nova CH UMa during quiescence. The identifications of the strongest neutral and
ionized line features as well as rotational-vibrational transitions of molecular hydrogen are
marked with vertical tick marks. Several terrestrial line features are indicated with the earth
symbol. There are some broad emission features associated with the source: we identify here
the Ovi doublet, C iii (at 977Å and 1175Å). There could be some N iv emission in the short
wavelengths (between 920Å and 925Å), but the increase of flux in the very short wavelengths
suggests that most the flux at λ < 930Å could be higher orders of the hydrogen Lyman series
associated with air glow contamination and possibly some detector noise.
Fig. 4.— A single temperature WD fit to the FUSE spectrum of UU Aql for a distance of 350
pc. The best-fit yielded Teff = 27, 000K ±3000K, log g = 9. The white dwarf contributes
100% of the FUV flux.
Fig. 5.— An accretion disk-only fit to the FUSE spectrum of UU Aql for a distance of 350
pc. The best-fit corresponded to Mwd = 0.8M⊙, i = 41 degrees, Ṁ = 5× 10−11M⊙/yr. The
accretion disk contributes 100 % of the FUV flux.
– 18 –
Fig. 6.— A WD + belt fit to the FUSE spectrum of UU Aql for a distance of 350 pc. The
dotted line is the white dwarf flux component, the dashed line is the belt flux component
and the solid line is the combined two-temperature fit. This best fit two temperature WD
yields χ2 = 0.74 with the cooler white dwarf portion ( Teff = 24, 000K) giving 57% of the
flux and the hotter belt (Tbelt = 33, 000K) providing 43%.
Fig. 7.— A single temperature WD fit to the FUSE spectrum of CH UMa. The best-
fit yielded Teff = 40, 000 ± 1, 000K, log g = 9.5 and a projected rotational velocity of
Vrot sin i = 200km/s. The portions of the spectrum that have been masked are shown in
blue and include emission lines (either intrinsic to the source or due to air glow) and ISM
molecular hydrogen absorption lines (which have been matched using a simple ISM model
to identify the exact location of the ISM lines). Therefore the fit is between the model and
the red portions of the observed FUSE spectrum. The absence of strong absorption features
in the lower panel were modeled assuming low Si and C abundances and led to a lower χ2
than the same model with solar C and Si abundances but with a high projected rotational
velocity (≈ 1, 000km/s).
Fig. 8.— A single temperature WD fit to the FUSE spectrum of BV Cen. The best-fit
yielded Teff = 40, 000K±1000K, a projected rotational velocity of 500km/s, and d=435pc,
assuming M = 0.83M⊙.
the FUSE plus IUE spectrum of UU
– 19 –
– 20 –
– 21 –
– 22 –
– 23 –
– 24 –
– 25 –
– 26 –
	Introduction
	Observations and Data Reduction
	Multi-Component Synthetic Spectral Fitting
	Synthetic Spectral Fitting Results
	Discussion
	Acknowledgements

  We report on FUSE spectra of three U Gem-type, long period, dwarf novae, UU
Aql, BV Cen and CH UMa taken during their quiescence intervals. We discuss the
line identifications in their spectra and attempt to characterize the source(s)
of their FUV flux distribution. Archival IUE spectrum of CH UMa and BV Cen in
quiescence were identified as having a matching flux level with the FUSE
spectra and these were combined with each FUSE spectrum to broaden the
wavelength coverage and further constrain model fits. Multi-component synthetic
spectral fits from our model grids, consisting of single temperature white
dwarfs, two-temperature white dwarfs, accretion disks and white dwarfs plus
accretion disks, were applied to the FUSE spectra alone and to the combined
FUSE + IUE spectra. We present the results of our model analyses and their
implications.

Introduction
Dwarf novae (DNe) are close interacting binaries in which a Roche-lobe filling main
sequence-like dwarf transfers matter with angular momentum through a disk onto a white
dwarf (WD). The rapid disk accretion during outburst, due to a thermal instability that
causes cyclic changes of the accretion rate, releases gravitational potential energy identified
as the DN outburst. The high accretion rate (∼ 10−8 to 10−9 M⊙/yr) outburst phase (which
lasts a few days to weeks) is preceded and followed by a low accretion rate (∼ 10−11M⊙/yr)
quiescence stage. This DN behavior is believed to be punctuated every few thousand years
or more by episodes of catastrophically unstable thermonuclear burning, the classical nova
explosion. Perhaps the least understood topic in CV/DN research (along with what drives the
wind outflow in outburst) is the state and structure of the boundary layer and accretion disk
during quiescence and the physics of how long term accretion of mass, angular momentum
and energy affects the WD. Our studies with archival IUE, and HST/STIS have found that
∼50% of the DNe in quiescence are dominated (i.e., > 60% of UV flux) by a component
of FUV flux other than the WD called the ”accretion disk”; ∼25% are dominated by the
WD and ∼25% have nearly equal contribution of WD and accretion disk (40-60% each)
(Urban & Sion 2006).
A number of studies (Araujo-Betancor et al. 2003; Sion 1991, 1999; Urban & Sion
2006) have shown that CV WDs above the gap are typically on-average 10,000K hot-
ter than CV WDs below the period gap (almost certainly a consequence of higher time-
averaged accretion rates of systems above the gap but possibly with system total age also
being a factor). Since the white dwarf surface temperature is crucial for understanding CV
evolution and whether CVs evolve across the period gap, the use of cooling ages and long
term evolutionary model sequences with accretion (including the effects of nova explosions,
Townsley & Bildsten (2003)) must rely on the empirical WD temperature of the photo-
sphere in equilibrium with long term compressional heating from accretion. The work of
Townsley & Bildsten (2002, 2003) allows measured Teff ’s of CV WDs to be converted to
the accretion rate per unit WD surface area averaged over the thermal time of the WD
envelope.
Unfortunately, there are far fewer systems with reliably known WD properties above
the period gap compared with below the gap, thus impeding detailed comparisons between
the two groups. For example, among CVs below the gap, there are now roughly 20 systems
with reliable WD temperatures but only 5 systems above the gap with reliable WD temper-
atures. The primary reason for this disparity is that in long period CVs with higher mass
transfer rates, the disks may remain optically thick even during quiescence, making the disk
contribution to the total flux typically larger in systems above the gap. Hence, it is more
– 3 –
difficult to disentangle the white dwarf flux contribution from that of the accretion disk.
As part of our effort to increase the sample of CV degenerates with known properties
above the gap, we have used FUSE and IUE archival spectra to analyze three long period
dwarf novae, UU Aql, BV Cen, and CH UMa. For UU Aql, system properties were adopted
from Ritter & Kolb (2003) and from Szkody (1987). For BV Cen parameters were adopted
from Ritter & Kolb (2003). For CH UMa, we adopted values from Friend et al. (1990).
For all three systems, the distances were the same as those in Urban & Sion (2006) where
the Warner (1995) and Harrison et al. (2004) Mv(max) versus Porb relations, calibrated with
trigonometric parallaxes, were used. The reddening values were the same as those quoted in
Urban & Sion (2006) which were from Verbunt (1987), laDous (1991) and Bruch & Engel
(1994).
The dwarf nova systems analyzed in this work are UU Aql, BV Cen, and CH UMa. In
Table 1, the observed properties of these dwarf novae are summarized by column as follows:
(1) system name; (2) dwarf nova subclass with UG denoting a U Gem-type system; (3)
orbital period in days; (4) the recurrence time of dwarf nova outbursts in days; (6) the
apparent magnitude at minimum (quiescence); (7) the apparent magnitude in outburst; (8)
secondary spectral type; (9) orbital inclination in degrees; (10) white dwarf mass in solar
masses; (11) secondary star mass in solar masses; (12) adopted reddening value and ; (13)
distance in parsecs.
2. Observations and Data Reduction
The instrumental setup and exposure details of the FUSE spectra of BV Cen, CH UMa
and UU Aql in quiescence are provided in Table 2. The LWRS was used in all cases since
it is least prone to slit losses due to the misalignment of the four FUSE telescopes. All
the spectra were obtained in time tag (TTAG) mode, and each one of them consists of 7
individual exposures (corresponding to 7 FUSE orbits). It is clear that the relatively poor
FUSE spectral quality of the spectra speaks to the requirement for more observing time.
Nevertheless, we deemed that there was sufficient S/N to warrant a first attempt multi-
component FUV analysis of each system
All the data were reduced using CalFUSE version 3.0.7. In this version of CalFUSE
the data are maintained as a photon list: the intermediate data file - IDF. Bad photons are
flagged but not discarded, so the user can examine and combined data without re-running
CalFUSE. For each target, we combined the individual exposures (using the IDF files) and
channels to create a time-averaged spectrum weighted in the flux in each output datum by
– 4 –
the exposure time and sensitivity of the input exposure and channel of origin. The details are
given here. The spectral regions covered by the spectral channels overlap, and these overlap
regions are then used to renormalize the spectra in the SiC1, LiF2, and SiC2 channels to the
flux in the LiF1 channel. We then produce a final spectrum that covers almost the full FUSE
wavelength range 905 − 1182Å. The low sensitivity portions of each channel are discarded.
In most channels there exists a narrow dark stripe of decreased flux in the spectra running
in the dispersion direction. This stripe has been affectionately known as the ”worm” and it
can attenuate as much as 50% of the incident light in the affected portions of the spectrum.
The worm has been observed to move as much as 2000 pixels during a single orbit in which
the target was stationary. The ”worm” appears to be present in every exposure and, at this
time, there is no explanation for it. Because of the temporal changes in the strength and
position of the worm, CalFUSE cannot correct target fluxes for its presence. Here we take
particular care to discard the portion of the spectrum where the so-called worm ’crawls’,
which deteriorates LiF1 longward of 1125Å . Because of this the 1182−1187Å region is lost.
We then rescale and combine the spectra. When we combine, we weight according to the
area and exposure time for that channel and then rebin onto a common wavelength scale
with a 0.1Å, 0.2Å, and 0.5Å resolution.
In the observing log given in Table 2, the entries are by column: (1) gives the target,
(2) FUSE spectral data ID, (3) the aperture used, (4) the date and time of observation, (5)
the (good) exposure time in seconds, (6) central wavelength, and (7) S/N.
The FUSE spectra for the three systems, UU Aql, BV Cen and CH Uma are displayed in
Figures 1, 2, and 3 respectively. A quantitative sense of the relative data quality is provided
by the signal to noise for the three FUSE spectra. We binned the data by 0.1Å for which
the S/N of UU Aql, BV Cen, and CH UMa is 5.15, 5.9, and 3.6, respectively.
For UU Aql, Fig. 1 reveals a rich line spectrum with numerous lines of molecular
hydrogen, interstellar species and possible accretion disk or photospheric absorption features.
The spectrum reveals a downturn in the continuum shortward of 1000Å. The spectrum does
not exhibit any evidence of emission lines from the source, the only emission lines are from
air glow and heliocoronal (e.g. sharp emissions lines from C iii (around 977Å) and the Ovi
doublet).
In figure 2, the FUSE spectrum of BV Cen has a variety of interstellar and stellar features
but has a continuum shape distinctly different from UU Aql. The broad C iii absorption
feature around 1175Å is definitely from the source, and the C iii (around 977Å) and the
Ovi doublet broad emission features could also possibly be associated with one of the FUV
components in BV Cen. All the other sharp emission features are either heliocoronal or
geocoronal in origin.
– 5 –
In figure 3, CH UMa reveals numerous absorption lines due to highly ionized and singly
ionized metals. It has a continuum energy distribution similar to UU Aql. There are def-
initely some broad emission lines from the source itself. The most prominent one is the
Ovi doublet (the right component being strongly attenuated by molecular hydrogen sharp
absorption lines), C iii (both around 977Å and 1175Å) which also seems to be in emission
and a tentative identification of N iv emission in the short wavelengths. The source is also
contaminated with sharp emission lines due to air glow. Here too the N i & N ii are geo-
coronal in origin, and the sharp peaks on top of the broad C iii (977Å) and Ovi emissions
are heliocoronal in origin. We note that ISM molecular hydrogen absorption is affecting the
continuum.
Since the wavelength range covered by FUSE overlaps with HST/STIS or IUE in the
region of C iii between 1170Å and 1180Å, a much broader FUV wavelength coverage is
afforded by combining the spectra when the flux levels of the two spectra in the wavelength
overlap region match closely enough. We found archival IUE spectra matching the FUSE
spectra of two of the three systems (CH UMa and BV Cen) but unfortunately no HST
spectra exist for the three systems.
The FUSE + IUE combination of spectra rests on the assumptions that (1) differences
between the two spectra in orbital phase and (2) in time after the last outburst can be
ignored. Given the long exposure times of the FUSE and IUE spectra, the questionable
reliability of the orbital ephemerides, including UU Aql’s and (3) the limited S/N of the
quiescent spectra, the influence of phase-dependent variations is not considered. However,
the time since the last outburst as well as the brightness state of the system at the times of
the FUSE and IUE observations for the three systems is considered in detail using AAVSO
archival light curve data. For UU Aql, the FUSE spectrum was obtained approximately
50 days after its last major dwarf nova outburst, however, the IUE spectrum was obtained
during the transition to a brightening that appeared not to be a major outburst. Therefore,
we have excluded the IUE spectrum from use in combination with the FUSE data for UU
Aql. For BV Cen both the FUSE and IUE spectra were obtained during quiescence but
the FUSE spectrum was acquired approximately 159 days after the last outburst while the
IUE spectrum was taken roughly 50 days after the last outburst. Thus, the FUSE spectrum
probably recorded a greater degree of white dwarf cooling than the IUE spectrum obtained
closer to the last outburst. Likewise for CH UMa, both the FUSE and IUE spectra were
obtained during quiescence but the FUSE spectrum appears to have been obtained roughly
125 days after the last outburst while the IUE spectra were acquired about 83 days after
the last outburst. Since the e-folding times for white dwarf cooling in both of these systems
following the outburst heating episode is typically shorter than the above two post-outburst
intervals, it is probably acceptable to combine the FUSE and IUE spectra for BV Cen and
– 6 –
CH UMa.
In Table 3 we present the observing log of IUE observations which matched the FUSE
flux level in the wavelength overlap region of these two systems. The entries by column are
(1) the target name, (2) the observation ID, (3) aperture, (4) dispersion mode, (5) date of
the observation, (6) time of mid-exposure, and (6) the exposure time in seconds.
Thus, our analysis was carried out first for the FUSE spectra of the three systems and
then separately for the combined FUSE plus IUE data of BV Cen and CH UMa.
3. Multi-Component Synthetic Spectral Fitting
Our data analysis and modeling involves the full suite of multi-component (accretion
disk, white dwarf photosphere, accretion belt) synthetic spectral codes, which we have uti-
lized in our spectral fitting of FUSE and IUE data. Based upon our expectation that the
accreting white dwarf is an important source of FUV flux in these systems during quies-
cence, we carried out a high gravity photosphere synthetic spectral analysis first. The model
atmosphere (TLUSTY200; Hubeny (1988)), and spectrum synthesis codes (SYNSPEC48
and ROTIN4 Hubeny & Lanz (1995)) and details of our χ2ν (χ
2 per degree of freedom)
minimization fitting procedures are discussed in detail in Sion et al. (1995) and will not be
repeated here. To estimate physical parameters, we generally took the white dwarf photo-
spheric temperature Teff , log g, and rotational velocity vrot and chemical abundances as free
parameters.
We normalize our fits to 1 solar radius and 1 kiloparsec such that the distance of a
source is computed from d = 1000(pc) ∗ (Rwd/R⊙)/
S, or equivalently the scale factor
, is the factor by which the theoretical flux (integrated over the entire
wavelength range) has to be multiplied to equal the observed (integrated) flux.
The grid of WD models extended over the following range of parameters: log g =
7.0, 7.5, 8.0, 8.5, 9.0; Teff/1000 (K) = 22, 23, ..., 75; Si = 0.1, 0.2, 0.5, 1.0, 2.0, 5.0; C = 0.1,
0.2, 0.5, 1.0, 2.0, 5.0; and vrot sin i (km s
−1) = 100, 200, 400, 600, 800.
For the synthetic accretion disk models, we used the latest accretion disk models from
the optically thick disk model grid of Wade & Hubeny (1998). The range of disk model
parameters varies as follows: WD mass (in solar units) value of 0.35, 0.55, 0.80, 1.03, and
1.21; orbital inclination (in degrees) of 18, 41, 60, 75 and 81. The accretion rate ranges from
10−10.5M⊙yr
−1 to 10−8.0M⊙yr
−1 by increments of 0.5 in logṀ .
For each dwarf nova, we adopted the following procedure. First, we masked out all of
– 7 –
the obvious emission features and artifacts in both the FUSE and IUE spectra of each object.
Second, we carried out synthetic spectral fits using and/or combining model components in
this order: a white dwarf model alone, accretion disk model alone, combination white dwarf
plus accretion disk model, and two-temperature white dwarf model (the latter to simulate a
hotter equatorial region as well as a cooler photosphere at higher latitudes). For accretion
disk fits, we ”fine-tuned” the derived accretion rate of the best-fitting disk model by changing
the accretion rate in increments of 0.1 over the range 0.1 to 10, on the assumption that the
disk fluxes scale linearly over that range.
In Table 4, we indicate where we masked any strong emission features, artifacts, or
negative fluxes in the FUSE and IUE spectra of each object.
4. Synthetic Spectral Fitting Results
The noise level of the FUSE spectra precludes the opportunity to extract reliable pa-
rameters for the accreting white dwarfs in these three systems. This is especially true for
deriving rotational velocities which rely on well-resolved, strong absorption lines arising in
the photosphere. The rotational velocity is also affected by underlying emission filling of
absorption features and by the chemical abundances one uses. With these caveats in mind,
we proceeded to apply our grid of WD photosphere models (keeping the chemical abundance
fixed at solar) and accretion disk synthetic spectra.
For any dwarf nova in quiescence, a single temperature white dwarf model should be
a reasonable first approximation as the source of the FUV flux. For UU Aql, we adopted
two possible distances, 150 pc and 350 pc and carried out detailed fits for both values. For
a distance of 350 pc, the best-fit WD model to the FUSE spectrum gave Teff = 27, 000K,
Rwd/R⊙ = 1.13× 10−2 and a χ2 = 0.963. This best-fitting model is shown in figure 4. The
continuum of the model gives a fair representation of the observed continuum down to about
1060Å but there is a shortfall of model flux relative to the data at wavelengths shorter than
1030Å. For the same distance, an accretion disk alone yielded a best fit with χ2 = 1.14, an
accretion rate of 5 × 10−11M⊙/yr and inclination i = 41 degrees, Mwd = 0.8M⊙. This disk
fit is shown in figure 5. The model disk continuum, unlike the WD, fails to match the flux
level of the data between about 1090Å and 1180Å and the solar abundance accretion disk
model fails to provide a sharp absorption features.
A combination white dwarf plus accretion disk yielded a modest improvement with
χ2 = 0.71, Ṁ = 1.6× 10−11M⊙/yr, i = 41 degrees and Mwd = 0.8M⊙. Finally we tried two
temperature (WD + belt) fits. The best fit two temperature WD yields χ2 = 0.74 with the
– 8 –
cooler white dwarf portion ( Teff = 24, 000K) giving 57% of the flux and the hotter belt
(Tbelt = 33, 000K) providing 43% (figure 6).
In general, for a distance d = 150 pc, the model fits to UU Aql’s FUSE spectrum are
worse than for 350 pc. Applying single temperature white dwarf fits, we obtained Teff =
17, 000K, Rwd/R⊙ = 1.06 × 10−2 and a χ2 = 1.13. An accretion disk alone yielded a
best fit with χ2 = 1.23, an accretion rate of 1.3 × 10−11M⊙/yr and inclination i = 41
degrees and Mwd = 0.8M⊙. A combination white dwarf plus accretion disk yielded a modest
improvement with χ2 = 1.13, Ṁ = 1.6× 10−11M⊙/yr, i = 41 degrees and Mwd = 0.8M⊙. A
two temperature white dwarf (WD + belt) gives a best-fit with a χ2 = 1.02, with a 17,000K
white dwarf providing 77% of the FUV flux and the belt giving 23% of the flux. In view of
the much better agreement of the models with the observations of UU Aql for our assumed
distance of 350 pc than for a distance of 150 pc, the closer distance can be ruled out.
For CH UMa, the FUSE spectrum is very noisy and underexposed. We took into account
our adopted distance of 300 pc in the model fitting, we fixed the WD mass at Mwd = 1.2M⊙
(Log g = 9.0, with a radius of ≈ 4, 000km), and fixed the disk inclination at the published
value of 18 degrees. A single temperature white dwarf fit to CH UMa had χ2 = 0.227,
and yielded a best fit Teff = 29, 000K, and a distance of 310pc. This model however did
not fit very well in the shorter wavelengths (<1020Å). A lowest χ2 fit for this same WD
mass was obtained for T=40,000K, with χ2 = 0.208, but it yielded a distance of 600pc,
twice the adopted estimate of the distance. Since the Ritter catalog give a mass of 1.95M⊙
(well above the Chandraskhar mass limit for a WD), we decided to try a larger mass with a
correspondingly smaller radius which lead to a smaller emitting surface area and therefore
a shorter distance. We assumed M = 1.38M⊙ (logg = 9.5, with a radius of ≈ 2, 000km)
and found that the best fit was for T = 40, 000K. This model yielded a distance of 307pc
with χ2 = 0.199. Since the absorption features around 1120Å - 1150Å are not pronounced
in the observed spectrum, we decided to increase the rotational velocity to improve the fit
of this model. However, a better result was obtained by simply reducing the abundances
of Si and C to 0.01 their solar value. This low Si and C model had a χ2 = 0.184 and a
distance of 314pc. This model is shown in Figure 7. Because of the low S/N of the spectrum
the assessment of the error on the temperature estimate is of the order of 5,000K, namely
Twd = 40, 000± 5, 000K.
Though the WD fit yielded the lowest value of χ2, we tried accretion disk fits alone to
CH UMa’s FUSE data. The best fit gave a χ2 = 0.213, an accretion rate of 5× 10−12M⊙/yr
for Mwd = 1.2M⊙ and i = 18 degrees.
Various attempts to fit the combined FUSE + IUE spectra of CH UMa met with limited
success as summarized in Table 5. The best combination fit was for an accretion disk plus
– 9 –
WD with the disk contributing 79% of the FUV flux. However, these fits were less satisfactory
than the fits to the FUSE spectrum alone.
Because the FUSE spectrum of CH UMa is very noisy, and because of the broad emission
lines and the absence of strong absorption lines, it is difficult to assess which model (disk
or WD) is the best solution. However, the absence of strong absorption lines would favor
the accretion disk model because of Keplerian broadening. Also because of the poor quality
of the spectrum, composite models (WD+accretion disk, two-temperature WD) did not
improve the fit. In view of all of the above, we cannot be confident that we have determined
the temperature of the white dwarf in CH UMa.
Next we analyzed the combined FUSE + IUE spectra of CH UMa. The best white
dwarf-only fit yielded Teff = 31, 000K, log g = 9 a χ
2 = 3.02 and Rwd/R⊙ = 6.02 × 10−3.
For accretion disk models only, the best fit occurred with a χ2 = 2.02, corresponding to
Ṁ = 3× 10−9M⊙/yr. This disk fit is a modest improvement of the fit to CH UMa since the
χ2 value was lowered to 2.02. The best-fitting white dwarf + accretion disk model resulted
in only a modest, statistically insignificant improvement. The χ2 value was lowered to 1.87,
the accretion rate was 6.4× 10−11M⊙/yr with the WD Teff = 22, 000K and the scale factor
yielding a white dwarf radius of 5.84 × 10−3R⊙. In this composite disk plus white dwarf
fit, the WD contributes 21% of the FUV flux while the accretion disk contributes 79% of
the FUV flux. We also tried a two-temperature WD solutions with the best-fitting model
consisting of a 26,000K WD providing 77% of the flux and a hot accretion belt/ring with
Tbelt = 50, 000K giving 23% of the UV flux. However, this two-temperature fit was no better
than the WD + accretion disk fit.
For BV Cen we dereddened the spectrum assuming E(B-V)=0.10, and first tried single
temperature white dwarf fits with a white dwarf mass = 0.83M⊙, and used solar chemical
abundances. We found the best-fitting white dwarf model to have Teff = 40, 000± 1000K,
log g = 8.3, V sin i = 500 km/s ±100 km/s. This fit yielded χ2 = 0.2701 and a distance
d=435pc (see figure 8). We note here that the WD solution fits the following absorption
features quite accurately: C ii (1066Å), S iv (1073Å), N ii (1084Å), S iiv (1122.5Å, 1128.3Å),
and S iiii (1140Å- 1146Å).
For an optically thick, steady state accretion disk alone, we chose Mwd = 0.80M⊙,
i = 60 degrees. The best fit we obtained has a mass accretion rate of 10−8.5M⊙yr
−1, too
large for dwarf nova quiescence. Moreover, the longer wavelength part of the spectrum is
rather flat and unable to fit the absorption features around 1120Å and 1130Å. This fit, with
i=60deg, leads to a distance of 1255pc. In order to fit these absorption features, one needs
to assume an inclination of 18 degrees, inconsistent with the known inclination of the system
(62 degrees). Such a model leads to a distance of more than 2000pc, again inconsistent
– 10 –
with all estimates of the system distance (500pc). Therefore, based on the parameters of
the system, the disk solution is completely inconsistent. We tried composite model fits, but
they also led to very poor results. We also combined BV Cen’s FUSE + IUE spectra but
they led to very poor fits, much worse than the fits to the FUSE spectrum alone. Since
the WD solution is consistent with the parameters of the system and fits the absorption
features of the spectra, it is clear that the favored solution of the FUSE spectrum of BV Cen
is Twd = 40, 000± 2, 000K.
5. Discussion
Our principal objective of determining the surface temperatures of the white dwarfs
in these three dwarf novae during quiescence has met with mixed results. From synthetic
spectral fits to the FUSE spectra of the three long period dwarf novae, UU Aql, CH UMa
and BV Cen, and the FUSE + IUE archival SWP spectra of CH UMa we have presented
preliminary evidence that during quiescence, their accreting white dwarfs all have surface
temperatures hotter than 20,000K. Unfortunately, all three temperatures have considerable
uncertainty due to the low S/N of the FUSE spectra and IUE spectra as well as the difficulty
of disentangling the flux contribution of the second component of FUV flux or ”accretion
disk” during quiescence. Of the three systems in this study, we regard our estimate of the
Teff of the WD in UU Aql to be the most reliable since that system appears to be dominated
in the FUV by the white dwarf flux.
For UU Aql, we used both assumed distances, 150 pc and 350 pc, and tried the composite
fits (WD + disk) to UU Aql’s FUSE spectrum. However, the quality of the fits for WD-only,
disk-only, WD + disk, and WD + ”belt” were all roughly comparable and thus it is difficult
to distinguish the best-fit case. For both distances, statistically insignificant improvements
in the fits result when a white dwarf and accretion disk are combined or a two-temperature
WD (WD + belt) is applied to the FUSE data. The results in Table 5 illustrate the difficulty.
Qualitatively, the composite fits involving combinations of white dwarf plus accretion disk
or accretion belt models look marginally more reasonable than the fits that involve single
component (WD or Disk). It appears that for both distances, the white dwarf component is
the dominant source of FUV flux and that the Teff of the WD is probably between 17,000K
and 27,000K (say 22,000±5,000K).
For BV Cen, the WD model fit to the FUSE spectrum gave the best result, both in
fitting actual features of the observed spectrum and in leading to consistent values of the
system parameters. This was not the case for the disk model and the composite models.
From the best WD model fit we obtained that the WD of BV Cen must have a temperature
– 11 –
of about 40,000K.
For CH UMa, the broad and dominant emission lines together with the poor S/N (and
possible detector noise at very short wavelengths) of the FUSE and IUE spectra precluded
the opportunity to obtain conclusive results. However, the results for the FUSE spectrum
were of a much higher quality than for the FUSE + IUE combined spectrum. Therefore we
adopt the FUSE results for CH UMa with the possibility that the WD temperature could
be as high as 40,000K.
In Table 6, we list the dwarf novae above the period gap whose white dwarfs have surface
temperature determinations. In Column (1) we give the system name, column (2) the orbital
period; column (3) the surface temperature and column (4) the temperature reference. As
seen in Table 6, there is now a sample of eight long period dwarf novae of which roughly
seven have relatively secure white dwarf temperatures obtained during quiescence. In the
case of BV Cen, the inclination is expected to be high. Thus, it is plausible to expect that
the quiescent accretion disk may be blocking the direct radiation from the accreting white
dwarf.
The relatively poor FUSE spectral quality of CH UMa underscores the need for re-
observation with longer exposure times. However, analysis will remain hindered until more
reliable information on white dwarf masses and distances becomes available. Until then, the
conclusions in this work must be regarded as preliminary.
6. Acknowledgements
We thank an anonymous referee for helpful comments and corrections. PG wishes to
thank the Space Telescope Science Institute for its kind hospitality. This work was supported
by NSF grant AST05-07514 and NASA grants NAG5-12067 and NNG04GE78 to Villanova
University. This research was based on observations made with the NASA-CNES-CSA Far
Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins
University under NASA contract NAS5-32985.
– 12 –
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– 14 –
Table 1: Dwarf Nova Parameters
System Porb trec Vq Vo Sec. i Mwd EB−V Distance
Name (days) (days) (deg) (M⊙) (pc)
BV Cen 0.610108 150 12.6 10.5 G5-8V 62± 5 0.83± 0.1 0.10 500
CH UMa 0.343 204 15.3 10.7 K4-M0V 21± 4 300
UU Aql 0.14049 71 16.0 11.0 M2-4V 150-350
Table 2. FUSE Observations
Target Data ID Aperture Time of Observation texp λcentered S/N(0.1Å)
UU Aql C1100301000 LWRS 30x30 2004-05-16 13h:48m:00s 16,121s 1035Å 5.15
BV Cen D1450301000 LWRS 30x30 2003-04-13 20h:26m:00s 26,545s 1035Å 5.9
CH UMa D1450201000 LWRS 30x30 2003-04-02 22h:00m:00s 17,311s 1035Å 3.6
Table 3. IUE Observations
Target SWP No. Aperture Dispersion Date Time 0f Mid-Exposure texp(s)
BV Cen 26623 Lg Low 08/16/85 17:17:00 14,400
CH UMa 56270 Lg Low 12/06/95 12:15:49 13,800
– 15 –
Table 4. Masked Spectral Regions
Target Emission Features
UU Aql for λ < 1050Å if Fλ > 6× 10−15ergs s−1cm−2Å−1
for 1050 Å < λ < 1185Å if Fλ > 1.8× 10−14ergs s−1cm−2Å−1
975-985Å
CH UMa for λ < 957Å if Fλ > 1× 10−14ergs s−1cm−2Å−1
970-980, 987-994, 1023-1042, 1071-1090, 1107-1136, 1167-1180, 1200-1260,
1285-1315, 1380-420, 1532-1560, 1630-1650, 1846-1940Å
BV Cen for λ < 1185Å if Fλ > 2.5× 10−14ergs s−1cm−2Å−1
1190-1220, 1320-1340, 1375-1415, 1530-1570, 1625-1650, 1835-1880Å
Table 5. UU Aql, BV Cen, and CH UMa Fitting Results
System Spectrum d χ2 Twd Tbelt Ṁ % Flux Fig
(pc) (103K) (103K) (M⊙/yr) (Contribution)
UU Aql FUSE 350 0.96 27 - - 100(WD) 4
FUSE 150 1.13 17 - - 100(WD) -
FUSE 350 1.13 - - 5× 10−11 100(Disk) 5
FUSE 150 1.24 - - 1× 10−11 100(Disk) -
FUSE 350 0.714 24 - 2× 10−11 73(WD)/27(Disk) -
FUSE 150 1.13 17 - 3× 10−11 60(WD)/40(Disk) -
FUSE 350 0.738 24 33 - 57(WD)/43(Belt) 6
FUSE 150 1.021 16 29 - 77(WD)/23(Belt) -
CH UMa FUSE 314 0.184 40 - - 100(WD) 7
FUSE 313 0.213 - - 5× 10−12 100(Disk) -
FUSE+IUE 300 3.02 31 - - 100(WD) -
FUSE+IUE 300 2.02 - - 3× 10−9 100(Disk) -
FUSE+IUE 300 1.87 22 - 6.4× 10−11 21(WD)/79(Disk) -
FUSE+IUE 300 2.00 26 50 - 77(WD)/23(Belt) -
BV Cen FUSE 435 0.27 40 - - 100(WD) 8
FUSE 1255 0.27 - - 3× 10−9 100(Disk) -
– 16 –
Table 6: Surface Temperatures of White Dwarfs in Dwarf Novae above the Period Gap
System Name Period Teff References
(min) (Kelvin)
BV Cen 878.6 40,000: This paper
RU Peg 539.4 49,000 Sion et al (2004)
Z Cam 417.4 57,000 Hartley et al. (2005)
RX And 302.2 34,000 Sion et al. (2001)
SS Aur 263.2 31,000 Sion et al (2004)
U Gem 254.7 31,000 Sion et al. (1998)
WW Ceti 253.1 26,000 Godon et al. (2006)
UU Aql 202.3 27,000: This paper
– 17 –
Figure Captions
Fig. 1.— The flux Fλ (ergs/cm
2/s/Å) versus wavelength (Å) FUSE spectrum of the U Gem-
type dwarf nova UU Aql during quiescence. The identifications of the strongest neutral and
ionized line features as well as rotational-vibrational transitions of molecular hydrogen are
marked with vertical tick marks. The molecular hydrogen absorption are modestly affecting
the continuum, slicing it at almost regular wavelength intervals. We identify the most
prominent molecular hydrogen absorption lines by their band (Werner or Lyman), upper
vibrational level (1-16), and rotational transition (R,P, or Q) with lower rotational state
(J=1,2,3). Air glow line features are indicated with the earth symbol. The C iii (977Å) and
Ovi doublet sharp emissions are due to heliocoronal emission often present in some of the
FUSE channels when the target (i.e. UU Aql) is faint.
Fig. 2.— The flux Fλ (ergs/cm
2/s/Å) versus wavelength (Å) FUSE spectrum of the U Gem-
type dwarf nova BV Cen during quiescence. The identifications of the strongest neutral and
ionized line features as well as rotational-vibrational transitions of molecular hydrogen are
marked with vertical tick marks. Air glow line features are indicated with the earth symbol.
The O i emissions are also due to air glow. The broad C iii (977Å) and Ovi (doublet)
features might be real broad emissions from the source, as these correspond to the left wing
of the Lyman γ & β (respectively) and not much flux is expected there. The N i and N ii
sharp emission features are geocoronal.
Fig. 3.— The flux Fλ (ergs/cm2/s/Å) versus wavelength (Å) FUSE spectrum of the U Gem-
type dwarf nova CH UMa during quiescence. The identifications of the strongest neutral and
ionized line features as well as rotational-vibrational transitions of molecular hydrogen are
marked with vertical tick marks. Several terrestrial line features are indicated with the earth
symbol. There are some broad emission features associated with the source: we identify here
the Ovi doublet, C iii (at 977Å and 1175Å). There could be some N iv emission in the short
wavelengths (between 920Å and 925Å), but the increase of flux in the very short wavelengths
suggests that most the flux at λ < 930Å could be higher orders of the hydrogen Lyman series
associated with air glow contamination and possibly some detector noise.
Fig. 4.— A single temperature WD fit to the FUSE spectrum of UU Aql for a distance of 350
pc. The best-fit yielded Teff = 27, 000K ±3000K, log g = 9. The white dwarf contributes
100% of the FUV flux.
Fig. 5.— An accretion disk-only fit to the FUSE spectrum of UU Aql for a distance of 350
pc. The best-fit corresponded to Mwd = 0.8M⊙, i = 41 degrees, Ṁ = 5× 10−11M⊙/yr. The
accretion disk contributes 100 % of the FUV flux.
– 18 –
Fig. 6.— A WD + belt fit to the FUSE spectrum of UU Aql for a distance of 350 pc. The
dotted line is the white dwarf flux component, the dashed line is the belt flux component
and the solid line is the combined two-temperature fit. This best fit two temperature WD
yields χ2 = 0.74 with the cooler white dwarf portion ( Teff = 24, 000K) giving 57% of the
flux and the hotter belt (Tbelt = 33, 000K) providing 43%.
Fig. 7.— A single temperature WD fit to the FUSE spectrum of CH UMa. The best-
fit yielded Teff = 40, 000 ± 1, 000K, log g = 9.5 and a projected rotational velocity of
Vrot sin i = 200km/s. The portions of the spectrum that have been masked are shown in
blue and include emission lines (either intrinsic to the source or due to air glow) and ISM
molecular hydrogen absorption lines (which have been matched using a simple ISM model
to identify the exact location of the ISM lines). Therefore the fit is between the model and
the red portions of the observed FUSE spectrum. The absence of strong absorption features
in the lower panel were modeled assuming low Si and C abundances and led to a lower χ2
than the same model with solar C and Si abundances but with a high projected rotational
velocity (≈ 1, 000km/s).
Fig. 8.— A single temperature WD fit to the FUSE spectrum of BV Cen. The best-fit
yielded Teff = 40, 000K±1000K, a projected rotational velocity of 500km/s, and d=435pc,
assuming M = 0.83M⊙.
the FUSE plus IUE spectrum of UU
– 19 –
– 20 –
– 21 –
– 22 –
– 23 –
– 24 –
– 25 –
– 26 –
	Introduction
	Observations and Data Reduction
	Multi-Component Synthetic Spectral Fitting
	Synthetic Spectral Fitting Results
	Discussion
	Acknowledgements
