The IUE spectra are presented for two old novae and for two of the symbiotic variables. Prominent emission line spectra are revealed as a continuum whose appearance is effected by the system inclination. These data provide evidence for hot companions in the symbiotic stars, making plausible the binary model for these peculiar stars. Recent IUE spectra of dwarf novae provide additional support for the existence of optically thick accretion disks in active binary systems. The ultraviolet data of the eclipsing dwarf novae EX Hya and BV Cen appear flatter than for the noneclipsing systems, an effect which could be ascribed to the system inclination

Ferland, G. J.

Lambert, D. L.

Shields, G. A.

Slovak, M. H.

English

NASA Technical Reports Server

ULTRAVIOLET SPECTROSCOPY OF OLD NOVAE
AND SYMBIOTIC STARS
David L. Lambert, Mark H. Slovak and Gregory A. Shields
University of Texas at Austin
Gary J. Ferland
Cambridge University
ABSTRACT
Low-dispersion, short-wavelength IUE spectra are presented
for two old novae, DQ Herculis (1934) and V603 Aquilae (1918) =
HD 174107, and for two symbiotic variables, AG Pegasi = HD 207757
and CI Cygni. The emission line spectra of the novae are rela-
tively sparse, dominated by a strong C IV (11549) feature and show-
ing weaker e/nissi_ns arising from NV(I1241), Si IV+ 0 I_ (111398,
1402), and He II (11640). The absence of intercombination lines
suggest electron densities greater than i0 _° cm -3 . The continua
of the two novae are remarkably different, the appearance seeming-
ly dictated by the inclination of the system. The symbiotic var-
iables display a more variegated emission line spectrum, exhibit-
ing features of O I (11304), N IV](I1485), N III](I1752), and C II_
(11909). The continua for the quiescent symbiotics are stellar
in appearance and change as a function of the system inclination.
These results confirm the existence of hot companions in the sym-
biotic stars, lending support to the binary hypothesis for these
variables. Model atmospheres are fit to the spectra to estimate
the properties of the hot components.
INTRODUCTION
Ultraviolet spectra have been obtained for several old novae
and various of the symbiotic stars in order to study the nature
of these objects in a region inaccessible to ground-based obser-
vations. The ultraviolet data are being combined with optical
spectra in order to provide absolute flux distributions extending
into the near infrared.
In order to ascertain the steady-state post-eruptive proper-
ties of these active systems, the data were obtained while the
stars were in their quiescent states. Hence, the observations
can be compared directly with various of the steady-state models
that have been proposed to explain the novae and nova-like vari-
ables.
The old novae are thought to be close binary systems (ref. i)
involved in active mass transfer via Roche lobe overflow of mat-
ter from a late type star onto a white dwarf. An accretion disk
is formed around the white dwarf as a result of the mass transfer
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process and is predicted to contribute an appreciable fraction of
the ultraviolet luminosity (ref. 2). The old novae V603 Aql (19]8)
and DQ Her (1934) were examined and their spectra are discussed in
the context of the binary model.
The exact nature of the symbiotic variables has been an unre-
solved puzzle for many years. The optical spectra are usually do-
minated by a late-type giant and a strong nebular line spectrum,
obscuring any contribution from a hot companion. The IUE data ob-
tained for various of the symbiotics, however, show clear evidence
for the existence of a hot component in many of these stars, sup--
porting the binary hypothesis discussed in reference 3. Data fo]_
the spectroscopic binary AG Peg and the eclipsing system CI Cyg
are presented as examples.
OBSERVATIONS
The low resolution (AI = 5 x i0 -mm) spectra were obtained with
the short wavelength prime (SWP) camera of the IUE satellite on
June 29 and December 14, 1979. The exposures were taken through
the i0 _ x 20 _ oval aperture and were untrailed with the exceptiorL
of SWP 7407 (AG Peg).
The data have been converted to absolute fluxes using the
calibration given in reference 4. Unfortunately, the June spec-
tra were processed using an incorrect intensity transfer function
(ITF). However, additional spectra of similar quality for V603
Aql (SWP 5921) and CI Cyg (SWP 5485) have been processed using
both the incorrect ITF and a corrected version (ref. 5). The a-
verage correction error between the two was 2.3%. Since the un-
certainty in the absolute flux calibration of the SWP spectra
longward of Le (11216) is estimated to be 10%, the results present-
ed here would appear to be uneffected by the ITF problem.
The fluxes have been de-reddened using the reddening curve
given in reference 6. The values of the color excess, E(B-V),
used to de-redden the data are indicated in each of the figures.
The color excesses are small for the novae, tending to be larger
for the more luminous symbiotic stars.
DISCUSSION
THE OLD NOVAE
The IUE data for V603 Aql are compared with earlier OAO-2
and ANS satellite measurements in figure i. The satellite data
have been de-reddened assuming E(B-V) = 0.07 mag, as have the IUE
fluxes. The systematic disagreement between the OAO-2 and IUE
values shortward of 11600 _ suggests real variability in the con-
tinuum of the hot component. The OAO-2 data were used (ref. 7)
to derive ultraviolet colors and estimate a color temperature of
25,000 K. A hot model atmosphere (Tef f = 25,000K ; log g =4.5)
(ref. 8) is compared to the IUE data in figure 1 as well as the
predicted flux distribution for an optically thick accretion disk
(fl _ I_)" Also shown are values from a white dwarf model
462
(Tef f = 25,000 K ; log g = 8) (ref. 9). The model fluxes have
been normalized to the stellar fluxes at i1700 _ where the con-
tinuum appears free of emission lines.
The continuum of DQ Her can be seen in figure 2 and offers a
startling contrast to that of V603 Aql. It is well represented
by a flux distribution of the form f_ l°= constant over the band-
pass of the SWP camera (111200-1900 _). Despite the difference
between the continua of the novae, however, the emission line
spectra are qualitatively similar at least in the appearance of
various of the lines. Note the strong NV (_1239, 1243 ) feature
present in DQ Her which is absent from V603 Aql.
It is interesting to note that DQ Her is an eclipsing system
seen at a relatively high inclination ( i = 7_ ) whereas V603 Aql
is viewed nearly pole-on (i = 150). The difference in the appear-
ance of the continua may thus reflect the differing aspects of
these systems. Those binaries seen at high inclinations apparent-
ly must present a larger optical depth in the disk to the obser-
ver than do the lower inclination systems. This simple geometri-
cal interpretation provides support for the models that posit the
existence of optically thick accretion disks surrounding the hot
white dwarf components.
THE SYMBIOTIC STARS
The IUE spectra of the symbiotic stars present a wealth of
observational material. These data show a much richer and more
diverse emission line spectrum than seen in the novae. Further-
more, the ultraviolet data provide direct confirmation, in many
cases, of the existence of a hot companion. Spectra were obtain-
ed on two systems known to be binaries from optical studies. In
figure 3 the low dispersion spectrum of AG Peg, a spectroscopic
binary, is seen, and figure 4 presents a similar spectrum for the
eclipsing system CI Cyg.
Satellite data also exist for AG Peg and are shown in figure
3 compared to the IUE fluxes after de-reddening using a value of
E(B-V) = 0.12 mag. Despite the large uncertainties associated
with the earlier measurements (_34% at _ = 1330 _), the continuum
appears to have systematically decreased by approximately 20%
since the epoch of the OAO-2 observations (May 1970). Such a
decline appears to be the source of the steady 0.025 mag/year de-
crease seen in the optical light curve which would also imply a
20% decrease in flux.
Model atmospheres from reference 8 were fit to the continuum
of AG Peg and serve to provide an estimate of the temperature of
the hot component ( Tef f = 40,000 K). This value is in accord
with the temperature estimated from the OAO-2 measurements, dis-
cussed in reference i0.
Both the continuum and the emission line spectrum of the
eclipsing system CI Cyg (figure 4) provide a marked contrast to
those of AG Peg. The emission lines are much narrower and gener-
ally weaker. Note the presence of the strong permitted nitrogen
lines in AG Peg and their weaker appearance or even absence in
463
CI Cyg.
Further support for the hypothesis developed above relating
system inclination to continuum appearance is provided from the
comparison of these two systems. CI Cyg appears at a very high
inclination showing flat-bottomed eclipses of variable depth.
AG Peg, on the other hand, is seen at a lower inclination of ap.-
proximately 36 ° . Thus, the material (disk?) surrounding the high
inclination systems would again appear to present a large optical
depth to the line of sight. It is expected that spectra of the
eclipsing nova T Aurigae and the other known eclipsing symbiotic
star AR Pavonis would show similar effects.
CONCLUSIONS
IUE spectra have been presented for two old novae and for
two of the symbiotic variables. Prominent emission line spectra
are revealed as is a continuum whose appearance is effected by
the system inclination. These data provide evidence for hot
companions in the symbiotic stars, making plausible the binary
model for these peculiar stars. It is also worthy to mention
that recent'IUE spectra of dwarf novae (ref. ii) provide addition-
al support for the existence of optically thick accretion disks
in active binary systems. The ultraviolet data of the eclipsing
dwarf novae EX Hya and BV Cen appear flatter than for the non-
eclipsing systems (e.g. VW Hyi), an effect which could be ascribed
to the system inclination.
464
REFERENCES
i. Robinson, E. L.: The Structure of Cataclysmic Variables.
Ann. Rev. Astr. Ap., vol. 14, 1976, pp. 119-142.
2. Herter, T; Lacasse, M. G.; Wesemael, F.; and Winget, D. E.:
Theoretical Photometric and Spectroscopic Properties of
Stellar Accretion Disks with Application to Cataclysmic
Variables. Ap. J. Suppl., vol. 39, April 1979, pp. 513-535.
3. Boyarchuk, A. A.: Symbiotic Stars. Non-Periodic Phenomena
in Variable Stars. Fourth Colloquium of Variable Stars,
1969, pp. 398-410.
4. Bohlin, R. C.; Holm, A. V.; Savage, B. D.; Snijders, M. A.;
and Sparks, W. M: Photometric Calibration of the Inter-
national Ultraviolet Explorer (IUE): Low Dispersion. NASA
X-681-79-19, June 1979.
5. Cassatella, A.; Holm, A.V.; Ponz, D.; and Schiffer, F. H.:
A Correction Algorithm for Low Dispersion SWP Spectra.
NASA IUE Newsletter, NO. 8, Feb. 1980, pp. 1-21.
6. Codej A. D.; Davis, J.; Bless, R. C.; and Brown, R. H.: Em-
pirical Effective Temperatures and Bolometric Corrections
for Early-Type Stars. Ap. J., vol. 203, Jan. 1976, pp. 417
-434.
7. Gallagher, J. S.; and Holm, A. V.: Ultraviolet Detection of
the Nova Variables V603 Aquilae and RR Pictoris. Ap. J.
Lett., vol. 189, May 1974, pp. 123-126.
8. Kurucz, R. L.: Model Atmospheres for G, F, A, B, and O Stars.
Ap. J. Suppl., vol. 40, May 1979, pp. 1-340.
9. Wesselius, P. R.; and Kooster, D.: Effective Temperatures
of Hot White Dwarfs. Astr. Ap., vol. 70, Dec. 1978, pp.
745-750.
10. Gallagher, J. S.; Holm, A. V.; Anderson, C. M.; and Webbink,
R.F.: Ultraviolet Photometry from the Orbiting Astron-
omical Observatory. XXXIII. The Symbiotic Star AG Pegasi.
Ap. J., vol. 229, May 1979, pp. 994-1000.
ii. Bath, G. T.; Pringle, J. E.; and Whelan, J. A. J.: Spectro-
photometry of Dwarf Novae in the Wavelength Range 1250 -
7500 A. Mon. Not. R. Astr. Soc., vol. 190, Jan. 1980, pp.
185-194.
' 465
Figure 1 - The low-dispersion IUE spectrum f o r  t h e  o l d  nova 
V 6 0 3  Aql. The IUE f l u x e s  a r e  compared wi th  t h e  
OAO-2 d a t a  ( f i l l e d  c i r c l e s ) ,  t h e  ANS d a t a  (open 
c i r c l e s  with d o t ) ,  and t h e  white dwarf model fluxes; 
( c i r c l e s  with s l a s h ) .  Dashed regions  of spectrum 
l o c a t e  reseau contaminated por t ions .  Also shown 
a r e  t h e  p red ic ted  f l u x e s  of a  s t e l l a r  model atmos- 
phere (0) and an o p t i c a l l y  t h i c k  a c c r e t i o n  d i sk  (0). 
See d i scuss ion  i n  t h e  t e x t .  
Figure 2 - A s  i n  f i g u r e  1 f o r  t h e  o ld  nova DQ Her. The d a t a  
have been de-reddened using a  va lue  of E(B-V) = 0.08 
mag. The s t rong  La f e a t u r e  i s  geocoronal i n  o r i g i n .  
{,e_;.a,e a, -- If,"l" ° ._, eoc v, ic_ _ ° ,0._,
t
2.57L
<])
I. 342 •
O
0, 727
1200 1300 1400 1500 1600 1700 1800 1900
,kt,q]
Figure 3 - Low-dispersion spectrum for the symbiotic variable
AG Peg. The lines with dashed tops are saturated.
OAO-2 data are represented by filled circles; ANS
values by open circles with dot. Two model atmo-
spheres are fit to the continuum: ® - Tef f = 35,000 K
and 0 - Tef f = 40,000 K.
ecs-vl • o,{o
_.491 Cl CYGNI IUF 5NP 5672
2. 085
II I I I(J
o'1 I. 6"/4
O
112
? h 0l_9
t
° LO. 462 I/
120(1 I $00 14 flO 1$00 | 600 | 7dO ! tlO0 t 900
.k[_l
Figure 4 - As in figure 3 for the eclipsing symbiotic Cl Cyg.
The fluxes have been de-reddened using a value of
E(B-V) = 0.10 mag.
467


Ultraviolet spectroscopy of old novae and symbiotic stars

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