Non-Periodic Phenomena in Variable Stars
IAU Colloquium, Budapest, 1968
SPECTROSCOPIC OBSERVATIONS OF THE RECURRENT NOVA
RS OPHIUCHI FROM 1959 TO 1968
R. BARBON, A. MAMMANO, L. ROSINO
Astrophysical Observatory Asiago, University of Padova
ABSTRACT
Spectra of RS Oph have been taken at Asiago with the 122 cm telescope during
the 1959-67 minimum and on the occasion of the 1967-68 outburst. At minimum the
variable has a composite spectrum, one component being of spectral type around
M2, while the other component gives a blue continuum with emission lines of
medium or high excitation. During the outburst the star has shown the same
spectral evolution as in the 1933 and 1958, with the development of bright
nebular and coronal lines, indicating an extremely high degree of ionization.
Further details will be given in a forthcoming paper.
A preliminary account is given here of the spectroscopic observations
of the recurrent nova RS Oph carried out at Asiago during its minimum from
1959 to 1967 and on the occasion of its recent outburst in 1967-1968. It is
not necessary perhaps to recall that RS Oph was caught at maximum four
times, in 1898, 1933, 1958 and more recently in 1967, rather surprisingly in
view of the short time-interval passed from the preceding maximum.
The nova was extensively observed at Asiago from July to October during its
1958 maximum. A detailed report on its spectral evolution is published in the
Contributions of Asiago, No. 113 (1960). At the end of October 1958, 104 days
past maximum, RS Oph had reached its highest stage of ionization, with an
emission spectrum characterized by the presence of strong coronal lines
of [FeXIV], [FeX] and [AX], forbidden lines of [OIII] and [NII] and permitted
lines of NIII, HeI and HeII, besides of course the Balmer series of hydrogen.
H_alpha, H_beta, [FeX] 6374, HeI 5875 and HeII 4686 were by that time prominent
in the spectrum of the nova.
THE MINIMUM 1959-1967
When the nova was again observed in 1959, after an intermission of several
months due to the seasonal period of invisibility, the situation was radically
changed. RS Oph had reached its minimum magnitude at about 12.5 and all of the
lines due to high ionized atoms had faded out or disappeared.
The object was since then included in a regular program of observations of
old novae, carried out at first with the prismatic spectrograph attached to the
newtonian focus of the 122 cm telescope (dispersion: 130 A/mm at H_gamma) and
later with an RCA Carnegie intensifier applied to the cassegrain spectrograph
giving a dispersion of 60 A/mm at H_gamma. Some cassegrain spectra were also
taken at 75 and 180 A/mm.
From 1959 to 1962 RS Oph, as shown by the AAVSO observations, was constantly
at a mean magnitude of about 11.5 visual, with slow irregular fluctuations of
luminosity from 10.5 to 12.5. The spectra obtained during this period,
independent of the intrinsic brightness of the star, show a gradual decline in
the degree of excitation, a progressive fading of the blue-visual continuum
and consequently a decisive strengthening in the red. In the 1959-1960 spectra,
the following emission lines have been recorded: the Balmer series from H_alpha
to H_delta with very steep decrement; HeI 6678 and 5876; [OIII] 5006 very faint,
partially in blend with HeI 5015; [NII] 5755. Some FeII permitted emission
lines of multiplets 26, 27, 37, 38, 48, 49 and 74 are also represented. HeII
4686 was not recorded, even as a trace, and also NIII 4640 and the coronal
lines had completely disappeared.
In 1961-62 a further general weakening of the emission lines was observed:
the Fell emissions faded out and only H_alpha, H_beta and lambda 5876 HeI, the
last two very faint, were still visible in our spectra. By this time wide
absorption bands of moderate strength (faintly recorded also in 1959-60) emerged
clearly at lambda lambda 6350-6158, 6030-5890, 5780-5635, 5460, 5167 etc.
The NaI doublet 5890-96 was prominent. Although some of the dark lines might be
tentatively attributed to interstellar absorption, the comparison with standard
spectra of advanced type leaves no doubt that the predominant spectrum of RS Oph
in 1962 is an early M type, very likely M2-III. The symbiotic nature of the
nova is therefore confirmed by the observations at minimum. In 1962 the red
companion was dominant and masked the spectrum of the blue component, except
for the H emission (Fig. 1).
A new phase of activity was displayed by RS Oph in 1965. Mrs. Mayall
announced on March 26 that the star was of magnitude 9.7. On spectra taken
at Asiago on April 2 a neat increase of the degree of excitation was clearly
indicated by the strengthening of the continuum at wave lengths shorter
than 5500 and by the emergence of relatively strong emission lines of H, HeI
and FeII. During the following weeks, while the nova maintained a visual
magnitude of about 10.2, the degree of excitation further increased. Noteworthy
was the reappearance of HeII 4686, about as strong as H_beta which had
never been observed since 1958. By May 14, however, 4686 faded out and
became just visible as a trace, while the Balmer lines, HeI and FeII still
appeared rather strong. Faint forbidden lines of FeII, SII and the blend
NIII 4640 were also weakly recorded. The degree of excitation, although
less than in April and May, was higher than at minimum, when the observations
were interrupted in July 1965.
THE 1967 MAXIMUM
The announcement of the new explosion of RS Oph was given by Dr. Beyer of
Hamburg-Bergedorf and by Fernald on October 26, 1967 (IAU Circ. 2040), who
estimated the nova of magnitude about 5.9. Three days before, the star was
still at its minimum, magnitude 10.7. I shall not speak here of the light
curve, which was found to be almost exactly the duplicate of the, light curves
of 1933 and 1958.
Four months before maximum, on June 27-29, some spectra of RS Oph had been
taken at Asiago with the new Carnegie intensifier. They show an intense
continuum, also in the blue, with emission lines of H, HeI (6678, 5876, 4922,
4471, 4384) and FeII, moderately strong. The spectra do not look different
from those taken during the secondary outburst of 1965, except for the absence
of HeII 4686, and indicate an unusual state of activity of the nova.
Fig. 1. Spectra of RS Oph at minimum, from 1959 to 1965.
Fig. 2. Spectral evolution of RS Oph during the 1967-68 outburst.
On the top a premaximum spectrum taken on 27 th June 1967.
After the explosion, the nova was followed from October 27 to Dec 2 and
again, from January 30 to July. It is presently under observation.
Its spectral evolution can be shortly described as follows. On October 27,
one day after maximum, the spectrum is characterized by wide emission bands
(halfwidth 30-60 A) bordered on the blue side by two systems of broad
absorptions, to which correspond mean radial velocities of -2700 and
-3900 km/s. The continuum is strong. Near the center of the emission bands
a narrow absorption line is visible, bordered on the red by a sharp emission.
On the highest member of the Balmer series only the narrow absorption is
recorded. The radial velocity of these sharp components is nearly -40 km/s,
coincident with the radial velocity of the star, as given by Sanford. There
seems to be no doubt that the sharp emission-absorption components, which
were observed also in 1958, are originated in a stationary envelope which
surrounds the nova.
On the following days, the absorption systems fade out and finally disappear.
At this point the flaring of the HeI lines takes place. In the space of a few
days, from October 27 to November 7, the HeI emission bands become outstanding
in strength surpassing all other emissions except H_alpha and N_beta. While
the excitation further increases, the HeI lines start to weaken and by November
8-9 some lines of highly ionized atoms, as HeII 4686 and NIII 4640 make their
appearance, and rapidly increase in intensity. Forbidden lines of [NII] 5755,
[FeII], [OI] weakly appear from November 3 to 13. The [FeX] 6374 coronal line
was already present from November 3, in blend with [OI] 6364. When the
observations were interrupted, on December 3, thirty days after maximum,
[FeX] was prominent, stronger than H_beta.
The new series of spectra, taken from February 3, 1968, one hundred days after
maximum, at about the same phase at which the observations were interrupted in
1958, show the latest stages of evolution of the nova. The early spectra
strongly resemble those taken in October 1958. The HeI lines (with the sole
exception of 5876) and the Fell lines have faded out and the spectrum discloses
the extremely high excitation of the nebular envelopes ejected by the star.
Besides the Balmer lines and Hell 4866 the most prominent features in the
spectrum are: 6827, [FeX] 6374 still stronger than H, [AX] 5535, [FeXIV] 5303,
[0III] 5006-4959, [0III] 4363, [NiXIII] 5116, [NiXII] 4231.
By March however, with the star well at its minimum, the excitation
begins to decrease. The coronal lines are rapidly weakening: [FeX] 6374 is
still conspicuous, in blend with 6364, but [AX] 5535 and [FeXIV] 5303 have
faded. It is the moment of the nebulium flaring. [OIII] 5006-4959 with
4363 become outstanding. As in 1958 they are rather fuzzy, as well as NII
5755, now stronger than 5876 HeI. Also [FeVII] 5160, which was barely visible
in February has become conspicuous. The last spectrum represented in Fig. 2
was taken on May 7 when the nova, as announced by Locker and confirmed
by Mrs. Mayall, was fainter than its normal brightness, about 13.3. The coronal
lines, FeX included, have completely disappeared, while [OIII] 5006 is still
conspicuous, stronger than H_gamma and so are HeII 4686, HeI 5876, [NII] 5755
and, in the ultraviolet, the two [NeIII] lines 3967, 3689. They are still present,
although very weak, on the last spectra of the nova, taken in August.
A more detailed description of the spectra obtained at Asiago from 1959
to 1968 and the complete discussion of the material will be given in a
forthcoming paper.
DISCUSSION
Bakos: Is the spectroscopic orbit of the system known?
Rosino: No, the N type secondary was never distinctly observed before 1962.
Herbig: Why should only RS Oph among all recurrent novae have these sharp
stationary emission lines near maximum?
Rosino: The presence of a stationary evelope has been supposed by Wallerstein
and by the writer. However, it is not clear why other recurrent novae do
not display this peculiarity. The astonishing fact in the recurrent novae
is the presence, in advanced phase of evolution past maximum, of coronal
lines, which may eventually derive by collision with a stationary envelope
around these novae.
Mrs. Mayall: Do you have any information on the magnitude of RS Oph at
the time of your last spectrum (August 3) ?
Rosinó: No, we did not make photometric observations. The star was very
faint, probably 12-13.
Mrs. Mayall: The last observations I received before I left Cambridge were
made in July, and it was still below 12.5, - unusually faint.