Non-Periodic Phenomena in Variable Stars
IAU Colloquium, Budapest, 1968
PRELIMINARY REPORT ON THE SPECTRUM OF NOVA VUL 1968
A. MAMMANO, R. MARGONI and L. ROSINO
Astrophysical Observatory of Asiago
ABSTRACT
Spectroscopic observations of Nova Vul 1968 made at Asiago from April
to August are reported in this paper. The nova belongs to the fast type.
Absorption systems with velocities of - 680, -800, -1380 and -2500 km/s
have been observed. Some peculiarities of the emission components of CaII
lambda lambda 8498, 8542 and 8662 (mult. 2) are pointed out. The evolution of
the nova from the premaximum to the nebular stage is shortly described.
1. Nova Vulpeculae 1968 has been discovered by Alcock on April 15
1968 during its rise to the maximum, reached on April 17 (Candy, 1968).
A comparison of blue photographs obtained at Asiago with the Palomar Sky
Atlas indicates that the prenova was fainter than 16m, in agreement. with
Herbig's (1968) estimate of 16.5m. Herbig pointed out that the prenova is one
component of a double star and that it is not the remnant of Nova Vul 1670.
The spectra discussed here have been obtained with the Cassegrain spectrograph
attached to the 122 cm reflector of Asiago. The following combinations
have been mostly used: Camera III, dispersion 40 A/mm at H_gamma, Cameras VII
and VI, associated with the Carnegie Intensifier, dispersion 45 and 60 A/mm
at H_gamma. Kodak blue, red and infrared sensitive material was employed.
The photoelectric light curve of the nova, kindly supplied by Prof. P. Tempesti,
Teramo Observatory (1968) is reproduced in Fig. 1; short vertical lines
indicate the epochs at which the most significant spectra have been taken.
2. At the moment of its maximum brightness, on April 17, the nova is still
characterized by a premaximum spectrum, with many absorption lines, shortwards
displaced of about -670 km/s. The lines are rather broad and diffuse and the
spectrum can be classified as a peculiar F5-F8. Noteworthy
is the presence of the infrared triplet 8498, 8542, 8662 of CaII (mult.2)
relatively weak, while H and K are very strong and wide. The two absorption
bands of OI at 7774 and 8446 have about the same intensity. Emission features
are represented by H_alpha with moderate intensity and by a weak component
of CaII 8542 and OI 7774 (fig. 2).
On April 18, one day after maximum, the nova develops the principal
spectrum (-1380 km/s), together with emission components of H, FeII, OI
(7774, 8446), etc. The number and strength of the emission bands increase on
April 19, when emission components of CaII 8498 and 8662 also appear (Fig. 2).
The infrared triplet of CaII is therefore present with emission and absorption
components; the emission became later stronger than the absorptions.
On the following day wide emission bands appear also for H and K, the
absorptions remaining, however, much stronger than the emissions.
Fig. 1. Light curve of N Vul 1968 obtained at Teramo Observatory
photoelectrically (courtesy of Prof. P. Tempesti). Short
vertical lines indicate the epochs of the most important
spectroscopic observations at Asiago.
Fig. 2. Microphotometric tracings of two near-infrared spectra of N Vul 1968,
obtained with camera III. Original dispersion 400 A/mm at lambda 8500 A.
From now on, the evolution of the absorption and emission lines will
be separately described.
3. The premaximum system persists for several days and is still recorded
when the nova has weakened of 0.8m (Apr. 24-25): On May 1, at 1.3m below
maximum, it is no more present. Its last recorded velocity has been -800 km/s.
The principal system has, at first, a velocity of -1380 km/s. Later it
splits into two components, whose velocities are -1000 and -2000 km/s,
the second being the strongest (diffuse enhanced system).
The Orion system, faintly recorded on May 6 (2.4m below maximum)
strengthens on May 15, when the nova has fallen by three magnitudes. At this
phase three absorption systems are recorded the principal system, with
velocity -1270 km/s; the diffuse enhanced system at -1850 km/s and the
Orion system at -2330 km/s. A few faint hydrogen components, if correctly
identified, may belong to a shell having a velocity of -4000 km/s.
The identification, however, is doubtful.
The absorption spectrum, which is still strong at the beginning of June,
slowly fades out and disappears on the second half of June. On June 3 hydrogen
components have been observed at -2500 km/s, together with the faint suspected
hydrogen lines at -4300 km/s.
Sharp stationary components of H and K, probably due to interstellar
absorption, have been recorded from the beginning.
4. The behaviour of the emission spectrum until April 20 has been
already described. On April 24 the forbidden line [OI] 6300 is faintly recorded,
while the broad blend of NIII at 4640 appears. The infrared emission bands
OI 8446, CaII 8542 and 8498 are partially resolved by using Camera IV (140 A/mm
at lambda 8600): all are displaced to the violet, because of absorptions
taking place at their red side.
On May 1, faint emissions due to [0I] are recorded at lambda 6364, 6300,
5578, while [NII] 5755 is just visible as a trace. By this time the OI 8446
emission has become stronger than OI 7774.
On May 8, 2.4m below maximum, the [0I] and [NIII 5755 lines strengthen and a
faint band due to NII appears at 5680. The [OI] flash occurs near May 14, when
also HeI 5876 and other NIT lines appear. On May 14, three magnitudes below
maximum, [OI] 6300 surpasses in intensity all of the other emission lines,
except H_alpha and H_beta.
The [NII] flash occurs on June 3 (3.7m from maximum). [NII] 5755 reaches
about the same intensity as H_beta, followed in order of decreasing strength
by [OI] 6300, 6364 and 5578. On June 9 the spectrum is dominated by wide
emission bands of H, HeI, NII, FeII, NIII. The forbidden line of [OIII] at 4363
is faintly recorded, in blend with H_gamma. Althought the identification is
somewhat uncertain, HeII 4686 is probably present in a wide blend at the red
side of 4640.
By this time the star has reached the nebular stage. On June 21 the N_1 band
of [OIII] at 5006 is prominent. In the infrared, the strong blend 7325 of [OII]
is recorded; OI 8446 has become about ten times as strong as OI 7774 and a
faint band due to HeI emerges at 7064.
A further strengthening of the nebular emissions is noticed on the spectra
taken on June 27: H_beta, however, is still brighter than [OIII] 5007, although
the band at 5007 appears stronger because of the contribution of HeI 5018.
On July 6 the N_1 line becomes definitely stronger than H_beta, while [OIII]
4363 has about the same intensity of H_gamma. On July 21, the most conspicuous
emissions are: H_alpha, [OIII] 5006, H_beta, [OI] 6300 and 6364, [NII] 5755,
[OIII] 4958, HeI 5874, [OIII] 4363 much stronger than H_gamma, NIII 4640,
NII 5680. Finally, in the last spectrum obtained on August 14, 1968 when the
nova has fallen by 5.4m, 5007 [OIII] is about two times stronger than H_beta.
The emission bands are very wide, the expansion velocity derived by their
halfwidth being of the order of 1200 km/s. It may be observed that the
halfwidths of the emission bands have increased with time, passing from
700 km/s when the principal spectrum was first recorded, to 900 km/s at
the appearance of the diffuse enhanced system and to 1100-1200 km/s at
the nebular phase.
Fig. 3. Spectrum of N Vul 1968 at nebular phase, obtained through
the Carnegie-RCA intensifier. Dispersion 60 A/mm at H_gamma.
5. Although the nova is still far from minimum, we may attempt to
sketch some of its main properties:
a) It is clear from the photometric and spectroscopic evolution that
N Vul 1968 is a fast nova, without strong deviations from the normal type. From
the velocity of decline (3 magnitudes in 30 days) a mean absolute magnitude
at maximum of -7.8m can be derived; the amplitude is 12^m and the photographic
magnitude at minimum about +4, as in other normal novae.
b) Infrared spectra taken near maximum have shown for the first time
in a nova the absorption lines of the CaII infrared triplet. Since N Vul 1968
is a common nova it is likely that this infrared triplet may be visible also
in the other normal novae, provided that infrared spectra are taken very
close to maximum. Now, the fact that the infrared triplet of CaII appears
in emission stronger than H and K is quite puzzling, because both multiplets
have in common their upper term and the transition probabilities for H and
K are much higher than for the infrared triplet lines. The same problem
apply with a few long period variables and the accepted explanation is that
of Herbig (1952), according to which, a self-absorption, due to a layer lying
above the emitting region, reduces the emission intensities at H and K much
more than at the infrared triplet of CaII. In the case of Nova Vulpeculae
1968 there is however no clear evidence of such an absorbing layer above the
emitting region.
Worthy of mention is also the great variation of the intensity ratio of
OI 8446 to OI 7774, which changed from 0.5 near the maximum of the nova,
to about 20, during the last recorded nebular stage. It is likely that the
theory of Pagel (1960) may account for the behaviour of this ratio
during the first days close to maximum, while Bowen's fluorescence mechanism
may become more important during the nebular stage.
A detailed study of the spectral evolution of this interesting nova will
be published in a forthcoming paper.
REFERENCES
Candy, M. P., 1968, I. A. U. Circ. 2066.
Herbig, G. H., 1952, Astrophys. J. 116, 369.
Herbig, G. H., 1968, I. A. U. Circ. 2072.
Pagel, B. E., 1960, Ann. Astrophys. 23, 850.
Tempesti, P., 1968, Private communication.