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Absorptional bands of several molecules (, CN, TiO,
etc.) are often present in the spectra of post-AGB stars (see,
for example, Hrivnak 1995; Bakker et al. 1997). However,
molecular emission features are only very rarely observed
in the optical spectra of PPNe. One such example is RAFGL2688
(the Egg Nebula) for which Crampton et al. (1975) observed
emission features of the
molecule in a medium
resolution spectrum. On the other hand, it is well known that
cometary nuclei spectra show prominent Swan band emission.
In both spectra of IRAS04296 we have discovered strong
emission in the (0;0) and (0;1) bands of the Swan system of
the molecule. On Figs. 2-4 we
present a comparison between the spectrum of IRAS04296
(observed on February, 26, 1997) and that of the Hale-Bopp comet
(observed on March 30, 1997 with the same spectrometer) around
bands (0;1), (0;0) and (1;0), respectively. From
Figs. 2-4 it is clear that emission band
(1;0) at 4735Å is absent in the spectrum of IRAS04296 while
the bands (0;1) at 5635Å and (0;0) at 5165Å are reliably
measured.
Hrivnak (1995) obtained the spectrum of IRAS04296
inside the blue spectral region, 3872-4870ÅÅ,
therefore he could not observe emission features of
at
5165 and 5635ÅÅ.
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To understand the observed ratios between different bands, we
have estimated the temperature function for monochromatic
coefficient of absorption per molecule () for
the Swan bands in the ``just overlapping'' approximation (JOA,
Golden 1967). For the microturbulent velocity
this approximation works well near the
band heads. Values of
for band heads of (0;0)
at 5165Å and (1;0) at 4735Å coincide within 0.2dex
for the temperatures range 3000-7000K, while
for the band (0;1) at 5635Å is
systemically lower by about 0.6dex. Taking into account these
relations between
's for different band heads
and since we do not observe the 4735Å band in RAFGL2688
and IRAS04296, we can conclude that it is impossible to
describe the intensity ratios of
emission bands for
these objects by means of the equilibrium vibrational
temperature in the 3000-7000K range.
To explain emission bands intensities for comets the mechanism
of resonance fluorescence has been proposed (Zanstra 1928, Swings
1941). In that case population of vibration-rotational levels
for the molecule is described by the Boltzmann approximation,
however the value of T in the exponent no longer has the
meaning of equilibrium temperature but it is a distribution
parameter only. We suggest that the same mechanism could be
responsible for the observed emission bands of IRAS04296.
However, it is clear from Figs. 2-4
that there are significant differences in the equivalent widths
of the emission bands in the spectra of our supergiant and of
the Hale-Bopp comet nucleus. They could be explained by a
difference of radiation fluxes which illuminate molecules in these objects. The temperature of IRAS04296
(
around 6300K) is sufficiently higher than that for
the Sun, therefore the band (1,0) at 4735Å for IRAS04296
should be stronger than that for the Hale-Bopp comet nuclei.
However, Fig.4 shows the opposite behaviour. It
could mean that radiation field of IRAS04296 which excite the
molecules is strongly reddened by matter located
between its photosphere and the region which produces the
emission.
Together with the emission bands of the Swan system (Klochkova
et al., 1997b) absorption bands of the Phillips system (1:0),
(2;0), (3;0)have been revealed in the spectrum of IRAS04296
(Bakker et al., 1997). Let us try to explain this phenomenon
within the resonance fluorescence mechanism ordinary used to
interpret comets' spectra. At first approximation, we can adopt
that the vibrational distribution has to be corresponding to the
effective temperature of a star illuminating a circumstellar
envelope if vibrational transitions in the low triplet state of
a homonuclear molecule are strictly forbidden. But even when
interpreting comets' spectra such an approach appears to be too
poor. The intensity distributions for different systems of bands
and for bands of individual systems of the resonance fluorescence
of the molecule have been considered in papers by
Krishna Swamy, O'Dell (1977, 1979, 1981). The intensities of
bands have been calculated taking into account the excitation of
the Swan, Ballick-Ramsay and Fox-Herzberg triplet systems,
Phillips and Milliken singlet systems as well as singlet-triplet
transitions in low states. It has been shown, in particular,
that at the value of the moment of singlet-triplet transitions
and at the heliocentric distance of a
comet d=1a.u. the ratios of intensities of sequences
in the Phillips system to the intensity
of sequence
of the Swan system is equal to
0.094, 0.11 and 0.04, correspondingly (Krishna Swamy, O'Dell,
1981). This agrees well with results of measurement of comets'
spectra. Using these results of Krishna Swamy, O'Dell
(1981), we may suppose that the intensity of main bands of the Swan
system is ten times higher than that in the Phillips system.
Now consider the case of IRAS04296. Let us add such an emission
spectrum of the on the stellar continuum.
In order to observe the emission bands of both the Swan and the
Phillips systems over the continuum in such a combined spectrum,
the stellar flux at
= 5165Å must be at least 10 times
higher than near
= 7720Å.
From Kurucz's (1979) tables it follows that the ratio of the fluxes
near these wavelenghts for the Sun (the emitter in the case of comets) is
equal to
. For the model with
= 6300K this ratio is equal to
. From the real spectral energy distrubution observed for
IRAS04296 (Kwok 1993) the ratio of the fluxes is essentially smaller:
. Therefore,
the conditions to observe the absorption bands of the Phillips
system and the emission bands of the Swan system may arise
inside the circumstellar envelope of IRAS04296.