In Fig. 1 the principal features formed by the molecular species C2, CO, and CN are displayed as separate spectra. Atomic features are also shown, as these are also present in Sakurai's Object (see Eyres et al. 1998; Geballe et al. 2002). As in the optical spectrum (Pavlenko et al. 2000), absorption of only a few molecular species accounts for the main features in the IR spectrum. Only the most abundant isotopic species of each molecule is shown. Of the less abundant isotopic species, only bands of 13CO have been detected in the infrared (Eyres et al. 1998).
The model spectra of Sakurai's Object display a strong dependence on
(Fig. 2).
In
general, the dependence of the IR SED on
is determined mainly by the
variations of the molecular densities with temperature. The band strengths
of CN, CO and C2 all increase as
decreases. Changes in the
continuum fluxes are much smaller. Similar effects are seen in model
optical spectra (Pavlenko & Yakovina 2000). However, there the molecular
bands are numerous, whereas in the infrared only the few strongest
vibration-rotation bands of CN, C2, and CO are prominent.
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Figure 1:
Model spectra of species that produced the
strongest absoption features in the 1.0-2.5 ![]() ![]() |
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Figure 2:
Dependence of the model IR spectrum on
![]() |
As can be seen in Fig. 3, the dependence of the spectrum on log g is generally rather weak. However, there are differences in the responses of different spectral regions. The strong molecular bands show rather weak dependence on log g, whereas the fluxes at 1.25-1.35, 1.60-1.75, 1.9-2.2 microns show more noticeable changes.
Previous abundance analyses of the spectra of Sakurai's Object and
related R CrB stars indicate microturbulent velocities in the range
5-8 km s-1 (cf. Asplund et al. 2000).
The value of
affects the spectral distribution, as is shown
in Fig. 4. The effect of
on the IR spectra of Sakurai's
Object is larger at the heads of molecular bands than elsewhere, because
the heads are formed by closely packed molecular lines whose overall
absorption is sensitive to
.
The main sources of line opacity in the model atmospheres
approximating Sakurai's Object in 1997 are molecular (Pavlenko et al.
2000). Thus it is not surprising that the optical spectra which match
Sakurai's Object respond weakly to changes in the hydrogen abundance.
This is in contrast to the behavior of models corresponding to the star a
year earlier (Asplund et al. 1997). Similarly, the model IR spectra of
Sakurai's Object for
= 5000-6000 K depend weakly on log N(H) (Fig. 5). The magnitude of the change in the spectrum when log N(H)
is changed from -2.42 (the Asplund et al. 1999 value for October
1996) to -0.97 (i.e, a change of 1.5 dex) is comparable (in a qualitative
sense) to lowering log g from 1 to 0 (Fig. 3).
Fits to the spectra of Sakurai's Object on April 21 and July 13 are shown
in Figs. 6 and 7. The long wavelength portion of
the H band is of special interest for the "carbon problem'', because the
strongest absorption bands of the C2 molecule, the Ballick-Ramsey
bands, occur just longward of 1.768 m. In the comparatively hot
atmosphere of Sakurai's Object log N(C) > log N(O) (Asplund et al. 1997,
1999) and the abundance of C2 depends mainly on the elemental abundance
of carbon. Therefore, these bands may provide the most accurate
determination of log N(C). The fits imply that the carbon abundance is in
the range log
.
The most likely value is 0.3 dex
higher than that found by Asplund et al. (1999). The accuracy of the
determination of log N(C) is limited mainly by the quality of the
molecular line list.
The effective temperatures that best fit the 1.0-2.0 m spectra in
1997 April and July are
K and
K,
respectively, indicating that the cooling evidenced by the dramatic
spectral changes seen between 1996 and 1997 (e.g., Geballe et al. 2002)
continued in 1997. Our estimated uncertainties in the above temperatures
are rather large, despite the comparatively good fits to the observed
spectra, because of questions concerning abundances, non-sphericity
effects, and dynamical phenomena, and because of contamination of the
spectra by dust emission (see below).
Emission by dust is evident in the 1997 spectra by the mismatch between
the synthetic and observed spectra longward of 2.0 m in
Figs. 6 and 7. The difference
between the observed and synthetic spectra is greater in the July
spectrum, attesting to an increase in the amount of dust. The thermal
emission from the dusty envelope overlaps the region of first overtone
bands of 12CO and 13CO at
2.3
m. Usually these
bands are used for determination of carbon abundances and isotopic ratios
(cf. Lazarro et al. 1991). The reduced equivalent widths of the CO bands
in July 1997 cannot be reasonably attributed to a large decrease in the
oxygen abundance, because (1) this is unlikely to have occurred in three
months and because the continuum shortward of the CO bands also shows an
excess. We note that in fitting spectra, the most frequent situation is
that the computed spectra have excess flux due to the deficit of known or
hypothetical opacities. To fit the observed spectra, opacities in the
model would need to be decreased at
2
m, an
unrealistic possibility.
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