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\IBVShead{4661}{19 January 1999}

\IBVStitle{The spectrum of FG~Sagittae in 1998}


\IBVSauth{T.~KIPPER$^1$, V.G.~KLOCHKOVA$^2$}

\IBVSinst{Tartu Observatory, T\~oravere, 61602, Estonia, e-mail: tk@aai.ee}
\IBVSinst{Special Astrophysical Observatory RAS, Nizhnij Arkhyz, 357147, Russia, e-mail: valenta@sao.ru}

\SIMBADobj{FG Sge}
\IBVStyp{ R~CrB }
\IBVSkey{spectroscopy}
\IBVSabs{The spectra obtained during a very deep minimum in 1998 are described.}
\IBVSabs{The spectrum consists mostly of narrow emission lines of rare earths}
\IBVSabs{and C$_2$ Swan bands in emission.}

\begintext

The interest in the variable star FG~Sge has considerably increased
after 1992 when it underwent dramatic photometric variations not seen
before. These variations have continued up to the present time.
Wallerstein (1990) and Jurcsik (1993) were the first who noted
similarities between low--amplitude variations and sharp brightness
declines of FG~Sge and those of R~CrB stars.  At present, the
universally accepted paradigm is that in the case of FG~Sge we witness the
result of a final helium shell flash after the star had departed from AGB
(Iben 1984). After the discovery of another late helium flash star ---
Sakurai's object (Nakano, Benetti \& Duerbeck 1996) the interest to
both stars is higher still. For the latest reviews see Kipper (1996)
and Gonzalez et al.\ (1998).

We have continued our routine monitoring of FG~Sge and starting from
1996 also Sakurai's object trying to obtain some spectra every year in
order to detect possible changes in their spectra. Two sets of spectra
of FG~Sge were obtained in 1998, on June,~19 and July,~11 with the
prime focus echelle spectrometer of RAS 6--m telescope. The resolution
of these spectra is 12,000. The spectral coverage is $\lambda \lambda
4200\div 7700$ for the first set and $\lambda \lambda 4700\div 8600$
for the second one. Both sets are usable redwards from $\lambda 4800$.
The reduction of the spectra  was performed using the image reduction
system IRAF\IBVSfm.
 At the time of our observations a deep brightness drop was in an
advanced stage and the star was as faint as it was during the previous
deep minimum 2 years ago when Gonzalez et al. (1998) observed rich
emission--line spectrum. The light--curve for dates near to our
observations is depicted in Fig.~1. The data for this figure were taken
from the AAVSO International Database (Mattei 1998). The moments of our
observations are indicated by vertical lines.

\IBVSfoot{IRAF is distributed by the National Optical
Astronomy Observatories, which are operated by the Association of
Universities for research in Astronomy, Inc., under cooperative
agreement with the National Science Foundation.}

The spectra of the first set which were observed about 60 days after
the onset of brightness decline are very rich emission--line spectra
with narrow lines.  The rich photospheric absorption spectrum
characteristic of light maxima is absent. Due to moderate resolution
the lines are not fully resolved but are not wider than 29~km\,s$^{-1}$.
Such a spectrum is referred to as E1 by Alexander et al.\ (1972). In
R~CrB stars the E1 lines are usually blueshifted by some 10~km\,s$^{-1}$
relative to stellar radial velocity.  We, however, found, that these
lines give the velocity $v_{\rm r}=41\pm 3$~km\,s$^{-1}$ close to the
velocity we found from absorption lines during relative light maxima
$v_{\rm r}=44.4$~km\,s$^{-1}$ (Kipper \& Kipper 1993). The lines
correspond mostly to low excitation ($\epsilon_{\rm u}<$~4~eV) of
singly ionized rare earths and few lines of Fe\,I, K\,I, Na\,I, Mg\,I, Ca\,I
and Ca\,II. Selected regions of the spectra are illustrated in Fig.~2
and Fig.~3.

The lines of Ti\,II, which are usually observed in the spectra of R~CrB
stars in minima, were not identified in FG~Sge spectrum at this
resolution.  Of usually strong Sc\,II lines only few weak ones were
found.  In the second set of spectra, obtained 20 days later, the
emission lines are much weaker or absent. The remaining lines were
initially stronger with the tendency of lower excitation lines
($\epsilon_{\rm u}<$~3~eV) being more persistent.

\IBVSfig{7.5cm}{4661-f1.ps}{Light curve of FG~Sge for summer 1998 
(Mattei 1998). Open symbols indicate the upper limits. Vertical lines indicate the
moments of spectral observations}

Two of Ca\,II ($\epsilon_{\rm u}=3.15$~eV) IR triplet lines at $\lambda
8498$ and $\lambda 8542$, which are close to the red wavelength limit
of our second set of spectra, remained quite strong with the line ratio
1:2.1 corresponding to optically thick case.

The Mg\,I~b triplet lines at 5167.3, 5183.6 and 5183.6 \AA , which
usually belong to broad line category during R~CrB stars minima, are
narrow and weaken considerably between the two observation sets.

C$_2$ Swan bands of $\Delta v=0,1,2$ sequences are strongly in emission
in our first set of spectra. We could not, however, see the bands of CH
and CN. In the case of FG Sge this phenomenon was first observed by
Gonzalez et al.\ (1998) during the deep brightness decline in May, 1994
and then again in June, 1996. In the case of V854~Cen (R~CrB star) Rao
\& Lambert (1993) found that C$_2$ bands in emission were not
rotationally resolved and the bandheads were somewhat blueshifted. No
such shift is observable in our spectra and some rotational structure
of bands is visible. In the second set of spectra the C$_2$ bands are
considerably weaker and the $\Delta v=2$ sequence is no more visible.

The only broad lines in the spectra are the Na\,I~D doublet lines and
K\,I doublet lines at 7664.9 and 7699.0 \AA. The Na\,I~D lines are also
the most prominent features in the entire visible spectrum.Their
profile is depicted in Fig.~3. The interstellar
absorption, which has been present in all FG~Sge spectra observed
so far, is indicated in the figure.  Broad and sharp emission features
are visible. Both broad and narrow emission components are nearly at the
photospheric velocity. The half--width of the broad emission is around
180~km\,s$^{-1}$.  The K\,I doublet shows the same behaviour with broad
and narrow components.

\IBVSfig{7.5cm}{4661-f2.ps}{A part of the spectra of FG~Sge on June,~19
(full line), and on July,~11, 1998 (dotted line). The relative flux is
given in continuum units}

\IBVSfig{7.5cm}{4661-f3.ps}{The same as in Fig.~2 for another wavelength
interval}

\vfill\eject

\IBVSfig{7.5cm} {4661-f4.ps}{Na\,I doublet in the spectrum of FG~Sge on
June,~19 (full line), and on July,~11, 1998 (dotted line)}

Some of the lines formed in FG~Sge nebula (He~1--5) are also visible.
Part of those lines (H$_\alpha$, H$_\beta$, $\lambda 6548.05$~[N\,II])
seem to be contaminated by stellar (``chromospheric'') emission lines
and their intensities are decreasing as the other stellar emission
lines do. Several lines ( $\lambda 5007.57$~ [O\,III], $\lambda
6583.45$~[N\,II], $\lambda 6716.4$ and $\lambda 6730.8$ of [S\,II]) are
quite free from stellar emission lines and their intensity does not
change during our observations.  All nebular lines and especially the
lines with stellar contribution show two--peaked profiles with the red
component weaker than the blue one. The radial velocity of the nebula
inferred from these lines is $45\pm 4$~km\,s$^{-1}$, in good accordance
with the stellar velocity, and the expansion velocity of the nebula
$33\pm 1$~km\,s$^{-1}$ is again close to the value of $35\pm
1$~km\,s$^{-1}$ found by Gonzalez et al.\ (1998).

\medskip
{\it Acknowledgements.} In this note, we have used, and
acknowledge with thanks, data from the AAVSO International Database.
T.K. acknowledges support by Estonian Science Foundation grant 3166.

\references

Alexander, J.B., Andrews, P.J., Catchpole, R.M., et al., 1972, {\it
MNRAS}, {\bf 158}, 305

Gonzalez, G., Lambert, D.L., Wallerstein, G., et al., 1998, {\it ApJS},
{\bf 114}, 133

Iben, I.Jr., 1984, {\it ApJ}, {\bf 277}, 333

Jurcsik, J., 1993, {\it Acta Astronomica}, {\bf 43}, 353

Kipper, T., 1996, {\it ASP Conf.\ Ser.}, {\bf 96}, 329

Kipper, T., Kipper, M., 1993, {\it A\&A}, {\bf 256}, 389

Mattei, J.A., 1998, Observations from the AAVSO International Database,
private communication

Nakano, S., Benetti, S., Duerbeck, H.W., 1996, {\it IAU Circ.}, 6322

Rao, N.K., Lambert, D.L., 1993, {\it AJ}, {\bf 105}, 1915

Wallerstein, G., 1990, {\it ApJS}, {\bf 74}, 755

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