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\IBVShead{4957}{20 September 2000}

\IBVStitle{Nova CI Aquilae in Decline}


\IBVSauth{Schmeja, S.; Armsdorfer, B.; Kimeswenger, S.}

\IBVSinsto{Institut f\"ur Astrophysik, Leopold-Franzens-Universit\"at 
Innsbruck, Technikerstr.\ 25, A-6020 Innsbruck, Austria\\
\indent e-mail: stefan.j.schmeja@uibk.ac.at, birgit.armsdorfer@uibk.ac.at, stefan.kimeswenger@uibk.ac.at}

\SIMBADobjAlias{CI Aql}{Nova Aql 1917}
\IBVSkey{recurrent novae - photometry, spectroscopy}

\IBVSabs{We present NIR photometries of the recurrent nova CI Aql and compare}
\IBVSabs{them to the visual light curve and to the one from the 1917 outburst.}
\IBVSabs{In addition, we obtained a spectrum of this nova.}

\begintext

The first known outburst of CI Aquilae was discovered on Heidelberg
plates recorded in June 1917 (Reinmuth 1925) and classified as a
possible nova by D\"urbeck (1987).  The measured maximum of the
outburst was $m_{\rm pg}\approx 11\mm$ and thus rather low, but
obviously the real maximum has been missed.  Williams (2000) found,
that this nova outburst had been recorded before on Harvard College
Observatory patrol plates from 1917; he measured a maximum of 8\fmm6.

On April~28, 2000 Takamizawa et al.\ (2000) discovered a probable nova
in Aquila with $m_{\rm V}\approx 10\mm$ which seems to be identical
with the 1917 nova. It reached its peak in the beginning of May at
about 8\fmm7, which is 7\fmm5 above the quiescent phase (Szkody
1994). If no outbursts were missed between 1917 and 2000, CI Aql has
the longest period (83~years) of all recurrent novae so far known.

We obtained near infrared photometries of this object using the DENIS
instrument (Epchtein et al.\ 1997) at the ESO 1-m telescope in La Silla
in the period from May~10 (about 12 days after outburst) to July~1,
2000. The images were taken simultaneously in all three bands Gunn-$I$
(0.82~$\mu$m), $J$ (1.25~$\mu$m) and $K_s$ (2.15~$\mu$m).  The exposure
time of each image was 9~seconds in $I$ and 11~seconds in $J$ and $K$.  Each
band was observed with five to seven images while moving the source
around in the field of view. This was used to eliminate errors due to
local flatfield effects, and to be able to obtain the sky background
using the iso-airmass median sky filtering. Thus, the intrinsic noise
within one set could be reduced to 0\fmm01. The fluxes of the nova
and two nearby comparison stars were measured using the SExtractor
software (Bertin \& Arnouts 1996). The magnitudes of the nova were
calculated relative to the two comparison stars, which were then
calibrated using the DENIS online zero points. The difference of the
measured magnitudes of the two comparison stars indicate the overall
quality of the measurements.  The errors are 0\fmm013 in $I$,
0\fmm031 in $J$ and 0\fmm023 in $K$.

\IBVSfig{22cm}{4957-f1.eps}{The light curve of CI~Aql (a) in 1917, 
(b) 2000 in the visual, (c) 2000 in near-infrared}

\vskip-11pt
Fig.\ 1 shows the light curve of CI~Aql (a) in 1917 as measured from
photographic plates, (b) in 2000 from visual data and (c) in 2000 from
our NIR data, all in the same scale. In each of the three cases the
abscissa covers a range of 120 days. The light curve (b) contains
mostly data contributed to the AAVSO database by amateur astronomers
(Mattei 2000). The filled symbols are photometries obtained using CCDs,
the open symbols are visual estimates and can therefore contain
significant errors. Especially after the end of July (MJD~51750), when
the magnitude of the nova has fallen below 13\mm, there is a pretty large
straggling of the data.  The light curve shows a rather slow decline
compared to other recurrent novae ($t_2\approx 25$~days).  The NIR
decline is of about the same order as the visual decline.  On June~27
(MJD~51722) we took three sets of images in intervals of about
20~minutes, and we could not find any short-term variation with this
period.

\IBVSfig{7cm}{4957-f2.eps}{The color indices $I-J$ and $J-K$ of CI Aql 
between June 14 and July 4, 2000}

While the nova gets bluer in $I-J$ (Fig.~2), the reddening in $J-K$
around June~25, 2000 (MJD~51720) may indicate the start of a dust
formation episode. $J-K$ was about 0\fmm36 on May~11/12.  The decrease
in $I-J$ is consistent with a recovery from the 0\fmm7 dip in the
visual (Fig.~1b) between MJD~51700 and 51720 caused by the formation of
a dust shell.  The increase of $J-K$ may be a sign of the onset of the
formation of very hot dust particles causing the next dip in the visual
light curve starting at MJD~51730.  Using typical dust formation radii
of $10^{11}$~m (Kimeswenger \& Koller 2000), this gives an estimate for
the velocity of the dust drift of a few hundreds of km/s.

Furthermore, we obtained a composite spectrum of CI Aql with the
Innsbruck 60-cm telescope on May~14, 2000 (MJD 51679), about two weeks
after the outburst of the nova (marked with an arrow in Fig.~1b).  We
took two or three spectra of every region with an exposure time of
1800~seconds each. The spectrograph was used with a grating of
240~lines/mm, giving a resolution of about 2.4~{\AA}/pixel on the CCD.
Different spectra of the same region show no differences, which
indicates that the error is smaller than 10\% of the continuum.
Therefore, the various features at the emission lines are real and not
caused by noise.  The spectrum, shown in Fig.~3, shows a flat continuum
from 4000 to 9000~{\AA} and strong emission lines (1.5 to 9 times the
continuum).  The FWHM range from 3400~km/s (in case of H$_\alpha$) up
to 7400~km/s (in case of [NII]).  The emission line profiles vary
significantly between the different species:  the lines with higher
velocities are asymmetrical and show multiple peaks, the lines with
lower velocities exhibit a single asymmetric peak.

\bigskip

{\it Acknowledgements:}
This work was supported by the FWF project P11675-AST.
We thank the DENIS consortium (PI N. Epchtein, Observatoire de la C\^ote 
d'Azur, Nice) for being able to do these additional observations.
We have used data from the AAVSO International Database, based on observations 
submitted to the AAVSO by variable star observers worldwide.

\IBVSfig{9cm}{4957-f3.eps}{Composite spectrum of CI Aql taken on 
May 14,~2000}

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