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\IBVShead{5604}{25 February 2005}

\IBVStitle{The dwarf nova RX Volantis in Quiescence}

\IBVSauth{Schmidtobreick, L.$^1$; Tappert, C.$^2$; Carver, A.J.$^3$; Baes, M.$^1$; Vidal Perez, E.$^2$}

\IBVSinst{European Southern Observatory, Casilla 19001, Santiago 19, Chile}
\IBVSinst{Departamento de Astronom\'{\i}a y Astrof\'{\i}sica, 
          Pontificia Universidad Cat\'olica, Casilla 306, Santiago 22, Chile}
\IBVSinst{Cerro Tololo Inter-American Observatory, 
          Casilla 603, La Serena, Chile}

\SIMBADobj{RX Vol}
\IBVStyp{UGSU}
\IBVSkey{stars: spectroscopy - stars: variables: cataclysmic variables}

\IBVSabs{We present the first optical spectrum of the dwarf nova RX Vol.} 
\IBVSabs{The system is found at a magnitude of R=20.0. Its spectrum is}
\IBVSabs{dominated by Balmer lines and HeI in emission, as expected for}
\IBVSabs{a low mass transfer system situated below the period gap.}
\IBVSabs{We tentatively derive a disc temperature around 8000K with moderate}
\IBVSabs{density. It is seen at low inclination.}


\begintext

The variable has been found as S\,6185 Vol by Hoffmeister (1963).
He also provided the finding chart which is given by Downes et al. (2001).
Munari \& Zwitter (1998) tried to take an optical spectrum with the ESO 1.5\,m 
telescope, but found the system to be too faint. They give an upper limit for
its brightness of 22\mm.
In May 2003, the system has been observed in outburst, reaching a magnitude
of about 15. Superhumps where detected with a period of 
0.06117\,d which showed a brightening near the termination of the outburst,
and a regrowth before the start of the final, rapid decline (Kato et al. 2003).

We performed spectroscopic observations using the
ESO Faint Object Spectrograph and Camera (EFOSC2) at
the 3.6\,m telescope on La Silla, Chile.
Three spectra, each of 30\,min exposure time, have been
obtained on 2005-02-15 starting at 02:47 UT using
grism \#6 and a $1\arcs$\ slit.

Standard reduction has been performed with IRAF.
The BIAS has been subtracted and the data have been divided by a flat field,
which was normalised by fitting Chebyshev functions of high order
to remove the detector specific spectral response.

The three spectra have been combined and then
optimally extracted (Horne, 1986).
Wavelength calibration yielded a final FWHM resolution of
1.2\,nm and a spectral range of 390\,nm to 790\,nm.
The spectrum has been corrected for the instrument function
and was flux--calibrated using the spectrophotometric standards LTT\,2415,
LTT\,3218, and LTT\,3864. From the variation between the three standard stars
we estimate the uncertainty of the flux--calibration as 3\%. 

The finding chart in Fig. 1 shows two stars at the position
of RX\,Vol with a separation of 2\farcs3. 
We first took the spectrum of the brighter one in the west, 
which turned out to be an early K-type star. The spectrum discussed in 
this paper is the one of the fainter star, which is marked in the chart.

\vspace*{-0.4cm}

\IBVSfig{8cm}{5604-f1.eps}{The R image of a 2\arcs $\times$ 2\arcs\ region around RX Vol. North is up, east is left. The one star of the older finding charts actually resolves into two, the fainter one being the variable.}
\IBVSfigKey{5604-f1.eps}{RX Vol}{finding chart}

This spectrum is plotted in Fig. 2. It is
dominated by the Balmer lines and He\,I lines in emission.
Also present, but very weak, is Fe\,II at 516.9\,nm. No indication
for any high excitation lines like He\,II are found.
The properties of the identified emission lines are listed in
Table 1.

\IBVSfig{9cm}{5604-f2.eps}{The optical spectrum of RX\,Vol, dominated by Balmer lines and He\,I in emission is typical for a dwarf nova in quiescence.}
\IBVSfigKey{5604-f2.eps}{RX Vol}{spectrum}

\vspace*{-0.4cm}

\begin{table}[t]
\caption{Measured line width, computed velocity, and measured
line flux and equivalent width are given for the major emission
lines in the spectrum of RX\,Vol.}
\centerline{
\begin{tabular}{l c c c c c}
 \noalign{\smallskip}
\hline
\hline
 \noalign{\smallskip}
Transition & $\lambda$ [nm] & FWHM [nm] & $v_{\rm rot} \sin i$ [$\rm km\, s^{-1}$] & F [$10^{-18}$\,W\,m$^{-2}$] & $-W$ [nm] \\
 \noalign{\smallskip}
\hline
 \noalign{\smallskip}
H$\alpha$   & 656.2 & 1.7245 &  789 & 0.875(4) & 12.57(8) \\
H$\beta$    & 486.1 & 1.5903 &  981 & 0.503(3) & 6.40(4) \\
H$\gamma$   & 434.0 & 1.5521 & 1072 & 0.337(7) & 3.97(7) \\
H$\delta$   & 410.2 & 1.4650 & 1072 & 0.241(5) & 2.91(3) \\
He\,I       & 447.1 & 1.6221 & 1088 & 0.072(3) & 0.85(3) \\
He\,I       & 492.1 & 1.1239 &  685 & 0.047(2) & 0.60(2) \\
He\,I       & 587.6 & 1.8518 &  946 & 0.180(9) & 2.35(9) \\
He\,I       & 667.8 & 1.6041 &  721 & 0.101(8) & 1.53(10) \\
He\,I       & 706.5 & 1.9725 &  837 & 0.085(7) & 1.38(15) \\
 \noalign{\smallskip}
\hline
\hline
\end{tabular}}
\end{table}

In order to derive information on the possible temperature range of
the disc of RX\,Vol, we have measured the Balmer decrement, which
is defined as ratio of line intensities $\rm H_\alpha : H_\beta : H_\gamma$.
In the case of RX Vol, we find $\rm H_\alpha/H_\beta = 1.74$ and 
$\rm H_\gamma/H_\beta = 0.67$.
A comparison of these ratios, as well as the equivalent widths of the
lines  with the model data
from Williams (1991) yields moderately high temperatures and densities. We
find the best match for a disc temperature of 8000\,K,
a density Log$N = 12.5$, and an inclination of 34\deg. A temperature up 
to 10000\,K is still in agreement with our data but yields a slightly 
lower density and higher inclination.

The high Balmer decrement, the presence of H\,I and He\,I emission lines
and the absence of
high excitation lines agree very well with the classification of RX\,Vol as a
dwarf nova of SU\,UMa subtype.

On average we find a projected
rotation velocity of about 1000\,km/s. From this moderate value, we conclude
that RX Vol is seen at rather low inclination. This agrees with the average
line profile, a single peak, as well as with the relatively 
high values of the equivalent widths. However, the Balmer lines seem 
to be slightly broader than the He\,I lines.
We have previously found a similar effect for the emission lines
of other cataclysmic variables, eg. AG\,Hya (Tappert \& Schmidtobreick, 2005)
and V842\,Cen (Schmidtobreick et al, 2005).
It probably indicates that the lines origin in different regions of the
accretion disc.

We measured the actual brightness of the star on our acquisition file 
(Fig. 1). By comparison with the magnitudes of
USNO 0237-0126168, 0237-0126141, 0236-0127843, and 0236-0127895,
we derive R = 20\fmm0(1).

\references

Downes R.A., Webbink R.F., Shara M.M., Ritter H., Kolb U., Duerbeck H.W., 2001,
{\it PASP}, {\bf 113}, 764, living edition.
%\BIBCODE{2001PASP..113..764D}

Hoffmeister C., 1963, {\it Veroeff. Sternw. Sonneberg}, {\bf 6}, 1
%\BIBCODE{1963VeSon...6....1H}

Horne K., 1986, {\it PASP}, {\bf 98}, 609
%\BIBCODE{1986PASP...98..609H}

Kato T., Nogami D., Moilanen M., Yamaoka H., 2003,
{\it PASJ}, {\bf 55}, 989
%\BIBCODE{2003PASJ...55..989K}

Munari U., Zwitter T., 1998, A\&AS, {\bf 128}, 277
%\BIBCODE{1998A&AS..128..277M}

Schmidtobreick L., Tappert C., Bianchini A., Mennickent R.E., 2005,
A\&A in press

Tappert C., Schmidtobreick L., 2005, IBVS {\bf 5587}, 1
%\BIBCODE{2005IBVS.5587....1T}
 
Williams G.A., 1991, {\it AJ}, 101, 1929
%\BIBCODE{1991AJ....101.1929W}


\endreferences

\end{document}

