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\IBVShead{5508}{12 March 2004}

\IBVStitletl{Spectroscopic classification of a suspected SU\,UM\lowercase{a} Star}{ in Libra}

\IBVSauth{Schmidtobreick, L.$^1$; Galli, L.$^{2,3}$; Whiting, A.$^3$; Tappert, C.$^4$}

\IBVSinst{European Southern Observatory, Casilla 19001, Santiago 19, Chile }
\IBVSinst{Colorado College, 14 East Cache La Poudre St., 
          Colorado Springs, CO 80903 USA. }
\IBVSinst{Cerro Tololo Inter-American Observatory, 
          Casilla 603, La Serena, Chile}
\IBVSinst{Grupo de Astronom\'{\i}a, Universidad de Concepci\'on,
          Casilla 160--C, Concepci\'on, Chile}

\IBVSobj{ASAS 153616-0839.1}
\IBVStyp{ UG }
\IBVSkey{spectroscopy}

\IBVSabs{We present optical spectroscopy of the suspected}
\IBVSabs{SU\,UMa type variable ASAS 153616-0839.1, recently discovered}
\IBVSabs{in Libra (Pojmanski, G. 2004, VSNET alert 7982).}
\IBVSabs{The object shows a very blue continuum, with hydrogen and He\,I lines}
\IBVSabs{in absorption, only H$\alpha$ showing an emission core. He\,II}
\IBVSabs{and higher ionised lines are present in emission.}
\IBVSabs{The spectrum is typical for a SU\,UMa type system in early outburst}
\IBVSabs{phase and thus confirms the photometric classification.}
\
\begintext

\noindent

During the REU (Research Experiences for Undergraduates) observation campaign 
at CTIO (Cerro Tololo Inter-American Observatory)
in February 2004, the VSNET alert 7982 by Pojmanski was received, indicating
the outburst of a new eruptive star (with designation ASAS\,153616-0839.1) 
in Libra.  Light curves taken all 
over the world revealed that the object showed periodic variability with
probably increasing period (Kiyota, 2004), which could thus been interpreted 
as growing superhumps with a final period of \hbox{$P_{sh} = 0.06501(3)$\,d} 
(Kato, 2004).

We observed the object in low resolution
with the  R-C spectrograph at the 1.5\,m telescope at CTIO, see Table \ref{obs}
for the details.
Standard data reduction was performed with IRAF
including bias and flatfield correction and wavelength calibration.
The standard star HR\,3454 has been used to correct for the instrument curve. 
The spectra have a resolution of
1.5\,nm and cover the range of 370--970\,nm.  Since we did not 
find variations in the spectra (apart from the increasing brightness), 
we averaged them to increase the S/N.
All subsequent analysis of the data has been done using MIDAS.
\begin{table}[b]
\caption{\label{obs} Observational details of the individual spectra.}
\centerline{
\begin{tabular}{c c c c}
\\
\hline
\hline
 \noalign{\smallskip}
Date & UT start & Exp-time [s]\\
 \noalign{\smallskip}
\hline
 \noalign{\smallskip}
2004-02-09 & 08:25:57.5 & 900 \\ 
2004-02-09 & 08:42:09.4 & 900 \\ 
2004-02-10 & 07:40:11.9 & 900 \\ 
2004-02-10 & 08:56:04.4 & 900 \\ 
 \noalign{\smallskip}
\hline
\hline
\end{tabular}}
\end{table}

\IBVSfig{9cm}{5508-f1.eps}{The spectrum is dominated by a very blue continuum
and the hydrogen lines in absorption. The feature at $\lambda = 418$\,nm is
due to a CCD artefact. The spectrum thus confirms the classification as a
dwarf nova in outburst.\label{lib_spec}}
\IBVSfigKey{5508-f1.eps}{ASAS 153616-0839.1}{spectrum}

In Fig.\,\ref{lib_spec}, the average spectrum of the two nights is plotted.
Note that the flux values should be taken as relative values only, 
the absolute flux calibration is just a rough estimate. The spectrum is
given as observed, without correction for interstellar reddening.
It shows a very blue continuum, with the characteristic dwarf nova lines 
(hydrogen and helium) present as broad
absorption features, indicating an optically thick disc of high temperature. 

We have tried to fit a $\lambda^\alpha$ power law to the blue part of 
the spectrum, leaving out the line features, and determined $\alpha = -4.5(2)$. 
The spectral energy distribution is thus much bluer than expected for
a steady state disc; for comparison, Lynden--Bell (1969) calculates the 
maximum slope to be $\lambda^{-7/3}$.
Furthermore, the fit showed clearly that a single power law is not
sufficient to fit even the blue slope of the continuum. This is not surprising,
if we assume the system to be of SU\,UMa type. The superhump during
the outburst phase gives an additional blue contribution to the 
continuum, which is thus no longer disc dominated.

\IBVSfig{6cm}{5508-f2.eps}{In the normalised spectrum, the weak lines
like H$\alpha$, He\,I (447.1\,nm), He\,II (468.6\,nm) blended with 
Bowen-blend, or N\,IV/C\,IV(583\,nm) blended with Na\,D in absorption become 
more apparent. The line at $\lambda = 628.3$\,nm is of atmospheric origin.\label{norm_spec}}
\IBVSfigKey{5508-f2.eps}{ASAS 153616-0839.1}{spectrum}

To check for other, weaker lines and in search for emission
features, we normalised the spectrum using high order splines.
A part of this normalised spectrum is plotted in Fig.\,\ref{norm_spec}. 
The emission core
of H$\alpha$ is clearly visible in the broad absorption trough,
as is emission of He\,II at 468.6\,nm blended with the Bowen emission feature
at 464\,nm. He\,I is present in absorption at $\lambda = 402.6$\,nm (outside
plotted range), 447.1\,nm, 492.1\,nm (barely visible in the red flank of
H$\beta$) and 667.8\,nm. 
A broad emission feature centred on 584\,nm is probably caused by C\,IV/N\,IV
blended with He\,I and the nearby Na\,D line in absorption. The half width of 
this emission feature is about 3400\,km/s indicating that the origin of these
highly ionised lines either lies in the boundary layer between the accretion 
disc and the white dwarf surface or at least in the very inner region of the
accretion disc.
The FWHM and the equivalent width of the main absorption features are given in
Table\,\ref{lines}.

\begin{table}[b]
\caption{\label{lines} The full width of half maximum  and the equivalent width 
(both in nm) are listed for the main absorption lines in the spectrum.} 
\centerline{
\begin{tabular}{c c c c c c c c c c c c}
\\
\hline
\hline
 \noalign{\smallskip}
 & H$_9$ & H$_8$ & H$\epsilon$ & H$\delta$ & H$\gamma$ & H$\beta$ 
 & He\,I 402.6 & He\,I 447.1& Na D\\
% & & & & & & & 402.6 & 447.1 & 667.8 & \\
 \noalign{\smallskip}
\hline
 \noalign{\smallskip}
FWHM & 2.38 & 2.92 & 3.03 & 4.06 & 4.13 & 5.75 & 1.03 & 2.45 & 2.18 \\
W    & 0.22 & 0.40 & 0.47 & 0.99 & 0.72 & 0.79 & 0.015& 0.088& 0.071\\
 \noalign{\smallskip}
\hline
\hline
\end{tabular}}
\end{table}

We have compared our spectrum to those of other dwarf novae in outburst.
Mennickent et al. (2002) published an analysis of V592\,Her during its 
outburst in August 1998 and their average spectrum looks very similar
to ours.  However, it shows a slightly less blue continuum
($\alpha = 3.50(1)$), the absorption lines are narrower, and emission
cores of He\,I are found in the absorption troughs. This behaviour can be 
explained with ASAS\,153616-0839 having a higher temperature and thus a
thicker disc. 

Similar spectra have also been obtained of WZ\,Sge during  the first days of
its super-outburst in 2001 (Nogami \& Iijima, 2004). Except for the 
double--peaked line--profile of He\,II and H$\alpha$, whose separation of
780 km/s would be  
below the detection limit of our resolution, and the absence of He\,II at
541.1\,nm in our spectrum, their average spectrum from
day 1--5 resembles the spectrum of ASAS\,153616-0839. However, the spectra of
WZ\,Sge evolved very rapidly during the outburst. After day 5, emission cores 
became visible for H$\beta$ and later also for the other blue Balmer lines
as well as for He\,I, 
while the emission features of He\,II and N\,IV/C\,IV disappeared.
The similarity to the first spectra would thus indicate that our observations
have been taken during the beginning of the super-outburst. This is consistent
with the photometric observations, as the superhumps were still evolving in
this epoch.

To summarise this investigation, we would like to state that the 
spectra of ASAS\,153616-0839
confirm its photometric classification as a cataclysmic variable of SU\,UMa 
type. 
Although rather blue, they are consistent with spectra observed during 
early outburst phases of similar systems. The FWHM of the lines indicate
that the system is seen at high inclination. As it has not been known before
the outburst, but should be reasonably bright also in quiescence, it will 
present an interesting object for further studies.

\references
Kato T., 2004, vsnet-campaign-dn 4126,7,8

Kiyota S., 2004, vsnet-campaign-dn 4123,4

Lynden--Bell D., 1969, Nature 223, 690

Mennickent R.E., Tappert C., Gallardo R., Duerbeck H.W., Augusteijn T., 2002,
A\&A 395, 557 %\BIBCODE{2002A&A...395..557M}

Nogami D., Iijima T., 2004, PASJ, in press (astro-ph/0401292)

Pojmanski G., 2004, vsnet-alert 7982 
\end{document}

