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\IBVShead{5138}{17 July 2001}

\IBVStitletl{The Light Curve and Red Spectrum of}{ Nova V4643 S\lowercase{gr} in Early Decline}

\IBVSauth{Bruch, A.}

\IBVSinsto{Laborat\'orio Nacional de Astrof\'{\i}sica, C.P.\ 21, 37500-000
Itajub\'a -- MG, Brazil, 
e-mail: albert@lna.br}

\SIMBADobjAlias{V4643 Sgr}{Nova Sgr 2001}
\IBVStyp{ N }
\IBVSkey{Nova - spectroscopy}

\IBVSabs{The early decline light curve of Nova V4643 Sgr and spectra in the}
\IBVSabs{range of H-alpha 19 and 68 after outburst maximum are presented.}
\IBVSabs{The t_2 (t_3) times of 4.8 (8.6) days make V4643 Sgr a very fast}
\IBVSabs{nova. The MMRD suggests an (upper limit) to the distance of}
\IBVSabs{2900 +/- 170 pc. The profile of the H-alpha emission}
\IBVSabs{line consists of a very broad, almost flat component and a narrower}
\IBVSabs{central emission (the equivalent width of the latter decreasing}
\IBVSabs{more rapidly than that of the former) suggesting a geometry of the}
\IBVSabs{ejecta consisting of an equatorial ring and polar blobs and a high}
\IBVSabs{orbital inclination of the underlying binary system.}


\begintext

V4643~Sgr (Nova Sgr 2001) was discovered by Liller (2001) on 2001,
Feb.~24.  The light curve compiled from magnitude estimates published
in IAU Circulars and observations of members of the V.S.S. of the
R.A.S.N.Z.  (Fig.~1) shows it to be a very fast nova. A parametric fit
of a second order polynomial to the first part of the light curve
(shown as a dashed line in Fig.~1) indicates that it takes $t_2=4.8$
days ($t_3 = 8.6$ days) for the nova to decline 2 (3) magnitudes from
maximum light, assuming the first point on the light curve
corresponding to outburst maximum.

\IBVSfig{5cm}{5138-f1.ps}{Early decline visual light curve of
V4643~Sgr. Dots are magnitude estimates published in IAU Circulars;
triangles are observations of members of the V.S.S. of the
R.A.S.N.Z\@.  The dashed line is a parametric second order polynomial
fit to the data. The chevron indicates the epoch of the last negative
observation before discovery}

The absolute magnitude of V4643~Sgr during maximum can be estimated
using the maximum magnitude--rate of decline (MMRD) relation of which
many versions are published in the literature (Schmidt-Kaler 1957, Pfau
1976, de Vaucouleurs 1978, Cohen 1988, Capaccioli et al.\ 1989, Della
Valle 1991).  Disregarding the slight dependence on the photometric
band, and rejecting a discrepant result based on the Capaccioli et
al.\ relation, all others yield a mean absolute magnitude at maximum of
$M = -9\fmm04 \pm 0\fmm08$. The error of the mean is significantly
smaller than the error of the individual relations. Thus, there is no
point in preferring one relation over the other and I adopt the mean as
the best result. Together with the apparent magnitude of the first
point in the light curve, $m = 8\fmm1$ (to which I arbitrarily assign
an error of $0\fmm1$), this yields a distance of $d = 2900 \pm 170~{\rm
pc}$. Since the interstellar extinction towards V4643~Sgr is unknown
and consequently is not considered here, this distance should be
regarded as an upper limit.

The various determinations of the absolute magnitude $M_{15}$ of novae
15 days after maximum (Buscombe \& de Vaucouleurs 1955, Schmidt-Kaler
1957, Pfau 1976, de Vaucouleurs 1978, Cohen 1985, van den Bergh \&
Younger 1987, van den Bergh 1988, Capaccioli et al.\ 1989) predict
$M_{15} = -5\fmm53 \pm 0\fmm24$. The light curve of V4643~Sgr at this
epoch exhibits substantial scatter permitting only to roughly estimate
$m_{15} = 11\fmm7 \pm 0\fmm5$ as the apparent magnitude 15 days after
maximum. This is $3\fmm6$ fainter than the first point on the light
curve which thus corresponds to $M = -9\fmm1$, well compatible with the
results obtained from the MMRD relation, suggesting that the first
observation of V4643~Sgr has been obtained close to maximum light.  The
error being significantly larger than in the case of the MMRD relation,
$m_{15}$ cannot yield an improved distance estimate.

\IBVSfig{10cm}{5138-f2.ps}{Emission line profiles of H$\alpha$ (stronger
line:  2001, March 16; fainter line: 2001, May 4) and He~I
$\lambda$~5876~{\AA} (plus N~II $\lambda$~5932~{\AA}; 2001 March 16) in
the spectra of V4643~Sgr on a velocity scale. Vertical bars indicate
the location of features for which radial velocities are quoted in the
text}

Spectra of V4643 Sgr in the range of H$\alpha$ were obtained at the
1.6-m telescope of the Laborat\'orio Nacional de Astrof\'{\i}sica,
Brazil on 2001, March 16 (JD~2451984.80; day 19 after maximum; two
exposures; 20 min total integration time) when the visual magnitude had
dropped to $m_{\rm v} \sim 11.5$, and on 2001 May 4 (JD~2452033.75; day
68; three exposures; 45 min total integration time; visual magnitude
unknown, probably $\sim 14^m$). A Cassegrain spectrograph equipped with
a thin, back-illuminated SITeSI003AB CCD was used to record the
spectra. The instrumental setup yielded a resolution (FWHM) of
2.1~{\AA}. The spectral coverage ranged from 5843~{\AA} to 6624~{\AA}
on March 16, and from 5924~{\AA} to 6664~{\AA} on May 4.

The spectra are dominated by strong and complex H$\alpha$ emission
which exhibits appreciable differences between the two observing
epochs. The profiles, normalized to the continuum, are shown on a
velocity scale in Fig.~2 (left). During both nights H$\alpha$ consists
of a very broad, almost flat component underlying a much narrower
strong central emission. The most obvious difference between the
observing epochs is the line strength (with respect to the continuum)
which is much stronger earlier in the outburst than later on. On March
16 the narrow component exhibits two peaks which, however, at
$-220~{\rm km\, sec}^{-1}$ and $+330~{\rm km\, sec}^{-1}$ (these and
the subsequently quoted velocities are marked by small vertical bars in
Fig.~2) are not symmetrical to the rest wavelength. The full width of
the central emission is about $3200~{\rm km\, sec}^{-1}$ (ranging from
$-1500~{\rm km\, sec}^{-1}$ to $1700~{\rm km\, sec}^{-1}$).  The broad
component reaches out to $\pm 3900~{\rm km\, sec}^{-1}$ from the rest
wavelength. It appears to be essentially flat with some low scale
structure, in particular a red peak at $3280~{\rm km\, sec}^{-1}$. The
overall line profile resembles very much the profile of emission lines
of Nova Ophiuchi 1998 a few days after outburst (Lynch et al.\ 2000).

On May 4 the total width of the broad component is practically
unchanged but the equivalent width has decreased by a factor of more
than 2. Details of its structure are largely preserved, with the red
peak now appearing at a slightly higher radial velocity: $3390~{\rm
km\, sec}^{-1}$. The equivalent width of the narrow component has
decreased by a factor of more than five, much more than the broad
component. It exhibits more fine-structure than during the earlier
epoch: there are peaks (or shoulders) at $-110~{\rm km\, sec}^{-1}$,
$+100\, {\rm km\, sec}^{-1}$ and $\pm 500~{\rm km\, sec}^{-1}$.
Furthermore, the total width of the narrow component has decreased to
$2370~{\rm km\, sec}^{-1}$ (ranging from $-1270~{\rm km\, sec}^{-1}$ to
$1100~{\rm km\, sec}^{-1}$).

The only other spectral features unambiguously present in the spectrum
of March 16 are emissions of He~I $\lambda$ 5876~{\AA} and N~II
$\lambda$ 5932~{\AA} as well as absorption lines of Na~I $\lambda
\lambda$ 5890, 5896~{\AA}, shown in the right frame of Fig.~2 on a
velocity scale centered on the rest wavelength of the helium line (note
the different intensity scale as compared to the left frame of the
figure). This spectral range was only observed on March 16.  Just as in
H$\alpha$ the structure of the He~I $\lambda$ 5876~{\AA} emission is
double-peaked with a peak separation consistent with that seen in
H$\alpha$.  The broad, flat component is not discernible, probably
because its blue edge is beyond the observed spectral range and the red
edge is beneath the N~II $\lambda$ 5932~{\AA} emission. This makes it
difficult to properly define the continuum level in this wavelength
range. The nitrogen line is faint and thus noisy, but it is probably
safe to say that it does not show a double peak. Therefore, its place
of origin is not the same as that of the hydrogen and helium lines.
Sharp Na~I $\lambda \lambda$ 5890, 5896~{\AA} lines cut into the red
flank of the He~I line. They are clearly of interstellar origin. They
radial velocity is $-48~{\rm km\, sec}^{-1}$.  The spectrum of May 4
shows (in a range not covered on March 16) a faint emission of He~I
$\lambda$ 6678~{\AA}, seen as a small hump at the extreme right of the
left frame of Fig.~2.

The equivalent widths (EWs) of the emission lines were measured and are listed 
in Table~1. In the case of H$\alpha$ the EW of the entire line as well as of
the broad and narrow components are listed. The EW of the sodium absorption
lines cannot reliably be measured because they are superposed upon the steep
red flank of the helium emission. 


\begin{table}
\begin{center}
\caption{Equivalent width (in {\AA}) of emission lines observed in V4643 Sgr}
\vspace{1ex}

\begin{tabular}{lcc}
\hline
Line & 2001, March 16 & 2001, May 4 \\
\hline
H$\alpha$ (entire line)        & 
219\phantom{.0}                & 
\phantom{0}68\phantom{.0}      \\
H$\alpha$ (broad component)    & 
\phantom{0}95\phantom{.0}      &
\phantom{0}45\phantom{.0}      \\
H$\alpha$ (narrow component)   & 
123\phantom{.0}                &
\phantom{0}23\phantom{.0}      \\
He~I $\lambda$ 5876 {\AA}      & 
\phantom{0}15\phantom{.0}      &
\phantom{00}--\phantom{.0}     \\
He~I $\lambda$ 6678 {\AA}      & 
\phantom{00}--\phantom{.0}     &
\phantom{00}0.8                \\
N~II $\lambda$ 5932 {\AA}      & 
\phantom{00}1.9                &
\phantom{00}--\phantom{.0}     \\
\hline
\end{tabular}
\end{center}
\end{table}

Qualitatively, the morphology of the H$\alpha$ emission can be
explained if the mass was not ejected spherically during the nova
outburst but mainly in the equatorial plane of the white dwarf and in
the polar regions. Such an outburst geometry is suggested by the
nebular remnants of numerous other novae (although Slavin et
al.\ (1995) found this type of morphology preferable in slower novae).
If the inclination of the rotation axis to the line of sight is high
(but not high enough for the approaching part of the matter ejected in
the equatorial plane to prevent the view of the receding part) the
broad component should be interpreted as being emitted by an equatorial
ring. The narrow component is then due to matter ejected along the
polar axis which (even if the true velocity is comparable to that of
the equatorially ejected matter) has a smaller radial velocity due to
the higher angle with respect to the line of sight. The two peaks in
the narrow component which are clearly present early on in the outburst
then indicate emission from opposite polar ejecta.
\vspace{1em}

\noindent
{\bf Acknowledgments:} I am grateful to Dr.\ Frank Bateson and the observers
of the Variable Star Section of the Royal Astronomical Society of New
Zealand for putting their observations of V4643~Sgr at my disposal.

\references

Buscombe, W., de Vaucouleurs, G., 1955, {\it Observatory}, {\bf 75},
170

Capaccioli, M., Della Valle, M., D'Onofrio, M., Rosino L., 1989, {\it
AJ}, {\bf 97}, 1622

Cohen, J.G., 1985, {\it ApJ}, {\bf 292}, 90

Cohen, J.G., 1988, {\it ASP Conf.\ Ser.}, {\bf 4}, 114

Della Valle, M., 1991, {\it A\&A}, {\bf 252}, L9

De Vaucouleurs, G., 1978, {\it ApJ}, {\bf 223}, 351

Liller, W., 2001, {\it IAU Circ.}, No.\ 7589

Lynch, D.K., Rudy, R.J., Mazuk S., Puetter, R.C., 2000, {\it ApJ}, {\bf
541}, 791

Pfau, W., 1976, {\it A\&A}, {\bf 50}, 113

Slavin, A.J., O'Brien, T.J., Dunlop, S.J., 1995, {\it MNRAS}, {\bf
276}, 353

Schmidt-Kaler, T., 1957, {\it Z.\ Astrophys.}, {\bf 41}, 182

van den Bergh, S., 1988, {\it PASP}, {\bf 100}, 8

van den Bergh, S., Younger, P.F., 1987, {\it A\&AS}, {\bf 70}, 125

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