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\IBVShead{4749}{16 August 1999}

\IBVStitletl{The Spectrum, Period, and Proper Motion of V893 Scorpii}

\IBVSauth{J. R. Thorstensen}

\IBVSinsto{Dept.\ of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755-3528, USA\\ e-mail: john.thorstensen@dartmouth.edu}

\SIMBADobj{V893 Sco}
\IBVStyp{ SU UMa }

\begintext

The dwarf nova V893 Sco was recently recovered by Kato et al.~(1998). Because its identification was obscure for so long, little is known about its outburst characteristics. I obtained spectra in 1999 June with
the 2.4 m Hiltner telescope at MDM Observatory on Kitt Peak; details of the instrumentation, reduction and analysis were essentially identical to those 
in Thorstensen et al.~(1998). There are 30 exposures of $\sim 180$ s each.

Fig.~1 shows the mean spectrum, and Table 1 lists the spectral features detected. He\,II $\lambda$ 4686 is somewhat stronger than usual for a dwarf nova, reminiscent of the X-ray emitter VZ Pyx (Remillard et al.~1994, Thorstensen 1997), and indeed Kato et al.~(1998) note that V893 Sco is the likely counterpart of a ROSAT bright source. The emission lines are all double-peaked, with separation $\sim 1200$ km\,s$^{-1}$ in He\,I $\lambda$5876. The dips in the central cores of the emission lines appear somewhat deeper than usual, with the cores of HeI extending down to the continuum in some exposures. The FWHM of the H$\alpha$ emission is 43 \AA . The continuum flux, which is uncertain by $\sim 30$ percent, implies $V = 15.5$. 

I measured radial velocities of H$\alpha$ (Table 2) using a convolution method (Schneider and Young, 1980). The convolution function used had
positive and negative Gaussians of 14 \AA\ FWHM separated by 56 \AA . This emphasized the steep sides of the line profile. Fig.~2 shows the period search `residual-gram' (Thorstensen et al.~1996). It indicates a period near 0.0760 d, though a daily cycle count alias at 0.0822 d is possible. The Monte Carlo test of Thorstensen \& Freed (1985) indicates that the 0\fday0760 period is preferred at the $\sim 98$ per cent confidence level, so the choice is fairly secure but not definitive. Table 3 gives sinusoidal fits to the two aliases, of the form 
$$v(t) = \gamma + K \sin [2 \pi (t - T_0)]$$
Fig.~3 shows the velocities folded on each of the two best periods.

In addition to the spectra I acquired three 15-second $V$-band direct frames 1999 June 7.25 UT. Fig.~4 shows a finding chart. A fit of this image to the USNO A2.0 catalogue (Monet et al.~1996) and the Digital Sky Survey revealed significant proper motion since the plate from which these were derived (epoch 1977.5). The implied motion is 0.067 arcsec\,yr$^{-1}$ in position angle 223 degrees, with an uncertainty of approximately 0.015 arcsec\,yr$^{-1}$.

All dwarf novae with periods similar to V893 Sco are SU UMa stars, which show superoutbursts and superhumps. As the photometric properties of V893 Sco are explored, it is very likely that it too will follow this pattern.
The substantial proper motion and bright apparent magnitude both suggest a relatively small distance.

\newpage

\IBVSfig{9.9cm}{4749-f1.ps}{Mean spectrum of V893 Sco at minimum light.}

\IBVSfig{9.9cm}{4749-f2.ps}{Periodogram of the H$\alpha$ velocities.}

\newpage

\IBVSfig{9.4cm}{4749-f3.ps}{Velocities folded on the best period (upper panel) and on the second-best alias (lower panel). All points are shown twice for
continuity.}

\IBVSfig{9.4cm}{4749-f4.ps}{Finding chart for V893 Sco derived from
a 1999 June $V$-band image. Orientation and scale are indicated, as
is the position derived from a fit to USNO A2.0 stars.}

\newpage

\begin{center}

{\normalsize Table 1: Spectral Features}
\vskip 3mm
\begin{tabular}{lcc}
\hline
Line & EW (\AA ) & $F_{\lambda}^{\rm a}$ \\
\hline
H$\gamma$ & 41 & $1.2 \times 10^{-13}$ \\
HeI 4471 & 8: & $2.4 \times 10^{-14}$ \\
HeII 4686 & 9 & $2.5 \times 10^{-14}$ \\
H$\beta$ & 59 & $1.5 \times 10^{-13}$ \\
HeI 4921 & 4: & $1.2 \times 10^{-14}$ \\
HeI 5015 & 7 & $1.7 \times 10^{-14}$ \\
HeI 5876 & 18 & $3.9 \times 10^{-14}$ \\
H$\alpha$ & 84 & $1.7 \times 10^{-13}$ \\
HeI 6678 & 11 & $2.1 \times 10^{-14}$ \\
HeI 7027 & 7 & $1.1 \times 10^{-14}$ \\
\hline
\end{tabular}
\vskip 1mm
{\normalsize $^{\rm a}$ Uncertain by $\sim 30$ per cent, 
in erg\,cm$^{-2}$\,s$^{-1}$\,\AA$^{-1}$}

\vfil

{\normalsize Table 2: Radial Velocities of H$\alpha$}
\vskip 3mm
\begin{tabular}{crcrcrcr}
\hline
HJD$^{\rm a}$ & $v^{\rm b}$ & HJD & $v$ & HJD & $v$ & HJD & $v$ \\
\hline
$1338.7901$ & $ 14$ & $1340.8314$ & $ 2$ & $1342.6608$ & $ -14$ & $1343.8856$ & $ 3$ \\ 
$1338.8547$ & $ -10$ & $1341.6885$ & $-107$ & $1342.7574$ & $ -39$ & $1343.8897$ & $ 23$ \\ 
$1339.7534$ & $-135$ & $1341.7262$ & $-178$ & $1342.8513$ & $-174$ & $1343.8937$ & $ -70$ \\ 
$1339.7890$ & $ -90$ & $1341.7510$ & $ 16$ & $1343.8654$ & $ -33$ & $1343.8978$ & $ -41$ \\ 
$1339.8784$ & $-120$ & $1341.7914$ & $-183$ & $1343.8694$ & $ -21$ & $1343.9018$ & $ -41$ \\ 
$1340.6832$ & $ -35$ & $1341.8900$ & $ -60$ & $1343.8734$ & $ 6$ & $1343.9059$ & $-112$ \\ 
$1340.7247$ & $-175$ & $1342.6540$ & $ 11$ & $1343.8775$ & $ -14$ & \\ 
$1340.7893$ & $-127$ & $1342.6574$ & $ -48$ & $1343.8816$ & $ 24$ & \\ 
\hline
\end{tabular}
\vskip 1mm
{\normalsize $^{\rm a}$ Heliocentric Julian date of mid-integration, minus 2450000. $^{\rm b}$ Units of $v$ are km\,s$^{-1}$.}

\vfil

{\normalsize Table 3 Fits to H$\alpha$ Velocities}
\vskip 3mm
\begin{tabular}{ccccc}
\hline
$T_0^{\rm a}$ & $P$ (d) & $\gamma$ (km\,s$^{-1}$) & $K$ (km\,s$^{-1}$) & 
$\sigma$ (km\,s$^{-1}$)\\
\hline
$1341.8858 \pm 0.0012$ & $0.07605 \pm 0.00006$ & $91 \pm 9$ & $-85 \pm 6$ & 30 \\
$1341.8903 \pm 0.0014$ & $0.08220 \pm 0.00007$ & $92 \pm 11$ & $-86 \pm 7$ & 36 \\ 
\hline
\end{tabular}
\vskip 1mm
{\normalsize $^{\rm a}$ Heliocentric Julian date minus 2450000.}
\end{center}

\vfil

\references

Kato, T., Haseda, K., Takamizawa, K., Kazarovets, E. V., \& Samus, N. N. 1998,
IBVS No.~4585

Monet, D. et al.\ 1996, USNO--SA2.0, (US Naval Observatory, Washington DC)

Remillard, R. A., Bradt, H. V., Brissenden, R. J. V., Buckley, D. A. H., Schwartz, D. A., Silber, A., Stroozas, B. A., and Touhy, I. R. 1994, ApJ, 428, 785

Schneider, D. and Young, P. 1980, ApJ, 238, 946

Thorstensen, J. R., \& Freed, I. W. 1985, AJ, 90, 2082 

Thorstensen, J. R., Patterson, J., Thomas, G., \& Shambrook, A. 1996,
PASP, 108, 73

Thorstensen, J. R. 1997, PASP, 109, 1241 

Thorstensen, J. R., Taylor, C. J., \& Kemp, J. 1998, PASP, 110, 1405

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