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\IBVShead{4617}{3 August 1998}
\IBVStitle{HD 6628: a new active, single-lined spectroscopic binary}

\IBVSauth{L.C.~WATSON, K.R.~POLLARD, J.B.~HEARNSHAW}
\IBVSinsto{Mt John University Observatory, Univ.\ of Canterbury, Private Bag 4800,
Christchurch, New Zealand, e-mail: L.Watson@csc.canterbury.ac.nz}

\SIMBADobj{HD 6628}
\IBVSkey{spectroscopy}

\begintext

The variable star HD\,6628 (SAO\,166806, CPD\,$-23^{\circ}\,130$, HIP\,5227,
{\sl IRAS}\,01043$-$2307, RE\,J010649\,$-$225156; listed by {\sl Simbad}
as CS Ceti) has been previously listed (SAO catalogue) as a G5\,IV single star,
$M_{\rm V}=7.9$ and $d = 75$\,pc.  The {\sl Hipparcos} magnitude and
parallax (Schrijver 1997) are $M_{\rm V}=7.77$ and $\pi =
0\farcs 00756\pm 0\farcs 00108$ ($d = 132\pm 22$ pc).
Schrijver (1997) lists HD\,6628 as a variable single star with an amplitude of
variation in $V$ of 0.10 mag.\ and a period of 4.50 days, and a light curve has
been fitted (Grenon, 1997) to the data. That HD\,6628 is chromospherically
active was shown by EUV emission in both the ROSAT (Pounds {\it et al.}, 1991;
Pounds {\it et al.}, 1993; Mason {\it et al.}, 1995; Pye {\it et al.}, 1995;
Kreysing {\it et al.}, 1995) and EUVE (Bowyer {\it et al.}, 1994; Malina {\it et
al}, 1994; Bowyer {\it et al.}, 1996) surveys, and has been confirmed by
medium-resolution spectra of the H and K lines (Figure 1) obtained at the Mount
John University Observatory (MJUO) at Lake Tekapo, New Zealand.


High-resolution CCD spectra of HD\,6628 were obtained at MJUO between August
1993 and August 1996 using the 1\,m McLellan telescope (Nankivell and Rumsey,
1986) and \'{e}chelle spectrograph (Hearnshaw, 1977 and 1978).  Most of the
data are at H$\alpha$ which shows strong and variable emission, the line
often appearing entirely in emission.  A Th-Ar calibration spectrum was
obtained immediately after each stellar spectrum.  The spectra were reduced
using the ESO Munich Image Data Analysis System ({\sl MIDAS}), the dispersion
solutions being computed using standard {\sl MIDAS} procedures in the
``\'{e}chelle'' context.  Radial velocities were measured from metal lines,
principally from the $\lambda 6200$, $\lambda 6219$, $\lambda 6394$, $\lambda
6569$ and $\lambda 6750$ lines of Fe\,I, and the $\lambda 6768$ line of Ni\,I.
The mean velocities for the individual spectra are listed in Table I. The radial
velocities were analysed using the {\sl Orbsol} program as modified and supplied
to MJUO by Tsevi Mazeh of Tel Aviv University.  A period of $27.3316\pm 0.0079$
days was found and Figure 2 shows the radial velocities of Table 1 plotted
against the periodic phase.  The best-fit orbital parameters as determined by
the {\sl Orbsol} program are listed in Table 2.



The {\sl Hipparcos} parallax and $V$ magnitude imply an absolute magnitude for
the HD\,6628 system of $M_{\rm V}=2.16\pm 0.29$.  Such a
magnitude is consistent with the presence of two G5\,IV stars of $M_{\rm
V}=2.91$ each, but the fact that the spectra are single-lined and not
double-lined implies that one of the stars is several magnitudes fainter than
the other.  We therefore prefer the interpretation that the star responsible
for the observed metal lines is a G5\,IV star of magnitude in the range
$M_{\rm V}=2.5$ to 2.3, for example a star of greater than solar mass
which is now a bright subgiant, and its companion is a main sequence star,
possibly a spectral type F star.

\IBVSfig{10cm}{4617-f1.ps}{Medium-resolution spectrum of the H and K lines of
HD\,6628, obtained at MJUO, 1995 December 30.}

\IBVSfig{10cm}{4617-f2.ps}{Phased radial velocities of HD\,6628.}

\vfil\eject



\begin{table}[!h]
\caption{Radial velocities of HD\,6628, computed from high-resolution spectra
obtained at MJUO.}
\begin{center}
\begin{tabular}{|l|r|r|}\hline
H.J.D.        & Mean      & Standard \\
              & velocity  & deviation\\
              & (km/s)    & (km/s)\\ \hline\hline
2449204.20136 & $-$11.058 & 0.548\\
2449290.02947 & $-$11.173 & 1.285\\
2449539.02793 &    +2.355 & 0.360\\
2449579.20077 &   +47.712 & 0.967\\
2449622.11782 &    +8.425 & 0.905\\
2449639.92354 &    +7.869 & 0.544\\
2449641.99194 & $-$11.588 & 0.511\\
2449859.21730 &  $-$0.627 & 0.540\\
2449860.24754 & $-$11.270 & 0.954\\
2449916.12831 & $-$17.814 & 0.205\\
2449941.09675 &    +1.822 & 0.102\\
2450011.89198 &   +36.646 & 0.588\\
2450012.97834 &   +38.892 & 0.072\\
2450075.98908 &   +22.879 & 1.375\\
2450081.91063 & $-$20.505 & 3.043\\
2450111.94092 &    +1.621 & 2.029\\
2450263.14307 &   +46.259 & 2.251\\
2450294.23803 &   +29.388 & 1.042\\
2450295.13675 &   +16.519 & 1.781\\
2450308.07385 &   +17.792 & 1.032\\ \hline
\end{tabular}
\end{center}
\end{table}

\IBVSfig{8.5cm}{4617-f3.ps}{High-resolution spectrum of the H$\alpha$ line of
HD\,6628, obtained at MJUO, 1995 July 17.}




\vfil\eject

\begin{table}[!h]
\caption{Orbital parameters of HD\,6628 as computed by the {\sl Orbsol} program}
\begin{center}
\begin{tabular}{|l|r|}\hline
Parameter & Value\\ \hline\hline
$P$ (days)                  & $27.3316\pm 0.0079$\\
$\gamma$ (km/s)             & $18.38\pm 0.59$\\
$K$ (km/s)                  & $13.99\pm 0.80$\\
$e$                         & $0.293\pm 0.026$\\
$\Omega$ ($^{\circ}$)       & $116.1\pm 4.0$\\ \hline
Mass function ($M_{\odot}$) & $0.078\pm 0.012$\\
$a\sin i$ (Gm)              & $11.49\pm 0.30$\\ \hline
\end{tabular}
\end{center}
\end{table}


A high-resolution H$\alpha$ spectrum (Figure 3), obtained at orbital phase
0.08 referred to Figure 2, shows the red wing of an H$\alpha$ absorption
line having a radial velocity approximately 70--80 km/s greater than that of
the metallic lines of the spectrum ($-$17.814 km/s) and, blueward of it,
emission almost filling in a second absorption line.  The radial velocity of
the latter is measurable only very imprecisely, but it is of the same order
as that of the metal-lines spectrum, and confirms that the observed metal
lines and the H$\alpha$ emission belong to the G5\,IV subgiant.  The
unfilled-in red absorption wing has a profile characteristic of an F-type dwarf
and is attributed to the companion star.

\references
Bowyer, S., {\it et al.,} 1994, {\sl Astrophys.\ J. Suppl.,} {\bf 93}, 569

Bowyer, S., {\it et al.,} 1996, {\sl Astrophys.\ J. Suppl.,} {\bf 102}, 129

Grenon, M., 1997, {\sl The Hipparcos Catalogue} vol.\,12, ESA Publications,
Noordwijk.

Hearnshaw, J. B., 1977, {\sl Proc.\ ASA,} {\bf 3}, 102

Hearnshaw, J. B., 1978, {\sl Sky and Telescope,} {\bf 56}, 6

Kreysing, H.-C., {\it et al.,} 1995, {\sl Astron.\ \& Astrophys.\ Suppl.,} {\bf
114}, 465

Malina, R. F., {\it et al.,} 1994, {\sl Astron.\ J.,} {\bf 107}, 751

Mason, K. O., {\it et al.,} 1995, {\sl Mon.\ Not.\ R. Astron.\ Soc.,} {\bf 274},
1194

Nankivell, G. R., and Rumsey, N. J., 1986, {\it IAU Symposium}, {\bf 118}, 101,
{\sl Instrumentation and Research Programmes for Small Telescopes}, 
ed.\ J. B. Hearnshaw and P. L. Cottrell, Reidel, Dortrecht
%Changed: Wed Jun 15 10:14:49 MEST 2005

Pounds, K. A., {\it et al.,} 1991, {\sl Mon.\ Not.\ R. Astron.\
Soc.,} {\bf 253}, 364

Pounds, K. A., {\it et al.,} 1993, {\sl Mon.\ Not.\ R. Astron.\ Soc.,} {\bf
260}, 77

Pye, J. P., {\it et al.,} 1995, {\sl Mon.\ Not.\ R. Astron.\ Soc.,} {\bf 274},
1165

Schrijver, H., 1997, {\sl The Hipparcos Catalogue} vol.\,5, ESA Publications,
Noordwijk.

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