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\IBVShead{5014}{16 January 2001}

\IBVStitletl{The EUV Source HD 52452:}{ Discovery of a Likely Triple System}

\IBVSauth{Messina, S.$^1$; Cutispoto, G.$^1$; Pastori, L.$^2$; Rodon\`o, M.$^{1,3}$; Tagliaferri, G.$^2$}
\IBVSinst{Catania Astrophysical Observatory, Via S.~Sofia 78, I-95125
Catania, Italy}
\IBVSinst{Brera Astronomical Observatory, Via Bianchi 46, I-22055
Merate, Italy}
\IBVSinst{Department of Physics and Astronomy, University of Catania,
Via S.~Sofia 78, I-95125 Catania, Italy}

\SIMBADobj{HD 52452}
\IBVStyp{ G5:V + G5:V + G5:V }
\IBVSkey{Photometry and Spectroscopy}
\IBVSabs{We present the results of a photometric and spectroscopic}
\IBVSabs{study of the EUV bright source HD 52452. This star results to be a}
\IBVSabs{triple system composed by a SB1 G5:V+G5:V close binary, which rotates}
\IBVSabs{with a period P=0.423 days and by a slower rotating G5:V companion.}

\begintext

HD 52452 (BD\,+26\deg1435 = SAO 78998, $V=7.96$) has been identified by
Mason et al.\ (1995) as the optical counterpart of the EUV bright
source RE~J70222+255054 detected in the Wide Field Camera all-sky
survey by ROSAT (Pounds et al.\ 1993). It is reported in the Tycho
Catalogue (TYC 1899 688 1) as a suspected variable with a parallax of
$17.0\pm6.4$ mas (Perryman et al.\ 1997) and it is classified as a G5
star in the SIMBAD database.  HD 52452 has been included since late
1994 in a program of spectroscopic and photometric observations aimed
at the classification of EUV stellar sources detected by EXOSAT and
ROSAT (Cutispoto et al.\ 1999, 2000).  In this paper we report on the
discovery of the optical variability of HD 52452 and on its inferred
spectral classification.

The $UBV$ photoelectric photometry presented here were collected
from 16 November 1994 to 23 February 1995 by the 80-cm  Automated
Photometric Telescope (APT-80) at the  {\it  M.~G.~Fracastoro\/}
station of Catania Astrophysical Observatory on Mt.~Etna (1725~m {\it
a.s.l.}).  The  APT-80 feeds a single channel charge-integration
photometer equipped with an  uncooled Hamamatzu R1414 SbCs
photomultiplier and  Johnson's  standard   $UBV$  filters.  The
differential photometry of HD 52452 ({\bf v}) was made using HD 52071
(K2III; $V=7.12$; $B-V=1.26$, $U-B=1.27$) as  comparison star ({\bf
c}), and HD 51530  (F7V; $V=6.21$; $B-V=0.51$, $U-B=-0.01$)  and HD
50692  (G0V; $V=5.77$; $B-V=0.60$, $U-B=0.05$) as check stars ({\bf
ck}$_1$ and {\bf ck}$_2$, respectively). Ten seconds integrations in
the $U$, $B$ and $V$ filters and an observing sequence {\bf
ck}$_2$--{\bf c}--{\bf ck}$_1$--{\bf c}--{\bf v}--{\bf v}--{\bf
v}--{\bf c}--{\bf v}--{\bf v}--{\bf v}--{\bf c} were adopted.  After
sky background subtraction, the measurements were corrected for
atmospheric extinction. Normal points were computed by averaging each
sequence of six ${\bf v-c}$ differential values   and transformed into
the $UBV$ Johnson standard system (Table~1). The  typical
standard deviations of the normal points are of the order of 0\fmm01 in
$V$ and $B$ filters and 0\fmm015 in the $U$ filter.  No significant light
variations were detected from the differential measurements of the
comparison and check stars. During the whole observing period these
stars were constant within about $\pm 0\fmm015$ in the $V$-band.
%\IBVSindexobj{HD 52071}
%\IBVSindexobj{HD 51530}
%\IBVSindexobj{HD 50692}

\begin{table}
\caption{Heliocentric Julian day, rotational phase, $V$ magnitude,
$B-V$ and $U-B$ colours of HD52452. Phases are reckoned from the
photometric ephemeris ${\rm HJD} = 2449672.0 + 0\fday42304\times E$}
\begin{center}
\begin{tabular}{ccccc}
\hline
 HJD & Phase & $V$ & $B-V$ & $U-B$ \\
\hline
 2449672.6213 &   0.468  &  8.005  &  0.691  &  0.221 \\
 2449682.6683 &   0.215  &  7.959  &  0.706  &  0.206 \\
 2449683.6579 &   0.554  &  7.986  &  0.720  &  0.221 \\
 2449699.6492 &   0.351  &  8.001  &  0.702  &  0.218 \\
 2449700.6261 &   0.660  &  8.042  &  0.702  &  0.227 \\
 2449701.4792 &   0.676  &  8.040  &  0.690  &  0.218 \\
 2449703.5877 &   0.660  &  8.032  &  0.698  &  0.234 \\
\hline
 2449752.5680 &   0.428  &  7.986  &  0.724  &  0.228 \\
 2449754.4465 &   0.868  &  8.125  &  0.718  &  0.213 \\
 2449756.4505 &   0.604  &  8.008  &  0.699  &  0.221 \\
 2449757.4588 &   0.987  &  8.070  &  0.723  &  0.238 \\
 2449761.4301 &   0.374  &  7.996  &  0.697  &  0.196 \\
 2449766.4177 &   0.162  &  7.957  &  0.705  &  0.223 \\ 
 2449768.4409 &   0.944  &  8.116  &  0.714  &  0.229 \\ 
 2449771.4734 &   0.112  &  7.972  &  0.690  &  0.214 \\
 2449772.4665 &   0.459  &  7.975  &  0.699  &  0.200 \\
 \hline
\end{tabular}
\end{center}
\end{table}

From the present data HD 52452 resulted to be variable with a
peak-to-peak amplitude of $\Delta V \simeq 0\fmm16$. The set of
photometric data  was analysed  using a Scargle--Press period search
routine (Scargle 1982, Horne \& Baliunas 1986) and a photometric
period  $P = 0.42304 \pm 0.00015$ day, with a {\it
false-alarm-probability\/} ${\rm FAP} = 8.1\%$, was found.
Fig.~1  shows the $V$-band,  $B-V$, and $U-B$ light curves
for the mean epochs 1994.92 (open triangles) and 1995.12 (filled
circles).  Phases are reckoned using the ephemeris ${\rm HJD} =
2449672.0 + 0\fday42304\times E$. The $V$-band light curve is double peaked
and shows no evidence of eclipses. Both $B-V$ and $U-B$ colours are
constant within the photometric precision.

HD 52452 was  observed spectroscopically with the McMath telescope
(Kitt Peak, AZ) in October 12, 14 and 17, 1994. The 12 and 14 October
high resolution spectra were collected in the Li I 6708 \AA\ region
(Fig.~2). These spectra show the existence of two
components: a rather fast ($v\sin i =14\pm2$~km\,s$^{-1}$) and a very
fast rotating  star ($v\sin i \geq 60$~km\,s$^{-1}$). The Li I line was
not detectable (${\rm EW}< 4$~m\AA). The 17 October spectrum was collected in
the H$\alpha$ region (Fig.~3): only the very fast rotating
component is  visible and the H$\alpha$ is partially filled-in. The
procedures of spectroscopic observation, reduction and analysis are
given  in Cutispoto et al.\ (1999, 2000).

The  lack of Li, implying that the star is not young, and the very high
rotation of one of the two visible components strongly suggest that the
very fast rotating star is  an  SB1 close binary system, whose high
rotation rate is attributable to tidal coupling.  Assuming for this SB1
component an inclination of the orbital plane $i<50$ degrees (which is
an upper limit for  the SB1 to be non eclipsing), and by using the
method described by Cutispoto et al.\ (1999, 2000), we infer that HD
52452 is  a triple system consisting of a G4V\,+\,late-G very fast
rotating SB1 and a G5:V slower rotating companion.  This spectral
classification ($M_V \simeq 3.8$, $D\simeq68$~pc) is in fairly
good agreement with the absolute magnitude of HD 52452 derived from the
distance listed in the Tycho catalogue ($D=59^{+35}_{-16}$~pc).

In order to compute the X-ray luminosity of HD 52452 we converted into
flux the Count Rate (CR) and Hardness Ratio (HR) values from RASS--BSC
(Rosat All Sky Survey--Bright Source Catalogue) using as conversion
factor ${\rm ECF = (8.31 + 5.30 HR)}\times 10^{-12}$~erg\,cm$^{-2}$
given by Fleming et al.\ (1995).  By adopting a distance of  68~pc,
the  X-ray luminosity, in the 0.2--2.5~keV energy band, turns out to be
$L_{\rm X}=5.0\times 10^{30}$~erg\,sec$^{-1}$.

\IBVSfig{10cm}{5014-f1.ps}{The $V$-band,  $B-V$, and $U-B$ light curves
for the mean epochs 1994.92 (open triangles) and 1995.12 (filled
circles).  Phases are reckoned using the ephemeris ${\rm HJD} =
2449672.0 + 0\fday42304\times E$}

\IBVSfig{8cm}{5014-f2.ps}{The October 12, 1994 spectrum of HD 52452 in
the Li 6708 \AA\ region.  The rather fast ($v\sin i = 14\pm
2$~km\,s$^{-1}$) and a very fast rotating  star's components  ($v\sin i
\geq 60$~km\,s$^{-1}$) are visible}

\IBVSfig{8cm}{5014-f3.ps}{The October 17, 1994 spectrum of HD 52452 in
the H$\alpha$ region. Only the very fast component is visible and the
H$\alpha$ line is partially filled-in}

\vskip-36pt
On the basis of the currently available data, we are confident that HD
52452 is a triple system consisting of a tidally coupled
G4V\,+\,late-G  SB1, which rotates with a period of $P\simeq 0\fday423$
and it is responsible for most of the observed optical variability, and
a G5:V companion. The  observed photometric variability is likely to be
attributable to the presence of cool spots on the photospheres of both
components of the SB1 system. However, a non negligible contribution to
the observed optical variability, though not  revealed by our
periodogram analysis, may come from the G5V star, whose rotation,
according to  the above mentioned {\it vsini}, spectral classification
and inclination of the orbital plane, is expected to be quite fast
($P\sim2\fday5$).


\IBVSedata{5014-t1.txt}

\references

Cutispoto, G., Pastori, L., Tagliaferri, G., et al., 1999, {\it A\&AS}, {\bf 138}, 87

Cutispoto, G., Pastori, L., Guerrero, A., et al.,  2000, {\it A\&A}, {\bf 364}, 205

Fleming, T.A., Molendi, S., Maccacaro, T., Wolter, A., 1995, {\it ApJS}, {\bf 99}, 701

Horne, J. H., Baliunas, S. L., 1986, {\it ApJ}, {\bf 302}, 757

Mason, K.O., Hassall, B.J.M., Bromage, G.E., et al., 1995, {\it MNRAS}, {\bf 274}, 1194

Perryman, M.A.C., and The Hipparcos Science Team, 1997, {\it ESA SP-1200}, 
Vol.\ 1-12, ESA Publ.\ Division, c/o ESTEC, Noordwijk, The Netherlands

Pounds, K.A., Allan, D.J., Barber, C., et al., 1993, {\it MNRAS}, {\bf 260}, 77

Scargle, J.D., 1982, {\it ApJ}, {\bf 263}, 835

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