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\IBVShead{4576}{2 April 1998}

\IBVStitle{ A newly discovered  BY D\lowercase{ra}-type star: HD\,134319}

\IBVSauth{S. Messina$^1$, E. F. Guinan$^2$}\\

\IBVSinst{ Istituto di Astronomia, Universit\`a degli Studi di Catania, Viale A.Doria 6, 95125 Catania, Italy. E-mail: sme@sunct.ct.astro.it}

\IBVSinst{ Astronomy and Astrophysics Dept., Villanova University, Villanova PA, USA. E-mail: Guinan@ucis.vill.edu}

\SIMBADobj{HD134319}
\IBVStyp{ BY }


\begintext

HD\,134319 (G5V; V $\simeq$ +8.42 mag; B$-$V = +0.68) is a main-sequence
G-type star with very strong chromospheric  line emissions,
most likely arising from strong magnetic activity
(Soderblom 1985).
 High precision radial velocity measures carried out in the CORAVEL program
show HD\,134319 to have a constant radial velocity V$_{r}$ = $-$6.38 $\pm$ 0.17 km s$^{-1}$,
indicating that HD\,134319 is unlikely a member of a close binary system (Duquennoy et al., 1991).
 Its  U, V, W space velocity components (39, $-$17, 2 km s$^{-1}$)  are very close to those
of the Hyades star cluster (40, $-$18, $-$2 km s$^{-1}$) (Eggen, 1960).  This indicates that it is a probable
member of the Hyades moving group with an age of about 625 Myr (Perryman et al., 1998).

The photoelectric photometry reported in this paper has been obtained in 1991 with an automatic photometric telescope (APT, see Boyd \& Genet 1986)  on Mt.\,Hopkins (AZ/USA): the {0.75m Four Colleges Consortium  APT}.

The differential photometry of HD\,134319 was  made in the {\sl u, v, b} and {\sl y} filters matching the Str{\"o}mgren system (Str{\"o}mgren, 1966) and using HD\,134851 (K0V; V = +7.1 mag; B$-$V = +0.90) as the comparison star and HD\,138852 (K0III-IV; V = +5.79 mag; B$-$V = +1.96) as the check star.
Ten second integrations were used in each filter and the usual observing sequence of {sky-comparison-check-variable-comparison-sky} was employed. The measures were corrected for the effects of the atmospheric extinction. No significant light variation
 was detected from the differential measures of the comparison
and check stars, indicating that the light of the comparison stars is constant,
within about $\pm$ 0.015 mag. Normal points were computed by averaging the observations obtained on each night. The  typical standard deviations of the averaged
data points are of the order of 0.008 mag in $y$ and $b$ filters and 0.012 mag in the $v$ and $u$ filters, because of the
lower signal/noise ratio at shorter wavelengths in late-type stars.

The full  set of photometric data was analysed  using a Scargle-Press period search routine (Scargle, 1982) and a photometric period  P = 4.448 $\pm$ 0.01 was found. Since the optical flux of HD\,134319 changes in amplitude and shape over a scale time of few rotation periods,  it proved possible to select two data sets,  along which the  flux modulation resulted to be  rather  stable.

\begin{table*}[t]
\caption{Mean epochs, number of observing nights, mean differential magnitudes and peak-to-peak amplitude of  the light curves of  HD\,134319 (subscripts $v$ and $c$ denote variable and comparison stars, respectively).}
\begin{center}
\begin{tabular}{ccc@{\hspace{.1cm}}cc@{\hspace{.1cm}}cc@{\hspace{.1cm}}cc@{\hspace{.1cm}}c}
\hline
 Mean & $ \# \,\,\,of$ & $ \langle u_{v} - u_{c}\rangle$ &$ \Delta  u$ &$ \langle v_{v} - v_{c}\rangle$  &  $ \Delta v$ &$\langle b_{v} - b_{c}\rangle$
 &  $ \Delta b$ &$ \langle y_{v} - y_{c}\rangle$ & $\Delta y$ \\
Epoch  & nights&(mag)&(mag)&(mag)&(mag)&(mag)&(mag)&(mag)&(mag)\\
 \hline
1991.18 & 14 & 0.34 & 0.06 & 0.75 & 0.05 & 1.14 & 0.03 & 1.36 & 0.03\\
1991.28 & 17 & 0.35 & 0.06 & 0.76 & 0.04 & 1.17 & 0.05 & 1.36 & 0.04\\
\hline
\end{tabular}
\end{center}
\label{tab:tabhd13}
\end{table*}

\vskip 2mm

\IBVSfig{21cm}{4576-f1.ps}{The $y$-band,  $b - y$, $c_1$ and $m_1$ light curves of HD\,134319 for the mean epoch  1991.18 ({\sl top}) and 1991.28 ({\sl bottom)}. Phases are reckoned from the first observed
light curve minimum at HJD = 2448367.766 using the 4\fday 448 photometric period}


Fig. 1  shows the $y$-band,  $b-y$, $m_1$ and $c_1$   light curves for the mean epochs 1991.18 and 1991.28, respectively.  Phases are reckoned from the first observed light curve minimum at HJD = 2448367.766 using the P=4\fday 448 rotation period. 


The 4\fday 448 continuum flux modulation is interpreted as arising from the presence of dark starspots, unevenly distributed on the stellar photosphere,  whose visibility is modulated by the stellar rotation. 

The wavelength dependence shown by the  light curves of HD\,134319,  whose peak-to-peak amplitude increases towards decreasing wavelengths (see Table 1), can be accounted for by the presence  of  spots cooler than the surrounding photosphere.

Singularity, youth and high level of chromospheric and photospheric magnetic activity make this newly discovered BY Dra-type star a proxy for the young Sun and, therefore, of interest for studying the magnetic activity of the young Sun.

\vskip 1cm

\references

Boyd, L., Genet, R.M., 1986, {\it PASP}, {\bf 98}, 618

 Duquennoy, A., Mayor, M., Halbwachs, J.L., 1991,  {\it A\&ASS}, {\bf 88}, 281

 Eggen, O.J., 1960, {\it MNRAS}, {\bf 120}, 540

 Perryman, M.A.C., Brown, A.G.A., Lebreton, Y., Gomez, A., Turon, C., De Strobel, G., Cayrel, Mermilliod, J.D., Robichon, N., Kovalevsky, J., Crifo, F., 1998, {\it A\&A}, {\bf 331}, 81

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

 Soderblom, D.R., 1985, {\it AJ}, {\bf 90}, 2103

 Str{\"o}mgren, B., 1966,  {\it ARAA}, {\bf 4}, 433


\IBVSedata{4576-t2.txt}

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