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\IBVShead{4286}{4 January 1996}

\centerline{\ft STARSPOTS ON THE YOUNG  SINGLE K0V STAR HD\,82443}
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\begintext

HD\,82443 (Gl 354.1; K0V; V= +7\fm 00; B$-$V = +0.76) is a single, main-sequence
K$-$star with very strong chromospheric and transition-region line emissions,
most likely arising from strong magnetic activity (Soderblom, 1985).
It is a high proper motion star with a relatively large parallax
of $\pi$ = 0\farcs 054 (Gliese, 1969).  High precision radial velocity
measures carried out on the CORAVEL program (Duquennoy et al., 1991; Duquennoy \& Mayor, 1991) 
show HD\,82443 to have a constant radial velocity of Vr = +8.2 $\pm$ 0.24 km/s,
indicating that HD\,82443 is probably not a member of a close binary system.
Independent radial velocity measures of the star obtained by Griffin (1994)
yield a mean value of Vr = +8.9 $\pm$ 0.5 km/s and also show no indication
of variability.  The projected rotational velocity of the HD 82443 has
been measured by Benz and Mayor (1991) as vsin i = 4.6 $\pm$1.0 km/s; while
Soderblom (1995) estimates a slightly larger value of vsini = 7.0 $\pm$ 2 km/s.
As pointed out by Soderblom and Clements (1987), HD\,82443 has U,V,W
space velocity components ($-$14, $-$24, $-$1) (km/s) that are very close to those
of the Pleiades star cluster ($-$9, $-$29, $-$12).  This indicates that it is a probable
member of the Pleiades moving group with an age of about 70 Myr.  The implied
young age of HD\,82443 is consistent with the observed high levels of
chromospheric, transition-region and coronal emissions.  Activity-Period-Age
relationships for cool stars indicate that HD\,82443 should have a rotation period
of a few days (Soderblom \& Clements, 1987).

The photometric observations of HD\,82443 reported here were obtained on 67
nights from 10 February through 21 May, 1989 using the Phoenix-10 automatic 
photoelectric telescope (APT), located on Mt. Hopkins, AZ.  The Phoenix-10
APT is a 25-cm reflecting telescope which is completely computer controlled
(see Boyd \& Genet, 1986). The photometer is equipped with standard U, B, V
filters and the observations were made using HD\,82191 (A0 V; V=+6.58;
B$-$V= +0.08) as the comparison star while HD\,81146 (K5 III; V= +4.46; B$-$V= 
+1.22) served as the check star. Ten second integrations were used
and the usual observing sequence of sky-comparison-check-variable-comparison-
sky was employed.  The observations were corrected for atmospheric extinction
and the instrumental differential magnitudes were converted to
the standard UBV system. Because of the close angular separations
between the comparison and the variable and check stars, the
atmospheric corrections were very small. No significant light variations
were detected from the differential measures of the comparison
and check stars, indicating that the comparison star is constant in light to
within about $\pm$ 0.015mag.

To determine the photometric period, the data in each filter
were analyzed using a Scargle-Press period search routine (Scargle, 1982). 
These period searches revealed a period of P = 5.400 $\pm$ 0.012 days. 
The observations from the first two months (February-March) are plotted
against phase in Figure 1 using this period; the phases are reckoned
from the first observed light minimum at HJD\,2447573.78. The observations
were split\break 
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Figure 1. The normalized fluxes from UBV photometry of HD\,82443 are plotted
versus photometric phase. These observations cover the interval from 10
February through 31 March 1989. The phases were computed with the 
photometric period of 5.40d. The best fitting spot model curves (see Table~1) 
are shown among the observations and the corresponding 3-dimensional
representation of the spot configuration on the star's surface is depicted
at three different phases: 0.25P, 0.50P and 0.75P.
\large
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\centerline{3}
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\centerline{Table 1}
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\begin{center}
\begin{tabular}{|l|l|l|l|l|l|}
\hline
\multicolumn{6}{|c|}{\bf HD\,82443 Spot parameters:}\\
\hline
\multicolumn{6}{|c|}{Rotational period = 5.40d}\\
\multicolumn{6}{|c|}{Effective temperature = 5370K}\\
\multicolumn{6}{|c|}{Inclination = 60\degr}\\
\multicolumn{6}{|c|}{Limb Darkening}\\
\multicolumn{6}{|c|}{0.68 ($\lambda$ = 5500\AA) $-$ 0.81 ($\lambda$ = 4500\AA) $-$ 0.83 ($\lambda$ = 3700\AA)}\\
\hline
\hline
\multicolumn{6}{|c|}{\bf 02.10.1989 -- 03.31.1989}\\
\hline
Spot&Longitude&Colatitude&Radius&Area&Temperature\\
\hline
1&80\degr&25\degr&10\degr&5.6\%&4300$\pm$100K\\
2&320\degr&38\degr&17\degr&9.5\%&4300$\pm$100K\\
\hline
\multicolumn{6}{|c|}{Total spotted surface = 15.1\%}\\
\hline
\hline
\multicolumn{6}{|c|}{\bf 04.02.1989 -- 05.21.1989}\\
\hline
Spot&Longitude&Colatitude&Radius&Area&Temperature\\
\hline
1&80\degr&25\degr&10\degr&5.6\%&4300$\pm$100K\\
2&332\degr&38\degr&16\degr&8.9\%&4300$\pm$100K\\
\hline
\multicolumn{6}{|c|}{Total spotted surface = 14.5\%}\\
\hline
\end{tabular}
\end{center}

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\noindent into two time intervals because the light curve slowly
changed with time.  As shown in the figure, the light variations are
quasi-sinusoidal and show a wavelength dependence; the light amplitudes in the
UBV bandpasses are: 0\fm 11, 0\fm 09, and 0\fm 09, respectively.
The theoretical spot model fits are also shown in  Figure 1  along with the
3-dimensional representations of the star with starspots depicted at three
different rotational phases. The periodic 5.400 day light
variation is interpreted to arise from the presence of starspots, unevenly
distributed over the surface of the chromospherically-active K-dwarf. The
photometric period is assumed to be the star's rotational period.

We analyzed the light curves on the assumption that the light variations
arise from dark starspots. The light curves were modelled using
Binary Maker V 2.0 (Bradstreet, 1994) in which the second component
is essentially ``turned off" (i.e. assigned a near zero mass
and luminosity) in order to model a single rotating star.
Circular, cool spots of uniform temperature were assumed and the
light curves were fit by manual iteration.  For the modelling
we adopted a temperature of $T_{eff}$= 5370K for the unspotted photosphere of
the star, inferred from its B$-$V index and K0\,V spectral type (see Novotny, 1973); the
limb-darkening coefficients of Al-Naimy (1978) were also adopted.  The
inclination of the star's rotational axis (relative to our line of
sight) was assumed to be 60\degr. This value was obtained from the
measured rotation period, assuming a mean vsini = 6 km/s and adopting
a stellar radius of R $\simeq$ 0.80 $R_{\odot}$, appropriate for its spectral type.
After a number of iterations, we were able to obtain satisfactory
fits to the U, B, V light curves. The best fits to the February-March
observations are shown in Figure 1 along with the geometric model of
the star with starspots.  Because of the asymmetries in the light
curves, it was necessary to model them with two dark (cool) spots,
separated in longitude by 120\degr\ for the February-March 1989 
data; the best fit to the April-May observations was achieved by
adopting a longitudinal separation of 132\degr. The temperatures
of the starspots were found to be about 1100K cooler than the
photospheric temperature. The two spots were located within 40\degr\ 
of the rotational pole and the total spotted area was about
15\%. Table 1 gives a summary of the modelling results for both
data sets along with\break
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\centerline{4}
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\noindent  the input parameters used in the analysis. The
values given for the spot properties are not unique, however, but should
be considered as representative of spot areas, distributions and
temperatures.

Additional photoelectric photometry of HD\,82443 has been obtained in subsequent
years.  These observations indicate systematic year-to-year changes
in the light amplitude and mean light level, possibly arising from a starspot
cycle.  We plan to present these results in the future in a more detailed paper.

This research was partially supported under NSF grant 86-16362 and
Catania University fellowship which we gratefully acknowledge. 
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\beginauthors
S. MESSINA
Inst. of Astronomy
University of Catania
Catania, Italy
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\authorsrightcol
E. F. GUINAN                  
Dept. of Astronomy and                
Astrophysics              
Villanova University
Villanova, PA
USA
\endauthors

\references

Al-Naimy, H. M., 1978, {\it Astrophys. Space Sci.}, {\bf 53}, 181

Benz, W., and Mayor, M., 1991, {\it A\&A}, {\bf 93}, 235

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

Bradstreet, D. H., 1993,  ``Binary Maker 2.0: Light Curve Synthesis Program", (Contact Software, Norristown, PA)

Duquennoy, A., and Mayor, M., 1991, {\it A\&A}, {\bf 248}, 485

Duquennoy, A. et al., 1991,  {\it A\&A Suppl. Ser.}, {\bf 88}, 281.

Gliese, W., 1969, {\it Ver\"off. Astr. Rechen-Inst. Heidelberg}, No.\,23

Griffin, R. F., 1994, {\it The Observatory}, {\bf 114}, 294

Novotny, E., 1973, Introduction to Stellar Atmospheres and Interiors,
    (Oxford Univ. Press: New York), p.10 

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

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

Soderblom, D. R., 1995, priv. commun.

Soderblom, D. R. and Clements, S. D., 1987, {\it AJ}, {\bf 93}, 920



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