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\IBVShead{5836}{20 June 2008}
\IBVStitletl{UX~A\lowercase{ri}: New Photometry and Longitudinal Asymmetry}{ in Spot Activity Fixed in Orbital Reference Frame}

\IBVSauth{Rosario, M. J.$^1$; Mekkaden, M. V.$^2$; Raveendran, A. V.$^2$}

\IBVSinst{Vainu Bappu Observatory, Indian Institute of Astrophysics,
   Kavalur 635701, India. e-mail: mjr@iiap.res.in}

\IBVSinst{Indian Institute of Astrophysics, Bangalore 560034, India.
             e-mail: mvm@iiap.res.in, avr@iiap.res.in}

\SIMBADobj{UX Ari}
\IBVSabs{We present new BV photometry of UX Ari obtained on 23 nights}
\IBVSabs{during January-March 2008. From an analysis of the long-term}
\IBVSabs{V band photometry available for this star we find strong evidence}
\IBVSabs{for the orbital modulation of spot activity in the cool component}
\IBVSabs{of UX Ari: the hemisphere facing the companion shows a higher level}
\IBVSabs{of spot activity, as indicated by a significantly larger spread in}
\IBVSabs{observed brightness, when compared to that away from it.}


\begintext

UX~Ari (HD~21242) is one of the brightest member of RS~CVn binaries,
and  has been
observed photometrically almost every season since the discovery of
its light variability in 1972 by Hall et al. (1975).
It is a non-eclipsing, double-lined
spectroscopic binary with a K0--K1 subgiant as the primary
and a G5 dwarf as the secondary in a near-circular orbit
(Carlos \& Popper, 1971; Duemmler \& Aarum, 2001).

We observed UX~Ari photometrically in $BV$ bands on 23 nights
during January--March~2008 with the 34-cm telescope of Vainu Bappu Observatory,
Kavalur. All the measurements were made with respect to the comparison 62~Ari.
HR~999 was also observed on several nights along with the variable as the check
star. Table~1 lists the results of our photometric observations. Each value
given in the table is a mean of 3-4 independent measurements. The typical 
uncertainty in both the differential $V$ and ($B-V$) values is $\sim$0.01 mag.

The differential $V$ and ($B-V$) values given in Table~1 are plotted in
Fig.~1 after
converting the Julian dates of observation to orbital phases with the
ephemeris:
JD~=~2450646.83~+~6\fday 4372703E.
The initial epoch corresponds to the conjunction with the more massive, cool
primary in front. The orbital period and the epoch of maximum radial
velocity of the
active star from which the above time of conjunction is derived are from
Duemmler \& Aarum (2001).
Figure~1 shows that the light variation during January--March~2008 
was highly asymmetric with a broad minimum and a
narrow maximum. The monotonic decrease in the brightness at light curve minimum
observed during 2001--07 (Rosario et al., 2007) seems to be over and the light
curve minimum seems to be getting brighter from this season onwards.
The trend in ($B-V$) variation over the photometric cycle is not well-defined;
however, there is some indication that the star is bluer at fainter
visual magnitudes as reported by several observers earlier.

\begin{table}
\centering
\caption[]{$BV$ photometry of UX~Ari.}
\begin{tabular}{lrrlrr}
\hline
JD &  &  & JD  & & \\
2450000.0$+$ & $V$ & ($B-V$) & 2450000.0$+$  & $V$ & ($B-V$)\\
\hline
  4475.1170  &  1.204 & --- & 4476.1592  &  1.089  &  $-$0.203 \\
  4477.1181  &  1.098  &  $-$0.225 & 4478.1951  &  1.211  &  $-$0.239 \\
  4480.1072  &  1.250  &  $-$0.257 & 4481.1188  &  1.239  &  $-$0.240 \\
  4485.1243  &  1.241  &  $-$0.253 & 4486.1181  &  1.255  &  $-$0.241 \\
  4487.1100  &  1.244  &  $-$0.262 & 4491.1278  &  1.214  &---\\
  4502.1311  &  1.081  &  $-$0.232 & 4514.0905  &  1.182  &  $-$0.255 \\
  4515.0924  &  1.081  &  $-$0.211 & 4516.0944  &  1.148  &  $-$0.217 \\
  4517.0917  &  1.227  &  $-$0.230 & 4522.0912  &  1.103  &  $-$0.224 \\
  4525.1090  &  1.221  &  $-$0.244 & 4528.0953  &  1.093  &  $-$0.254 \\
  4529.0993  &  1.165  &  $-$0.248 & 4530.1026  &  1.245 &---\\
  4531.1078  &  1.237  &--- & 4532.1035 &   1.222   & $-$0.235 \\
  4534.1023  &  1.126  &  $-$0.211   &&& \\
\hline
\end{tabular}
\end{table}

\IBVSedata{5836-t1.txt}
\IBVSdataKey{5836-t1.txt}{UX Ari}{photometry}

Aarum Ulv\aa s \& Henry (2003), who analysed the individual light curves 
of UX~Ari, have reported that there is no clear correlation between the
orbital phase of light minimum and time, except that during 1982--90
the orbital phase seemed to decrease linearly from about 0\fper 75 to 0\fper 40. 
They also reported that the migration rate of phase of light minimum varied
from $-$0.1157 yr$^{-1}$ to $+$0.2605 yr$^{-1}$, and most of the time
the rate had a negative value.

All the differential $V$ magnitudes of UX~Ari with respect to
62~Ari obtained during 1972--2008
(Aarum Ulv\aa s \& Henry, 2003; Rosario et al., 2007; Table~1) are plotted in
Fig.~2 after converting the Julian dates of observation to orbital phases
with the above ephemeris.
The uncertainty in the orbital period quoted by Duemmler \& Aarum (2001)
is
only 0\fday 0000069 and the accumulated error in orbital phase over 36 years,
over which the $V$ band observations of UX~Ari span, is only 0\fper 002.
Figure~2 shows that the range in
the observed differential $V$ magnitudes of UX~Ari has a
clear orbital modulation, implying that the spot activity in the star
has a longitudinal asymmetry that is fixed in the orbital frame of reference.
The upper envelope of the $\Delta V$ values shows a maximum around 0\fper 50.
The lower boundary shows a minimum around the
same phase and a maximum around 0\fper 0 where the upper envelope shows a
minimum. It is remarkable that the total spread in $\Delta V$ magnitudes
at 0.$^p$0 observed so far is only around 0.18 mag while that at 0\fper 5 it is
around 0.48 mag.

\IBVSfig{8cm}{5836-f1.eps}{Plot of $\Delta V$ and $\Delta$($B-V$)
values of UX~Ari obtained during January--March~2008 against
the corresponding orbital phase computed using the ephemeris
JD = 2450646.83 + 6\fday 4372703E.}
\IBVSfigKey{5836-f1.eps}{UX Ari}{light curve}

The fainter secondary component of the binary system also shows a low
level of chromospheric activity
as indicated by the variation in the Ca II K core emission from it
(Aarum Ulv\aa s \& Engvold, 2003).
UX~Ari appears bluer at fainter visual magnitudes, which is unusual
for a spotted star. Rosario et al. (2007) have shown that the bluer
colour of UX~Ari at fainter $V$ magnitudes results because of
the increased fractional contribution to the total
light in the blue spectral region by the hotter G5 companion
as the cooler component becomes fainter. Hence,
most of the light variation observed in UX~Ari can be
attributed to the intrinsic light variability of the cool primary component.

The active stars in RS~CVn binaries are presumed to undergo
highly enhanced solar-like activity. Starspots, which are analogues to
sunspots, distributed asymmetrically across the stellar surface rotationally
modulate the observed flux, thereby producing light variability observed
in these objects. The variations in light curves are attributed to changes
in sizes of spots or spot groups and their distribution on the stellar surface.
The phases of light minimum in many of these objects are found to migrate
along the orbital phases at different rates. In some of the objects the
migration can be traced continuously for several years while in some
it can be traced only for a few years. To account for this, in analogy
with the sun, Hall (1972, 1991) proposed that there
is differential rotation in
the active stars and only a particular latitude co-rotates synchronously with
the orbital motion, and hence spots present in different latitudes would
produce light curves with slightly different periods.
The existence of differential
rotation in components of close binaries, especially
in active stars of RS~CVn systems, is not well-established observationally.
Almost all information on the differential rotation of spotted stars is
based on the migration of the phase of the minimum of light curves of these
stars.

\IBVSfig{10cm}{5836-f2.eps}{Plot of $\Delta V$ values of UX~Ari obtained so far
against the corresponding orbital phase computed using the ephemeris
JD = 2450646.83 + 6\fday 4372703E.}
\IBVSfigKey{5836-f2.eps}{UX Ari}{light curve}

Figure~2 clearly shows that there is enhanced spot activity, as indicated
by a larger spread in observed magnitudes,
in the hemisphere of the active star facing the hotter
companion when compared to that away from it. The minimum and maximum of
a light curve obtained during a particular epoch
may occur over a large range of orbital phases (Aarum Ulv\aa s \& Henry, 2003).
But the fainter light minima and brighter light maxima among them
always occur at orbital phases close to 0\fper 5.
The orbital inclination of UX~Ari is around 60\deg\ (Duemmler \& Aarum, 2001). 
Hence, the existence of a significant orbital modulation in
spot activity implies that the regions of enhanced spot activity
are located closer to the equator rather than the poles
and that these regions
rotate in near-perfect synchronism with the orbit. Any slight difference
in the rotational period would completely smear out the modulation when data
spread over such a time interval as long as 35 years
($\sim$ 2000 photometric cycles) are combined.

The large spread of about 0.5 mag in $V$ magnitudes close to
0\fper 5 requires that the regions that produce the enhanced activity
have an appreciable latitudinal extent on the surface of the active star.
The rotation of a large latitudinal zone in near-perfect
synchronism with the orbit would mean that differential rotation
in the active star is either absent or really small.

Another implication of the existence of the longitudinal asymmetry
in spot activity, which is fixed in the orbital frame of reference,
is that the spots do not appear and disappear
with equal probability at all longitudes on the
surface of the active star in the UX~Ari system;
the presence of the companion significantly affects
the physical processes that produce spots and modulates the
surface distribution of spots on the active star.


\references 

Aarum Ulv\aa s, V. \& Henry, G. W., 2003, {\it A\&A}, {\bf 402}, 1033

Aarum Ulv\aa s, V. \& Engvold, O., 2003, {\it A\&A}, {\bf 402}, 1043

Carlos, R. C. \& Popper, D. M., 1971, {\it PASP}, {\bf 83}, 504

Duemmler, R. \& Aarum, V., 2001, {\it A\&A}, {\bf 370}, 974

Hall, D. S., 1972, {\it PASP}, {\bf 84}, 323

Hall, D. S., Montle, R. E. \& Atkins, H. R., 1975, {\it Acta Astron.}, {\bf 25}, 125

Hall, D. S., 1991, {\it IAU Coll.}, {\bf 130}, 353, 
in: The Sun and Cool Stars: activity, magnetism, dynamos, ed.: I. Tuomien,
D. Moss, G. Rudiger, Springer-Verlag \BIBCODE{1991LNP...380..353H}

Rosario, M. J., Raveendran, A. V. \& Mekkaden, M. V., 2007, {\it A\&A}, {\bf 474}, L41

\endreferences 

\end{document}
