COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                     Number 3437

        	                       Konkoly Observatory
	                               Budapest
                	               2 March 1990
	
        	                       HU ISSN 0374 - 0676


   CHANGES IN THE LIGHT CURVE OF AB DORADUS (HD 36705)

The rapidly rotating single star HD 36705 (AB Dor) has been
under observation by the Monash group for approximately ten
years. Presented here (see fig.1) are the V and B-V light
curves recorded during October/November 1989 at Siding
Spring observatory. Results were obtained with a cooled
extended-red S-20 tube and a motorized filter box on the 0.6
m Boller and Chivens telescope. Calculated phases for AB Dor
are in accordance with epoch and period HJD 2449296.575 and
0.51479 day as discussed by Innis et al.(1988), with the
main comparison star being HD 35537. The data were
transformed to the standard UBV system via a set of
calibration equations obtained by the author in June 1989 on
the same instrument.

From fig.1(a) it can be seen that the light curve is
dominated by a large decrease in brightness at an
approximate phase of 0.2, presumably due to the effect of a
large starspot (or group of smaller spots) at this phase of
the star's rotation. The portion of the curve between phases
0.7 and 0.9 is unusually flat, possibly indicating either
the immaculate (unspotted) surface of the star, or a
uniformly spotted section of the stellar surface. There may
also be a small dip in the light curve at phase 0.6, but a
lack of complete phase coverage prevented confirmation of
this. The B-V curve in 1(b) shows that AB Dor is slightly
redder at minimum light.


                        [FIGURE 1]

 Figure 1  (a),(b) : the light curves in V and B-V for AB Dor 
 in Oct/Nov. 1989. Phases calculated with respect to period
 0.51479 d and epoch 2444296.575.


The most interesting aspect of these data becomes apparent
when it is compared with previous light curves from
1986,1987 and 1988 (see fig.2). From the March/April 1986
data, it can be seen that the light curve is quite flat,
possessing little modulation due to starspot activity. In
September/November 1986 and January 1987, however, a
definite minimum is seen to emerge at a phase of about 0.5.
The December 1988 data shows a significant change from this,
in that the curve now exhibits a peak at phase 0.4 and a
minimum at phase 0.7 (Thompson and Thompson 1988). The 1989
data, taken only 10 months later, shows quite a dramatic
change in the shape of the light curve, with a deep minimum now
at phase (approximately) 0.2. Since the migration of a starspot
by 0.5 phase in less than a year is quite unlikely, one must
assume (after verifying that each set of data was correctly
reduced) that the change in the shape of the curve is the
result of the formation and decay of one or more spotted
regions on the star.


                        [FIGURE 2]

 Figure 2  Light curves in V for AB Dor since March 1986.
 The vertical scale is given by the bar at the side of each 
 curve which represents 0.02 mag. All data shown were collected
 by the Monash group, and cover two complete rotational cycles.


The level of the flat portion of the 1989 light curve is
quite similar to the level of the peak in the 1988 data,
implying perhaps that the spot causing the minimum at phase
0.7 (1988) has completely disappeared in 1989, and a new
spot has formed at phase 0.2. There was some hope that the
flat part of the 1989 light curve may be indicative of the
unspotted star brightness, but AB Dor has been observed as
being up to 0.15 magnitudes brighter in previous years (see
fig.3). The mean light level of AB Dor in 1989 further
emphasizes the continual dimming of the star over the past
ten years, however it is yet to be seen whether this is evidence
of a solar-type spot cycle.


                        [FIGURE 3]

 Figure 3:  The variation of mean light level in V for AB Dor since 1979. The
      data was collected from many different sources (for references
      see Innis et al., 1988)


                       G. ANDERS

                       Dept. of Physics, Monash Univ.
                       Clayton 3168, Australia


References:


Innis, J.L., Thompson, K., Coates, D.W., Lloyd
 Evans, T. (1988) Observations of active
 chromosphere stars - II. Photometry of AB Dor,
 1978-1987. Mon. Not. R. astr. Soc. 235, 1411-1422. [BIBCODE 1988MNRAS.235.1411I ]

Thompson, K., Thompson, I. (1988)
 December 1988 Photometry on AB Dor. Inf. Bull. Var. Stars. No. 3320.