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Astron. Astrophys. 338, 1031-1040 (1998)

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3. Photometric characteristics

The photometric parameters, the brightness at light maximum and minimum, [FORMULA] and [FORMULA], the phase of light minimum [FORMULA], and the amplitude of light variation estimated from the present data and those available in the literature are given in Table 2.


[TABLE]

Table 2. Photometric characteristics of HD 81410


The shape of the light curve of HD 81410 is found to change even within a few orbital cycles. The observations (see Table 2 for the references) of HD 81410 during the closely spaced epochs 1988.01, 1988.23, 1988.41, 1988.89, 1989.12 and 1989.20, which are plotted in Figs. 1b-g, clearly bring out the large-scale, short-term variability that occurs in its light curve. During this period the light curve changed from being of double minima to a single minimum and again back to double minima. It is interesting to see that all the while the deeper minimum remained more or less at the same phase, between [FORMULA] and [FORMULA].

The variations in [FORMULA], [FORMULA], [FORMULA] and [FORMULA] shown by HD 81410 are in phase with the V light curve in the sense that the colours tend to be redder at the light minimum. The amplitude of variation is the largest in [FORMULA] colour. The colours [FORMULA], [FORMULA], and [FORMULA] also show variations over the photometric phase in the same sense as the broadband colours with the [FORMULA] colour showing the largest amplitude.

Fig. 2a is a plot of [FORMULA] and [FORMULA], the brightness at light maximum and minimum, against the corresponding mean epoch of observations. The largest amplitude of light variation so far observed occurred during 1971 and 1972 (0.45 mag), and the smallest during March 1989 (0.05 mag). Both [FORMULA] and [FORMULA] show a large range in magnitudes. The unspotted brightness, which is an important parameter in quantitative spot modeling, can be determined if photometry spanning over a large time interval is available. As seen from Table 2 HD 81410 has been observed photometrically almost every year starting from 1978 till 1996. The maximum [FORMULA] mag observed during 1990 probably corresponds to the unspotted photospheric magnitude. An inspection of Fig. 2a (see also Fig. 11 of Strassmeier et al. 1997) shows that during the period 1971-1985 the [FORMULA] remained more or less constant around 0.20 mag below the maximum, whereas the [FORMULA] monotonically became brighter from 1971 till 1987. The increase in [FORMULA] during this interval was more than 0.5 mag. After 1987 both [FORMULA] and [FORMULA] did not change appreciably, even though there were small fluctuations in their values.

[FIGURE] Fig. 2. a Plots of [FORMULA] (open circles ) and [FORMULA] (filled circles ) of HD 81410 against the mean epoch of observation. b  Plot of phase of light minimum against the mean epoch. Open circles denote the secondary minima

The total range in brightness shown by HD 81410, in the sense [FORMULA] (brightest) minus [FORMULA] (faintest), is around 0.70 mag. This is comparable to that observed in active RS CVn systems like II Peg and DM UMa (Mohin & Raveendran 1993, 1994), and therefore, the [FORMULA] observed in 1971 by Eggen (1973) is probably close to its saturation value.

From 1971 to around 1985 the [FORMULA] of HD 81410 was about 0.20 mag below the maximum brightness so far observed, indicating that the starspots never disappeared from the field of view. A drastic change which resulted in a substantial reduction in the overall spot activity seems to have happened sometime after 1982 because both [FORMULA] and [FORMULA] became brighter by almost 0.20 mag.

Table 2 shows that the light curves of HD 81410 display two minima quite often, implying a highly asymmetrical surface brightness distribution most of the time. The observations obtained so far do not show any flat-topped light curve with [FORMULA] close to its maximum observed value, indicating that the spots responsible for the light modulation always had large longitudinal extents. The [FORMULA] determined from such light curves would give the effective longitude of the spot or spot group. Similarly, from the large light amplitudes observed it is reasonable to expect a large latitudinal extent also for the spots. Fig. 2b is a plot of the phase of light minimum against the corresponding mean epoch of observations. There is an indication of two preferred effective longitudes about which spots are generally formed, one around 0p.50 and the other around 0p.95. The scatter about these values seen in the figure partly arises from the errors in the estimation of [FORMULA] because of the large intrinsic scatter in the light curves as a result of folding the observations over several photometric cycles and the large longitudinal extent of the light minimum. Probably, several spots are involved in producing the observed light modulation, and the small short-term fluctuations in the effective longitudes, which might also be occurring, is caused by the rather short life-times (a couple of rotational cycles) of individual spots. The migration of [FORMULA], arising from a difference in the orbital and photometric periods, that are usually observed in RS CVn systems, is not very prominent in the case of HD 81410, which implies that the effective latitude of the spot or spot groups responsible for the light modulation is in synchronous rotation with the orbit.

In Fig. 3 we have plotted the brightness at light maximum and minimum, [FORMULA] and [FORMULA], given in Table 2 against the corresponding amplitudes. There is an indication that at amplitudes larger than 0.2 mag an increase in amplitude occurs more as a result of a decrease in the brightness at light minimum; at amplitudes smaller than this apparently there is no such correlation. From the figure it is seen that at low amplitudes ([FORMULA] 0.1 mag) both the brightness at light maximum and minimum converge to the same value. Therefore a smaller amplitude, mostly likely, arises not from a reduction in the spot activity, but rather from a more uniform distribution of spots across the longitude.

[FIGURE] Fig. 3. Plots of [FORMULA] (open circles ) and [FORMULA] (filled circles ) against the corresponding amplitude of light variation

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© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998
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