The observations of FG Hya in the V-band were carried out over 4 nights in
March, 1999, with the PI1024 TKB CCD photometric system attached to the 100-cm
reflector telescope at the Yunnan Observatory in China. The effective field
of view of the photometric system is 6.5 square arc minutes at the Cassegrain
focus and its BV color system approximates the standard Johnson BV photometric system (Yang & Li 1999). The coordinates of the comparison star
and the check star used are listed in Table 1, respectively. The comparison star
and the check star are so close to the variable that they are
in the same field of the observation together with the program star.
star | R.A.(1999.0) | Dec.(1999.0) |
variable |
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comparison star | 08 26 57 | 03 32 08 |
check star | 08 26 56 | 03 29 46 |
The integration time for each image is 100 s. A total of 207 images in Vband were obtained for 4 nights in March, 1999. The aperture photometry package of
IRAF was used to reduce the images. The reduced results show that
the difference between the magnitude of the check star and that of the comparison
star is constant within probable error of
magnitude. Extinction
corrections were not made since the comparison star was so close to the variable.
From the observations, four times of minimum light were derived using
parabola fitting. The new times of the minimum light are listed in Table 2,
in which the (O-C)1 values are calculated by means of the light element
formula given by Smith (1963)
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(1) |
HJD 2451240+ | Min. | (O-C)1 | (O-C)2 |
5.2079(4) | I | -0.0282 | -0.0007 |
6.1925(3) | I | -0.0261 | 0.0004 |
7.1762(9) | I | -0.0269 | 0.0006 |
8.1592(4) | I | -0.0274 | 0.0001 |
From 36 photoelectric times of minimum light collected in the references and the new ones in the present article, the (O-C) values of the minima computed with the above ephemeris are plotted in Fig. 1. This diagram shows that several sudden jumps in the orbital period of FG Hya may occur. The timings of Mahdy et al. (1985) may be crucial in the O-C diagram (see Fig. 1). When their observations were checked, one of their timings (the first one listed) was found to be incorrect. They gave -0.0017 for the O-C whereas the calculated O-C is -0.031! Their other values are correct as given. But the question remains, their light curves appear to be distorted by heavy and asymmetrical spot(s). Therefore, the timings from the most extreme light curves could be suspected to be off because of the substantial spot(s). If the timings near epoch 28000 from Mahdy et al.'s observations are not taken into account, one can see clearly that the period of FG Hya was decreasing (see Fig. 2) and then a parabola ephemeris can be derived as the following:
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(2) |
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(3) |
A total of 207 individual observations have been obtained and listed in Table 4
with their Heliocentric Julian Day, phases and magnitude differences between
the variable and the comparison star. The light curve of the system is shown
as the solid circle points in Fig. 3. Since the period of FG Hya is close to
one third of a day, it is difficult to obtain a complete light curve in a
season. Most of the phases of the present light curve were covered in the four
observing sessions, but an interval of 0.1 in phase (from 0.35p to 0.45p) is still is not
covered.
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Figure 3: The light curve of FG Hya. The solid circle points indicate the observations and the line shows those from the model (see the text) |
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Figure 4: The variation of the lignt curves of FG Hya. The explanation of the different synbols used can be seen in the text |
As noted by Binnendijk (1963), the shape of the light curve of the system was changing. A graphical depiction of the variable shape of the light curves in Vband is shown in Fig. 4, which gives the observations carried out by Smith (X's) in 1955, by Binnendijk (open circles) in 1961-62, by Yang et al. (solid circles) in 1982, by Mahdy et al. (open diamonds) in 1985 and by the present authors (solid diamonds) in 1999. For comparison, all light curves are reduced assuming that they could have the same light level at the phase of 0.5. This level was chosen to normalize all of the light curves to simply demonstrate the substantial variability of the light curve over time. As shown in Fig. 4, the variation of the light curve of the system is very obvious. Some parameters of the light curves for FG Hya are listed in Table 3, from which one can see that the difference of the eclipsing depth between the primary eclipse and the secondary one is considerably changing while neither the O'Connell effect (two maxima in the light curves are unequal), nor its variation is obvious.
obs. date | 1955 | 1961-62 | 1982 | 1985 | 1999 |
Max.I-Min.I | -0.350 | -0.339 | -0.369 | -0.397 | -0.357 |
Max.I-Min.II | -0.333 | -0.310 | -0.314 | -0.327 | -0.350 |
Max.II-Min.I | -0.372 | -0.352 | -0.363 | -0.394 | -0.353 |
Max.II-Min.II | -0.355 | -0.323 | -0.314 | -0.324 | -0.346 |
Max.I-Max.II | 0.022 | 0.013 | -0.006 | -0.003 | -0.004 |
Min.I-Min.II | 0.017 | 0.029 | 0.055 | 0.070 | 0.007 |
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