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Astron. Astrophys. 328, 311-320 (1997)

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3. Photometric variations

Here we present the analysis of the photometric variations which could be connected with the transit of either a giant planet or a scattering dust cloud in front of the star.

In order to study the inhomogeneities in the edge-on disk of [FORMULA]  Pic, we decided to look for the stellar light variations to scan the dust disk structure through its variable extinction. We analysed photometric measurements of the Geneva-photometry from Nov 18, 1975 to Feb 27, 1992 (JD 2734 to JD 8679, where JD is for Julian Day - 244000) (Rufener 1989). Since 1975, [FORMULA]  Pic had been used as a photometric reference star. In 1982 it was noted that [FORMULA]  Pic was showing some unexplained variations and thus was eliminated from the list of the Geneva Observatory reference stars in 1982. At that time the "Vega-like" phenomenon of [FORMULA]  Pic was not yet known. Unfortunately, the consequence was that this star was not photometrically observed any more until 1988, when we suggested to start again its survey, because by then the star was known to have a disk seen edge-on.

3.1. Long time scale variations

The data are composed of 238 magnitude measurements between 1975 and 1992, in seven bands: three wide U, B and V bands, and four narrower B1, B2, V1 and G bands. They are associated with a quality factor from 0 to 4, from lowest to highest accuracy. The detailed analysis of the photometric variations can be found elsewhere (Lecavelier des Etangs et al. 1995). Here we present a summary of these variations and new possible interpretations.

The main result of the analysis of the long time scale variations is that [FORMULA]  Pic shows an achromatic linear brightness increase from Feb 21, 1979 to Feb 17, 1982, with a slope [FORMULA] mag. per day in all seven bands (Fig. 1). In contrast eight other stars used as references did not show any variation at all.

[FIGURE] Fig. 1. Photometric measurements of [FORMULA]  Pic from 1975 to 1982. The linear fit gives evidence that the long term variations are significant. The plot of the V-B index shows that the variations are achromatic.

We cannot completely exclude the presence of intrinsic stellar variations, although such achromatic stellar variations would be surprising. On the other hand, these variations could be compatible with disk inhomogeneities of 10% to 20% in the azimuthal distribution of the dust in the disk. Indeed, it has been demonstrated that the presence of a planet in the disk can produce inhomogeneities, such as arcs or accumulation of matter following planet trajectories (Scholl et al. 1993, Roques et al. 1994). These structures could explain the variations on a long time scale. However, due to the collisional destruction of dust particles, these structures can be supplied by mean motion resonances only if the vertical optical depth is not larger than a critical one: [FORMULA] (Artymowicz 1995, Lecavelier des Etangs 1996b). The consistency of a small dust vertical optical depth and variations of about 0.01 magnitude is questionable. The disk thickness would have to be smaller than 1 AU at 20 AU from the star, which corresponds to an opening angle smaller than 2 degrees. These quantities are different from the large disk thickness and opening angle measured on the disk image at larger distances. But the observation does not constrain the vertical structure of the inner disk. The apparent paradox could thus be solved if some extinction is due to a flat ring of larger particles (up to few centimeters) similar to the planetary rings in the Solar System which cannot be detected by infrared observations.

3.2. Short term variations

Among the stars we have analysed in the Geneva photometry, only two have shown short term variations each on one occasion with a confidence level of 99%: HR 10 with a strong color dependency and [FORMULA]  Pic during November 1981 with little or no color dependency. For [FORMULA]  Pic, the probability that these variations are produced by only statistical noise is less than [FORMULA] (Lecavelier des Etangs et al. 1995). We emphasize that short term variation of that type is an unique case observed in all Geneva Observatory data. The photometric observations around this period and the mean magnitudes outside this short interval are listed in Table 1.


Table 1. Magnitude and colors measurements on [FORMULA]  Pic between JD 4914 and JD 4926 with quality factor greater than 2.

The variation of [FORMULA]  Pic in November 1981 can be separated into two parts (Fig. 2). The brightness of [FORMULA]  Pic slowly increased by about 0.06 magnitude in about 10 days, and symmetrically slowly returned to normal. But at the center of this variation, near the maximum increase, on JD 4918 (= Nov 10, 1981), there was a sharp decrease.

[FIGURE] Fig. 2. The short term magnitude variations of [FORMULA]  Pic in the seven bands around JD 4918. The dashed line represents the variation in U band. Good quality measurements (quality factor greater than 2) are connected and represented by dots, the others ones are represented by open squares. These variations have no color dependency (except for the point when the brightness increase was at its maximum: there the increase could be larger in U.) Note the slow increase and decrease in brightness with a dip in the middle. The bottom panel gives for the same period the difference between the individual values and the mean magnitude for all the standard stars. The measurements of these stars are normal with a relative dispersion of the measurements due to statistical noise of only [FORMULA] mag.

The first measurement of this particular night gives the brightest measurement of [FORMULA]  Pic: the star was about 0.06 magnitude brighter than the mean magnitude in all seven bands except for the U band, for which the variation rises 0.09 magnitude above average level. Unfortunately the quality of these measurements obtained with an air mass of 2.4 is not guaranteed at the same level of confidence and thus they must be considered with care. However a plot of the observations of eight other stars used as references clearly shows that no systematic errors are present and that the air mass correction even at the 2.4 level is accurate (Fig. 3). This measurement on JD 4918.563 is thus significant although the accuracy is probably smaller than for the following measurements. 1.5 hours later the star was 0.04 magnitude brighter than the mean magnitude, except in the U band where the variation was 0.05 magnitude. The three last measurements were obtained between 4 and 7 hours later. At that time [FORMULA]  Pic was close to its normal brightness but still decreasing (Fig. 4). Given that the variance of the data in a single day is about [FORMULA] (a value smaller than the dispersion related to all the data in which there are systematic errors from day to day), a [FORMULA] test gives a confidence level of 83 % to consider that this slope is real and not due to statistical fluctuations. We will see in Sect.  6 that this slope could be related to the limb-darkening of the stellar photosphere.

[FIGURE] Fig. 3. Plot of magnitudes, color variations and air mass for 9 stars observed from 1970 until 1992. Squares are for the two first measurements for [FORMULA]  Pic on JD 4918. Triangles are for the two extreme measurements of the star HR10, which also showed photometric variability and spectroscopic similarities with [FORMULA]  Pic . a B variations versus V variations. The statistical variations around the mean magnitude in B band are strongly correlated to variations on V band. The measurements of [FORMULA]  Pic are within the diagonal correlation, which implies that there is no color variation in B-V. b U variations versus V variations. There is again a correlation between V and U bands. [FORMULA]  Pic shows a color variation, but it is within the uncertainty of the U-band measurements. c V variations versus air mass. The solid lines represent the standard deviation [FORMULA] as a function of the air mass. This plot shows that air mass corrections are accurate up to large zenithal distances. The variations of [FORMULA]  Pic (and HR10) are obviously outside the [FORMULA] statistical noise and are clearly not due to the atmospheric extinction. d U-V variations versus air mass. The variations of [FORMULA]  Pic in U-V are at about the [FORMULA] noise level.
[FIGURE] Fig. 4. Plot of [FORMULA]  Pic magnitude variations on JD 4918. The time is given in hours relative to the time of the first measurements of this particular day. The dashed line represents the variation in U band. The first measurements have been obtained through an air mass of 2.4 and are represented by open squares. The other data have very good accuracy (quality factor equal to 3)

It is interesting that the variations of [FORMULA]  Pic around JD 4918 are approximately symmetrical around the dip in the light curve (see Fig. 2). We stress the fact that the atmospheric conditions were very good during this particular night, and for all of the other stars observed during that night the measurements were all totally normal. The measurements of the standard stars during this period are also plotted for comparison in Fig. 2.

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998