Using standard reduction programs, the uvby observations were reduced by the SAT group of the Copenhagen University Observatory (see Crawford & Barnes 1970; Olsen 1994). The data are available in files containing the Henry Draper (HD) number of the star, the Julian Date (JD) of the observation, the air mass factor of the observation, the Strömgren V magnitude, the b-y color index, the m1=(v-b)-(b-y) and the c1=(u-v)-(v-b) parameters.
From our set of data, it was possible to search for short term as well as long term variations. The data analysis were performed on differences between stellar measurements and the corresponding measurements made on a comparison star. Short term variations would be detected if two subsequent measurements differ by more than 3 sigma from the average value of the data set. An eventual detection can be confirmed when comparing the star to the other comparison star. Over our three campaigns, no fluctuation was observed in any of our program stars except two of them, HR 10 and HD 38392 which will be discussed below.
This result already shows the quality of the survey since we obtained on the
average the following precisions on the different parameters evaluated:
mag for V
and
mag for the b-y color index.
Within this accuracy, most of our surveyed stars did not present any short term
variations, including
Pictoris.
Concerning long term variations, we will illustrate here our approach directly
in the case of Pictoris, since it is also the only case where some long
range variation were detected.
Figure 1 shows the Strömgren V magnitude evolution between October
1995 (JD 2450030) and March 1998 (JD 2450880) for Pictoris and
its two comparison stars HD 35580 and HD 40200.
It is clear that the three stars present similar variations
which are thus not intrinsic variations but systematic residuals.
However it is noteworthy that the statistical dispersion of individual data
is at this stage of the order of only
mag.
To detect possible weak variations, as in the case of our search for
short term variations,
we evaluate the difference between the data of a given star and its comparison star,
provided that the measurements are contiguous in time.
Practically, we set a maximum time difference between two measurements
and we consider only the couples of measurements separated by less
than 0.004 day (6 min). This process is done
for the Strömgren Vmagnitude, the b - y color index, the m1 parameter and the c1 parameter
of all the stars.
Figure 2 shows the magnitude difference between
Pictoris
and its two comparison stars (HD 35580 and HD 40200), whereas
Fig. 3
shows the difference for the b-y color index.
As already mentioned, no short-term variations are detected in the case of
Pictoris. To detect long-term variations,
we used the same method as described in Lecavelier et al. (1995a). We fitted
the data of each star (s) by a linear function of the time (t):
(
)
and evaluate the probability
that the calculated slope
for the star s
is far from 0 assuming that b=0 is true.
For a Gaussian distribution of the statistical noise in the data,
the distribution of
must follow a Student law.
Here we find that
the Student law is well followed and consequently that the probabilities
are uniformly distributed between 0 and 1
(see an example in Lecavelier et al. 1995a).
For all the stars (s) except
Pictoris, we do not find a
value
significantly
different from 0. This shows that our program stars as well as the
comparison stars had no significant
light variations during the survey. Again this underlines the quality of the
survey and the strength of the very few real detections that are presented
now.
star | * |
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mV | Spectral type | ![]() |
|
HR 10 | HD 256 | PT | 00h 07m 18.3s | -17![]() |
6.23 | A2IV/V | 220 |
HD 693 | CMP | 00h 11m 15.8s | -15![]() |
4.89 | F5V | 8 | |
![]() |
HD 39060 | PT | 05h 47m 17.1s | -51![]() |
3.80 | A5V | 104 |
HD 35580 | CMP | 05h 22m 22.1s | -56![]() |
6.08 | B8.5V | ||
HR 2174 | HD 42111A | PT | 06h 08m 57.8s | +02![]() |
5.73 | A3Vn | 120 |
HD 41692 | CMP | 06h 06m 38.7s | -04![]() |
5.39 | B5IV | 20 | |
51 Ophiuchi | HD 158643 | PT | 17h 31m 24.8s | -23![]() |
4.81 | A0V | 210 |
HD 155450 | CMP | 17h 12m 58.6s | -32![]() |
6.01 | B1II | ||
TY CrA | CD -37![]() |
PT | 19h 01m 40.1s | -36![]() |
9.54 | Ae | |
HD 180885 | CMP | 19h 19m 39.9s | -35![]() |
5.59 | B3V | 158 | |
68 Ophiuchi | HD 164577 | PT | 18h 01m 45.1s | +01![]() |
4.45 | A2Vn | 232 |
HD 157089 | CMP | 17h 21m 07.0s | +01![]() |
6.95 | F9V |
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