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Astron. Astrophys. 345, 505-513 (1999)

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6. UBV photometry

6.1. Transformations

As was already mentioned in Sect. 2, our observations were carried out from two sites: Mt. Suhora ([FORMULA]) and Bial ków ([FORMULA]). We calculated the instrumental magnitudes in all bands (separately for each site) with respect to five relatively bright stars in the field. Next, the differential magnitudes were corrected for second-order extinction effects and averaged. These mean values were used in subsequent transformations.

Because the [FORMULA] data carried out at Suhora were the most numerous, we transformed them first. In the transformations, we used mainly the photoelectric photometry of Johnson & Morgan (1955) and Schild (1965). Because of the lack of red photoelectric standards, we also applied the photographic photometry of some bright, well-isolated stars from Moffat & Vogt (1974) giving them half the weight given to photoelectric measurements. The resulting transformation equations follow:

[EQUATION]

[EQUATION]

[EQUATION]

where numbers in parentheses are the rms errors of the preceding numbers and the lowercase symbols denote the instrumental magnitudes. The internal standard deviations are 0.031, 0.044, and 0.059 mag, for Eqs. (6), (7), and (8), respectively. The above three equations were used to transform only the Suhora measurements. Then, using [FORMULA] magnitudes and [FORMULA] colour indices obtained in this way, we transformed Bial ków [FORMULA] photometry with equations:

[EQUATION]

[EQUATION]

This time, the lowercase letters stand for Bial ków instrumental magnitudes. The standard deviations are equal to 0.024 and 0.017 mag for Eqs. (9) and (10), respectively. Subsequently, for stars observed in both sites, transformed V magnitudes and [FORMULA] colour indices were combined together. Resulting standard [FORMULA] magnitudes and [FORMULA] colour indices for 184 stars as well as [FORMULA] colour indices for 79 stars are given in Table 4. The full version of this table is available only in electronic form at CDS via anonymous ftp to 130.79.128.5. Here - for reader's convenience - we present only the data for 10 variable stars.


[TABLE]

Table 4. [FORMULA] photometry of stars in h Persei. The columns are: (1),(2), X and Y coordinate in Fig. 1, (3), Oosterhoff (1937) number, (4), Wildey (1964) number, (5) Moffat & Vogt (1974) number, (6), [FORMULA] magnitude, (7), [FORMULA] colour index, (8), reference to [FORMULA] photometry (`S' means that the star was observed from Suhora only, `B', from Bial ków only, `S+B', both from Suhora and Bial ków. The [FORMULA] photometry transformed with the use of I-filter measurements [Eqs. (11)-(14)] are additionally flagged with `I'), (9), [FORMULA] colour index, (10), right ascension (epoch 2000.0), (11), declination (epoch 2000.0), and (12), remarks.


The [FORMULA]-filter observations were not transformed to the standard Cousins [FORMULA] system because there are no [FORMULA] standards in the field. However, because the I-filter photometry is the deepest, we incorporated these measurements in another way, namely using them for the transformations for faint stars for which the [FORMULA] photometry was not reliable. The transformation equations

[EQUATION]

[EQUATION]

for Bial ków and

[EQUATION]

[EQUATION]

for Suhora give us additional [FORMULA] photometry for 74 faint stars. The standard deviations for Eqs. (11)-(14) are, in succession, equal to 0.014, 0.024, 0.004, and 0.025 mag. This photometry is also included in Table 4. One may doubt the reality of the [FORMULA] colour indices transformed from [FORMULA] indices if red and reddened stars are used in transformation, but in our case both the stars used to obtain the transformations and those transformed later are mostly reddened cluster members. It follows that the systematic effects can be important only for a few faint non-members.

Because the mean instrumental magnitudes were calculated from all search frames, the internal accuracy of our [FORMULA] photometry is very good - it is better than 1 mmag for brightest stars and reaches about 0.02 mag for the faintest. Obviously, owing to the transformation errors, the magnitudes given in Table 4 are much less accurate.

Magnitudes and colours of eclipsing variables in Table 4 were calculated for the phases of maximum light. For remaining variables mean magnitudes and colours calculated from all our observations are given. Equatorial coordinates given in the tenth and eleventh column of Table 4 were derived using the Guide Star Catalogue (GSC) positions of 57 stars in the observed field. Regarding the accuracy of the stars' positions in GSC, the coordinates given in Table 4 are accurate to within 0[FORMULA]1.

6.2. Colour-magnitude and colour-colour diagrams

The cluster colour-magnitude (CM) and colour-colour diagrams are shown in Fig. 9. [FORMULA] photometry of faint stars obtained with the use of [FORMULA] indices is shown with plus signs in the CM diagram.

[FIGURE] Fig. 9. a  The colour-magnitude diagram for the observed field in h Persei. The two [FORMULA] Cephei stars are denoted by open, and the other stars by filled circles. Variables are enclosed in large open squares. Stars for which [FORMULA] photometry was transformed from the instrumental [FORMULA] measurements are shown with plus signs. b  Colour-colour diagram for stars which [FORMULA] photometry is available. Our photometry is shown with the same symbols as in the left diagram. Crosses are points in which the [FORMULA] indices were taken from other sources, photoelectric (large symbols) or photographic (small symbols). The intrinsic colour-colour relation for dwarfs, shown with dashed line, was taken from Caldwell et al. (1993). The same relation for the mean value of reddening, [FORMULA] = 0.52 mag (long solid line), and [FORMULA] = 0.47 and 0.57 mag (short solid lines) are also plotted.

As in [FORMULA] Persei, both the h Persei [FORMULA] Cephei stars, Oo 692 and Oo 992, lie close to the cluster turn-off point (Fig. 9a). The positions of these two stars bracket a few other stars, including the [FORMULA] Eri star Oo 922. The V magnitudes of the [FORMULA] Cephei stars (9.35 for Oo 692, and 9.90 for Oo 992) are in the same range as in [FORMULA] Persei. The scatter in the cluster main sequence is real and is a result of small differences in reddening. The leftmost star in the CM diagram is Oo 622. The [FORMULA] colour index for this star is equal to -0.53 (Tapia et al. 1984). This means that it is probably an early-type star, either slightly less reddened than the other cluster stars or simply a foreground object.

In order to derive the average reddening of the cluster, we moved the intrinsic [FORMULA] vs. [FORMULA] relation for dwarfs taken from Caldwell et al. (1993) (dashed line in Fig. 9b) along the reddening line with the average parameters derived by Turner (1989), i.e.,

[EQUATION]

The best agreement was obtained for [FORMULA] = 0.52 mag (long solid line in Fig. 9b). As was shown by Turner (1989), the slope of the reddening line is not unique throughout the sky. This, however, does not greatly affect our result: even if the extreme values of the slope of reddening line derived by Turner (1989) are assumed, the best-fit value of [FORMULA] for h Persei differs from 0.52 mag by no more than 0.01 mag. The average value of [FORMULA] is in general agreement with previous determinations (see Tapia et al. 1984and references therein; Pandey et al. 1989; Natali et al. 1994). As can be seen in Fig. 9b, most of the cluster stars have reddenings which differ from the mean value by no more than 0.05 mag. This range of individual reddenings is smaller than obtained by Wildey (1964), but this disagreement will become understandable in the view of the accuracy of his photometry (see next subsection).

Unfortunately, no Strömgren photometry is available for the two [FORMULA] Cephei stars we discovered. Therefore, we could not convert Strömgren indices to [FORMULA] and compare their positions with those in other open clusters, as we did in Paper I (see Fig. 9 there in). It seems, however, rather certain that both these stars are still in the core-hydrogen burning phase.

6.3. Comparison with previous work

We compared our [FORMULA] photometry with previous photoelectric (Johnson & Morgan 1955; Schild 1965; Tapia et al. 1984) and photographic (Wildey 1964; Moffat & Vogt 1974) [FORMULA] studies. The result of these comparisons is shown in Fig. 10, and the mean differences are given in Table 5.

[FIGURE] Fig. 10. Comparison of different [FORMULA] photometries. All the differences are in the sense our minus Johnson & Morgan (1955, filled circles), Schild (1965, open circles), Tapia et al. (1984, open squares), Wildey (1964, plus signs), and Moffat & Vogt (1974, dots).


[TABLE]

Table 5. Mean differences between our and other photometric studies. SD stands for the standard deviation from the mean difference which is given in the preceding column. N is the number of stars in common with a given author. Both the mean differences and SD are expressed in mag.
Notes:
a The mean differences, [FORMULA] and [FORMULA], and the corresponding standard deviations, were in each case calculated without the one star that deviated most. This was Oo 837 in the [FORMULA] and Oo 885 in the [FORMULA].


Our photometry agrees quite well with all photoelectric studies. Out of the two photographic [FORMULA] data sets, the one of Moffat & Vogt (1974) is much better. The differences between our and Wildey's (1964) V measurements are mostly positive and have very large scatter, even for bright stars. On the other hand, his [FORMULA] indices are systematically larger than ours, and have a large scatter too. This means that this photometry is very uncertain, as was already claimed by Tapia et al. (1984). Consequently, Wildey's (1964) conclusions concerning the cluster parameters and reddening should be viewed with caution. The photographic [FORMULA] photometry of Moffat & Vogt (1974) have a much smaller scatter than Wildey's, and agrees better with the photoelectric measurements. This is why some of the stars measured by them were used in our transformations.

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Online publication: April 19, 1999
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