2. Observations

The observations for this long term study were carried out during three different seasons: the spring of 1982, in February, 1985 and February, 1997. The photometric data was reduced in two different ways: i) by differential photometry to link the data strings of the observatories when more than one is involved and to get more accurate measurements and ii) by absolute photometry to determine the physical characteristics. These latter observations were reduced with a standard method which has already been described (Peniche et al. 1990). Emphasis should be made, however, that in the case of differential photometry the mean for each night was subtracted from the data so long term variations cannot be detected.

The first campaign was designed to increase the accuracy in the period determination of KW 204 and thus, a simultaneous observational season was planned between the Ege University Observatory, Turkey, and the Observatorio Astronomico Nacional, at San Pedro Martir, Mexico. The dates of observations and the telescopes employed have been summarized in Table 2 (click here). In Turkey, the observations of KW 204 were made at the Ege University Observatory; an unrefrigerated EMI 9781 photomultiplier with the Johnson V filter along with a 48 cm Cassegrain reflector telescope was utilized. Correction for atmospheric extinction with respect to the primary comparison was carried out. Errors in each figure are 0.002 day and 0.002 magnitude. Two telescopes were used in Mexico, the 1.50 m and the 0.84 m. A single-channel photometer with a dry-ice-cooled 1P21 photomultiplier was utilized on each one; the photocells were used with the Johnson V filter. Two comparison stars were chosen for all the observed runs, KW 328 and KW 284 with spectral types of A9 III and A9 V, respectively. The highest possible precision is achieved by the frequent monitoring of the two nearby comparison stars, similar in brightness and color to the program stars. Since in Mexico two program stars were observed in the observing procedure, with different amplitudes, 0.07 mag (KW 204) and 0.02 mag (KW 207), the latter star was monitored more frequently, with a consequent increase in the time span between consecutive observations on the large amplitude variable. Each measurement consisted of five ten-second integrations of each star and one ten-second integration on the sky. A subtraction of the sky measurement from the average of the star's integrations was done. The photometric values reported are the magnitude differences between each variable star and the average of the standard stars interpolated to the time of the observation of the variables. Then, an average V was subtracted so a zero baseline could be established.

A second observing run was carried out in February, 1985. At this time in Mexico, better instrumentation was available and thus, different stars were considered. Also, the collaboration program with the Konkoly Observatory allowed the continuous monitoring of two variable stars. The 1.5 m telescope was employed with a photometer that allows the simultaneous observations in the uvby filters of the Strömgren system and, almost simultaneously, in the N and W filters that define H. Two comparison stars were chosen for the observed runs, KW 150 and KW 284. Each measurement consisted of four ten-second integrations of each star and one ten-second integration of the sky for the uvby filters and three ten-second integrations for the narrow and wide filters one ten-second integration of the sky. The accuracy of each value is, for the faintest star, KW 154, millimag and the accuracy in time is 0.0015 d. The photometric values analyzed were the magnitude differences between each variable star and that of the mean of the reference stars interpolated to the time of the observation of the variables. Then, the average magnitude of the season was subtracted so a zero baseline could be established. The sequence was the following: KW 284, KW 323, KW 207, KW 45, KW 154, KW 150, KW 445, KW 323 uninterruptedly during the whole season except for the two last nights on which KW 45 was substituted for KW 204. A few standard stars were observed to transform the instrumental observations into the standard system. In Hungary a 1.0 m telescope of the Piszkéstetö observing station provided with a one channel photometer was employed. The reference star was KW 284 and the V filter was utilized; the integration time for each star was 10 s for each one of the three measurements and the sequence of observation was KW 284, sky, KW 323 and KW 445.

In 1997, the 0.84 m telescope provided with a one channel cooled phototube at SPM was utilized in differential photometry. Two stars were considered as references: KW 150 and KW 284. The reduction was done in the customary way. The accuracy of each point, taken from the dispersion of the three measurements, was 0.003 mag and 0.004 d in time. All the photometric data considered in the present work has been submitted to the IAU archives.

 

HJD- observed stars observatory photometry

2440000 (KW)

5039 204, 284 Ege V

5046 204, 284 Ege V

5047 204, 207,284 SPM V

5048 204, 207,284 SPM V

5056 204, 284 Ege V

5076 204, 207,284 SPM V

6106 045,150,154,207,284,323,445 SPM uvby

6107 045,150,154,207,284,323,445 SPM uvby

6108 045,150,154,207,284,323,445 SPM uvby

6109 045,150,154,207,284,323,445 SPM uvby

6110 045,150,154,207,284,323,445 SPM

6111 045,150,154,207,284,323,445 SPM

6112 150,154,204,207,284,323,445 SPM

6113 150,154,204,207,284,323,445 SPM

6113 284,323,445 Piszkéstetö V

10509 150,204,284 SPM V

10510 150,204,284 SPM V

 

2.1. Absolute photometry

Strömgren photometry provides unique opportunities for determining both membership to the cluster and physical characteristics of the observed star. In the present study, besides observing the variable stars in differential photometry, several standard stars were observed in order to transform their photometric values into the standard system. The reduction to the instrumental system followed the method described by Gronbech et al. (1976). The description of the photometric equipment can be found in Schuster & Nissen (1988). The transformation from the instrumental system to the standard one was done considering the computing package of Parrao et al. (1988). The night of February 13th was not included in H because very few photometric standards were measured so only the nights between 14 and 17 of February were considered. The quality of each night is presented in Table 3 (click here) in which Col. 1 provides the date; the number of standard stars observed per night is given in columns two and seven for uvby and H, respectively. Columns three to six and eight provide the errors of the night in each index.

 

100285 21 0.008 0.002 0.002 0.005

110285 61 0.005 0.001 0.002 0.005

120285 57 0.012 0.001 0.002 0.005

130285 56 0.006 0.002 0.002 0.005

140285 58 0.005 0.002 0.003 0.009 12 0.006

150285 68 0.006 0.002 0.002 0.005 32 0.004

160285 78 0.006 0.002 0.003 0.006 31 0.008

170285 89 0.005 0.002 0.003 0.008 48 0.007

 

Instrumental and standard errors for the colors and indexes are presented in Table 4 (click here).

 

system V (b-y) m₁ c₁

inst 0.007 0.002 0.003 0.006 0.007

std dev 0.008 0.004 0.017 0.007 0.008

Table 4: Instrumental and standard errors for the colors and indexes

 

The obtained coefficients to the standard system are:

The values were transformed into the standard system in the following fashion: V magnitude from the UBV of Johnson (1952); (b-y), m₁, c₁ from Crawford & Barnes (1970) and the H values from Crawford & Mander (1966). The standard photometric values utilized for the transformation were those listed by Olsen (1983) except for KW 284 and KW 150 whose photometric values were from Crawford & Barnes (1969). The final transformations as well as the errors found and the number of observations for each standard star are listed in Table 5 (click here). In Table 6 (click here) the corresponding values for the program stars are presented. The mean values of the program stars have been compared to those of Crawford & Barnes (1969). Both sets gave a regression coefficient of 0.99 when considering the values of (b-y), m₁, c₁ and H. However, since the uncertainties in magnitude and color indexes are higher than the amplitude of variation, particularly in H, a mean value of the measurements was taken to extract the physical characteristics of the stars from the theoretical grids.

 

object V b-y m₁ c₁

KW 284 6.782 0.159 0.193 0.932 -0002 0.002 -0.013 0.005 221 2.758 -0.002 48

KW 150 7.450 0.155 0.186 0.946 0.000 0.002 -0.005 -0.004 222 2.756 0.003 49

HD 84937 8.332 0.303 0.051 0.361 0.007 0.000 0.003 -0.007 22 2.618 0.001 12

HD 87195 8.258 0.421 0.226 0.330 0.003 -0.003 0.013 0.006 17 2.601 -0.013 10

BS 1662S 6.177 0.397 0.193 0.351 -0.011 0.000 -0.007 -0.002 1

BS 1861 5.340 -0.072 0.066 0.002 0.002 -0.001 0.007 0.002 1

HD 107550 8.360 0.494 0.154 0.429 0.007 0.004 -0.013 0.002 2 2.553 0.004 2

HD 117243 8.355 0.408 0.216 0.396 -0.005 -0.004 0.015 -0.002 2 2.598 0.007 2

 

 

45 8.263 0.012 0.132 0.003 0.216 0.004 0.854 0.006 145 2.793 0.002 21

154 8.515 0.012 0.145 0.003 0.211 0.004 0.803 0.007 220 2.786 0.001 34

204 6.677 0.018 0.144 0.003 0.199 0.003 0.988 0.007 76 2.763 0.001 37

207 7.683 0.037 0.105 0.003 0.208 0.003 0.970 0.010 220 2.804 0.001 28

323 7.825 0.007 0.120 0.002 0.208 0.003 0.902 0.006 373

445 7.981 0.010 0.110 0.003 0.219 0.003 0.914 0.007 201