|The WBVR Photometry of Northern Bright Stars|
Photoelectric measurements of more than 13 500 stars have been made with a four-channel photometer attached to 48-cm reflector of the Tien Shan High Altitude Station of the Sternberg Astronomical Institute during 1985 - 1988. The program, observational details, photometric equipment and reduction procedures used are described. Particular attention is paid to definition of the photometric WBVR system of the catalogue. The stability of photometric system has been assured by taking into account temporal and temperature shifts of the photometric passbands. These shifts were obtained from the analysis of observational data. Estimates of the accuracy of the photometry are presented. A study of completeness of the catalogue shows that it contains more than 95% of all stars with V < 7m2. The transformation to the photometric system UBV is discussed.
Photometric observations in WBVR system were started at the Sternberg Astronomical Institute in 1976. The choose of this broad-band photometric system was influenced by Straizys (1973) who has suggested the system as an alternative to Johnson's UBV system to avoid its shortcomings.
From 1976 till 1984, WBVR observations were obtained for more than 2000 stars with magnitudes in the range from 0m to 8m (Mironov et al. 1984, Khaliullin et al. 1985). In 1984 we started a program of magnitude limited WBVR photometry of bright stars in the northern sky. The total number of the program stars was limited by the equipment efficiency, number of observers, planned observation period planned(four years):it contained about 15 thousand stars down to 7m2 in the V passband.
During the period of observation, WBVR photometry of 13 560 objects (stars and multiple systems) has been obtained in the range of declination from -14° to +90°. The results have been published in the Catalogue of WBVR Magnitudes of Bright Northern Stars by Kornilov et al. (1991).
The observations have been made at the Tien Shan High Altitude Station of the Sternberg Astronomical Institute near Alma-Ata, Kazakhstan (at the altitude of 3000 m above sea level) using a 48-cm reflector. The latitude of site is about +43°. The site has a high transparency of the atmosphere (the contribution of aerosols in absorption is about 0m01 on some nights) and good stability. The observations were started in January 1985 and completed at October 1988. The photometric measurements of all program stars were carried out during 360 nights with total obsevational time of 2400 hours. About 57 000 independent measurements of program stars and more than 13 000 of standard stars have been obtained. The distribution of observational time over years and seasons is listed in Table I.
For observations, only the nights with complete absence of cloudiness and with a transparency variation less than 0m01 during 20 minutes were used. Further analysis of obtained data confirmed the stability of transparency for 95% of observational time. Only 0.5% of observational data were rejected due to instability of the atmospheric extinction.
About 65% of observations were made in the autumn and winter seasons. The great majority of observations 80%) was obtained at temperatures from +5°C to -15°C (i.e. at -3.8°C in average). Small temperature variations resulted in the homogeneity of observational data.
The program stars were chosen from the input catalogue based the CSI (Ochsenbein & Bishoff, 1975). All stars brighter than 7m2 in V passband (or in the visual passband) and with declinations within +90° and -14° (the whole -14 BD zone)were included into our observational program. Thus, the program covers about 63% of all sky. Some fainter stars from HD catalogue, which were near to the main program stars, have been mesured too. Additionally, we included some stars of late spectral type brighter than 9m. A few stars brighter than 6m5 have not been measured due to mistakes in the input catalogue.
Our intention was to obtain at least four measurements for each program star: two measurements with an interval 15 — 20 min during one night and next two measurements during other night in other season (see Table IV). Such a method has been accepted to control both the stability of the atmospheric extinction within a time-scale of about 10 min and the stability of the photometric system over all period of observations. Also, such distribution gave us possibility to identify new variable stars and to reveal mistakes in measurements.
Two or three standard stars were usually observed each ~15 min with the program stars. Pairs of standards with equal altitude were observed more or less periodically. To obtain the parameters of atmospheric extinction, a pair of standard stars, with difference in air masses ~0.6, were observed two or three times per night.
The 30" diaphragm was used most frequently. In the case of close multiple stars, we tried to measure both the total brightness and the individual components if it was possible.
For the photometry we used a four-channel photometer providing simultaneous measurements in W, B, V and R photometric bands, connected with a computer for measurement, recording and preliminary analysis of the data. The main feature of the photometer (Kornilov & Krylov, 1990) is a division of the light beam among the channels by means of semi-transparent aluminium films. Despite losses of light, the efficiency of this photometer for measurements of stars brighter 9m (with a 0.5 m telescope) is higher than that with the successive changes of filters because, in our case, the accuracy and integration time for bright stars is defined mainly by atmospheric scintillations and practically do not depend on the brightness of a star. The use of the grey (quasi-neutral) beam-splitter, reducing the light considerably, made it possible to decrease the effects of non-linearity. In our case, the accuracy of the correction for non-linearity was better than 0m005 for all stars.
The equipment for measurement, recording and preliminary analysis of data was based on a mini-computer LSI-11 (Kornilov et al., 1990). The hardware and software used helped to increase the efficiency of photoelectric observations and to decrease the probability of wrong measurements. The main source of mistakes was in the hand-pointing of the telescope.
The response functions of the WBVR system shown in Fig.1. have been set up by means of the glass filters described by Meistas et al. (1975) and kindly presented by V.Straizys. The passbands of the UBV system are plotted too. The V passband of Johnson's system practically coincides with V band in our system. The transmittance curves of our filters were measured woth a laboratory spectrophotometer and monochromator calibrated with the optical radiometer. The curves were reduced to a temperature of -4°C.
A network of standard stars was set up simultaneously with the observations of program stars. To define the magnitudes of 228 standard stars, more than 13 000 measurements have been used.
For the definition of the zero-point of the V magnitude, the star HD 5015 (F8 V) was used: for it V = 4m797 is taken in accordance with Khaliullin et al. (1985). For the definition of the zero-point of color indices, 14 A0 V stars were used: their average color indices are taken to be zero. With this normalization, color indices of HD 5015 have to following values: W-B = -0m082, B-V = +0m553, and V-R = +0m456. The average color indices of six A0 V stars, used for the normalization of color indices by H.L.Johnson in UBV system, are: W-B = +0m028, B-V = +0m000, V-R = -0m007.
The magnitudes of the standards and estimates of their errors were derived simultaneously. About 50% of 192 standard stars have rms errors of V magnitudes smaller than 0m0016, 90% of them — less than 0m0023.
The basic ideas of the method used for reduction of observations to outside the atmosphere were presented by Moshkalev & Khaliullin (1985). The extinction of light in each passband was calculated by the numerical convolution of typical energy distributions in the stellar spectra, the model curve of atmospheric extinction and the response functions of the bands. The difference between the real extinction coefficient and the theoretical one was modeled by a two-parameter function of wavelength. These parameters were found by minimizing the deviations of measurements of standard stars. Special attention was paid to the choice of the suitable energy distribution adequate to the observed photometric data.
Some changes of the photometric system has been suspected during preliminary data processing. This changes have affected especially the W and B magnitudes of the red stars. The most probable explanation of this effect is a temperature shift of the response functions. It was possible to evaluate this effect from the same observational material, since most stars have been measured in different seasons, with different outside temperatures.
However, an additional effect was found: the shift of passband with time. The detected dependence of shifts in the W and B passbands with time is shown in Fig. 2a, b. We did not find any significant changes in the V and R passbands. A shift of the response function by 1 nm causes changes of the measured magnitudes for an M0III star up to 0m013 in W and 0m017 in B. Apparently, these shifts can be explained by the changes of spectral properties of semitransparent aluminium layers and the optical glue in the photometer beamsplitter. The jump in those shifts in September 1986 can be explained by the readjustment of the photometer optics and the replacement of glue in beamsplitter.
The evaluated temperature coefficients are: 0.005, 0.020, 0.036 and 0.056 nm/°C for W, B, V and R passbands. The maximum change of magnitude in the B passband for M0III stars is about 0m003 for each 10°C.
The data have been reduced to the fixed period (September 1987) and to the average temperature of observational nights (-4°C). The influence of the temperature effects for the standards stars (spectral classes from A0 to G5) was small and was not taken into account. The azimuth effect did not exceed 0m003 for more than 99% standard and program stars.
The rms errors of magnitudes were calculated for all stars except for the multiple and variable stars. The errors and their correlation coefficients for different groups of stars are shown in Table II and III. A significant correlation between the errors of W, B, V and R magnitudes is due to the simultaneous measurements in the four spectral passbands under quasi-neutral variations in atmospheric transparency. This correlation decrease errors in the color indices.
The accuracy of photometry of binary stars with close components (angular distance less than 5 - 10"; only total magnitude can be measured) or of binary stars with detached conponents (angular distance more than 30" - 40"; the individual components were observed) is the same as for single stars. The errors can be larger in the intermediate case.
The accuracy class is introduced to define the quality of measurements for each star. This is only a ranking estimate, not the rms error, which is difficult to estimate since there are strong correlations of different colors and too small number of measurements for each star. Number distributions of the accuracy classes for different groups of stars is shown in Fig. 3. This distributions may be may be used as quantitative estimations of the quality of our observational data. The statistical characteristics of the distributions correspond to the accuracy estimates of individual measurements (Table II), taking into account that the mean number of measurements for a star is 4.
As it was mentioned above, our intendion was to obtain four independent measurements for each program star. We did not succeed to do this for some stars. On the other hand, a considerable part of the stars has been measured six and more times (see Table IV).
The investigation of the completeness of our catalogue shows that it contains more than 95% of stars brighter than V =7m2 (see Kornilov & Sementsov, 1992). Differential star counts of the WBVR catalogue in Fig.4. are compared with the Catalogue of Bright Stars and its Suppliment (Hoffleit, 1982, Hoffleit et al., 1984), the HD catalogue and the classic data by F.Seares (from Allen, 1977).
For a comparison of our results with magnitudes and color indices in the UBV system
obtained by other authors, the catalogue compiled by Mermilliod (1991) was used.
The total number of common stars is 6511, but 475 of them with
the differences V* - V exceeding 0m1 have been excluded in
obtaining transformation equations (the asterisk denotes magnitudes from Mermilliod
catalogue). The correlation of magnitudes is found in the form
Transformation equations between ours B-V and W-B color indices and the (B - V)* and (U - B)* color indices of the UBV system also habe been determined (Leontiev & Zakharov, 1995). The stars used in determination of the color equations were not separated into luminosity classes, and the interstellar reddening was not taken into account. The rms scatter of residuals (B - V)*c - (B - V)* after the transformation is equal to 0m014.
The rms scatter of residuals (U - B)*c - (U - B)* is equal 0m026. The transformation by a qubic polinomial with three color indices was used in this case. As Fig.5 shows, no systematic residuals are present.
An independent comparison of WBVR and UBV magnitudes and color indices was accomplished recently by Warren (1994).
The WBVR system was proposed in 1973 to replace the illdefined Johnson's system UBV. Unfortunately, the new system is not sufficiently known among astronomers. Therefore, more information about the system is necessary. One must realize that the replacement of the defective U passband by revised W passband and introduction of the exact transformation of the W - B color index to outside the atmosphere should increase its accuracy considerably (Straizys 1973, 1977, 1983, 1992). Let us hope that a homogeneity and a significant volume of the WBVR published by the authors will be a good start for a wide use of this variant of the UBV system. The complete WBVR catalogue on a disk may be obtained from the authors. Soon it will be aviable at the Strasbourg Stellar Data Center.