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Baltic Astronomy, vol. 2, 246-255, 1993. THE SOUTHERN VILNIUS PHOTOMETRIC SYSTEM. I. TRANSFORMATION TO THE STANDARD SYSTEM M.C. Forbes2, R.J. Dodd1 and D.J. Sullivan2 1 Carter Observatory, P. 0. Box 2909, Wellington, New Zealand 2 Victoria University, P.O. Box 600, Wellington, New Zealand Received July 15, 1993. Abstract. This paper is the first in a series on the extension of the Vilnius photometric system to the southern hemisphere. Observa- tions of a common set of 73 stars measured in both hemispheres are described and an analysis of the differences given. Key words: Vilnius photometric system - transformation equations - response functions - southern hemisphere 1. Introduction The seven filter intermediate bandpass Vilnius photometric system was expressly designed to allow purely photometric determina- tion of spectral type, absolute magnitude and metallicity of a star while correcting for interstellar reddening (Straizys and Sviderskiene, 1972; Straizys, 1973, 1992a,b) The system can also identify pecu- liar stars such as subdwarfs, white dwarfs, blue horizontal branch stars, Be stars, Herbig Ae/Be stars, Τ Tauri type stars, metal poor stars, carbon and barium stars and some types of unresolved bina- ries (Straizys, 1992a,b). Hence, the system is well suited to studying star clusters, interstellar matter and galactic structure, as shown by investigations listed by Straizys (1992b). In view of this, it was felt desirable to extend the use of the Vil- nius system to the southern hemisphere. An extensive set of standard stars was established around the North Celestial Pole (Cernis et al., 1989), the Kapteyn areas (Zdanavicius et al., 1978; Cernis and Jasevicius, 1992), the Cygnus Standard Region (Zdanavicius et al., 1969; Zdanavicius and Cerniene, 1985) and Aquila Standard Region The Southern Vilnius Photometric System. I. 247 (Zclanavicius et al., 1969). As none of these standard regions is south of the celestial equator it became necessary to establish first a set of southern standard stars. A set of stars that are observable from the southern hemisphere was selected from those previously measured in the Vilnius system (North, 1980) to be used as primary standards. This paper describes the analysis of 73 of these stars which were measured using a local version of the Vilnius filters, being the first step in the establishment of a Southern Vilnius Photometric System. 2. Observations The observations were made at the Mt. John University Obser- vatory (University of Canterbury), situated near Lake Tekapo in the South Island of New Zealand, over the period 1987 to 1992. Approx- imately 10% of the nights scheduled for observing were photometric (Dodd, Forbes and Sullivan, 1992). The photometer housed a cooled EMI 9558 photomultiplier tube mounted on a 61 cm (Boiler and Chivens) Cassegrain telescope. A descriminator/amplifier drives a pulse counter, with the integrated count being logged by a computer. The local filter set was manufactured by the Vilnius Observa- tory to closely match the standard system. The variation of filter transmission with wavelength, shown in Fig. 1, was measured using a double beam monochromator at Victoria University of Wellington. A comparison with the Vilnius standard response functions is given in Fig. 2. Apart from the U filter which has a peak transmission at a shorter wavelength than the Vilnius standard, and the S filter which has a peak transmission to the red of the Vilnius standard (which also has more pronounced wings), the two systems match well. A typical observation consisted of three measurements per filter, with the filter sequence being U, Ρ, Χ, Υ, Ζ, V, S. As the measured stars are quite bright (\Λ = 4—7 mag), 10 second integration times for the U and S filters and 5 seconds for the remainder were sufficient to ensure a photon count greater than 10 000 (i.e. photon statistics better than 1%). Two consecutive observations were usually made per star each night. The sky background was measured at 15 to 60 minute intervals, dependent on the sky stability and proximity to twilight or moon light. Extinction stars were measured every hour, using a variant of the Nikonov's method (Nikonov, 1953; Straizys, 1992b) described in Section 3. The measurements of the primary standard stars were interspersed with the program stars, each star being observed near Μ.C: Forbes, R.J. Dodd and D.J. Sullivan 3000 4000 5000 6000 7000 A(Ä) Fig. 1. The spectral transmittance of the southern Vilnius filters. 3000 4000 5000 6000 7000 A(Ä) Fig. 2. The normalised spectral response functions of the southern (solid lines) and northern (dashed lines) Vilnius systems. The standard Vilnius system responses are from Straizys and Zdanavicius (1970). The Southern Vilnius Photometric System. I. 249 culmination. During summer, the short nights and scarcity of equatorial standards permitted the measurement of only 6 or so primary standards; the situation in winter was more satisfactory with 16 standards commonly observed per night. 3. Reductions First, the count-rates were corrected for the dead-time of the instrument system and the sky background subtracted. The raw magnitudes were then corrected for atmospheric extinction using a variant of the Nikonov's method: the control star was measured to- gether with each extinction star measurement rather than only twice (i.e. at the beginning and end of each night). This allowed a least squares fit to be made to the magnitudes of both the extinction and control stars each night. Where appropriate, averages of magnitudes were taken over the entire observing run. It should be noted that extinction has generally increased and become less stable when the dust from the Mt. Pinatubo eruption reached New Zealand (Dodd et al., 1992). By measuring a. minimum of 6 extinction stars of differing spectral types over a wide range of airmasses on a photometric night, the colour dependence of the extinction coefficients was found: ku-p = kffjp - 0.020{(-Y-K) + 0.03} e kp-χ = k P% + 0.017{(X-F) + 0.03} kx_Y = Jfe«jy - 0.013 {(X-F) + 0.03} with the magnitudes and other colour indices having no colour dependence. Unreddened GO V stars were used as extinction stars, having an X-Y colour in the instrumental system (i.e. before trans- forming to the standard system) of -0.03. The standard deviations are ±0.006 in the colour coefficients and ±0.01 in the colour zero- point. These colour extinction coefficients compare favourably with the corresponding coefficients -0.027, +0.013, and -0.011 of the standard Vilnius system (Straizys, 1992b). Finally, the observations were transformed to the primary Vil- nius system by least square fitting of the following equations: 250 M.C. Forbes, R.J. Dodd and D.J. Sullivan (Y-V)0 =a0 + a1(Y-V) V + -V)0 (U-P)-(U- Ρ)ο = Co + Cl(Y - V)o (Ρ - X) - (P - X)0 =d0+d1 (Y- V)0 (X-Y)-(X- Υ)ο = eo + ei(Y - V)0 (Y-Z)-(Y- Ζ)ο =f0+ fi(Y - V)0 (Z-V)-(Z-V)o=g0+9i(Y-V)o (V -S)-(V- S)0 = ho + h\(Y - V)o with the primary standard magnitudes and colours indicated by the zero subscript. As there are inherent systematic errors in any under-sampled filter system (Young, 1974, 1992), the transformations will not per- fectly map onto the standard Vilnius system but will produce a similar system (called the southern Vilnius photometric system) which on average matches the standard system. A plot of the residuals between the standard and southern systems is shown in Fig. 3. 4. Catalogue Table 1 lists the observations of the 73 standard stars measured in the southern system. The first column is the star name (HD number unless otherwise stated). The next two columns are the Epoch 2000 equatorial coordinates. These are followed by the Vilnius V magnitude and the six colour indices. If a measurement is followed by a colon, then it differs from the value in the standard (northern) system by 0.05 to 0.10 mag; a double colon means a measured dif- ference greater than 0.10 mag. The number of observations of each star is recorded in the next column, with the final column being the spectral type as given in the Vilnius catalogue (North, 1980). The overall internal accuracy and the precision of the transformation between the standard and the southern Vilnius systems can be seen in Table 2. This shows the standard deviation ε of each colour calculated using σ ε = The Southern Vilnius Photometric System. I. 251 Fig. 3. The residuals between the standard (northern) system and the southern Vilnius system. The ordinate interval is 0.10 mag. where σ is the typical standard deviation of a single observation for the southern system or residuals between the standard and southern systems respectively, η is the number of observations of a star and Ν is the number of observed stars. The largest error is in U-P. This is seen in both the internal and external errors, implying that longer integration times are required for observations in the U band if a similar precision to observations in the other bands is required. 252 M.C. Forbes, R.J. Dodd and D.J. Sullivan Table 1. Catalog of standard stars in the southern Vilnius photometric system (equatorial region) HD α(2000) (5(2000) V U-Ρ P-X X-Y Y-Z Z-V V-S η Sp 26912 04h15m32s+08°53.'5 4.29 0.34 0.40 0.20 0.11 0.07 0.15 2 Β3 IV 28556 04 30 37 +13 43.0 5.43 0.61 0.74 0.53 0.20 0.12 0.30 2 F0 V 31237 04 54 16 +02 26.4 3.71 0.18 0.21 0.13 0.07 0.03 0.08 2 B3 III+BO V 31283 04 54 47 +11 25.4 5.19 0.69 0.84 0.42 0.14 0.10 0.19 2 A3 V 50082 06 51 54 +06 36.0 7.41 0.48 0.84 1.28 0.49 0.29 0.71 2 G5 III(Ba) 52382 07 00 39 - 09 12.2 6.44:0.21 0.18 0.28

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