Baltic Astronomy, vol. 5, 445~450, 1996. OBSERVATIONS OF CLUSTERS USING THE STRÖMVIL SYSTEM. I. STANDARD AREAS A. G. Davis Philip1-2, R. P. Boyle3 and V. Straizys4 1 Union College, Schenectady, NY 12308, U.S.A. 2 Institute for Space Observations, 1125 Oxford PL, Schenectady, NY 12308, U.S.A. 3 Vatican Observatory Research Group, Steward Observatory, University of Arizona, Tucson, AZ 85721, U.S.A. 4 Institute of Theoretical Physics and Astronomy, Gostauto 12, Vilnius 2600, Lithuania Received September 1, 1995. Abstract. A set of standard areas of the Strömvil system is de- scribed. Preliminary values of the color indices of standard stars for photoelectric work may be obtained from their published values in both parent (uvby and Vilnius) systems. Stays in 19 open clusters already observed in both systems may be used as preliminary stan- dards. Standard areas for the CCD work are to be measured anew. For this, a set of 12 compact open clusters and globular clusters have been selected, six areas in each hemisphere, mostly at declinations +30 deg and -30 deg. The 10-20 arcmin areas will contain stan- dard stars of 10-16 mag. They will be good for fixing the zero-point of magnitudes and color indices, for the determination of extinction coefficients in the Earth's atmosphere and for the determination of color equations between instrumental CCD systems and the standard Strömvil system. The first observations in this program are planned with the new 1.8-m Vatican telescope on Mt. Graham, Arizona. Key words: techniques: photometric — stars: standard — Galaxy: areas, open clusters, globular clusters 446 A. G. Davis Philip, R. P. Boyle and V. Straizys 1. INTRODUCTION Straizys, Crawford & Philip (1996) have proposed a photometric system, named Strömvil, which combines the Vilnius and Strömgren systems. Table 1 presents data concerning the passbands in the Strömvil system. The physical justification of the passbands in the new system is described in the paper cited above. The system will achieve better classification possibilities than the uvby system alone over a larger range of spectral types and luminosities, especially in the presence of interstellar reddening. Although the Vilnius system gives classification for stars of all spectral types, by switching four of its passbands to four passbands of the Strömgren system we can make use of the very extensive uvby photometric database. For thousands of stars only three additional measures (in Ρ, Ζ and S) will create a set of seven color indices which will allow a classification of stars of all spectral types. Table 1. Mean wavelengths and half-widths of passbands of the Strömvil photometric system. Passband 35 37 41 47 52 55 66 λο (nm) 350 374 411 467 516 547 656 Δλ (nm) 30 26 19 23 21 23 20 2. REQUIREMENTS FOR THE STANDARD STARS For calibration of the system, we need photoelectric observa- tions of relatively bright stars of various spectral classes, luminosi- ties, metallicities and peculiarities. These stars should have accu- rately determined temperatures, surface gravities, absolute magni- tudes, metallicities, rotational velocities and other parameters. For this, as well as for determination of the intrinsic color indices, stars of open and globular clusters are suitable. Since the Strömvil system will be used for the study of faint stars in relatively small areas, we need CCD areas with standard stars for the following purposes: (a) establishing the zero-point of the magnitude scales, (b) determination of color equations between Observations of clusters in the Strömvil system. I. 447 the instrumental and standard systems, (c) determination of the ex- tinction coefficients in the Earth's atmosphere. These stars should be sufficiently faint (>10 mag) to be on the linear part of the CCD response but also sufficiently bright (<16 mag) to be measurable with high precision (< 0.01 mag) on short exposure frames. The areas should contain stars of a wide range of color indices for the determination of color equation. The stars should be almost unred- dened to be useful in finding the intrinsic color equations. Also, these stars should be not of very different apparent brightness, so they can be observed with similar accuracy in one exposure. And last, the standard areas should be sufficiently small (10-20 arcmin) to be used for different sizes of CCD chips on telescopes of different focal lengths. It is not easy to find areas satisfying all these requirements. Areas at high galactic latitudes usually do not contain hot stars and at low galactic latitudes faint stars are all reddened by interstellar dust. Most of the close and unreddened open clusters have much too large angular sizes to be imaged on a single CCD frame. Distant open clusters are usually considerably reddened. In open clusters the reddest main sequence stars are very faint relative to member early-type stars. However, there are some old open clusters of the M 67 type which contain both red giants and blue stragglers at about the same appar- ent visual magnitude. Some of them are at high galactic latitudes and, therefore, unreddened. Also, they are mostly distant objects and thus have small angular size. The M 67 cluster contains seven or eight blue stragglers and is a very good area for CCD cameras embracing a 20' field. In some cases, the intermediate-age open clus- ters of the NGC 7789 type with small interstellar reddening are also acceptable. However, unreddened open clusters of small angular size are rare and it is not possible to find them in various right ascensions to be comfortably accessible all the year round. Other candidates for standards are globular clusters. Many of them contain horizontal branches with blue and red stars at simi- lar apparent magnitudes. In the ultraviolet, the blue HB stars are sufficiently bright, being of comparable brightness as the red giants. The angular sizes of globular clusters match the areas covered by CCD chips. The considerable crowding of stars in a globular cluster core mandates that standard stars should be selected in their outer regions. Globular clusters have a metal deficiency relative to the majority of field stars and open clusters. However, color equations 448 A. G. Davis Philip, R. P. Boyle and V. Straizys Table 2. Open clusters observed in the Strömgren and Vilnius systems Name RA(2000) Dec(2000) NGC 129 00 29.9 +60 14 NGC 752 01 57.8 +37 41 Alpha Persei 03 22 +48 40 Pleiades 03 47 +24 08 Hyades 04 20 +15 39 Praesepe (M 44) 08 40.1 +20 00 NGC 2682 (M 67) 08 50.4 +12 00 Coma Berenices 12 26 +26 NGC 6494 (M 23) 17 56.8 -19 02 NGC 6611 (M 16) 18 18.8 -13 47 NGC 6633 18 27.9 +06 34 NGC 6705 (M 11) 18 51.1 -06 17 NGC 6811 19 38.2 +46 34: NGC 6910 20 23.1 +40 47 NGC 6913 (M 29) 20 23.9 +38 32 NGC 7039 21 22.2 +45 39" NGC 7092 (M 39) 21 32.2 +48 26 NGC 7686 23 30.2 +49 08 NGC 7789 23 57.0 +56 44 between close photometric systems are not very sensitive to metal- licity differences (Straizys, Kazlauskas et al. 1996). 3. PRELIMINARY SELECTION OF THE STANDARD AREAS Color indices for the Strömvil system can be obtained from the existing measurements in both the parent systems. Strömvil indices 35-55, 41-55 and -55 can be obtained from the Strömgren indices u — y, ν — y and b — y with a correction in zero-point. The Strömvil indices 37-55, 52-55 and 55-66 can be obtained from the Vilnius indices Ρ — V, Ζ — V and V — S without any changes. This has been done by Straizys et al. (1996) in obtaining Strömvil indices for three standard areas. There are about 2000 stars observed in both systems for which we are able to form Strömvil indices. Among them are stars of 19 open clusters which are listed in Table 2. These clusters, especially Observations of clusters in the Strömvil system. I. 449 Table 3. Preliminary standard areas for the Strömvil system in the northern hemisphere Name of central Type of RA (2000) DEC (2000) EB-V Purpose object object h m 0 / mag * NGC 752 OCl 01 57.8 +37 41 0.03 1, 2 NGC 2099 (M 37) OCl 05 52.4 +32 33 0.31 1, 2, 3 NGC 2682 (M 67) OCl 08 50.4 +12 00 0.06: 1, 2, 3 SA 57 Field 13 08.4 +29 28 0.02 1, 2 NGC 6341 (M 92) GC1 17 17.1 +43 09 0.01 1,2,3 NGC 7078 (M 15) GC1 21 30.0 +12 10 0.08 1, 2, 3 * 1. Zero-point of magnitudes and color indices, 2. Atmospheric extinction coefficients, 3. Color equations between the instrumental and standard systems. Table 4. Preliminary standard areas for the Strömvil system in the southern hemisphere Name of central Type of RA (2000) DEC (2000) EB-V Purpose object object h m 0 / mag * NGC 288 GCl 00 52.6 -26 36 0.03 1,2,3 NGC 1904 (M 79) GCl 05 24.3 -24 31 0.01 1, 2, 3 NGC 2818 OCl 09 16.0 -36 36 0.18 1, 2 NGC 5139 (Ω Cen) GCl 13 26.8 -47 19 0.11 1, 2, 3 NGC 6405 (M 6) OCl 17 40.1 -32 13 0.15 1, 2, 3 NGC 7099 (M 30) GCl 21 39.1 -23 12 0.01 1, 2, 3 * see notes to Table 3. those having small angular sizes, can be used as the existing pre- liminary standards of the Strömvil system for CCD work.