3. JHKLM photometry of solar type stars

BS 334, BS 458, BS 4031, BS 6075, BS 7328, BS 7957 and BS 8499 were used as standard stars. These stars were chosen from the catalog by Johnson et al. (1966). As H, M, and for many stars L magnitudes are absent in this catalog, missing magnitudes were calculated using formulae presented in the paper by Koorneeff (1983). The accuracy of observations is 0.^m02 in J,H and K bands, 0.^m03 in L band and 0.^m05 in M band.

Table 4 contains infrared color indices V-J, V-H, V-K, V-L and V-M obtained on the basis of different authors' observations. We include in this table not only the stars of our observational program but also some stars which are considered to be the best solar analogs (BS 2290, stars in Hyades) or have energy distribution in the range 3400 - 7500 AA near enough to the solar one (BS 1729, BS 5072; Glushneva, 1994).

In spite of the accuracy of observations declared by most of the authors, which is 0^m.02 - 0^m.03 in J,H,K,L bands in some cases observations of the different authors are in worse agreement. As for color indices of the Sun in the infrared, many former determination are based not on the direct observations of the Sun, but are obtained as a result of the comparison with the stars -- solar analogs. The energy distribution of these stars in the infrared range was considered to be similar to solar energy distribution (Johnson 1965; Campins et al. 1985; Wamsteker 1981; A'Hearn et al. 1984). Kharitonov & Knyazeva (1996) calculated synthetic V-J, V-H,V-K, V-L and V-M indices using energy distribution of the Sun from the monograph by Makarova et al. Response curves were taken from the paper by Bessell and Brett (1988). Color indices of the Sun in the JHKLM system obtained by different authors are presented in Table 5. The differences between minimal and maximal values of color index V-J are 0.^m08, V-H - 0.^m12, V-K - 0.^m07, V-L - 0.^m11, and V-M - 0.^m14.

Table 6 contains ratios of the normalized fluxes from the Sun to the fluxes from solar type stars in the infrared bands according to the data of different authors. These values were obtained from color indices by means of equating of J magnitudes of stars and the Sun. The flux of the Sun is more than the flux of solar type stars in H band while in K, L, M the flux of the Sun is less. For 16 Cyg B, BS 2290, vB 64, vB 106 and vB 142 it was noted by Kharitonov & Knyazeva (1996).

Table 7 presents color indices J-H, J-K, J-L and J-M of the Sun and solar type stars. Color indices for the stars of our observational program are given in the upper part of the table. It is interesting to note that J-K, J-L and J-M of 16 Cyg A (BS 7503) and 16 Cyg B are equal, and the difference between their J-H indices is only 0.^m02. For stars considered to be the best solar analogs color indices observed by different authors are presented.

The lower part of Table 7 includes synthetic color indices of the Sun calculated by Kharitonov & Knyazeva and solar color indices obtained by different authors by means of the comparison with solar type stars. It is fully natural that these color indices obtained for the Sun are in the agreement with observed color indices of solar type stars. But synthetic color indices calculated on the basis of solar energy distribution differ from the observed color indices of solar type stars. Discrepancies between these stellar and solar color indices may be due to the differences of response curves used in the process of integrating and realized in observations. Therefore, direct determinations of color indices of the Sun in the JHKLM photometric system are very necessary and urgent.