Selection of Solar analogues on the basis of various color indices

A. V. Kharitonov

Fessenkov astrophysical institute, Almaty, Kazakhstan

A. V. Mironov

Sternberg astronomical institute, Moscow, Russia

It is possible to choose stars similar to the Sun, using various parameters as a background. However, to find a solar ``twin" it is necessary first of all that the colour indices of a star obtained in different photometric systems are be similar to solar ones.

We have made a search of stars for which the colorimetric parameters are closest to those for the Sun in three catalogs: the famous catalogue of Mermilliod, containing measurements in Johnson's UBV system (we have chosen only stars from the Mermilliod catalogue which are included in the Henry Draper catalogue), the catalogue of measurements in the Vilnius UPXYZVS system [1], and in the catalogue of measurements in WBVR} system, which recently was created at the Sternberg astronomical institute [2], [3]. One can do this as the color excesses of G-dwarfs which are brighter than the limit of the HD catalogue are practically equal to zero.

The parameter

where i is a counter of color indices, K is a number of independent color indices of a photometric system, ci,sun are solar color indices, and ci, * are stellar color indices, was a closeness criterion of solar and stellar color indices.

The color indices of the Sun in some photometric system may be carried out from observations or may be calculated by integration of product of the energy distribution function of the Sun Esun(lambda) and the response curves of the given system phii(lambda). In Table 1 the summary of results of the main determinations is shown.

Table 1. Summary of observed and synthetic color indices of the Sun
author method U-B B-V energy distrib. source response curves source
Gallouet observ.0.15 0.68 - -
Tug, Schmidt-Kaler observ.0.1830.686 - -
Kharitonov, Kniazevacalcul.0.0330.648 [9] [12]
-"- calcul.0.0670.652 [9] [13]
Straizys, Valiaugacalcul.0.140.64 [11] [12]
-"- calcul.0.13 0.60 [14] [12]
-"- calcul.0.15 0.62 [15] [12]
WBVR System
author method W-B B-V V-R energy distrib. source response source
Mironov,Kharitonov observ.-0.0500.6670.531 - -
Kharitonov,Kniazevacalcul.-0.0720.6740.505 [9] [3]
Vilnius system
author method U-V P-V X-V Y-V Z-V V-S energy distrib. source response curves source
Straizys,Valiauga calcul.2.512.061.410.550.210.59 [11] [12]
-"- calcul.2.482.011.350.540.220.51 [14] [12]
-"- calcul.2.482.061.390.540.210.59 [15] [12]
Kharitonov,Kniazevacalcul.2.462.021.370.540.210.55 [9] [12]

The direct measurements are now made in the UBV and WBVR systems only.

The direct measurements of the Solar color indices in the UBV system were made by Gallouet in 1964 [5] and by Tug and Schmidt-Kaler in 1982 [6]. However, a comparison of color indices of any two stars which were measured in the UBV system at different times, with different equipment and at different observatories, is not reliable because of known uncertainties of passbands and the method of determination of extra-atmospheric magnitudes.

The direct measurements of the Solar color indices in the WBVR system were carried out by the authors of the present report in 1996 [7]. The measurements were made with the same equipment (the four-channel photoelectric photometer [4] fixing in 48-cm reflector), with which the catalogue of WBVR-magnitudes was created.

The observations were executed on five dates in August and September 1996 in Northern Tien-Shan (altitude 2750 m above sea level).

To decrease the solar radiative flux the little screen of BaSO4 (with the diameter of 27 mm) was used. It was placed 200 m from the telescope. The screen was illuminated by the Sun, but it was protected from scattered light of the sky by special construction. The screen was fixed in equatorial support and always was directed perpendicularly towards the solar rays. The observations of the Sun were executed immediately after sunrise, when the solar air mass changed from 6 down to 2. The observations of the stars were always preceded the observations of the Sun. We carried out special measures to investigate and control the apparatus sensibility and to determine atmospheric extinction.

Many investigators have calculated synthetic color indices for the Sun and a lot of versions of Esun(lambda) and response curves are used. We want to point, for example, to detailed article by Straizys and Valiauga [8] where solar color indices in UBV and Vilnius systems were calculated for several assumptions. Recently Makarova et al. [9] have obtained a new version of the solar energy distribution on a basis of five most reliable observation sets. However, as one can see from Table 1, synthetic color indices in the UBV system, calculated under different cases, show considerable discrepancies.

To choose the solar analogues the following color indices have been accepted:

A value of the rho parameter was given by rsqrt(K), where K is the number of independent color indices of a photometric system; (K=2 for UBV, K=3 for WBVR and K=6 for the Vilnius system), and r=2 sigma= 0.04 mag is a factor, which is approximately equal to twice the standard error of the color indices in the UBV system. Under the given values of that rho parameter we have found 346 stars from the UBV system catalogue, 140 stars from the WBVR system catalog, and 259 stars from Vilnius system catalogue.

Only 6 stars are at once in all three samples. They are listes in Table 2. They will be apparently consideres as optimal candidates chosen by closeness of color indices. Two of them are 16 Cyg A and B (HD 186408 and HD 186427), which usually are included in the number of best analogues of the Sun. But the star HD 28099 = vB 64 (Hyades), which is usually considered to be a best analogue too, is not found in this list. The latter star is a member of only one sample: the sample from the Vilnius catalogue, where it takes 180th place among 259 stars according to value of the rho parameter.

If we rank chosen stars in increasing order of the rho parameter and put an upper bound on the rho parameter as a value which is in agreement with standard error r=0.01 mag, thw samples from the three catalogues have no common stars. If we increase r to 0.017 mag then two common stars appear in the samples from the Vilnius and WBVR catalogues: HD 4915 and HD 146233. Of course, these facts are a consequence that all color indices both for the Sun and for stars contain errors. It is known that the errors in the Vilnius and WBVR catologues are less than in the Mermilliod catalogue, therefore we think that stars which are at once in the samples from Vilnius catalogue and from WBVR catalogues may be considered as candidates for solar analogues too. These stars are listed in Table 3.

It is impossible not to note the fact that there are stars in a wide interval of spectral subclasses (from G0 to G5) in the Tables 2 and 3. It is obvious that the fact is a consequence of errors of spectral classification on the one hand, and of natural differences of spectral line parameters of stars having identical color indices (i.e. having similar energy distribution) on the other hand.

Pay attention that there are two stars, 16 B Cyg and 47 UMa, in tables 2 and 3, which, as found recently, have planetary systems. The whole sample from the Vilnius catalogue (259 stars) contain stars with planetary system 51 Peg and rho CrB too.

Table 2. List of stars whose color indices are close to solar ones
in all three catalogues in the three photometric systems.
6060146233A 16 15.6 -08 22 G2Va 5.499-0.0280.6500.524
6538159222 17 32.0 +34 16 G5V 6.537-0.0060.6460.510
7503186408 19 41.8 +50 32 G1.5V 5.986-0.0040.6590.521
7504186427 19 41.9 +50 31 G2.5V 6.244+0.0080.6710.531
187237 19 48.0 +27 52 G2III 6.896-0.0130.6540.512
7569187923A 19 52.1 +11 38 G0V 6.164-0.0320.6640.520

Table 3. List of stars whose color indices are close to solar ones
in two catalogues: in the Vilnius and WBVR photometric systems
203 4307A 00 45.5 -12 53 G2V 6.158-0.0860.6120.512
4915 00 51.2 -05 02 G0V 6.982-0.0270.6660.543
8262 01 22.3 +18 41 G3V 6.973-0.0790.6300.513
483 10307 01 41.8 +42 37 G1.5V 4.965-0.0490.6230.499
29310*04 37.5 +15 09 G1V 7.547-0.0450.6080.516
1729 34411A 05 19.1 +40 06 G2IV/V4.705-0.0330.6220.499
4277 95128 10 59.5 +40 26 G0V 5.037-0.0560.6220.505
5384126053 14 23.3 +01 14 G1V 6.266-0.1100.6440.527
5596133002 14 50.3 +82 31 F9V 5.643 0.0030.6820.555
5868141004 15 46.4 +07 21 G0V 4.419-0.0640.6110.494
152792 16 53.5 +42 49 G0V 6.827-0.0930.6450.535
6573160269AB 17 35.0 +61 52G0Va+K3V5.233-0.0820.6080.525
168874AB 18 20.8 +27 32 G2IV 7.014-0.0460.6370.512
177082 19 02.6 +14 34 G2V 6.895-0.0760.6410.518
7683190771AB 20 05.2 +38 29 G5IV 6.185+0.0000.6680.523
7914197076A 20 40.8 +19 56 G5V 6.444-0.0820.6280.505
8964222143 23 38.0 +46 12 G5 6.591-0.0180.6520.522
9107225239 00 04.9 +34 40 G2V 6.111-0.0980.6400.541
*var. V998 Tau


[1] Straizis V., & Kazlauskas A. 1993, ``General photometric catalogue of stars observed in the Vilnius system,'' Baltic Astronomy, 2, 1.

[2] Khaliullin, Kh., Mironov, A.V., Moshkalyov, V.G. 1985, Astrophys.Sp.Sc., 111, 291.

[3] Kornilov, V.G., Volkov, I.M., Zakharov, A.I., Kozyreva, V.S., Kornilova, L.N., Krutjakov, A.N., Krylov, A.V., Kusakin, A.V., Leontiev, S.E., Mironov, A.V., Moshkaliov, V.G., Pogrosheva, T.M., Sementsov, V.N., & Khaliullin Kh.F. 1991, ``Catalogue of WBVR magnitudes of Northern Sky Bright Stars,'' Proceedings of Sternberg Astronomical Institute. Vol.62. (Moscow: Moscow university press)

[4] Kornilov, V.G., & Krylov, A.V. 1990, Astronomicheskij Zhurnal (Soviet Astronomy) 67, 173.

[5] Gallouet, L. 1964, Ann.Astrophys. 27, 423.

[6] T\"ug, H., & Schmidt-Kale,r T. 1982, A&A, 105, 400.

[7] Mironov, A.V., & Kharitonov, A.V. 1997, ``The Standard Star Newsletter. An electronic publication of the Working Group on Standard Stars,'' (IAU Comm. 29.30,45.) 22, 6.

[8] Straizys, V., Valiauga, G. 1994, Baltic Astronomy, 3, 282.

[9] Makarova, E.A., Kazachevskaya, T.V., Kharitonov, A.V. 1994. Solar Phys. 152, 195.

[10] Kharitonov, A.V., & Kniazeva, L.N. 1997. Astronomicheskij Zhurnal (Soviet Astronomy) (in press).

[11] Neckel, H., & Labs, D. 1984, Solar Phys., 90, 205.

[12] Straizys, V. 1992. ``Multicolor Stellar Photometry,'' (Tucson: Pachart)

[13] Bessell, M.S. 1990, PASP, 102, 1181.

[14] Lockwood, G.W., Tug, H., & White, N.M. 1992. 1pJ, 390, 668.

[15] Makarova, E.A., Kniazeva, L.N., Kharitonov, A.V. 1989. Astronomicheskij Zhurnal (Soviet Astronomy) 66, 583.