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2 Photometry

In order to inspect whether the main star (comp. A) is variable or not, we measured it in 30 nights during 6 observing periods (1985 to 1992) at the European Southern Observatory, La Silla, Chile. The measurements were made in the UBV system with the Bochum 61 cm telescope and the ESO 50 cm telescope. Other components were measured too. The results for components AB are given in Table 2 and for the other components in Table 3.

Table 2: UBV photometry - components AB

Date & $V$\space & $B{-}V $&...
 ... &7.544 &+.002& $-$\space .92 & 2 & 2 \\ \hline\noalign{\smallskip}\end{tabular}
a Telescope: 1 - Bochum 61 cm (dia = 18.2''),

2 - ESO 50 cm (dia = 20.9'').

Table 3: UBV photometry of individual components

Component & ...
 ... & +&77:& \multicolumn{2}{c}{}& & 10.0\\ \hline\noalign{\smallskip}\end{tabular}
aThe components are very close but it is possible to separate them using a small diaphragm; C2 intensity ratio I(C)/I(D) is 3.9/1.0. bThe bright components C+D are in close vicinity.

One can see that the measurements by Lindroos (A, CD) agree well with our data. The measurement by Rufener (AB) is also close to our value, however the discrepancies of the earlier data remain unexplained. In particular, we cannot agree with magnitudes and colours as given by Echevarria et al. Using values from Table 3, integral magnitudes of various component combinations can be calculated (V magnitudes follow):
AB: 7.538, ABCD: 7.208, CDE: 8.621, ABCDE: 7.197.

The multiple system is located in the brightest, albeit homogenous, part of the nebula. The components C, D and E are rather close to one of the well-known dark lanes (see e.g. Lynds et al. 1985). The measurements were made with small diaphragms and the sky was measured in points close to the system; then the effect of the background brightness on measurements of the components AB can be estimated as not larger than about 0$.\!\!^{\rm m}$01. Nevertheless, the scatter of our measurements of the B and V magnitudes in individual nights is slightly larger than expected. The U data obtained in the observing epochs 1986 and 1987 show a rather large scatter, which cannot completely be understood. Probably, the reason is a non-sufficient number of measurements of standard stars with strongly negative U-B index, which caused some uncertainties during the reduction procedure. Hence, these U data were discarded.

The UBV magnitudes of individual components (Table 3) were determined from measurements made in two observing periods (1985-86) - in nights with good seeing - using the Bochum 61 cm telescope with small diaphragm (diameter 11.3$^{\prime\prime}$). The sky was measured in two or four points located close to and symmetrically around the stars. In spite of this, the data for fainter components might be affected by the nonhomogenous background.

We made also CCD observations with the 2.2 m telescope at ESO, La Silla, in April 1986. In order to diminish the brightness of the surrounding nebula, we used the IHW cometary filter C2 with the central wavelength 513.6 nm and FWHM 9.0 nm. Two frames with integration times 15 s and 8 s were taken; the A component is saturated on both frames. The C2 intensities are given in Table 3 (intensity of G component was set to 10.0). The positions of the components are shown in Fig. 1 and given in Table 4. They are nearly identical with values as given in ADS.

\includegraphics [width=6cm]{1495f1.eps}\end{figure} Figure 1: Central part of nebula M 20 (Trifid). 2.2 m telescope + CCD at ESO, La Silla. IHW cometary filter C2, April 1986

Table 4: Positions of individual components of HD 164492 (relative to component A; in footnotes, positions relative to component C are given)

Component & $d$\space & PA & D...
 ...1 & 108.1 & \\ G $^c$& 40.51 & 212.3 &\\ \hline\noalign{\smallskip}\end{tabular}
a d=2.39'', PA = 280.8$^\circ$

b d=5.98'', PA = 191.6$^\circ$

c d=29.86'', PA = 212.3$^\circ$.

Table 5: Positions and C2 intensities of surrounding stars (relative to HD 164492 component A)

Star & \multicolumn{...
 ...& 3.41\\ 10 & +43&6: & $-45$&3: & 0.60\\ \hline\noalign{\smallskip}\end{tabular}

From the dereddened colour indices (B-V)0 it appears that the brighter components A to E are of early spectral type and probably form a physical system - the nucleus of the nebula. However, these dereddened indices disagree with the classification by Gahm et al. (1983) mentioned in Introduction. On the other hand, Lindroos 1983) gives uvby values for CD, which also correspond to a spectral type of about B0.5, as our UBV values do.

We also measured positions (relative to the component A) and C2 intensities (supposing intensity of G 10.0) of several stars in the field believing that this can be useful for further investigation. The results are given in Table 5.

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