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Interstellar Reddening in the Direction of the Globular Cluster Ngc 6712

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Interstellar Reddening in the Direction of the Globular Cluster Ngc 6712 Baltic Astronomy, vol.1, 430-438, 1992. INTERSTELLAR REDDENING IN THE DIRECTION OF THE GLOBULAR CLUSTER NGC 6712 R. Janulis1 and F. Smriglio2 1Institute of Theoretical Physics and Astronomy, Gostauto 12, Vilnius 2600, Lithuania 2Istituto Astronomico, Università di Roma "La Sapienza", via Lancisi 29, 00161 Roma, Italy Received June 15, 1992. Abstract. Interstellar extinction as a function of distance in a 1 sq. degree area around the globular cluster NGC 6712 is determined us- ing two-dimensional quantification of 69 foreground stars observed in the Vilnius photometric system. The northern part of the area, in- cluding the globular cluster, seems to be more transparent. However, the extinction plot is affected by a considerable limiting magnitude selection effect, excluding the most reddened stars. Key words: globular clusters - Vilnius photometric system - pho- tometric classification - interstellar reddening This paper continues the investigation of interstellar extinction in the direction of low galactic latitude globular clusters. Previ- ous studies were concerned with the globular clusters M 71 (Janulis and Straizys, 1984; Janulis, 1984; Boyle et al., 1992; Smriglio et al., 1992b), M 56 (Janulis, 1986; Smriglio et al., 1991) and M 15 (Janulis, 1992). Here the results of investigation of interstellar extinction in the vicinity of the globular cluster NGC 6712* are given. The cluster is situated behind the Scutum stellar cloud, which is one of the high- est surface brightness regions in the Milky Way. This makes precise photometry of faint stars difficult due to the background problems. *a(1950) = 18h50in3, ¿(1950) = -8°47', I = 25.3°, b = -4.3° Send offprint requests to: R. Janulis Interstellar reddening in the direction of NGC 6712 431 Most stars down to 12 mag in the 1 square degree area around NGC 6712 were observed by one of us (R.J.) in the Vilnius photomet- ric system using the 1-rrieter telescope of the Institute of Theoretical Physics and Astronomy at the Maidanak Observatory in Uzbekistan. Details of observations, reduction outside the atmosphere and sys- tem standardization are not very different from those described by Janulis and Straizys (1984). The results of photometry of 69 stars are presented in Table 1 where the star number corresponds to the identification map in Fig. 1. The table contains HD and BD numbers, coordinates, magni- tude V, colour indices and the number of independent measurements. The mean square errors of magnitudes and colour indices are of the order of ±0.01 mag. Stars are quantified in spectral classes and absolute magnitudes by using the following methods: (1) diagrams of reddening-free Q-parameters (QJJPY,QPYV', QUPYIQXYV', QUXY,QUPYV', QUPY,QXZS and Qxzs, QXYZ) calibrated in terms of spectral type and absolute magnitude (Straizys et al., 1982), (2) the sigma-Q method, by matching five reddening-free Q- parameters (QUPYV, QPXYV, QXYV, QZVYV and QVSYV) of the program stars to those of the standard stars of different spectral types and luminosities (Smriglio et al., 1991, 1992a). (3) comparison of dereddened colour indices of the program stars with the mean intrinsic colour indices in every point of the HR di- agram, using the program prepared by Vansevicius and Bridzius (1993). All these methods give the results in good agreement. The fi- nal spectral classes and absolute magnitudes given in this paper are based on method (3) with some corrections by methods (1) and (2) where quantification by method (3) shows a somewhat lower accu- racy. The results are given in Table 2 together with colour excesses Ey-v, extinctions Ay and distances r of the stars. The distance of every star is rounded to the nearest numeral multiple to 10. In quantification of stars and in extinction determination, the interstel- lar extinction law in the area was considered to be normal, i.e. the ratio Ay /Ey-v was taken to be 4.5. Interstellar extinction Ay is plotted against distance r in Fig. 2. Very large scatter of the extinction values at a fixed distance is seen. It is more than 5 times larger than the estimated rms error of Ay (±0.1 mag). This can be caused either by the cloudy microstructure 432 R. Janulis and F. Smriglio Table 1. Results of photometry Nr. BD HD a(1950) ¿(1950) V U-PP-XX-Y Y-ZZ-V V-Sn 1. 18h48m28s -8"' 26'.5 10.51 0.39 0.43 0.26 0.21 0.10 0.25 2 2. -8°4734 18 48 34 -8 48.2 10.49 0.62 1.02 1.35 0.53 0.40 0.84 3 3. 18 48 43 -8 52.9 10.64 0.78 0.82 0.39 0.26 0.15 0.32 3 4. -8°4736 174628 18 48 57 -8 28.0 9.06 0.45 0.54 0.27 0.16 0.11 0.23 2 5. 18 49 07 -8 56.3 11.96 0.61 0.75 0.38 0.23 0.11 0.26 2 6. -8°4737 18 49 19 -8 31.2 9.17 0.79 1.44 2.02 0.58 0.56 1.08 3 7. 18 49 22 -8 46.5 10.74 0.78 1.32 1.79 0.60 0.51 1.01 3 8. 18 49 25 -8 20.5 11.58 0.78 0.92 0.54 0.26 0.15 0.32 2 9. 18 49 35 -8 40.2 11.88 0.68 0.78 0.61 0.31 0.17 0.45 3 10. -8°4740 174738 18 49 36 -8 50.1 8.90 0.55 0.83 1.14 0.46 0.32 0.75 3 11. -9°4870 18 49 42 -9 15.1 10.89 0.76 0.76 0.38 0.20 0.13 0.29 1 12. 18 49 43 -8 36.5 10.67 0.71 1.20 1.59 0.59 0.39 0.89 4 13. -8°4741 174753 18 49 49 -8 24.0 9.14 0.70 0.68 0.62 0.32 0.19 0.49 2 14. -9°4871 174770 18 49 51 -8 59.7 9.61 0.71 0.87 0.56 0.28 0.13 0.40 1 15. 18 49 59 -8 51.7 11.28 0.64 0.86 0.40 0.22 0.12 0.23 2 16. -8°4742 174817 18 49 59 -8 34.8 9.88 0.85 0.82 0.38 0.20 0.12 0.28 3 17. -8°4744 174816 18 50 00 -8 29.9 10.39 0.76 0.81 0.36 0.19 0.12 0.25 3 18. 18 50 07 -8 50.1 11.09 0.66 0.92 1.26 0.54 0.33 0.81 3 19. 18 50 08 -8 49.5 12.53 0.54 0.60 0.84 0.42 0.25 0.59 2 20. 18 50 11 -8 37.7 11.92 0.72 0.85 0.60 0.28 0.15 0.43 2 21. -9°4875 18 50 12 -8 57.7 9.86 1.04 1.53 2.24 0.82 0.61 1.35 3 22. -9°4874 18 50 12 -9 19.8 10.28 0.29 0.28 0.24 0.15 0.11 0.25 1 23. 18 50 13 -8 56.7 11.00 0.59 0.67 0.89 0.45 0.26 0.66 3 24. 18 50 15 -8 39.6 11.32 0.82 0.86 0.42 0.22 0.12 0.31 3 25. 18 50 17 -8 40.5 11.40 0.69 0.73 0.74 0.38 0.22 0.55 4 26. 18 50 19 -8 23.9 11.67 0.74 1.29 1.71 0.61 0.43 0.94 2 27. 18 50 20 -8 48.3 11.85 0.70 1.27 1.79 0.60 0.45 0.96 3 28. 18 50 21 -8 50.1 11.82 0.67 1.18 1.53 0.59 0.42 0.90 3 29. -8°4746 18 50 23 -8 38.8 10.58 0.49 0.59 0.72 0.34 0.21 0.52 4 30. 18 50 25 -8 22.5 11.70 0.64 0.97 1.34 0.57 0.34 0.89 2 31. 18 50 25 -8 43.6 11.75 0.54 0.62 0.77 0.43 0.23 0.62 3 32. -9°4877 18 50 29 -9 05.2 10.43 0.71 0.77 0.40 0.25 0.15 0.36 1 33. 18 50 29 -8 48.4 11.63 0.68 1.01 1.31 0.57 0.34 0.83 3 34. -9°4878 18 50 30 -9 07.1 10.33 0.69 1.23 1.66 0.63 0.44 1.00 2 35. 18 50 31 -8 58.3 11.21 0.86 1.27 1.80 0.69 0.50 1.10 2 36. 18 50 32 -8 21.9 11.73 0.76 1.30 1.81 0.60 0.46 1.00 2 37. -8°4748 174918 18 50 37 -8 44.4 9.07 0.73 1.29 1.82 0.60 0.48 1.00 4 38. 18 50 40 -8 21.9 10.48 0.87 1.45 2.12 0.66 0.55 1.16 2 39. -8°4749 174971 18 50 45 -8 38.1 9.03 0.81 0.87 0.41 0.21 0.11 0.27 3 40.
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