NGC 6558 / C1807-317 (Updated November 2024) RA: 18:10:18.38 DEC: -31:45:48.6 (J2000) ============================================================ Bytes Format Explanation 1-8 A8 Star ID 10-32 A11,1x,A11 Position 34-35 A2 Units for position R0 denotes RA, DEC in the J2000 coordinates R5 denotes RA, DEC in the 1950 coordinates XA denotes X, Y in arcseconds XP denotes X, Y in pixels 37-44 F8.4 Period (days) 46-51 F6.3 Mean magnitude (or maximum magnitude if "max" is indicated in the remarks column) 53-57 F5.3 Light amplitude (range of variability) 59 A1 Colour for mean magnitude and amplitude e.g. B, V, R, I, J, K or P (for photographic). 61-65 A5 Type of variable (draft 2006 GCVS classifications) CST denotes non variable stars previously designated as variables 67-80 A19 Notes and Remarks (f denotes field star) "--" or "----" indicates no data available ========================================================================= ID Position Period ampl C Type Notes/ RA/X Dec/Y Units Remarks ========================================================================= 1 18:10:15.59 -31:45:54.1 R0 0.4666 16.634 1.154 V RR0 2 18:10:16.26 -31:45:03.5 R0 ---- 13.553 -- CST Note 3 18:10:21.64 -31:45:15.6 R0 0.5880 16.280 1.285 V RR0 4 18:10:13.27 -31:46:16.0 R0 0.5039 16.439 1.262 V RR0 5 18:10:13.48 -31:43:45.1 R0 0.7487 16.836 0.863 V RR0 f;Note 6 18:10:15.88 -31:46:41.4 R0 0.3125 16.466 0.572 V RR1 7 18:10:25.61 -31:43:33.2 R0 ---- 14.603 -- CST f;Note 8 18:10:17.87 -31:48:55.3 R0 0.5745 16.478 1.308 V RR0 f;Note 9 18:09:50.47 -31:46:37.3 R0 0.4546 16.514 1.322 V RR0 f;Note 10 18:10:36.26 -31:45:31.4 R0 ---- 13.978 -- V L f 11 18:10:01.62 -31:42:05.4 R0 ---- 15.230 -- V L f 12 18:09:53.86 -31:43:25.8 R0 ---- 16.037 -- V L f 13 18:10:04.75 -31:40:24.9 R0 0.3610 16.431 0.384 V RR1 f 14 18:10:45.76 -31:46:20.3 R0 0.2650 16.412 0.514 V RR1 f 15 18:09:54.16 -31:48:10.9 R0 ---- 15.230 -- V L f 16 18:10:19.63 -31:46:06.0 R0 0.6876 16.579 0.386 V RR0 17 18:10:16.21 -31:45:42.4 R0 0.7558 16.250 0.229 V RR0 18 18:10:12.50 -31:47:33.8 R0 0.4060 19.549 0.543 V E 19 18:10:17.67 -31:45:39.4 R0 346.19 13.755 -- V SR/L ======================================================================== Supplementary Notes NGC 6558 is a relatively metal poor, core collapsed cluster in the Galactic bulge. According to the 2010 version 0f the Harris (1996) catalogue, it has [Fe/H] = -1.32, a tidal radius of 9.5 arcminutes, and a half-light radius of 2.15 arcminutes. The most comprehensive investigation of the variable stars in NGC 6558 was by Arellano Ferro et al. (2024, hereafter AF24). They observed the cluster with the 1.54 m telescope at Bosque Alegre in Argentina and with Carnegie's 1 m Swope telescope in Chile and published RA, dec (J2000) and ID charts for V1-19. They also carried out an analysis of membership status, using positions and proper motions in the Gaia DR3 data base, and found that many of the variables are not cluster members. The data in the above table are from their paper. In addition, they cross-matched their variables with photometric data from the OGLE and Gaia DR3 surveys and listed these identifications in their paper. ====================================================================== Notes on individual stars V2, V7: Hazen (1996) could not confirm the variability of either of these stars and noted that both of them were seriously blended with nearby stars. AF24 also observed both stars and did not detect any variabiity. Furthermore, neither of them has been detected in other surveys of variable stars in the region. V5: AF24 found that V5 was a field star, a conclusion reached previously by Hazen (1996) and Rich et al. (1998) on the basis of its apparent magnitude. It is fainter than the other RR Lyrae variables. V5 is V4301 in the GCVS (Samus et al. 2017) V8, V9 These are V4601 and V4595 Sgr in the GCVS (Samus et al. 2017). ======================================================================== Discovery of the variable stars in NGC 6558: V1-9 Rosino (1962) with x,y coordinates and ID charts He also provided identifications for 14 variables in the surrounding field. They were listed as V10-22 and V738 Sgr. Some of Rosino's variables were later recovered in a survey by Blanco & Blanco (1997): V1=BB#25, V3=BB#30, V4=BB#24, V6=BB#E2, V8=BB#26, V9=BB#16 V10-15 Hazen (1996) with x,y coordinates and an ID chart Hazen also identified V1-9. Hazen's V10-15 were NOT the stars that were listed as V10-15 by Rosino (1962) because his were considered to be field stars. V1-15 all lie within 6.3 arcminutes of the cluster centre, which was assumed to be the tidal radius when Hazen published her paper. Hazen also announced discovery of 36 new variables in the field around the cluster with x,y coordinates and ID chart. Eleven of Rosino's 14 field variables were in the field of Hazen's investigation. V16-17 = OGLE-BLG-RRLYR-14912, OGLE-BLG-RRLYR-14892 These two variables were discovered in the Ogle III survey (Soszynski et al. 2011) and the numbers V16 and 17 were assigned. V16 was previously announced as star #29 by Blanco & Blanco (1997) who published Epoch 1950 RA and dec and an ID chart. V18-19 AF24 with RA, dec (J2000) and ID charts, as well as Gaia IDs for both stars and Ogle BLG ID for V18 They also discovered two variables, N1 and N2, not reported by Gaia or Ogle. Both were found to be field stars and are therefore not listed in the above table. AF24 also searched the Ogle III (Soszynski et al. (2013), Ogle IV (Soszynski et al. 2014) and Gaia DR3 (Gaia Collaboration 2018) data bases for additional variable stars in the field of NGC 6558 and matched the IDs with stars in their field. They tabulated data for these stars, but none turned out to cluster members. ======================================================================== References Arellano Ferro, A., Zerpa Guillen, L. J., Yepez, M.A., Bustos Fierro, I. H., Prudil, Z., Perez Parra, C. E. 2024, MNRAS, 532, 2159 Blanco, B. M., Blanco, V. M. 1997, AJ, 114, 2596 Gaia Collaboration 2018, A&A, 616, A1 Harris, W. E. 1996, AJ, 112, 1487 Hazen, M. L. 1996, AJ, 111, 1184 Rich, R. M., Ortolani, S., Bica, E., Barbuy, B. 1998, AJ, 116, 1295 Rosino, L. 1962, Mem. SAI, 33, 351 = Asiago Contr. 132 Samus, N. N., Kazarovets, E. V., Durlevich, O.V., Kireeva, N. N., Pastukhova, E. N. 2017, General Catalog of Variable Stars: Version GCVS 5.1, Astronomy Reports, Vol. 61, No. 1, 80 Soszynski, I., Dziembowski, W. A., Udalski, A., Poleski, R., Szymanski, M. K., Kubiak, M., Pietrzynski, G. and 4 coauthors, 2011, Ac. A., 61, 1 Soszynski, I., Udalksi, A., Szymanski, M. K., Kubiak, M., Pietrzynski, G., Wyrzykowski, L., Ulaczyk, K. and 4 coauthors, 2013, Acta A., 63, 21 Soszynski, I., Udalski, A., Szymanski, M. K., Pietrukowicz, P., Mroz, P., Skowron, J., Kozlowski, S. and 6 coauthors, 2014, Ac. A. 64, 177 ========================================================================