NGC 6626 / Messier 28 / C1821-249 (Updated June 2014) RA: 18:24:32.81 DEC: -24:52:11.2 (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 variablility) 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 18:24:45.23 -24:48:57.9 R0 0.4916 15.91 1.1 V RR0 Note 2 18:24:29.16 -24:51:06.6 R0 58.0 12.67 0.7 V L P12 3 18:24:30.18 -24:50:18.5 R0 49.0 12.67 0.25 V L Note;P12 4 18:24:30.11 -24:51:36.5 R0 13.462 14.21 1.0 B CW 5 18:24:29.50 -24:51:53.0 R0 0.6444 15.52 0.8 V RR0 6 18:24:35.16 -24:51:18.1 R0 ---- 12.63 0.7 V LB 7 18:24:44.48 -24:50:25.1 R0 320.0 15.1 3.3 V M f;Note 8 18:24:49.61 -24:55:48.3 R0 0.5660 15.71 1.3 V RR0 9 18:24:21.26 -24:56:24.4 R0 0.6615 14.60 0.4 V RR0 10 18:24:39.52 -24:53:26.3 R0 ---- 12.14 0.6 V LB Note 11 18:24:31.49 -24:51:33.2 R0 0.5428 15.78 1.2 V RR0 12 18:24:43.31 -24:52:56.0 R0 0.5782 15.61 1.4 V RR0 13 18:24:25.77 -24:52:33.6 R0 0.6549 15.69 1.0 V RR0 14 18:24:23.86 -24:53:54.5 R0 ---- 16.02 0.2 V CST Note 15 18:23:56.81 -24:55:10.3 R0 0.4489 16.73 1.1 V RR0 f 16 18:25:04.86 -24:58:20.4 R0 0.6727 16.63 0.8 V RR0 f 17 18:24:35.84 -24:53:15.8 R0 62.0 11.9 0.75 V RV? Note 18 18:24:36.56 -24:51:50.1 R0 0.6402 15.75 1.15 V RR0 19 18:24:30.84 -24:51:56.3 R0 0.335 16.05 0.5 B RR1 W&SH 20 18:24:33.01 -24:51:43.3 R0 0.4977 15.4 0.9 B RR0 W&SH 21 18:24:33.0 -24:51:59 R0 ---- -- -- L? Note;P12 22 18:24:30.97 -24:52:02.6 R0 0.323 -- -- RR1 P12 23 18:24:30.27 -24:52:02.9 R0 0.2923 15.9 0.5 V RR1 24 18:23:59.58 -24:49:35.6 R0 0.5104 16.45 1.2 V RR0 f 25 18:24:28.89 -24:52:09.9 R0 0.748 -- -- RR0 NV1 26 18:24:33.59 -24:52:32.3 R0 0.297 -- -- RR1 NV2 27 18:24:32.31 -24:52:32.2 R0 0.323 -- -- RR1 f;NV3 28 18:24:24.13 -24:52:36.6 R0 0.261 -- -- RR1 NV4 29 18:24:27.67 -24:52:15.0 R0 0.311 -- -- RR1 NV5 30 18:24:33.45 -24:52:10.6 R0 0.308 -- -- RR1 NV6 31 18:24:32.17 -24:52:00.4 R0 0.329 -- -- RR1 NV7 32 18:24:33.60 -24:52:07.1 R0 13.53 -- -- CW NV8 33 18:24:33.05 -24:51:28.9 R0 ---- -- -- L NV9 PSR Note ============================================================================= Supplementary Notes For V1-24, the RA and dec are from Samus et al. (2009). The period, magnitude, amplitude, variability type and membership status are from Wehlau & Butterworth (1990) unless designated W&SH or P12 in the remarks column: W&SH indicates that they are from Wehlau & Sawyer Hogg (1984) P12 indicates that the period and classification are from Prieto et al. (2012). Samus et al. (2009) noted that V7=V4137 Sgr, V15=V4128 Sgr, V16=V4130 Sgr, V17=V2342 Sgr and V24=V4129 Sgr. All the data for V25-V33 are from the discovery paper by Prieto et al. (2012). The numbers that these authors assigned to these stars, NV1-9 are indicated in the remarks column. A VI CM diagram by Testa et al. (2001) shows an extended blue HB and a few BS stars. ====================== Notes on individual stars V1, V3, V10, V17: In a proper motion study of M28, Rees & Cudworth (1991) found membership probabilities for V1 (90%), V3 (57%), V10 (90%) and V17 (94%). V17 was classified as an RV Tauri variable by Wehlau & Sawyer Hogg (1984) who derived a period of approximately 92 days for the full cycle. In contrast to this, Prieto et al. (2012) derived a period of 62 days and their light curve has very little scatter. On the other hand, Sloan et al. (2010) classified it as a type II Cepheid and derived P=48.6, but their light curve had more scatter than that of Prieto et al. Thus the classification for V17 is uncertain. It could be an SR. V7: The RA and dec refer to the SE component of a close pair. Wehlau & Sawyer Hogg (1984) postulated that V7 might be a U Gem variable because, on many of their photographs, it was fainter than the plate limit. This motivated Margon & Anderson (1985) to obtain a spectrum which showed an M5 spectral type with TiO bands. This is a characteristic of Mira variables. The 320 day period subsequently derived by Wehlau & Clement (1990) and the large amplitude confirmed that V7 is a Mira variable. It is considered to be a field star because it is too faint to be a cluster member. Furthermore, it would be unsusual to find a Mira variable in such a metal poor cluster. [Fe/H]= -1.32 according to the 2010 version of the Harris (1996) catalogue. V14: This star is no longer considered to be variable. Although Wehlau & Sawyer Hogg (1984) derived a period, Wehlau & Butterworth (1990) questioned its variability and Prieto et al. (2012) did not detect any variation in their image subtraction study, even though the star was in their field of view. V21: This star was classified as a type II Cepheid by Wehlau & Sawyer Hogg (1984) who derived a period of 29.93 days. However, Prieto et al. (2012) made CCD observations over an interval of approximately 50 days and their light curve indicates that the period must be longer than 50 days. PSR: A number of millisecond pulsars have been discovered in M28. Further information about these is given at the end of the "Discovery of the variable stars" section. ============================================================================= Discovery of the variable stars in M28 V1-9 Bailey (1902) with x,y positions (page 242) and ID chart (Plate XI, Fig. 3) V10-16 Sawyer (1949) with x,y positions V17 = Star #5 of Hoffleit (1965) and star 63a of Hoffleit (1972) with 1900 RA and dec in both papers and an ID chart in 1972 The number V17 was assigned by Sawyer Hogg (1973) in her 3rd catalog and x,y coordinates were later derived by Wehlau & Sawyer Hogg (1982). V18, V20, V21, V24 Wehlau & Sawyer Hogg (1982) with x,y positions These authors pointed out that the star listed as V18 in Sawyer Hogg's (1973) 3rd catalogue is V12. They also listed coordinates for three other variables that they designated V19, V22 and V23. However, in a subsequent study (Wehlau & Sawyer Hogg 1984), they found that these three stars were not variable and so they assigned these numbers to three other variables. V19, V22, V23 Wehlau & Sawyer Hogg (1984) with x,y positions for these variables and ID charts for V1-24 PSR A millisecond pulsar, PSR 1821-24, was discovered in the core of M28 by Lyne et al. (1987). They published 1950 RA, dec and an ID chart. Additional pulsars have been discovered since then. The RA and dec for most of them have been published by Becker & Hui (2013). According to Freire (2014), there are now 12 known pulsars in M28. Optical counterparts that are variable stars have been identified by Pallanca et al. (2010) for PSR J1824-2452H and by Pallanca et al. (2013) for PSR J1824-2452I. ------------------------------ Rees & Cudworth (1991) found strong evidence for variation of 3 additional stars: their 2-56 and RC11 which are near the RG tip and RC133 which might be an RR Lyrae. However, Prieto et al. (2012) could not confirm the variability of star 2-56 or RC11, but they concluded that RC133 might be the same star as their NV5. =================================================================== References Bailey, S. I. 1902, Harvard Ann., 38 Becker, W. & Hui, C. Y. 2013, arXiv:0705.0119v1 Freire, P. 2014, website: www.naic.edu/~pfreire/GCpsr.html (May 2014) Harris, W. E. 1996, AJ, 112, 1487 Hoffleit, D. 1965, AJ, 70, 307 Hoffleit, D. 1972, IBVS, 660 Lyne, A. G., Brinklow, A., Middleditch, J., Kulkarni, S. R., Backer, D. C., Clifton, T. R. 1987, Nature, 328, 399 Margon, B. & Anderson, S. F. 1985, PASP, 97, 962 Pallanca, C., Dalessandro, E., Ferraro, F. R., Lanzoni, B., Rood, R. T., Possenti, A., D'Amico, N., Freire, P. C., Stairs, I., Ransom, S. M., Begin, S. 2010, ApJ, 725, 1165 Pallanca, C., Dalessandro, E., Ferraro, F. R., Beccari, G. 2013, ApJ, 773, 122 Prieto, G., Catelan, M., Contreras Ramos, R., Pritzl, B. J., Smith, H. A., Alonso-Garcia, J. 2012, A&A, 543, A148 Rees, R. F. & Cudworth, K. M. 1991, AJ, 102, 152 Samus, N. N., Kazarovets, E. V., Pastukhova, E. N., Tsvetkova, T. M., Durlevich, O. V. 2009, PASP, 121, 1378 Sawyer, H. B. 1949, AJ, 54, 193 Sloan, G. C., Matsunaga, N., Matsuura, M., Zijlstra, A. A., Kraemer, K. E., Wood, P. R., Nieusma, J., Bernard-Salas, J., Devost, D., Houck, J. R. 2010, ApJ, 719, 1274 Testa, V., Corsi, C. E., Andreuzzi, G., Iannicola, G., Marconi, G., Piersimoni, A. M., Buonanno, R. 2001, AJ, 121, 916 Wehlau, A. & Butterworth, S. 1990, AJ, 100, 686 Wehlau, A. & Clement, C. 1990, IBVS, 3450 Wehlau, A. & Sawyer Hogg, H. 1982, IBVS, 2171 Wehlau, A. & Sawyer Hogg, H. 1984, AJ, 89, 1005 ============================================================