Arp 2 / C1925-304 (Updated July 2019) RA: 19:28:44.11 DEC: -30:21:20.3 (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 19:28:36.4 -30:20:56.2 R0 0.5684 18.18 1.15 V RR0 2 19:28:39.4 -30:20:07.0 R0 0.811 18.03 0.49 V RR0 3 19:28:56.9 -30:21:29.6 R0 0.565 17.99 0.90 V RR0 4 19:29:01.5 -30:20:56.1 R0 0.458 18.23 1.06 V RR0 5 19:28:56.3 -30:26:32.0 R0 0.760 18.04 0.59 V RR0 6 19:28:30.2 -30:23:11.3 R0 0.4472 18.22 0.90 V RR0 7 19:29:04.8 -30:16:04.6 R0 0.527 17.86 0.56 V RR0 Note 8 19:28:44.9 -30:21:54.7 R0 0.2921 18.18 0.49 V RR1 9 19:28:52.3 -30:22:10.7 R0 0.515 18.16 1.28 V RR0 10 19:28:52.1 -30:23:13.3 R0 0.0474 20.47 0.50 V SXPHE 11 19:28:45.6 -30:20:05.8 R0 0.0612 20.14 0.35 V SXPHE 12 19:28:42.4 -30:25:15.9 R0 0.0604 20.26 0.28 V SXPHE ================================================================= Supplementary Notes Arp 2 (along with M54, Terzan 7, Terzan 8 and Pal 12) is associated with the Sagittarius (Sgr) dwarf galaxy. All of the data in the above table are from Pritzl et al. (2019) who also published a finding chart for the variables. Their study is based on BVI photometry. Salinas et al. (2012) presented BV photometry for Arp 2 and derived structural parameters for the cluster:  1.64 +/- 0.14 and 4.61 +/-0.30 arcminutes for the core and tidal radii and a central concentration of 0.45 +/- 0.05. From an analysis of the blue straggler distribution, they concluded that the cluster has not yet relaxed and is probably relatively young. =================================================================== Notes on individual stars V7: Pritzl et al. (2019) questioned the membership status of V7 based on the distance modulus they computed for it and on the fact that it lies outside the cluster's tidal radius. (See the discussion in section 4 of their paper.) They noted that, in future, when more accurate proper motions can be determined for stars like V7, from Gaia data, this issue will be resolved. =================================================================== Discovery of the variable stars in Arp 2: V1-12 Salinas et al. (2005) with an ID chart and x, y coordinates with respect to the cluster centre. They pointed out that V1-4 were V4, V5, V25 and V28, respectively in the 2001 Univ. of Bologna thesis of Valenti, but they did not confirm the variation of the other variables reported by Valenti. Pritzl et al. (2019) reported that some of the RR Lyrae variables were independently discovered in the Catalina survey and in the Gaia mission. ================================================================= References Pritzl, B. J., Gehrman, T. C., Salinas, R., Catelan, M., Smith, H. A., Borissova, J. 2019, PASP, 131, 054202 Salinas, R., Catelan, M., Smith, H. A., Pritzl, B. J., Borissova, J. 2005, IBVS, 5640 Salinas, R., Jilkova, L., Carraro, G., Catelan, M., Amiga, P. 2012, MNRAS, 421, 960 ====================================================================