NGC 6218 / C1644-018 / Messier 12 (Updated May 2022) RA: 16:47:14.18 DEC: -01:56:54.7 (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 16:47:16.81 -01:57:59.6 R0 15.542 11.4 0.8 V CW Note 2 16:47:22.91 -01:55:36.0 R0 0.2432 18.93 0.25 V EC max;Note 3 16:47:12.69 -01:57:26.1 R0 0.2106 17.86 0.08 V EC max 4 16:47:36.53 -01:54:53.3 R0 0.2339 19.45 0.46 V EC max;f 5 16:47:10.39 -01:56:10.4 R0 0.2255 18.71 0.13 V EC max;f?;Note 6 16:47:21.25 -01:54:52.6 R0 0.2562 18.80 0.34 V EC max 7 16:46:54.67 -01:59:26.0 R0 0.2571 16.65 0.07 V EC max;f;Note 8 16:47:21.25 -01:57:24.3 R0 0.4352 16.48 0.07 V EC max 9 16:47:16.82 -01:56:31.1 R0 0.4445 17.12 0.03 V EC max 10 16:47:05.46 -01:59:35.9 R0 4.5951 19.06 0.34 V EA max 11 16:47:15.02 -01:57:05.8 R0 5.2186 18.26 0.36 V EA max 12 16:47:17.27 -01:55:42.7 R0 1.0251 16.09 0.10 V EA max 13 16:47:14.78 -01:56:44.6 R0 0.7341 17.07 0.26 V EA max 14 16:47:13.51 -01:57:02.1 R0 0.4638 17.58 0.62 V EA max;f 15 16:47:14.03 -01:56:28.1 R0 0.5417 17.13 0.16 V EA max 16 16:47:28.65 -01:55:36.5 R0 2.5003 17.30 0.45 V EA max 17 16:47:19.66 -01:51:57.9 R0 2.2797 20.00 0.82 V EA max;f?;Note 18 16:46:51.54 -01:57:34.1 R0 17.4710 16.46 0.65 V EA max;f 19 16:46:55.92 -02:00:32.8 R0 ---- 19.64 0.97 V EA max 20 16:47:20.48 -01:57:10.3 R0 ---- 19.72 0.62 V EA max;Note 21 16:47:13.54 -01:56:28.1 R0 0.0196 17.11 0.06 V SX max 22 16:47:23.30 -01:55:40.7 R0 0.0344 17.25 0.05 V SX max 23 16:47:17.28 -01:57:42.0 R0 0.0443 16.70 0.08 V SX max 24 16:47:14.19 -01:53:52.3 R0 0.0450 17.20 0.08 V SX max 25 16:47:02.89 -01:57:37.8 R0 0.0490 17.02 0.17 V SX max 26 16:47:14.07 -01:59:37.1 R0 ---- 16.57 0.12 V sin max;Note 27 16:47:22.63 -02:01:03.9 R0 11.3607 17.89 0.44 V ? max;f 28 16:47:16.14 -01:56:01.8 R0 0.9499 13.85 0.04 V ? max;Note 29 =V26 30 16:47:14.87 -01:56:06.8 R0 0.4007 16.39 0.03 V Ell? max 31 16:47:15.25 -01:57:18.6 R0 ---- 17.61 0.08 V sin max;Note 32 16:47:13.03 -01:57:14.8 R0 0.4991 17.23 0.07 V Ell? max 33 =V31 34 16:47:12.88 -01:58:00.8 R0 0.3523 19.55 0.41 V ? max;Note 35 16:47:12.27 -01:57:19.7 R0 0.9623 17.95 0.07 V Ell? Note 36 16:47:06.19 -01:57:34.4 R0 0.7748 16.81 0.02 V var? max PSR:A,B -- -- ---- -- -- PSR ==================================================================== Supplementary Notes NGC 6218 (M12) is a moderately metal poor globular cluster, with [Fe/H] = -1.37 and tidal radius = 17.6 arcminutes (Harris 1996 - 2003 and 2010 updates). Its CM diagram is characterized by a predominantly blue horizontal branch (Kaluzny et al. 2015, hereafter Kal15). Gontcharov et al. (2021) made isochrone fits to the CMD to estimate the relative age of M12 compared to other GCs with similar metallcity, but different CMD mrphology. Only one variable, a type II Cepheid, was discovered from photographic studies. However, numerous eclipsing and SX Phe variables were identified when Kaluzny and his collaborators (Kal15) observed the cluster using CCD detectors in the first decade of the 21st century. The data for V2-V36 are from Kal15. In their paper, Kal15 listed the membership status for most of the variables. These were based on proper motion investigations by Zloczewski et al. (2012, hereafter Zlo12) or by Narloch et al. (2017, hereafter Nar17). Even though the Narloch paper was still in preparation when the Kaluzny paper was published, they had access to some of their results. In the above table, the membership status listed in the Remarks column is from Nar17. An "f" in the remarks column indicates field status; otherwise it is considered to be a cluster member. Any exceptions are discussed in the notes on individual stars. Other important resources for M12 are papers by Sariya et al. (2018, hereafter Sar18) and by Stetson et al. (2019, hereafter Stet19). Sar18 carried out a proper motion investigation that included many of the variable stars and Stet19 published homogeneous photometry for approximately 69000 stars in M12. ================================================================= Notes on individual stars V1: The variability classification is from Clement et al. (1988). The RA and dec are from Samus et al. (2009). The remaining data are from Karmakar et al. (2019). The period is the one they derived from ASAS-SN observations obtained in the years 2013 to 2018. The V magnitude and amplitude are from the light curve they plotted in Figure 3 of their paper, based on observations obtained at Michigan State. Note: This is not the star that Kal15 listed as V1. V2: The data in the above table are the values that Kal15 listed for V1 in their Table 1. This star was initially listed as M12-V1 by von Braun et al. (2002) in their discovery paper. However, it was assigned the number V2 in an earlier version of this electronic catalogue because another star, designated as V1 in M12, was discovered many years earlier by Sawyer (1938). Furthermore, Sawyer's V1 had already been listed in a number of catalogues and papers. V5: The membership status for V5 is uncertain because Nar17 listed a membership probablility of 44%. V7: Kal15 listed this as a field star and contact binary with V=16.65 mag at maximum and V amplitude = 0.07 mag. It appears that the membership status was based on the star's position in the CM diagram. No proper motion was published by Zlo12 or by Nar17. The closest star in the Nar17 study is located 2 arcseconds further south, has V = 19.16 mag and is a cluster member. V17: Kal15 did not list membership status for this star. However, its location, 0.4 mag to the red of the lower main sequence, on the CM diagram they published indicates that it is probably a field star. It was not included in the proper motion studies of Zlo12, Nar17 or Sar18. V20: Kal15 did not list membership status for this star because it was not included in any of the proper motion studies. However, its B and V magnitudes were confirmed by Stet19. It is a detached eclipsing binary located on the main sequence in the CM diagram and probably a cluster member. V26: In their erratum published in 2016, Kal15 pointed out that V26 and V29 are the same object. They also noted that the star's light curve, which is sinusoidal, can be phased with two different periods: 1.843644 and 0.646669 days, which are aliasses of one another. Therefore no period has been listed for V26 in the above table and its classification is uncertain. The proper motion studies of Zlo12, Nar17 and Sar18 all indicate that V26 is a cluster member. V28: Kal15 listed a V magniitude of 13.85 mag for this star and claimed it was a cluster member. However, such a bright star was not found at Kal15's published position in any of the other studies. Zlo12, Nar17, Sar18 and Stet19 all list a star with V=15.2 mag, located about 3 arseconds west and the proper motion studies all indicate that it is a cluster member. It is located on the giant branch in the CM dagram. V31: In their erratum published in 2016, Kal15 pointed out that V31 and V33 are the same object. They also pointed out that the star's light curve, which is sinusoidal, can be phased with two periods: 0.405218 and 0.682552 days respectively, which are aliasses of one another. A close examination of the light curves they published for V31 and V33, particularly around the time of maximum light, indicates that both might be aliasses of the correct period. Therefore no period has been listed for V31 in the above table and its classification is uncertain. V31 is probably a cluster member. Even though Zlo12 classified it as a field star, the more recent study by Nar17 indicated it is a cluster member and it is located in the BS region of the CM diagram. V34: Kal15 lists a V magnitude of 19.55 and amplitude of 0.41 mag for this star and according to the CM diagram that they published, it is a main sequence star. However, the other investigators, Zlo12, Nar17, Sar18, and Stet19, all list a V magnitude between 15.55 and 15.59 mag for the star located at Kal15's published position for V34. According to these other studies it a red giant and cluster member. Thus the status of V34 is uncertain. V35: The membership status of V35 is unknown because it was not included in the proper motion studies of Zlo12, Nar17 or Sar18. However, the B and V magnitudes published by both Kal15 and by Stet19 indicate that it is a blue straggler and probably a cluster member. ================================================================ Discovery of the variable stars in M12: V1 Sawyer (1938) with ID chart and x,y coordinates V2 = von Braun's M12-V1 von Braun et al. (2002) with ID chart, RA and dec von Braun et al. also discovered a field variable which they called M12-V2. It is a foreground WUMa and is not included in the above table. Kal15 followed von Braun's numbering system for V1 and V2 in their paper. V3-36 Kaluzny et al. (2015) with RA and dec (2000) and individual finding charts PSR: According to Paulo Freire's website, there are 2 milli-second pulsars in NGC 6218. (www.naic.edu/~pfreire/GCpsr.html) ----------------------------------------------------------------- Malakhova et al. (1997) announced two suspected RR Lyrae (based on their positions in the CM diagram). They published their X,Y positions in pixels and an ID chart and designated them as V2 and V3. Kinman (2016) provided further information about these two stars and concluded that neither one is a cluster RR Lyrae variable. Their V2 is a non-member according to the proper motion study of Zlo12 and their V3 consists of two member stars separated by 1.1 arcseconds. Kinman also noted that neither star appeared on the list of variables in the M12 field published by Kal15. Thus there is no evidence that either one is a variable that is a cluster member. -------------- Pietrukowicz et al. (2008) searched for dwarf novae in M12 and found none. ------------- The Second GALEX Ultraviolet Variability Catalog published by Wheatley et al. (2008) included five stars in M12. Kinman (2016) noted that none of these stars were among the variables that Kal15 studied. Since their amplitudes of variation in the near ultraviolet ranged from 0.88 to 2.11 mag, their variability should have been readily detected by Kal15. Kinman therefore raised the possibility that the Kal15 compilation of variables was incomplete. However, another possibility is that there are errors in the GALEX catalog. For each suspected variable, Wheatley et al. published B, R and I magnitudes from the USNO-B1.0 catalog of Monet et al. (2003), when available, in addition to the GALEX ultraviolet data. Unfortunately, not one of the USNO positions for these stars matches the GALEX positions. They all differ by about 7 arcseconds. Furthermore these differences are not due to a systematic shift. They are random, as indicated in the following table. GALEX RA and dec (J2000) USNO RA and dec (J2000) USNO B,R,I #333 16:47:14.3 -2:00:35.7 16:47:14.17 -2:00:44.4 -,15.29,- #334 16:47:18.2 -1:56:03.4 -,-,- #335 16:47:18.8 -1:52:21.4 16:47:18.57 -1:52:14.9 14.75,14.48,15.19 #336 16:47:25.0 -1:55:05.9 16:47:25.02 -1:55:13.9 12.35,11.90,15.98 #337 16:47:27.8 -2:00:40.7 16:47:27.31 -2:00:36.1 18.80,16.41,- Therefore it is not appropriate at this time to include these new variables in the list of M12 variables. However, it should be acknowledged that NOT ALL of the positions listed in the GALEX catalog are problematic. The USNO position for another candidate variable star in the vicinity of NGC 6218, GALEX #330 (J164536.6-014631.6), matches the GALEX position. GALEX #330 is a known variable according to several catalogues listed on the CDS Vizier website. It is a fundamental mode RR Lyrae variable with a period of 0.454 days. However, it is not a cluster member because it is located about 26.5 arcminutes from the centre of the cluster, well beyond the 17.6 arcminute tidal radius, and outside the field of the proper motion studies of Zlo12, Nar17 and Sar18. Also, with a V magnitude fainter than 16 mag, GALEX #330 is too faint to be an RR Lyrae cluster member. ----------------------- Lu et al. (2009) identified 6 X-ray sources inside the half-mass radius (2.16 arcminutes) of M12 and suggested that there may be 2-5 active binaries or cataclysmic variables in the cluster. None of these were listed by Kal15. Sar18 estimated membership probability for four of the X-ray sources and concluded that CX1, CX2 and CX4 are probably cluster members and that CX3 is not. ---------------------- Kinman (2016) announced two candidate variable stars. They were designated as CSS_J164714.5-020046 and CSS_J164719.0-015229 in the Catalina survey (Drake et al. 2009). He derived periods of 4.951404 and 0.277440 days respectively for these two stars from Catalina data and the light curves he plotted indicate that both stars might be eclipsing binaries. CSS_J164714.5-020046 is a cluster member, based on the proper motion studies of Zlo12, Nar17 and Sar18. CSS_J164719.0-015229 is also confirmed as a cluster member by Nar17 and Sar18. It was not included in the study of Zlo12. Neither of these stars was listed as a variable by Kal15. However, there is an interesting possibility, given the uncertainties in the GALEX RA and dec values. Is CSS_J164714.5-020046 the same object as GALEX #333 and is CSS_J164719.0-015229 the same object as GALEX #335? ============================================================ References Clement, C. M., Sawyer Hogg, H., Yee, A. 1988, AJ, 96, 1642 Drake et al. 2009, ApJ, 696, 870 Harris, W. E. 1996, AJ, 112, 1487 Gontcharov, G. A., Khovritchev, M. Y., Mosenkov, A. V., Ill'in, V. B., Marchuk, A. A., Savchenko, S. 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