NGC 6397 / C1736-536 (Updated December 2021) RA: 17:40:42.09 DEC: -53:40:27.6 (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 17:41:04.75 -53:32:58.2 R0 314.6 13.36 3.94 V M f?;Note 2 17:40:10.87 -53:47:33.8 R0 39.5977 12.88 0.47 V SRB? f;max;Note 3 17:40:16.88 -53:41:03.6 R0 0.4940 16.07 1.20 V RR0 f;K97 4 17:41:08.71 -53:42:34.6 R0 0.4227 16.35 0.73 V EW f;max;Note 5 17:41:05.46 -53:33:36.2 R0 0.27 18.70 0.8 V E f;max;K97 6 17:40:53.24 -53:43:39.8 R0 0.5 17.16 0.13 V EW f;max;K97 7 17:40:43.74 -53:40:35.6 R0 0.2699 17.09 0.47 V EW max 8 17:40:39.14 -53:38:47.1 R0 0.2712 16.24 0.38 V EW max 9 17:40:02.14 -53:35:45.4 R0 0.5802 16.25 0.65 V EW f;max;K97 10 17:40:37.43 -53:40:36.4 R0 0.0308 16.0 0.13 V SXPHE 11 17:40:43.95 -53:40:40.9 R0 0.0383 15.43 0.05 V SXPHE 12 17:40:41.42 -53:40:19.6 R0 0.4712 17.52 0.49 V CV? max;Note 13 17:40:48.82 -53:39:49.0 R0 0.2352 19.43 0.37 V CV? max;Note 14 17:40:46.31 -53:41:15.9 R0 0.3351 19.25 0.16 V E max 15 17:40:45.24 -53:40:25.2 R0 0.0238 15.43 0.06 V SXPHE 16 17:40:44.44 -53:40:42.0 R0 1.3541 16.65 0.15 V ELL max;Note 17 17:40:43.63 -53:41:16.8 R0 1.061 16.23 0.04 V Ell? 18 17:40:43.45 -53:40:28.1 R0 0.7867 15.75 0.14 V E max 19 17:40:42.81 -53:40:23.3 R0 0.2538 17.12 0.10 V E max;Note 20 17:40:41.51 -53:40:33.7 R0 0.8612 15.83 0.08 V GDOR? 21 17:40:41.40 -53:40:23.9 R0 0.0390 15.47 0.30 V SXPHE 22 17:40:41.02 -53:40:42.2 R0 ---- 16.15 0.18 V RR01? f;Note 23 17:40:39.21 -53:40:46.9 R0 0.0372 15.52 0.03 V SXPHE 24 17:40:38.97 -53:40:23.3 R0 0.4572 16.17 0.02 V GDOR? 25 17:41:10.16 -53:39:30.5 R0 1.2306 17.98 0.17 V ? f;Note 26 17:40:43.05 -53:38:31.2 R0 ---- 16.24 0.09 V ? f;Note 27 17:41:13.80 -53:41:14.1 R0 0.5561 18.25 0.05 V Ell? f 28 17:41:02.70 -53:39:47.0 R0 ---- 15.13 0.28 V ? f;Note 29 17:40:59.64 -53:40:38.6 R0 1.2434 19.72 0.19 V Ell? 30 17:40:54.51 -53:40:44.4 R0 ---- 17.64 0.56 V E f 31 17:40:42.59 -53:40:27.1 R0 1.3100 15.99 0.08 V Ell? 32 17:40:40.30 -53:41:25.2 R0 9.8783 16.13 0.03 V E max;Note 33 17:40:42.62 -53:40:19.0 R0 ---- 18.3 4.2 V CV? max;Note 34 17:40:42.29 -53:40:28.6 R0 ---- 16.2 5.8 V CV max;Note 35 17:40:43.31 -53:41:55.2 R0 0.2984 18.79 0.17 V Ell 36 17:40:44.10 -53:42:11.3 R0 0.5487 16.53 0.02 V ? pulsator? 37 17:40:42.61 -53:40:27.6 R0 1.96 17.56 0.2 U E PSRA -- -- ---- -- -- msP see V16 PSRB -- -- ---- -- -- msP ======================================================================= Supplementary Notes NGC 6397 is a core-collapsed globular cluster with at least 79 Chandra X-ray sources within its half-mass radius. These sources are the subject of numerous investigations. Variability in the optical counterparts of some of these sources has already been detected, but investigations continue and more variables may be identified in future. In the above table, the RA and dec for V1-36 are from the following sources: V1 (V639 Ara) and V2 (V825 Ara) from (Samus et al. (2009) V3-V11 (Kaluzny 1997) V12-V24 (Kaluzny & Thompson 2003) V25-V36 (Kaluzny et al. 2006). Note: There is a systematic shift in RA and dec between the 2003 and 2006 coordinates published for V12-24, in the sense that the 2003 RA values are ~ 0.1 to 0.15 lower and the 2003 dec values are ~0.5 arcseconds further south. A similar shift occurs between Kaluzny's 1997 coordinates and the values published in 2006 for the stars invesigated in both papers (V4, V7, V8, V10, V11). The remaining data for V1-36 are from Kaluzny et al. (2006) unless indicated otherwise in the Notes/Remarks. K97 in the Notes column refers to the paper by Kaluzny (1997). The data for V37 are from the discovery paper (Pichardo Marcano et al. 2021). -------------- Ahumada et al. (2021) carried out a VI study of variable stars in NGC 6397. They published an identification chart that included all of the variables except V1 which was outside their field of view. They also established membership status for many of the variables based on astrometric data available in Gaia DR2. Members: V1, V7, V8, V10, V11, V15, V16, V17, V20, V32, V36 Field stars: V2, V3, V4,V5, V6, V9, V22, V25, V26, V27, V28, V30 Stars not studied: V12, V13, V14, V18, V19, V21, V23, V24, V29 V31, V33, V34, V35, V37 The stars they found to be field stars are all indicated as "f" in the above table. The Ahumada et al. investigation included detailed analysis for only about half of the variables because the others were either too faint or outside the field of view on most of their frames. Therefore, most of the elements of the variables listed in the above table are from the Kaluzny et al. (2006) paper. ===================================================================== Notes on individual stars V1: The membership status of V1 is uncertain. Sawyer (1931), Swope & Greenbaum (1952) and El-Worfaly & Budding (1983) all pointed out that it might belong to the rich field around the cluster and a radial velocity by Feast (1996) indicated that V1 is unlikely to be a cluster member. However, a proper motion study by Cudworth & Benensohn (1994) indicated a fairly high membership probability. Ahumada et al. (2021) also concluded that V1 is probably a member, based on Gaia DR2 data. With a distance of approximately 8.3 arcminutes The period listed for V1 was published by Sawyer (1931). According to El-Worfaly & Budding (1983), the period might be slightly longer (315.3 days). The V magnitude and amplitude in the above table are from Woolley et al. (1961). V1 is listed in the Moscow GCVS as V639 Ara. V2 (= V825 Ara in the Moscow GCVS): The period, magnitude and amplitude are from the ASAS survey (Pojmanski 2002) who tentatively classified the star as an RV Tauri variable. However, Samus et al. (Moscow GCVS 5.1, version Oct 2020) classified V825 Ara as a semi-regular (SRB) and their classification is listed in the above table. V2 (V825 Ara) has always been considered to be a field star and this was recently confirmed by Ahumada et al. (2021) based on Gaia DR2 data. V4: In the above table, V4 refers to a variable announced by Kaluzny (1997). In an independent study, El-Worfaly & Budding (1983) discussed and identified a variable which they called V4. It is a different star and is considered to be a field star based on its distance from the cluster. According to Samus (2018, private communication) it is listed as V827 Ara in the Moscow GCVS, an RR Lyrae with Vmax=16.25 which is too faint for cluster membership. V12, V13: These are CV1 and CV6 in the list of possible cataclysmic variables detected by Grindlay et al. (2001) with the Chandra telescope. Kaluzny & Thompson (2003) detected periodic variability for these stars in the V band. They attributed this variability to the ellipsoidal effect in a binary system. They did not observe outbursts in either of these variables. V16: This ellipsoidal variable is the optical component of the millisecond pulsar J1740-5340 (D'Amico et al. 2001) and Chandra X-ray source U12 (Grindlay et al. 2001). Photometric variations of this star have been discussed by Kaluzny et al. (2003, 2008) and by Orosz & van Kerkwijk (2003). In their discovery paper, D'Amico et al. derived an orbital period of 1.35 days from changes in the pulsar period and this period was confirmed in the subsequent photometric investigations. Kaluzny et al. (2003) also derived its radial veloctiy and concluded that it is a cluster member, an assumption that was confirmed by Ahumada et al. (2001) from astrometric data. It is listed as J1740-5340(A) on Paulo Freire's website (www.naic.edu/~pfreire/GCpsr.html) V19: Samus (2018, private communication) pointed out that there was a a typo in the RA and dec that Kaluzny & Thompson (2003) published for V19. After consultation with Thompson, he concluded that the coordinates for V19 published by Kaluzny et al. (2006) were the correct ones. V22: Kaluzny et al. (2006) classified V22 as a double mode RR Lyrae variable with first overtone period P= 0.344: and fundamental mode P=0.52 days respectively. On the other hand, Ahumada et al. (2021) derived periods of 0.46497 and 0.30669 days respectively. In any case, Ahumada et al. showed that V22 is not a cluster member, based on astrometric data available in Gaia DR2. V25: Kaluzny et al. (2006) reported that a period (1.2306 days) was derived from their 2004 observations. However, the periodicity was absent in 2003 so they were unable to determine the variability type. V26: Kaluzny et al. (2006) classified V26 as an irregular variable. However, Ahumada et al. (2021) did not detect any variability in their data. V28: Kaluzny et al. (2006) concluded that V26 might be a pulsating variable but did not have enough data to derive a reliable period. The observational data of Ahumada et al. (2021) did not confirm Kaluzny's provisional period (25.997 days). They found that the period must be longer than 97 days. V31: Kaluzny et al. (2006) derived a period of 1.31 days for V31 and classified the star as an ellipsoidal variable. They suggested that it might be the optical counterpart of Chandra X-ray source U18 (Grindlay et al. 2001). However, they also pointed out that their position for V31 deviated from U18 by more than expected for a secure identification. In a later investigation, Pichardo Marcano et al. (2021) showed that V31 and U18 are indeed different sources and that U18 is about 0.4 arcseconds south of V31. They derived a period of 1.96 days for the optical counterpart of U18 and the star is listed as V37 in the above table. V32: The period, magnitude, amplitude and classification are from Kaluzny et al. (2008). V33, V34: These are CV3 and CV2 in the list of possible cataclysmic variables detected by Grindlay et al. (2001) with the Chandra telescope. Kaluzny et al. (2006) detected non-periodic variability for these stars in the V band. For V33 (CV3), they observed non-periodic changes of V exceeding 0.8 mag on some nights. For V34 (CV2), they observed a probable dwarf nova outburst in May 2003. Shara et al. (2005) found dwarf nova-like eruptions in both stars. V37: The period, magnitude, amplitude and classification are from Manuel Pichardo et al. (2021) who determined that this is the optical component of the X-ray source U18. They also cited evidence, based on radio and X-ray measurements, that this object is similar to V16 (U12) and therefore concluded that it is part of a binary milliscond pulsar system. ==================================================================== Discovery of the variable stars in NGC 6397: V1&2 (=V639 and V825 Ara) Bailey (1902) with x,y coordinates (page 241) and an ID chart (Plate XI, Fig. 2, caption on page 250) Bailey (1923) published data for 4 additional variables in the surrounding field. V3 Swope (unpublished correspondence with Sawyer) The number V3 was assigned by Sawyer (1955) in her 2nd catalogue where she published the x,y coordinates V3 is considered to be a field star and is designated as V826 Ara in the Moscow GCVS. V4-11 Kaluzny (1997) with RA and dec, as well as X,Y pixel numbers that refer to a template image that they submitted to the data centre at Strasburg. V10 is BS#11 that was identified by Lauzeral et al. (1992). El-Worfaly & Budding (1983) discussed and identified a variable which they called V4. It is a different star and is considered to be a field star based on its distance from the cluster. According to Samus (2018, private communication) it is listed as V827 Ara in the Moscow GCVS, an RR Lyrae with Vmax=16.25 which is too faint for cluster membership. V12-13 Kaluzny & Thompson (2003) with RA, dec and X,Y pixel numbers that refer to an HST archival image These two variables are the optical counterparts to CV1 and CV6, candidate cataclysmic variables detected by Grindlay et al. (2001) with the Chandra telescope. V14-24 Kaluzny & Thompson (2003) with RA, dec and X,Y pixel numbers that refer to an HST archival image for V14-21. V22 and V23 are BS#8 and BS#16 that were identified by Lauzeral et al. (1992). V25-36 Kaluzny et al. (2006) with RA, dec They published finding charts for most of these variables and for the others (V31, V33 and V34) they cited sources where the charts can be located (see page 549). V37 Pichardo Marcano et al. (2021) These authors showed that this star is the optical companion to the Chandra X-ray source U18 (Grindlay et al. 2001). The RA and dec listed in the above table are the values that Cohn et al. (2010) published for the optical counterpart to U18, based on HST data. PSRA = millisecond pulsar J1740-5340 D'Amico et al. (2001) It is listed as millisecond pulsar J1740-5340A with a period of 3.65033 milliseconds on Paulo Freire's website (www.naic.edu/~pfreire/GCpsr.html) It has been established that this pulsar is associated with V16 in the above table and with Chandra X-ray source U12 (Grindlay et al. 2001). This is discussed in the note on V16 (above). PSRB = millisecond pulsar J1740-5340B It is listed as millisecond pulsar J1740-5340B with a period of 5.787 milliseconds on Paulo Freire's website Paulo Freire's website (Dec 2021) indicates that J1740-5340B was discovered with the Parkes telescope. No discovery paper is cited. ------------------------------------------------------ Cohn et al. (2010) published optical identifications for 79 Chandra X-ray sources, based on HST data. Some of these (V7, V12, V13, V14, V16, V17, V20, V24, V26, V30, V31, V33, V34, V35, V36) are known variables listed in the above table. Using their H_alpha time series, they searched for variability and found that some of the the other sources exhibited evidence for variability, including the sources that Grindlay et al. (2001) listed as CV4 and CV5. CV4 and CV5 were not detcted as variables in the investigations by Kaluzny and his collaborators, but they did detect variability in CV1 (V12), CV2 (V34), CV3 (V33) and CV6 (V13). ------------------------------------------------------ In an investigation to detect planetary transits among low main sequence stars in NGC 6397, Nascimbeni et al. (2012) identified 12 new variable stars. All of them were considered to be field stars, based on the proper motions and position in the CM diagram. ------------------------------------------------------ Martinazzi et al (2017) conducted a photometric survey of 8391 stars on the lower main sequence of NGC 6397 on two consecutive nights in July 2009 and identified 412 suspected variable stars. The stars in their sample had magnitudes ranging from 17 to 23 with a 465-nm filter. 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B., Mather, L., Epps, E. 1961, RGO Bull., 43, 303 ======================================================================