NGC 6656 / C1833-239 / Messier 22
RA: 18:36:23.94 DEC: -23:54:17.1 (J2000)
(Most recent updates:
Membership probability flag - 2024; Remaining data - May 2018)
============================================================
Bytes Format Explanation
1-8 A8 Star ID
10-32 A11,1x,A11 Position
34-35 A2 Membership probability flag based on data published
by Prudil & Arellano Ferro (2024)
m1: prob >=0.85; m2: prob >=0.7 and <0.85;
m3: prob >= 0.5 and <0.7; f: prob <0.5
u: no data available
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 Mem Period <mag> ampl C Type Notes/
RA Dec Flag Remarks
=========================================================================
1 18:36:19.56 -23:54:32.9 m1 0.6155 14.19 1.00 V RR0
2 18:36:34.91 -23:53:06.5 m1 0.6417 13.97 0.85 V RR0
3 18:36:38.00 -23:47:15.3 f 0.5154 15.77 0.93 V RR0 f
4 18:36:23.36 -23:55:29.2 m1 0.7164 14.08 0.76 V RR0
5 18:36:10.58 -23:55:00.6 m1 93.103 11.02 0.47 V SR Note
6 18:36:18.25 -23:56:03.6 m1 0.6385 14.02 0.66 V RR0
7 18:35:57.60 -23:47:40.8 m1 0.6495 14.08 1.02 V RR0
8 18:36:20.72 -23:55:27.1 m1 58.480 10.84 0.58 V SR Note
9 18:36:08.20 -23:55:02.9 m1 94.252 11.07 0.43 V SR Note
10 18:36:20.93 -23:56:27.3 m1 0.6460 14.13 1.00 V RR0
11 18:36:22.71 -23:54:09.1 m1 1.6905 12.77 0.70 V CW Note
12 18:36:23.76 -23:55:39.0 m1 0.3226 14.15 0.43 V RR1
13 18:36:28.65 -23:51:39.7 m1 0.6725 14.02 0.99 V RR0
14 18:36:40.51 -23:46:07.4 f 200.00 14.68 4.03 V M f;Note
15 18:36:32.08 -23:55:40.9 m1 0.3732 14.24 0.43 V RR1
16 18:36:36.97 -23:54:33.5 m1 0.3253 14.14 0.41 V RR1
17 18:35:51.37 -23:52:24.2 f 113.3 15.85 1.4 P SR f;Note
18 18:36:16.15 -23:47:15.1 m1 0.3249 14.07 0.43 V RR1
19 18:36:20.59 -23:52:17.4 m1 0.3840 14.13 0.42 V RR1
20 18:36:14.80 -23:56:33.0 m1 0.7561 14.03 0.61 V RR0
21 18:36:25.75 -23:52:58.1 m1 0.3271 14.02 0.41 V RR0
22 18:35:03.46 -23:51:15.6 f 0.6245 14.372 1.04 V RR0 f?;Note
23 18:36:23.03 -23:54:41.5 m1 0.5516 14.29 1.12 V RR0
24 18:36:21.79 -23:54:13.4 m1 1.7149 13.42 0.65 V CW
25 18:36:46.29 -23:48:02.5 m1 0.4024 14.09 0.41 V RR1
26 18:35:26.30 -23:47:45.8 f 309.0 16.45 2.0 P SR f;Note
27 18:35:28.33 -23:45:29.9 m1 0.3428 13.979 0.47 V RR1 f?;Note
28 18:34:52.07 -23:52:56.4 u 424.0 14.25 1.0 P SR f;Note
29 18:36:30.58 -23:41:03.5 m1 0.3043 14.28 0.42 V RR f?;Note
30 18:36:41.06 -23:58:19.5 m1 82.500 10.64 0.76 V SR Note
31 18:36:10.12 -24:05:49.1 m1 95.8253 10.83 0.57 V SR max;Note
32 18:35:37.85 -24:00:02.5 f 233.5 16.5 2.6 P M f;Note
33 18:36:14.09 -24:07:51.0 f 251.1 15.8 2.7 P M f;Note
34 18:36:26.07 -23:55:33.9 m1 30.300 11.27 0.17 V SR
35 18:36:24.04 -23:54:29.4 m1 31.900 11.32 0.25 V SR
36 18:35:39.30 -23:54:06.5 f 0.6264 16.733 0.9 V RR0 f;Note
37 18:35:48.72 -23:40:58.9 f 1.1968 15.391 1.774 V E f?;max;Note
38 18:35:48.95 -23:42:43.4 f 0.5738 15.847 1.1 V RR0 f;Note
39 18:36:09.73 -23:59:35.6 f 65.640 16.82 0.61 V SR f
40 18:36:34.82 -23:47:23.3 f 37.841 15.33 0.26 V SR f
41 18:36:58.94 -23:52:49.1 f ---- 17.15 0.5 V ? f;Note
42 18:36:59.59 -23:47:16.0 f ---- 17.15 0.5 V ? f;Note
43 18:36:59.83 -23:52:19.6 f ---- 16.65 0.5 V ? f;Note
KT-01 18:36:43.34 -23:56:25.7 m1 0.3139 18.62 0.19 V EW f;max
KT-02 18:36:41.82 -23:56:21.2 m1 0.4906 17.33 0.28 V EA/EB max
KT-03 18:36:41.26 -23:52:19.9 f 0.3650 18.70 0.24 V EW f;max
KT-04 18:36:39.37 -23:52:25.8 m1 0.0357 16.83 0.16 V SX
KT-05 18:36:39.08 -23:50:27.9 f 0.0608 17.21 0.20 V SX f
KT-07 18:36:36.85 -23:57:00.8 f 0.3298 17.67 0.49 V EW max
KT-08 18:36:35.67 -23:55:15.4 f 0.3639 19.68 0.64 V EW max
KT-10 18:36:32.30 -23:54:28.0 f 0.0375 16.60 0.09 V SX
KT-12 18:36:30.94 -23:53:49.0 f 0.4436 16.46 0.92 V RR0 f
KT-13 18 36 30.86 -23 53 46.2 m1 0.2817 17.22 0.45 V EW max
KT-14 18:36:30.67 -23:53:54.0 m1 0.3747 14.13 0.25 V RR1
KT-15 18:36:30.12 -23:49:59.3 f 0.3381 16.45 0.55 V EW f;max
KT-16 18:36:30.36 -23:57:13.2 m1 0.2819 14.12 0.07 V RR1
KT-18 18:36:29.03 -23:49:59.4 f 2.7642 17.65 1.08 V EA f;max
KT-20 18:36:26.09 -23:51:27.0 m1 0.2885 16.66 0.17 V EW max
KT-23 18:36:23.84 -23:51:16.4 m1 0.2985 16.47 0.24 V EW max
KT-26 18:36:23.16 -23:53:23.5 f 0.3614 14.04 0.21 V RR1
KT-27 18:36:22.54 -23:55:12.4 m1 0.0422 16.99 0.39 V SX
KT-28 18:36:22.04 -23:52:06.5 m1 0.0556 16.17 0.06 V SX
KT-29 18:36:20.96 -23:55:48.2 u 0.0443 16.40 0.18 V SX
KT-33 18:36:16.86 -23:53:54.5 u 0.2441 16.96 0.08 V EW max
KT-34 18:36:16.89 -23:55:25.0 m1 0.0473 16.81 0.11 V SX
KT-36 18:36:15.89 -23:56:07.1 m1 0.3133 14.13 0.35 V RR1
KT-37 18:36:13.18 -23:53:47.0 m1 0.2961 14.09 0.11 V RR1
KT-38 18:36:11.43 -23:56:47.5 m1 0.0328 16.88 0.06 V SX
KT-39 18:36:09.99 -23:51:57.8 f 1.4748 17.28 0.19 V EA max
KT-40 18:36:08.13 -23:51:49.1 f 0.4372 17.65 0.21 V EW f;max
KT-41 18:36:07.09 -23:54:13.2 m1 0.2939 17.66 0.57 V EW f;max
KT-42 18:36:34.67 -23:52:31.2 f 0.5549 17.29 0.11 V EW max
KT-43 18:36:24.28 -23:56:19.1 m1 0.2205 17.36 0.10 V EW max
KT-45 18:36:22.04 -23:54:41.9 m1 0.0501 16.59 0.23 V SX
KT-46 18:36:21.76 -23:58:25.1 f 0.6102 19.41 0.99 V EA max
KT-48 18:36:16.54 -23:57:36.5 m1 0.3389 20.11 0.96 V EW f;max
KT-51 18:36:35.08 -23:53:03.6 m1 0.1034 14.62 0.015 V ? Note
KT-54 18:36:24.95 -23:50:50.2 f 0.0836 16.36 0.09 V SX
KT-55 18:36:23.24 -23:53:58.1 m1 0.6587 14.11 0.91 V RR0
PK-04 18:36:22.79 -23:52:48.3 m1 0.3123 18.46 0.32 I EW max;PK
PK-05 18:36:22.26 -23:54:32.9 u 0.2428 18.31 0.25 V EW max
PK-06 18:36:25.19 -23:54:37.3 u ---- 17.14 0.47 I EW max;Note
PK-07 18:36:26.89 -23:53:43.3 m1 0.3557 17.87 0.11 I BY? max;PK
PK-08 18:36:24.17 -23:54.10.1 u 0.3273 18.70 0.20 I BY? max;PK
PK-09 18:36:29.99 -23:55:42.8 m1 1.4791 17.93 0.16 I BY? max;PK
PK-10 18:36:27.12 -23:52:59.7 f 5.2500 17.32 0.32 I BY? max;PK
PK-11 18:36:30.32 -23:55:23.0 m1 0.0662 19.85 0.16 I ELL? max;PK
CV1 18:36:24.66 -23:54:35.5 u ---- 17.24 4.60 V UG
CV2 18:36:02.72 -23:55:24.6 f ---- 19.35 0.85 V UG
P1 18:36:22.40 -23:56:29.4 u ---- 19.62 0.99 V mlens
Ku-1 18:35:59.12 -23:57:13.4 m1 0.3058 14.09 0.23 V RR1
Ku-2 18:36:02.96 -23:50:29.6 m1 0.3352 14.10 0.16 V RR1
Ku-3 18:36:29.53 -24:01:33.0 m1 0.3340 14.01 0.39 V RR1
Ku-4 18:36:31.68 -23:49:30.6 m1 0.2902 14.17 0.13 V RR1
SLW-4 18:36:17.51 -23:54:26.3 m1 61.500 11.15 0.26 V SR
SLW-5 18:36:18.38 -23:54:01.3 m1 38.000 11.33 0.12 V SR
SLW-6 18:36:19.27 -23:53:26.7 m1 ---- 11.18 0.07 V SR Note;P=73?
SLW-7 18:36:21.01 -23:54:42.5 m1 61.0 11.28 0.1 V SR Note
SLW-8 18:36:21.64 -23:55:57.0 m1 51.530 11.24 0.24 V SR
SLW-9 18:36:25.42 -23:54:35.6 m1 ---- 13.23 0.6 V L? f?;Note
SLW-11 18:36:28.05 -23:53:23.2 m1 39.500 11.29 0.15 V SR
102 18 36 36.91 -23 53 47.1 m1 0.0244 16.51 0.03 V SX Note
103 18 36 30.96 -23 52 52.8 m1 0.0345 16.47 0.09 V SX
104 18 36 23.86 -23 54 19.0 u 0.0353 16.93 0.18 V SX
105 18 36 19.88 -23 55 27.5 f 0.0354 16.72 0.15 V SX
106 18 36 12.43 -23 51 12.2 f 0.0366 16.58 0.04 V SX
107 18 36 27.17 -23 55 27.8 m2 0.0367 16.85 0.05 V SX
108 18 36 19.03 -24 0 27.4 m1 0.0371 16.89 0.04 V SX
109 18 36 3.38 -23 53 37.4 m1 0.0374 16.56 0.11 V SX
110 18 36 13.34 -23 44 58.6 m1 0.0374 16.50 0.12 V SX
111 18 36 21.35 -23 54 38.6 u 0.0498 16.64 0.29 V SX
112 18 36 25.17 -23 54 1.1 u 0.0623 15.73 0.38 V SX
113 18 36 23.80 -23 55 5.4 m1 0.1346 15.03 0.02 V ? max
114 18 36 35.71 -24 2 5.5 m1 0.1380 19.30 0.18 V ? max
115 18 36 33.23 -23 59 58.2 m1 0.1507 19.55 0.21 V ? max
116 18 36 34.69 -23 57 52.5 u 0.1670 19.74 0.13 V var? max
117 18 36 16.16 -23 56 11.1 m1 0.3133 15.84 0.02 V EW? max
118 18 36 6.19 -23 48 19.6 m1 0.2803 18.55 0.13 V EW max
119 18 36 5.18 -23 53 14.2 u 0.3082 19.43 0.22 V EW max
120 18 36 22.29 -23 54 46.9 u 0.3130 17.42 0.09 V EW max
121 18 36 14.47 -23 52 11.0 u 0.3883 19.21 0.26 V EW max
122 18 36 30.07 -23 56 10.6 u 0.3886 19.25 0.56 V EW max
123 18 36 34.58 -23 52 58.4 m1 0.5087 17.00 0.05 V ? max
124 18 36 23.79 -23 52 24.7 u 0.5093 15.89 0.02 V ? max
125 18 35 57.48 -23 47 59.2 m1 0.5429 14.52 0.02 V ? max
126 18 36 23.25 -23 53 53.6 m1 0.5481 14.85 0.03 V ? max
127 18 36 55.75 -23 48 23.9 m1 0.6218 17.32 0.05 V EW max
128 18 36 36.23 -23 55 21.0 m1 0.8818 14.55 0.02 V ? max
129 18 36 21.23 -23 51 37.5 f 1.3948 15.80 0.05 V EW max
130 18 36 17.59 -23 57 14.4 m1 1.4460 16.63 0.04 V EA/EB max
131 18 36 24.18 -23 54 26.7 f 1.7336 16.00 0.26 V EA max
132 18 36 25.73 -23 56 54.5 m1 1.8429 13.34 0.04 V var? max
133 18 36 37.30 -23 53 38.8 m1 2.2442 18.99 0.34 V EA max
134 18 36 16.63 -23 55 58.2 m1 2.3309 16.82 0.14 V ? max
135 18 36 55.52 -23 55 32.3 m1 4.9280 18.79 0.18 V EA max
136 18 36 17.52 -23 57 31.5 m1 11.6962 15.59 0.06 V SR? Note
137 18 36 21.69 -23 53 35.1 m1 14.3226 16.05 0.09 V SR? Note
138 18 36 25.51 -23 52 38.6 f 34.9127 14.91 0.05 V SR? Note
139 18 36 15.10 -23 54 55.0 m1 80.8938 11.07 0.21 V SR
140 18 36 15.38 -23 52 51.9 f ---- 18.03 0.77 V L P=3100?
PSR -- -- -- -- -- -- u ---- -- -- 4 msPs
===================================================================
Supplementary Notes
M22 is located in the foreground of a rich field in the Galactic bulge.
The most comprehensive investigation of the variables in M22 was
carried out by Rozyczka et al. (2017, hereafter R17) and most of the
data in the above table are from their study.
There are a few exceptions:
15 of the previously known variables (V17, V22, V26, V27,
V28, V29, V31, V32, V33, V36, V37, V38, V41, V42, V43)
were not included in the R17 field of view. For these
variables, the positions are from Samus et al. (2009) and
the sources for the remaining data are listed in the notes
on individual stars.
7 of the variables discovered by Pietrukowicz & Kaluzny (2003)
were not studied by R17 because they were too close to the
cluster centre. For these stars, the data of Pietrukowicz &
Kaluzny have been listed and are indicated "PK" in the
remarks column.
3 of the long period variables (#6, #7, #9) announced by Sahay
et al. (2014) were excluded in the R17 study because they
were overexposed in their frames. These are discussed in the
notes on individual stars.
Another CCD investigation of M22 variable stars was made by Kunder et al.
(2013), but their study dealt only with RR Lyrae variables. In general,
their results are in good agreement with those of R17. They determined
the same periods, but in some cases, their magnitudes and amplitudes
differ by as much as 0.05 mag.
The field of the OGLE IV Bulge survey includes M22 and results have been
published for RR Lyrae variables (Soszynski et al 2014) and for
eclipsing binaries (Soszynski et al 2016). Consequently, the RR Lyrae
variables (V22, V27, V29, V36, V38) and eclipsing binary (V37) outside
the R17 field were observed in the OGLE survey and this is discussed in
the notes on individual stars.
In addition, a number of the M22 variables are included in the Moscow
GCVS. Samus et al. (2009) provided a cross reference.
V14 = V1311 Sgr, V17 = V4068 Sgr, V22 = V3853 Sgr,
V26 = V2007 Sgr, V28 = V2367 Sgr, V29 = NSV 11080,
V30 = NSV 11084, V31 = V3855 Sgr, V32 = V4067 Sgr,
V33 = V4069 Sgr
Also V27 = V2592 Sgr (Samus, private communication 2012)
Harris (1996 - 2010 update) listed a half-light radius of 3.36' and
tidal radius of 32.0' for M22. However, it is highly unlikely that
any cluster members are located as far as 32' from the cluster centre.
In fact, most of the variables beyond the half-light radius are probably
field stars.
CM diagrams published by Lee (2015) and by R17 show strong contamination
by Galactic bulge stars at much smaller distances. This strong
contamination by field stars is also apparent in the vector point
diagrams published in proper motion studies by Zloczewski et al.
(2012) and by Narloch et al. (2017).
R17 published data for 291 variable stars, with r < 12.4', for which
they listed membership status, based on proper motions: 102 members
and 189 field stars. Of the 214 variables with r > 3.36', the
half-light radius, 30 were members and 184 were field stars.
========================================================================
Notes on individual stars
V5, V8, V9, V11:
The cluster membership of V5, V8, V9 and V11 was established
in a radial velocity study by Joy (1949). Radial velocities
confirming the membership of V5, V8 and V9 were subsequently
obtained by Peterson & Cudworth (1994). These findings were
confirmed by R17 based on proper proper motion data.
V9: Wehlau & Sawyer Hogg (1977) observed V9 over an interval of
more than 80 years (1893-1975) and found that its amplitude
of light variation changed from time to time. In fact, in
1968-1969, the star's magnitude was almost constant, but then
the amplitude increased again. Sahay et al. (2014) did not
detect significant variation in V9 when they observed it for
six months in 2002 and concluded that it must be in a quiescent
phase.
The data for V9 listed above are from R17.
V12: According to Sawyer (1944), Bailey doubted the variability of
V12. Sawyer also found that its variability was not confirmed
on her plates. Wehlau & Sawyer Hogg (1978) measured V12 on
many of their plates but the data were not good enough to
confirm variability.
However, Kravtsov et al. (1994) questioned the non-variable
status of V12 and Kunder et al. (2013) found that it is an RR1
variable with a V amplitude of more than 0.4 mag, a result that
was subsequently confirmed by R17.
The data for V9 listed above are from R17.
V14 (V1311 Sgr):
The non-membership of V14 was established in a radial velocity
study by Joy (1949) and confirmed by R17 based on proper
motion data.
V17 (V4068 Sgr):
The period, magnitude and amplitude are from Wehlau & Sawyer
Hogg (1977). The classification (SR) is from the GCVS and
from Marinchev (1983) who also published magnitudes for V17.
Observations by Monaco et al. (2004) and by Lee (2015) indicate
that it is a field star based on its location in the CM diagram.
V22, V27 (OGLE-BLG-RRLYR-36640 and 36648):
The periods, magnitudes and amplitudes are from the OGLE IV
survey (Soszynski et al 2014). These two variables were not
observed by Kunder et al. (2013) or by R17.
Since the OGLE website does not list V amplitudes, these were
calculated by applying empirical amplitude ratios derived by
Kunder et al. (2013) for M22:
A_V/A_I= 1.66 for RR0 and A_V/A_I= 1.61 for RR1 variables.
Kunder et al. published data from ASAS (Pojmanski 2002) for
these two stars and considered them both to be field stars
because of their bright V magnitudes and their great distance
from the cluster centre (18.7' and 15.5' respectively).
However, the V magnitudes from the OGLE IV survey are appropriate
for cluster membership. Their membership status is therefore
uncertain.
V24: Kunder et al. (2013) stated that V24 is not variable. However,
the coordinates that they listed for V24 pertain to a different
star. According to R17, the star listed as V24 in the above
table is variable. This misidentifcation might have come about
because, according to Samus et al. (2009), Kravtsov et al. (1994)
listed the wrong position for V24.
V26 (V2007 Sgr)
The period, magnitude and amplitude are from Wehlau & Sawyer
Hogg (1977). The classification is from the GCVS.
Observations by Lee (2015) indicate that V26 is a field star
based on its location in the CM diagram.
V28 (V2367 Sgr)
The period, magnitude and amplitude are from Wehlau & Sawyer
Hogg (1977). The classification is from the GCVS and from
Marinchev (1983) who published magnitudes for V28.
V28 was also observed by Pojmanski (2002) who designated
the star as ASAS183452-2353.0. On the Vizier website, there
is a link to a light curve that clearly demonstrates that
the max and min magnitudes vary from cycle to cycle,
characteristics of an SR variable.
Observations by Lee (2015) indicate that V28 is a field star
based on its location in the CM diagram.
V29 (= NSV 11080 = OGLE-BLG-RRLYR-36678):
The projected radial distance of V29 from the cluster centre is
13.3 arcmin. Based on the proper motion study of Zloczewski et
al. (2013), Kunder et al. (2013) concluded it is unlikely that a
star with such a large radial distance is a cluster member.
However, the V and I magnitudes are appropriate for cluster
membership. The period, V magnitude and amplitude listed above
are from the Ogle IV survey (Soszynki et al. 2014). Since the
OGLE website does not list V amplitudes, the V amplitude was
calculated by applying empirical amplitude ratio derived by
Kunder et al. (2013) for M22: A_V/A_I= 1.61 for RR1 variables.
Kunder et al. published a different period (0.47 days) for V29
but their data were very sparse. The OGLE period agrees with
the period derived earlier by Wehlau & Sawyer Hogg (1978).
The membership status for V29 is uncertain.
V30: The cluster membership of V30 was established in a radial
velocity study by Peterson & Cudworth (1994). This result
was confirmed by R17 based on proper proper motion data.
V31 (V3855 Sgr):
Marinchev (1983) classified V31 as an SR variable.
The period, magnitude, and amplitude are from Pojmanski (2002)
who designated the star as ASAS183610-2405.8. On the Vizier
website, there is a link to a light curve that clearly
demonstrates that the max and min magnitudes vary from cycle to
cycle, characteristics of an SR variable.
Observations by Monaco et al. (2004) and by Lee (2015) indicate
that V31 is located near the red giant tip in the CM diagram so
it is undoubtedly a cluster member.
V32 (V4067 Sgr)
The period, magnitude and amplitude are from Wehlau & Sawyer
Hogg (1977). The Mira classification and non-membership status
are from the GCVS.
V33 (V4069 Sgr)
The period, magnitude and amplitude are from Wehlau & Sawyer
Hogg (1977). The Mira classification and non-membership status
are from the GCVS. The Mira classification has been
confirmed by Catchpole et al. (2016) who published a period,
P=772.0.
V35: Figuera Jaimes et al. (2016) observed V35 and derived a 141 days
period..
V36, V38 (OGLE-BLG-RRLYR-36651, OGLE-BLG-RRLYR-36657):
V36 and V38 were not studied by Kunder et al. (2013) or by R17.
The periods, magnitudes and amplitudes are from the OGLE IV
survey (Soszynski et al 2014). Since the OGLE website does
not list V amplitudes, these were calculated by applying
empirical amplitude ratios derived by Kunder et al. (2013) for
M22: A_V/A_I= 1.66 for RR0 variables.
Both stars are considered to be field stars because they are
much fainter than the RR Lyrae cluster members. They are
located 10.2 and 14.1 arcminutes from the cluster centre.
Although this is well within the tidal radius, proper motion
data (R17) shows that, at this distance, most stars are not
cluster members.
V37 (OGLE-BLG-ECL-423040)
The period, maximum magnitude and amplitude are from the
OGLE IV survey of eclipsing binaries in the Galactic Bulge
(Soszynski et al. 2016). With a distance of 15.6 arcminutes
from the cluster centre, V37 is unlikely to be a cluster
member. However, its VI magnitudes indicate that the star
might be a blue straggler. Thus its membership status is
uncertain.
V41, V42, V43:
The magnitudes are from the discovery paper by Kravtsov et al.
(1994). All of these stars have been detected as variables in
the Pan-STARRS1 surveys (Chambers et al. 2016).
Observations by Lee (2015) indicate that they are all field
stars based on their locations in the CM diagram.
KT-51: R17 were unable to classifiy this variable. They noted
that it is located on the blue HB and has a sinusoidal light
curve.
PK-06: The maximum magnitude and amplitude is from the study by
Pietrukowicz & Kaluzny (2003). They derived P= 0.239431, but
no period has been listed because Figuera Jaimes et al. (2016)
observed this star and derived a different period, 0.140851 days.
The period derived by Pietrukowicz & Kaluzny (2003) did not
produce a good light curve in their data.
CV1: The V magnitude and amplitude for CV1 are from Pietrukowicz et
al. (2005). An outburst was also observed in the near-infrared
(an increment of more than 1 mag in the K_s band) in the summer
of 2014 in the VVV survey (Minniti et al. 2010) and reported
by Alonso-Garcia et al. (2015). Figuera Jaimes et al. (2016)
detected a 3 mag outburst in the I-band. CV1 was also observed
in the OGLE IV survey where it is OGLE-BLG-DN-1057.
CV2: CV2 was also observed in the OGLE IV survey where it is
OGLE-BLG-DN-1056.
SLW-6, SLW-7:
The data for these two stars are from the discovery paper by
Sahay et al. (2014) because they were not studied by R17.
Observations by Monaco et al. (2004) and by Lee (2015) indicate
that both stars are located near the red giant tip in the CM
diagram and are considered to be SR cluster members.
SLW-9: The data for this star are also from the Sahay et al. (2014)
discovery paper. They derived a provisional period of 122 days.
The membership of SLW-9 is uncertain. Although it is located
on the giant branch, it is approx 2 mags below the tip.
V102: For the SX variables (V102-V112), the V magnitudes are assumed
to be mean magnitudes. R17 (page 207 of their Acta Ast paper)
indicated that the V band magnitudes listed in their table
correspond to maximum magnitude for the eclipsing binaries and
mean magnitude for the other variables.
V136, V137, V138:
R17 found that these 3 variables are proper motion cluster
members and classified them as SR variables. They all lie on
the lower giant branch, below the HB, in the CM diagram and
are therefore faint compared with other SR cluster members.
Thus their classification is uncertain.
The V magnitudes listed for these variables are assumed
to be mean magnitudes. R17 (page 207 of their Acta Ast paper)
indicated that the V band magnitudes listed in their table
correspond to maximum magnitude for the eclipsing binaries and
mean magnitude for the other variables.
========================================================================
Discovery of the variable stars in M22:
V1-16
Bailey (1902) with ID chart
V17
by Swope and announced by Shapley (1927) with x,y positions for V1-17
V18-25
Sawyer (1944) with x,y positions for V1-25 and ID chart for V1-21
and V23-25. (V22 was outside the field.)
V26-31 = #181a,181b,173a,187b,191,185
by Hoffleit (1972) with 1900 RA & dec and GCVS numbers for
some of them (181a=V2007 Sgr, 181b=V2592 Sgr, 173a=V2367 Sgr).
These variables were later identified on an ID chart by Wehlau &
Sawyer Hogg (1977) who also published x,y coordinates. The
numbers V26-31 were assigned by Sawyer Hogg (1973) in her 3rd
catalogue.
V32-33
V. Sherwood, an assistant to Sawyer Hogg (unpublished) and
identified on the chart by Wehlau & Sawyer Hogg (1977).
who also publsihed x,y coordinates
V34-35
Lloyd Evans (1978) reported (see page 300) that Kukarkin had
discovered two red variables not previously known.
These were star IV-17 labelled on a plate by Arp & Melbourne (1959)
and star V-9 labelled by Lloyd Evans (1975). Wehlau & Sawyer Hogg
(1978) numbered these two variables as V34 and V35 respectively and
identified them on their ID chart. Both stars are near the red giant
tip.
V36-43
Kravtsov et al. (1994) with ID chart and the RA & dec (Epoch 1950).
They noted that these are faint for RR Lyrae cluster members, but they
could be members if they are subgiants.
KT-01 to KT-55 (CASE M22 variables)
Kaluzny & Thompson (2001) with RA & dec
The designation KT-01 to KT-55 has been assigned in this catalogue.
These authors initially selected 55 candidate variables, but only
36 of them turned out to be new variables: 6 were spurious and 13
were RR Lyrae variables that were already known.
PK-04 to PK-11
Pietrukowicz & Kaluzny (2003) with RA & dec
P&K referred to these stars as M22_04 to M22_11. The designation
PK-04 to PK-11 has been assigned in this catalogue.
P&K also listed data for three stars numbered 1 to 3 which are
the same stars as KT-27, 13 and 43 respectively.
CV1
Anderson et al. (2003) with RA, dec and ID chart
This object had been previously announced as a microlensing event
in the Galactic bulge by Sahu et al. (2001), but Anderson et al.
concluded that it was more likely the outburst of a cataclysmic
variable because its proper motion indicated it was a cluster
member. Also it exhibited H_alpha emission and variability in
quiescence and was associated with an X-ray source.
Cataclysmic outbursts of the star were subsequently detected in
other studies including that of Pietrukowicz et al. (2005) and most
recently by Alonso-Garcia et al. (2015) who reported an increment
in brightness of more than 1 mag in K mag, the first dwarf nova
eruption observed at near infra-red wavelengths in a globular
cluster.
CV2
Pietrukowicz et al. (2005) with RA, dec and ID chart.
Webb et al.(2004) announced that there are 5 (+/-3) X-ray sources
associated with M22 and Pietrukowicz concluded that CV2 is
associated with one of them.
P1 is a probable microlensing event detected by
Pietrukowicz et al. (2005) who listed the RA & dec and published
an ID chart.
Ku-1 to Ku-4 = NV1-4
Kunder et al. (2013) with RA and dec
SLW-4, SLW-5, SLW-6, SLW-7, SLW-8, SLW-9, SLW-11
Sahay et al. (2014) with RA and dec.
All except SLW-9 are located near the RGB tip in the CM diagram
and are considered to be cluster members. The membership of SLW-9
is uncertain.
Sahay et al. announced 4 other new long period variables:
SLW 1, 2, 3, 10. Based on their location in the CM diagram,
these variables are considered to be field stars. Furthermore, R17
reported that SLW-2 (=N166) and SLW-10 (=N159) were not members
based on proper motion.
SLW-1,3,6,7,9 were not investigated by R17.
Sahay et al. also discovered 11 candidate long period variables
for which they could not determine periods.
V102-V140
R17 with RA & dec for all of the variables, and finding charts for
V112, V116, V117, V125, V129, V130, V131, V133, V134, V135
These variables were all considered to be cluster members based on
a proper motion study by Narloch et al. (2017).
The R17 investigation was based on observations with a field of
view 14.8 by 22.8 arcminutes and they discovered 244 additional
variables:
69, numbered U01 to U69, for which no proper motion data were
available and
175, numbered N01 to N175, whose proper motions indicated that
they were not cluster members.
The data for all of the 359 variables (previously known and new
discoveries) studied by R17 are listed on the CASE website at
http://case.camk.edu.pl/results/Photometry/M22/index.html
Light curves and other data for a selection of the variables were
published in their 2017 Acta Astronomica paper.
PSR: According to Paulo Freire's website at the Max Planck Institute
in Bonn (July 2024 version), there are 4 millisecond pulsars
in NGC 6656 (M22).
==================================================================
The field of the OGLE IV Bulge survey includes M22 and 23 of the
RR Lyrae variables in the above table were included in the OGLE IV
investigation (Soszynski et al 2014) where:
V1=OGLE-BLG-RRLYR-36670, V2=OGLE-BLG-RRLYR-36681,
V4=OGLE-BLG-RRLYR-36673, V6=OGLE-BLG-RRLYR-36669,
V12=OGLE-BLG-RRLYR-36674, V13=OGLE-BLG-RRLYR-36677,
V15=OGLE-BLG-RRLYR-36680, V16=OGLE-BLG-RRLYR-36682,
V18=OGLE-BLG-RRLYR-36668, V19=OGLE-BLG-RRLYR-36671,
V20=OGLE-BLG-RRLYR-36666, V21=OGLE-BLG-RRLYR-36675,
V22=OGLE-BLG-RRLYR-36640, V23=OGLE-BLG-RRLYR-36672,
V25=OGLE-BLG-RRLYR-36683, V27=OGLE-BLG-RRLYR-36648,
V29=OGLE-BLG-RRLYR-36678, V36=OGLE-BLG-RRLYR-36651,
V38=OGLE-BLG-RRLYR-36657, KT-12 =OGLE-BLG-RRLYR-36679,
KT-36=OGLE-BLG-RRLYR-36667, KT-37=OGLE-BLG-RRLYR-36665,
Ku2=OGLE-BLG-RRLYR-36662.
The OGLE IV website lists data for an additional 27 RR Lyrae variables
within the tidal radius (32'), but all of them are too faint for
cluster membership.
-------------------------------------------------------
The OGLE IV Bulge survey of eclipsing binaries (Soszynski et al. 2016)
includes 19 of the eclipsing variables in the above table:
V37=OGLE-BLG-ECL-423040, KT-02=OGLE-BLG-ECL-423217,
KT-03=OGLE-BLG-ECL-423216, KT-07=OGLE-BLG-ECL-423211,
KT-08=OGLE-BLG-ECL-423208, KT-13=OGLE-BLG-ECL-423192,
KT-15=OGLE-BLG-ECL-423188, KT-18=OGLE-BLG-ECL-423184,
KT-20=OGLE-BLG-ECL-423175, KT-23=OGLE-BLG-ECL-423159,
KT-39=OGLE-BLG-ECL-423107, KT-40=OGLE-BLG-ECL-423102,
KT-41=OGLE-BLG-ECL-423099, KT-42=OGLE-BLG-ECL-423203,
KT-43=OGLE-BLG-ECL-423166, KT-46=OGLE-BLG-ECL-423150,
V118=OGLE-BLG-ECL-423093, V122=OGLE-BLG-ECL-423187,
V134=OGLE-BLG-ECL-423136.
The OGLE IV website lists data for a total of 510 eclipsing binaries
that are within the M22 tidal radius (32') and 147 of them are within
16', the radius listed by Soszynski et al (2014). Most of these are
probably field stars.
The study by R17 included data for 59 of these OGLE variables with
radial distances from the cluster centre ranging from 1.6' to 11.9'.
Of these, 52 had known membership status, based on proper motions, and
only 13 were considered to be cluster members. V118, with r = 7.2',
was the furthest 'member' from the cluster centre and the closest field
star was KT-41 with r = 3.85'.
--------------------------------------------------------------
Figuera Jaimes et al. (2016) searched for new variables in a 41 by 41
arcsec field around the cluster centre and found none.
==========================================================
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