NGC 6624 / C1820-303
RA: 18:23:40.69 DEC: -30:21:38.8 (J2000)
(Most recent updates:
Membership probability flag - 2024; Remaining data - January 2025)
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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
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ID Position Mem Period <mag> ampl C Type Notes/
RA Dec Flag Remarks
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1 18:23:53.52 -30:18:42.3 u 292.6 12.08 1.555 I M Note
2 18:23:49.35 -30:17:52.2 u 194.95 11.944 2.751 I M f;Note
3 18:23:39.84 -30:20:50.3 m1 132.50 12.5 1.1 V SR Note
4 18:23:37.52 -30:22:01.8 m1 ---- -- -- CST? Note
5 18:23:30.00 -30:19:04.1 f 0.3467 16.4 0.6 B RR1? f?;Note
FL09 18:23:46.13 -29:58:55.6 u 156.0 12.40 0.26 G L Note;mem?
PSR -- -- -- -- -- -- u ---- -- -- 11 msPs
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Supplementary Notes
NGC 6624 is a metal rich bulge cluster with [Fe/H] = -0.44 according to
the 2010 revision of the Harris (1996) catalgue. A VI CM diagram
published by Heasley et al. (2000) shows a prominent red HB.
In the above table, the RA and dec for V1-5 are from Samus et al.
(2009). The sources for the remaining data are indicated in the Notes on
individual stars.
The data for FL09 are from Layden et al. (2025, hereafter L25).
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Notes on individual stars
V1: V1=OGLE-BLG-LPV-263031
The period, magnitude, amplitude and classification are from
the OGLE collection of Mira Stars in the Milky Way (Iwanek et al.
2022).
V2: V2=OGLE-BLG-LPV-263012
The period, magnitude, amplitude and classification are from
the OGLE collection of Mira Stars in the Milky Way (Iwanek et al.
2022).
V2 is considered to be a field star because magnitudes published
by Liller & Liller (1976) indicated that the star lies well to
the red of the cluster's CM diagram at a V magnitude level
comparable to the HB stars.
V3: V3=V7047 Sgr
The period, magnitude, amplitude and classification are from the
Moscow GCVS.
Liller & Carney (1978) showed that V3 lies near the RG tip in the
CM diagram. Thus it was considered to be a cluster member, a
result confirmed by Prudil & Arellano Ferro (2024).
V4: Liller & Liller (1976) noted that this star is part of a close
pair and questioned its variable status. It has not been
recovered as a variable in any other surveys.
V5: The magnitude and colour derived by Liller & Liller (1976) for V5
are appropriate for cluster membership. However, they also
pointed out that the star was outside the cluster's tidal radius.
It is therefore possible that the star belongs to the rich field
surrounding the cluster, which is confirmed by the membership flag
published by Prudil & Arellano Ferro (2024).
FL09: L25 suggested that this star might be a cluster member based on
its radial velocity (Gaia Collaboration 2023) and location in
the CMD, but they acknowledged that the proper motion differed
from the mean value for the cluster. Nevertheless, they
recommended that it be included in this catalogue so that it
might receive additional attention regarding its cluster
membership.
L25 obtained their magnitude, amplitude and classification listed
in the above tabel are from the ASAS-SN survey (Jayasinghe et
al. 2020).
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Discovery of the variable stars in NGC6624:
V1-4 = V1, V2, V11 and V14 of Fourcade et al. (1966)
with x,y coordinates and an ID chart
Fourcade et al. announced 33 variables, but only 4 of them were
considered to be cluster members. Sawyer Hogg (1973) assigned the
numbers V3 and V4 to FL #11 and #14.
V5: Liller & Liller (1976) with an ID chart for V1-5
FL09:
This was one of the 33 variables in NGC 6624 announced by Fourcade
et al. (1966), but because of its large angular offset from the
cluster centre, it was not included in Sawyer Hogg's (1973)
catalogue.
L25 derived its RA and dec (J2000) by matching the stars in Fourcade's
charts with modern digital images.
FL09 is now listed in this catalogue because L25 proposed that its
membership status should be revisited in view of developments
in the study of globular cluster structure since 1973 when Sawyer Hogg
published her catalogue.
The Fourcade catalogue is not available in many libraries and has not
been digitized. As a result, many of the stars listed in their
catalogue will soon become lost to the literature on globular
cluster variable stars. Because of this, L25 also listed the RA,
dec (J2000) and Gaia ID for the remaining "Fourcade" variables in
their paper.
PSR: According to Paulo Freire's website at the Max Planck Institute
in Bonn (July 2024 version), NGC 6624 has 11 millisecond pulsars
and one long period pulsar (P=2.497 sec) which might not be
associated with the cluster.
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A possible quiescent cataclysmic variable (or LMXB) was announced by
Deutsch et al. (1999). Dalessandro et al. (2014) followed up on this
system with HST observations and identified a star that appeared to be
the optical counterpart of the X-ray source Star 1. They called it
COM_Star1. It exhibited sinusoidal variations with an amplitude of ~0.7
mag in F435W and an orbital period of ~98 minutes. However, they could
not make a definitive conclusion on the nature of the system: whether it
was a CV or LMXB.
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References
Dalessandro, E., Pallanca, C., Ferraro, F. R., Lanzoni, B., Castiglione,
C., Vignali, C., Fiorentino, G. 2014, ApJ, 784, L29
Deutsch, E. W., Margon, B., Anderson, S. F., Downes, R. A. 1999, AJ, 118,
2888
Fourcade, C. R., Laborde, J. R., Albarracin, J. 1966, Atlas y Catalogue
de Estrellas Variables en Cumulos Globulares al sur de -29 degrees, Cordoba
Gaia Collaboration: Vallenari, A., Brown, A. G. A. et al. 2023, A&A,
674, A1
Harris, W. E. 1996, AJ, 112, 1487
Heasley, J. N., Janes, K. A., Zinn, R., Demarque, P., Da Costa, G. S.,
Christian, C. A. 2000, AJ, 120, 879
Iwanek, P., Soszynski, I., Kozlowski, S., Poleski, R., Pietrukoowicz,
P., Skowron, J., Wrona, M. and 8 coauthors, 2022, ApJS, 260, 461
Jayasinghe, T., Stanek, K. Z., Kochanek, C. S., Shappee, B. J.,
Holoien, T. EW.-S., Thompson, T. A., Prieto, J. L. and 9 coauthors,
2020, MNRAS, 491, 13
Layden, A. C., Revangelista, J., Gupta, S., Mittameda, M., Mulloy, E.,
Muslim, S. I., Olushola-Alao, T., Rishi, A. 2025, Astron. Nachr., 346,
Issue 2
Liller, M. H. & Carney, B. W. 1978, ApJ, 224, 383
Liller, M. H. & Liller, W. 1976, ApJ, 207, L109
Prudil Z., Arellano Ferro, A. 2024, MNRAS, 534, 3654
Samus, N. N., Kazarovets, E. V., Pastukhova, E. N., Tsvetkova, T. M.,
Durlevich, O. V. 2009, PASP, 121, 1378
Sawyer Hogg, H. 1973, Publ. DDO, 3, No. 6
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