Arp 2 / C1925-304
RA: 19:28:44.11 DEC: -30:21:20.3  (J2000)

(Most recent updates: 
Membership probability flag - 2024; Remaining data - July 2019)
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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
=========================================================================
1        19:28:36.4  -30:20:56.2 m1   0.5684 18.18  1.15  V RR0   
2        19:28:39.4  -30:20:07.0 m1   0.811  18.03  0.49  V RR0   
3        19:28:56.9  -30:21:29.6 m1   0.565  17.99  0.90  V RR0   
4        19:29:01.5  -30:20:56.1 u    0.458  18.23  1.06  V RR0   
5        19:28:56.3  -30:26:32.0 u    0.760  18.04  0.59  V RR0    
6        19:28:30.2  -30:23:11.3 u    0.4472 18.22  0.90  V RR0    
7        19:29:04.8  -30:16:04.6 u    0.527  17.86  0.56  V RR0   Note
8        19:28:44.9  -30:21:54.7 m1   0.2921 18.18  0.49  V RR1   
9        19:28:52.3  -30:22:10.7 m1   0.515  18.16  1.28  V RR0   
10       19:28:52.1  -30:23:13.3 m2   0.0474 20.47  0.50  V SXPHE 
11       19:28:45.6  -30:20:05.8 f    0.0612 20.14  0.35  V SXPHE 
12       19:28:42.4  -30:25:15.9 u    0.0604 20.26  0.28  V SXPHE 
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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.
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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.
     However, Prudil and Arellano Ferro (2024) were unable to assess the
     membership status of V7 in their investgation.
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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.

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References

Pritzl, B. J., Gehrman, T. C., Salinas, R., Catelan, M., Smith, H. A.,
Borissova, J. 2019, PASP, 131, 054202
    
Prudil Z., Arellano Ferro, A. 2024, MNRAS, 534, 3654

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
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