Астронет > Coordinate and Finding-chart Errors for Several Variables in the Young LMC Globular Cluster NGC 1866

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Peremennye Zvezdy (Variable Stars) 45, No. 11, 2025
Received 3 October; accepted 14 October.
Article in PDF

DOI: 10.24412/2221-0474-2025-45-106-109

Coordinate and Finding-chart Errors for Several Variables in the Young LMC Globular Cluster NGC 1866

A. K. Dambis, L. N. Berdnikov

Sternberg Astronomical Institute, Moscow State University, Universitetskij pr. 13, Moscow 119992, Russia; leonid.berdnikov@gmail.com

We report significant consistent systematic errors in the equatorial coordinates

(1950.0) and

(1950.0) of 10 variables (V1-V10) in the young LMC cluster NGC 1866 in the discovery paper by Storm et al. (1988). The authors incorrectly mark one of these variables in their finding chart. We provide correct Gaia DR3 identifications for the stars considered here, together with the corrected finding chart.

1. Introduction

Variable stars in clusters inherit a number of important characteristics of their host systems, such as distance, age, metallicity, and amount of interstellar extinction, and therefore are of great value for the study of the properties of both individual objects and the clusters they belong to. However, the crowded nature of cluster fields may result in position and identification errors, especially in distant clusters observed against crowed backgrounds, as is the case for stellar systems in the LMC.

2. Variable Stars in NGC 1866

Storm et al. (1988), in the announcement of their discovery of 10 new variable stars in the young LMC globular cluster NGC 1866, which is now known to contain at least 20 Cepheids, provided a finding chart and a table with the equatorial coordinates (1950.0) and (1950.0), mean magnitudes, and color indices. We compared the coordinates and finding-chart positions presented by the above authors with Gaia DR3 data (Gaia Collaboration, 2023a) and found the reported coordinates to be systematically and consistently offset by and with respect to Gaia DR3 positions propagated to the epoch of Storm et al. observations (December 1983 - December 1985) (see Fig. 1). With these offsets applied, the scatter of the positions reported by Storm et al. (1988) about those implied by Gaia DR3 becomes 0 6 and 0 3 along right ascension and declination, respectively.

The resulting Gaia DR3 identifications are further confirmed by Gaia DR3 synthetic photometry (Gaia Collaboration, 2023b): all ten stars have Gaia DR3-based synthetic -band magnitude estimates, and 9 of them (except V2) also have Gaia DR3-based synthetic -band magnitude estimates, such that and (we plot individual and offsets for 9 stars with both - and -band synthetic Gaia DR3 photometry in Fig. 2). The scatter of individual-star offsets is and . Furthermore, in the finding chart of Storm et al. (1988), the variable V4 is incorrectly identified with a nearby -fainter star located at an angular separation of 3 77 (Gaia DR3 4663636160446653184), which is inconsistent with the relative arrangment defined by tabulated coordinates. This star is marked by the black arrow and original arc in Fig. 3, which shows an enlarged portion of the Storm et al. (1988) finding chart featuring V4 and neighboring stars. The star indicated in the original paper has and according to Gaia DR3 synthetic photometry, which is evidently inconsistent with the and reported in Table 1 of the same paper. The red arrow indicates the correct star (V4 = Gaia DR3 4663636156133536000) whose Gaia DR3 synthetic-photometry magnitude and color index, and , are consistent with the Strom et al. (1988) estimates.

In our Table, we list the correct Gaia DR3 identifications for the ten NGC 1866 variables V1-V10, together with their Gaia DR3 ICRS positions for the epoch of 2016.0 and equinox J2000.0, and Gaia DR3 synthetic - and -band photometry.

Acknowledgments: The study was conducted under the state assignment of Lomonosov Moscow State University.

This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

Fig. 1. Coordinate offsets and in the sense Storm et al. (1988) minus Gaia DR3 coordinates. The open blue circles show the offsets computed using Gaia DR3 identifications listed in the Table. The filled red circle marked by red "V4" shows the offsets computed assuming the identification of variable V4 shown in the original finding chart. The magenta arrow points to the offsets of variable V4 computed using identification listed in the Table.

Fig. 2. The -band magnitude and color index differences in the sense Storm et al. (1988) minus Gaia DR3 synthetic-photometry values. The open blue circles show the offsets computed using Gaia DR3 identifications listed in Table 1. The filled red circle marked by red "V4" shows the offsets computed assuming the identification of variable V4 shown in the original finding chart. The magenta arrow points to the offsets of variable V4 computed using identification listed in the Table.

Fig. 3. A fragment of the Storm et al. (1988) finding chart showing variable V4 and the central part of the NGC 1866 cluster. The original identification of V4 is indicated by the small arc around it and with our black arrow. The red arrow shows the correct star (V4 = Gaia DR3 4663636156133536000).

Table. Gaia DR3 data for 10 variable stars in NGC 1866 discovered by Strom et al. (1988)

Variable
Gaia DR3 id R.A. Decl.

(Equinox 2000.0, epoch 2016.0) (Gaia DR3 synthetic photometry)

V1
4663636229174240512 078.37748773965 -65.43218890210 16.823790 16.184423 0.639367

V2 4663636126094988032 078.39092479449 -65.46005585302 16.373241

V3 4663636057376591232 078.39755894669 -65.45893738559 17.612946 16.829468 0.783478

V4 4663636156133536000 078.40693712081 -65.45403489676 16.702795 16.104410 0.598385

V5 4663541842986446848 078.41684490468 -65.51473094774 15.981842 15.652927 0.328915

V6 4663636057367262080 078.42435252493 -65.47206987396 16.612055 16.178410 0.433645

V7 4663636160446465792 078.42629561573 -65.45832557643 16.636070 16.068666 0.567404

V8 4663636156133535360 078.42836066552 -65.45545791154 16.737722 16.129217 0.608505

V9 4663635851217101696 078.44963558263 -65.46900612096 16.497131 16.068040 0.429091

V10 4663635919936613120 078.50506799094 -65.46187682794 16.632813 15.815446 0.817367

References:

Gaia Collaboration, Vallenari, A., Brown, A. G. A., Prusti, T., et al. 2023a, Astron. & Astrophys., 674, id. A1

Gaia Collaboration, Montegriffo, P., Bellazzini, M., De Angeli, F., et al. 2023b, Astron. & Astrophys., 674, id. A33

Storm, J., Andersen, J., Blecha, A., & Walker, M. F. 1988, Astron. & Astrophys., 190, No. 1-2, L18

Variable	Gaia DR3 id	R.A.	Decl.
		(Equinox 2000.0, epoch 2016.0)		(Gaia DR3 synthetic photometry)
V1	4663636229174240512	078.37748773965	-65.43218890210	16.823790	16.184423	0.639367
V2	4663636126094988032	078.39092479449	-65.46005585302		16.373241
V3	4663636057376591232	078.39755894669	-65.45893738559	17.612946	16.829468	0.783478
V4	4663636156133536000	078.40693712081	-65.45403489676	16.702795	16.104410	0.598385
V5	4663541842986446848	078.41684490468	-65.51473094774	15.981842	15.652927	0.328915
V6	4663636057367262080	078.42435252493	-65.47206987396	16.612055	16.178410	0.433645
V7	4663636160446465792	078.42629561573	-65.45832557643	16.636070	16.068666	0.567404
V8	4663636156133535360	078.42836066552	-65.45545791154	16.737722	16.129217	0.608505
V9	4663635851217101696	078.44963558263	-65.46900612096	16.497131	16.068040	0.429091
V10	4663635919936613120	078.50506799094	-65.46187682794	16.632813	15.815446	0.817367