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Peremennye Zvezdy (Variable Stars) 45, No. 11, 2025 Received 3 October; accepted 14 October. |
Article in PDF |
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DOI: 10.24412/2221-0474-2025-45-106-109
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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.
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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.
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Fig. 1.
Coordinate offsets
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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.
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Fig. 2.
The |
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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)
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Gaia DR3 id | R.A. | Decl. | |
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| (Equinox 2000.0, epoch 2016.0) | (Gaia DR3 synthetic photometry) | |||||
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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 |
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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