A Revised Shapley-Lindsay Catalogue of LMC Clusters
By Max Gardner AM
In the last two centuries numerous attempts have been made to catalogue the various interesting objects (star clusters, nebulae and variable stars etc) in the LMC. Unfortunately the accuracy and precision of published positions has often left much to be desired. Even in the 1960s, when surveys were probing fainter objects, some positions being published were still rather rough, making it difficult to identify some objects in the absence of reliable charts.
The present work has concentrated on a very useful dataset, the Shapley-Lindsay (SL) catalogue of 898 LMC star clusters in order to:
- facilitate identification by presenting positions of higher precision
- ensure that each SL position corresponds closely to an intended object
- produce a dataset which can be further manipulated electronically, if required eg to be imported into popular planetarium-type programs
- ensure that all SL objects are included, such as those which have been omitted from the Hodge-Wright (HW) atlas of the LMC (1967), and the omitted clusters in the LMC Bar (KMHK catalogue,1990).
The Shapley-Lindsay Catalogue
This compilation of 898 LMC clusters was published by H. Shapley and E.M. Lindsay (1963), based on searches of ADH Schmidt plates. Their catalogue covers the whole range of morphologies, from the large bright aggregations now known as stellar associations, through the dense swarms of stars we recognize as globular clusters, to the smaller gatherings which are the counterparts of our galactic clusters (open clusters). The listed objects generally lie between RA 4h24m and 6h34m, and Dec. -63° and -75°. The two main problems with the original SL catalogue, as I see it, are:
- positions which are approximate, and subject to both systematic and random errors. These problems are alluded to in the HW Atlas (p15, p37), and I concur. Right Ascensions are frequently out by 10 or 20s, and declinations are out by 4-5', or 10', which helps explain why HW could not find particular SL clusters, and were led to misidentify others.
- difficulty in distinguishing between small clusters and background galaxies, especially in the outer reaches of the LMC. Clearly, this was a limitation of their plate material. Images from the large Schmidt telescopes (via the Digitized Sky Survey) make it easy to distinguish the two.
While the SL catalogue gave diameters for all objects, I found that very small values (5-10 arcsec) would just as likely indicate a galaxy as a cluster.
Catalogues consulted in preparation of "SL-Revised".
- "A Catalog of LMC Star Clusters outside the Hodge-Wright Atlas", Olszewski, Edward W., Harris, Hugh C., Schommer, Robert A. and Canterna, Ronald W. 1988, Astron. J. 95,84-90. [OHSC]
- "The Cluster system of the Large Magellanic Cloud", Kontizas,M., Morgan,D., Hatzidimitriou,D. and Kontizas, E. 1990, Astron. Astrophys. Suppl. 84, 527-547. [KMHK].
- "Catalog of Extended Objects in the Magellanic Clouds", Bica, E., Schmitt, H.R., Dutra, C.M. and Oliveira, H. 1998, Catalogue II. Astron. J. December 1998, in press. [BICA] - See also http://www.if.ufrgs.br/~dutra/catpage.html
- The Guide Star Catalog, v1.1, Space Telescope Science Institute 1989. [GSC]
- USNO-A2.0 Catalog, Monet, D., Bird, A., Canzian, B. et al 1998, US Naval Observatory, Washington DC. [USNO-A2.0] - See also http://www.usno.navy.mil/pmm
The BICA catalogue, being very extensive (7847 objects), was deemed to include most, if not all, SL objects, while providing semi-precise positions, dimensions and alternative designations. The BICA data was selected as a suitable starting point. All SL objects were extracted from the BICA catalogue, and imported to a spreadsheet file for further editing. Each SL object was then located in the KMHK catalogue, and the position was corrected for precession (1950 --> 2000) and compared with the BICA catalogue. [The KMHK catalogue couldn't identify all SL objects with certainty.] If KMHK and BICA were in good agreement, the former was accepted. In cases of disagreement, ie when the RA differed by more than 4s, and/or declination by more than 10", the object was set aside for further examination. In cases of very severe disagreement it was found prudent to download the field from the Digitized Sky Survey (DSS). About 90 problem objects were inspected on DSS images. Not every SL object required a DSS image (and time did not allow), as it was assumed that if KMHK and BICA positions agreed, then the identification was secure. Furthermore, if a check of the GSC revealed a non-stellar object at the same spot, then the identification was hardly in doubt. The original SL coordinates were also frequently referred to, as about 10% of SL objects are missing in the KMHK catalogue.
To rely only on the HW Atlas for identifications was found to be a little hazardous, as misidentifications are far too common, and it seems that the compilers of the KMHK and BICA catalogues have been led astray, in a few instances, by the HW charts. Particularly where the faintest SL clusters are concerned, which are barely visible. A notable instance is SL 834. The original SL position is actually quite close to the mark, but the HW chart erroneously marks a spot 30' due S, where nothing can be seen. The KMHK and BICA cataloguers, obviously guided by the HW position, reported the positions of two different asterisms in the near vicinity.
The catalogue is saved as a PDF file here: https://www.asnsw.com/content.cfm?page_id=1342110¤t_category_code=21965