The Pyxis cluster: a newly identified galactic globular cluster

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Session 49 -- Star Clusters in the Milky Way and Other Galaxies
Display presentation, Thursday, June 15, 1995, 9:20am - 4:00pm

[49.06] The Pyxis cluster: a newly identified galactic globular cluster

Serge Demers (U. de Montr\'eal), M.J. Irwin (RGO), W.E. Kunkel (OCIW)

In a recent paper Weinberger (1995 PASP 107 58) produced a catalogue of several new interesting objects discovered from a systematic search of sky survey plates. One of these objects appeared to just resolve into stars at the plate limit, suggesting it might be a previously unknown distant Galactic satellite. We carried out a preliminary investigation of this object using UKST Bj and R sky survey copies of the region digitised with the APM facility in Cambridge. The object just resolved into stars on both digitised plates supporting the idea that it is either a distant star cluster or even a small dwarf spheroidal galaxy. To investigate this object further we obtained deep CCD images in B, R and I on the Du Pont telescope at Las Campanas during March 1995. A B,R CMD to R=23 reveals a well defined main sequence with a turnoff at R=21.5. A sparsely populated sub-giant branch leads up to a horizontal branch at R=18.8. Assuming a reddenning of E(B-V) = 0.27 read from the maps of Burstein \& Heiles (1982 AJ, 87, 1165) and a horizontal branch magnitude of $M_R = 0.2$ gives a true distance modulus of $\approx 18.0$ and hence a distance of 40 kpc. The cluster of stars is some 2 arcmin in core size, or 23 pc if it is at a distance of 40 kpc.

The object lies in the constellation of Pyxis and we propose to call it the Pyxis ``globular'' cluster. It is similar in morphological appearance to the outer halo Pal globular clusters. Interestingly, the Pyxis cluster lies close to the plane of the Magellanic Clouds orbit (Jones {\it et al.} 1994) and might be a detached cluster of this system. A radial velocity is urgently needed to examine this possibility.

We will present further analysis of our data at the meeting, including better reddenning estimates from the B,R,I data, an age estimate obtained by fitting to the revised Yale isochrones and an analysis of the cluster morphology from structure counts of detected images.

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