An X-ray investigation of the NGC 346 field in the small magellanic cloud I. The luminous blue variable HD 5980 and the NGC 346 cluster

Abstract

We present results from a <i>Chandra</i> observation of the NGC 346 cluster. This cluster contains numerous massive stars and is responsible for the ionization of N66, the most luminous H II region and the largest star formation region in the SMC. In this first paper we will focus on the characteristics of the main objects of the field. The NGC 346 cluster itself shows only relatively faint X-ray emission (with L<SUP>unabs</SUP><SUB>x</SUB> ∼1.5 × 10<SUP>34</SUP> ergs s<SUP>-1</SUP>, tightly correlated with the core of the cluster. In the field also lies HD 5980, a luminous blue variable star in a binary (or possibly a triple) system that is detected for the first time at X-ray energies. The star is X-ray bright, with an unabsorbed luminosity of L<SUP>unabs</SUP><SUB>x</SUB> ∼ 1.7 × 10<SUP>34</SUP> ergs s<SUP>-1</SUP>, but needs to be monitored further to investigate its X-ray variability over a complete 19 day orbital cycle. The high X-ray luminosity may be associated either with colliding winds in the binary system or with the 1994 eruption. HD 5980 is surrounded by a region of diffuse X-ray emission, which is a supernova remnant. While it may be only a chance alignment with HD 5980, such a spatial coincidence may indicate that the remnant is indeed related to this peculiar massive star.