SIMBAD references

Soft X-ray images of the intermediate-age (∼250 Myr) NGC 1039 and NGC 3532 open star clusters have been acquired with the ROSAT High Resolution Imager, resulting in the detection of 75 sources above a 3 σ detection level of L_X≃2.5x10²⁹ ergs.s^–1. Roughly half the sources detected in both images have likely optical counterparts from earlier ground-based surveys. The remainder are either prospective cluster members or foreground/background stars, which can be decided only through additional photometry, spectroscopy, and proper-motion studies. There is some indication (at the 98% confidence level) that solar-type stars in both clusters, and in the similarly aged NGC 6475 cluster, may lack the extreme rotation and activity levels shown by those in the much younger Pleiades and α Persei clusters, but a detailed assessment of the coronal X-ray properties of these clusters must await more sensitive observations in the future. If confirmed, this finding could help to rule out the possibility that stellar dynamo activity and rotational braking are controlled by a rapidly spinning central core as stars pass through this phase of evolution from the Pleiades stage to that represented by the Hyades.