Ultraviolet Observations of Blue Stragglers and Halo K Dwarfs

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We propose to carry out IUE observations of two classes of old, cool stars in order to search for white-dwarf companions. Blue stragglers are stars in globular clusters and old open clusters that lie on or near the main sequence but are well above the turnoff point for the majority of the cluster members. One proposed explanation for the origin of blue stragglers is that they are the former secondary stars of close-binary systems; if they have received sufficient mass via mass transfer from their companions, they could have moved up the main sequence to their observed positions. If this mechanism is correct, the former primary star might now be a white dwarf that could be detected with IUE. We plan to carry out IUE observations of three field blue stragglers that are already known to be radial velocity variables, and of a member of the old open cluster NGC 7789 with anomalous optical colors, to see whether they have the white-dwarf companions predicted by this scenario. Recently, several field halo K dwarfs have been discovered to have abnormally large ultraviolet excesses(from ground-based photometry). The UV excesses might be due to ultra-low metallicity, so that these K dwarfs could represent a heretofore unknown class of objects. However, an alternative explanation could be that these K dwarfs have white-dwarf companions that contaminate the optical colors. IUE observations will provide a straightforward test of these two possibilities, thus shedding light on the question of the existence of "Population III" stars.

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