GALEX photometry of the open cluster NGC 752

Abstract

Ultraviolet photometry from the GALEX mission is compiled for a sample of main sequence stars and red giants in the open cluster NGC 752. This is one of the few ensembles of coveal near-solar abundance FGK stars for which near-ultraviolet and far-ultraviolet magnitudes are available from GALEX. Cluster members are identified on the basis of Gaia proper motion and parallax data. Two-colour diagrams, including (NUV‑B,B‑V) and (FUV‑B,B‑V), are presented by combining the GALEX photometry with Johnson BV photometry. In all diagrams which include the NUV magnitude in one of the colours a clear offset is seen between the unevolved main sequence and the main sequence turn-off stars. However, in the (FUV‑B,B‑V) diagram the main sequence and the turn-off stars overlap. There is evidence of scatter in the (FUV‑B) colours of main sequence stars redward of (B‑V)∼0.50 that may be related to differences in stellar activity. A GALEX-only colour-magnitude diagram of NGC 752 in which NUV apparent magnitude is plotted against the (FUV‑NUV) colour shows much less separation of the main sequence turn-off stars and red giants from the dwarf sequence than does a traditional (V,B‑V) colour-magnitude diagram. Indeed, in the former diagram the red giants of NGC 752 fall very close to the main sequence.