GALEX far-ultraviolet observations of metal-poor subdwarfs

Abstract

Far-ultraviolet photometry derived from the GALEX satellite observatory has been compiled for a sample of metal-poor subdwarfs with [Fe/H]<‑1.0. The FUV properties of these subdwarfs are compared with those of a set of Population I dwarfs that are known to have low levels of chromospheric activity. Comparisons are made via a number of photometric plots, including an absolute FUV magnitude versus (V‑Ks) diagram, two-colour diagrams involving both (mFUV‑B) and (mFUV‑V) versus B‑V, and a two-colour diagram composed of (mFUV‑V) versus (V‑Ks). The warmest subdwarfs with (V‑Ks)∼1.2–1.4 show FUV excesses ranging from ∼2–3mag relative to the Population I dwarfs, with the amount of FUV enhancement decreasing among subdwarfs of decreasing effective temperature. The coolest dwarfs that are compared have (V‑Ks)∼1.8, and among these stars the subdwarfs with ‑2.0≤[Fe/H]≤‑1.0 approach the locus of low activity Population I dwarfs in the (mFUV‑V,V‑Ks) diagram. In the (mFUV‑B,B‑V) diagram the subdwarfs in this metallicity range overlap the Population I dwarf sequence for (B‑V)>0.6. The behaviour of the subdwarfs is consistent with their FUV fluxes being determined by a combination of a photospheric FUV spectrum, the strength of which diminishes towards cooler effective temperatures, and a spectrum of emission lines arising from a chromosphere and/or transition region which are of comparable strength between the coolest dwarfs and subdwarfs.