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 $\mathrm{[Fe/H]} < -1.0$. 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 $(V-K_{s})$ diagram, two-colour diagrams involving both $(m_{ \mathrm{FUV}}-B)$ and $(m_{\mathrm{FUV}}-V)$ versus $B-V$, and a two-colour diagram composed of $(m_{\mathrm{FUV}}-V)$ versus $(V-K_{s})$. The warmest subdwarfs with $(V-K_{s}) \sim1.2\mbox{--}1.4$ show FUV excesses ranging from $\sim2\mbox{--}3~\mbox{mag}$ 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 $(V-K_{s}) \sim1.8$, and among these stars the subdwarfs with $-2.0 \leq{\mathrm{[Fe/H]}} \leq-1.0$ approach the locus of low activity Population I dwarfs in the $(m_{\mathrm{FUV}}-V, V-K_{s})$ diagram. In the $(m_{\mathrm{FUV}}-B, B-V)$ 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.