Discovery of a variable lead-rich hot subdwarf: UVO 0825+15

Abstract

UVO0825+15 is a hot bright helium-rich subdwarf which lies in {\it K2} Field 5 and in a sample of intermediate helium-rich subdwarfs observed with {\it Subaru/HDS}. The {\it K2} light curve shows low-amplitude variations, whilst the {\it Subaru} spectrum shows Pb{\sc iv} absorption lines, indicative of a very high lead overabundance. UVO0825+15 also has a high proper motion with kinematics typical for a thick disk star. Analyses of ultraviolet and intermediate dispersion optical spectra rule out a short-period binary companion, and provide fundamental atmospheric parameters of $T_{\rm off}=38\,900\pm270$\,K, $\log g/{\rm cm\,s^{-2}}=5.97\pm0.11$, $\log n_{\rm He}/n_{\rm H}=-0.57\pm0.01$, $E_{B-V}\approx0.03$, and angular radius $\theta = 1.062\pm0.006\times10^{-11}$ radians (formal errors). The high-resolution spectrum shows that carbon is $>2$ dex subsolar, iron is approximately solar and all other elements heavier than argon are at least 2 -- 4 dex overabundant, including germanium, yttrium and lead. Approximately 150 lines in the blue-optical spectrum remain unidentified. The chemical structure of the photosphere is presumed to be determined by radiatively-dominated diffusion. The {\it K2} light curve shows a dominant period around 10.8\,h, with a variable amplitude, its first harmonic, and another period at 13.3\,h. The preferred explanation is multi-periodic non-radial oscillation due to g-modes with very high radial order, although this presents difficulties for pulsation theory. Alternative explanations fail for lack of radial-velocity evidence. UVO0825+15 represents the fourth member of a group of hot subdwarfs having helium-enriched photospheres and 3--4 dex overabundances of trans-iron elements, and is the first lead-rich subdwarf to show evidence of pulsations.