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

Abstract

UVO 0825+15 is a hot bright helium-rich subdwarf which lies in <it>K2</it> Field 5 and in a sample of intermediate helium-rich subdwarfs observed the <it>Subaru</it> High Dispersion Spectrograph. The <it>K2</it> light curve shows low-amplitude variations, whilst the <it>Subaru</it> spectrum shows Pb <scp>iv</scp> absorption lines, indicative of a very high lead overabundance. UVO 0825+15 also has a high proper motion with kinematics typical for a thick disc star. Analyses of ultraviolet and intermediate dispersion optical spectra rule out a short-period binary companion and provide fundamental atmospheric parameters of <f>${\\,T_{\\rm eff}}=38\\,900\\pm 270$</f> K, <f>${\\,\\log g / {\\rm cm\\,s^{-2}}}=5.97\\pm 0.11$</f>, log <it>n</it><inf>He</inf>/<it>n</it><inf>H</inf> = −0.57 ± 0.01, <it>E</it><inf><it>B</it> − <it>V</it></inf> ≈ 0.03, and angular radius θ = 1.062 ± 0.006 × 10−11 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</it> 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 multiperiodic 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. UVO 0825+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.