2015A&A...576A..65R

The intermediate helium subdwarf B star LS IV-14°116 is a unique object showing extremely peculiar atmospheric abundances as well as long-period pulsations that cannot be explained in terms of the usual opacity mechanism. One hypothesis invoked was that a strong magnetic field may be responsible. We discredit this possibility on the basis of FORS2 spectro-polarimetry, which allows us to rule out a mean longitudinal magnetic field down to 300 G. Using the same data, we derive the atmospheric parameters for LS IV-14°116 to be T_eff=35150±111K, logg=5.88±0.02 and logN(He)/N(H)=-0.62±0.01. The high surface gravity in particular is at odds with the theory that LS IV-14°116 has not yet settled onto the helium main sequence, and that the pulsations are excited by an ε mechanism acting on the helium-burning shells present after the main helium flash. Archival UVES spectroscopy reveals LS IV-14°116 to have a radial velocity of 149.1±2.1km/s. Running a full kinematic analysis, we find that it is on a retrograde orbit around the Galactic centre, with a Galactic radial velocity component U=13.23±8.28km/s and a Galactic rotational velocity component V=-55.56±22.13km/s. This implies that LS IV-14°116 belongs to the halo population, an intriguing discovery.