The enigmatic He-sdB pulsator LS IV-14°116: new insights from the VLT^⋆

¹ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
e-mail: srandall@eso.org
² Armargh Observatory, College Hill, Armargh BT61 9DG, Northern Ireland, UK
³ Dr. Remeis-Observatory & ECAP, Astronomical Institute, Friedrich-Alexander University Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
⁴ Département de Physique, Université de Montréal, CP 6128, Succ. Centre-Ville, Montréal, QC H3C 3J7, Canada

Received: 31 October 2014
Accepted: 18 February 2015

Abstract

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 = 35 150 ± 111 K, log g = 5.88 ± 0.02 and log N(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 ± 2.1 km s^-1. 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 ± 8.28 km s^-1 and a Galactic rotational velocity component V = −55.56 ± 22.13 km s^-1. This implies that LS IV-14°116 belongs to the halo population, an intriguing discovery.