The enigmatic He-sdB pulsator LS IV$-$14$^\circ$116: new insights from the VLT

TL;DR

This study investigates the peculiar helium subdwarf B star LS IV-14°116, ruling out magnetic fields as the cause of its pulsations, and providing new atmospheric and kinematic insights that suggest it belongs to the Galactic halo.

Contribution

The paper presents spectropolarimetric and spectroscopic analyses that disprove magnetic field influence and reveal the star's atmospheric parameters and halo membership, offering new understanding of its nature.

Findings

Magnetic fields are not responsible for pulsations in LS IV-14°116.

Atmospheric parameters: Teff=35150K, logg=5.88, He/H=-0.62.

LS IV-14°116 is part of the Galactic halo.

Abstract

The intermediate Helium subdwarf B star LS IV$-$14$^\circ$116 is a unique object showing extremely peculiar atmospheric abundances as well as long-period pulsations that cannot be explained in terms of the usual opacity mechanism. One hypothesis invoked was that a strong magnetic field may be responsible. We discredit this possibility on the basis of FORS2 spectro-polarimetry, which allows us to rule out a mean longitudinal magnetic field down to 300 G. Using the same data, we derive the atmospheric parameters for LS IV$-$14$^\circ$116 to be $T_{\rm eff}$ = 35,150$\pm$111 K, $\log{g}$ = 5.88$\pm$0.02 and $\log{N(\rm He)/N(\rm H)}$ = $-$0.62$\pm$0.01. The high surface gravity in particular is at odds with the theory that LS IV$-$14$^\circ$116 has not yet settled onto the Helium Main Sequence, and that the pulsations are excited by an $\epsilon$ mechanism acting on the Helium-burning shells present after the main Helium flash. Archival UVES spectroscopy reveals LS IV$-$14$^\circ$116 to have a radial velocity of 149.1$\pm$2.1 km/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$\pm$8.28 km/s and a Galactic rotational velocity component $V$=$-$55.56$\pm$22.13 km/s. This implies that LS IV$-$14$^\circ$116 belongs to the halo population, an intriguing discovery.