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

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 $T_{\mathrm{eff}}$ = 35,150$\pm$111 K, $\log g$ = 5.88$\pm$0.02 and $\log N(\mathrm{He})/N(\mathrm{H})$ = $-$0.62$\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.

accepted for publication in A&A

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