PHL 417: a zirconium-rich pulsating hot subdwarf (V366 Aquarid) discovered in K2 data

TL;DR

This paper reports the discovery and analysis of a zirconium-rich pulsating hot subdwarf star, PHL 417, observed by K2, revealing unique pulsation modes and heavy-metal overabundance, challenging existing stellar pulsation models.

Contribution

The study identifies PHL 417 as a rare heavy-metal pulsator with unusual pulsation periods and heavy-metal overabundance, providing new insights into stellar pulsation mechanisms.

Findings

17 pulsation modes detected with periods 38-105 minutes

High-temperature, helium-rich, zirconium-overabundant star characterized

Stable pulsation mode with minimal period drift observed

Abstract

The Kepler spacecraft observed the hot subdwarf star PHL 417 during its extended K2 mission, and the high-precision photometric lightcurve reveals the presence of 17 pulsation modes with periods between 38 and 105 minutes. From follow-up ground-based spectroscopy we find that the object has a relatively high temperature of 35 600 K, a surface gravity of $\log g / {\rm cm\,s^{-2}}\,=\,5.75$ and a super-solar helium abundance. Remarkably, it also shows strong zirconium lines corresponding to an apparent +3.9 dex overabundance compared with the Sun. These properties clearly identify this object as the third member of the rare group of pulsating heavy-metal stars, the V366 Aquarii pulsators. These stars are intriguing in that the pulsations are inconsistent with the standard models for pulsations in hot subdwarfs, which predicts that they should display short-period pulsations rather than the observed longer periods. We perform a stability analysis of the pulsation modes based on data from two campaigns with K2. The highest amplitude mode is found to be stable with a period drift, $\dot{P}$, of less than $1.1\cdot10^{-9}$ s/s. This result rules out pulsations driven during the rapid stages of helium flash ignition.