Whole Earth Telescope Observations of the subdwarf B star KPD 1930+2752: A rich, short period pulsator in a close binary

TL;DR

This study presents extensive Whole Earth Telescope observations of the pulsating subdwarf B star KPD 1930+2752, revealing a complex spectrum of pulsation frequencies, mode structures, and amplitude variations influenced by its close binary companion.

Contribution

First detailed multi-site photometric analysis of KPD 1930+2752, identifying numerous pulsation modes, including tidally-induced and rotationally split modes, and exploring their stability and mechanisms.

Findings

Detected 68 pulsation frequencies with 13 additional candidate frequencies.

Identified rotationally split and tidally-induced pulsation modes.

Observed complex amplitude variability and mode geometries.

Abstract

KPD 1930+2752 is a short-period pulsating subdwarf B (sdB) star. It is also an ellipsoidal variable with a known binary period just over two hours. The companion is most likely a white dwarf and the total mass of the system is close to the Chandresakhar limit. In this paper we report the results of Whole Earth Telescope (WET) photometric observations during 2003 and a smaller multisite campaign from 2002. From 355 hours of WET data, we detect 68 pulsation frequencies and suggest an additional 13 frequencies within a crowded and complex temporal spectrum between 3065 and 6343 $\mu$Hz (periods between 326 and 157 s). We examine pulsation properties including phase and amplitude stability in an attempt to understand the nature of the pulsation mechanism. We examine a stochastic mechanism by comparing amplitude variations with simulated stochastic data. We also use the binary nature of KPD 1930+2752 for identifying pulsation modes via multiplet structure and a tidally-induced pulsation geometry. Our results indicate a complicated pulsation structure that includes short-period ($\approx 16$ h) amplitude variability, rotationally split modes, tidally-induced modes, and some pulsations which are geometrically limited on the sdB star.