Subaru and Swift observations of V652 Herculis: resolving the photospheric pulsation

TL;DR

This study combines Subaru high-resolution spectroscopy and Swift ultraviolet photometry to analyze the pulsation of V652 Herculis, revealing a shock at minimum radius and detailed photospheric dynamics during pulsation cycles.

Contribution

It provides the first high-resolution, phase-resolved spectral analysis of V652 Her's pulsation, identifying a shock and describing the photospheric compression and phase-dependent radius variations.

Findings

Photosphere compressed by at least a factor of two at minimum radius

Detected a shock with line profile jumps over 70 km/s

Pulse speed exceeds local sound speed by at least ten times

Abstract

High resolution spectroscopy with the Subaru High Dispersion Spectrograph, and Swift ultraviolet photometry are presented for the pulsating extreme helium star V652\,Her. Swift provides the best relative ultraviolet photometry obtained to date, but shows no direct evidence for a shock at ultraviolet or X-ray wavelengths. Subaru has provided high spectral and high temporal resolution spectroscopy over 6 pulsation cycles (and eight radius minima). These data have enabled a line-by-line analysis of the entire pulsation cycle and provided a description of the pulsating photosphere as a function of optical depth. They show that the photosphere is compressed radially by a factor of at least two at minimum radius, that the phase of radius minimum is a function of optical depth and the pulse speed through the photosphere is between 141 and 239 km/s (depending how measured) and at least ten times the local sound speed. The strong acceleration at minimum radius is demonstrated in individual line profiles; those formed deepest in the photosphere show a jump discontinuity of over 70 km/s on a timescale of 150 s. The pulse speed and line profile jumps imply a shock is present at minimum radius. These empirical results provide input for hydrodynamical modelling of the pulsation and hydrodynamical plus radiative transfer modelling of the dynamical spectra.