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.
Highlights
Just over 50 years ago, Berger & Greenstein (1963) used the Palomar 5-m (200-inch) telescope to show that the early B star BD+13◦3224 is hydrogen deficient and a probable subdwarf
We have presented Swift ultraviolet photometry and Subaru High Dispersion Spectrograph (HDS) spectroscopy of the pulsating helium star V652 Her
The 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, demonstrating how it is compressed by a factor of at least 2 at minimum radius
Summary
Just over 50 years ago, Berger & Greenstein (1963) used the Palomar 5-m (200-inch) telescope to show that the early B star BD+13◦3224 is hydrogen deficient and a probable subdwarf. In obtaining the highest precision measurement of the radial velocity curve to date, Jeffery et al (2001) noted that the extreme acceleration at minimum radius could be associated with a shock travelling outwards through the photosphere. Application of the classical period–meandensity relation P ρ /ρ = Q (Shapley 1914) using the Jeffery et al (2001) mass and radius and with a pulsation constant Q ≈ 0.033 d (Fadeyev & Lynas-Gray 1996) indicates an unacceptably long period of 0.15 d and implies that the measured surface gravity is too low by a factor of 2. The result is a unique empirical description of the behaviour of the outer layers of a pulsating star, resolved over nearly three decades of optical depth
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