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Multi-periodic pulsations of a stripped red-giant star in an eclipsing binary system
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Multi-periodic pulsations of a stripped red-giant star in an eclipsing binary system
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Journal Article
Multi-periodic pulsations of a stripped red-giant star in an eclipsing binary system
2013
Overview
Measurements of a precursor to a low-mass white-dwarf star reveal that such white-dwarf stars probably had a thick hydrogen envelope, which was lost by irradiation or shell flashes in the case of rapidly cooling white-dwarf stars.
A pulsating star in red-to-white transition
Before becoming low-mass white dwarfs, stripped red-giant stars evolve at nearly constant luminosity towards higher effective temperatures. The system known as J0247-25 was recently found to be a binary in which a star in this unusual evolutionary state (J0247-25B) is totally eclipsed by an apparently normal A-type star (J0247-25A). New spectroscopic and photometric observations have been used to derive precise astrophysical parameters for both stars. The data fit models in which the hotter white-dwarf precursor has a thick hydrogen envelope. This suggests that very cool low-mass white dwarfs have lost their thick hydrogen envelopes by irradiation from pulsar companions or by episodes of unstable hydrogen fusion (shell flashes). The discovery of pulsations in J0247-25B opens up new observational opportunities for the study of the structure of a low-mass white dwarf.
Low-mass white-dwarf stars are the remnants of disrupted red-giant stars in binary millisecond pulsars
1
and other exotic binary star systems
2
,
3
,
4
. Some low-mass white dwarfs cool rapidly, whereas others stay bright for millions of years because of stable fusion in thick surface hydrogen layers
5
. This dichotomy is not well understood, so the potential use of low-mass white dwarfs as independent clocks with which to test the spin-down ages of pulsars
6
,
7
or as probes of the extreme environments in which low-mass white dwarfs form
8
,
9
,
10
cannot fully be exploited. Here we report precise mass and radius measurements for the precursor to a low-mass white dwarf. We find that only models in which this disrupted red-giant star has a thick hydrogen envelope can match the strong constraints provided by our data. Very cool low-mass white dwarfs must therefore have lost their thick hydrogen envelopes by irradiation from pulsar companions
11
,
12
or by episodes of unstable hydrogen fusion (shell flashes). We also find that this low-mass white-dwarf precursor is a type of pulsating star not hitherto seen. The observed pulsation frequencies are sensitive to internal processes that determine whether this star will undergo shell flashes.
Publisher
Nature Publishing Group UK,Nature Publishing Group
Subject
