Astrophysics Group, Keele University, Keele, Staffordshire ST5 5BG, UK.
Nature. 2013 Jun 27;498(7455):463-5. doi: 10.1038/nature12192.
Low-mass white-dwarf stars are the remnants of disrupted red-giant stars in binary millisecond pulsars and other exotic binary star systems. Some low-mass white dwarfs cool rapidly, whereas others stay bright for millions of years because of stable fusion in thick surface hydrogen layers. 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 or as probes of the extreme environments in which low-mass white dwarfs form 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 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.
低质量白矮星是双星毫秒脉冲星和其他奇异双星系统中破坏的红巨星的残余物。一些低质量白矮星迅速冷却，而另一些则由于厚的表面氢层中的稳定融合而保持数百万年的亮度。这种二分法还没有得到很好的理解，因此无法充分利用低质量白矮星作为独立时钟来测试脉冲星的自旋下降年龄或作为探测低质量白矮星形成的极端环境的探针。在这里，我们报告了低质量白矮星前体的精确质量和半径测量结果。我们发现，只有在这种破坏的红巨星具有厚氢包层的模型中，才能与我们的数据提供的强约束相匹配。因此，非常冷的低质量白矮星必须通过脉冲星伴星的辐照或不稳定的氢融合（壳闪光）事件失去其厚的氢包层。我们还发现，这个低质量白矮星前体是一种迄今未见的脉动星。观测到的脉动频率对决定这颗恒星是否会经历壳闪光的内部过程敏感。
