近日，美国加州理工学院的Ilaria Caiazzo及其研究小组取得一项新进展。经过不懈努力，他们发现了一颗旋转的白矮星在其对立面上表现出不同的成分。相关研究成果已于2023年7月19日在国际权威学术期刊《自然》上发表。

该研究团队报道了对ZTFJ203349.8+322901.1的观测结果，这是一颗过渡白矮星，具有两个截然不同的面：大气层的一面由氢主导，另一面由氦主导。这种特殊的性质可能是由于表面存在一个小磁场，导致温度、压力或混合强度的不均匀性。ZTFJ203349.8+322901.1可能是磁性过渡白矮星中最极端的成员之一，类似的还有GD323，这是一颗表现出类似但更微妙变化的白矮星。这类白矮星的研究有助于揭示白矮星光谱演化背后的物理机制。

据悉，白矮星是大多数恒星死亡后留下的残留物，具有极高的密度，质量与太阳相当，但体积只有类地行星大小。在白矮星的强引力作用下，重元素会向中心沉降，导致大气层中只含有最轻的元素，通常是氢或氦。在白矮星冷却的过程中，引力沉降与其他机制竞争，从而改变白矮星的表面组成。已知，在温度低于30000开尔文时，具有氦大气层的白矮星的比例会增加约2.5倍。因此，一些在温度高于30000开尔文的白矮星中，它们的大气主要是氢，但当温度降至30000开尔文以下时，大气层的主要成分将转变为氦。

附：英文原文

Title: A rotating white dwarf shows different compositions on its opposite faces

Author: Caiazzo, Ilaria, Burdge, Kevin B., Tremblay, Pier-Emmanuel, Fuller, James, Ferrario, Lilia, Gnsicke, Boris T., Hermes, J. J., Heyl, Jeremy, Kawka, Adela, Kulkarni, S. R., Marsh, Thomas R., Mrz, Przemek, Prince, Thomas A., Richer, Harvey B., Rodriguez, Antonio C., van Roestel, Jan, Vanderbosch, Zachary P., Vennes, Stphane, Wickramasinghe, Dayal, Dhillon, Vikram S., Littlefair, Stuart P., Munday, James, Pelisoli, Ingrid, Perley, Daniel, Bellm, Eric C., Breedt, Elm, Brown, Alex J., Dekany, Richard, Drake, Andrew, Dyer, Martin J., Graham, Matthew J., Green, Matthew J., Laher, Russ R., Kerry, Paul, Parsons, Steven G., Riddle, Reed L., Rusholme, Ben, Sahman, Dave I.

Issue&Volume: 2023-07-19

Abstract: White dwarfs, the extremely dense remnants left behind by most stars after their death, are characterized by a mass comparable to that of the Sun compressed into the size of an Earth-like planet. In the resulting strong gravity, heavy elements sink towards the centre and the upper layer of the atmosphere contains only the lightest element present, usually hydrogen or helium. Several mechanisms compete with gravitational settling to change a white dwarf’s surface composition as it cools, and the fraction of white dwarfs with helium atmospheres is known to increase by a factor of about 2.5 below a temperature of about 30,000kelvin; therefore, some white dwarfs that appear to have hydrogen-dominated atmospheres above 30,000kelvin are bound to transition to be helium-dominated as they cool below it. Here we report observations of ZTFJ203349.8+322901.1, a transitioning white dwarf with two faces: one side of its atmosphere is dominated by hydrogen and the other one by helium. This peculiar nature is probably caused by the presence of a small magnetic field, which creates an inhomogeneity in temperature, pressure or mixing strength over the surface. ZTFJ203349.8+322901.1 might be the most extreme member of a class of magnetic, transitioning white dwarfs—together with GD323 (ref. ¹²), a white dwarf that shows similar but much more subtle variations. This class of white dwarfs could help shed light on the physical mechanisms behind the spectral evolution of white dwarfs.

DOI: 10.1038/s41586-023-06171-9

Source: https://www.nature.com/articles/s41586-023-06171-9