Environmental effects on the ionisation of brown dwarf atmospheres
Environmental effects on the ionisation of brown dwarf atmospheres
复制标题
环境对褐矮星大气电离的影响
DOI：
10.1051/0004-6361/201832685
复制
发表时间：
2018
影响因子：
6.5
通讯作者：
U. Groningen
中科院分区：
文献类型：
--
作者：
M. I. Rodriguez;Christiane Helling;Kenneth Wood Centre for Exoplanet Science;U. S. Andrews;K. Institute;U. Groningen
语境。褐矮星会发射 Hα 爆发、白光耀斑，并显示射电耀斑和静态射电发射。人们认为它们会形成极光，类似于太阳系中的行星，但能量更高。所有这些过程都需要具有适当电离程度的源气体，到目前为止，大多数情况下认为这是足够的。
目标。我们的目标是证明银河环境影响大气电离，从而放大或实现超冷物体（如褐矮星和自由漂浮行星）大气层的磁耦合。
方法。我们以之前关于超冷大气热电离的工作为基础，探索环境高能辐射对大气电离程度的影响。我们考虑莱曼连续谱辐射在三种不同环境中的光电离效应：星际辐射场 (ISRF)、恒星形成区的 O 型和 B 型恒星以及双星系统中的白矮星伴星。我们应用蒙特卡罗辐射传输代码来研究莱曼连续光电离对极低质量物体的规定大气结构的影响。
结果。外部辐射环境对于极低质量、超冷物体的大气电离起着重要作用。莱曼连续谱辐射极大地提高了最高层大气区域的电离水平。我们的结果表明，恒星形成区的褐矮星和白矮星双星系统中的褐矮星会出现几乎完全电离的大气气体外壳。因此，褐矮星大气层可以磁耦合，这是色球加热发生和极光出现的假设。对假设的类色球温度值的首次测试表明，所得的自由 X 射线光度与从恒星形成区域的非吸积褐矮星观察到的光度相当。
Context. Brown dwarfs emit bursts of Hα, white-light flares, and show radio flares and quiescent radio emission. They are suggested to form aurorae, similar to planets in the solar system, but much more energetic. All these processes require a source gas with an appropriate degree of ionisation, which, so far, is mostly postulated to be sufficient.
Aims. We aim to demonstrate that the Galactic environment influences atmospheric ionisation, and that it hence amplifies or enables the magnetic coupling of the atmospheres of ultra-cool objects, like brown dwarfs and free-floating planets.
Methods. We build on our previous work on thermal ionisation of ultra-cool atmospheres and explore the effect of environmental high-energy radiation on the degree of ionisation in the atmosphere. We consider the effect of photoionisation by Lyman-continuum radiation in three different environments: in the interstellar radiation field (ISRF), O and B stars in star-forming regions, and in white dwarf companions in binary systems. We apply our Monte Carlo radiation transfer code to investigate the effect of Lyman-continuum photoionisation for prescribed atmosphere structures for very low-mass objects.
Results. The external radiation environment plays an important role for the atmospheric ionisation of very low-mass, ultra-cool objects. Lyman-continuum irradiation greatly increases the level of ionisation in the uppermost atmospheric regions. Our results suggest that a shell of an almost fully ionised atmospheric gas emerges for brown dwarfs in star-forming regions and brown dwarfs in white dwarf binary systems. As a consequence, brown dwarf atmospheres can be magnetically coupled, which is the presumption for chromospheric heating to occur and for aurorae to emerge. First tests for assumed chromosphere-like temperature values suggest that the resulting free-free X-ray luminosities are comparable with those observed from non-accreting brown dwarfs in star-forming regions.
影响因子：
4.8
作者：
S. Casewell;I. Braker;S. Parsons;J. Hermes;M. Burleigh;C. Belardi;A. Chaushev;N. Finch;M. Roy;S. Littlefair;M. Goad;E. Dennihy
通讯作者：
S. Casewell;I. Braker;S. Parsons;J. Hermes;M. Burleigh;C. Belardi;A. Chaushev;N. Finch;M. Roy;S. Littlefair;M. Goad;E. Dennihy
DOI：
10.3847/0004-637x/829/2/80
发表时间：
2016-01
期刊：
The Astrophysical Journal
影响因子：
--
作者：
C. Brady;T. Arber
通讯作者：
C. Brady;T. Arber