The minimum mass for star formation, and the origin of binary brown dwarfs
The minimum mass for star formation, and the origin of binary brown dwarfs
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恒星形成的最小质量以及双星褐矮星的起源
DOI：
10.1051/0004-6361:20065806
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发表时间：
2006
影响因子：
6.5
通讯作者：
D. Stamatellos
中科院分区：
文献类型：
--
作者：
A. Whitworth;D. Stamatellos
上下文星星形成的最小质量是一个重要的参数，具有深远的天体物理学、宇宙学和人择学意义。目标。我们的第一个目标是计算在各种潜在的恒星形成的情况下，即（a）分层碎裂的三维介质，（B）一次拍摄，二维破碎的冲击压缩层，（c）破碎的拱星盘的PrimaryFragmentation的最小质量。我们所说的初级碎裂是指由有效的辐射冷却促进的碎裂。我们的第二个目标是评估的作用，氢解离促进二次碎裂，从而产生密切的，低质量的双星。方法.我们使用幂律拟合的本构物理，一个区域模型的凝聚碎片，和扩散近似的辐射传输的光学厚度的限制，以制定简单的分析估计。结果(i)对于当代的局部星星形成，初级碎裂的最小质量在0.001到0.004 M ²的范围内，与恒星形成的情况无关。这一结果是显着的，因为无论是重力不稳定性的条件，和辐射输运制度的最小质量的碎片，在不同的情况下是不同的。(ii)星周盘的辐射速度只能快到在其较冷的外部（R > 100 Au）进行初级碎裂。因此，褐矮星在R 100 Au的初级碎裂作用下很难形成，这可能是类太阳恒星周围宽轨道褐矮星的来源，也可以解释为什么很少看到超过这个半径的大质量圆盘。(iii)H2的离解会导致坍缩和二次碎裂，从而将一次碎裂转化为紧密的低质量双星，其半长轴为1.5A U（m SYSTEM /0.1 M�），与观测结果非常吻合;在这种情况下，一次碎裂的最小质量就变成了最小系统质量，而不是最小恒星质量。(iv)任何初级碎片都可以进行二次碎裂，产生一个紧密的低质量双星，只要初级碎片旋转。因此，次级碎裂最有可能是在星周盘的外部形成的初级碎片（因为这些碎片不可避免地会旋转），这可以解释为什么在类太阳星星的宽轨道上的棕矮星比在视野中的棕矮星更有可能拥有棕矮星伴星--正如我们所观察到的那样。此外，我们表明，以这种方式形成的双褐矮星有时可以被弹射到外地没有分手。
Context. The minimum mass for star formation is a critical parameter with profound astrophysical, cosmological and anthropic consequences. Aims. Our first aim is to calculate the minimum mass for PrimaryFra gmentation in a variety of potential star-formation scenarios, i.e. (a) hierarchical fragmentation of a 3D medium; (b) one-shot, 2D fragmentation of a shock-compressed layer; (c) fragmentation of a circumstellar disc. By Primary Fragmentation we mean fragmentation facilitated by efficient radiative cooling. Our second aim is to evaluate the role of H2 dissociation in facilitating Secondary Fra gmentation and thereby producing close, low-mass binaries. Methods. We use power-law fits to the constitutive physics, a one-zone model for condensing fragments, and the diffusion approximation for radiative transport in the optically thick limit, in order to formulate simple analytic estimates. Results. (i) For contemporary, local star formation, the minimum mass for Primary Fragmentation is in the range 0.001 to 0.004 M� , irrespective of the star-formation scenario considered. This result is remarkable since, both the condition for gravitational instability, and the radiation transport regime operating in a minimum-mass fragment, are different in the different scenarios. (ii) Circumstellar discs are only able to radiate fast enough to undergo Primary Fragmentation in their cool outer parts (R > 100 AU). Therefore brown dwarfs should have difficulty forming by Primary Fragmentation at R 100 AU could be the source of brown dwarfs in wide orbits about Sun-like stars, and could explain why massive discs extending beyond this radius are rarely seen. (iii) H2 dissociation can lead to collapse and Secondary Fragmentation, thereby converting primary fragments into close, low-mass binaries, with semi-major axes a ∼ 5A U (m SYSTEM /0.1 M� ), in good agreement with observation; in this circumstance, the minimum mass for Primary Fragmentation becomes a minimum system mass, rather than a minimum stellar mass. (iv) Any primary fragment can undergo Secondary Fragmentation, producing a close low-mass binary, provided only that the primary fragment is spinning. Secondary Fragmentation is therefore most likely in primary fragments formed in the outer parts of circumstellar discs (since such fragments inevitably spin), and this could explain why a brown dwarf in a wide orbit about a Sun-like star has a greater likelihood of having a brown-dwarf companion than a brown dwarf in the field – as seems to be observed. Moreover, we show that binary brown dwarfs formed in this way can sometimes be ejected into the field without breaking up.
影响因子：
3.2
作者：
V. Mannings;A. Boss;S. Russell
通讯作者：
V. Mannings;A. Boss;S. Russell