Thermodynamics play an important role in determining the way a protostellar disc fragments to form planets, brown dwarfs and low-mass stars. We explore the effect that different treatments of radiative transfer have in simulations of fragmenting discs. Three prescriptions for the radiative transfer are used, (i) the diffusion approximation of Stamatellos et al., (ii) the barotropic equation of state (EOS) of Goodwin et al., and (iii) the barotropic EOS of Bate et al. The barotropic approximations capture the general evolution of the density and temperature at the centre of each proto-fragment but (i) they do not make any adjustments the particular circumstances of a proto-fragment forming in the disc, and (ii) they do not take into account thermal inertia effects that are important for fast-forming proto-fragments in the outer disc region. As a result, the number of fragments formed in the disc and their properties are different, when a barotropic EOS is used. This is important not only for disc studies but also for simulations of collapsing turbulent clouds, as in many cases in such simulations stars form with discs that subsequently fragment. We also examine the difference in the way proto-fragments condense out in the disc at different distances from the central star using the diffusion approximation and following the collapse of each proto-fragment until the formation of the second core (~10^{-3} g/cm3). We find that proto-fragments forming closer to the central star tend to form earlier and evolve faster from the first to the second core than proto-fragments forming in the outer disc region. The former have a large pool of material in the inner disc region that they can accrete from and grow in mass. The latter accrete more slowly and they are hotter because they generally form in a quick abrupt event.
Fragmentation favoured in discs around higher mass stars
我们研究了原行星盘对引力不稳定性和碎片的敏感性如何取决于其宿主恒星的质量。我们使用1D光盘模型结合3D SPH模拟，以确定临界盘星质量比，在该光盘变得不稳定的碎片，发现光盘变得越来越容易受到自引力的影响，因为我们增加了主机恒星的质量。稳定性的实际极限对盘的温度很敏感，所以如果盘是光学薄的，恒星辐射可以显着稳定盘对抗引力不稳定性。然而，即使是这种情况，我们发现$2$ M $_{\odot}$恒星周围的圆盘也容易碎裂，这将产生宽轨道的巨行星和棕矮星。这项工作的结果是双重的：低质量的恒星原则上可以支持高的盘星质量比，而更高质量的恒星具有更容易碎裂的盘，这与有利于高质量恒星周围的高质量宽轨道行星的观测结果在质量上是一致的。我们还发现，这些行星的初始质量取决于在大半径的光盘，这本身取决于恒星辐射的水平的温度。
Disc formation and fragmentation using radiative non-ideal magnetohydrodynamics
我们调查的形成和破碎的光盘使用一套三维平滑粒子辐射磁流体动力学模拟。我们的模型被初始化为1 M $_\odot$旋转Bonnor-Ebert球线程与均匀的磁场。我们研究的效果，包括理想和非理想的磁场，方向和强度的磁场，和初始旋转速率。我们遵循的引力坍缩和早期演化的每个系统，直到最终分类的原恒星盘可以确定。在我们的105个模型中，41个碎片，21个形成螺旋结构但不碎片，另外12个形成光滑盘。当初始旋转速率较快且磁场较弱时，更容易发生碎裂。对于更强的磁场强度，包括非理想的MHD促进磁盘的形成，其中几个模型的碎片，而他们的理想MHD同行没有。对于碎片化的模型，我们的参数与碎片化发生的地点或时间之间没有相关性。双极流出仅在17个型号中发射，这些型号具有最初平行于旋转轴的强磁场。反向旋转的信封形式在四个缓慢旋转，强场模型-包括一个理想的MHD模型-表明它们只形成在一小部分的参数空间调查。
Disc fragmentation. I. Ejection of Jupiter-mass Free Floating Planets from growing binary systems
School of Physics and Astronomy, University of Leicester（莱斯特大学物理与天文学学院）；Dep. de Física Quàntica i Astrofísica (FQA), Univ. de Barcelona (UB)（巴塞罗那大学量子物理与天体物理学系）；Institut de Ciències del Cosmos (ICCUB), Univ. de Barcelona (UB)（巴塞罗那宇宙科学研究所）
