圆盘碎裂的下限和观察碎裂圆盘的前景
The lower limits of disc fragmentation and the prospects for observing fragmenting discs
摘要：
很大一部分褐矮星和低质量的氢燃烧恒星可能是由原恒星盘的引力破碎形成的。我们探索了圆盘碎裂的条件，我们发现当圆盘足够大（>100 AU）时，它们是满足的，这样它的外部区域可以有效地冷却，并且它有足够的质量在这样的半径下是引力不稳定的。我们进行辐射流体动力学模拟，并表明，即使是一个光盘质量0.25 Msun和大小100 AU的碎片。盘的质量，半径和盘星质量比（Mdisc/Mstar~0.36）都小于以前的研究。我们发现，碎片盘的质量和大小在其形成后的几个10^4年内会减小，因为它们的一部分质量，特别是在100天文单位以外的部分，会被新形成的恒星和棕矮星所消耗。碎片盘最终质量约为0.001-0.1 Msun，大小约为20-100 AU。另一方面，稍微稳定的光盘寿命更长。我们产生模拟图像的碎片光盘，并发现，观察光盘正在经历的碎片是可能的，使用当前（例如IRAM-PdBI）和未来（例如ALMA）的干涉仪，但极不可能由于这个过程的持续时间短。与观测结果的比较表明，许多观察到的光盘可能是残留的光盘，已在较早的阶段破碎。然而，只有少数候选者可能质量和足够大，目前引力不稳定。大质量（>0.2兆太阳）、扩展（>100天文单位）盘的稀有性表明，此类盘要么是高度瞬时的（即，由于周围包层的物质落入，质量增加而变得重力不稳定，并迅速碎裂），要么它们的形成被抑制（例如，通过磁场）。我们的结论是，目前的观察早期光盘不能排除光盘碎片的机制。
英文摘要：
A large fraction of brown dwarfs and low-mass H-burning stars may form by gravitational fragmentation of protostellar discs. We explore the conditions for disc fragmentation and we find that they are satisfied when a disc is large enough (>100 AU) so that its outer regions can cool efficiently, and it has enough mass to be gravitationally unstable, at such radii. We perform radiative hydrodynamic simulations and show that even a disc with mass 0.25 Msun and size 100 AU fragments. The disc mass, radius, and the ratio of disc-to-star mass (Mdisc/Mstar~0.36) are smaller than in previous studies. We find that fragmenting discs decrease in mass and size within a few 10^4 yr of their formation, since a fraction of their mass, especially outside 100 AU is consumed by the new stars and brown dwarfs that form. Fragmenting discs end up with masses ~0.001-0.1 Msun, and sizes ~20-100 AU. On the other hand, discs that are marginally stable live much longer. We produce simulated images of fragmenting discs and find that observing discs that are undergoing fragmentation is possible using current (e.g. IRAM-PdBI) and future (e.g. ALMA) interferometers, but highly improbable due to the short duration of this process. Comparison with observations shows that many observed discs may be remnants of discs that have fragmented at an earlier stage. However, there are only a few candidates that are possibly massive and large enough to currently be gravitationally unstable. The rarity of massive (>0.2 Msun), extended (>100 AU) discs indicates either that such discs are highly transient (i.e. form, increase in mass becoming gravitationally unstable due to infall of material from the surrounding envelope, and quickly fragment), or that their formation is suppressed (e.g. by magnetic fields). We conclude that current observations of early-stage discs cannot exclude the mechanism of disc fragmentation.
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