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arXiv 0907.3886astro-ph.SR
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低质量恒星和棕矮星演化早期阶段的幕式吸积：HR图中观测到的光度分布的解？

Episodic accretion at early stages of evolution of low mass stars and brown dwarfs: a solution for the observed luminosity spread in HR diagrams?

I. Baraffe, G. Chabrier, J. Gallardo

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8 pages, 5 figures, accepted for publication in APJ Letter
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摘要：

我们提出了年轻低质量恒星和褐矮星的进化模型，考虑到了进化早期阶段的吸积阶段，最近对嵌入原恒星的大规模调查支持了这一假设。一个演化，包括短期的剧烈吸积（$\mdot \ge 10^{-4} \msolyr$），其次是较长的静止阶段（$\mdot < 10^{-6} \msolyr$）可以解释所观察到的亮度分布在HR图的恒星形成区的年龄为几百万年，从几个木星的质量到太阳质量的十分之几。这些吸积物体的引力收缩强烈地偏离了标准的Hayashi轨道，并保持恒定的$\te$。如果大部分吸积激波能量被辐射出去，则与观测到的光度散射最吻合。在HR图中得到的100万年时的光度分布相当于非吸积天体的$\sim$ 1000万年的年龄分布。我们还预测，在一个给定的$\te$的半径显着蔓延，建议由最近的观察。这些计算对我们理解恒星的形成和演化的早期阶段，以及对年轻（$\le$几百万年）星系团的IMF的确定具有重要意义。我们的研究结果还表明，低质量天体的恒星出生线的概念没有有效的支持。

英文摘要：

We present evolutionary models for young low mass stars and brown dwarfs taking into account episodic phases of accretion at early stages of the evolution, a scenario supported by recent large surveys of embedded protostars. An evolution including short episodes of vigorous accretion ($\mdot \ge 10^{-4} \msolyr$) followed by longer quiescent phases ($\mdot < 10^{-6} \msolyr$) can explain the observed luminosity spread in HR diagrams of star forming regions at ages of a few Myr, for objects ranging from a few Jupiter masses to a few tenths of a solar mass. The gravitational contraction of these accreting objects strongly departs from the standard Hayashi track at constant $\te$. The best agreement with the observed luminosity scatter is obtained if most of the accretion shock energy is radiated away. The obtained luminosity spread at 1 Myr in the HR diagram is equivalent to what can be misinterpreted as a $\sim$ 10 Myr age spread for non-accreting objects. We also predict a significant spread in radius at a given $\te$, as suggested by recent observations. These calculations bear important consequences on our understanding of star formation and early stages of evolution and on the determination of the IMF for young ($\le$ a few Myr) clusters. Our results also show that the concept of a stellar birthline for low-mass objects has no valid support.

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