OBSERVED LUMINOSITY SPREAD IN YOUNG CLUSTERS AND FU Ori STARS: A UNIFIED PICTURE
OBSERVED LUMINOSITY SPREAD IN YOUNG CLUSTERS AND FU Ori STARS: A UNIFIED PICTURE
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DOI：
10.1088/0004-637x/756/2/118
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发表时间：
2012-09-10
影响因子：
4.9
通讯作者：
Chabrier, G.
中科院分区：
文献类型：
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作者：
Baraffe, I.;Vorobyov, E.;Chabrier, G.
原恒星演化嵌入阶段的非稳态吸积可以产生年轻星系团赫茨普朗-拉塞尔图（HRD）中观察到的光度分布，这一观点最近受到质疑。例如，FU Ori的观测表明，在强烈的吸积事件中，星星会膨胀，而光度分布则意味着吸积物体会收缩，从而减少了它们的辐射表面。在本文中，我们提出了一个全球性的情况下，计算耦合幕吸积的历史来自各种质量的坍缩云星前核和随后的原恒星演化的数值模拟。我们的计算表明，假设初始原恒星质量M-i类似于1 M-Jup，典型的第二个拉森的核心，无论是在HRD和FU Ori事件（质量，半径，吸积率）的推断属性的光度传播可以解释这种情况下，提供两个条件。首先，在吸积过程中，原恒星所吸收的吸积能量的比例一定会发生变化。第二，这种变化的范围应该随着吸积爆发强度的增加而增加，从而随着初始核心质量和最终星星质量的增加而增加。对星前核坍缩的数值流体力学模拟表明，星前核的质量和初始角动量与吸积爆发的强度有关。大规模的星前核与高初始角动量被发现产生强烈的爆发特征的FU Ori-like事件。因此，我们的研究结果表明，爆发强度和吸积能量被原恒星吸收的比例之间存在联系，吸积速率中有一定的阈值，数量级为10（-5）M圈·年（-1），界定了从“冷”到“热”吸积的过渡。这种转变可能反映了随着吸积速率的增加，吸积几何形状的变化，即，从磁层或薄盘到厚盘吸积的过渡，或者在星星和盘之间的磁层相互作用中。相反地，光度扩散也可以用原恒星初始质量在1-5 M-Jup范围内的变化来解释，虽然目前还不清楚第二个拉森核心的光度扩散是否会在星前核心的第二次坍缩中产生。这个统一的图像证实了这样的想法，即在原恒星和原棕矮星形成/演化过程中的早期吸积可以解释在年轻星团中观察到的光度分布，而不会引起任何显着的年龄分布，并且定义明确的出生线的概念不适用于低质量物体。最后，我们研究吸积的影响，在年轻的集群的初始质量函数的确定。
The idea that non-steady accretion during the embedded phase of protostar evolution can produce the observed luminosity spread in the Herzsprung-Russell diagram (HRD) of young clusters has recently been called into question. Observations of FU Ori, for instance, suggest an expansion of the star during strong accretion events, whereas the luminosity spread implies a contraction of the accreting objects, decreasing their radiating surface. In this paper, we present a global scenario based on calculations coupling episodic accretion histories derived from numerical simulations of collapsing cloud prestellar cores of various masses and subsequent protostar evolution. Our calculations show that, assuming an initial protostar mass M-i similar to 1 M-Jup, typical of the second Larson's core, both the luminosity spread in the HRD and the inferred properties of FU Ori events (mass, radius, accretion rate) can be explained by this scenario, providing two conditions. First, there must be some variation within the fraction of accretion energy absorbed by the protostar during the accretion process. Second, the range of this variation should increase with increasing accretion burst intensity and thus with the initial core mass and final star mass. The numerical hydrodynamics simulations of collapsing cloud prestellar cores indeed show that the intensity of the accretion bursts correlates with the mass and initial angular momentum of the prestellar core. Massive prestellar cores with high initial angular momentum are found to produce intense bursts characteristic of FU Ori-like events. Our results thus suggest a link between the burst intensities and the fraction of accretion energy absorbed by the protostar, with some threshold in the accretion rate, of the order of 10(-5) M-circle dot yr(-1), delimitating the transition from "cold" to "hot" accretion. Such a transition might reflect a change in the accretion geometry with increasing accretion rate, i.e., a transition from magnetospheric or thin-disk to thick-disk accretion, or in the magnetospheric interaction between the star and the disk. Conversely, the luminosity spread can also be explained by a variation of the initial protostar mass within the similar to 1-5 M-Jup range, although it is unclear for now whether such a spread among the second Larson's core can be produced during the prestellar core second collapse. This unified picture confirms the idea that early accretion during protostar and proto-brown dwarf formation/evolution can explain the observed luminosity spread in young clusters without invoking any significant age spread, and that the concept of a well-defined birthline does not apply for low-mass objects. Finally, we examine the impact of accretion on the determination of the initial mass function in young clusters.