Protostellar half-life: new methodology and estimates
Protostellar half-life: new methodology and estimates
复制标题
原恒星半衰期：新方法和估计
DOI：
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发表时间：
2018
影响因子：
6.5
通讯作者：
M. Dunham
中科院分区：
文献类型：
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作者：
L. Kristensen;M. Dunham
原恒星系统从星前核开始演化，经过深埋阶段，然后是盘主导阶段，最后进入主序星。了解原恒星系统在每个阶段花费多少时间对于理解恒星和相关行星系统如何形成至关重要，因为一个关键的约束是形成此类系统的时间。同样重要的是理解这些推断的时间尺度的扩展或不确定性是什么。推断原恒星年龄最常用的方法是假设一个演化阶段的寿命，然后将此寿命换算为其他阶段原恒星的相对数量，即，该方法假设种群处于稳定状态。计数法没有考虑到星星形成的潜在年龄分布和明显的随机性，也没有考虑到星星形成是连续的，即，人口并不稳定。为了克服这一点，我们提出了一个新的计划，其中每个原恒星阶段的寿命遵循一个分布的基础上的形式主义的顺序核衰变。在这种形式主义中，主要的假设是：0类源遵循一条直线路径到III类源，年龄分布遵循二项分布，恒星形成率始终是常数。结果是，0类、I类和扁平源的半衰期分别为（2.4 ± 0.2）%，（4.4 ± 0.3）%，II类半衰期的（4.3 ± 0.4）%，分别相当于47 ± 4、88 ± 7和87 ± 8 kyr，对于古尔德带中有超过100颗原恒星的云，其II类半衰期为200万年。这些云的平均年龄为1.2 ± 0.1百万年，而对于平均质量为0.5 M <$的原恒星，总的推断星星形成速率为（8.3 ± 0.5）× 10−4 M <$yr−1。得出这些数字的关键参数是假设II类阶段的半衰期，以及恒星形成率和半衰期恒定的假设。这种方法提出了第一步，从稳态到非稳态解决方案的原恒星种群。
Protostellar systems evolve from prestellar cores, through the deeply embedded stage and then disk-dominated stage, before they end up on the main sequence. Knowing how much time protostellar systems spend in each stage is crucial for understanding how stars and associated planetary systems form, because a key constraint is the time available to form such systems. Equally important is understanding what the spread or uncertainty in these inferred time scales is. The most commonly used method for inferring protostellar ages is to assume the lifetime of one evolutionary stage, and then scale this lifetime to the relative number of protostars in the other stages, i.e., the method assumes populations are in steady state. The number-counting method does not take into account the underlying age distribution and apparent stochasticity of star formation, nor that star formation is sequential, i.e., populations are not in steady state. To overcome this, we propose a new scheme where the lifetime of each protostellar stage follows a distribution based on the formalism of sequential nuclear decay. In this formalism, the main assumptions are: Class 0 sources follow a straight path to Class III sources, the age distribution follows a binomial distribution, and the star-formation rate is constant throughout. The results are that the half-life of Class 0, Class I, and Flat sources are (2.4 ± 0.2)%, (4.4 ± 0.3)%, and (4.3 ± 0.4)% of the Class II half-life, respectively, which translates to 47 ± 4, 88 ± 7, and 87 ± 8 kyr, respectively, for a Class II half-life of 2 Myr for protostars in the Gould Belt clouds with more than 100 protostars. The mean age of these clouds is 1.2 ± 0.1 Myr, and the total inferred star formation rate is (8.3 ± 0.5) × 10−4 M⊙ yr−1 for a mean protostellar mass of 0.5 M⊙. The critical parameters in arriving at these numbers are the assumed half-life of the Class II stage, and the assumption that the star-formation rate and half-lives are constant. This method presents a first step in moving from steady-state to non-steady-state solutions of protostellar populations.