The stellar initial mass function (IMF), along with the star formation rate, is one of the fundamental properties that any theory of star formation must explain. An interesting feature of the IMF is that it appears to be remarkably universal across a wide range of environments. Particularly, there appears to be little variation in either the characteristic mass of the IMF or its high-mass tail between clusters with different metallicities. Previous attempts to understand this apparent independence of metallicity have not accounted for radiation feedback from high-mass protostars, which can dominate the energy balance of the gas in star-forming regions. We extend this work, showing that the fragmentation of molecular gas should depend only weakly on the amount of dust present, even when the primary heating source is radiation from massive protostars. First, we report a series of core collapse simulations using the ORION AMR code that systematically vary the dust opacity and show explicitly that this has little effect on the temperature or fragmentation of the gas. Then, we provide an analytic argument for why the IMF varies so little in observed star clusters, even as the metallicity varies by a factor of 100.
Low-Metallicity Star Formation Survey in Sh2-284 (LZ-STAR): The Core Mass Function
National Astronomical Observatory of Japan（日本国立天文台）；University of Virginia（弗吉尼亚大学）；Chalmers University of Technology（查尔姆斯理工大学）；European Southern Observatory（欧洲南方天文台）；Instituto de Astrofísica de Andalucía, CSIC（安达卢西亚天体物理研究所，西班牙高等科学研究委员会）；Institute of Science Tokyo（东京科学大学）；Shanghai Jiao Tong University（上海交通大学）
HST survey of the Orion Nebula Cluster in the H$_2$O 1.4 $μ$m absorption band: II. The substellar IMF down to planetary masses
我们利用哈勃太空望远镜探测低质量恒星、棕矮星和行星质量天体大气中近红外水吸收的能力，创建一个非常纯净的猎户座星云团（ONC）成员样本，不受缺乏水吸收的背景恒星和星系污染的影响。由于这些数据，我们推断出ONC的初始质量函数（IMF）在$0.005 - 1.4$ M $_{\odot}$制度，即下降到几个木星质量。ONC的年轻年龄，$\sim1$ Myr，提供了一个快照的结果，恒星形成的现代条件（金属丰度，温度，压力）的典型银河系盘分子云。我们证明，ONC的IMF是很好地描述了一个对数正态函数或破幂律，与规范的Chabrier或Kroupa形式的银河系磁盘IMF的参数值定性一致。这种质量分布的连续性提供了线索，说明同样的物理过程可能正在调节恒星、褐矮星和行星质量物体的形成。这两种典型的IMF形式都低估了观测到的极低质量成员的数量（低于$0.1$ M $_\odot$），我们的数据允许更精确的约束。然而，我们没有观察到棕矮星或行星质量体系的上升或第二峰。因此，我们的研究与基于宽波段近红外地基测光的发现相矛盾，后者预测了极高数量的自由浮动行星，但可能受到不明背景污染的影响。
Probing the IMF in the Early Universe -- Direct measurements in the Boötes I UFD with JWST/NIRCam
University of Maryland, College Park（马里兰大学帕克分校）；Space Telescope Science Institute（太空望远镜科学研究所）；INAF – Osservatorio Astronomico di Abruzzo（意大利国家天体物理研究所阿布鲁佐天文台）；INFN – Sezione di Pisa, Universita’ di Pisa（意大利国家核物理研究所比萨分部，比萨大学）
通过JWST/NIRCam对Boötes I UFD中低质量IMF进行研究，结合机器学习和统计方法，发现单幂律IMF无法再现观测到的光度函数，而分段幂律和对数正态IMF参数与银河系一致，表明低金属度下恒星形成遵循相似IMF。
