arXiv 2004.139202020-04-29astro-ph.SRastro-ph.GA

在H $_2$ O 1.4 $μ$ m吸收带对猎户座星云团的HST巡天：II。亚恒星IMF下降到行星质量

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

Mario Gennaro, Massimo Robberto

备注：: Accepted for publication in the Astrophysical Journal
DOI：: 10.3847/1538-4357/ab911a

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摘要：

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。

英文摘要：

We exploit the ability of the Hubble Space Telescope to probe near infrared water absorption present in the atmosphere of low-mass stars, brown dwarfs and planetary mass objects to create a very pure sample of Orion Nebula Cluster (ONC) members, not affected by contamination from background stars and galaxies which lack water absorption. Thanks to these data we infer the Initial Mass Function (IMF) of the ONC in the $0.005 - 1.4$M$_{\odot}$ regime, i.e. down to few Jupiter masses. The young age of the ONC, $\sim1$ Myr, provides a snapshot of the outcome of star formation for the present-day conditions (metallicity, temperature, pressure) of typical Milky Way disk molecular clouds. We demonstrate that the IMF of the ONC is well described by either a log-normal function or a broken power-law, with parameter values qualitatively in agreement with the canonical Chabrier or Kroupa forms for the Milky Way disk IMF. This continuity in the mass distribution provides clues to the fact that the same physical processes may be regulating formation of stars, brown dwarfs, and planetary mass objects. Both the canonical IMF forms under-predict the observed number of very low mass members (below $0.1$ M$_\odot$), a regime where our data allows more precise constraints. Nevertheless, we do not observe a rise or secondary peak in the brown dwarfs or planetary mass regimes. Our study thus contradicts findings based on broad-band near infrared ground-based photometry, which predict an extremely high number of free-floating planets, but likely suffer from unaccounted background contamination.

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