The mass spectrum in the young cluster NGC1333

The mass spectrum in the young cluster NGC1333 in the star forming complex in Perseus With an age of around 1Myr and a distance of only 300pc, NGC1333 is an excellent laboratory to study the outcome of star formation. In our long-term project SONYC (Substellar Objects in Nearby Young Clusters) we have identified more than 50 new very low mass members in this cluster, down to masses of 6 Jupiter masses (see link). The plot shows the number of objects in this cluster per mass bin derived from our survey in combination with the results from other groups. In the most recent paper, we put strong limits on the number of planemos - free-floating objects with planetary masses - in this cluster. We give an upper limit of 8 planemos for NGC1333. Based on a very recent study with new information on the mid-infrared colours of young ultracool objects (Faherty et al. 2012, submitted to AJ), our estimate is probably too conservative, an upper limit of 4 might be more realistic. In other words, planemos are very rare, at least down to about 5 Jupiter masses (the limit of our survey). Our upper limit for planemos is shown with a red arrow in the figure. The plot demonstrates that the mass spectrum continues smoothly down to masses of 0.02 Msol, corresponding to ~20 Jupiter masses, with a possible decline in the planemo domain. This is in contrast to recent findings based on microlensing surveys (Sumi et al., 2011, Nature, 473, 349), which indicate that 'unbound or distant Jupiter-mass objects' could be 'almost twice as common as main-sequence stars'. If the microlensing result is confirmed, those isolated Jupiter-mass objects must have a different origin from brown dwarfs and planemos observed in young clusters. More information: Scholz, Jayawardhana, Muzic, Geers, Tamura, Tanaka, 2012: "Substellar Objects in Nearby Young Clusters (SONYC) VI: The planetary-mass domain of NGC1333", Astrophysical Journal, in press, appears on arxiv on 09/07/2012

英仙座恒星形成复合体中年轻星团NGC1333的质量谱。该星团年龄约1百万年（Myr），距离仅300秒差距（pc），是研究恒星形成产物的绝佳实验室。在长期项目SONYC（近邻年轻星团中的亚恒星天体，Substellar Objects in Nearby Young Clusters）中，我们已在该星团内识别出50余个新的极低质量成员星，质量下限可达6倍木星质量（详见链接）。本图展示了结合本团队巡天数据与其他研究组结果后得到的该星团内各质量区间的天体数量分布。在最新发表的论文中，我们对该星团内行星质量自由漂浮天体（planemos）的数量给出了严格约束：NGC1333内的planemos数量上限为8个。基于一项针对年轻超冷天体中红外颜色的最新研究（Faherty等，2012年，投稿至《天文期刊》（AJ）），我们此前的估算或许过于保守，更合理的上限值应为4个。换言之，planemos极为罕见——至少在本巡天所能覆盖的约5倍木星质量质量下限以上的范围内如此。我们给出的planemos数量上限在图中以红色箭头标示。本图表明，质量谱可平滑延伸至0.02倍太阳质量（Msol），对应约20倍木星质量，在planemos质量区间内可能存在数量下降趋势。这与基于微引力透镜巡天得到的近期研究结果（Sumi等，2011年，《自然》（Nature），第473卷，第349页）形成鲜明反差：后者提出“未束缚或遥远的木星质量天体”的出现频率“几乎是主序星的两倍”。若微引力透镜的研究结果得到证实，那么这些孤立的木星质量天体的起源，必然与年轻星团中观测到的褐矮星和planemos截然不同。更多详细信息详见：Scholz、Jayawardhana、Muzic、Geers、Tamura、Tanaka，2012年："Substellar Objects in Nearby Young Clusters (SONYC) VI: The planetary-mass domain of NGC1333"，《天体物理学杂志》（Astrophysical Journal），已付印，2012年7月9日发布于arXiv预印本平台。