The frequency of discs around young stars, a key parameter for understanding planet formation, is most readily determined in young stellar clusters where many relatively coeval stars are located in close proximity. Observational studies seem to show that the disc frequency decreases rapidly with cluster age with <10% of cluster stars retaining their discs for longer than 2-6 Myr. Given that at least half of all stars in the field seem to harbor one or more planets, this would imply extremely fast disc dispersal and rapid planet growth. Here we question the validity of this constraint by demonstrating that the short disc dissipation times inferred to date might have been heavily underestimated by selection effects. Critically, for ages >3Myr only stars that originally populated the densest areas of very populous clusters, which are prone to disc erosion, are actually considered. This tiny sample may not be representative of the majority of stars. In fact, the higher disc fractions in co-moving groups indicate that it is likely that over 30% of all field stars retain their discs well beyond 10 Myr, leaving ample time for planet growth. Equally our solar system, with a likely formation time > 10 Myr, need no longer be an exception but in fact typical of planetary systems.
Early Science with the Large Millimetre Telescope: Molecules in the Extreme Outflow of a proto-Planetary Nebula
极高速度的发射可能与喷流有关，已知在一些原行星状星云中发生。然而，这种运动学成分的分子复杂性在很大程度上是未知的。我们观察到已知的极端流出的原行星状星云IRAS 16342-3814，一个原型喷泉，在全频率范围从73至111 GHz的RSR接收器的大毫米望远镜。我们检测到的分子SiO，HCN，SO，和$^{13}$ CO。所有的分子跃迁，除了后者是第一次在这个源中检测到，和所有目前的排放速度高达几百公里秒$^{-1}$。因此，IRAS 16342-3814是唯一一个同时在所有这些分子中表现出极端外流活动的源，SO和SiO的发射显示了这些物质在原行星状星云中的最高速度。待确认的是一个半高宽$\sim$为700 km s $^{-1}$的暂定弱SO分量。极端流出气体由稠密气体（n $_{\rm H_2} >$ 10 $^{4.8}$ --10 $^{5.7}$ cm $^{-3}$）组成，其质量大于$\sim$ 0.02--0.15 M $_{\odot}$。相对高丰度的SiO和SO可能是富氧极高速气体的指示。
