JWST reveals excess cool water near the snowline in compact disks, consistent with pebble drift

Previous analyses of mid-infrared water spectra from young protoplanetary disks observed with the Spitzer-IRS found an anti-correlation between water luminosity and the millimeter dust disk radius observed with ALMA. This trend was suggested to be evidence for a fundamental process of inner disk water enrichment, used to explain properties of the Solar System 40 years ago, in which icy pebbles drift inward from the outer disk and sublimate after crossing the snowline. Previous analyses of IRS water spectra, however, were very uncertain due to the low spectral resolution that blended lines together. We present new JWST-MIRI spectra of four disks, two compact and two large with multiple radial gaps, selected to test the scenario that water vapor inside the snowline is regulated by pebble drift. The higher spectral resolving power of MIRI-MRS now yields water spectra that separate individual lines, tracing upper level energies from 900 K to 10,000 K. These spectra clearly reveal excess emission in the low-energy lines in compact disks, compared to the large disks, establishing the presence of a cooler component with T ≈ 170–400 K and equivalent emitting radius Req ≈ 1-10 au. We interpret the cool water emission as ice sublimation and vapor diffusion near the snowline, suggesting that there is indeed a higher inwards mass flux of icy pebbles in compact disks. Observation of this process opens up multiple exciting prospects to study planet formation chemistry in inner disks with JWST.

此前利用斯皮策红外光谱仪（Spitzer-IRS）对年轻原行星盘的中红外水光谱开展的分析发现，水光度与阿塔卡马大型毫米波/亚毫米波阵列（ALMA）观测到的毫米级尘埃盘半径之间存在负相关关系。这一趋势被认为是内盘水富集基本过程的证据——该过程于40年前被用于解释太阳系的相关性质，其机制为冰质砾石从外盘向内漂移，越过雪线后发生升华。不过此前针对斯皮策红外光谱仪水光谱的分析受限于较低的光谱分辨率，多条光谱线发生混叠，导致结果存在较大不确定性。本研究展示了4个原行星盘的全新詹姆斯·韦布空间望远镜—中红外仪器（JWST-MIRI）光谱数据：其中2个为紧凑盘，另外2个为带有多个径向间隙的大尺寸盘，我们选取这些源旨在验证“雪线内的水蒸气由砾石漂移调控”这一假说。中红外中分辨率光谱仪（MIRI-MRS）具备更高的光谱分辨能力，如今可分离出单条水光谱线，其探测的上能级能量范围为900 K至10000 K。相较于大尺寸盘，紧凑盘的低能级谱线明显呈现超额发射，这表明存在温度约为170~400 K、等效发射半径R_eq≈1~10 au的低温成分。我们将这类低温水发射归因于雪线附近的冰质升华与蒸气扩散过程，这意味着紧凑盘内确实存在更高通量的冰质砾石向内物质流。对这一过程的观测为利用詹姆斯·韦布空间望远镜研究内盘行星形成化学过程提供了诸多极具前景的研究方向。