High-contrast JWST-MIRI spectroscopy of planet-forming disks for the JDISC Survey

The JWST Disk Infrared Spectral Chemistry Survey (JDISCS) aims to understand the evolution of the chemistry of inner protoplanetary disks using the Mid-InfraRed Instrument (MIRI) on the James Webb Space Telescope (JWST). With a growing sample of > 30 disks, the survey implements a custom method to calibrate the MIRI Medium Resolution Spectrometer (MRS) to contrasts of better than 1:300 across its 4.9-28 µm spectral range. This is achieved using observations of Themis-family asteroids as precise empirical reference sources. High spectral contrast enables precise retrievals of physical parameters, searches for rare molecular species and isotopologues, and constraints on the inventories of carbon- and nitrogen-bearing species. JDISCS also offers significant improvements to the MRS wavelength and resolving power calibration. We describe the JDISCS calibrated data and demonstrate its quality using observations of the disk around the solar-mass young star FZ Tau. The FZ Tau MIRI spectrum is dominated by strong emission from warm water vapor. We show that the water and CO line emission originates from the disk surface and traces a range of gas temperatures of ∼500–1500 K. We retrieve parameters for the observed CO and H2O lines, and show that they are consistent with a radial distribution represented by two temperature components. A high water abundance of n(H2O) ∼ 10−4 fills the disk surface at least out to the 350 K isotherm at 1.5 au. We search the FZ Tau environs for extended emission detecting a large (radius of ∼300 au) ring of emission from H2 gas surrounding FZ Tau, and discuss its origin.

詹姆斯·韦布空间望远镜（James Webb Space Telescope, JWST）盘体红外光谱化学巡天（JDISCS）旨在借助JWST上的中红外仪器（Mid-InfraRed Instrument, MIRI），研究原行星盘内部区域的化学演化历程。当前该巡天的样本规模已超过30个原行星盘，并采用定制化方法对MIRI中分辨率光谱仪（Medium Resolution Spectrometer, MRS）进行校准，使其在4.9~28 µm的光谱波段内的对比度优于1:300。该校准效果依托忒弥斯族小行星的观测数据作为高精度经验参考源得以实现。优异的光谱对比度可实现物理参数的高精度反演、稀有分子物种与同位素取代物的搜寻，以及对含碳、含氮分子丰度的约束。JDISCS同时还对MRS的波长与分辨率校准方案做出了显著优化。本文阐述了JDISCS的校准后数据，并通过对太阳质量级年轻恒星FZ Tau周围原行星盘的观测，验证了该数据集的质量。FZ Tau的MIRI光谱以暖水汽产生的强发射信号为主导。研究表明，水汽与一氧化碳的谱线发射源自原行星盘表面，其气体温度范围约为500~1500 K。我们对观测到的CO与H2O谱线进行参数反演，结果显示其径向分布符合双温度成分模型。水的数密度丰度约为10^-4，该丰度遍布原行星盘表面，至少延伸至1.5天文单位处的350 K等温线。我们对FZ Tau周围区域进行延展发射搜寻，发现了一个半径约300天文单位的氢气发射环环绕该恒星，并对其起源展开了讨论。