Probing the Composition of Planet-Building Material with ALMA

像我们这样的行星系统，是由围绕新生星星的原行星盘中的物质--尘埃、气体和冰--组合而成的。研究围绕附近年轻恒星运行的尘埃和气体，让我们对太阳系在行星诞生之前的样子有了独特的了解。这使我们能够回答一些关于我们起源的基本问题。我们的太阳系是独一无二的吗？年轻的行星系统是否包含有关纳塔尔时原行星盘的物理和化学条件的信息？行星的大气成分是由行星盘中行星形成区的分子含量决定的吗？我们正处于一个开始解决这些问题的独特时代。开普勒太空天文台（现在的K2）不仅揭示了系外行星系统的极端多样性，我们现在手头上还有工具来绘制附近原行星盘行星形成区域的物理结构和化学。我们使用的主要工具是阿尔马，阿塔卡马大型毫米/亚毫米阵列。阿尔马是一种由多达66个天线组成的干涉仪，具有放大到行星形成区（< 50 Au）的灵敏度和空间分辨率，并揭示新生行星诞生环境的分子组成。阿尔马最近的亮点包括第一次检测到所谓的复杂有机分子（COM），由6个或更多原子组成的分子，也被认为是较大（益生元）分子的主要原料。COMs还追踪了冷尘埃（<< 100 K）上的冰库的成分，这些尘埃颗粒是冰冷的小行星（如彗星）的组成部分。通过高空间分辨率观测（~0.1”-0.01”）还发现了尘埃间隙和陷阱，其中一种解释是由看不见的形成行星雕刻而成。尚待确定的是，尘埃雕刻可以影响化学成分的程度，从而改变行星建造材料（气体和冰）的组成。该项目将研究附近行星形成盘中有机分子的化学性质。阿尔马观测我们最近的太阳系类似物TW Hya的气相甲醇（CH 3OH），将具有揭示其分布和丰度所需的灵敏度和空间分辨率。特别感兴趣的是甲醇排放是否符合CO雪线（在19 Au）和/或行星雕刻的尘埃腔（在22 Au）。使用详细的化学模型对数据进行分析将揭示其化学来源，并为推动未来阿尔马寻找更大（更接近于益生元）的分子提供重要数据。与此同时，阿尔马观测已知的分子盘示踪剂，CO，CN，HCN和HCO+，原行星盘包围HD 100546，将被用来研究行星雕刻的磁盘化学结构上的差距的影响。HD 100546是一个独特的测试案例，因为它仍然是唯一已知的在外盘（~50 Au）中检测到嵌入式（原）行星的来源，并且现有的阿尔马数据已经揭示了形成行星位置外部的尘埃陷阱。新的阿尔马数据，结合详细的化学模型，将揭示形成的行星是否在其位置的气体结构中雕刻了一个很深的缺口，而穿过差距的分子发射将揭示其中和与行星位置接壤的尘埃陷阱中引起的化学变化。这项关于行星雕刻间隙的潜在分子诊断的试点研究，将激励未来阿尔马对附近原行星盘的研究，这些原行星盘可能是尚未看到的形成行星的宿主。

Planetary systems, such as our own, are assembled from the material - dust, gas, and ice - contained in the protoplanetary disk around a newly-born star. Studying the dust and gas orbiting nearby young stars gives us unique insight into how our solar system might have looked prior to the inception of the planets. This enables us to answer some fundamental questions on our origins. Is our solar system unique? Do young planetary systems contain information on the physical and chemical conditions in the natal protoplanetary disk? Are planetary atmospheric compositions set by the molecular content of the planet-forming zone in the disk? We are in a unique era to begin to address these questions. Not only is the Kepler space observatory (now K2) revealing the extreme diversity of exoplanetary systems, we now have tools at hand to map the physical structure and chemistry across the planet-forming region of nearby protoplanetary disks. The primary tool we use for this is ALMA, the Atacama Large Millimeter/submillimeter Array. ALMA, an interferometer composed of up to 66 antennas, has the sensitivity and spatial resolution to zoom into the planet-forming zone (< 50 au) and reveal the molecular composition of the birth environment of nascent planets. Recent highlights from ALMA include the first detections of so-called complex organic molecules (COMs), molecules consisting of 6 or more atoms which are also thought to be the main feedstock of larger (prebiotic) molecules. COMs also trace the composition of the ice reservoir hosted on cold (<< 100 K) dust grains which are the building blocks of icy planetesimals such as comets. Also revealed via high spatial resolution observations (~0.1"-0.01") are dust gaps and traps, for which one explanation is sculpting by unseen forming planets. What remains to be determined, is the degree to which dust sculpting can influence the chemistry, thus altering composition of planet-building material (both gas and ice) in situ. This project will investigate the chemistry of organic molecules in nearby planet-forming disks. ALMA observations of gas-phase methanol (CH3OH) of our nearest solar-system analogue, TW Hya, will have the sensitivity and spatial resolution necessary to reveal its distribution and abundance. Of particular interest is whether the methanol emission is coincident with the CO snowline (at 19 au) and/or a planet-carved dust cavity (at 22 au). Analysis of the data using detailed chemical models will reveal its chemical origin and provide vital data for motivating future ALMA searches for even larger (and closer to prebiotic) molecules. In parallel, ALMA observations of known molecular disk tracers, CO, CN, HCN, and HCO+, of the protoplanetary disk encompassing HD 100546, will be used to investigate the effects of planet-carved gaps on the disk chemical structure. HD 100546 is a unique test case because it remains the only known source for which an embedded (proto)planet has been detected in the outer disk (~50 au) and for which existing ALMA data have revealed a dust trap external to the location of the forming planet. The new ALMA data, in combination with detailed chemical models, will reveal whether the forming planet has carved a deep gap in the gas structure at its location, and the molecular emission across the gap will reveal chemical changes induced therein and in the dust traps bordering the planet location. This pilot study, on potential molecular diagnostics of planet-carved gaps, will motivate future ALMA studies of nearby protoplanetary disks which may be host to as yet unseen forming planets.