Cometary compositions compared with protoplanetary disk midplane chemical evolution
Cometary compositions compared with protoplanetary disk midplane chemical evolution
复制标题
彗星成分与原行星盘中面化学演化的比较
DOI：
10.1051/0004-6361/201935812
复制
发表时间：
2019
影响因子：
6.5
通讯作者：
E. Dishoeck
中科院分区：
文献类型：
--
作者：
C. Eistrup;C. Walsh;E. Dishoeck
上下文。彗星是太阳系行星形成过程中遗留下来的行星体。随着在许多彗星中观测到的分子丰度越来越多，以及对原行星盘中面化学演化的更好理解，可以在模型和观测之间进行比较，这可能会限制彗星的形成历史。
目标。我们的目标是进行彗星挥发冰丰度与原行星盘中面模拟演化的丰度之间的首次统计比较。
方法：研究方法。用χ-2方法确定了14颗不同彗星的最大似然面，这些彗星在给定的时间(最多8Myr)形成，并位于太阳前星云中面(超出CO冰线)。这是利用观测到的14颗彗星的挥发分丰度和盘中面化学模拟的挥发分丰度的演变来完成的。对化学模拟起始条件的两个假设(云遗传或化学重置)以及两组不同的彗星分子(母体物种，含硫物种或不含硫物种)进行了调查。
结果。考虑到重置情景中的所有母体物种(10个分子)，χ2似然面在参数空间中显示了一条特征轨迹，对于10颗彗星，早期形成的可能性约为30 AU，后期形成的可能性约为12 AU。这条踪迹在时间上大致追踪到了CO冰线附近。
结论。在彗星和彗星形成区域观测到的化学丰度和模拟的化学丰度之间的统计比较可能是限制彗星形成历史的有力工具。所有彗星的形成历史都被限制在CO冰线附近，假设化学成分在太阳前星云早期部分重置。无论是考虑含碳、含氧和含硫的分子(总共10个)，还是只考虑含碳和含氧的分子(总共7个)，都能发现这一点。由于这14颗彗星以前并不属于相同的分类类别，这一化学约束可能被建议作为彗星的另一种分类。根据这些彗星在盘面化学演化过程中最有可能形成的时间，提出了这14颗彗星的形成时间分类。
Context. Comets are planetesimals left over from the formation of planets in the solar system. With a growing number of observed molecular abundances in many comets, and an improved understanding of chemical evolution in protoplanetary disk midplanes, comparisons can be made between models and observations that could potentially constrain the formation histories of comets.
Aims. Our aim is to carry out the first statistical comparison between cometary volatile ice abundances and modelled evolving abundances in a protoplanetary disk midplane.
Methods. A χ2-method was used to determine maximum likelihood surfaces for 14 different comets that formed at a given time (up to 8 Myr) and place (out to beyond the CO iceline) in the pre-solar nebula midplane. This was done using observed volatile abundances for the 14 comets and the evolution of volatile abundances from chemical modelling of disk midplanes. Two assumptions for the chemical modelling starting conditions (cloud inheritance or chemical reset), as well as two different sets of cometary molecules (parent species, with or without sulphur species) were investigated.
Results. Considering all parent species (ten molecules) in the reset scenario, χ2 likelihood surfaces show a characteristic trail in the parameter space with high likelihood of formation around 30 AU at early times and 12 AU at later times for ten comets. This trail roughly traces the vicinity of the CO iceline in time.
Conclusions. A statistical comparison between observed and modelled chemical abundances in comets and comet-forming regions could be a powerful tool for constraining cometary formation histories. The formation histories for all comets were constrained to the vicinity of the CO iceline, assuming that the chemistry was partially reset early in the pre-solar nebula. This is found, both when considering carbon-, oxygen-, and sulphur-bearing molecules (ten in total), and when only considering carbon- and oxygen-bearing molecules (seven in total). Since these 14 comets did not previously fall into the same taxonomical categories together, this chemical constraint may be proposed as an alternative taxonomy for comets. Based on the most likely time for each of these comets to have formed during the disk chemical evolution, a formation time classification for the 14 comets is suggested.