無中文摘要

As primitive members of our solar system, comets are remnants from early solar system formation hence retain primordial compositions of proto-solar nebula. Comet study therefore can help us to understand the chemical and physical conditions in the early of solar system. To date, study of cometary compositions is mainly via ground-based observations of cometary coma, dust and ion tails. Unfortunately, with limited angular resolution, direct observations of cometary nucleus is unachievable in spite of a handful of comets were visited by spacecraft. Details of comet nucleus hence can only be inferred circuitously by observing coma activities; consequently our knowledge about the structure and chemical compositions of comet nuclei is still wanting. To better understand the nature of comet hence solar system formation, we have thus carried out millimeterwave observations of one Jupiter Family comet: 103P/Hartley 2, and three Oort Cloud comets: C/2007 N3 (Lulin), C/2009 P1 (Garradd) and C/2012 F6 (Lemmon), using the Kitt Peak 12m, SMT and ALMA. Additional comet data of two JFCs: 73P/Schwassmann-Wachmann 3 and 17P/Holmes were also included in our study.

We observed multiple CH3OH transitions simultaneously toward comet 103P, Lulin and Garradd, in order to derive rotational temperatures which could be utilized as a thermometer of coma gas. Accurately measured excitation temperatures are important for deriving CH3OH column densities which are essential for determining temporal variations and possible periodicities of comets. Day-to-day variations of comet 103P and Garradd follow the trend of heliocentric distances. CH3OH data were split further into hourly intervals to investigate likely short-term variations of 103P and Garradd. By fast-Fourier transforming CH3OH column densities, we found temporal variations in periods of 6.7 and 13.5 hours for 103P, and 5.5 and 7.5 hours for Garradd. The multi-periodicity unveiled may result from multi-vent outgassing of CH3OH on nuclear surface. Better determined periodicity of Garradd was attained via HCN measurements instead of CH3OH; the periodicity of HCN variation is comparable with that of CH3OH which suggests HCN and CH3OH molecules are closely correlated in the nucleus. On the other hand, we may apply the Cometary Halo Icy Particles (CHIPs) scenario to explain the moderate variation of CH3OH emission and indistinct periodicity of Garradd when compared to 103P.

Primary volatiles are molecular species stored as ices originally in cometary nucleus hence observations of gas-phase primary molecules offer the most direct connection to the proto-solar nebula. A large fraction of comets exhibit distinct spatial distributions of polar and apolar primary volatiles when volatiles were released into the inner coma. Native species sublimating from common nucleus ices situated in the same region of the comet nucleus would result in similar spatial distributions. By comparing the spatial distributions of various native molecules in the inner coma simultaneously, we will be able to unveil whether these primary species are related to distinct nucleus sources or not. We therefore used ALMA to observe the native volatiles HCN and CH3OH simultaneously in the inner coma of comet Lemmon and imaged their spectral variations in 18-minute time span. HCN and CH3OH are found to be concentrated on comet nucleus as anticipated. However, detailed spectral analysis indicates the apparent double-peak line profile of HCN actually consists of four velocity components in two separate sets, i.e. two velocity components in each set, or simply a line profile of “paired double peaks”, which imply strongly the existence of dual HCN outgassing vents in Lemmon’s nucleus. The highly similar HCN and CH3OH line profiles toward comet nucleus also suggests that HCN and CH3OH share a common nucleus source. Spectral images of HCN show outgassing from comet nucleus constantly while CH3OH emission reveals its clumpy distribution due to additional extended sources and fast-dispersing nature. Our high-resolution ALMA results support the CHIPs scenario that considers large amount of methanol may be sublimated from the surface of icy particles originated from nucleus outbursts.

Last but not least, we have also detected a new cometary organic molecule, cyclopropenylidene (c-C3H2), in five different comets: 73P/S-W 3, 17P/Holmes, Lulin, 103P and Lemmon. We have successfully imaged cyclopropenylidene and disclosed its clumpy spatial distributions with the ALMA. Our ALMA images indicate that a large portion of c-C3H2 is either secondary or evaporated from the extended source in the coma. Potential sources of cometary cyclopropenylidene thus include: primordial c-C3H2 trapped in icy grains, decomposition of macromolecular organics from dust grains, and photodestruction product of large hydrocarbon polymers or aromatic compounds. Deuterated c-C3H2 is also detected in comet Lemmon with ALMA. The D/H ratio thus derived is of ~0.20 to 0.32 which is relatively high compared to the VSMOW value. The high D/H ratio, if confirmed, will be extremely important for our understanding of solar-system formation.

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