彗星C/2001 Q4 (NEAT) 及 C/2002 T7 (LINEAR) 之毫米波光譜研究

Abstract

Comets are thought to be pristine relics from the birth of our Solar System and they have experienced the least thermal alteration. Comets, in particular long-period ones, thus keep the information of the time when they were formed. In spring 2004, the apparitions of two long-period comets C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR), at a distance of 0.3 and 0.27 AU respectively from the Earth at perigee, provided us a rare opportunity to have a detailed study cometary inner coma where photodissociation is ineffective and most cometary parent molecules remain. We successfully imaged the spatial distribution of CS and CH3OH of C/2002 T7 (LINEAR) using the Submillimeter Array (SMA). Detailed analysis of SMA images of Comet/T7 suggests strongly that methanol molecules are sublimated directly from the cometary nucleus. Earlier studies implied that CS is a daughter molecule of CS2. Even with our SMA observations at 1500×900 km high-linear resolution, however, we were not able to discriminate whether CS is a parent or a daughter molecule, i.e., whether the CS molecule seen in the inner coma was directly sublimated from the cometary nucleus or it was produced via photodissociation of CS2 molecule. Single-dish observations using the Kitt Peak 12m radio telescope (KP12M) were also carried out. Molecular emission of CS, HCN, H2CO in Comet C/2001 Q4 (NEAT), and HCN, H2CO, HDCO and CH3OH in Comet C/2002 T7 (LINEAR) were detected. HDCO has never been detected in any comet previously. The D/H ratio of Comet C/2002 T7 derived is 0.28 based on our measurements of HDCO and H2CO. The high D/H ratio observed is analogous to the values of Class 0 solar-type protostellar sources reported, and therefore implies that the Solar Nebula may have gone through evolutionary stages similar to that of these protostellar sources.