彗星C/2004 Q2（MACHHOLZ）：母体挥发物，寻找氘代甲烷和对CH4自旋温度的限制
Comet C/2004 Q2 (MACHHOLZ): Parent Volatiles, a Search for Deuterated Methane, and Constraint on the CH4 Spin Temperature
摘要：
2004年11月28日至29日和2005年1月19日（UT日期），在莫纳克亚山的凯克-2望远镜上用NIRSPEC获得了彗星C/2004 Q2（Machholz）的高色散（l/dl ~ 25，000）红外光谱。我们检测了H2O、CH 4、C2 H2、C2 H6、CO、H2 CO、CH 3OH、HCN和NH3，并对CH 3D进行了灵敏搜索。我们报告旋转温度，生产率，和混合比（相对于H2O）在日心距离为1.49 AU（2004年11月）和1.21 AU（2005年1月）。我们强调三个主要结果：（1）在1.49 AU和1.21 AU测量的母挥发物的混合比同意在置信限内，一致的均匀组成的平均挥发物从C/2004 Q2的核释放。值得注意的是，C2 H6/C2 H2的相对丰度远高于其他彗星，而C2 H6/H2O、CH 3OH/H2O和HCN/H2O的混合比与彗星中观察到的相似，被称为“有机物正常”。(2)CH 4的自旋温度> 35-38 K，这一估计与H2O的更稳定的自旋温度一致。(3)我们得到了3s的上限CH_3D/CH_4 < 0.020（D/H < 0.005）。这一限制表明，从C/2004 Q2的核释放的甲烷并不是由在极冷（近10 K）的环境中形成的成分所主导的。形成途径的星际和星云的起源与测量的D/H在甲烷中的一致进行了讨论。评估这些途径的相对贡献，需要进一步的化学建模，包括气相和气体颗粒的过程中的出生星际云和原行星盘。
英文摘要：
High-dispersion (l/dl ~ 25,000) infrared spectra of Comet C/2004 Q2 (Machholz) were acquired on Nov. 28-29, 2004, and Jan. 19, 2005 (UT dates) with NIRSPEC at the Keck-2 telescope on Mauna Kea. We detected H2O, CH4, C2H2, C2H6, CO, H2CO, CH3OH, HCN, and NH3 and we conducted a sensitive search for CH3D. We report rotational temperatures, production rates, and mixing ratios (with respect to H2O) at heliocentric distances of 1.49 AU (Nov. 2004) and 1.21 AU (Jan. 2005). We highlight three principal results: (1) The mixing ratios of parent volatiles measured at 1.49 AU and 1.21 AU agree within confidence limits, consistent with homogeneous composition in the mean volatile release from the nucleus of C/2004 Q2. Notably, the relative abundance of C2H6/C2H2 is substantially higher than those measured in other comets, while the mixing ratios C2H6/H2O, CH3OH/H2O, and HCN/H2O are similar to those observed in comets, referred to as "organics-normal". (2) The spin temperature of CH4 is > 35-38 K, an estimate consistent with the more robust spin temperature found for H2O. (3) We obtained a 3s upper limit of CH3D/CH4 < 0.020 (D/H < 0.005). This limit suggests that methane released from the nucleus of C/2004 Q2 is not dominated by a component formed in extremely cold (near 10 K) environments. Formation pathways of both interstellar and nebular origin consistent with the measured D/H in methane are discussed. Evaluating the relative contributions of these pathways requires further modeling of chemistry including both gas-phase and gas-grain processes in the natal interstellar cloud and in the protoplanetary disk.
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