Thermal Physics of the Inner Coma: ALMA Studies of the Methanol Distribution and Excitation in Comet C/2012 K1 (PanSTARRS)
Thermal Physics of the Inner Coma: ALMA Studies of the Methanol Distribution and Excitation in Comet C/2012 K1 (PanSTARRS)
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内彗发的热物理：彗星 C/2012 K1 中甲醇分布和激发的 ALMA 研究 (PanSTARRS)
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发表时间：
2017
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通讯作者：
J. Boissier
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作者：
M. Cordiner;M. Cordiner;N. Biver;J. Crovisier;D. Bockelée;M. Mumma;S. Charnley;G. Villanueva;L. Paganini;L. Paganini;D. Lis;S. Milam;A. Remijan;I. Coulson;Y. Kuan;Y. Kuan;J. Boissier
我们在2014年6月28日至29日利用阿塔卡马大型毫米/亚毫米阵列对彗星C/2012 K1 （PanSTARRS）的CH3OH发射进行了空间和光谱分辨观测。推导了视距平均旋转温度（Trot）的二维图，其空间尺度为0.″3-1。″8（对应于天空投影距离ρ ~ 500-2500公里）。CH3OH柱密度分布与各向同性一致，从细胞核均匀流出，没有证据表明昏迷中有扩展的CH3OH来源。在距离原子核几千公里的范围内，T - rot （ρ）的径向分布有显著的下降，6月28日在ρ = 0和2500 km之间从60 K下降到20 K，而6月29日在ρ = 0和1000 km之间从120 K下降到40 K。观察到的Trot行为主要被解释为由于流出气体的绝热冷却以及CH3OH旋转水平的辐射冷却而导致的昏迷动力学温度变化的结果。我们的激发模型表明，辐射冷却对J = 7−6跃迁（在338 GHz）比K = 3−2跃迁（在252 GHz）更重要，导致在ρ > 1000 km处J = 7−6波段的能级有强烈的亚热分布。对于这两个波段，观测到的温度随距离的下降比标准彗发理论模型预测的要小，这表明除了通常认为的光解热源外，还存在重要的热源。
We present spatially and spectrally resolved observations of CH3OH emission from comet C/2012 K1 (PanSTARRS), using the Atacama Large Millimeter/submillimeter Array on 2014 June 28–29. Two-dimensional maps of the line-of-sight average rotational temperature (Trot) were derived, covering spatial scales 0.″3–1.″8 (corresponding to sky-projected distances ρ ∼ 500–2500 km). The CH3OH column density distributions are consistent with isotropic, uniform outflow from the nucleus, with no evidence for extended sources of CH3OH in the coma. The T rot ( ρ ) radial profiles show a significant drop within a few thousand kilometers of the nucleus, falling from about 60 to 20 K between ρ = 0 and 2500 km on June 28, whereas on June 29, Trot fell from about 120 to 40 K between ρ = 0 km and 1000 km. The observed Trot behavior is interpreted primarily as a result of variations in the coma kinetic temperature due to adiabatic cooling of the outflowing gas, as well as radiative cooling of the CH3OH rotational levels. Our excitation model shows that radiative cooling is more important for the J = 7 − 6 transitions (at 338 GHz) than for the K = 3 − 2 transitions (at 252 GHz), resulting in a strongly sub-thermal distribution of levels in the J = 7 − 6 band at ρ ≳ 1000 km . For both bands, the observed temperature drop with distance is less steep than predicted by standard coma theoretical models, which suggests the presence of a significant source of heating in addition to the photolytic heat sources usually considered.