The Activity of Comet 29P/Schwassmann–Wachmann 1 Monitored through Its CO J=2→1 Radio Line☆
The Activity of Comet 29P/Schwassmann–Wachmann 1 Monitored through Its CO J=2→1 Radio Line☆
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通过CO J=2→1射电线监测彗星29P/Schwassmann-Wachmann 1的活动☆
DOI：
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发表时间：
2001
期刊：
影响因子：
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通讯作者：
G. Tancredi
中科院分区：
文献类型：
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作者：
M. Festou;M. Gunnarsson;H. Rickman;A. Winnberg;G. Tancredi
摘要：在1996年12月4日至1997年1月1日期间，用15米的卫星天线对29P/ Schwassmann-Wachmann 1号彗星进行了18天的观测。在昏迷的四个不同位置测量了CO排放。在整整一个月里，在望远镜光束的所有位置上，这些线的形状都保持了惊人的恒定。在这些条件下，每日光谱的性质几乎可以独立于任何彗发和激发模型进行比较。我们的结论是，核和昏迷的物理条件在一个月内是稳定的。每天的核能产量变化很小，可能不到月平均产量的一半。因此，光谱可以结合起来产生具有前所未有的信噪比的线剖面。我们的数据和之前对彗星的毫米观测使我们能够描述彗星在静止状态下的特性。在这种状态下，核上线表现出以下特征：线面积为0.04 K km s−1；在- 0.45 km s - 1附近有一个向太阳移动的速度峰；速度裙边向正值延伸，这可能表明CO在夜侧产生。我们已经用流动方向和彗星-太阳线之间夹角的余弦值推导出了单位表面和时间的CO排放率和气体径向流出速度的简单表达式。我们发现，从亚太阳区域到终点，速度单调递减，而在反向阳半球，速度基本上是恒定的。大约75%的气体释放发生在向阳半球。静止状态下CO的平均产率为1.8±0.1×10 28分子s−1。这种稳态放气可以被描述为由两个组成部分组成：(1)低强度、几乎各向同性的CO源；(2)在亚太阳点附近达到峰值的主要源，显示出小空间范围生产区域的特性。前一种源可能是核源，也可能是分布源。解释后一种来源所需的适当的表面和近表面性质是由太阳能量不断落在旋转的原子核上维持的。虽然彗核可能被视为昏迷物质的单一来源，在太阳能量流入最高的地方，其产量会增加，但从这些观测推断，如果存在类似哈雷的活动区域，则很有可能发挥作用。
Abstract The 230-GHz emission from CO was observed on 18 different days between 4 December 1996 and 1 January 1997 with the 15-m SEST antenna pointed toward Comet 29P/Schwassmann–Wachmann 1. The CO emission was measured at four different locations within the coma. During the entire month, the shape of the lines remained remarkably constant for all positions of the telescope beam. Under these conditions, the properties of the daily spectra can be compared almost independent of any coma and excitation model. We conclude that the physical conditions for both the nucleus and the coma were stable over one month. Day-to-day nuclear output variations were small, probably less than half the monthly average production rate. The spectra could therefore be combined to produce line profiles with an unprecedented SNR. Our data and previous millimeter observations of the comet allow us to describe the properties of the comet in its quiescent state. In that state, the on-nucleus line presents the following characteristics: The line area is 0.04 K km s −1 ; there is a sunward-displaced velocity peak near −0.45 km s −1 ; a velocity skirt extends toward positive values, which might indicate CO production on the night side. We have derived simple expressions for both the CO emission rate per unit surface and time and the gas radial outflow velocity in terms of the cosine of the angle between the flow direction and the comet–Sun line. We find that the velocity decreases monotonically from the subsolar region to the terminator and is essentially constant over the anti-sunward hemisphere. Roughly 75% of the outgassing occurs on the sunward hemisphere. The mean CO production rate in the quiescent state is 1.8±0.1×10 28 molecules s −1 . This steady-state outgassing can be described as comprising two components: (1) a low-intensity, nearly isotropic source of CO and (2) a major source peaking near the subsolar point that exhibits the properties of a production region of small spatial extent. The former source may be of either nuclear or distributed nature. The appropriate surface and near-surface properties required to explain the latter source are maintained by the constant infall of solar energy on the rotating nucleus. While the nucleus may be viewed as a single source of coma materials, with enhanced production where the influx of solar energy is highest, the possible role of Halley-like active regions, if they exist, as inferred from these observations is very likely.