宇宙射线对冰质星际物体的侵蚀及其对“Oumuamua”的影响
Erosion of Icy Interstellar Objects by Cosmic Rays and Implications for `Oumuamua
摘要：
我们研究了宇宙射线和气体碰撞对冰质星际物体的破坏和改变。利用在本地星际介质中测量的宇宙射线通量以及从不同银河中心半径的伽马射线观测中推断的宇宙射线通量，我们发现宇宙射线侵蚀对于由常见类型的冰组成的星际物体是重要的。有趣的是，宇宙射线加热可能会非常有效地破坏冰冷的星际物体，因此，\citet{jackson2021}在解释“Oumuamua”的组成时提出的N $_2$碎片的初始大小至少应为0.5公里，以便在ISM中的大约0.5 Gyr的旅程中幸存下来，如果它起源于宇宙射线通量增强的区域，则可能更大。这意味着初始N $_2$质量至少比最终值大一个数量级，加剧了解释“Oumuamua”的N $_2$质量预算不足。由于宇宙射线加热和气体碰撞而产生的侵蚀时间也使我们能够对其他类型的冰质星际天体的初始大小设定近似的限制，例如由CO、CO $_2$或CH $_4$组成的天体。对于一个给定的初始大小，我们约束的最大距离的诞生地点的星际物体具有不同的速度。我们还发现，宇宙射线和气体加热可以在摧毁星际物体之前完全改变冰的结构。
英文摘要：
We study the destruction and modification of icy interstellar objects by cosmic rays and gas collisions. Using the cosmic-ray flux measured in the local interstellar medium as well as inferred from gamma-ray observations at the different galactocentric radii, we find that cosmic-ray erosion is significant for interstellar objects made of common types of ices. Interestingly, cosmic-ray heating might destroy icy interstellar objects very efficiently such that the initial size of an N$_2$ fragment as suggested by \citet{jackson2021} to explain the composition of `Oumuamua should be at least 0.5 km in size in order to survive the journey of about 0.5 Gyr in the ISM and might be even larger if it originated from a region with an enhanced cosmic-ray flux. This implies an initial N$_2$ mass that is at least an order of magnitude larger than the final value, exacerbating the N$_2$ mass budget deficiency for explaining `Oumuamua. The erosion time due to cosmic-ray heating and gas collisions also allows us to set approximate limits on the initial size for other types of icy interstellar objects, e.g. composed of CO, CO$_2$, or CH$_4$. For a given initial size, we constrain the maximum distance to the birth site for interstellar objects with different speeds. We also find that cosmic-ray and gas heating could entirely modify the ice structure before destroying interstellar objects.
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