Milky Way星系γ射线暴的天体生物学效应
Astrobiological Effects of Gamma-Ray Bursts in the Milky Way Galaxy
摘要：
在伽马射线暴（GRB）的准直光束内具有保护性臭氧的行星可能会遭受臭氧消耗，可能导致行星表面和海洋上现有生命的大规模灭绝事件。我们模拟了长伽玛暴对银河系行星的危险，并利用了一个与主要可观测性质相匹配的银河系静态统计模型，如由内而外的恒星形成历史，金属丰度演化和3维恒星数密度分布。伽玛射线暴的形成率是恒星形成历史和金属丰度的函数;然而，化学演化在多大程度上降低了银河系中伽玛射线暴的形成率仍然是一个悬而未决的问题。因此，我们用两种模型比较了伽玛暴对银河系生物圈的破坏作用。一个模型产生的伽玛暴是由内而外的恒星形成历史的函数。另一个模型遵循恒星形成的历史，但产生的伽玛射线暴的祖先作为金属丰度的函数，从而有利于金属贫乏的主机区域的银河系随着时间的推移。如果伽玛暴的发生率只与恒星形成历史相一致，那么绝大多数伽玛暴都发生在银河系内部。然而，如果伽玛暴的祖先被限制在低金属丰度的环境中，那么伽玛暴只形成在金属贫乏的郊区在最近的时代。有趣的是，在过去的1 Gyr中，在两个模型中，在GRB中幸存下来的恒星（及其相应的行星）的表面密度仍然是内星系中最大的。目前，在太阳半径（$R_\odot=8$ kpc）范围内，长伽玛暴对生命的威胁很低。我们发现，至少有$\sim$ 65%的恒星在过去的1 Gyr伽玛暴中幸存下来。此外，当伽玛射线暴率预计已在较高的红移，如$z\gtrsim0.5$增强，我们的研究结果表明，大部分的行星将生存这些致命的伽玛射线暴事件。
英文摘要：
A planet having protective ozone within the collimated beam of a Gamma Ray Burst (GRB) may suffer ozone depletion, potentially causing a mass extinction event to existing life on a planet's surface and oceans. We model the dangers of long GRBs to planets in the Milky Way and utilize a static statistical model of the Galaxy that matches major observable properties, such as the inside-out star formation history, metallicity evolution, and 3-dimensional stellar number density distribution. The GRB formation rate is a function of both the star formation history and metallicity; however, the extent to which chemical evolution reduces the GRB rate over time in the Milky Way is still an open question. Therefore, we compare the damaging effects of GRBs to biospheres in the Milky Way using two models. One model generates GRBs as a function of the inside-out star formation history. The other model follows the star formation history, but generates GRB progenitors as a function of metallicity, thereby favoring metal-poor host regions of the Galaxy over time. If the GRB rate only follows the star formation history, the majority of the GRBs occur in the inner Galaxy. However, if GRB progenitors are constrained to low metallicity environments, then GRBs only form in the metal-poor outskirts at recent epochs. Interestingly, over the past 1 Gyr, the surface density of stars (and their corresponding planets) that survive a GRB is still greatest in the inner galaxy in both models. The present day danger of long GRBs to life at the solar radius ($R_\odot=8$ kpc) is low. We find that at least $\sim$65% of stars survive a GRB over the past 1 Gyr. Furthermore, when the GRB rate was expected to have been enhanced at higher redshifts, such as $z\gtrsim0.5$, our results suggest that a large fraction of planets would have survived these lethal GRB events.
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