Roman FFP革命：两个、三个、许多冥王星
Roman FFP Revolution: Two, Three, Many Plutos
AI总结：
Roman微透镜观测的束缚行星策略与自由漂浮行星探测存在冲突，本文推导方程揭示该冲突，并指出需加倍观测频率以避免探测数量减半及丢失最低质量自由漂浮行星。
摘要：
Roman微引力透镜观测正处于其最初规划的路径与自由漂浮行星（FFP）路径的十字路口。最初规划的路径是编目冷行星族群，该族群随后已被精确测量至超级地球级别；而自由漂浮行星路径在2010年选择Roman任务时并不存在，但如今有望通过其与跨越18个数量级质量的天体光谱的可能联系，为行星形成与演化提供革命性见解。直到现在，人们甚至没有意识到这两条路径存在冲突：Roman的策略是为探测束缚行星而优化的，自由漂浮行星仅在该策略背景下被考虑能从中获得什么信息。我们推导出一个简单方程，从数学上表达了这一冲突，并解释了为何当前方法严重抑制了5个数量级潜在自由漂浮行星质量中2个数量级的探测，即恰好是$M_{\rm Pluto}< M <2\\,M_{\rm Mars}$这两个数量级，它们将类地行星与其形成的原行星物质联系起来。自由漂浮行星可以是真正的自由漂浮，也可以束缚在“宽距”、“柯伊伯”和“奥尔特”轨道中，对这些轨道的单独识别将有助于进一步理解行星形成。在（低质量）极限下，当源半径远大于爱因斯坦半径时，$θ_*\ggθ_{\rm E}$，自由漂浮行星光变曲线上显著放大点的数量为$N=2Γθ_*\sqrt{1-z^2}/μ$，当归一化到所采用的Roman观测频率$Γ=4$/小时、源半径$θ_*=0.3\\,$μas、透镜-源自行$μ=6\\,$mas/年、源碰撞参数$z=0.5$（均为典型值）时，$N$ --> 3.0。相比之下，自由漂浮行星探测需要$N=6$。因此，除非$Γ$加倍，否则自由漂浮行星探测将被推向参数空间（大$θ_*$、小$μ$）的角落，使探测数量净减少2倍，并切断最低质量自由漂浮行星的探测。
英文摘要：
Roman microlensing stands at a crossroads between its originally charted path of cataloging a population of cool planets that has subsequently become well-measured down to super-Earths, and the path of free-floating planets (FFPs), which did not exist when Roman was chosen in 2010, but by now promises revolutionary insights into planet formation and evolution via their possible connection to a spectrum of objects spanning 18 decades in mass. Until now, it was not even realized that the 2 paths are in conflict: Roman strategy was optimized for bound-planet detections, and FFPs were considered only in the context of what could be learned about them given this strategy. We derive a simple equation that mathematically expresses this conflict and explains why the current approach severely depresses detection of 2 of the 5 decades of potential FFP masses, i.e., exactly the two decades, $M_{\rm Pluto}< M <2\,M_{\rm Mars}$, that would tie terrestrial planets to the proto-planetary material out of which they formed. FFPs can be either truly free floating or can be bound in "Wide", "Kuiper", and "Oort" orbits, whose separate identification will allow further insight into planet formation. In the (low-mass) limit that the source radius is much bigger than the Einstein radius, $θ_*\ggθ_{\rm E}$, the number of significantly magnified points on the FFP light curve is $N=2Γθ_*\sqrt{1-z^2}/μ$ --> 3.0, when normalized to the adopted Roman cadence $Γ=4/$hr, and to source radius $θ_*=0.3\,μ$as, lens-source proper motion $μ=6\,$mas/yr, and source impact parameter $z=0.5$, which are all typical values. By contrast $N=6$ are needed for an FFP detection. Thus, unless $Γ$ is doubled, FFP detection will be driven into the (large-$θ_*$, small-$μ$) corner of parameter space, reducing the detections by a net factor of 2 and cutting off the lowest-mass FFPs.
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