用于打破直接成像系外行星中半径-反照率简并性的多epoch紫外观测量
Multi-epoch ultraviolet observables for breaking the radius-albedo degeneracy in directly imaged exoplanets
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中文总结 AI 辅助
该研究针对直接成像系外行星的半径-反照率简并问题，提出多epoch紫外观测量，结合vSmartMOM模型和光谱偏振测量，可在HWO观测条件下以低误差确定行星半径，提供了可行的观测路径。
中文摘要 AI 辅助
直接成像通过行星反射恒星光的亮度测量其视亮度，但仅亮度无法区分行星是大而暗还是小而亮。这种半径-反照率简并性限制了对非凌日系外行星的表征，包括宜居世界天文台（HWO）的目标行星。我们从多epoch观测中构建出与行星半径无关的紫外观测量：归一化400 nm光变曲线形状、紫外强度和偏振色，以及线偏振度。在360-400 nm的清晰光谱窗口中，瑞利散射强烈且明亮的紫外表面几乎无色，这些观测量可约束大气柱、表面反射率，必要时还可在推导半径前约束观测相位。我们使用GPU加速的矢量辐射传输模型vSmartMOM，在信噪比（SNR）为5、20和100时测试这些无半径观测量确定散射状态的唯一性。对于选定的6个相位序列，已知相位时，光谱偏振测量给出的半径中值误差分别为9.5%、3.3%和0.45%；若相位未知，6个不同epoch在低SNR时保持误差适度，中高SNR时可忽略，中值误差分别为14%、3.2%和0.47%。移除偏振测量会降低结果精度，尤其是在HWO模拟中，内工作角限制了相位覆盖范围。对于6和12秒差距的太阳孪生系统，所需的6相位观测活动在数百小时的分配范围内，这为HWO反射光探测确定行星半径提供了明确的观测路径。
英文摘要
Direct imaging measures how bright a planet appears in reflected starlight, but brightness alone cannot tell whether the planet is large and dark or small and bright. This radius-albedo degeneracy limits the characterization of non-transiting exoplanets, including those targeted by the Habitable Worlds Observatory (HWO). From multi-epoch observations we construct ultraviolet observables that are independent of planetary radius: the normalized 400 nm lightcurve shape, the ultraviolet intensity and polarization colors, and the degree of linear polarization. In a clear 360-400 nm spectral window where Rayleigh scattering is strong and bright ultraviolet surfaces are nearly colorless, these observables constrain the atmospheric column, the surface reflectivity, and, when relevant, the observed phases before radius is inferred. Using the GPU accelerated vector radiative transfer model vSmartMOM, we test how uniquely these radius-free observables determine the scattering state at signal-to-noise ratios (SNRs) of 5, 20, and 100. For the chosen six phase sequence, spectropolarimetry gives median radius consequences of 9.5%, 3.3%, and 0.45% when the phases are known. If the phases are unknown, six distinct epochs keep the penalty modest at low SNR and negligible at moderate-high SNR, with median consequences of 14%, 3.2%, and 0.47%, respectively. Removing polarimetry degrades the result, especially in the HWO simulations where phase coverage is restricted by inner working angle. For solar twin systems at 6 and 12 parsecs, the required six-phase campaigns fall in the allotted range of a few hundred hours, making this a clear observing path to planetary radii from HWO's reflected-light detections.
