Multi-wavelength time-resolved observations of rotationally modulated variability from brown dwarfs and giant exoplanets are the most effective method for constraining their heterogeneous atmospheric structures. In a companion paper (Bowler et al. 2020), we reported the discovery of strong near-infrared variability in HST/WFC3/G141 light curves of the very red L-dwarf companion VHS J1256-1257b. In this paper, we present a follow-up 36-hr Spitzer/IRAC Channel 2 light curve together with an in-depth analysis of the HST and the Spitzer data. The combined dataset provides time-resolved light curves of VHS1256b sampling 1.1 to 4.5 $\mu$m. The Spitzer light curve is best-fit with a single sine wave with a period of $22.04\pm0.05$ hr and a peak-to-peak amplitude of $5.76\pm0.04$%. Combining the period with a previously measured projected rotational velocity ($v\sin i$), we find that VHS1256b is most consistent with equatorial viewing geometry. The HST/G141+Spitzer spectral energy distribution favors a $T_{\rm eff}$ of 1000~K, low surface gravity model with disequilibrium chemistry. The spectral variability of VHS1256b is consistent with predictions from partly cloudy models, suggesting heterogeneous clouds are the dominant source of the observed modulations. We find evidence at the $3.3\sigma$ level for amplitude variations within the 1.67$\mu$m CH$_{4}$ band, which is the first such detection for a variable L-dwarf. We compare the HST/G141 time-resolved spectra of three red L-dwarfs with high-amplitude near-infrared rotational modulations and find that although their time-averaged spectra are similar, their spectroscopic variabilities exhibit notable differences. This diversity reinforces the advantage of time-resolved spectroscopic observations for understanding the atmospheres of brown dwarfs and directly-imaged exoplanets.

中文翻译：

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VHS J1256–1257b 从 1 到 5 μm 的光谱变化

对来自褐矮星和巨型系外行星的旋转调制变化的多波长时间分辨观测是限制其异质大气结构的最有效方法。在一篇配套论文（Bowler 等人，2020 年）中，我们报告了在非常红的 L 矮星伴星 VHS J1256-1257b 的 HST/WFC3/G141 光变曲线中发现了强烈的近红外变化。在本文中，我们展示了后续的 36 小时 Spitzer/IRAC Channel 2 光变曲线以及对 HST 和 Spitzer 数据的深入分析。组合数据集提供了 VHS1256b 采样 1.1 至 4.5 $\mu$m 的时间分辨光变曲线。斯皮策光曲线最适合周期为 $22.04\pm0.05$ hr 和峰峰值幅度为 $5.76\pm0.04$% 的单个正弦波。将该周期与先前测量的投影旋转速度 ($v\sin i$) 相结合，我们发现 VHS1256b 与赤道观测几何最一致。HST/G141+Spitzer 光谱能量分布有利于 $T_{\rm eff}$ 为 1000~K，具有不平衡化学的低表面重力模型。VHS1256b 的光谱变异性与部分多云模型的预测一致，表明异质云是观测到的调制的主要来源。我们在 $3.3\sigma$ 水平上发现了 1.67$\mu$m CH$_{4}$ 波段内振幅变化的证据，这是对可变 L 矮星的首次此类检测。我们比较了三个红 L 矮星的 HST/G141 时间分辨光谱与高振幅近红外旋转调制，发现虽然它们的时间平均光谱相似，它们的光谱变异表现出显着差异。这种多样性加强了时间分辨光谱观测在了解褐矮星和直接成像系外行星大气方面的优势。