Recent studies revealed viewing-angle-dependent color and spectral trends in brown dwarfs, as well as long-term photometric variability (∼100 hr). The origins of these trends are yet unexplained. Here, we propose that these seemingly unrelated sets of observations stem from the same phenomenon: the polar regions of brown dwarfs and directly imaged exoplanets are spectrally different from lower-latitude regions, and they evolve over longer timescales, possibly driven by polar vortices. We explore this hypothesis via a spatiotemporal atmosphere model capable of simulating time series and disk-integrated spectra of ultracool atmospheres. We study three scenarios with different spectral and temporal components: a null hypothesis without polar vortex, and two scenarios with polar vortices. We find that the scenarios with polar vortex can explain the observed infrared color–inclination trend and the variability amplitude–inclination trend. The presence of spectrally distinct, time-evolving polar regions in brown dwarfs and giant exoplanet atmospheres raises the possibility that one-dimensional static atmospheric models may be insufficient for reproducing ultracool atmospheres in detail.

中文翻译：

---

极涡假说：进化的、光谱不同的极地区域解释了褐矮星和巨型系外行星的短期和长期光曲线演变和颜色倾斜趋势


最近的研究揭示了褐矮星中与视角相关的颜色和光谱趋势，以及长期光度变化（∼100 小时）。这些趋势的起源尚不清楚。在这里，我们提出这些看似无关的观测集源于同一个现象：褐矮星和直接成像的系外行星的极地区域在光谱上与低纬度区域不同，它们在更长的时间尺度上演变，可能是由极地涡旋驱动的。我们通过能够模拟超冷大气的时间序列和磁盘积分光谱的时空大气模型来探索这一假设。我们研究了三种具有不同光谱和时间分量的情景：一个没有极地涡旋的零假设，以及两种有极地涡旋的情景。我们发现极地涡旋的情景可以解释观察到的红外颜色-倾角趋势和变率振幅-倾角趋势。褐矮星和巨型系外行星大气中存在光谱不同的、时间演变的极地区域，这增加了一维静态大气模型可能不足以详细再现超冷大气的可能性。