论文标题
直接成像系外行星上生物签名的K矮人优势
The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets
论文作者
论文摘要
氧气和甲烷被认为是现代地球的规范生物签名，并且在行星大气中同时检测这些气体是一种特别强大的生物签名。但是，这些气体在行星大气中一起检测可能具有挑战性，因为光化学自由基破坏了甲烷。先前的工作表明，含氧气氛中甲烷的光化学寿命在M矮人周围更长，但是M Dwarf Planet的可居住性可能会受到极端的恒星活动和进化的阻碍。在这里，我们使用1-D光化学气候模型来表明，与G矮人相比，在存在氧气的情况下，K矮星的甲烷还提供了更长的光化学寿命。例如，我们表明，与绕G2V恒星的等效行星相比，绕着K6V恒星的行星在大气中的甲烷中可以支撑大气中的数量级。在绕着K矮星恒星的世界的反射光谱中，可以在可见的和近红外波长处观察到强的氧气和甲烷特征。由于K矮人比G矮人昏暗，因此它们提供了更好的行星主对比比，从而增强了给定观察中可能的信噪比（SNR）。例如，在7米处的7个行星上观察50小时的行星在绕行太阳能G2V恒星的行星附近的SNR = 9.2接近1 UM，对于绕K6V恒星绕的同一行星，SNR = 20。尤其是，附近的中线k矮人，例如61 CYG A/B，Epsilon Indi，Groombridge 1618和HD 156026，可能是未来生物签名搜索的绝佳目标。
Oxygen and methane are considered to be the canonical biosignatures of modern Earth, and the simultaneous detection of these gases in a planetary atmosphere is an especially strong biosignature. However, these gases may be challenging to detect together in the planetary atmospheres because photochemical oxygen radicals destroy methane. Previous work has shown that the photochemical lifetime of methane in oxygenated atmospheres is longer around M dwarfs, but M dwarf planet habitability may be hindered by extreme stellar activity and evolution. Here, we use a 1-D photochemical-climate model to show that K dwarf stars also offer a longer photochemical lifetime of methane in the presence of oxygen compared to G dwarfs. For example, we show that a planet orbiting a K6V star can support about an order of magnitude more methane in its atmosphere compared to an equivalent planet orbiting a G2V star. In the reflected light spectra of worlds orbiting K dwarf stars, strong oxygen and methane features could be observed at visible and near-infrared wavelengths. Because K dwarfs are dimmer than G dwarfs, they offer a better planet-star contrast ratio, enhancing the signal-to-noise (SNR) possible in a given observation. For instance, a 50 hour observation of a planet at 7 pc with a 15-m telescope yields SNR = 9.2 near 1 um for a planet orbiting a solar-type G2V star, and SNR = 20 for the same planet orbiting a K6V star. In particular, nearby mid-late K dwarfs such as 61 Cyg A/B, Epsilon Indi, Groombridge 1618, and HD 156026 may be excellent targets for future biosignature searches.
