Time Variability of FUV Emission from Cool Stars on Multi-year Timescales

IF 3.3 3区 物理与天体物理 Q2 ASTRONOMY & ASTROPHYSICS Publications of the Astronomical Society of the Pacific Pub Date : 2024-02-20 DOI:10.1088/1538-3873/ad119f
Leo Kamgar, Kevin France, Allison Youngblood
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Abstract

The physical and chemical properties of planetary atmospheres are affected by temporal evolution of ultraviolet (UV) radiation inputs from their host stars at all time scales. While studies of X-ray/UV flare properties and long-term stellar evolution of exoplanet host stars have provided new constraints regarding stellar inputs to exoplanetary systems, the UV temporal variability of cool stars on the timescale of stellar cycles remains largely unexplored. To address this gap in our understanding of the UV temporal variability of cool stars, we analyze far-ultraviolet (FUV) emission lines of ions that trace the chromosphere and transition region of nearby stars (C ii, Si iii, Si iv, and N v; formation temperatures ∼ 20–150 kK) using data from the Hubble Space Telescope (HST) and International Ultraviolet Explorer (IUE) archives spanning temporal baselines of months to years. We select 33 unique stars of spectral types F-M with observing campaigns spanning over a year, and create ionic light curves to evaluate the characteristic variability of cool stars on such timescales. Screening for large flare events, we observe that the relative variability of FUV light curves decreases with increasing stellar effective temperature, from 30% to 70% variability for M-type stars to <30% variability for F and G-type stars. We also observe a weak trend in the temporal variability with the Ca ii RHK stellar activity indicator, suggesting that stars with lower Ca ii activity exhibit a smaller range of FUV flux variability. Screening for data sets with optimal temporal spread, and a sufficient number of individual observations, we select 5 data sets for further periodicity analysis (HST α Centauri A, HST α Centauri B, IUE α Centauri B, IUE ϵ Eri, IUE ξ Boo). Various periodic structures within the FUV flux were detected, with most significant being a 79 days frequency present within the IUE observations of ξ Boo, with a significance of 6σ, and a periodic signal in the FUV observations of α Centauri B, for both HST and IUE measurements, at ≈210 days frequency with significance of 3σ and 3.7σ, respectively. Our results suggest that extreme ultraviolet flux from cool stars varies by less than a factor of two on decade timescales, significantly smaller than variations on flare or stellar evolutionary timescales.
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冷恒星在多年时间尺度上的 FUV 发射时间变异性
行星大气的物理和化学性质受到来自其宿主恒星的紫外线(UV)辐射输入在所有时间尺度上的时间演化的影响。虽然对系外行星宿主恒星的 X 射线/紫外线耀斑特性和长期恒星演化的研究为系外行星系统的恒星输入提供了新的约束条件,但在恒星周期的时间尺度上,冷恒星的紫外线时变性在很大程度上仍未得到探索。为了弥补我们对冷恒星紫外线时变性认识上的这一空白,我们利用哈勃太空望远镜(HST)和国际紫外线探测器(IUE)档案中的数据,分析了追踪附近恒星(C ii、Si iii、Si iv 和 N v;形成温度 ∼ 20-150 kK)色球层和过渡区的离子远紫外(FUV)发射线,其时间基线跨度从数月到数年不等。我们选择了 33 颗光谱类型为 F-M 的独特恒星,观测活动跨度超过一年,并绘制了离子光曲线,以评估冷恒星在这种时间尺度上的特征变异性。通过对大型耀斑事件的筛选,我们观察到 FUV 光曲线的相对可变性随着恒星有效温度的升高而降低,从 M 型恒星的 30% 到 70% 可变性到 F 型和 G 型恒星的 <30% 可变性。我们还观察到随着 Ca iiRHK′ 恒星活动性指标的变化,时间变率呈微弱趋势,这表明 Ca ii 活动性较低的恒星表现出的 FUV 通量变率范围较小。在筛选具有最佳时间分布和足够数量的单个观测数据集时,我们选择了 5 个数据集进行进一步的周期性分析(HST α 半人马座 A、HST α 半人马座 B、IUE α 半人马座 B、IUE ϵ Eri、IUE ξ Boo)。在 FUV 流量中探测到了各种周期性结构,其中最重要的是ξ Boo 的 IUE 观测中出现的频率为 79 天的周期信号,其显著性为 6σ;在半人马座α B 的 FUV 观测中,HST 和 IUE 的测量都出现了频率为≈210 天的周期信号,其显著性分别为 3σ 和 3.7σ。我们的结果表明,来自冷恒星的极端紫外通量在十年时间尺度上的变化不到2倍,明显小于耀斑或恒星演化时间尺度上的变化。
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来源期刊
Publications of the Astronomical Society of the Pacific
Publications of the Astronomical Society of the Pacific 地学天文-天文与天体物理
CiteScore
6.70
自引率
5.70%
发文量
103
审稿时长
4-8 weeks
期刊介绍: The Publications of the Astronomical Society of the Pacific (PASP), the technical journal of the Astronomical Society of the Pacific (ASP), has been published regularly since 1889, and is an integral part of the ASP''s mission to advance the science of astronomy and disseminate astronomical information. The journal provides an outlet for astronomical results of a scientific nature and serves to keep readers in touch with current astronomical research. It contains refereed research and instrumentation articles, invited and contributed reviews, tutorials, and dissertation summaries.
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