{"title":"Time Variability of FUV Emission from Cool Stars on Multi-year Timescales","authors":"Leo Kamgar, Kevin France, Allison Youngblood","doi":"10.1088/1538-3873/ad119f","DOIUrl":null,"url":null,"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 <sc>ii</sc>, Si <sc>iii</sc>, Si <sc>iv</sc>, and N <sc>v</sc>; 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 <sc>ii</sc>\n<inline-formula>\n<tex-math>\n<?CDATA ${R}_{\\mathrm{HK}}^{{\\prime} }$?>\n</tex-math>\n<mml:math overflow=\"scroll\"><mml:msubsup><mml:mrow><mml:mi>R</mml:mi></mml:mrow><mml:mrow><mml:mi>HK</mml:mi></mml:mrow><mml:mrow><mml:mo accent=\"true\">′</mml:mo></mml:mrow></mml:msubsup></mml:math>\n<inline-graphic xlink:href=\"paspad119fieqn1.gif\" xlink:type=\"simple\"></inline-graphic>\n</inline-formula> stellar activity indicator, suggesting that stars with lower Ca <sc>ii</sc> 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 <italic toggle=\"yes\">α</italic> Centauri A, HST <italic toggle=\"yes\">α</italic> Centauri B, IUE <italic toggle=\"yes\">α</italic> Centauri B, IUE <italic toggle=\"yes\">ϵ</italic> Eri, IUE <italic toggle=\"yes\">ξ</italic> Boo). Various periodic structures within the FUV flux were detected, with most significant being a 79 days frequency present within the IUE observations of <italic toggle=\"yes\">ξ</italic> Boo, with a significance of 6<italic toggle=\"yes\">σ</italic>, and a periodic signal in the FUV observations of <italic toggle=\"yes\">α</italic> Centauri B, for both HST and IUE measurements, at ≈210 days frequency with significance of 3<italic toggle=\"yes\">σ</italic> and 3.7<italic toggle=\"yes\">σ</italic>, 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.","PeriodicalId":20820,"journal":{"name":"Publications of the Astronomical Society of the Pacific","volume":"46 1","pages":""},"PeriodicalIF":3.3000,"publicationDate":"2024-02-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Publications of the Astronomical Society of the Pacific","FirstCategoryId":"101","ListUrlMain":"https://doi.org/10.1088/1538-3873/ad119f","RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"ASTRONOMY & ASTROPHYSICS","Score":null,"Total":0}
引用次数: 0
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 iiRHK′ 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.
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.