Magnetic Activity of F-, G-, and K-type Stars in the LAMOST–Kepler Field

Abstract

Monitoring chromospheric and photospheric indexes of magnetic activity can provide valuable information, especially the interaction between different parts of the atmosphere and their response to magnetic fields. We extract chromospheric indexes, S and + R HK , for 59,816 stars from LAMOST spectra in the LAMOST-Kepler program, and photospheric index, R eff , for 5575 stars from Kepler light curves. The log R eff shows positive correlation with log + R HK . We estimate the power-law indexes between R eff and + R HK for F-, G-, and K-type stars, respectively. We also confirm the dependence of both chromospheric and photospheric activity on stellar rotation. Ca II H and K emissions and photospheric variations generally decrease with increasing rotation periods for stars with rotation periods exceeding a few days. The power-law indexes in exponential decay regimes show different characteristics in the two activity-rotation relations. The updated largest sample including the activity proxies and reported rotation periods provides more information to understand the magnetic activity for cool stars.

No Sign of G2's Encounter Affecting Sgr A*'s X-Ray Flaring Rate from Chandra Observations

“…This emission is spatially extended and well modeled by thermal bremsstrahlung emission arising from thermal plasma in the accretion flow around the Bondi radius (Quataert 2002;Baganoff et al 2003;Yuan et al 2003;Liu et al 2004;Xu et al 2006;Wang et al 2013). This quiescent behaviour is punctuated by X-ray flares of luminosities varying from ∼ 10s to 100s of times quiescence (or 100s to 1000s of times their locally-emitted background since the inner accretion flow contributes 10% of the quiescent emission; Baganoff et al 2001;Goldwurm et al 2003;Porquet et al 2003Porquet et al , 2008Belanger et al 2005;Nowak et al 2012;Neilsen et al 2013Neilsen et al , 2015Degenaar et al 2013;Barriere et al 2014;Ponti et al 2015;Mossoux et al 2016;Yuan & Wang 2016;Zhang et al 2017;Yuan et al 2018;Boyce et al 2019;Haggard et al 2019).…”

“…Possible sources of flares include magnetic reconnection, stochastic acceleration, shocks from jets or the accretion flow, or even tidal disruption of asteroids (Markoff et al 2001;Liu & Melia 2002;Yuan et al 2003;Liu et al 2004;Čadež et al 2008;Kostić et al 2009;Zubovas et al 2012;Dibi et al 2014Dibi et al , 2016Ball et al 2016Ball et al , 2018 whereas the responsible radiation mechanisms are likely synchrotron or synchrotron self-Compton in nature (Marrone et al 2008;Dodds-Eden et al 2009;Eckart et al 2009;Witzel et al 2012;Nowak et al 2012;Barriere et al 2014;Neilsen et al 2015;Ponti et al 2017;Zhang et al 2017) but not inverse-Compton where the photons that get up-scattered to X-rays come from an external region (Boyce et al 2019). Flares share a consistent spectrum with a photon index Γ ∼ 2 (Nowak et al 2012;Neilsen et al 2013;Ponti et al 2017;Yuan et al 2018) and their timescale of minutes to hours points to an origin at ∼ 10's of Schwarzschild radii Quataert 2003).…”

Stellar Rotation in the Gaia Era: Revised Open Clusters’ Sequences

“…& Egeland 2019;van Saders et al 2019); and lastly, the importance of all of the above in regards to the deep connection between rotation and the various aspects of stellar activity (e.g., coronal X-ray and chromospheric Ca ii and Hα emission, UV excess, flares;Stelzer et al 2013;Zhang et al 2020;Dixon et al 2020;Ilin et al 2020; Godoy-Rivera et al 2020, submitted), and its dependence on stellar structure, the presence of a tachocline, and the evolutionary phase (e.g.,Wright et al 2018;Lehtinen et al 2020). …”

天体物理中自组织临界现象的观测和模拟

“…对恒星磁活动和恒星爆发的研究目前已成为一 个热点研究领域, 包括恒星磁场演化 [71,72,73,74,75] 、恒 星耀斑 [76,77,78,79,80] 和星冕物质抛射 [81,82,83,84] [68,85] ), 可观测的恒星耀斑的能量范围在 10 32 -10 38 erg [86] ,因而可称之为"超级耀斑". 近年来随 着 Kepler 和 TESS 卫星的陆续发射,对恒星耀斑的研 究也越来越多.…”