Stellar Chromospheric Variability

Grijs, Richard de; Kamath, D.

doi:10.3390/universe7110440

Cited by 10 publications

(4 citation statements)

References 269 publications

(390 reference statements)

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“…Numerous efforts have therefore been dedicated to investigate different activity diagnostics, especially at spectral ranges accessible from the Earth. These include the Hydrogen Balmer lines, Na D1 and D2, the Ca II infrared triplet and He I D3, among others (e.g., Maldonado et al, 2019;Thompson et al, 2020;de Grijs and Kamath, 2021).…”

Section: Introductionmentioning

confidence: 99%

“…Similarly, the Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs (CARMENES, Quirrenbach et al, 2016) does not observe in the Ca II K range but observes at wavelengths longer than 520 nm. The small (compared to other ranges) contamination from telluric and photosperic lines in the Ca II infrared triplet range, also facilitates the interpretation of measurements of these lines (de Grijs and Kamath, 2021). Previous studies indicate that the Ca II infrared triplet lines are indeed excellent activity indicators, meaning that indices extracted from these lines are positively correlated with other activity indicators as those derived from the Ca II UV lines (Martin et al, 2017;Long et al, 2021;Lanzafame et al, 2023), especially for late type stars (e.g., Dempsey et al, 1993;Zhang et al, 2016).…”

Section: Introductionmentioning

confidence: 99%

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Sun-as-a-star variability of Hα and Ca II 854.2 nm lines

Zills,

Criscuoli,

Bertello

et al. 2024

Front. Astron. Space Sci.

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Studies of stellar magnetic fields mostly rely on proxies derived from chromospheric lines, typically forming in the UV and shorter wavelengths and therefore accessible only from space based observatories. Even Ca II K or H observations, forming in regions accessible from the ground, are not always available. As a result, there is a crucial need to explore alternative activity proxies to overcome the limitations posed by observational constraints. Using sun-as-a-star observations acquired with the ISS at SOLIS we investigated the correlation between the Ca II K emission index and indices derived from the Hα 656.3 nm and Ca II 854.2 nm lines, which are well known chromospheric diagnostics. We found that both the core intensities and widths of the two lines are positively correlated with the Ca II K emission index (ρ ≳ 0.8), indicating their suitability as reliable indicators of magnetic activity, the width of the Hα line showing the highest correlation (ρ = 0.9). We also found that such correlations vary with the activity cycle. Specifically, during the analyzed cycle 24, the correlations with the Ca II K index varied 14% for the Hα width, 33% for the Hα core intensity, and doubled for the two Ca II 854.2 nm line indices. These results suggest that, among the investigated indices, the Hα width best traces magnetic activity. Results are discussed at the light of current knowledge of the formation heights of the two lines, and of spatially resolved solar observations.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Sun-as-a-star variability of Hα and Ca II 854.2 nm lines

Zills,

Criscuoli,

Bertello

et al. 2024

Front. Astron. Space Sci.

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“…Chromospheric indices, such as those derived from the Ca II H&K, Mg II h&k, and Hα lines (a ratio of an average line-core flux to average line-wing flux), are often employed in many of the applications described above, as they are known to track the magnetic activity of the Sun and other stars very well (e.g., Egeland et al 2017;Linsky 2017;Lovric et al 2017;de Grijs & Kamath 2021). In particular, in the case of the Sun, chromospheric indices are fundamental to reconstructing the solar irradiance (the radiant solar energy impinging at the top of the Earth atmosphere) at times when irradiance measurements were not available (e.g., Coddington et al 2016;Yeo et al 2017a;Penza et al 2022), thus allowing for production of the long and uniform time series necessary to estimate the effects of solar variability on the Earth's climate (e.g., Lean 2017).…”

Section: Introductionmentioning

confidence: 99%

Understanding Sun-as-a-Star Variability of Solar Balmer Lines

Criscuoli

Марченко

DeLand

et al. 2023

ApJ

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Precise, high-cadence, long-term records of stellar spectral variability at different temporal scales lead to better understanding of a wide variety of phenomena including stellar atmospheres and dynamos, convective motions, and rotational periods. Here, we investigate the variability of solar Balmer lines (Hα, β, γ, δ) observed by space-borne radiometers (OSIRIS, SCIAMACHY, OMI, and GOME-2), combining these precise, long-term observations with high-resolution data from the ground-based NSO/ISS spectrograph. We relate the detected variability to the appearance of magnetic features on the solar disk. We find that on solar-rotational timescales (about 1 month), the Balmer line activity indices (defined as line-core to line-wing ratios) closely follow variations in the total solar irradiance (which is predominantly photospheric), thus frequently (specifically, during passages of sunspot groups) deviating from behavior of activity indices that track chromospheric activity levels. On longer timescales, the correlation with chromospheric indices increases, with periods of low correlation or even anticorrelation found at intermediate timescales. Comparison of these observations with estimates from semiempirical irradiance reconstructions helps quantify the contributions of different magnetic and quiet features. We conclude that both the lower sensitivity to network and in part the higher sensitivity to filaments and prominences, may result in complex, time-dependent relationships between Balmer and other chromospheric indices observed for the Sun and solar-like stars. The fact that core and wings contribute in a similar manner to the variability, and current knowledge of Balmer-lines formation in stellar atmospheres, supports the notion that Balmer line core-to-wing ratio indices behave more like photospheric rather than chromospheric indices.

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“…Therefore, we expect young, rapidly rotating late-type stars to exhibit significant magnetic activity. There exist strong correlations among age, stellar activity and rotation period (Hall 2008;de Grĳs & Kamath 2021). Magnetic activity is closely linked to variations in the stellar magnetic field, and it is therefore connected to the structure of the stellar subsurface zone, stellar rotation and the regeneration of the magnetic field through self-sustaining dynamo activity.…”

Section: Introductionmentioning

confidence: 99%

Statistics of BY Draconis Chromospheric Variable Stars

Chahal,

de Grijs,

Kamath

et al. 2022

Preprint

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We present an extensive catalogue of BY Draconis (BY Dra)-type variables and their stellar parameters. BY Dra are mainsequence FGKM-type stars. They exhibit inhomogeneous starspots and bright faculae in their photospheres. These features are caused by stellar magnetic fields, which are carried along with the stellar disc through rotation and which produce gradual modulations in their light curves (LCs). Our main objective is to characterise the properties of BY Dra variables over a wide range of stellar masses, temperatures and rotation periods. A recent study categorised 84,697 BY Dra variables from Data Release 2 of the Zwicky Transient Facility based on their LCs. We have collected additional photometric data from multiple surveys and performed broad-band spectral energy distribution fits to estimate stellar parameters. We found that more than half of our sample objects are of K spectral type, covering an extensive range of stellar parameters in the low-mass regime (0.1-1.3 M ). Compared with previous studies, most of the sources in our catalogue are rapid rotators, and so most of them must be young stars for which a spin-down has not yet occurred. We subdivided our catalogue based on convection zone depth and found that the photospheric activity index, 𝑆 ph , is lower for higher effective temperatures, i.e., for thinner convective envelopes. We observe a broad range of photospheric magnetic activity for different spectral classes owing to the presence of stellar populations of different ages. We found a higher magnetically active fraction for K-than M-type stars.

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Stellar Chromospheric Variability

Cited by 10 publications

References 269 publications

Sun-as-a-star variability of Hα and Ca II 854.2 nm lines

Sun-as-a-star variability of Hα and Ca II 854.2 nm lines

Understanding Sun-as-a-Star Variability of Solar Balmer Lines

Statistics of BY Draconis Chromospheric Variable Stars

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