SIMBAD references

1996A&A...305..284H - Astronomy and Astrophysics, volume 305, 284-295 (1996/1-1)

Coronal X-ray emission of cool stars in relation to chromospheric activity and magnetic cycles.

HEMPELMANN A., SCHMITT J.H.M.M. and STEPIEN K.

Abstract (from CDS):

We study the relationship between the coronal X-ray emission of single, main-sequence F-K stars and the characteristics of their magnetic cycles. We use X-ray data primarily from the ROSAT all-sky survey (RASS) as well as data acquired by us in the ROSAT pointed program, and the published data of the Mt. Wilson CaII H+K monitoring program. According to their CaII H+K long-term variability characteristics, we divide the stars into three groups: non-variable, regular variable and irregular (chaotic) variable stars. We show that the regular and the irregular stars differ mainly in their Rossby-numbers (Ro): regular stars have almost always Ro<1 whereas the irregular group is characterized by Ro>1; further, the X-ray surface flux distributions differ significantly between these three groups. We discuss to what extent stars exhibiting constant Ca II fluxes can be considered "Maunder minimum" stars, and demonstrate - in a statistical sense - that cyclic chromospheric activity also implies cyclic coronal activity. From a reanalysis of the flux-flux relation between the calcium excess flux density ({DELTA}FCa) and FX, we find different relations between the regular and the constant stars on one hand and the irregular stars on the other hand. Performing regression analysis in the form of a power law, the coefficient κ is derived to be κ≃1 for constant and regular stars whereas κ≃2 for the more active irregular stars. We discuss our findings in the context of a transition from a nonlinear to a linear dynamo regime when going from irregular to regular stars.

Abstract Copyright:

Journal keyword(s): stars: activity - stars: late-type - stars: coronae - X-rays: stars

Simbad objects: 79

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2020.02.23-00:31:57

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