Abstract

Cool stars with convective envelopes of spectral types F and later tend to exhibit magnetic activity throughout their atmospheres. The presence of strong and variable magnetic fields is evidenced by photospheric starspots, chromospheric plages and coronal flares, as well as by strong Ca ii H+K and Hα emission, combined with the presence of ultraviolet resonance lines. We review the drivers of stellar chromospheric activity and the resulting physical parameters implied by the observational diagnostics. At a basic level, we explore the importance of stellar dynamos and their activity cycles for a range of stellar types across the Hertzsprung–Russell diagram. We focus, in particular, on recent developments pertaining to stellar rotation properties, including the putative Vaughan–Preston gap. We also pay specific attention to magnetic variability associated with close binary systems, including RS Canum Venaticorum, BY Draconis, W Ursae Majoris and Algol binaries. At the present time, large-scale photometric and spectroscopic surveys are becoming generally available, thus leading to a resurgence of research into chromospheric activity. This opens up promising prospects to gain a much improved understanding of chromospheric physics and its wide-ranging impact.

Highlights

  • To fully understand solar dynamo theory we have to consider the key ingredients through which a convecting, rotating and electrically conducting fluid can maintain a magnetic field over cosmic timescales

  • We briefly present two sets of dynamo models for the solar cycle discussed at length in the recent literature, the first based on mean-field theory and the second based on the Babcock–Leighton mechanism

  • We will address the chromospheric activity and variability associated with RS Canum Venaticorum (RS CVn), BY Draconis (BY Dra), W Ursae Majoris (W UMa) and Algol binary systems

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Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. In the solar chromosphere magnetic heating causes the temperature to increase to a plateau of approximately 7000 K, with the ambient density falling by orders of magnitude with respect to the region near the photosphere (see Figure 2) [20,21]. Chromospheric activity relates the changes observed at the stellar surface to changes occurring deeper into the stellar interior [42], e.g., in the context of non-convective mixing of abundances in advanced evolutionary stages, in particular for stars exhibiting solar-like dynamos ([43,44], and references therein). Following a high-level overview of the theory of stellar dynamo physics, we will discuss the broad range of observational diagnostics employed to trace and understand stellar chromospheric activity and variability: see Section 3.

The Solar Activity Cycle
The Solar Dynamo
Key Components of Dynamo Theory
Mean-Field Models
Babcock–Leighton Flux Transport Models
Spectroscopic Diagnostics
The Mt Wilson S Index
The Chromospheric Activity–Age Relation
Correlations with Starspot Number
The Vaughan–Preston Gap
Solar-Type FGK Stars
Late-Type Giant and Supergiant Stars
M Dwarfs and L-Type Brown Dwarfs
Close Binarity and Stellar Chromospheric Activity
Rs Canum Venaticorum Systems
By Draconis Systems
W Ursae Majoris Common Envelope Systems
Algol Binaries
Findings
Open Questions and Current Trends

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