The structures and extents of the chromospheres of late-type stars

Abstract

The chromosphere is the region of a star, above what is traditionally defined as the stellar surface, from which photons freely escape. As the definition implies, this region is characterised by complexity, non-equilibrium, and specifying its structure is a vastly non-linear, nonlocal problem. In this work we are concerned with the chromospheres of late-type stars, objects of spectral type K to M, the thermodynamic structure, extent, and heating mechanisms of whose chromospheres are not well understood. We use a number of observational and computational methods in order to gain a detailed quantitative understanding of these chromospheres. We construct a model to compute the mm, thermal bremsstrahlung flux from the chromospheres of late-type objects, based on a number of simplifying assumptions concerning their thermodynamic structure. We compare this model with archival and recent observations, and find that the model is capable of reproducing the observed flux from objects of spectral type K to mid-M in the frequency range 100 GHz -350 GHz. We suggest that, while this model is quite simple, it provides an accurate reflection the structure of the chromosphere of late-type stars. We also make use of the unique insights into stellar structure provided by the eclipsing Aurigae binaries. We present the ‘rediscovery’ of periodic Si I emission in these objects. We hypothesise that this effect arises as a result of the UV radiation of the secondary object, which photo-ionizes Si I in the chromosphere of the primary. Since this UV radiation falls only on a portion of the visible hemisphere of the primary at a given phase, the line is periodic, and as a locally formed line in disk-averaged spectrum this is a novel and powerful diagnostic. While we rule out this line being a result of simple reflection, we expect it to have the same phase variation as broadband reflection. We construct a geometric model of the system, and find that the computed reflection curve matches the observed line periodicity very well. As the primary of Aurigae undergoes an eclipse it is possible to effectively resolve its chromosphere, which allows us to place direct constraints on its structure. However, despite the unique insights afforded by the study of this system, no semi-empirical model has yet been constructed. Using archival observations of Aurigae A we construct a full one-component, semi-empirical model of its chromosphere. As this binary system is detached its chromosphere is thought to be comparable to that of single stars - Velorum being a specific example - and we use the model constructed to comment on the structure of late-type chromospheres generally.