Astron. Astrophys. 324, 196-202 (1997)

1. Introduction

One of the main outstanding problems in stellar astrophysics is the nature of the non-radiative heating that gives rise to the chromospheric and coronal temperature inversion in late-type stars. In order to constrain heating theories it is necessary to have accurate and detailed semi-empirical models of the outer atmospheres of these stars. Strong resonant absorption lines such as the H I Lyman , Ca II HK, and Mg II hk lines have been the main chromospheric and transition region diagnostics because their cores form at relatively small column mass densities and, therefore, carry information about the gas above (for a review see Avrett (1990)). Also, late-type dwarfs can exhibit high levels of transient chromospheric activity, such as flares, and the cores of strong lines have been used to model the flaring state of the plasma (for a review see, Haisch et al. 1991).

In order to further constrain chromospheric models, it is necessary to develop additional spectral diagnostics for which the formation depends on the chromospheric structure in a way that differs somewhat from that of previous diagnostics, and, therefore, provide diagnostic complements. Also, because the Ca II HK and Mg II hk lines are either far out on the Wien side of the emergent flux distribution where the star is faint, or not accessible from the ground, it is desirable to find other diagnostics that are more observable. For example, Andretta et al. (1996) (henceforth ADB) have performed a theoretical investigation of the Na I D lines in a grid of chromospheric models corresponding to an early M star with a range of chromospheric activity levels. They show that the Na I D lines provide a useful diagnostic for semi-empirical chromospheric models. The purpose of this investigation is to study the behavior of the Ca I 4227 line and assess its utility as a chromospheric diagnostic. Recently Ca I 4227 has been used by Mauas & Falchi (1996) to track the time development of a flare on the dMe star AD Leo, and is already known to be responsive to the high activity state of flaring plasma.

In general, the cores of strong lines that form at relatively low gas densities high in the atmosphere differ greatly from those predicted by calculations done with the approximation of Local Thermodynamic Equilibrium (LTE) (see the review by Avrett 1990). The line under investigation may depend on radiative rates in other transitions of the atom, and these rates may be sensitive to the non-local radiation field. Therefore, a detailed description of the background radiation field may be important for an accurate solution of the non-LTE problem (see, for example, Mihalas (1978)). Many previous investigations of chromospheric line formation have either ignored line blanketing in the background radiation field, or, as in the case of ADB, have only included photospheric line blanketing. The multi-line chromospheric modelling of g Her (M6 III) by Luttermoser et al. 1994, which included Ca I 4227, included background line opacities. However, it is unclear if this line opacity corresponds to a photospheric or a chromospheric model, and, in any case, they do not compare the calculated line with and without the inclusion of line blanketing. Because strong lines form well above , they may be affected by line blanketing that arises in the chromosphere and transition region as well as blanketing in the photosphere. Short & Doyle (1996) have used the recently developed PHOENIX model atmosphere code of Allard & Hauschildt (1995) to compute the line opacity in a grid of M dwarf models that take into account the chromospheric and transition region temperature structure. We include this blanketing opacity in our calculations and investigate its effect on the predicted behavior of Ca I 4227.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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