The observed relation for the Li i 671 nm line is rather flat and falls in between the theoretical LTE and the NLTE relations, though closer to the latter. It seems that the results exclude LTE as a line-formation hypothesis, something that is of interest in principle, though of course not very surprising. The NLTE modelling of this paper is at best only marginally confirmed, however. It is the extreme weakness of the Li i line that makes definite conclusions difficult to make. This is both because the necessary binning makes it impossible to study the spread in the plot and because the equivalent widths are sensitive to errors in the continuum placement. Spectra with better spatial resolution and/or higher S/N would probably allow firmer conclusions if they could be acquired.
The idea that spatially resolved spectroscopy may be helpful in revealing NLTE effects is still viable. It should be tested on somewhat stronger lines of other elements, even though their NLTE modelling is intrinsically more complicated and uncertain than for the relatively simple Li i case. A coming paper (Kiselman, in preparation) will discuss similar solar observations and possible departures from LTE of a large set of Fe lines of different strengths.
© European Southern Observatory (ESO) 1998
Online publication: April 20, 1998