4. Discussion and conclusions
We have presented here results obtained using a relatively simple method which shows that the Li I resonance doublet may form in absorption in the spectrum of X-ray novae accretion discs. Its strength may allow direct detection with present observational facilities. No any emission component in the calculations of the line has been considered. Such emission components may arise if there is a "chromosphere-like" increase in the temperature distribution of the accretion disc atmosphere caused by irradiation or, in the outer optically thin parts of the disc. The first is important if the disc luminosity is high, , and the second case plays a dominant role if the disc luminosity is low, . As a consequence, our computations of equivalent widths for the doublet should be considered as upper limits, but they can be close to the real values when the luminosity of the disc has intermediate values 0.01 .
In conclusion, the performed exact calculations of the Li I resonance doublet have shown that this line may be strong enough to be detected in the disc spectra of X-ray novae during outburst decay, if 0.01 and if the Li abundance is sufficiently large (log N (Li) 3-4). The expected can be higher than 100 m in the most favourable cases, when of the disc is larger than 1011 cm and cos i 1. For example, we note that the Li I 6708 absorption component may be found in the spectrum of the X-ray nova GRO J0422+32, during the mini-outburst of December 1993. In this spectrum the equivalent width is about 10 in absorption. This is possible, of course, if the Li abundance is sufficiently large (log N (Li) 2).
In general, a major difficulty for detection will be the Doppler broadening of the line due to the rotation of the disc. The typical rotational broadening of Balmer lines in the spectra of discs is in the range 500-1000 km/s. Such broadening will be smaller for the Li I line, as it is expected to form near the outer radius of the disc, but it still will make spectroscopic observations of very high S/N ( 500 or better) and at least modest spectral dispersion (better than 1Å/pixel) necessary for reaching detections. The new generation of large diameter telescopes may provide the detection of lithium during outburst decay in systems with discs of suitably high inclination angle with respect to the line of sight.
© European Southern Observatory (ESO) 1998
Online publication: January 27, 1998