Astrophysical Lasers Operating in optical Fe II Lines Lines in Stellar Ejecta of Eta Carinae

摘要

After the discovery of space masers based on OH radicals (Weaver et al, 1965)and H2O (Cheung et al, 1969) such microwave lasers have been found to work inmore than 100 molecular species (Elitzur, 1992; Townes, 1997), as well as inhighly excited H atoms (Strelnitski et al,1996). In the IR region (10 microns),the effect of stimulated emission of radiation in the CO2 molecule has beendiscovered in the Martian and Venus' atmospheres (Betz et al, 1976; Mumma etal, 1981). We report here on the discovery of laser action in the range 0.9-2micr. in several spectral lines of Fe II, which are associated with transitionsfrom "pseudo-metastable" states populated by spontaneous transitions fromLy-alpha pumped Fe II levels. The intense Ly-alpha radiation is formed in theHII region of gas condensations close to the star Eta Carinae. The lasertransitions form together with spontaneous transitions closed radiative cycles,one of which includes the extremely bright 2507/09 A lines. Closed radiativecycles, together with an accidental mixing of energy levels, may provide anexplanation of the abnormal intensities of these UV non-lasing lines. Using thecomplicated energy level diagram of Fe II we present those peculiar features,which are essential for the inverted population and laser effect: the pumping,the level mixing, and the "bottle neck" for spontaneous decay. The laser actionis a new indicator of non-equilibrium and spatially non-homogeneous physicalconditions as well as a high brightness temperature of Ly-alpha in ejecta fromeruptive stars. Such conditions are very difficult to probe by existingmethods, and we propose some future experiments. The fact, that the lasingnear-IR lines appear in the spectrum with about the same inten- sity asnon-lasing lines is discussed and compared with the situation in masers.