Astrophysical lasers operating in optical Fe II lines in stellar ejecta of η Carinae

Abstract:
After the discovery of space masers based on OH radicals (Weaver et al.cite{Wea65}) and H<SUB>2</SUB>O (Cheung et al. cite{Che69}) such microwavelasers have been found to work in more than 100 molecular species(Elitzur cite{Eli92}; Townes cite{Tow97}), as well as in highlyexcited H atoms (Strelnitski et al. cite{Str96}). In the IR region (10mum), the effect of stimulated emission of radiation in the CO<SUB>2</SUB>molecule has been discovered in the Martian and Venus' atmospheres (Betzet al. cite{Bet76}; Mumma et al. cite{Mum81}). We report here on thediscovery of laser action in the range 0.9-2 μm in several spectrallines of Fe II, which are associated with transitions from``pseudo-metastable'' states populated by spontaneous transitions fromLyα pumped Fe II levels. The intense Lyα radiation is formedin the HII region of gas condensations close to the star η Car. Thelaser transitions form together with spontaneous transitions closedradiative cycles, one of which includes the extremely bright 2507/09Å lines. Closed radiative cycles, together with an accidentalmixing of energy levels, may provide an explanation of the abnormalintensities of these UV non-lasing lines. Using the complicated energylevel diagram of Fe II we present those peculiar features, which areessential for the inverted population and laser effect: the pumping, thelevel mixing, and the ``bottle neck'' for spontaneous decay. The laseraction is a new indicator of non-equilibrium and spatiallynon-homogeneous physical conditions as well as a high brightnesstemperature of Lyα in ejecta from eruptive stars. Such conditionsare very difficult to probe by existing methods, and we propose somefuture experiments. The fact, that the lasing near-IR lines appear inthe spectrum with about the same intensity as non-lasing lines isdiscussed and compared with the situation in masers.