On the 1.7 [mu]m Fe ii and other natural lasers

• Published in: Monthly notices of the Royal Astronomical Society 2010-05, Vol.404 (3), p.1545
• Main Authors: Messenger, Stephen J, Strelnitski, Vladimir
• Format: Article
• Language: English
• Subjects: Astrophysics; Atoms & subatomic particles; Infrared imaging systems; Lasers; Stars & galaxies
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2010.16385.x

We confirm, by numerical modelling, the suggestion by Johansson & Letokhov (JL) that some infrared (IR) transitions of circumstellar Fe ii may be inverted by radiative pumping and can, in principle, act as lasers. In contrast with JL, we show that the inversion of populations can be created directly, by the diluted blackbody continuum of the nearby star, without the intermediacy of trapped Lyα photons. Besides η Car (the target object of the JL model), IR Fe ii lasers should be searched for in other objects with large detected amounts of Fe ii- quasars, supernova remnants, B[e] stars, T Tau stars, etc. However, assuming that the pumping cycle contains radiative steps (the most probable situation for an interstellar or circumstellar medium), we demonstrate that the probability of creating a high-gain natural laser is low (as opposed to the probability of creating a high-gain maser), which can explain the relative scarcity of the detected astrophysical lasers. In particular, the available indirect observational evidence indicates that the gain of the 1.7 μm Fe ii lasers in η Car must be low. IR observations with high spatial and spectral resolution are needed to verify this conclusion. [PUBLICATION ABSTRACT]