helium 1083 nm line

The helium 1083 nm line (metastable helium line, sometimes referred to as the helium 1080 nm line) is an absorption line of interest in the study of extra-solar planet atmospheres. It is a near infrared line but within visible-light atmospheric window, and little affected by the interstellar medium, thus useful for ground observation. Its detection through transmission spectroscopy of the planet's atmosphere (during a transit) is an indication of atmospheric escape of helium.

The 1083 nm line is produced when helium atoms in a particular metastable state of excitation absorb photons, raising the energy level of one electron in each such atom. This helium metastable state includes an electron in the atom's lowest electron shell and the other in its next higher shell. Such a configuration of electrons is metastable specifically if the two electrons have parallel spin, and given the Pauli exclusion principle, the excited electron cannot relax to the ground state in the straightforward manner. In consequence, its relaxation is much less probable, with a half-life of over an hour (rather than a fraction of a second). Some atoms enter this metastable state during recombination of singly ionized atoms. When a significant number of the atoms are entering this metastable state, a 1083 nm absorption line is produced in the continuum emission of the star passing through the planet's atmosphere. Atoms in the ground state are not in a position to absorb such photons and thus produce no such a line, so the line gives us the chance to detect the presence of continual ionization, which commonly occurs during the conditions producing atmospheric escape.

Atmospheric escape can be modeled using plausible values of temperature, pressure and constituents, that calculate how much helium would be ionized and how much would be in this metastable state, which would be reflected by the depth of the line.