Detection of polarization from the E^4Π-A^4Πsystem of FeH in sunspot spectra

Abstract

Here we report the first detection of polarization signals induced by the Zeeman effect in spectral lines of the E^4Π-A^4Πsystem of FeH located around 1.6 $\mu$m. Motivated by the tentative detection of this band in the intensity spectrum of late-type dwarfs, we have investigated the full Stokes sunspot spectrum finding circular and linear polarization signatures that we associate with the FeH lines of the E^4Π-A^4Πband system. We investigate the Zeeman effect in these molecular transitions pointing out that in Hund's case (a) coupling the effective Landé factors are never negative. For this reason, the fact that our spectropolarimetric observations indicate that the Landé factors of pairs of FeH lines have opposite signs, prompt us to conclude that the E^4Π-A^4Πsystem must be in intermediate angular momentum coupling between Hund's cases (a) and (b). We emphasize that theoretical and/or laboratory investigations of this molecular system are urgently needed for exploiting its promising diagnostic capabilities.

11 pages, 4 figures, accepted for publication in Astrophysical Journal Letters

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