Detection of Polarization from the E 4 - A 4 System of FeH in Sunspot Spectra

Here we report the first detection of polarization signals induced by theZeeman effect in spectral lines of the E^4\Pi-A^4\Pi system of FeH locatedaround 1.6 $\mu$m. Motivated by the tentative detection of this band in theintensity spectrum of late-type dwarfs, we have investigated the full Stokessunspot spectrum finding circular and linear polarization signatures that weassociate with the FeH lines of the E^4\Pi-A^4\Pi band system. We investigatethe Zeeman effect in these molecular transitions pointing out that in Hund'scase (a) coupling the effective Land\'e factors are never negative. For thisreason, the fact that our spectropolarimetric observations indicate that theLand\'e factors of pairs of FeH lines have opposite signs, prompt us toconclude that the E^4\Pi-A^4\Pi system must be in intermediate angular momentumcoupling between Hund's cases (a) and (b). We emphasize that theoretical and/orlaboratory investigations of this molecular system are urgently needed forexploiting its promising diagnostic capabilities.Comment: 11 pages, 4 figures, accepted for publication in Astrophysical Journal Letter