4. Concluding remarks
As mentioned in the introduction, the problem is to determine whether the collisions with H atoms depolarize or not the ground state of the Na atoms. As the linear polarization of the ground state is due to an optical pumping process by the radiation field, the collisional rates have to be compared to BJ where B is the Einstein coefficient in absorption and J is the mean intensity of the radiation field (Landi Degl'Innocenti 1999). Assuming that where is the spontaneous emission rate, the collisional depolarization occurs if . As the coefficients are of the order of magnitude as and assuming a temperature of 5000 K, we can conclude that the hydrogen density cannot be larger than a fraction of in the region of formation of the and lines.
Of course, this upper limit of is just a crude estimate, and the correct diagnostic requires the resolution of the non LTE multi-level rate equations that govern the temporal evolution of the atomic density matrix (Trujillo Bueno & Landi Degl'Innocenti 1997). This implies that all the inelastic collision cross sections that contribute to the population of the Na level are known. This work is in progress.
© European Southern Observatory (ESO) 2000
Online publication: June 26, 2000