Collisional depolarization of molecular lines. Application to the SiO+H isotropic collisions

Abstract

This paper reports the first theoretical study on the collisional depolarization and polarization transfer for rovibrational levels of diatomic molecules in the infinite-order-sudden approximation. As an application, we compute depolarization and polarization transfer rates for rotational levels J = 1,..., 99 within the vibrational quantum numbers v = 0, 1, 2, 3 of the SiO molecules in collisions with neutral hydrogen atoms for temperatures 400 ≤ T ≤ 1500 K. Analytical expressions of the collisional depolarizing rates of rovibrational levels as a function of local temperature and rotational number J are obtained. We note that the collisional depolarization rates decrease when J increases and that the fractional effect of the collisions on the J-levels can be characterized by a simple ratio. Especially for high J values, we find that alignment and population transfer rates between J to J + 1 inside the same vibrational number v are higher than the elastic depolarizing rates. This work might help to gain more understanding of the role of isotropic collisions in the formation of the very interesting polarized lines of C 2 and MgH observed in the solar photosphere.

Solar turbulent magnetic fields: surprisingly homogeneous distribution during the solar minimum

“…Another possibility is depolarization owing to collisions. There are indications that collision rates depend on the total angular momentum J and can be higher for lower J numbers (Derouich 2006). In this case, the depolarization would be stronger for the R-triplet than for the P-triplet, which is in accord with our measurements.…”

Solar turbulent magnetic fields: Non-LTE modeling of the Hanle effect in the C2molecule

“…The numbers ρ J denote the factor between the collision rates. While Derouich (2006) predicted a factor of ∼5.5, we found that a lower factor is needed (see Table 1). Because we cannot distinguish between elastic and inelastic collisions in the code and the prediction was for elastic collisions only, this difference is well possible.…”

“…A possible explanation for their difference might be that collisions affect the triplets differently. For instance, Derouich (2006) predicted that collision rates should decrease with increasing J. Since J of our P lines (40 ≤ J ≤ 42) are significantly higher than those of the R lines (12 ≤ J ≤ 14), one can expect higher collision rates for the R-triplet.…”

“…Therefore, there are big uncertainties in their values as well as dependence on temperature and pressure. Following a theoretical development by Derouich (2006), we assumed that C 2 collision rates vary with temperature T and the hydrogen concentration N H as γ C ∝ T 0.3 N H . Then, we chose the branching coefficient δ th at the height of 220 km above τ 5000 = 1 as a free parameter ρ C = δ th (220 km).…”

“…When the collision rates of the R triplet are enhanced by a factor of ρ J compared to the P triplet, the R lines show additional depolarization. These J-dependent collisions were predicted by Derouich (2006) and were necessary to obtain good fits to the observed spectra. The parameters used to produce this figure are ρ C = 0.05, B = 7 and μ = 0.11. calculated R 2 and P amplitudes agreed with the observations for a non-zero value of ρ C .…”