On inelastic hydrogen atom collisions in stellar atmospheres

TL;DR

This paper compares the classical Drawin formula with quantum mechanical calculations for inelastic hydrogen atom collisions in stellar atmospheres, highlighting significant discrepancies and the importance of quantum physics.

Contribution

It provides a detailed comparison showing the Drawin formula's inadequacy and emphasizes the need for quantum mechanical data in stellar abundance analyses.

Findings

Drawin formula overestimates collision rates.

Quantum mechanical calculations reveal essential physics missing in Drawin.

Discrepancies vary significantly between different transitions.

Abstract

The influence of inelastic hydrogen atom collisions on non-LTE spectral line formation has been, and remains to be, a significant source of uncertainty for stellar abundance analyses, due to the difficulty in obtaining accurate data for low-energy atomic collisions either experimentally or theoretically. For lack of a better alternative, the classical "Drawin formula" is often used. Over recent decades, our understanding of these collisions has improved markedly, predominantly through a number of detailed quantum mechanical calculations. In this paper, the Drawin formula is compared with the quantum mechanical calculations both in terms of the underlying physics and the resulting rate coefficients. It is shown that the Drawin formula does not contain the essential physics behind direct excitation by H atom collisions, the important physical mechanism being quantum mechanical in character. Quantitatively, the Drawin formula compares poorly with the results of the available quantum mechanical calculations, usually significantly overestimating the collision rates by amounts that vary markedly between transitions.