Rotationally Warm Molecular Hydrogen in the Orion Bar

TL;DR

This study develops advanced models of the Orion Bar PDR, incorporating magnetic fields and cosmic-ray effects, to explain the observed warm molecular hydrogen rotational temperatures that standard models cannot reproduce.

Contribution

The paper introduces comprehensive models including magnetic fields and cosmic-ray enhancements to better match observed warm H2 rotational lines in the Orion Bar.

Findings

Models with magnetic fields and cosmic-ray heating reproduce observed H2 line temperatures.

Inclusion of new collisional rate coefficients affects the predicted H2 excitation.

Standard assumptions on grain photoelectric emission are insufficient to explain observations.

Abstract

The Orion Bar is one of the nearest and best-studied photodissociation or photon-dominated regions (PDRs). Observations reveal the presence of H2 lines from vibrationally or rotationally excited upper levels that suggest warm gas temperatures (400 to 700 K). However, standard models of PDRs are unable to reproduce such warm rotational temperatures. In this paper we attempt to explain these observations with new comprehensive models which extend from the H+ region through the Bar and include the magnetic field in the equation of state. We adopt the model parameters from our previous paper which successfully reproduced a wide variety of spectral observations across the Bar. In this model the local cosmic-ray density is enhanced above the galactic background, as is the magnetic field, and which increases the cosmic-ray heating elevating the temperature in the molecular region. The pressure is further enhanced above the gas pressure in the H+ region by the momentum transferred from the absorbed starlight. Here we investigate whether the observed H2 lines can be reproduced with standard assumptions concerning the grain photoelectric emission. We also explore the effects due to the inclusion of recently computed H2 + H2, H2 + H and H2 + He collisional rate coefficients.