Temperature stratification in a molecular shock: Analysis of the emission of H₂ pure rotational lines in IC443G

¹ Laboratoire de Physique de l'École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 75005 Paris, France
e-mail: pierre.dellova@ens.fr
² Observatoire de Paris, PSL University, Sorbonne Université, LERMA, 75014 Paris, France
³ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, 75014 Paris, France
⁴ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁵ William H. Miller III Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
⁶ Universities Space Research Association, NASA Ames Research Center, Moffett Field, CA 94035, USA
⁷ Max Planck Institute for Radio Astronomy, 53121 Bonn, Germany
⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁹ Observatoire de Paris, PSL Research University, LUTH, 5 Place J. Janssen, 92195 Meudon, France

Received: 15 September 2023
Accepted: 8 February 2024

Abstract

Context. Supernovae remnants (SNRs) represent a major source of feedback from stars on the interstellar medium of galaxies. During the latest stage of supernova explosions (which lasts 10–100 kyr), shock waves produced by the initial blast modify the chemistry of gas and dust, inject kinetic energy in the surroundings, and may alter star formation characteristics. Simultaneously, γ-ray emission is generated by the interaction between the ambient medium and cosmic rays, in particular those locally accelerated in the early stages of the explosion.

Aims. We aim to estimate the total molecular mass, local density, and total column density of H₂ and the temperature structure in a shocked clump interacting with the supernova remnant IC443 located in a region where cosmic rays interact with the interstellar medium. Measuring the mass of the dense and neutral component of the medium is a prerequisite to understanding the chemistry, energetics, and GeV to TeV γ-ray emission.

Methods. Assuming that the emission of H₂ pure rotational lines is produced by a collection of gas layers with variable temperature, we compared Spitzer/IRS emission maps for the ν = 0–0 S(0) to S(7) lines with a thermal admixture model. Our description is based on a power-law distribution of thermalized components with temperatures varying between T_min = 25 K and T_max = 1500 K.

Results. Our thermal admixture model allows the level populations of H₂ to be described by a power-law distribution dN = ΛT^−ΓdT, with Γ ~ 2.2−4.7. We measured a total mass M_H2 = 220₋₈₀⁺¹¹⁰ M_⊙ across the Spitzer/IRS field of observations.

Conclusions. Our analysis shows that an estimate of the cold molecular gas temperature is paramount to accurately constraining the H₂ mass, although the mass remains affected by significant uncertainties due to the assumptions on the gas temperature distribution.