Interstellar anatomy of the TeV gamma-ray peak in the IC443 supernova remnant^★

¹ Laboratoire de Physique de l’École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université de Paris, 75005 Paris, France
e-mail: pierre.dellova@ens.fr
² Observatoire de Paris, PSL University, Sorbonne Université, LERMA, 75014 Paris, France
³ Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
⁴ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
⁵ Stratospheric Observatory for Infrared Astronomy, Universities Space Research Association, NASA Ames Research Center, MS 232-11, Moffett Field, 94035 CA, USA
⁶ University of Science and Technology of Hanoi, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Vietnam
⁷ Department of Physics and Astronomy, The University of Western Ontario, London, Ontario, N6A 3K7, Canada
⁸ Institut d’Astrophysique de Paris, CNRS UMR 7095, Sorbonne Université, 75014 Paris, France
⁹ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
¹⁰ Laboratoire Univers et Particules de Montpellier (LUPM) Université Montpellier, CNRS/IN2P3, CC72, place Eugène Bataillon, 34095, Montpellier Cedex 5, France
¹¹ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 4 May 2020
Accepted: 13 October 2020

Abstract

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

Aims. We study the stellar and interstellar contents of IC443, an evolved shell-type SNR at a distance of 1.9 kpc with an estimated age of 30 kyr. We aim to measure the mass of the gas and characterize the nature of infrared point sources within the extended G region, which corresponds to the peak of γ-ray emission detected by VERITAS and Fermi.

Methods. We performed 10′ × 10′ mapped observations of ¹²CO, ¹³CO J = 1–0, J = 2–1, and J = 3–2 pure rotational lines, as well as C¹⁸O J = 1–0 and J = 2–1 obtained with the IRAM 30 m and APEX telescopes over the extent of the γ-ray peak to reveal the molecular structure of the region. We first compared our data with local thermodynamic equilibrium models. We estimated the optical depth of each line from the emission of the isotopologs ¹³CO and C¹⁸O. We used the population diagram and large velocity gradient assumption to measure the column density, mass, and kinetic temperature of the gas using ¹²CO and ¹³CO lines. We used complementary data (stars, gas, and dust at multiple wavelengths) and infrared point source catalogs to search for protostar candidates.

Results. Our observations reveal four molecular structures: a shocked molecular clump associated with emission lines extending between −31 and 16 km s⁻¹, a quiescent, dark cloudlet associated with a line width of ~2 km s⁻¹, a narrow ring-like structure associated with a line width of ~1.5 km s⁻¹, and a shocked knot. We measured a total mass of ~230, ~90, ~210, and ~4 M_⊙, respectively, for the cloudlet, ring-like structure, shocked clump, and shocked knot. We measured a mass of ~1100 M_⊙ throughout the rest of the field of observations where an ambient cloud is detected. We found 144 protostar candidates in the region.

Conclusions. Our results emphasize how the mass associated with the ring-like structure and the cloudlet cannot be overlooked when quantifying the interaction of CRs with the dense local medium. Additionally, the presence of numerous possible protostars in the region might represent a fresh source of CRs, which must also be taken into account in the interpretation of γ-ray observationsin this region.