Dense Molecular Clouds in the Crab Supernova Remnant

TL;DR

This study uses ALMA to image molecular clouds in the Crab Nebula, revealing their properties and interactions with energetic particles, and compares observations with models to understand their environment.

Contribution

It provides detailed ALMA observations of molecular clouds in the Crab Nebula and compares them with Cloudy models, highlighting a novel molecular phase in supernova remnants.

Findings

Seventeen molecular clouds identified with CO emission.

Observations consistent with cosmic-ray heated, mostly-neutral zones.

Molecular knots represent a new phase of the ISM in supernova remnants.

Abstract

Molecular emission was imaged with ALMA from numerous components near and within bright H2-emitting knots and absorbing dust globules in the Crab Nebula. These observations provide a critical test of how energetic photons and particles produced in a young supernova remnant interact with gas, cleanly differentiating between competing models. The four fields targeted show contrasting properties but within them, seventeen distinct molecular clouds are identified with CO emission; a few also show emission from HCO+, SiO and/or SO. These observations are compared with Cloudy models of these knots. It has been suggested that the Crab filaments present an exotic environment in which H2 emission comes from a mostly-neutral zone probably heated by cosmic rays produced in the supernova surrounding a cool core of molecular gas. Our model is consistent with the observed CO J=3-2 line strength. These molecular line emitting knots in the Crab present a novel phase of the ISM representative of many important astrophysical environments.