W49B’s shocked clouds point to intense cosmic-ray ionization

The molecular clouds beside supernova remnant W49B carry two different records of the explosion’s aftermath. Their motions reveal gas disturbed by shocks. Their chemistry suggests exposure to unusually intense cosmic rays: energetic particles capable of penetrating molecular material and knocking electrons from its constituents.

In a preprint submitted to The Astrophysical Journal, Wenjin Yang and colleagues identify twelve compact regions of shocked molecular gas on W49B’s western side. Modeling six of those regions points to cosmic-ray ionization rates roughly 1,000–10,000 times the standard Galactic value adopted in the paper. That is an inference from molecular emission and chemical simulations, not a direct measurement of the particles themselves.

Separating shocked gas from overlapping clouds

The team used ALMA and its Atacama Compact Array, supplemented by APEX and other single-dish observations. They measured several rotational transitions—emission produced as molecules change rotational energy—of carbon monoxide, CO, and the positively charged molecular ion HCO+.

Different transitions respond differently to temperature and density. Observing several therefore provides more information than measuring a single line. Combining telescope arrays with single-dish data also helps recover extended emission that the arrays can miss.

In the twelve western regions, molecular lines have full widths at half maximum of about 25–60 kilometres per second. These broad profiles show gas moving over a substantial range of speeds, supporting an interaction between the remnant and molecular clouds. The molecular emission overlaps infrared emission from hydrogen molecules, but generally sits away from radio and ionized-iron brightness peaks. The different tracers appear to reveal different shock components rather than one identical layer.

Overlapping foreground gas complicates the spectra. For detailed modeling, the researchers selected six regions with prominent emission above a line-of-sight velocity of +75 kilometres per second, where unrelated cloud contamination is substantially reduced. These velocities are not direct measurements of shock speed: viewing angle and unresolved motions also shape the profiles.

Why the molecular mixture matters

Using RADEX, a program that models molecular excitation and radiation without assuming local thermodynamic equilibrium, the authors inferred molecular-hydrogen densities of approximately 50,000–160,000 molecules per cubic centimetre.

They also estimated the relative column densities of HCO+ and CO: the quantities of each molecule along the viewing direction. Their inferred HCO+/CO ratios range from 0.00042 to 0.00145, unusually high compared with the other interacting remnants discussed in the paper. These abundance estimates should not be confused with directly observed brightness ratios, which also depend on excitation and radiation transfer.

Ionization offers a chemical explanation. Ionizing molecular hydrogen initiates reactions that can form HCO+, while energetic particles can also alter or reduce CO. The researchers tested their inferred ratios with Paris–Durham shock simulations that follow gas dynamics, temperature and chemical reactions.

Within the explored models, reproducing the ratios requires cosmic-ray ionization rates of a few times 10⁻¹⁴ to about 10⁻¹³ per second per hydrogen molecule. Their separate estimate suggests X-rays contribute to ionization but are unlikely to dominate under the assumed shielding conditions.

Faster-moving gas is not chemically identical

Two regions show increasing HCO+/CO brightness ratios in their high-velocity wings. For one, B12, modeling two velocity intervals gives comparable temperatures and densities but an approximately 50% higher molecular column-density ratio in the faster-moving gas.

That favors a chemical difference over excitation alone as the explanation. It does not uniquely identify the cause. Several unresolved shocks passing through gas of different densities, spatial changes in cosmic-ray ionization, or both could produce the pattern.

Elsewhere, nearly constant ratios allow a common ionization rate. A conditional transport calculation suggests that low-energy protons would need replenishment and transport beyond simple carriage with the downstream gas to cross the modeled shocked layer before losing their energy. The layer itself remains unresolved.

Strong chemistry does not mean dominant heating

For one representative clump, the authors estimate that its bulk kinetic-energy reservoir substantially exceeds the accumulated heating supplied by cosmic rays and X-rays, with cosmic-ray heating larger than X-ray heating. This is not a direct division of the explosion’s energy: a present-day motion reservoir is being compared with heating integrated over uncertain exposure times.

The ionization inference has important limits too. Gas temperatures are poorly constrained in several regions, and the shock simulations omit both external ultraviolet radiation and ultraviolet light generated by shocks. Those omissions could affect the chemistry. Eastern clouds show narrow lines and too few strong transitions for similarly reliable abundance estimates.

The preprint therefore supports a specific picture, not a universal verdict: shocks strongly disturb W49B’s western molecular gas, while cosmic rays plausibly account for much of its unusual chemistry. The exact ionization rates remain model-dependent.