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Can Shock Waves Create the Conditions for Molecule Formation?

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spitzer space telescope infrared image of L1157
This infrared image from the Spitzer Space Telescope shows the dark, dusty cloud Lynds 1157, which hosts young protostars. [NASA/JPL-Caltech/UIUC]

The dark, dusty clouds surrounding young, hot protostars are the sites of molecule formation. What can new radio observations tell us about the potential for molecule formation in the shocked surroundings of a nearby protostar system?

Making Molecules

A visible-light image of the interstellar dark cloud Lynds 1157. Infrared or radio observations are needed to reveal the young stars hidden by the dust. [NASA/JPL-Caltech/AURA]

Over the past century, astronomers have discovered more than a hundred kinds of molecules in space. Exactly how these molecules form and survive in the cold, tenuous gas of the interstellar medium is an active area of research. One of several ways that molecules are thought to form is in the wake of a shock wave, which condenses and warms the interstellar medium, helping lone atoms link up in the vastness of space.

Shock waves can be produced by outflows from newly forming stars called protostars, which are still wrapped in dense clouds of gas and dust. Luckily, infrared and radio observations allow us to draw back this dusty curtain and peer into the birthplaces of young stars and watch as they collect gas and shoot out jets of material. In a new publication, a team led by Siyi Feng (冯思轶) from Xiamen University presents new radio data that probes the surroundings of a young protostellar system at the heart of the dark cloud Lynds 1157 — one of the best places to study how shocks impact interstellar chemistry.

maps of the Lynds 1157 jet in ammonia emission

Example maps of an outflowing jet from Lynds 1157 in two emission lines of ammonia. The shocks are located at the places labeled B0, B1, and B2, while smaller structures are labeled with additional letters. The protobinary is labeled “mm.” [Adapted from Feng et al. 2022]

Peering at Protostars

Previous observations of Lynds 1157 have shown that the region hosts organic molecules like methanol and cyanoacetylene — a clear sign of ongoing interstellar chemistry. What makes the region especially interesting is the series of shocks that have formed along a jet that flows outward from the central source, which is likely a protobinary system. Observations show that the outermost shock is 1,000 years old, while the inner shocks are younger, allowing us to study how the temperature and density of the gas changed over time as the shocks passed through.

Using the Karl G. Jansky Very Large Array, Feng and collaborators observed emission lines of ammonia (NH3) to make high-resolution maps of Lynds 1157 and measure how the temperature and density of the gas vary throughout the cloud.

Studying Shocks

maps of temperature, density, and the ratio of ortho to para ammonia

Maps of the mean temperature (left), density (center), and ratio of ammonia molecules in an excited state to those in an unexcited state (right). Click to enlarge. [Adapted from Feng et al. 2022]

The ammonia emission lines trace the jet as it moves outward from the central protobinary, and the observations show that the gas is warmest close to the protobinary, cooler farther out along the jet, and densest at the locations of the shocks. And at the locations of the shocks, the team found evidence for ammonia molecules in an excited state, a clear indication that the gas has been heated by the shocks.

The team’s observations show that the passage of shocks heated and compressed the gas, and that as the shocks moved outward, the gas cooled. This illustrates that shocks can provide the warm, dense environment needed for molecules to form. The measurements made in this work should enable detailed chemical modeling, allowing for an even better understanding of how shocks have transformed the gas around these young protostars and paved the way for molecule formation.

Citation

“A Detailed Temperature Map of the Archetypal Protostellar Shocks in L1157,” S. Feng et al 2022 ApJL 933 L35. doi:10.3847/2041-8213/ac75d7