Ammonia observations of bright-rimmed clouds: establishing a sample of triggered protostars

TL;DR

This study used ammonia line observations to identify and characterize star-forming regions within bright-rimmed clouds, confirming triggered star formation likely caused by external shocks and refining the sample to the most probable candidates.

Contribution

It provides the first ammonia-based physical characterization of bright-rimmed clouds, identifying those most likely undergoing triggered star formation due to external shocks.

Findings

Triggered sources show higher turbulent velocity dispersions.

Confirmed dense gas presence towards IRAS sources.

Refined the catalog to 15 star-forming, shock-influenced clouds.

Abstract

We observed 42 molecular condensations within previously identified bright-rimmed clouds in the ammonia rotational inversion lines NH3 (1,1), (2,2), (3,3) and (4,4) using the Green Bank Telescope in Green Bank, West Virginia. Using the relative peaks of the ammonia lines and their hyperfine satellites we have determined important parameters of these clouds, including rotational temperatures and column densities. These observations confirm the presence of dense gas towards IRAS point sources detected at submillimetre wavelengths. Derived physical properties allow us to refine the sample of bright-rimmed clouds into those likely to be sites of star formation, triggered via the process of radiatively-driven implosion. An investigation of the physical properties of our sources show that triggered sources are host to greater turbulent velocity dispersions, likely indicative of shock motions within the cloud material. These may be attributed to the passage of triggered shocks or simply the association of outflow activity with the sources. In all, we have refined the Sugitani et al. (1991) catalogue to 15 clouds which are clearly star-forming and influenced by external photoionisation-induced shocks. These sources may be said, with high confidence, to represent the best examples of triggering within bright-rimmed clouds.