The Elephant Trunk Nebula and the Trumpler 37 cluster: Contribution of triggered star formation to the total population of an HII region

TL;DR

This study quantifies the role of triggered star formation in the Elephant Trunk Nebula, revealing that over 14-25% of stars in the HII region may form through this process, significantly impacting galactic star formation.

Contribution

It provides the first comprehensive census of triggered stars in IC 1396A using X-ray, optical, and infrared data, and estimates the overall contribution of triggered star formation in the HII region.

Findings

Over 250 young stars identified, doubling previous counts.

Triggered star formation contributes over 14-25% of the total stellar population.

Radiation-driven implosion is the likely mechanism for star formation in IC 1396A.

Abstract

Rich young stellar clusters produce HII regions whose expansion into the nearby molecular cloud is thought to trigger the formation of new stars. However, the importance of this mode of star formation is uncertain. This investigation seeks to quantify triggered star formation (TSF) in IC 1396A (a.k.a., the Elephant Trunk Nebula), a bright rimmed cloud (BRC) on the periphery of the nearby giant HII region IC 1396 produced by the Trumpler 37 cluster. X-ray selection of young stars from Chandra X-ray Observatory data is combined with existing optical and infrared surveys to give a more complete census of the TSF population. Over 250 young stars in and around IC 1396A are identified; this doubles the previously known population. A spatio-temporal gradient of stars from the IC 1396A cloud toward the primary ionizing star HD 206267 is found. We argue that the TSF mechanism in IC 1396A is the radiation-driven implosion process persisting over several million years. Analysis of the X-ray luminosity and initial mass functions indicates that >140 stars down to 0.1 Msun were formed by TSF. Considering other BRCs in the IC 1396 HII region, we estimate the TSF contribution for the entire HII region exceeds 14-25% today, and may be higher over the lifetime of the HII region. Such triggering on the periphery of HII regions may be a significant mode of star formation in the Galaxy.