Westerhout 40

Lore & Background

W40 is projected on the sky in the direction of the Serpens-Aquila Rift, a mass of dark clouds above the Galactic plane. The high extinction from interstellar clouds makes the nebula unimpressive in visible light, despite being one of the nearest sites of massive star formation. Dust from the molecular cloud in which W40 formed obscures the nebula, so X-ray, infrared, and radio observations are used to study the star-formation processes within.

The star cluster at the center of the W40 H II region contains approximately 520 stars. Age estimates indicate that stars in the center are about 0.8 million years old, while those on the outside are slightly older at 1.5 million years. The cluster is roughly spherically symmetric and is mass segregated, with more massive stars more likely found near the center.

The cause of mass segregation in very young clusters like W40 is an open theoretical question. The cloud is ionized by several O and B-type stars, including one late-O type star (IRS 1A South) and three early B-type stars (IRS 2B, IRS 3A, IRS 5). IRS 1A North and IRS 2A are Herbig Ae/Be stars.

The interstellar medium in W40 includes a molecular cloud with an estimated mass of 10^4 solar masses. The core of the molecular cloud has a shepherd's crook shape and is currently producing new stars.

A weak, bipolar outflow of gas flows out of the core, likely driven by a young stellar object. It was in this region that the striking prevalence of filamentary cloud structures seen by ESA's Herschel Space Observatory was first noted. These filaments have dense cores embedded within them, many likely to gravitationally collapse and form stars.

Reader's Guide

W40 is notable as one of the closest sites of high-mass star formation, allowing detailed study of processes that are typically observed only at much greater distances. The region's significance is underscored by its role in revealing the fundamental importance of molecular-cloud filaments in star formation.

Herschel observations of W40 and the Aquila Rift, compared to those of the Polaris region, suggested that star formation occurs when the linear density of filaments exceeds a threshold making them susceptible to gravitational instability. This accounts for the high star-formation rate in W40 and the Aquila Rift, in contrast to the low rate in the Polaris clouds. These observational results complement computer simulations that also emphasize the role of filaments in star birth.

The region also hosts a young stellar cluster with evidence of mass segregation, a phenomenon whose cause in very young clusters remains an open theoretical question. Observations of circumstellar disks and Class-0 protostars support the view that the region is very young and actively forming stars. The presence of a diffuse X-ray glow from the H II region, likely due to a multi-million Kelvin plasma produced by winds from massive stars, further adds to the understanding of feedback effects in star-forming regions. W40 appears near to other star-forming regions—Serpens South and the Serpens Main Cluster—with similar distances suggesting they are part of the same larger-scale Serpens Molecular Cloud.

Did You Know?

• The ionizing radiation from massive OB stars has created an H II region with an hour-glass morphology.