Westerhout 40

Westerhout 40 or W40 (also designated Sharpless 64, Sh2-64, or RCW 174) is a star-forming region in the Milky Way located in the constellation Serpens. In this region, interstellar gas forming a diffuse nebula surrounds a cluster of several hundred new-born stars. The distance to W40 is 436 ± 9 pc (1420 ± 30 light years), making it one of the closest sites of formation of high-mass O-type and B-type stars. The ionizing radiation from the massive OB stars has created an H II region, which has an hour-glass morphology.

Dust from the molecular cloud in which W40 formed obscures the nebula, rendering W40 difficult to observe at visible wavelengths of light. Thus, X-ray, infrared, and radio observations have been used to see through the molecular cloud to study the star-formation processes going on within.

W40 appears near to several other star-forming regions in the sky, including an infrared dark cloud designated Serpens South and a young stellar cluster designated the Serpens Main Cluster. Similar distances measured for these three star-forming regions suggests that they are near to each other and part of the same larger-scale collection of clouds known as the Serpens Molecular Cloud.

The W40 star-forming region is projected on the sky in the direction of the Serpens-Aquila Rift, a mass of dark clouds above the Galactic plane in the constellations Aquila, Serpens, and eastern Ophiuchus. The high extinction from interstellar clouds means that the nebula looks unimpressive in visible light, despite being one of the nearest sites of massive star formation.

Like all star-forming regions, W40 is made up of several components: the cluster of young stars and the gaseous material from which these stars form (the interstellar medium). Most of the gas in W40 is in the form of molecular clouds, the coldest, densest phase of the interstellar medium, which is made up of mostly molecular hydrogen (H₂). Stars form in molecular clouds when the gas mass in part of a cloud becomes too great, causing it to collapse due to the Jeans instability. Stars usually do not form in isolation, but rather in groups containing hundreds or thousands of other stars, as is the case of W40.

In W40, feedback from the star cluster has ionized some of the gas and blown a bipolar bubble in the cloud around the cluster. Such feedback effects may trigger further star-formation but can also lead to the eventual destruction of the molecular cloud and an end of star-formation activity.

A cluster of young stars lies at the center of the W40 HII region containing approximately 520 stars down to 0.1 solar masses (M_☉). Age estimates for the stars indicate that the stars in the center of the cluster are approximately 0.8 million years old, while the stars on the outside are slightly older at 1.5 million years. The cluster is roughly spherically symmetric and is mass segregated, with the more massive stars relatively more likely to be found near the center of the cluster. The cause of mass segregation in very young star clusters, like W40, is an open theoretical question in star-formation theory because timescales for mass segregation through two-body interactions between stars are typically too long.

The cloud is ionized by several O and B-type stars. Near-infrared spectroscopy has identified one late-O type star named IRS 1A South, and 3 early B-type stars, IRS 2B, IRS 3A, and IRS 5. In addition, IRS 1A North and IRS 2A are Herbig Ae/Be stars. Radio emission from several of these stars is observed with the Very Large Array, and may be evidence for ultra-compact H II regions.