Westerhout 40 or W40 (also designated Sharpless 64, Sh2-64, or RCW 174) is a star-forming region in the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

located in the constellation

Serpens Serpens ( grc, , , the Serpent) is a constellation in the northern celestial hemisphere. One of the 48 constellations listed by the 2nd-century astronomer Ptolemy, it remains one of the 88 modern constellations designated by the International ...

. In this region, interstellar gas forming a

diffuse nebula A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming region ...

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 star A B-type main-sequence star (B V) is a main-sequence (hydrogen-burning) star of spectral type B and luminosity class V. These stars have from 2 to 16 times the mass of the Sun and surface temperatures between 10,000 and 30,000 K. B-type star ...

s. The ionizing radiation from the massive OB stars has created an

H II region An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds ...

, which has an hour-glass morphology.

Dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ho ...

from the

molecular cloud A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydroge ...

in which W40 formed obscures the nebula, rendering W40 difficult to observe at visible wavelengths of light. Thus,

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

, and

radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmi ...

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.

On the Sky

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 The galactic plane is the plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galactic poles'' usual ...

in the constellations Aquila, Serpens, and eastern Ophiuchus. The high extinction from interstellar clouds means that the nebula looks unimpressive in

visible light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

, despite being one of the nearest sites of massive star formation. Sh2-64 map

Star Formation in W40

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 Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, ...

(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 In stellar physics, the Jeans instability causes the collapse of interstellar gas clouds and subsequent star formation, named after James Jeans. It occurs when the internal gas pressure is not strong enough to prevent gravitational collapse of ...

. 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.

Star cluster

cluster may refer to: Science and technology Astronomy * Cluster (spacecraft), constellation of four European Space Agency spacecraft * Asteroid cluster, a small asteroid family * Cluster II (spacecraft), a European Space Agency mission to study t ...

of young stars lies at the center of the W40 HII region containing approximately 520 stars down to 0.1 solar masses (). 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 B type or Type B may refer to: Astronomy * B-type asteroid, a type of relatively uncommon type of carbonaceous asteroid * B-type giant, a type of blue giant star * B-type star, a type of star Biology * B type blood, a type in the ABO blood grou ...

. 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 The Karl G. Jansky Very Large Array (VLA) is a centimeter-wavelength radio astronomy observatory located in central New Mexico on the Plains of San Agustin, between the towns of Magdalena and Datil, ~ west of Socorro. The VLA comprises twent ...

, and may be evidence for ultra-compact H II regions. Excess light in the

indicates that a number of stars in the cluster have

circumstellar disks A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the re ...

, which may be in the process of forming planets.

Millimeter 330px, Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 meter to 1 millimeter. The millimetre (American and British English spelling differences#-re, -er, ...

observations from the

IRAM 30m telescope The IRAM 30-meter telescope is a radio telescope used for astronomical observations in the millimeter range of wavelengths, operated by the Institut de radioastronomie millimétrique, Institute for Radio Astronomy in the Millimeter Range (IRAM) an ...

show 9 Class-0 protostars in the Serpens South region and 3 Class-0 protostars in W40, supporting the view that the region is very young and actively forming stars.

Interstellar medium

W40 lies in a molecular cloud with an estimated mass of 10⁴ . The core of the molecular cloud has a shape like a shepherd's crook and is currently producing new stars. The cluster of OB and pre–main-sequence (PMS) stars lies just eastward of the bend in this filament. The cloud core was also observed in radio light produced by CO, which allows the mass of the core to be estimated at 200–300 . A weak, bipolar outflow of gas flows out of the core, likely driven by a young stellar object, with two lobes differing in velocity by 0.5

km/s The metre per second is the unit of both speed (a scalar quantity) and velocity (a vector quantity, which has direction and magnitude) in the International System of Units (SI), equal to the speed of a body covering a distance of one metre in a ...

It was in this region where the striking prevalence of filamentary cloud structures seen by

ESA , owners = , headquarters = Paris, Île-de-France, France , coordinates = , spaceport = Guiana Space Centre , seal = File:ESA emblem seal.png , seal_size = 130px , image = Views in the Main Control Room (120 ...

's Herschel Space Observatory was first noted. These filaments of cloud have dense "cores" of gas embedded within them—many of which are likely to gravitationally collapse and form stars. The Herschel results for this region, and subsequently reported results for other star-forming regions, imply that fragmentation of molecular-cloud filaments are fundamental to the star-formation process. The Herschel results for W40 and the Aquila Rift, compared to those for molecular clouds in the Polaris region, suggest that star-formation occurs when the linear density (mass per unit length) 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 star-formation rate in the Polaris clouds. These observational results complement computer

simulations A simulation is the imitation of the operation of a real-world process or system over time. Simulations require the use of models; the model represents the key characteristics or behaviors of the selected system or process, whereas the s ...

of star-formation, which also emphasize the role that molecular-cloud filaments play in the birth of stars. Observations by the space-based

Chandra X-ray Observatory The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources 1 ...

have shown a diffuse X-ray glow from the H II region, which is likely due to the presence of a multi-million Kelvin plasma. Such hot plasmas can be produced by winds from massive stars, which become shock heated.

Gallery

Image:W40_star-forming_region.jpg, Spitzer Space Telescope mosaic image of W40. Image:Infrared Dark Cloud (IRDC) in the W40 region.jpg, A close-up of an

IRDC IRDC (Iraq Reconstruction and Development Council) was a group of 150 Iraqi expatriates —most of whom were American citizens— recruited by the Pentagon in March 2003 to assist the Coalition Forces with post-war reconstruction planning after th ...

in the Spitzer image. Image:A Chandra image of the W40 star-forming region.jpg, The W40 cluster in the X-ray. Image: Infrared_Image_of_Dark_Cloud_in_Aquila.jpg , W40 and its environs seen by Herschel. Image: W40 - Sofia.jpg ,

SOFIA Sofia ( ; bg, София, Sofiya, ) is the capital and largest city of Bulgaria. It is situated in the Sofia Valley at the foot of the Vitosha mountain in the western parts of the country. The city is built west of the Iskar river, and h ...

view of the center of W40. Image:Sh2-64 (= Westerhout 40) from Mount Lemmon SkyCenter.jpg, An optical image of W40 from Mount Lemmon SkyCenter.

References