Akari infrared space telescope: Latest science highlights

Its on-board supply of liquid helium ran out on 26 August 2007, and the spacecraft entered a new mission phase. The liquid helium was required to keep Akari cold enough to observe in the far-infrared. The warm phase now uses the surviving instrument, the near-infrared mode of the infrared camera, which can operate under the warmer conditions provided by the on-board mechanical cooler for near-infrared observations.

Akari achieved its planned 'cold' lifetime of 550 days. During this time, it conducted an all-sky survey in the infrared, covering about 94% of the entire sky, with larger wavelength coverage and better spatial resolution than its predecessor, IRAS.

Splashes in the interstellar medium

The interstellar medium, a tenuous mix of gas and tiny solid dust particles, permeates space. As stars age, they spew out gas and dust in a flow called stellar wind, which eventually mixes with the interstellar medium. At the interface between stellar wind and the interstellar medium, physical conditions such as density and pressure change dramatically, creating what is called a bow shock.

Akari observations of Betelgeuse, a bright red supergiant star located in the constellation Orion about 200 light-years from Earth, show the star making a big splash by creating a bow shock as it crosses the interstellar medium. Researchers have found a strong flow of the interstellar medium around the star which originates from star-forming regions in Orion's Belt.

Stars condense out of the interstellar medium at birth, and old stars like Betelgeuse spew out matter into surrounding space, enriching the interstellar medium. This process is repeated by generations of stars and assists the chemical evolution of the Universe. Akari has found a number of such bow shocks and investigation into these processes will further our understanding of the cosmic recycling of matter.

Credit: Ueta et al, PASJ, 2008.

Akari observations of Betelgeuse, a bright red supergiant star located in the constellation Orion about 200 light-years from Earth, show the star making a big splash by creating a bow shock as it crosses the interstellar medium.

This is a three-colour composite image of Betelgeuse and its surroundings is composed of images taken at at 65 (blue), 90 (green) and 140 (red) micrometers taken by Akari's Far-Infrared Surveyor FIS.

The arc-like structure to the upper left direction of the star is the bow shock formed due to collision of stellar wind and the interstellar matter in the direction of the star's motion (from lower left to upper right). The diameter of the bow shock is about three light-years. The blue-white linear structure from lower left to upper right through the star is an effect of the instrument.

Researchers have found a strong flow of the interstellar medium around the star which originates from star-forming regions in Orion's Belt and has a velocity of 11 km/s. Betelgeuse is crossing this river at 30 km/s, while spewing out wind at 17 km/s.




Credit: JAXA.

This artist's impression shows how the bow shock structure is oriented with respect to Betelgeuse, the flow of the interstellar medium, and the Earth.

A discontinuity in density and pressure appears at the boundary where stellar wind from Betelgeuse collides into interstellar matter. Betelgeuse moves in space from lower right to upper left in this figure.

Mysterious missing dust

Globular clusters are spherical groups of a hundred thousand to a million stars that are found throughout our and other galaxies. A one-off star formation process in each system about 10 thousand million years ago triggered the formation of these globular clusters.

Aged stars often eject large amounts of gas and dust into interstellar space, which eventually forms a new generation of stars and planets, so scientists expected to detect cold dust in the 12 globular clusters observed by Akari. But high-sensitivity observations with the Far-Infrared Surveyor onboard Akari yielded no evidence of cold dust in any of the clusters.

One possibility is that the dust accreted on to the stellar surface. But such a process would take much longer than the lifetime of the cluster. The new Akari observations pose new questions to astronomers.

Credit: JAXA.

Three-colour composite of the Globular Cluster NGC 1261, composed of images taken at 4 (blue), 15 (green), and 90 (red) micrometers using Akari's near- and mid-Infrared Camera (IRC). The cluster is located in the constellation Horologium, about 53 000 light-years from us.

A cloud of stars seen in blue at the centre of the image is the globular cluster. Seven red sources around the clusters are all galaxies lying behind. These galaxies possess plenty of interstellar dust that emits brightly in the far-infrared. In contrast, only stars are seen in the globular cluster itself. Akari has confirmed that there is no dust present in NGC 1261.

Warm dust in supernova remnants

At the end of their lives, massive stars explode in a catastrophic explosion, returning a huge amount of energy and heavy elements into space. Scientists believe that the explosion destroys the surrounding interstellar dust grains, leaving behind a supernova remnant which can be studied to understand the explosion itself as well as its role on the evolution of the interstellar medium.

The study of interstellar dust is important because that very dust is a seed of another star as well as of a planet like Earth.

The Large Magellanic Cloud is a companion galaxy of the Milky Way located at a distance of about 160 000 light-years from us. Its relatively short distance and unique location provides a unique view of the entire galaxy from Earth and the possibility to study the interstellar medium.

Akari observations of about eight of the 20 reported supernova remnants in the region have revealed unexpected details.

Akari has found that supernova remnants in the Large Magellanic Cloud are surrounded by previously-unknown warm dust. This suggests that some dust grains survive the shock of the supernova explosion. Further analysis with the Akari data will greatly improve our knowledge on supernova remnants and their influence on their surroundings.

Credit: Seok et al., PASJ 2008.

Three-colour composite images of eight supernova remnants in the Large Magellanic Cloud composed of images taken at 7 (blue), 11 (green) and 15 (red) micrometers, with the near- and mid-Infrared Camera (IRC) onboard Akari. Contours indicate the intensity of the X-ray emission observed by the NASA's Chandra X-ray observatory. The line at the bottom of each image indicates a distance of 20 light-years.

Source: European Space Agency
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