AKARI unveils the birth and death of stars

AKARI was launched on February 21st 2006 from Uchinoura Space Centre, Japan, and is now about half way to completing its first map of the sky in infrared light. UK astronomers, who are collaborating with Japanese, Korean and Dutch Partners in the far-infrared all sky survey experiment on the satellite, have been supported by the Particle Physics and Astronomy Research Council (PPARC) to develop the software used to process the data returned from the telescope. The European Space Agency contributes technical and ground station support in return for access to some of the observations.

Star-birth in Cepheus

An area of approximately 3 square degrees around the reflection nebula IC 1396 in the constellation Cepheus has been observed by the AKARI Infrared Camera (IRC) in its scanning mode at wavelengths of 9 and 18 micrometres (10-6 metres).

IC 1396 is a bright star formation region located within our galaxy, about 3000 light years from our Solar System in a region where very massive (several tens of solar masses) stars are presently being born. In addition to a massive star that ionises most of the gas in the nebula, several other young stars in the central region of the image have swept out the gas and dust to the periphery of the nebula, creating a hollow shell-like structure.

The gas that has been swept out creates the bright filament-like structures seen in the surrounding regions. The dust in the gas is heated by the intense light coming from both the massive star at the centre of the nebula and the newly born stars in the dense gas itself, and emits infrared light.

The formation of a new generation of stars is now taking place within this compressed gas in these outer shell structures. The AKARI data reveals for the first time, the detailed distribution of this swept out gas and dust over the entire nebula, and sees many previously unknown newly born stars, enabling a census to be made of the star formation in this area.

Glenn White, Professor of Astronomy at the Open University, and The CCLRC Rutherford Appleton Laboratory said “This image is extremely impressive – the infrared radiation has penetrated through the obscuring dust clouds between the Earth and the nebula allowing us to measure the whole star formation history in the region. Observing star forming regions over large areas lets us study the physics of stars that are born and examine how earlier generations of stars can feedback to and trigger the next burst of star birth. PPARC’s role has enabled UK astronomers to work at the forefront of the data analysis of the AKARI all sky far-infrared survey”

Dr Richard Savage, postdoctoral research fellow at the University of Sussex and co-author of the far-infrared survey software said “These images are a glimpse of a vast range of science that AKARI is beginning to address. It's fantastic to think how far we've come since launch and even more exciting how much further we'll be able to go in the coming months. Although the telescope has almost the same size mirror as the earlier all-sky survey mission, IRAS (InfraRed Astronomy Satellite), the improvements in detector technology and software processing since IRAS are truly staggering, as can be seen by comparing this IRAS image at 12 micrometres wavelength (figure 3) with the new AKARI image (figure 2)”

Dr Stephen Serjeant, Senior Lecturer in Astronomy at the Open University said “I find it quite amazing that we can now see star formation in action at close quarters in our own Galaxy with such clarity. The dust in these stellar birthplaces absorbs the light and hides much of the action from optical telescopes. With data like the beautiful AKARI image we can detect the reprocessed energy from the dust and infer what's hidden. AKARI is a superb telescope to watch the birth of stars like our Sun, and the birth of galaxies like our own”

A star in its death throes

The AKARI Far-Infrared Surveyor (FIS) instrument has been used to take an image of the red giant star U Hydrae. This star is located at about 500 light years from our Sun. These observations have detected extended dust clouds surrounding this star, shown by the halo of emission around the central star in the new AKARI image (figure 4).

Stars with masses close to the Sun will expand during the later stages of their lives becoming so called “red-giant” stars. These stars often eject gas from their surface into interstellar space during the final phases of their life. Dust is formed in the ejected gas, and this mixture of gas and dust expands outward and escapes from the star.

This “mass loss” process is so important that it may well determine the final stage of the star’s life. Stars with masses close to that of our Sun will expand during the later stages of their life becoming so-called 'red-giant' stars. During this final phase of their life such stars often eject gas from their surface into interstellar space – dust is formed in the ejected gas, and this mixture of gas and dust expands and escapes from the star (figure 5).

Since most of the ejected material consists of small grains with temperatures below about 100 K, the natural regime to study these is at far infrared wavelengths. The AKARI image clearly detects a detached shell-like dust cloud surrounding U Hydrae at a distance of about 0.3 light years from the central star, and implies that a short and violent ejection of mass took place in the star some 10,000 years ago.

Michael Rowan-Robinson, Professor of Astronomy at Imperial College, London said “Observations at infrared wavelengths provide a unique view of the final stages of the evolution of stars – in particular as they eject of shells of dusty material which go on to seed future generations of star formation in the Galaxy. This ejected material spreads throughout star forming clouds and may eventually be assimilated into a new generation of stars, enriching them with heavy elements. Observations like this help us understand what galaxies look like when seen from large distances”.

Dr Seb Oliver, Reader in Astronomy at The University of Sussex added “Results like this help us understand the relation between mass and light in our own Galaxy. This understanding can then be used to help us weigh galaxies earlier in the history of the Universe which in turn helps us model how galaxies form and evolve”.

Dr Chris Pearson of the European Space Agency and JAXA said “AKARI’s unique ability to map large areas of the sky is producing extraordinary insight into the entire life cycle of stars in our Galaxy, providing for the first time high resolution images that are revealing the very moment of the birth of stars in stellar nurseries as well as the final stages of the life-cycle of stars like the Sun.”.

Dr Do Kester of SRON, The Netherlands Institute for Space Research, who has been working with the UK team on the AKARI far infrared sky survey experiment, said “The gain in image quality compared to the IRAS all-sky survey is quite astounding and now allows AKARI to make direct observations of the circumstellar shells around evolved stars. Features like this in circumstellar shells which could previously only be inferred by using sophisticated image reconstruction are now directly accessible to AKARI’s instruments (figure 6).

The AKARI spacecraft is continuing its mission of exploration the Universe and has already scanned more than half the sky. After completing a first pass of the whole sky, it will repeat this several times to confirm sources, improve the sensitivity, and search for variable objects. AKARI is an international mission with strong collaboration with ESA, and a consortium of UK institutes (Imperial College London, Sussex University, and The Open University) and the SRON / Kapteyn Institute in the Netherlands. The project also includes contributions from Seoul National University, South Korea.

AKARI is developed by the Japan Aerospace Exploration Agency (ISAS/JAXA), with the participation of Japanese universities and institutes. The FIS instrument is developed by Nagoya University, JAXA, the University of Tokyo & the National Astronomical Observatory of Japan and other institutes, with contributions of NICT to the development of the detectors. The IRC instrument is developed by JAXA and the University of Tokyo and other supporting institutes.