Studying distant stars and their planets in different states of evolution is of great interest to astronomers and of immense importance for astronomy. XMM-Newton has added its contribution with an observation of a stellar system, L1551 IRS5. It has enabled ESA's X-ray observatory to detect -- for the first time -- the X-ray emission from its spectacular jets.

The L1551 cloud as seen by XMM-Newton's EPIC-pn camera. The left-most X-ray point source is the one associated with L1551 IRS5. The others are background sources. Photo: ESA

Lynds 1551 IRS5 is well known for its large-scale outflows or 'jets' of molecular gas. It is located about 500 light-years away in an interstellar cloud of dust and gas that is producing Sun-like stars at a high rate. This young stellar system - in fact consisting of two stars - is less than a million years old.

Each star of IRS5 is surrounded by compact discs of matter that have the potential to condense into families of planets. The accretion discs are 20 AU across, approximately the diameter of Saturn's orbit around the Sun. (1 AU, or Astronomical Unit, is the distance between our planet and the Sun.)

But, the parallel jets shoot far beyond into the interstellar medium, to a distance of 1000 AU. At the same time, a stellar wind evacuates matter, creating a kind of cavity around the jets which reflects the light from the two stars. It is possible that similar outflows exist along the same axis on the far sides of the young stars.

Invisible stars lead to X-ray discovery
In most cases of young stellar objects, the X-ray luminous stars are visible and therefore outshine any jets which the objects may produce. But the particular configuration of IRS5 provided a rare opportunity for XMM-Newton.

The L1551 IRS5 protostellar system as seen with the Subaru optical telescope. Photo: ESA

With the X-rays from the stars blocked out by the absorbing material of the discs, the three EPIC cameras were able, during a 50,000 s exposure, to detect X-ray emission coming from the jets.

"The X-ray emission which we have observed is certainly of thermal origin and can probably be associated with the shock wave, the interface between the jets and the interstellar medium," explains Fabio Favata, lead author of a paper to be published in the journal Astronomy and Astrophysics.

Analysis of the data has provided new information on the composition of the jets and the way that they interact with their immediate environment. The IRS5 stars appear to be powering -- jointly or separately -- at least two observable jets, which are moving outwards at a speed of between 200-400 km/s. The observation succeeded in measuring, for the first time, the temperature of these jets which is about 100,000 K.

Illustration of the L1551 IRS5 system -- a binary protostellar system. Credit: K. Borozdin, Los Alamos National Laboratory

The X-ray observation, which took place in September 2000, has been complemented with data obtained by the 2.6 m Nordic Optical Telescope in the Canary Islands. The visible-light spectra and images from observations in December 2000 and March 2001 have allowed the detection of regions, or 'knots' along the jets, where the density varies. They also highlight faint background or embedded sources that shine through the molecular cloud.

"One thing we have discovered is that the X-rays from the jets illuminate the accretion discs from above -- which a star cannot do -- and change the physical conditions, such as ionisation. This could have an influence on the way planets form in the protoplanetary disk," says Fabio Favata. "The thermal nature of the emission, with the associated shock temperatures, also raises the question of whether X-rays associated with jets could be a common feature of stellar formation."

The paper "Discovery of X-ray emission from the protostellar jet L1551 IRS5 (HH154)" by F. Favata, C.V.M. Fridlund, G. Micela, S. Sciortino and A. Kaas will be published in Astronomy and Astrophysics.