X-rays in the Solar System

April 7 – 9, 2003

From April 7-9 2003, a 3--day workshop on "X-rays in the Solar System'' took place in the Lorentz Center, organized by the 2003 J.H. Oort professor, Alexander Dalgarno from Harvard University. In this highly interdisciplinary topic, both astrophysicists and atomic physicists closely interacted to interpret the latest X-ray observational results.

The main driver for the workshop was to discuss the unexpected discovery that comets, which are conglomerates of ice and dust, are sources of X-rays. Given the fact that these objects are very cold, the emission is unexpected. The probable mechanism is electron capture by highly stripped ions in the solar wind impacting the neutral material of the cometary atmospheres. The charge transfer process populates highly excited states of the ions created by capture which then radiate in the X-ray region of the spectrum. An alternative mechanism involving energetic electrons is less likely.

Observations of the X-ray spectra from comets were presented at the workshop together with theoretical models. The models made use of experimental laboratory and theoretical cross sections which were reported at the workshop. It was demonstrated that by comparing the measured comet spectra with the model results the composition of the solar wind could be derived. The capture process leads to a radiative cascade that produces emissions not only of X-rays but also of extreme ultraviolet and far ultraviolet radiation. Data from the Far Ultraviolet Spectroscopic Explorer for several comets were reported and a tentative identification of one line that may be attributable to cascading was proposed.

The contributions of electron capture by solar wind ions to X-rays from the planets and from other objects in the solar system including the heliosphere in which neutral atoms of the interstellar medium collide with the solar wind ions were examined. Measurements of the diffuse soft X-ray background were presented and the role of capture by solar wind ions was considered.

The workshop demonstrated the value of bringing together observers and modelers of comets as well as experimentalists and theorists studying basic processes in an environment that encouraged interactions between them.

A. Dalgarno (Harvard, United States)

E.F. van Dishoeck (Leiden University, Netherlands)