1. Institut d'Astrophysique de Paris, CNRS, 98 bis bld Arago, F-75014 Paris, France
2. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218,USA
3. Laboratoire d'Astrophysique de Marseille, BP 8, F-13376 Marseille Cedex 12, France

Correspondence to: A. Lecavelier des Etangs¹ Correspondence and requests for materials should be addressed to A.L.d.E. (e-mail: Email: lecaveli@iap.fr).

Abstract

Molecular hydrogen (H₂) is by far the most abundant material from which stars, protoplanetary disks and giant planets form, but it is difficult to detect directly. Infrared emission lines from H₂ have recently been reported¹ towards Pictoris, a star harbouring a young planetary system². This star is surrounded by a dusty 'debris disk' that is continuously replenished either by collisions between asteroidal objects³ or by evaporation of ices on Chiron-like objects⁴. A gaseous disk has also been inferred from absorption lines in the stellar spectrum^{5, 6, 7, 8}. Here we present the far-ultraviolet spectrum of Pictoris, in which H₂ absorption lines are not seen. This allows us to set a very low upper limit on the column density of H₂: N(H₂)  10¹⁸ cm^-2. This non-detection is puzzling when compared to the quantity of H₂ inferred from the infrared observations, but it does show that H₂ is not in the disk on the direct line of sight. Carbon monoxide (CO) has been seen in absorption against the star^{8, 9, 10}, yielding a ratio of CO/H₂ > 6  10^-4. As CO would be destroyed under ambient conditions in about 200 years (refs 9, 11), our result demonstrates that the CO in the disk arises from evaporation of planetesimals.