We thank Bradley Whitmore and an anonymous referee for useful comments. This work was based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with participation of ISAS and NASA. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under the NASA contract NAS 5-26555.
© European Southern Observatory (ESO) 1998
Online publication: April 15, 1998