Introduction

It is widely accepted that once OB stars form in a molecular cloud, they disperse it by ionization or energetic stellar winds. But giant molecular clouds are usually very clumpy (e.g., Tatematsu et al. 1991) and so some of the high density cores/clumps can persist. They are often found as bright-rimmed clouds or cometary globules in or around HII regions. Bright-rimmed clouds are surrounded and compressed by a curved ionization/shock front on at least one side of their surface. Cometary globules are also surrounded by an ionization front, but are found in very extended HII regions like the Gum Nebula or may not be directly associated with an HII region (see the list by Zealey et al. 1983). Therefore they probably correspond to a stage after bright-rimmed clouds. Due to their enhanced gas density which hydrodynamical simulations of radiation driven implosion indicate (e.g., Lefloch and Lazareff 1994), retarded star formation takes place in both bright-rimmed clouds (Sugitani, Fukui and Ogura 1991, Sugitani and Ogura 1994, Sugitani, Tamura and Ogura 1995) and cometary globules (Reipurth 1983).

After OB stars evolve into B giants and stop radiating strong Lyman continuum, cometary globules lose their ionized envelope and so are no longer prominent. Still they could be optically visible by reflecting interstellar light in the OB association, although presumably with a reduced brightness compared, in particular, to bright rims. In an attempt to detect such clouds and examine the above scenario we have undertaken a survey of Schmidt atlases and plates in the Orion OB 1 association and have found many likely objects. We propose calling them as ''reflection clouds'' and all the three categories collectively as ''remnant clouds''. Our follow-up observations for some of them confirm their molecular nature and show convincing evidence for low-mass star formation at least in some of them.

© Copyright Astronomical Society of Australia 1997