Solid-state CO toward rho Ophiuchi Elias 25: Ice thresholds revisited

In this paper we report the detection of CO ice at 4.67 mu m toward Elias 2 5 in the rho Ophiuchi cloud. The data were obtained using the UKIRT CGS4, a nd the CO-ice band is among the weakest yet observed. We compare our result to previous CO and H2O ice detections in an effort to understand the extin ction thresholds for ice growth (A(th)). Our data imply A(th) < 11 magnitud es for CO ices in rho Oph, much less than previously thought. We tentativel y suggest that the OB star lines of sight reflect ice growth trends for pri stine molecular cloud material not associated with star formation.