Abundant H 2D^+ in the pre-stellar core L1544

We have detected the 372 GHz line of ortho- H2D^+ towards the pre-stellar core L1544. The strongest emission (Tmb~1 K) occurs at the peak of the millimeter continuum emission, while measurements at offset positions indicate that H2D^+ is confined within ~20´´, where CO is highly depleted. The derived H2D^+ abundance of ~10^-9 is comparable with previous estimates of the electron abundance in the core, which suggests that H2D^+ is the main molecular ion in the central 20ʹʹ (2800 AU) of L1544. This confirms the expectations that H2D^+ is dramatically enhanced in gas depleted of molecules other than H2. The measured abundance even exceeds the present model predictions by about a factor ten. One possibility is that all CNO-bearing neutral species, including atomic oxygen, are almost completely ( (greater than)98%) frozen within a radius of ~2800 AU.