2003A&A...403L..37C

We have detected the 372 GHz line of ortho-H₂D⁺ towards the pre-stellar core L1544. The strongest emission (T_mb∼1K) 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 20'' (2800AU) 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 (>98%) frozen within a radius of ∼2800AU.