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We have detected the 372 GHz line of ortho-H2D+ towards the pre-stellar coreL1544. The strongest emission (T_mb ~ 1 K) occurs at the peak of the millimetercontinuum emission, while measurements at offset positions indicate that H2D+is confined within ~ 20 arcsec, where CO is highly depleted. The derived H2D+abundance of ~ 10^{-9} is comparable with previous estimates of the electronabundance in the core, which suggests that H2D+ is the main molecular ion inthe central 20 arcsec (2800 AU) of L1544. This confirms the expectations thatH2D+ is dramatically enhanced in gas depleted of molecules other than H2. Themeasured abundance even exceeds the present model predictions by about a factorten. One possibility is that all CNO-bearing neutral species, including atomicoxygen, are almost completely (> 98%) frozen within a radius of ~2800 AU.Comment: A&A Letters, in pres

Abundant H$\mathsf{_{2}}$D$\mathsf{^{+}}$ in the pre–stellar core L1544