L1521E: The first starless core with no molecular depletion

L1521E seems unique among starless cores. It stands out in a distribution of a ratio (R) that we define to asses core evolution, and which compares the emission of the easily-depleted (CO)-O-18 molecule with that of the hard to deplete, late-time species N2H+. While all cores we have studied so far have R ratio lower than 1, L1521E has an R value of 3.4, which is 8 times the mean of the other cores. To understand this difference, we have modeled the (CO)-O-18 and N2H+ abundance profiles in L1521E using a density distribution derived from 1.2 mm continuum data. Our model shows that the (CO)-O-18 emission in this core is consistent with constant abundance, and this makes L1521E the first core with no (CO)-O-18 depletion. Our model also derives an unusually low N2H+ abundance. These two chemical peculiarities suggest that L1521E has contracted to its present density very recently, and it is therefore an extremely young starless core. Comparing our derived abundances with a chemical model, we estimate a tentative age of less than or equal to1.5 x 10(5) yr, which is too short for ambipolar diffusion models.