  Excess levels of 15N isotopes which have been detected in primitive solar
system materials are explained as a remnant of interstellar chemistry which
took place in regions of the protosolar nebula. Chemical models of nitrogen
fractionation in cold clouds predict an enhancement in the gas-phase abundance
of 15N-bearing molecules, thus we have searched for 15N variants of the N2H+
ion in L1544, which is one of the best candidate sources for detection owing to
its low central core temperature and high CO depletion. With the IRAM 30m
telescope we have obtained deep integrations of the N2H+(1-0) line at 91.2 GHz.
The N2H+(1-0) line has been detected toward the dust emission peak of L1544.
The 14N/15N abundance ratio in N2H+ resulted 446+/-71, very close to the
protosolar value of ~450, higher than the terrestrial ratio of ~270, and
significantly lower than the lower limit in L1544 found by Gerin et al. (2009,
ApJ, 570, L101) in the same object using ammonia isotopologues.
