  High-resolution (R ~ 100 000), high signal-to-noise spectra of M71 giants
have been obtained with HIRES at the KeckI Telescope in order to measure their
Mg isotopic ratios, as well as elemental abundances of C, N, O, Na, Mg, Al, Si,
Ca, Ti, Ni, Zr and La. We demonstrate that M71 has two populations, the first
having weak CN, normal O, Na, Mg, and Al, and a low ratio of 26Mg/Mg (~4%)
consistent with models of galactic chemical evolution with no contribution from
AGB stars. The Galactic halo could have been formed from the dissolution of
globular clusters prior to their intermediate mass stars reaching the AGB. The
second population has enhanced Na and Al accompanied by lower O and by higher
26Mg/Mg (~8%), consistent with models which do incorporate ejecta from AGB
stars via normal stellar winds. All the M71 giants have identical [Fe/H],
[Si/Fe], [Ca/Fe], [Ti/Fe] and [Ni/Fe] to within sigma = 0.04 dex (10%). We
therefore infer that the timescale for formation of the first generation of
stars we see today in this globular cluster must be sufficiently short to avoid
a contribution from AGB stars, i.e. less than ~0.3Gyr. Furthermore, the Mg
isotopic ratios in the second M71 population, combined with their elemental
abundances for the light elements, demonstrate that the difference must be the
result of adding in the ejecta of intermediate mass AGB stars. Finally we
suggest that the low amplitude of the abundance variations of the light
elements within M71 is due to a combination of its low mass and its relatively
high Fe-metallicity.
