DETECTION OF HE-3 IN A PLANETARY-NEBULA

HELIUM-3 is created by cosmological nucleosynthesis with an abundance He-3/H almost-equal-to 2 x 10(-5) (ref. 1), but then augmented by stellar nucleosynthesis. Stars comparable in mass to the Sun should contribute a large fraction of the present He-3 abundance in interstellar material 2: winds from these stars during their main-sequence lifetime, as well as planetary nebulae created by more rapid mass loss later in the stars' lives, are expected to have He-3/H about 100 times the cosmic value. These stars are also thought to be the principal source of new material to the interstellar medium 3, and measurement of the present He-3 abundance should therefore be an important diagnostic of chemical evolution in the Galaxy, as well as an essential prelude to determining the primordial cosmic abundance 4. Over a decade ago we began a programme 5-7 to measure the galactic He-3 abundance, but until recently it had been possible to detect it only in giant H II regions, where it is already well mixed into the interstellar medium. We report here its first detection in a He-3 source, the planetary nebula NGC3242. We measure He-3/H greater-than-or-similar-to 10(-3), consistent with stellar models.