The Milky Way 3-Helium Abundance

We are making precise determinations of the abundance of the light isotope of helium, 3 He. The 3 He abundance in Milky Way sources impacts stellar evolution, chemical evolution, and cosmology. The abundance of 3 He is derived from measurements of the hyperfine transition of 3 He + which has a rest wavelength of 3.46 cm (8.665 GHz). As with all the light elements, the present interstellar 3 He abundance results from a combination of Big Bang Nucleosynthesis (BBNS) and stellar nucleosynthesis. We are measuring the 3 He abundance in Milky Way H ii regions and planetary nebulae (PNe). The source sample is currently comprised of 60 H ii regions and 12 PNe. H ii regions are examples of zero-age objects that are young relative to the age of the Galaxy. Therefore their abundances chronicle the results of billions of years of Galactic chemical evolution. PNe probe material that has been ejected from low-mass ( M ≤ 2 M ⊙ ) to intermediate-mass ( M ∼2–5 M ⊙ ) stars to be further processed by future stellar generations. Because the Milky Way ISM is optically thin at centimeter wavelengths, our source sample probes a larger volume of the Galactic disk than does any other light element tracer of Galactic chemical evolution. The sources in our sample possess a wide range of physical properties (including object type, size, temperature, excitation, etc.). The 3 He abundances we derive have led to what has been called “The 3 He Problem”.