Observations of $^{\bf 3}$He$^{\bf +}$ Emission in the Milky Way
Session 92 -- Dust and Gas in the ISM
Oral presentation, Friday, January 14, 10:15-11:45, Salon VI Room (Crystal Gateway)

## [92.02] Observations of $^{\bf 3}$He$^{\bf +}$ Emission in the Milky Way

T.M.Bania, D.S.Balser (Boston Univ.), R.T.Rood (Univ. of Virginia)

The only practical way to measure the cosmic abundance of $^{3}$He is via observations of the hyperfine transition of $^{3}$He$^+$ at 8.665 GHz (3.46 cm). We summarize the present status of our emission line observations of $^{3}$He$^+$ in galactic H\thinspace II regions and planetary nebulae. In addition to obtaining $^{3}$He$^+$ spectra we have simultaneously measured various recombination line transitions to use as input for our source models and to test radiometer performance. For all practical purposes our spectra of the $91\alpha$ ($\Delta N = 1$), $114\beta$ ($\Delta N = 2$), and $130\gamma$ ($\Delta N = 3$) transitions are noiseless.

Also descibed are the techniques we have been forced to develop to achieve the precision we require in the measurement of the $^{3}$He$^+$ line parameters. Over a decade of experimental tests definitively show that significant instrumental frequency structure exists in spectral baselines and results from a complex combination of effects. We have determined that the limiting factor on the accuracy of our spectra is now the imprecision inherent in modeling the instrumental spectral baselines produced by standing waves caused by both stray and direct radiation. We now find that the complexity of baseline models is compromised by real spectral features limiting the spectral regions available for determining the baseline model fits. For some sources the signal-to-noise ratio of our spectra is now so good that very high order recombination transitions (e.g. , the $H213\xi$ [$\Delta N = 14$] recombination line) produce real frequency structure that must be excluded from the baseline fitting region.

Despite these observational challenges, in some H\thinspace II regions we have measured the $^{3}$He$^+$ line to high enough precision that the line parameters are no longer the limiting factor in the accuracy of the abundance determinations. The details of improved source models and the $^{3}$He abundances derived from these models are reported by Balser et al. at this meeting. Also at this meeting Rood et al. discuss the astrophysical implications of these $^{3}$He abundances.