AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 47. Between the Stars I: The ISM, Galactic and Extragalactic
Display, Tuesday, June 1, 1999, 10:00am-7:00pm, Southwest Exhibit Hall

## [47.02] 3-Helium in Obscure H\thinspace II Regions

T.M. Bania (BU), R.T. Rood (UVa), D.S. Balser (NRAO-GB)

The light isotope of helium, {}3{\rm He}, can serve as a probe of cosmology, the evolution of low mass stars, and the chemical evolution of the Galaxy. Its abundance can be determined via measurements of the 3.46\ cm hyperfine transition of {}3{\rm He}+. Potentially observable sources of ionized gas include H~{\sc ii}\ regions and planetary nebulae. The selection of {}3{\rm He}\ targets is counter-intuitive because the {}3{\rm He}+\ hyperfine line strength is proportional to the source density, while one usually thinks of H~{\sc ii}\ regions in terms of radio continuum or recombination line strength both of which depend on the square of the density. The {}3{\rm He}+\ line strength depends on the {}3{\rm He}+\ abundance ratio and a number of other factors:

TLA({}3{\rm He}+) \propto N({}^3{\rm He}^+)/N({\rm H}^+) ({T_{C}^{A}}D)^{1/2}~T_{e}^{1/4}~(\theta_{\rm obs}^{2} - \theta_{\rm beam}^{2})^{3/4}/\Delta{v}({}^3{\rm He}^+) [{\rm ln}(5.717 \times 10^{-3}T_{e}^{3/2})]^{1/2}~\theta_{\rm obs}

\noindent where T\rm L\rm A and \Delta v are the antenna temperature and FWHM of the {}3{\rm He}+\ line, D is the nebular distance, T\rm C\rm A and \theta\rm obs are the antenna temperature and observed FWHM angular size of the continuum emission, \theta\rm beam is the telescope's FWHM beam, and Te is the nebular electron temperature. For H~{\sc ii}\ regions much larger than the telescope beam we can select targets using the criterion: TLA({}3{\rm He}+)~sqrt{TCA D\theta\rm obs}. This is the case since we can neglect the weak dependence on Te and because we do not know either {}3{\rm He}+/{\rm H+}\ or \Delta v. Thus big, distant H~{\sc ii}\ regions could be potential {}3{\rm He}+\ targets even if their continuum emission is weak.

Armed with this knowledge we included H~{\sc ii}\ regions like S209 in our early observing list along with more famous sources like W43. Still we did not have the temerity to push this reasoning to the limit. We have now found, however, that this selection criterion is valid for even the wimpiest known H~{\sc ii}\ regions. Here we report on the detection of {}3{\rm He}+\ emission in 8 distant, low density H~{\sc ii}\ regions.

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