Primordial Helium-3 Redux: The Helium Isotope Ratio of the Orion Nebula*

Abstract

<jats:title>Abstract</jats:title> <jats:p>We report the first direct measurement of the helium isotope ratio, <jats:sup>3</jats:sup>He/<jats:sup>4</jats:sup>He, outside of the Local Interstellar Cloud, as part of science-verification observations with the upgraded CRyogenic InfraRed Echelle Spectrograph. Our determination of <jats:sup>3</jats:sup>He/<jats:sup>4</jats:sup>He is based on metastable He <jats:sc>i</jats:sc>* absorption along the line of sight toward Θ<jats:sup>2</jats:sup>A Ori in the Orion Nebula. We measure a value <jats:sup>3</jats:sup>He/<jats:sup>4</jats:sup>He = (1.77 ± 0.13) × 10<jats:sup>−4</jats:sup>, which is just ∼40% above the primordial relative abundance of these isotopes, assuming the Standard Model of particle physics and cosmology, (<jats:sup>3</jats:sup>He/<jats:sup>4</jats:sup>He)<jats:sub>p</jats:sub> = (1.257 ± 0.017) × 10<jats:sup>−4</jats:sup>. We calculate a suite of galactic chemical evolution simulations to study the Galactic build up of these isotopes, using the yields from Limongi &amp; Chieffi for stars in the mass range <jats:italic>M</jats:italic> = 8–100 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and Lagarde et al. for <jats:italic>M</jats:italic> = 0.8–8 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. We find that these simulations simultaneously reproduce the Orion and protosolar <jats:sup>3</jats:sup>He/<jats:sup>4</jats:sup>He values if the calculations are initialized with a primordial ratio <jats:inline-formula> <jats:tex-math> <?CDATA ${\left({}^{3}\mathrm{He}{/}^{4}\mathrm{He}\right)}_{{\rm{p}}}=(1.043\pm 0.089)\times {10}^{-4}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mfenced close=")" open="("> <mml:mrow> <mml:msup> <mml:mrow /> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> <mml:mi>He</mml:mi> <mml:msup> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> <mml:mi>He</mml:mi> </mml:mrow> </mml:mfenced> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">p</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mn>1.043</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.089</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac6503ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. Even though the quoted error does not include the model uncertainty, this determination agrees with the Standard Model value to within ∼2<jats:italic>σ</jats:italic>. We also use the present-day Galactic abundance of deuterium (D/H), helium (He/H), and <jats:sup>3</jats:sup>He/<jats:sup>4</jats:sup>He to infer an empirical limit on the primordial <jats:sup>3</jats:sup>He abundance, <jats:inline-formula> <jats:tex-math> <?CDATA ${\left({}^{3}\mathrm{He}/{\rm{H}}\right)}_{{\rm{p}}}\leqslant (1.09\pm 0.18)\times {10}^{-5}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mfenced close=")" open="("> <mml:mrow> <mml:msup> <mml:mrow /> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> <mml:mi>He</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> </mml:mfenced> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">p</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≤</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mn>1.09</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.18</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>5</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac6503ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>, which also agrees with the Standard Model value. We point out that it is becoming increasingly difficult to explain the discrepant primordial <jats:sup>7</jats:sup>Li/H abundance with nonstandard physics, without breaking the remarkable simultaneous agreement of three primordial element ratios (D/H, <jats:sup>4</jats:sup>He/H, and <jats:sup>3</jats:sup>He/<jats:sup>4</jats:sup>He) with the Standard Model values.</jats:p>