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There might be twice as much neon in the Universe than currently thought

posted May 5, 2013, 8:11 PM by Jeremy Drake   [ updated Jun 3, 2013, 11:57 AM ]
We can measure how much of the different elements there are in the Sun and stars using optical spectroscopy - detailed analysis of the visible light spectrum.  The electrons in each element are only allowed to jump around between discrete energy levels dictated by quantum mechanics.  Each element has its own fingerprint of these atomic transitions.  When an atom of a given element sees light of the same precise colour that corresponds to the energy difference of one of its allowed electron jumps, this light can be absorbed by the atom.  The more of a given atom in the atmosphere of a star, the more the light of these specific colours is absorbed. We can see these atomic  fingerprints in star's spectrum as dark bands at specific colours where the light has been absorbed.  Provided we have an understanding of the conditions in the stellar atmosphere, such as temperature and pressure, we can build models that allow us to convert the amount of absorption into the amount of the element causing it. 

The most precise measurements are for the Sun because we can measure its spectrum in great detail and understand its atmosphere. Such studies of the Sun are used as a benchmark to infer the abundances of elements in other stars, and hence the rest of the Universe.  This works for just about all the important elements, except neon. Neon has no visible light absorption bands in the solar spectrum, and instead the emission from neon atoms in the Sun's outer atmosphere, or "corona", in the UV and X-ray bands is used.  The problem with this technique is that as gas moves from the lower atmosphere, or photosphere (where the visible light comes from), to the corona, its chemical composition changes.  There can, for example, be 2-4 times more iron in the corona per atom of hydrogen than in the photosphere.  Since we cannot see neon in the photosphere, we cannot tell if its abundance is modified in the corona.  

The figure shown here is from a new analysis of old data on the ratio of neon to oxygen in the solar corona obtained by satellite measurements from the late 1970's and 1980's in the X-ray range.  I have looked at the Ne/O abundance ratio as a function of the gas temperature and find that it increases toward hotter temperatures.  The blue and grey points represent the same data, just analysed in a slightly different way.  The hashed region "Drake & Testa (2005)" refers to an X-ray study we did using NASA's Chandra X-ray Observatory of the Sun of Ne/O in other stars.  In these stars, we found there is nearly always twice as much neon than current estimates for the Sun that are indicated by the dashed and dotted green and blue horizontal lines.  The new analysis of the solar data shows that we do not really understand the neon abundance in the solar corona yet.  Since it changes with temperature, we cannot be sure what temperature, if any, reflects the abundance of the rest of the Sun.  The Drake & Testa (2005) stars all had much hotter coronae than the Sun: it seems to me that solar trend points to a solar corona depleted of neon, and the true neon abundance in the Sun - and the rest of the Universe - is twice the current estimate and similar to measurements for the coronae of other stars.