Dr. Ayres of the University of Colorado will undertake a multifaceted study of the outer layers of the Sun. A controversy has shaken solar physics, over what ordinarily might be seen as the most arcane of details: the oxygen abundance. In the past few years, the measured abundance in the photosphere has plummeted nearly a factor of two, driven mainly by application of sophisticated 3D convection models to key atomic tracers such as atomic oxygen and oxygen-bearing molecules like OH and CO. The new low value is said to resolve several outstanding questions in the abundances in hot stars and the interstellar medium, but at the expense of wrecking the previous spectacularly good agreement between theoretical models of the solar interior (where oxygen is a crucial opacity source) and highly precise internal sound speed profiles from helioseismology. The solar low-O problem borders on a crisis, when one considers ramifications for diverse areas of astrophysics that rely fundamentally on the Sun to provide a baseline chemical composition; and even a catastrophe if the studies of solar abundances and helioseismology cannot be reconciled.
The present work will address these deficiencies with a multi-pronged strategy. A key objective will be to develop a series of new solar atlases at the highest possible spectral resolution and signal-to-noise, exploiting the recently upgraded Fourier transform spectrometer at Kitt Peak Observatory. A second objective will be to carefully test the most recent generation of 3D convection models, and to make such empirical modifications to the upper layers as necessary to apply them to molecular formation problems. A third, related objective, is to continue exploring the outer boundary of the photosphere, where the highly ordered fibril fields of the deep atmosphere become severely twisted and tangled, causing local heating, propelling jets of gas into the corona, and generally wreaking havoc.
This study will advance knowledge of our Sun, specifically atmospheric structure and energy transport from the deep photosphere out to the magnetically complex low chromosphere; relevant to space weather, one of few areas in astronomy that has direct societal impact. Beyond the Sun, better determinations of the oxygen (and carbon) abundance, and the rare isotopes of C and O, are crucial to many areas of astronomy that rely on the solar baseline. Ongoing instrumental developments, in partnership with the National Solar Observatory, will serve the broader NSO user community; as well as path finding for the Advanced Technology Solar Telescope (ATST), slated for operation in the next decade. ATST will benefit not only solar observers, but also specialized nighttime projects such as coronography of faint IR sources near bright objects. Finally, a variety of outreach projects and community service activities relevant to NSF will be carried out.
?" was a research project designed to study several aspects of an on-going controversy revolving around the amount of elemental oxygen in the Sun. Our star, like most others, is made up almost entirely of the simplest element, hydrogen; with a small amount of the second lightest element, helium, about 10%. Oxygen is the next most abundant element, but its concentration is only 0.06% that of hydrogen. Nevertheless, the tiny amount of oxygen plays a key role in setting the structure of the inside of the Sun, because it strongly controls the flow of radiant energy from the nuclear furnace at the very center. This is because at the very hot temperatures inside the Sun, hydrogen and helium have had all their electrons stripped away, but the stronger nuclear charge of oxygen allows it to hang on to enough electrons to impede the flow of radiant energy from the interior (by absorption processes that involve promoting the captured electrons to higher orbits around the nucleus, when bombarded by energetic light rays). In fact, oxygen is so important for the interior structure of the Sun that helioseismologists, who measure internal vibrations of the Sun by their imprint on surface motions, were able to confidently state a very precise value for the oxygen abundance. However, solar astronomers who study spectral signatures of elemental oxygen from the atmosphere of the Sun, came up with a very different conclusion, advocating a much lower oxygen abundance. Their conclusion was based partly on a new, very sophisticated generation of computer models, which more accurately capture the complexity of the dynamic atmospheric layers of the Sun that give rise to the spectral signatures recorded remotely by earth- and space-based telescopes. This was a catastrophe, of sorts, because now two forefront solar research areas were obtaining very conflicting answers concerning the pivotal oxygen abundance, and certainly both could not be right. This was a crisis, so to speak, as well because the Sun represents the "gold standard" relative to which all other cosmic oxygen abundances are measured, so changing the solar value has ramifications far beyond the Sun itself. At the same time, the Solar Oxygen Problem represented an opportunity, because whenever there are strongly conflicting results in any area of science, resolving the conflict often leads to valuable new insight and further progress. The purpose of the "Catastrophe, Crisis, or Opportunity?" project was to investigate the solar oxygen problem from a new direction: exploiting the new class of atmospheric computer models, but not applied to oxygen itself, but rather to the oxygen-bearing molecule carbon-monoxide, or CO for short. CO is well known on Earth as a toxic pollutant, and in fact the concentration of CO, in parts per million, is about the same in the air over Denver, Colorado, during a bad winter temperature inversion, as it is in the solar atmosphere every day. CO is the most abundant molecule in the solar atmosphere because it is the mostly tightly bound of the "di-atoms," and can survive the very hot temperatures, about 6000 C, whereas other less durable molecules break apart in the great heat. Not only is CO a valuable proxy for the oxygen abundance, but also because of the special characteristics of its infrared spectrum, the molecule can reveal the abundances of the trace isotopes of both oxygen and carbon, for example 18O and 13C. These isotopes are slightly heavier than normal O and C, by the addition of extra neutrons in the nucleus (2 extra for 18O, 1 extra for 13C). These heavy isotopes are prized by researchers who study the origins of planets in our solar system, because the contemporary concentrations of the isotopes in the crusts or atmospheres of the planets can tell a story of how the body was assembled from more primitive material in the early solar "nebula." However, even here there was a mini-crisis, of sorts, because earlier studies of solar CO found more 13C and 18O in the Sun than there should be, based on the current thinking in cosmo-chemistry circles. The new "Oxygen Problem" project made significant progress toward resolving both of these crises, by showing that solar CO spectral strengths were more consistent with the higher oxygen abundance favored by helioseismology; and further that the spectra of the isotopes of C and O indicated that the Sun was "lighter" than the Earth, isotopically speaking (in the language of cosmo-chemistry). The study revealed some minor flaws in the new generation of solar atmospheric computer models, which could have impacted some of the earlier "low oxygen" evaluations; and efforts are underway to seek further improvements in these sophisticated numerical simulations. This work was published in 2013 in the Astrophysical Journal.
