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Observations of Solar Carbon Monoxide with an Imaging Infrared Spectrograph. I. Thermal Bifurcation Revisited
Ayres, Thomas R.; Rabin, Douglas
Astrophysical Journal v.460, p.1042 (ApJ Homepage)
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We describe long-slit spectroscopy of the solar 4.7 mum carbon monoxide (CO) Deltaupsilo = 1 bands at the Main spectrograph of the NSO McMath-Pierce telescope. We utilized stigmatic imaging of the temperature- and velocity-sensitive CO absorptions to map quiet regions near disk center and at the extreme limb. At Sun center the dominant long-lived spatial structures are small-scale hot spots associated with fragments of the supergranulation network seen in cotemporal Ca II filtergrams. Oscillatory thermal and velocity fluctuations of the global p-mode interference pattern are a pervasive feature of the maps, but the rms amplitudes (≈70 K and ≈240 m s-1) are perturbations on a relatively unstructured outer photosphere. We occasionally see small-scale transient cooling episodes longer lived than the p-mode wavepackets that might be overshooting granules or rising magnetic flux ropes. The events are too rare, however, to influence the global properties of the CO fundamental bands.

Seeing-selected frames of the off-limb CO emissions show a typical extension of 0".6 for the strongest lines, with little variation along the limb. The off-limb extensions indicate the presence of cool gas up to 350 km above the "Tmin" of popular reference models of the solar chromosphere.

We carried out two-dimensional model atmosphere simulations to study the effects of thermal inhomogeneities on the disk-center, extreme-limb, and off-limb behavior of the CO lines. The models are spherically symmetric, static, and in LTE. Our data favor a scenario in which the bulk of the low chromosphere below the base of the magnetic "canopy" is in reality a "COmosphere" dominated by gas colder than the minimum temperature in conventional models.

The moderate-scale (≈5"), mild thermal perturbations of the p-mode pattern have little influence on the CO Deltaupsilo = 1 spectra. Small-scale (≈1") hot regions embedded in a cool average atmosphere are strongly "shadowed" at the extreme limb. The shadowing is of little consequence, however, because the atmosphere already is dominated by the cool component. The opposite scenario-small-scale cold regions in a warm average produce effective shadowing at the limb for granule-size (≈1"-2") dark points only if the covering fraction is relatively large (f &#8819 0.2). That scenario is ruled out: it predicts high-contrast dark spots at disk center, contrary to our observations. We also argue against the possibility of shadowing by even smaller, subresolution (≈0"3) cold spots with f &#8819 0.1.

We show that multistep reactions, rather than direct radiative associations, dominate the gas-phase chemistry of CO molecules under conditions typical of the outer photosphere. The CO formation and radiative cooling timescales are fast enough that low-temperature plasma conditions can be restored following disruption by a localized heating event such as a Ca II K2v "flash." In cool giant stars, the chemical formation timescales are much longer than in dwarfs like the Sun. Nevertheless, the density dependence is such that the molecular cooling proceeds proportionately more rapidly than the gas dynamics, ensuring an even more important role for autocatalyzed "thermal bifurcation."

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