Knowledge Bank

Improvements in infrared detectors used with the McMath Solar Telescope and spectrograph of the Kitt Peak National Observatory have made it possible to obtain spectra of sunspots between 1.4 and 2.6 microns. When used in first order, single pass, the spectrograph affords a resolution of about $0.05cm-1$ in this spectral region and high S/N spectra can be obtained from an area on the sun as small as 1 arc/sec square. Under favorable conditions it is possible to record about 0.2 microns of spectrum during one day's observing. Sunspots are much cooler than the photsphere (they have an effective temperature of about $4000\circ K$ as compared with $5800\circ K$ for the photosphere) and conditions are very favorable for the formation of molecules, particularly $H2,CO,OH$ and $H2O$. Molecular lines are sensitive indicators of the physical conditions in a sunspot and their principal use has been to test various sunspot models. The dominant features of the umbral sunspot spectrum are the $\Delta v=2$ vibration rotation lines of OH (1.4-1.8 microns) and CO (2.3-2.6 microns); the stronger lines of both molecules are saturated in darker spots. For both OH and CO, accurate wave numbers can be obtained for transitions involving much higher quantum numbers than those appearing in laboratory spectra and the spin and lambda doubling of OH can be determined for these levels. Weaker lines of two other molecules have been identified in the spectra. Lines of the 1-0 vibration-rotation band of HF can be distinguished and although many of these are strongly blended with CO lines, the few lines that are in the clear will yield the first accurate fluorine ratio for the sun. Many lines of the 0-1, 1-2 and 2-3 bands of the CN red system are clearly displayed and will be used to verify energy levels derived from near infrared spectra. No lines of the two other abundant molecules, $H2$ and $H2O$, have been seen. For $H2$ this is not unexpected as the 1-0 lines are quadrupole transitions and the electronic transitions have a highly excited lower state. Considering the strength of the OH lines and the high predicted abundance of $H2O$, the failure to distinguish any lines of the molecules is disconcerting. The problem is complicated by terrestial absorption but even high excitation lines do not appear to show any solar component. A large grating spectrograph designed to operate between 1 and 25 microns is nearing completion and will be used to survey this portion of the sunspot spectrum. Molecular lines that might be detectable with this instrument include the fundamental vibration-rotation bands of OH, CH, NH, CO, CN and SiO and some pure rotational lines of OH.