Confirmation of the presence of iron hydride in sunspots and cool stars

A high-resolution sunspot spectrum clearly shows the presence of the 9896 Â and 8691 Â bands of iron hydride (FeH). At least 332 lines of the two bands, accounting for 807o of the lines registered in the laboratory, can be identified with certainty in the spot spectrum ; most of these are unblended. Our estimate that the 9896 Â band is 2.0 ± 0.2 times stronger than the 8691 Â band supports the idea, suggested earlier by laboratory deutende experiments, that the 9896 Â band is the (0, 0) transition. No evidence for FeH is found in the spectrum of the solar disk. Image-tube spectrograms of M dwarfs and S stars have been taken in the near-infrared at 86 Â mm to examine the feature which Nordh, Lindgren, and Wing have proposed is due to the 9896 Â band of FeH. At this dispersion enough band structure is evident to confirm the identification in both kinds of stars. Subject headings: line identifications — molecular processes — stars : late-type — Sun : spectra — Sun : sunspots Carroll, McCormack, and O’Connor (1976) found that their statistical evidence for the blue and green bands of FeH was strongest when they limited consideration to solar lines that are enhanced in spot spectra or seen only in spots. Clearly, if the identifications of the blue and green bands are correct, the FeH bands in the much cleaner infrared spectrum should be easily visible in spot spectra of sufficient resolution. Such a spectrum is now available, and our study of it is discussed in § II. Nordh, Lindgren, and Wing (1977) have recently proposed that the 9896 Â band of FeH is responsible not only for a feature centered at 9910 Â found by Wing and Ford (1969) in the spectra of late M dwarfs but also for a band noted by Wing (1972) in the spectra of certain cool stars of type S. They found that the shape of the band in a photoelectric scan of an S star agreed with the degraded laboratory spectrum, but the available observations of M dwarfs had too low resolution to show any structure. We feel that it is important to remove the remaining uncertainty in the identification of the Wing-Ford band in M dwarfs, not only because of its potential value in studies of the atmospheres of these stars but also because of its use as an indicator of the stellar content of external galaxies (Whitford 1977). It is also important, from the viewpoint of atmospheric structure studies, to know if it is the same band that is so greatly enhanced in both M dwarfs and S-type giants, relative to M giants. In § III we present spectroscopic observations of sufficient resolution to confirm the identification of FeH in both kinds of stars.