2. Atomic and molecular data
The vibration-rotation H2 lines are found through the 1-3 µm region of the infrared spectrum. A comprehensive line list has been compiled by Black & van Dishoeck (1987). The current paper is limited to the 1.5-2.5 µm region, which includes low excitation S and Q-branch H2 lines. All the Q-branch H2 lines are in a spectral region heavily blanketed by CO v=2 lines and, in the coolest stars with C/O 1, by stellar H2O lines. As a result, the Q-branch lines were not investigated. The 1-0 S(0) and S(1) lines are well placed in the 2 µm telluric window ("K-band"), 1-0 S(0) is well removed from any telluric lines, while a few weak (depth 10%) telluric lines are near S(1). 1-0 S(2) is in the region of a telluric CO2 band with telluric lines of about 30% central depth (at 1 airmass) and 1 cm-1 spacing crossing the rest frequency of the line. The 1-0 S(3), S(4), and S(5) lines are hopelessly blended with strong telluric CO2 and H2O lines. The higher excitation 1-0 S(6) and S(7) lines lie well into the 1.8 µm telluric window ("H-band") and both are separated by several wavenumbers from strong telluric lines. More highly vibrationally excited H2 lines are also in the 1.5-2.5 µm region. The 2-1 S(1) and S(2) lines are both well placed in the K-band.
Bragg et al. (1982) provide excellent laboratory frequencies, with precisions exceeding 0.1 mK, for the 1-0 S(0), S(1), and S(2) lines. For the remaining lines, the frequencies must be calculated from the molecular constants. For these lines we have used the list computed by Black & van Dishoeck (1987). The excitation potentials were computed for the 1-0 band from the constants provided by Jennings et al. (1984). For the 2-1 band the constants of Bragg et al. (1982) were used. Oscillator strengths for H2 were computed from the Einstein A coefficients of Turner et al. (1977). The 1-0 band values are in excellent agreement with the values compiled using independent input by Lambert et al. (1973). For H2 it is important to recall that homonuclear diatomic molecules have nuclear spin statistical weights three times larger for odd J lines than for even J lines. Thus 1-0 S(1) is a much stronger transition than 1-0 S(0). The molecular data are presented in Table 1.
Table 1. H2 Laboratory Line Data
In addition to the H2 lines, supporting measurements were made of CO, CN, and atomic Ti lines. The CO frequencies and oscillator strengths are those used by Hinkle et al. (1984). The CO v=3 measurements were taken mainly from HSH and Hinkle et al. (1997). In the K band the CO 2-0 R branch lines J=79, 81, 83, 86, and 87 were measured because they are unblended and free of circumstellar contamination (HHR). CN 0-2 red system data were provided by Smith & Lambert (1989). From the extensive Smith and Lambert CN line list a sample of 20 strong, unblended lines in the 4400-4800 cm-1 region were selected. The strong Ti lines in the K band were taken as a representative sample of metal lines (Hinkle & Barnes 1979b). Ti frequencies were taken from Forsberg (1987).
© European Southern Observatory (ESO) 2000
Online publication: December 5, 2000