Astron. Astrophys. 363, 1065-1080 (2000)

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
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