Detection of the S(1) Rotational Line of H2 toward IRC+10216: A Simultaneous Measurement of the Mass-loss Rate and CO Abundance

We report the first detection of the S(1) pure rotational line of ortho-H₂ at 17.04 μm in an asymptotic giant branch star, using observations of IRC+10216 with the Echelon-cross-echelle Spectrograph (EXES) mounted on the Stratospheric Observatory for Infrared Astronomy. This line, which was observed in a very high-sensitivity spectrum (rms noise ≃0.04% of the continuum), was detected in the wing of a strong telluric line and displayed a P Cygni profile. The spectral ranges around the frequencies of the S(5) and S(7) ortho-H₂ transitions were observed as well but no feature was detected in spectra with sensitivities of 0.12% and 0.09% regarding the continuum emission, respectively. We used a radiation transfer code to model these three lines and derived a mass-loss rate of (2.43 ± 0.21) × 10^-5 M _⊙ yr^-1 without using the CO abundance. The comparison of this rate with previous estimates derived from CO observations suggests that the CO abundance relative to H₂ is (6.7 ± 1.4) × 10^-4. From this quantity and previously reported molecular abundances, we estimate the O/H and C/H ratios to be (3.3 ± 0.7) × 10^-4 and >(5.2 ± 0.9) × 10^-4, respectively. The C/O ratio is >1.5 ± 0.4. The absence of the S(5) and S(7) lines of ortho-H₂ in our observations can be explained by the opacity of hot dust within 5 R _⋆ from the center of the star. We estimate the intensity of the S(0) and S(2) lines of para-H₂ to be ≃0.1% and 0.2% of the continuum, respectively, which are below the detection limit of EXES.