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We report the first detection of the S(1) pure rotational line of ortho-H2 at17.04 um in an asymptotic giant branch star, using observations of IRC+10216with the Echelon-cross-echelle Spectrograph (EXES) mounted on the StratosphericObservatory for Infrared Astronomy (SOFIA). This line, which was observed in avery high sensitivity spectrum (RMS noise ~0.04% of the continuum), wasdetected 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-H2transitions were observed as well but no feature was detected in spectra withsensitivities of 0.12% and 0.09% regarding the continuum emission,respectively. We used a radiation transfer code to model these three lines andderived a mass-loss rate of 2.43(0.21)E-05 M_sun/yr without using the COabundance. The comparison of this rate with previous estimates derived from COobservations suggests that the CO abundance relative to H2 is 6.7(1.4)E-04.From this quantity and previously reported molecular abundances, we estimatethe O/H and C/H ratios to be 3.3(0.7)E-04 and >5.2(0.9)E-04, respectively. TheC/O ratio is >1.5(0.4). The absence of the S(5) and S(7) lines of ortho-H2 inour observations can be explained by the opacity of hot dust within 5R* fromthe center of the star. We estimate the intensity of the S(0) and S(2) lines ofpara-H2 to be ~0.1% and 0.2% of the continuum, respectively, which are belowthe detection limit of EXES.

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