  We report the first detection of the S(1) pure rotational line of ortho-H2 at
17.04 um 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 (SOFIA). 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)E-05 M_sun/yr 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)E-04.
From this quantity and previously reported molecular abundances, we estimate
the O/H and C/H ratios to be 3.3(0.7)E-04 and >5.2(0.9)E-04, 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 5R* 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.
