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

Abstract

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.

7 pages, 4 figures, accepted for publication in ApJ Letters

References in corpus (11)

• Imaging the circumstellar envelopes of AGB stars
• Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles I. Theoretical model -- Mass-loss history unravelled in VY CMa
• (Sub)stellar companions shape the winds of evolved stars
• A Detailed Analysis of the Dust Formation Zone of IRC+10216 Derived from Mid-IR Bands of C2H2 and HCN
• Chemical equilibrium in AGB atmospheres: Successes, failures, and prospects for small molecules, clusters, and condensates
• Molecular shells in IRC+10216: tracing the mass loss history
• The millimeter IRAM-30m line survey toward IK Tau
• The complex dust formation zone of the AGB star IRC+10216 probed with CARMA 0.25 arcsec angular resolution molecular observations
• Photodissociation of CO in the outflow of evolved stars
• The detached dust shells around the carbon AGB stars R Scl and V644 Sco
• Multi-frequency high spectral resolution observations of HCN toward the circumstellar envelope of Y CVn

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• Exploring circumstellar chemistry in X-ray emitting AGB stars
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