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Maret, S., Bacmann, A., Bottinelli, S., Parise, B., Caux, E., Faure, A., et al. (2010). Nitrogen hydrides in the cold envelope of IRAS 16293-2422. Astron. Astrophys., 521, L52.
Abstract: Nitrogen is the fifth most abundant element in the Universe, yet the gas-phase chemistry of N-bearing species remains poorly understood. Nitrogen hydrides are key molecules of nitrogen chemistry. Their abundance ratios place strong constraints on the production pathways and reaction rates of nitrogen-bearing molecules. We observed the class 0 protostar IRAS 16293-2422 with the heterodyne instrument HIFI, covering most of the frequency range from 0.48 to 1.78 THz at high spectral resolution. The hyperfine structure of the amidogen radical o-NH2 is resolved and seen in absorption against the continuum of the protostar. Several transitions of ammonia from 1.2 to 1.8 THz are also seen in absorption. These lines trace the low-density envelope of the protostar. Column densities and abundances are estimated for each hydride. We find that NH:NH2:NH3 â‰<2c6> 5:1:300. Dark clouds chemical models predict steady-state abundances of NH2 and NH3 in reasonable agreement with the present observations, whilst that of NH is underpredicted by more than one order of magnitude, even using updated kinetic rates. Additional modelling of the nitrogen gas-phase chemistry in dark-cloud conditions is necessary before having recourse to heterogen processes.
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Justtanont, K., Decin, L., Schöier, F. L., Maercker, M., Olofsson, H., Bujarrabal, V., et al. (2010). A HIFI preview of warm molecular gas around χ Cygni: first detection of H2O emission toward an S-type AGB star. Astron. Astrophys., 521, L6.
Abstract: Aims. A set of new, sensitive, and spectrally resolved, sub-millimeter line observations are used to probe the warm circumstellar gas around the S-type AGB star χ Cyg. The observed lines involve high rotational quantum numbers, which, combined with previously obtained lower-frequency data, make it possible to study in detail the chemical and physical properties of, essentially, the entire circumstellar envelope of χ Cyg.
Methods. The data were obtained using the HIFI instrument aboard Herschel, whose high spectral resolution provides valuable information about the line profiles. Detailed, non-LTE, radiative transfer modelling, including dust radiative transfer coupled with a dynamical model, has been performed to derive the temperature, density, and velocity structure of the circumstellar envelope.
Results. We report the first detection of circumstellar H2O rotational emission lines in an S-star. Using the high-J CO lines to derive the parameters for the circumstellar envelope, we modelled both the ortho- and para-H2O lines. Our modelling results are consistent with the velocity structure expected for a dust-driven wind. The derived total H2O abundance (relative to H2) is (1.1±0.2) × 10-5, much lower than that in O-rich stars. The derived ortho-to-para ratio of 2.1±0.6 is close to the high-temperature equilibrium limit, consistent with H2O being formed in the photosphere.
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Chavarr'ıa, L., Herpin, F., Jacq, T., Braine, J., Bontemps, S., Baudry, A., et al. (2010). Water in massive star-forming regions: HIFI observations of W3 IRS5. Astron. Astrophys., 521, L37 (1 to 5).
Abstract: We present Herschel observations of the water molecule in the massive star-forming region W3 IRS5. The o-H217O 110-101, p-H218O 111-000, p-H2O 202-111, p-H2O 111-000, o-H2O 221-212, and o-H2O 212-101 lines, covering a frequency range from 552 up to 1669 GHz, have been detected at high spectral resolution with HIFI. The water lines in W3 IRS5 show well-defined high-velocity wings that indicate a clear contribution by outflows. Moreover, the systematically blue-shifted absorption in the H2O lines suggests expansion, presumably driven by the outflow. No infall signatures are detected. The p-H2O 111-000 and o-H2O 212-101 lines show absorption from the cold material (T ~ 10 K) in which the high-mass protostellar envelope is embedded. One-dimensional radiative transfer models are used to estimate water abundances and to further study the kinematics of the region. We show that the emission in the rare isotopologues comes directly from the inner parts of the envelope (T â‰<b3> 100 K) where water ices in the dust mantles evaporate and the gas-phase abundance increases. The resulting jump in the water abundance (with a constant inner abundance of 10-4) is needed to reproduce the o-H217O 110-101 and p-H218O 111-000 spectra in our models. We estimate water abundances of 10-8 to 10-9 in the outer parts of the envelope (T â‰<b2> 100 K). The possibility of two protostellar objects contributing to the emission is discussed.
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