Astronomy and Astrophysics – Astrophysics
Scientific paper
Mar 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000a%26a...355..699l&link_type=abstract
Astronomy and Astrophysics, v.355, p.699-712 (2000)
Astronomy and Astrophysics
Astrophysics
65
Ism: Molecules, Ism: Clouds, Stars: Formation, Molecular Processes, Methods: Data Analysis
Scientific paper
Observations of gas-phase C_2H_2 and HCN along the line of sight toward a large sample of deeply embedded massive young stellar objects (YSOs) have been performed using the Short Wavelength Spectrometer on board the Infrared Space Observatory. The nu_5 vibration-rotation band of C_2H_2 around 13.7 μm and the nu_2 band of HCN around 14.0 μm have been detected for most lines of sight. These wavelength regions are heavily affected by instrumental fringing and a detailed discussion of the data reduction techniques is given. Comparison with model spectra allows the excitation temperatures and the abundances of the molecules to be determined. The inferred excitation temperatures range from <100 to 1000 K, and correlate well with each other, indicating that the two molecules probe the same warm gas component. The C_2H_2 and HCN column densities increase by more than an order of magnitude with increasing excitation temperature, and with the amount of heating of the ices. The corresponding abundances of C_2H_2 and HCN in the warm gas increase from ~ 10-8 to ~ 10-6 with increasing temperatures. The enhanced abundances are compared with a variety of chemical models. The observed gas-phase C_2H_2 most likely results from direct evaporation of interstellar ices, where C_2H_2 must be present at an abundance of ~ 0.1-0.5 % with respect to H_2O ice. This abundance is consistent with the measured amount of C_2H_2 in cometary ices. The observed gas-phase HCN abundance shows a stronger increase with temperature and results from a combination of evaporation of ices and high-temperature gas-phase chemistry in the hot core. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom), with participation of ISAS and NASA.
Lahuis Fred
van Dishoeck Ewine F.
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