RESUMEN
Recent high-resolution in situ mass spectrometry at comet 67P/Churyumov-Gerasimenko visited by European Space Agency's Rosetta spacecraft raised the question, if sublimating ammonium salts can unequivocally be detected in the cometary coma. In laboratory experiments with the twin model of the space instrument, two prototypic ammonium salts NH4B, namely, ammonium chloride (B = Cl-) and ammonium formate (B = HCOO-) (as well as methodologically relevant isotopologues), were allowed to sublimate in vacuum while mass spectra were collected. High-resolution electron-impact ionization mass spectrometry provides an outstanding experimental tool to investigate the complex physicochemical processes occurring during the sublimation of ammonium salts. Sublimation of ammonium chloride led to the observation of the ammonium cation NH4+ and the chloramide molecule NH2Cl in the neutral gas mode of the instrument. These observations could be jointly interpreted as indirect evidence for the existence of a neutral gaseous parent species (either as the molecular complex NH3···HB or the double-ionic species NH4+···B-). However, the qualitative fragmentation pattern we present for 13C15N-ammonium formate suggests an alternative route of NH4+ production within the ionization region of the instrument, namely, by protonation/hydrogenation. Besides NH4+, other species were observed that were formed in protonation/hydrogenation reactions. Moreover, together with the two major species from the decomposition of the salt, ammonia and formic acid, three minor species also contributed to the fragmentation pattern: HCN/HNC, HOCN/HNCO, and CH3NO. Like chloramide, formamide (CH3NO) also is a secondary species probably formed in a pseudo-intramolecular chemical reaction while ammonia and the respective acid are in a state of association. HCN/HNC and HOCN/HNCO are ternary products coming out of formamide decomposition reactions. We discuss our experimental findings, summarized in a tentative chemical reaction network, in light of the available theoretical literature and highlight their relevance for the interpretation of in situ measurements in space research.
RESUMEN
The cosmic abundance of hydrides depends critically on high-energy UV, X-ray, and particle irradiation. Here we study hydrides in star-forming regions where irradiation by the young stellar object can be substantial, and density and temperature can be much enhanced over interstellar values. Lines of OH, CH, NH, and SH and their ions OH(+), CH(+), NH(+), SH(+), H2O(+), and H3O(+) were observed in star-forming regions by the HIFI spectrometer onboard the Herschel Space Observatory. Molecular column densities are derived from observed ground-state lines, models, or rotational diagrams. We report here on two prototypical high-mass regions, AFGL 2591 and W3 IRS5, and compare them to chemical calculations by making assumptions on the high-energy irradiation. A model assuming no ionizing protostellar emission is compared with (i) a model assuming strong protostellar X-ray emission and (ii) a two-dimensional (2D) model including emission in the far UV (FUV, 6-13.6 eV), irradiating the outflow walls that separate the outflowing gas and infalling envelope material. We confirm that the effect of FUV in two-dimensional models with enlarged irradiated surfaces is clearly noticeable. A molecule that is very sensitive to FUV irradiation is CH(+), enhanced in abundance by more than 5 orders of magnitude. The HIFI observations of CH(+) lines agree with the two-dimensional FUV model by Bruderer et al., which computes abundances, non-LTE excitation, and line radiative transfer.20 It is concluded that CH(+) is a good FUV tracer in star-forming regions. The effect of potential X-ray irradiation is not excluded but cannot be demonstrated by the present data.