SPATIALLY RESOLVED l-C{sub 3}H{sup +} EMISSION IN THE HORSEHEAD PHOTODISSOCIATION REGION: FURTHER EVIDENCE FOR A TOP-DOWN HYDROCARBON CHEMISTRY
- Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
- Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, F-38406 Saint Martin d’Hères (France)
- Instituto de Ciencia de Materiales de Madrid (CSIC), E-28049 Cantoblanco, Madrid (Spain)
- LERMA, Observatoire de Paris, École Normale Supérieure, PSL Research University, CNRS, UMR8112, F-75014 Paris (France)
- Sorbonne Universités, UPMC Univ. Paris 06, UMR8112, LERMA, F-75005 Paris (France)
Small hydrocarbons, such as C{sub 2}H, C{sub 3}H, and C{sub 3}H{sub 2} are more abundant in photo-dissociation regions (PDRs) than expected based on gas-phase chemical models. To explore the hydrocarbon chemistry further, we observed a key intermediate species, the hydrocarbon ion l-C{sub 3}H{sup +}, in the Horsehead PDR with the Plateau de Bure Interferometer at high-angular resolution (6″). We compare with previous observations of C{sub 2}H and c-C{sub 3}H{sub 2} at similar angular resolution and new gas-phase chemical model predictions to constrain the dominant formation mechanisms of small hydrocarbons in low-UV flux PDRs. We find that at the peak of the HCO emission (PDR position), the measured l-C{sub 3}H{sup +}, C{sub 2}H, and c-C{sub 3}H{sub 2} abundances are consistent with current gas-phase model predictions. However, in the first PDR layers, at the 7.7 μm polycyclic aromatic hydrocarbon band emission peak, which are more exposed to the radiation field and where the density is lower, the C{sub 2}H and c-C{sub 3}H{sub 2} abundances are underestimated by an order of magnitude. At this position, the l-C{sub 3}H{sup +} abundance is also underpredicted by the model but only by a factor of a few. In addition, contrary to the model predictions, l-C{sub 3}H{sup +} peaks further out in the PDR than the other hydrocarbons, C{sub 2}H and c-C{sub 3}H{sub 2}. This cannot be explained by an excitation effect. Current gas-phase photochemical models thus cannot explain the observed abundances of hydrocarbons, in particular, in the first PDR layers. Our observations are consistent with a top-down hydrocarbon chemistry, in which large polyatomic molecules or small carbonaceous grains are photo-destroyed into smaller hydrocarbon molecules/precursors.
- OSTI ID:
- 22518808
- Journal Information:
- Astrophysical Journal Letters, Vol. 800, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 2041-8205
- Country of Publication:
- United States
- Language:
- English
Similar Records
FAR-INFRARED DETECTION OF NEUTRAL ATOMIC OXYGEN TOWARD THE HORSEHEAD NEBULA
An observational investigation of the identity of B11244 (l-C{sub 3}H{sup +}/C{sub 3}H{sup -})