Abstract
Small hydrocarbons, such as C2H, C3H and C3H2 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-C3H+, in the Horsehead PDR with the Plateau de Bure Interferometer at high-angular resolution (6''). We compare with previous observations of C2H and c-C3H2 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-C3H+, C2H and c-C3H2 abundances are consistent with current gas-phase model predictions. However, in the first PDR layers, at the 7.7 mum PAH band emission peak, which are more exposed to the radiation field and where the density is lower, the C2H and c-C3H2 abundances are underestimated by an order of magnitude. At this position, the l-C3H+ abundance is also underpredicted by the model but only by a factor of a few. In addition, contrary to the model predictions, l-C3H+ peaks further out in the PDR than the other hydrocarbons, C2H and c-C3H2. 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.
Highlights
Displayed are the 7.7 μm emission arising from polycyclic aromatic hydrocarbons (PAH) imaged with ISO at 6′′ angular resolution (Abergel et al 2003), and the integrated intensity maps of the DCO+ J = 2−1 line observed with the Institut de Radioastronomie Millimetrique (IRAM)-30m telescope (Pety et al 2007) and the H2 v = 1 − 0 S(1) line obtained with SOFI/NTT at 1′′ angular resolution (Habart et al 2005)
The first one mainly coincides with the photo-dissociation regions (PDRs), where intense emission is seen both in the H2 rovibrational line and in the PAH mid-infrared band
The l-C3H+ emission reaches the red line in Fig. 1, which traces the edge of the PDR, while the emission of the two other hydrocarbons is shifted left of this line
Summary
Using single-dish observations, Pety et al (2012) reported the first detection of l-C3H+, a key intermediate species in the gas-phase formation of small hydrocarbons, toward the Horsehead PDR. Fig. 1.— Integrated intensity maps of the small hydrocarbons l-C3H+, C2H and c-C3H2 lines (upper row), as well as that of the DCO+. The Horsehead Nebula provides an ideal test-bed because it is viewed almost edge-on, providing easy access to the warm surface layer of a cloud where C+ and C3H+ is predicted to be the most abundant As this warm photo-active layer is spatially narrow (∼ 5′′, Guzman et al 2012), high angular resolution is needed to resolve the steep gradients in this region. To explore the relationship of C3H+ with its environment and with neutral carbon chains, we present in this letter the first spatially resolved observations of l-C3H+
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