Observations in the far-IR window should make a major contribution to solving two major problems in molecular astrophysics - the identity of the carriers of the Aromatic Infrared emission Bands (AIBs) and the Diffuse Interstellar absorption Bands (DIBs). The DIBs are generally attributed to carbon-based molecules but none has been assigned. Polycyclic aromatic hydrocarbons (PAHs) are commonly accepted to be the carriers of the AIBs but the hypothesis suffers from the lack of identification of individual species. This seriously limits the potential of these spectral signatures as probes of astrophysical conditions and processes.
We propose to exploit the unique capabilities of Herschel to record the far-IR emission features of PAHs in the Red Rectangle. These features are very specific to the exact molecular identity and are a very attractive route for the spectroscopic identification of PAHs. Most of them carry a sharp Q branch increasing the contrast for their detection. We ask for 24.8 hours to perform deep PACS and SPIRE FTS spectroscopy to search for these Q branches.
The unique carbon-rich Red Rectangle nebula is in an active stage of dust condensation and displays the strongest AIBs known as well as emission bands connected to the DIBs. Small molecules are underabundant and the Red Rectangle is clearly the place where the formation of large molecules can be tracked and the link between the AIB and DIB carriers should be explored.
The analysis of the bands will be performed using spectroscopic data, both theoretical and experimental, as well as Monte Carlo modelling to simulate the emission process. Even in the worst case of no band identification, comparison of model predictions with the Herschel spectra will strongly constrain the PAH model. If some lines are identified, we will apply for follow-up observations... with HIFI to resolve the hot band structure of the Q branches and structure in the P and R branches.
Considering the short life of Herschel, this proposal is therefore very timely.
Herschel was launched on 14 May 2009! It is the fourth 'cornerstone' mission in the ESA science programme. With a 3.5 m Cassegrain telescope it is the largest space telescope ever launched. It is performing photometry and spectroscopy in approximately the 55-671 µm range, bridging the gap between earlier infrared space missions and groundbased facilities.
