| Description |
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. |
