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P. P. Sorokin, J. H. Glownia (IBM Research Division)
An explanation based upon nonlinear photoexcitation of H2 is offered for the most striking features seen in the visible emission spectrum of the Red Rectangle (RR) nebula - three intense, relatively sharp, red-shaded bands appearing near 5798 Å, 5851 Å, and 6615 Å.
In the model, the \lambda5851 band is assigned as an emission step in a Ly-\alpha-pumped, four-wave parametric oscillation (FWPO) process originating from the level X14, J''=1. Pumping of this FWPO process occurs on the strong transition B'6-14R1 [Ly\alpha-57], producing broadband Stokes-wave IR emission on the strong 8.12- \mum transition B'6, J'=2 to J2(\Pig+), J=3. The FWPO step producing light at ~5851 Åinvolves the strong transition J2(\Pig+), J=3 to B8, J'=2. The fourth step in this FWPO scheme occurs on B8-14R1 and generates broadband VUV emission at ~1565 Å.
The \lambda5798 band occurs as a step in a six-wave parametric oscillation process, with the first two steps being identical to those in the FWPO scheme just described. Here two additional IR emission steps occur between the ~8.12-\mum IR Stokes-wave emission and the visible RR emission, which in this case occurs on the strong transition J1(\Pig+), J=3 to B6, J'=2. The first of these extra IR steps occurs on the strong transition J2 (\Pig+), J=3 to B'3, J'=2 (also at 8.12 \mum). The second extra IR step occurs on a strong transition at 12.44 \mum. Broadband emission at ~1621 Åis generated on B6-14R1.
The \lambda6615 emission is produced in an eight-wave parametric oscillation process on the transition GK0, J=1 to B5, J'=2. The first three steps are identical to those in the \lambda5798 scheme. Three IR steps occurring on strong transitions at 6.8 \mum, 7.57 \mum, and 7.80 \mum link B'3, J'=2 with GK0, J=1. Emission at ~1652 Åoccurs on B5-14R1.
Since the pumping transition is the same in all three schemes, one can understand why the intensities of all three RR emission bands vary together with changing offsets from HD 44179. The requirement for phase-matching in a parametric oscillation process can explain why the RR bands appear red shaded, and shift to longer wavelengths as HD 44179 is approached.