Infrared Spectroscopy of the Transition Objects CRL 618 and CRL 2688

Extract

We have observed CRL 618, an infrared object believed to be in transition between the red giant and planetary nebula stages, in seven lines of vibrationally excited H₂ and the Brackett α and γ lines of ionized hydrogen. The H₂ observations were made in 1979 and repeated in 1982, extending over more than four years monitoring of H₂ emission from this source. The intensity of H₂ emission approximately tripled between August 1977 and November 1979 and may have increased by another 15-20 per cent between November 1979 and February 1982. This increase in emission occurred during the same time period that the radio free-free flux roughly doubled (Kwok and Feldman, 1981). The object may now be in a more quiescent phase following a strong flare. The H₂ lines appear to be in thermal equilibrium at ≈ 2000 K and to suffer only small extinction. The Brackett α and γ lines are more heavily obscured, with ≈ 2 mag. of 2.17 μm extinction. These results support the model that the compact H II region is behind and expanding into the dense molecular material where the H₂ is found. The mass of hot H₂ is, ≈ 4 × 10⁻⁴ M and of total ejecta ≈ .1 M_⊙, so the mass-loss rate is probably > .