Abstract
Sensitive observations of the 21-cm neutral hydrogen line toward the compact planetary nebula IC418 reveal an absorption feature at the velocity of the near side of the expanding nebula, plus an emission line red-shifted with respect to the system velocity. For a distance of 2 kpc, the emission line flux implies a total neutral hydrogen mass of 0.07 solar masses, similar to the mass of the central ionized nebula. If the neutral gas is undergoing symmetric outflow, the relative intensity of the emission and absorption features indicate a spin-excitation temperature of about 1000 K.
| Original language | English |
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| Pages (from-to) | L5-L8 |
| Number of pages | 4 |
| Journal | Astronomy & Astrophysics |
| Volume | 176 |
| Issue number | 1 |
| Publication status | Published - Apr-1987 |