VLA Observations of H I in the Helix Nebula (NGC 7293)

摘要

We report the detection of 21 cm line emission from H I in the planetary nebula NGC 7293 (the Helix Nebula). The observations, made with the Very Large Array, show the presence of a ring of atomic hydrogen that is associated with the outer portion of the ionized nebula. This ring is most probably gas ejected in the asymptotic giant branch phase that has been subsequently photodissociated by radiation from the central star. The H I emission spreads over ~50 km s-1 in radial velocity. The mass in H I is ~0.07 M☉, about 3 times larger than the mass in molecular hydrogen and comparable to the mass in ionized hydrogen.