Kinematics of molecular gas in the proto-planetary nebula CRL 2688 (the "egg")

John H. Bieging, Nguyen Q. Rieu

Research output: Contribution to journalArticlepeer-review

19 Scopus citations


We present aperture synthesis images of the HCN J = 1 - 0 emission line toward the proto-planetary nebula CRL 2688 (the "Egg Nebula"). The images were obtained with the BIMA mm interferometer, and have angular and velocity resolutions of 3″ and 1.3 km s-1. The emission is well resolved spatially and is extended mainly in the east-west direction, coincident with the optical dark lane. There are also extensions of the HCN emission to the north and south, along the symmetry axis of the optical bipolar nebula. The north-south extensions are not consistent with predictions of photodissociation models for HCN. The morphology of the HCN maps shows a striking correspondence with the four bright lobes of the S(1) line of H2, which lie at the ends of the extended HCN emission. The velocity centroid of the HCN J = 1-0 line shows a significant gradient across the nebula. The magnitude of the gradient is larger than was found in previous work, attributable to improved angular resolution. The most blueshifted emission is to the north and east of the central star, while the most redshifted emission is to the south and west. The steepest gradient lies along a line intermediate between the bipolar axis (at P.A. 15°) and the dark equatorial lane (P.A. 105°). The gradients along each of these two axes are nearly equal, with a value of 0.9 km s-1 arcsec-1. We suggest that the observed velocity gradient may be attributed to two kinematic components: (1) a radial outflow in which the velocity along or near the bipolar axis exceeds the flow velocity in the equatorial plane by at least a factor of 1.5; and (2) a rotational component in the equatorial plane. The implied specific angular momentum in the rotational component to the gas motion could be supplied from the orbital angular momentum of a binary companion. We discuss some possible mechanisms for transferring angular momentum, and argue that gravitational torques are unlikely to be effective enough to produce the observed velocity gradient, but that magnetic fields, acting through Alfvén waves, could do so if the stellar surface field is sufficiently high (of order several kilogauss).

Original languageEnglish (US)
Pages (from-to)706-716
Number of pages11
JournalAstronomical Journal
Issue number2
StatePublished - Aug 1996

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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