MOLECULAR GAS and DUST at the CENTER of the EGG NEBULAMOLECULAR GAS and DUST at the CENTER of the EGG NEBULAJeremy Lim and Dinh-V-Trung (Institute of Astronomy & Astrophysics, Academia Sinica, Taiwan)

Introduction

The Egg Nebula (CRL 2688) is a bipolar proto-planetary nebula (PPN) that exhibits many intriguing features seen also in more distant bipolar PPN. In the optical, its most prominent features are bipolar lobes that are bisected by a dark lane. Straddling and extending beyond these lobes are two “searchlight beams,” with the darker inter-searchlight region extending into both lobes (Sahai et al. 1998a, ApJ, 493, 301). Finally, the entire nebula is crisscrossed by concentric arcs.

In the H2 =1-0 S(1) 2.122 m line, shocked molecular hydrogen gas is detected in the bipolar lobes, as well as along the equatorial region (Sahai et al. 1998b, ApJ, 492, L63). This gas delineates the interaction between fast polar and equatorial collimated outflows and the slower progenitor red-giant wind. Molecular gas thought to be entrained in these fast outflows has been imaged in 12CO (Cox et al. 2000, A&A, 353, L25). The geometric center of the bipolar lobes, searchlight beams, and collimated outflows all intersect at the location of the central illuminating post-AGB star as indicated by near-IR polarimetric imaging (Weintraub et al. 2000, ApJ, 531, 401).

Observations and Results

Here, we present observations of the Egg Nebula in the molecular line of HC3N (5-4) and dust emission at 7 mm with the VLA. Our objectives are to map the distribution and kinematics of dense molecular gas and dust at the center of the Egg Nebula, especially within the dark lane. This dark lane is thought to be produced by a dust cocoon that is flattened in the equatorial direction, and which has annular ‘holes’ centered on the polar axis through which the searchlight beams escape (Sahai et al. 1998a,b).

We find that the HC3N (5-4) traces a shell of gas with a diameter of ~5000 AU expanding at ~17 km/s, implying that the outflow from the central star was still spherically symmetric 700 years ago. The inner region of this gas shell appears to have been evacuated by the fast equatorial outflow. The dust emission originates from within the gas shell, and comprises several clumps that may lie at the ‘walls’ of the northern polar outflow. One of these dust clumps coincides with a near-IR source thought to be the binary companion of the central post-AGB star. Puzzlingly, the observed dust distribution is not able to explain the presence of either the dark lane or searchlight beams.

Dust emission at 7 mm, where features marginally detected at 1.3 mm with the IRAM PdBI (Cox et al. 2000) are imaged here at higher angular resolution and sensitivity. From the SED of the centimeter to millimeter wavelength emission, Jura et al. (2000, ApJ, 528, L105) estimate a dust mass of ~0.01 M (and hence a total gas and dust mass of ~1 M).

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Channel maps of the HC3N (5-4) molecular gas, showing the characteristic spatial-kinematic structure of an expanding envelope (with clumpy structure). The envelope is disrupted at velocities close to its systemic velocity (about –34 km/s), where the gas is concentrated in two roughly co-linear structures aligned in the equatorial direction.

Contours of the dust emission at 7 mm superposed on the integrated intensity of the HC3N (5-4) gas. Both the dust and molecular gas have a clumpy distribution, with the dust originating from within the gas shell. The synthesized beams of the gas and dust images are shown at the lower left corner, and have an angular resolution of ~0.6 (600 AU).

Composite color image of the H2 =1-0 S(1) 2.122 m line emission (red) and the 2.15 m continuum emission (green) (from Sahai et al. 1998b). Shocked molecular hydrogen gas is concentrated at the periphery of both bipolar lobes, and is especially bright at the tips. Shocked molecular hydrogen gas also is detected in the equatorial region. This gas traces the interaction between fast polar and equatorial collimated outflows and the slower progenitor red-giant wind. The near-IR source immediately south of the northern lobe is thought to be a binary companion of the central post-AGB star.

Contours showing the integrated intensity of the 12CO (2-1) line mapped with the IRAM Plateau de Bure Interferometer superposed on the H2 S(1) line and 2.15 m continuum emission (from Cox et al. 2000). The molecular gas traces fast polar and equatorial collimated outflows, and is thought to comprise entrained material. The synthesized beam of the 12CO (2-1) image is shown at the lower left corner, and has an angular resolution of ~1.0 (~1000 AU). Ignoring the dashed lines, distinct spatial-kinematic features in the 12CO (2-1) molecular gas are shown by the black arrows in the two images below.

Contours of the dust emission at 7 mm superposed on the H2 S(1) line and 2.15 m continuum emission. The two most northern dust clumps straddle the northern nebular lobe at its base. Another dust clump north-east of center coincides with the putative binary companion of the central post-AGB star. All the observed dust clumps may lie at the ‘wall’ of the northern polar outflow. No corresponding dust features, however, are detected around the southern polar outflow.

Contours showing the integrated intensity of the HC3N (5-4) line. We have not plotted the smoother large-scale structure of the molecular gas, but instead emphasized its clumpy nature. The central region of this expanding gas shell appears to have been evacuated by the fast equatorial collimated outflow. Weak spurs are visible extending beyond the shell along the northern and southern nebular lobes, and may comprise gas entrained by the fast bipolar outflow.

Composite color image of the Egg Nebula showing the NICMOS continuum-subtracted H2 S(1) image in red, the 1.65 m image in green, and the WFPC2 0.6 m image in blue (from Weintraub et al. 2000). Prominent features are the bright bipolar lobes, the searchlight beams, and the concentric arcs. Note that the darker inter-searchlight region extends into the bipolar lobes. Shocked molecular hydrogen gas is detected in the bipolar lobes, as well as the equatorial region.

HCHC33N (5-4) Molecular Gas and N (5-4) Molecular Gas and DustDust

The Optical/Near-IR NebulaThe Optical/Near-IR Nebula Molecular Gas and Dust correspondence with Optical/Near-IR Molecular Gas and Dust correspondence with Optical/Near-IR NebulaNebula

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Companion Star

Searchlight Beams

Inter-Searchlight Region

Bipolar Lobes

Equatorial Region

Concentric Arcs

Companion Star

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