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A "firework" of H2 knots in the planetary nebula NGC 7293 (the helix nebula)

Matsuura, M. ORCID:, Speck, A. K., McHunu, B. M., Tanaka, I., Wright, N. J., Smith, M. D., Zijlstra, A. A., Viti, S. and Wesson, R. 2009. A "firework" of H2 knots in the planetary nebula NGC 7293 (the helix nebula). The Astrophysical Journal 700 (2) , 1067–1077. 10.1088/0004-637x/700/2/1067

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We present a deep and wide field-of-view (4' × 7') image of the planetary nebula (PN) NGC 7293 (the Helix Nebula) in the 2.12 μm H2 v = 1 → 0 S(1) line. The excellent seeing (0farcs4) at the Subaru Telescope, allows the details of cometary knots to be examined. The knots are found at distances of 2farcm2–6farcm4 from the central star (CS). At the inner edge and in the inner ring (up to 4farcm5 from the CS), the knot often show a "tadpole" shape, an elliptical head with a bright crescent inside and a long tail opposite to the CS. In detail, there are variations in the tadpole shapes, such as narrowing tails, widening tails, meandering tails, or multipeaks within a tail. In the outer ring (4farcm5–6farcm4 from the CS), the shapes are more fractured, and the tails do not collimate into a single direction. The transition in knot morphology from the inner edge to the outer ring is clearly seen. The number density of knots governs the H2 surface brightness in the inner ring: H2 exists only within the knots. Possible mechanisms which contribute to the shaping of the knots are discussed, including photoionization and streaming motions. A plausible interpretation of our images is that inner knots are being overrun by a faster wind, but that this has not (yet) reached the outer knots. Based on H2 formation and destruction rates, H2 gas can survive in knots from formation during the late asymptotic giant branch phase throughout the PN phase. These observations provide new constraints on the formation and evolution of knots, and on the physics of molecular gas embedded within ionized gas.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Physics and Astronomy
Publisher: American Astronomical Society
ISSN: 1538-4357
Date of Acceptance: 28 May 2009
Last Modified: 13 May 2024 13:30

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