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Knots in the Helix Nebula found in H2 | | M. Matsuura
; A.K. Speck
; B.M. McHunu
; I. Tanaka
; N.J. Wright
; M.D. Smith
; A.A. Zijlstra
; S. Viti
; R. Wesson
; | | Date: |
16 Jun 2009 | | Abstract: | We present a deep and wide field-of-view (4’x 7’) image of the planetary
nebula (PN) NGC 7293 (the Helix Nebula) in the 2.12 micron H2 v=1-0 S(1) line.
The excellent seeing (0.4’’) at the Subaru Telescope, allows the details of
cometary knots to be examined. The knots are found at distances of 2.2’-6.4’
from the central star (CS). At the inner edge and in the inner ring (up to 4.5’
fromthe 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 multi-peaks within a tail. In the outer ring (4.5’-6.4’
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 photo-ionization 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 (AGB) 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. | | Source: | arXiv, 0906.2870 | | Services: | Forum | Review | PDF | Favorites | |
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