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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents



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ID: 50773.0, MPI für Astronomie / Publikationen_mpia
Knots in Nearby Planetary Nebulae
Authors:O'Dell, C. R.; Balick, B.; Hajian, A. R.; Henney, W. J.; Burkert, A.
Date of Publication (YYYY-MM-DD):2002
Title of Journal:Astronomical Journal
Volume:123
Start Page:3329
End Page:3347
Review Status:not specified
Audience:Not Specified
Abstract / Description:HST emission-line images of five of the
arguably closest planetary nebulae have shown
that there is a progression of characteristics of
their knots. This progression begins with dark
tangential structures showing no alignment with
the central star and location near the main
ionization front. At the end of the progression
in the largest nebulae, the knots are located
throughout much of the ionized zone, where they
are photoionized on the side facing the central
star and accompanied by long tails well aligned
radially. This modification of characteristics
is what would be expected if the knots were
formed near or outside the main ionization
front, obtaining densities high enough to lead to
their being only partially ionized as they are
fully illuminated by the Lyman continuum (Lyc)
radiation field. Their expansion velocities must
be lower than that of the main body of the
nebular shell. Their forms are altered by
exposure to the radiation field from the star,
although it is not clear as to the relative role
of radiation pressure acting on the dust
component vis-à-vis ionization shadowing. The one
object that does not fit into this sequence is
NGC 2392, which is the most complex nebula in
our sample. In this case the inner part of the
nebula is composed of a series of loops of
material, some being ionization bounded, which
cover only a small fraction of the area
illuminated by the star. This complex structure
may be what gives rise to the large variations
in electron temperature inferred from low spatial
resolution observations. Cometary-form knots are
seen in the outer part of this object, with
these objects closely resembling those found in
the largest nebula in our sample, NGC 7293.
Based in part on observations with the NASA/ESA
Hubble Space Telescope, obtained at the Space
Telescope Science Institute, which is operated
by the Association of Universities for Research
in Astronomy, Inc., under NASA contract NAS
5-26555.
Free Keywords:ISM: Planetary Nebulae: Individual: NGC
; Number: NGC 2392; ISM: Planetary Nebulae:
; Individual: NGC Number: NGC 6720; ISM: Planetary
; Nebulae: Individual: NGC Number: NGC 6853; ISM:
; Planetary Nebulae: Individual: NGC Number: NGC
; 7293; ISM: Planetary Nebulae: Individual:
; Alphanumeric: IC 4406
Comment of the Author/Creator:Date: 2002, June 1, 2002
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie
External Affiliations:Vanderbilt University, Box 1807-B, Nashville, TN
37235; <EMAIL>cr.odell@vanderbilt.edu</EMAIL>),
AB(Department of Astronomy, University of
Washington, Box 351580, Seattle, WA 98195-1580),
AC(US Naval Observatory, 3450 Massachusetts
Avenue, NW, Washington, DC 20392-5420),
AD(Instituto de Astronomía, Universidad Nacional
Autónoma, Apdo. Postal 3-72 (Xangari), 58089
Morelia, Michoacan, Mexico),
AE(Max-Planck-Institut für Astronomie, Königstuhl
17, D-69117, Heidelberg, Germany)
Identifiers:URL:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bi... [ID No:1]
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