Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...503..792b&link_type=abstract
Astrophysical Journal v.503, p.792
Astronomy and Astrophysics
Astronomy
17
Ism: Structure, Ism: Planetary Nebulae: Individual Ngc Number: Ngc 7293, Stars: Mass Loss
Scientific paper
The H alpha surface brightness distribution of cometary knots in the Helix Nebula is investigated. The brightness profiles of their photoionized surfaces (cusps) drop exponentially with increasing distance r from the center of the knots: Sigma (r) ~ exp (-r/h). An r-3 profile, which would be expected for a freely expanding ionization region, in general does not provide a good fit. The cusps of all the knots have the same universal exponential brightness scale height h of 2 pixels (6.3 x 1014 cm), which is also independent of the direction with respect to the central star. The origin of the cometary knots and the formation and nature of their luminous cusps is investigated. We propose that the cusps are in hydrostatic equilibrium rather than freely expanding, but we have been unable to identify a physical mechanism that will constrain them. Ly alpha radiation pressure acting on the dust is evaluated as a mechanism to do the confinement, but it lacks sufficient energy density, and the scale height expected disagrees with what is observed.
Burkert Andreas
O'Dell Charles Robert
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