Modelling the brightness profiles of the Orion proplyds
W. J. Henney & S. J. Arthur
Instituto de Astronomía, UNAM, México
Submitted to The Astronomical Journal, 20 Feb 1998
Abstract:
We investigate whether simple models of a photoevaporated flow from
an externally ionized neutral clump or wind can reproduce the
observed H$\alpha$ intensity profiles of the proplyds in the inner
Orion nebula. We find that models fit to the bright ``cusp'' at the
head of each proplyd successfully predict the brightness
distribution of the extended diffuse ``atmosphere'' between the
proplyd and the ionizing star of the nebula (theta 1 Ori C). This is strong
evidence that the ``atmospheres'' are freely expanding
photoevaporated flows and are not confined, hence contradicting the
claims of a recent study by O'Dell.
The model fits imply that the initial Mach number of the flow from
the ionization front is close to unity, which is consistent with
theoretical predictions that such fronts should be approximately
D-critical. The dust-gas ratio in the flow is not tightly
constrained by the models. The distribution of projected radii of
sources in the inner Trapezium cluster indicates (i) that both the
proplyds and non-proplyd stars in the cluster are strongly peaked
about theta 1 Ori C on a 0.02~pc scale, much smaller than has previously
been claimed, and (ii) that the fraction of low-mass stars that are
proplyds probably decreases smoothly with increasing physical
distance from theta 1 Ori C. Both these statements are supported by the
distribution of inclination angles of the best-fit proplyd models,
which allow the 3-dimensional distribution of proplyds to be
constrained.
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Will Henney <will@astrosmo.unam.mx>
Last modified: 30 Mar 1998