742 research outputs found
Polarization Profiles of Scattered Emission Lines. II. Upstream Dust Scattering in the HH 1 Jet
Detailed comparisons are made between observations of scattered light
upstream of the head of the HH~1 jet and predictions of simple scattering
models. It is shown that, in order to unambiguously determine the velocity of
the head of the jet (bow shock) with respect to the upstream dust, existing
spectroscopic observations are insufficient and that spectropolarimetric
observations of the scattered light are necessary. Such an independent measure
of the bow shock velocity is important in order to test ``multiple outflow''
theories of Herbig-Haro jets. It is also shown 2that the scattering dust must
have a very forward-throwing scattering phase function
(\langle\cos\theta\rangle\msim 0.7) and slight evidence is found for a
dust-gas ratio that is higher than average.Comment: 11 pages, uuencoded compressed postscript (including 9 figures),
accepted for publication in Ap.J., IAUNAM_contrib.#34
Determination of the Physical Conditions of the Knots in the Helix Nebula from Optical and Infrared Observations
[Abridged] We use new HST and archived images to clarify the nature of the
knots in the Helix Nebula. We employ published far infrared spectrophotometry
and existing 2.12 micron images to establish that the population distribution
of the lowest ro-vibrational states of H2 is close to the distribution of a gas
in LTE at 988 +- 119 K. We derive a total flux from the nebula in H2 lines and
compare this with the power available from the central star for producing this
radiation. We establish that neither soft X-rays nor FUV radiation has enough
energy to power the H2 radiation, only the stellar EUV radiation shortward of
912 Angstrom does. Advection of material from the cold regions of the knots
produces an extensive zone where both atomic and molecular hydrogen are found,
allowing the H2 to directly be heated by Lyman continuum radiation, thus
providing a mechanism that can explain the excitation temperature and surface
brightness of the cusps and tails. New images of the knot 378-801 reveal that
the 2.12 micron cusp and tail lie immediately inside the ionized atomic gas
zone. This firmly establishes that the "tail" structure is an ionization
bounded radiation shadow behind the optically thick core of the knot. A unique
new image in the HeII 4686 Angstrom line fails to show any emission from knots
that might have been found in the He++ core of the nebula. We also re-examined
high signal-to-noise ratio ground-based telescope images of this same inner
region and found no evidence of structures that could be related to knots.Comment: Astronomical Journal, in press. Some figures are shown at reduced
resolution. A full resolution version is available at
http://www.ifront.org/wiki/Helix_Nebula_2007_Pape
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