360 research outputs found
Polarization Profiles of Scattered Emission Lines. I. General Formalism for Optically Thin Rayleigh Scattering
A general theoretical framework is developed for interpreting
spectropolarimetric observations of optically thin emission line scattering
from small dust particles. Spatially integrated and spatially resolved line
profiles of both scattered intensity and polarization are calculated
analytically from a variety of simple kinematic models. These calculations will
provide a foundation for further studies of emission line scattering from dust
and electrons in such diverse astrophysical environments as Herbig-Haro
objects, symbiotic stars, starburst galaxies and active galactic nuclei.Comment: 18 pages, LaTeX, (uuencoded compressed postscript figures [648kB]
available from author [[email protected]]), accepted for publication in
Ap.J., IAUNAM contrib.#33
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
Studies of NGC 6720 with Calibrated HST WFC3 Emission-Line Filter Images--I: Structure and Evolution
We have performed a detailed analysis of the Ring Nebula (NGC 6720) using HST
WFC3 images and derived a new 3-D model. Existing high spectral resolution
spectra played an important supplementary role in our modeling. It is shown
that the Main Ring of the nebula is an ionization-bounded irregular
non-symmetric disk with a central cavity and perpendicular extended lobes
pointed almost towards the observer. The faint outer halos are determined to be
fossil radiation, i.e. radiation from gas ionized in an earlier stage of the
nebula when it was not ionization bounded.
The narrow-band WFC3 filters that isolate some of the emission-lines are
affected by broadening on their short wavelength side and all the filters were
calibrated using ground-based spectra. The filter calibration results are
presented in an appendix.Comment: Accepted for publication by the Astronomical Journa
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