4,874 research outputs found
Tangential Motions and Spectroscopy within NGC 6720, the Ring Nebula
We have combined recent Hubble Space Telescope WFPC2 images in the [O III]
5007 and [N II] 6583 lines with similar images made 9.557 years earlier to
determine the motion of the Ring Nebula within the plane of the sky. Scaled
ratio images argue for homologous expansion, that is, larger velocities scale
with increasing distance from the central star. The rather noisy pattern of
motion of individual features argues for the same conclusion and that the
silhouetted knots move at the same rate as the surrounding gas. These
tangential velocities are combined with information from a recent high
resolution radial velocity study to determine a dynamic distance, which is in
basic agreement with the distance determined from the parallax of the central
star. We have also obtained very high signal to noise ratio moderate resolution
spectra (9.4 Angstrom) along the major and minor axes of the nebula and from
this determined the electron temperatures and density in the multiple
ionization zones present. These results confirm the status of the Ring Nebula
as one of the older planetary nebulae, with a central star transitioning to the
white dwarf cooling curve. (Based 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 No. NAS 5-26555 and the San Pedro Martir Observatory
operated by the Universidad Nacional Autonoma de Mexico.)Comment: Astronomical Journal, in pres
Which Stars are Ionizing the Orion Nebula ?
The common assumption that Theta-1-Ori C is the dominant ionizing source for
the Orion Nebula is critically examined. This assumption underlies much of the
existing analysis of the nebula. In this paper we establish through comparison
of the relative strengths of emission lines with expectations from Cloudy
models and through the direction of the bright edges of proplyds that
Theta-2-Ori-A, which lies beyond the Bright Bar, also plays an important role.
Theta-1-Ori-C does dominate ionization in the inner part of the Orion Nebula,
but outside of the Bright Bar as far as the southeast boundary of the Extended
Orion Nebula, Theta-2-Ori-A is the dominant source. In addition to identifying
the ionizing star in sample regions, we were able to locate those portions of
the nebula in 3-D. This analysis illustrates the power of MUSE spectral imaging
observations in identifying sources of ionization in extended regions.Comment: 7 pages with 5 figure
Studies of NGC 6720 with Calibrated HST WFC3 Emission-Line Filter Images--II:Physical Conditions
We have performed a detailed analysis of the electron temperature and density
in the the Ring Nebula using the calibrated HST WFC3 images described in the
preceding paper. The electron temperature (Te) determined from [N II] and [O
III] rises slightly and monotonically towards the central star. The observed
equivalent width (EW) in the central region indicates that Te rises as high as
13000 K. In contrast, the low EW's in the outer regions are largely due to
scattered diffuse Galactic radiation by dust. The images allowed determination
of unprecedented small scale variations in Te. These variations indicate that
the mean square area temperature fluctuations are significantly higher than
expected from simple photoionization. The power producing these fluctuations
occurs at scales of less than 3.5E15 cm. This scale length provides a strong
restriction on the mechanism causing the large t^2 values observed.Comment: Accepted for publication in the Astronomical Journa
Studies of NGC 6720 with Calibrated HST WFC3 Emission-Line Filter Images--III:Tangential Motions using AstroDrizzle Images
We have been able to compare with astrometric precision AstroDrizzle
processed images of NGC 6720 (the Ring Nebula) made using two cameras on the
Hubble Space Telescope. The time difference of the observations was 12.925 yrs.
This large time-base allowed determination of tangential velocities of features
within this classic planetary nebula. Individual features were measured in [N
II] images as were the dark knots seen in silhouette against background nebular
[O III] emission. An image magnification and matching technique was also used
to test the accuracy of the usual assumption of homologous expansion. We found
that homologous expansion does apply, but the rate of expansion is greater
along the major axis of the nebula, which is intrinsically larger than the
minor axis.
We find that the dark knots expand more slowly that the nebular gas, that the
distance to the nebula is 720 pc +/-30%, and the dynamic age of the Ring Nebula
is about 4000 yrs. The dynamic age is in agreement with the position of the
central star on theoretical curves for stars collapsing from the peak of the
Asymptotic Giant Branch to being white dwarfs
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
Spectrophotometry of comets Giacobini-Zinner and Halley
Optical window spectrophotometry was performed on comets Giacobini-Zinner and Halley over the interval 300-1000 nm. Band and band-sequence fluxes were obtained for the brightest features of OH, CN, NH, and C2, special care having been given to determinations of extinction, instrumental sensitivities, and corrections for Fraunhofer lines. C2 Swan band-sequence flux ratios were determined with unprecedented accuracy and compared with the predictions of the detailed equilibrium models of Krishna Swamy et al. (1977, 1979, 1981, and 1987). It is found that these band sequences do not agree with the predictions, which calls into question the assumptions made in deriving the model, namely resonance fluorescence statistical equilibrium. Suggestions are made as to how to resolve this discrepancy
Pumping up the [N I] nebular lines
The optical [N I] doublet near 5200 {\AA} is anomalously strong in a variety
of emission-line objects. We compute a detailed photoionization model and use
it to show that pumping by far-ultraviolet (FUV) stellar radiation previously
posited as a general explanation applies to the Orion Nebula (M42) and its
companion M43; but, it is unlikely to explain planetary nebulae and supernova
remnants. Our models establish that the observed nearly constant equivalent
width of [N I] with respect to the dust-scattered stellar continuum depends
primarily on three factors: the FUV to visual-band flux ratio of the stellar
population; the optical properties of the dust; and the line broadening where
the pumping occurs. In contrast, the intensity ratio [N I]/H{\beta} depends
primarily on the FUV to extreme-ultraviolet ratio, which varies strongly with
the spectral type of the exciting star. This is consistent with the observed
difference of a factor of five between M42 and M43, which are excited by an O7
and B0.5 star respectively. We derive a non-thermal broadening of order 5 km/s
for the [N I] pumping zone and show that the broadening mechanism must be
different from the large-scale turbulent motions that have been suggested to
explain the line-widths in this H II region. A mechanism is required that
operates at scales of a few astronomical units, which may be driven by thermal
instabilities of neutral gas in the range 1000 to 3000 K. In an appendix, we
describe how collisional and radiative processes are treated in the detailed
model N I atom now included in the Cloudy plasma code.Comment: ApJ in press. 8 pages of main paper plus 11 pages of appendices, with
13 figures and 12 table
A Keck High Resolution Spectroscopic Study of the Orion Nebula Proplyds
We present the results of spectroscopy of four bright proplyds in the Orion
Nebula obtained at a velocity resolution of 6 km/s. After careful isolation of
the proplyd spectra from the confusing nebular radiation, the emission line
profiles are compared with those predicted by realistic dynamic/photoionization
models of the objects. The spectral line widths show a clear correlation with
ionization potential, which is consistent with the free expansion of a
transonic, ionization-stratified, photoevaporating flow. Fitting models of such
a flow simultaneously to our spectra and HST emission line imaging provides
direct measurements of the proplyd size, ionized density and outflow velocity.
These measurements confirm that the ionization front in the proplyds is
approximately D-critical and provide the most accurate and robust estimate to
date of the proplyd mass loss rate. Values of 0.7E-6 to 1.5E-6 Msun/year are
found for our spectroscopic sample, although extrapolating our results to a
larger sample of proplyds implies that 0.4E-6 Msun/year is more typical of the
proplyds as a whole. In view of the reported limits on the masses of the
circumstellar disks within the proplyds, the length of time that they can have
been exposed to ionizing radiation should not greatly exceed 10,000 years - a
factor of 30 less than the mean age of the proplyd stars. We review the various
mechanisms that have been proposed to explain this situation, and conclude that
none can plausibly work unless the disk masses are revised upwards by a
substantial amount.Comment: 23 pages, 8 figures, uses emulateapj.sty, accepted for publication in
The Astronomical Journal (scheduled November 1999
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