1,066 research outputs found
Angular diameters, fluxes and extinction of compact planetary nebulae: further evidence for steeper extinction towards the Bulge
We present values for angular diameter, flux and extinction for 70 Galactic
planetary nebulae observed using narrow band filters. Angular diameters are
derived using constant emissivity shell and photoionization line emission
models. The mean of the results from these two models are presented as our best
estimate. Contour plots of 36 fully resolved objects are included and the low
intensity contours often reveal an elliptical structure that is not always
apparent from FWHM measurements. Flux densities are determined, and for both
H-alpha and O[III] there is little evidence of any systematic differences
between observed and catalogued values. Observed H-alpha extinction values are
determined using observed H-alpha and catalogued radio fluxes. H-alpha
extinction values are also derived from catalogued H-alpha and H-beta flux
values by means of an Rv dependent extinction law. Rv is then calculated in
terms of observed extinction values and catalogued H-alpha and H-beta flux
values. Comparing observed and catalogue extinction values for a subset of
Bulge objects, observed values tend to be lower than catalogue values
calculated with Rv = 3.1. For the same subset we calculate = 2.0,
confirming that toward the Bulge interstellar extinction is steeper than Rv =
3.1. For the inner Galaxy a relation with the higher supernova rate is
suggested, and that the low-density warm ionized medium is the site of the
anomalous extinction. Lowvalues of extinction are also derived using dust
models with a turnover radius of 0.08 microns.Comment: Accepted by MNRAS. 17 pages, 9 figures (including 36 contour plots of
PNe), 5 Tables (including 2 large tables of angular diameters, fluxes and
extinction
Chemical Abundances of Planetary Nebulae in the Sagittarius Dwarf Elliptical Galaxy
Spectrophotometry and imaging of the two planetary nebulae He2-436 and
Wray16-423, recently discovered to be in the Sagittarius dwarf elliptical
galaxy, are presented. Wray16-423 is a high excitation planetary nebula (PN)
with a hot central star. In contrast He2-436 is a high density PN with a cooler
central star and evidence of local dust, the extinction exceeding that for
Wray16-423 by E(B-V)=0.28. The extinction to Wray16-423, (E(B-V)=0.14), is
consistent with the extinction to the Sagittarius (Sgr) Dwarf. Both PN show
Wolf-Rayet features in their spectra, although the lines are weak in
Wray16-423. Images in [O III] and H-alpha+[N II], although affected by poor
seeing, yield a diameter of 1.2'' for Wray16-423 after deconvolution; He~2-436
was unresolved. He2-436 has a luminosity about twice that of Wray16-423 and its
size and high density suggest a younger PN. In order to reconcile the differing
luminosity and nebular properties of the two PN with similar age progenitor
stars, it is suggested that they are on He burning tracks
The abundance pattern is very similar in both nebulae and shows an oxygen
depletion of -0.4 dex with respect to the mean O abundance of Galactic PN and
[O/H] = -0.6. The Sgr PN progenitor stars are representative of the higher
metallicity tail of the Sgr population. The pattern of abundance depletion is
similar to that in the only other PN in a dwarf galaxy companion of the Milky
Way, that in Fornax, for which new spectra are presented. However the
abundances are larger than for Galactic halo PN suggesting a later formation
age. The O abundance of the Sgr galaxy deduced from its PN, shows similarities
with that of dwarf ellipticals around M31, suggesting that this galaxy was a
dwarf elliptical before its interaction with the Milky Way.Comment: 24 pages, Latex (aas2pp4.sty) including 5 postscript figures. To
appear in Ap
The Acceleration of the Nebular Shells in Planetary Nebulae in the Milky Way Bulge
We present a systematic study of line widths in the [\ion{O}{3}]5007
and H lines for a sample of 86 planetary nebulae in the Milky Way bulge
based upon spectroscopy obtained at the \facility{Observatorio Astron\'omico
Nacional in the Sierra San Pedro M\'artir (OAN-SPM)} using the Manchester
Echelle Spectrograph. The planetary nebulae were selected with the intention of
simulating samples of bright extragalactic planetary nebulae. We separate the
planetary nebulae into two samples containing cooler and hotter central stars,
defined by the absence or presence, respectively, of the \ion{He}{2}
6560 line in the H spectra. This division separates samples of
younger and more evolved planetary nebulae. The sample of planetary nebulae
with hotter central stars has systematically larger line widths, larger radii,
lower electron densities, and lower H luminosities. The distributions of
these parameters in the two samples all differ at significance levels exceeding
99%. These differences are all in agreement with the expectations from
hydrodynamical models, but for the first time confirmed for a homogeneous and
statistically significant sample of galactic planetary nebulae. We interpret
these differences as evidence for the acceleration of the nebular shells during
the early evolution of these intrinsically bright planetary nebulae. As is the
case for planetary nebulae in the Magellanic Clouds, the acceleration of the
nebular shells appears to be the direct result of the evolution of the central
stars.Comment: accepted for publication in Astrophysical Journa
Three-micron spectra of AGB stars and supergiants in nearby galaxies
The dependence of stellar molecular bands on the metallicity is studied using
infrared L-band spectra of AGB stars (both carbon-rich and oxygen-rich) and
M-type supergiants in the Large and Small Magellanic Clouds (LMC and SMC) and
in the Sagittarius Dwarf Spheroidal Galaxy. The spectra cover SiO bands for
oxygen-rich stars, and acetylene (C2H2), CH and HCN bands for carbon-rich AGB
stars. The equivalent width of acetylene is found to be high even at low
metallicity. The high C2H2 abundance can be explained with a high
carbon-to-oxygen (C/O) ratio for lower metallicity carbon stars. In contrast,
the HCN equivalent width is low: fewer than half of the extra-galactic carbon
stars show the 3.5micron HCN band, and only a few LMC stars show high HCN
equivalent width. HCN abundances are limited by both nitrogen and carbon
elemental abundances. The amount of synthesized nitrogen depends on the initial
mass, and stars with high luminosity (i.e. high initial mass) could have a high
HCN abundance. CH bands are found in both the extra-galactic and Galactic
carbon stars. None of the oxygen-rich LMC stars show SiO bands, except one
possible detection in a low quality spectrum. The limits on the equivalent
widths of the SiO bands are below the expectation of up to 30angstrom for LMC
metallicity. Several possible explanations are discussed. The observations
imply that LMC and SMC carbon stars could reach mass-loss rates as high as
their Galactic counterparts, because there are more carbon atoms available and
more carbonaceous dust can be formed. On the other hand, the lack of SiO
suggests less dust and lower mass-loss rates in low-metallicity oxygen-rich
stars. The effect on the ISM dust enrichment is discussed.Comment: accepted for A&
Critical Race Theory and Education: racism and anti-racism in educational theory and praxis
What is Critical Race Theory (CRT) and what does it offer educational researchers and practitioners outside the US? This paper addresses these questions by examining the recent history of antiracist research and policy in the UK. In particular, the paper argues that conventional forms of antiracism have proven unable to keep pace with the development of increasingly racist and exclusionary education polices that operate beneath a veneer of professed tolerance and diversity. In particular, contemporary antiracism lacks clear statements of principle and theory that risk reinventing the wheel with each new study; it is increasingly reduced to a meaningless slogan; and it risks appropriation within a reformist âcan doâ perspective dominated by the de-politicized and managerialist language of school effectiveness and improvement. In contrast, CRT offers a genuinely radical and coherent set of approaches that could revitalize critical research in education across a range of inquiries, not only in self-consciously "multicultural" studies. The paper reviews the developing terrain of CRT in education, identifying its key defining elements and the conceptual tools that characterise the work. CRT in education is a fast changing and incomplete project but it can no longer be ignored by the academy beyond North America
Bright Planetary Nebulae and their Progenitors in Galaxies Without Star Formation
We present chemical abundances for planetary nebulae in M32, NGC 185, and NGC
205 based upon spectroscopy obtained at the Canada-France-Hawaii Telescope
using the Multi-Object Spectrograph. From these and similar data compiled from
the literature for other Local Group galaxies, we consider the origin and
evolution of the stellar progenitors of bright planetary nebulae in galaxies
where star formation ceased long ago. The ratio of neon to oxygen abundances in
bright planetary nebulae is either identical to that measured in the
interstellar medium of star-forming dwarf galaxies or at most changed by a few
percent, indicating that neither abundance is significantly altered as a result
of the evolution of their stellar progenitors. Several planetary nebulae appear
to have dredged up oxygen, but these are the exception, not the rule. The
progenitors of bright planetary nebulae typically enhance their original helium
abundances by less than 50%. In contrast, nitrogen enhancements can reach
factors of 100. However, nitrogen often shows little or no enhancement,
suggesting that nitrogen enrichment is a random process. The helium, oxygen,
and neon abundances argue that the typical bright planetary nebulae in all of
the galaxies considered here are the progeny of stars with initial masses of
approximately 1.5 Msun or less, based upon the nucleosynthesis predictions of
current theoretical models. These models, however, are unable to explain the
nitrogen enrichment or its scatter. Similar conclusions hold for the bright
planetary nebulae in galaxies with ongoing star formation. Thus, though
composition varies significantly, there is unity in the sense that the
progenitors of typical bright planetary nebulae appear to have undergone
similar physical processes. (Abridged)Comment: accepted for publication in the Astrophysical Journa
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