4,196 research outputs found
Proposed nomenclature for Extragalactic Planetary Nebulae
The ability to identify and distinguish between the wide variety of celestial
objects benefits from application of a systematic and logical nomenclature.
This often includes value-added information within the naming convention which
can aid in placing the object positionally either via an RA/DEC or l,b
concatenation. All new nomenclatures should be created following IAU
guidelines. However as the number density of specific object types on the sky
increases, as in the case of PN in external galaxies, a useful positional
identifier becomes problematic. This brief but timely paper attempts to
progress the debate on this vexing issue for the case of extragalactic
planetary nebulae (EPN). There is a clear need to rationalise the current
ad-hoc system now that many thousands of Extragalactic PN are being discovered.Comment: 3 pages, no figures, LaTeX, to be published in Proceedings of the ESO
workshop on Planetary Nebulae beyond the Milky Way held at ESO, Garching, May
19-21, 200
Chemical compositions and plasma parameters of planetary nebulae with Wolf-Rayet and wels type central stars
Aims: Chemical compositions and other properties of planetary nebulae around
central stars of spectral types [WC], [WO], and wels are compared with those of
`normal' central stars, in order to clarify the evolutionary status of each
type and their interrelation. Methods: We use plasma diagnostics to derive from
optical spectra the plasma parameters and chemical compositions of 48 planetary
nebulae. We also reanalyze the published spectra of a sample of 167 non-WR PN.
The results as well as the observational data are compared in detail with those
from other studies of the objects in common. Results: The central star's
spectral type is clearly correlated with electron density, temperature and
excitation class of the nebula, [WC] nebulae tend to be smaller than the other
types. All this corroborates the view of an evolutionary sequence from cool [WC
11] central stars inside dense, low excitation nebulae towards hot [WO 1] stars
with low density, high excitation nebulae. The wels PN, however, appear to be a
separate class of objects, not linked to WRPN by evolution, --abridged--Comment: 17 pages, 28 figures, Accepted in A&A. Accepted in A&
Newly discovered halos and outer features around southern planetary nebulae
We have used the SuperCOSMOS H-alpha Survey to look for faint outer
structures such as halos, ansae and jets around known planetary nebulae across
4000 square degrees of the southern Milky Way. Our search will contribute to a
more accurate census of these features in the Galactic PN population. Candidate
common-envelope PNe have also been identified on the basis of their
microstructures. We also intend to determine more reliable distances for these
PNe, which should allow a much better statistical basis for the post-AGB total
mass budget. Our survey offers fresh scope to address this important issue.Comment: 2 pages, 1 figure. To be published in Planetary Nebulae: an Eye to
the Future, Proceedings of IAU Symposium 283, held in Puerto de la Cruz,
Tenerife, Spain, July 25-29 201
Binary planetary nebulae nuclei towards the Galactic bulge. II. A penchant for bipolarity and low-ionisation structures
Considerable effort has been applied towards understanding the precise
shaping mechanisms responsible for the diverse range of morphologies exhibited
by planetary nebulae (PNe). A binary companion is increasingly gaining support
as a dominant shaping mechanism, however morphological studies of the few PNe
that we know for certain were shaped by binary evolution are scarce or biased.
Newly discovered binary central stars (CSPN) from the OGLE-III photometric
variability survey have significantly increased the sample of post
common-envelope (CE) nebulae available for morphological analysis. We present
Gemini South narrow-band images for most of the new sample to complement
existing data in a qualitative morphological study of 30 post-CE nebulae.
Nearly 30% of nebulae have canonical bipolar morphologies, however this rises
to 60% once inclination effects are incorporated with the aid of geometric
models. This is the strongest observational evidence yet linking CE evolution
to bipolar morphologies. A higher than average proportion of the sample shows
low-ionisation knots, filaments or jets suggestive of a binary origin. These
features are also common around emission-line nuclei which may be explained by
speculative binary formation scenarios for H-deficient CSPN.Comment: Accepted for publication in A&
Combining local- and large-scale models to predict the distributions of invasive plant species
Habitat-distribution models are increasingly used to predict the potential distributions of invasive species and to inform monitoring. However, these models assume that species are in equilibrium with the environment, which is clearly not true for most invasive species. Although this assumption is frequently acknowledged, solutions have not been adequately addressed. There are several potential methods for improving habitat-distribution models. Models that require only presence data may be more effective for invasive species, but this assumption has rarely been tested. In addition, combining modeling types to form ‘ensemble’ models may improve the accuracy of predictions. However, even with these improvements, models developed for recently invaded areas are greatly influenced by the current distributions of species and thus reflect near- rather than long-term potential for invasion. Larger scale models from species’ native and invaded ranges may better reflect long-term invasion potential, but they lack finer scale resolution. We compared logistic regression (which uses presence/absence data) and two presence-only methods for modeling the potential distributions of three invasive plant species on the Olympic Peninsula in Washington State, USA. We then combined the three methods to create ensemble models. We also developed climate-envelope models for the same species based on larger scale distributions and combined models from multiple scales to create an index of near- and long-term invasion risk to inform monitoring in Olympic National Park (ONP). Neither presence-only nor ensemble models were more accurate than logistic regression for any of the species. Larger scale models predicted much greater areas at risk of invasion. Our index of near- and long-term invasion risk indicates that \u3c4% of ONP is at high near-term risk of invasion while 67-99% of the Park is at moderate or high long-term risk of invasion. We demonstrate how modeling results can be used to guide the design of monitoring protocols and monitoring results can in turn be used to refine models. We propose that by using models from multiple scales to predict invasion risk and by explicitly linking model development to monitoring, it may be possible to overcome some of the limitations of habitat-distribution models
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