533 research outputs found
The ISO-SWS spectrum of planetary nebula NGC 7027
We present the infrared spectrum of the planetary nebula NGC7027 observed with the Short Wavelength Spectrometer (SWS), on board the Infrared Space Observatory (ISO). These data allow us to derive the electron density and, together with the IUE and optical spectra, the electron temperature for several ions. The nebular composition has: been determined, the evolutionary status of the central star is discussed and the element depletion in the nebula is given. We conclude that the progenitor was: a C-rich star with a mass: between 3 and 4 M-circle dot
Properties of Dust Grains in Planetary Nebulae -- I. The Ionized Region of NGC 6445
In this paper we study new infrared spectra of the evolved planetary nebula
NGC 6445 obtained with ISO. These data show that the thermal emission from the
grains is very cool and has a low flux compared to H beta. A model of the
ionized region is constructed, using the photo-ionization code CLOUDY 90.05.
Based on this model, we show from depletions in the gas phase elements that
little grain destruction can have occurred in the ionized region of NGC 6445.
We also argue that dust-gas separation in the nebula is not plausible. The most
likely conclusion is that grains are residing inside the ionized region of NGC
6445 and that the low temperature and flux of the grain emission are caused by
the low luminosity of the central star and the low optical depth of the grains.
This implies that the bulk of the silicon-bearing grains in this nebula were
able to survive exposure to hard UV photons for at least several thousands of
years, contradicting previously published results. A comparison between optical
and infrared diagnostic line ratios gives a marginal indication for the
presence of a t^2-effect in the nebula. However, the evidence is not convincing
and other explanations for the discrepancy are also plausible. The off-source
spectrum taken with ISO-LWS clearly shows the presence of a warm cirrus
component with a temperature of 24 K as well as a very cold component with a
temperature of 7 K. Since our observation encompasses only a small region of
the sky, it is not clear how extended the 7 K component is and whether it
contributed significantly to the FIRAS spectrum taken by COBE. Because our line
of sight is in the galactic plane, the very cold component could be a starless
core.Comment: 36 pages, 8 tables, 7 figures, accepted for publication in Ap
Infrared High-Resolution Spectroscopy of Post-AGB Circumstellar Disks. I. HR 4049 - The Winnowing Flow Observed?
High-resolution infrared spectroscopy in the 2.3-4.6 micron region is
reported for the peculiar A supergiant, single-lined spectroscopic binary HR
4049. Lines from the CO fundamental and first overtone, OH fundamental, and
several H2O vibration-rotation transitions have been observed in the
near-infrared spectrum. The spectrum of HR 4049 appears principally in emission
through the 3 and 4.6 micron region and in absorption in the 2 micron region.
The 4.6 micron spectrum shows a rich 'forest' of emission lines. All the
spectral lines observed in the 2.3-4.6 micron spectrum are shown to be
circumbinary in origin. The presence of OH and H2O lines confirm the
oxygen-rich nature of the circumbinary gas which is in contrast to the
previously detected carbon-rich material. The emission and absorption line
profiles show that the circumbinary gas is located in a thin, rotating layer
near the dust disk. The properties of the dust and gas circumbinary disk and
the spectroscopic orbit yield masses for the individual stars, M_AI~0.58 Msolar
and M_MV~0.34 Msolar. Gas in the disk also has an outward flow with a velocity
of 1 km/s. The severe depletion of refractory elements but near-solar
abundances of volatile elements observed in HR 4049 results from abundance
winnowing. The separation of the volatiles from the grains in the disk and the
subsequent accretion by the star are discussed. Contrary to prior reports, the
HR 4049 carbon and oxygen isotopic abundances are typical AGB values:
12C/13C=6^{+9}_{-4} and 16O/17O>200.Comment: 42 pages, 14 figures, Accepted by Ap
Geolocators lead to better measures of timing and renesting in black-tailed godwits and reveal the bias of traditional observational methods
Long-term population studies can identify changes in population dynamics over time. However, to realize meaningful conclusions, these studies rely on accurate measurements of individual traits and population characteristics. Here, we evaluate the accuracy of the observational methods used to measure reproductive traits in individually marked black-tailed godwits (Limosa limosa limosa). By comparing estimates from traditional methods with data obtained from light-level geolocators, we provide an accurate estimate of the likelihood of renesting in godwits and the repeatability of the lay dates of first clutches. From 2012 to 2018, we used periods of shading recorded on the light-level geolocators carried by 68 individual godwits to document their nesting behaviour. We then compared these estimates to those simultaneously obtained by our long-term observational study. We found that among recaptured geolocator-carrying godwits, all birds renested after a failed first clutch, regardless of the date of nest loss or the number of days already spent incubating. We also found that 43% of these godwits laid a second replacement clutch after a failed first replacement, and that 21% of these godwits renested after a hatched first clutch. However, the observational study correctly identified only 3% of the replacement clutches produced by geolocator-carrying individuals and designated as first clutches a number of nests that were actually replacement clutches. Additionally, on the basis of the observational study, the repeatability of lay date was 0.24 (95% CI 0.17-0.31), whereas it was 0.54 (95% CI 0.28-0.75) using geolocator-carrying individuals. We use examples from our own and other godwit studies to illustrate how the biases in our observational study discovered here may have affected the outcome of demographic estimates, individual-level comparisons, and the design, implementation and evaluation of conservation practices. These examples emphasize the importance of improving and validating field methodologies and show how the addition of new tools can be transformational
Determination of the high-pressure crystal structure of BaWO4 and PbWO4
We report the results of both angle-dispersive x-ray diffraction and x-ray
absorption near-edge structure studies in BaWO4 and PbWO4 at pressures of up to
56 GPa and 24 GPa, respectively. BaWO4 is found to undergo a pressure-driven
phase transition at 7.1 GPa from the tetragonal scheelite structure (which is
stable under normal conditions) to the monoclinic fergusonite structure whereas
the same transition takes place in PbWO4 at 9 GPa. We observe a second
transition to another monoclinic structure which we identify as that of the
isostructural phases BaWO4-II and PbWO4-III (space group P21/n). We have also
performed ab initio total energy calculations which support the stability of
this structure at high pressures in both compounds. The theoretical
calculations further find that upon increase of pressure the scheelite phases
become locally unstable and transform displacively into the fergusonite
structure. The fergusonite structure is however metastable and can only occur
if the transition to the P21/n phases were kinetically inhibited. Our
experiments in BaWO4 indicate that it becomes amorphous beyond 47 GPa.Comment: 46 pages, 11 figures, 3 table
Black Terns <i>Chlidonias niger</i> and their dietary problems in Dutch wetlands
Black Terns Chlidonias niger have shown a decrease of well over 90% as a breeding bird in The Netherlands during the twentieth century. Two hypotheses have been put forward for this decline: the disappearance of the floating plant Water Soldier Stratiotes aloides, which used to be the favourite nesting substrate of the terns, and a decrease of available insect food for the chicks, notably dragonflies. Both effects are attributed to eutrophication of surface waters. Reproductive bottlenecks vary greatly among areas and habitats. In river landscapes, no signs of food shortage could be found, and loss of nesting substrate has been successfully compensated for by offering artificial nest rafts. Extremely low fledging success in moors and in lowland grasslands is caused by food problems. In this case, artificial rafts are less successful. With decreased insect availability, fish and earthworms have become more important in the chicks' diet, but these are less reliable as a food source. Fledging success greatly depends on the amount of fish in the diet. Also, a minimum amount of fish is always needed to cover the calcium need of the chicks. In north-eastern Poland, there were no problems with either nesting places or food for the chicks.</p
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