291 research outputs found
Atomic Processes in Planetary Nebulae and H II Regions
Spectroscopic studies of Planetary Nebulae (PNe) and H {\sc ii} regions have
driven much development in atomic physics. In the last few years the
combination of a generation of powerful observatories, the development of ever
more sophisticated spectral modeling codes, and large efforts on mass
production of high quality atomic data have led to important progress in our
understanding of the atomic spectra of such astronomical objects. In this paper
I review such progress, including evaluations of atomic data by comparisons
with nebular spectra, detection of spectral lines from most iron-peak elements
and n-capture elements, observations of hyperfine emission lines and analysis
of isotopic abundances, fluorescent processes, and new techniques for
diagnosing physical conditions based on recombination spectra. The review is
directed toward atomic physicists and spectroscopists trying to establish the
current status of the atomic data and models and to know the main standing
issues.Comment: 9 pages, 1 figur
Absolute Spectrophotometry of Northern Compact Planetary Nebulae
We present medium-dispersion spectra and narrowband images of six northern
compact planetary nebulae (PNe): BoBn 1, DdDm 1, IC 5117, M 1-5, M 1-71, and
NGC 6833. From broad-slit spectra, total absolute fluxes and equivalent widths
were measured for all observable emission lines. High signal-to noise emission
line fluxes of H-alpha, H-beta, [OIII], [NII], and HeI may serve as emission
line flux standards for northern hemisphere observers. From narrow-slit
spectra, we derive systemic radial velocities. For four PNe, available emission
line fluxes were measured with sufficient signal-to-noise to probe the physical
properties of their electron densities, temperatures, and chemical abundances.
BoBn 1 and DdDm 1, both type IV PNe, have an H-beta flux over three sigma away
from previous measurements. We report the first abundance measurements of M
1-71. NGC 6833 measured radial velocity and galactic coordinates suggest that
it is associated with the outer arm or possibly the galactic halo, and its low
abundance ([O/H]=1.3x10E-4) may be indicative of low metallicity within that
region.Comment: 9 pages, 1 figure, accepted in A&A (03/14/2005
Improved Neutron-Capture Element Abundances in Planetary Nebulae
Spectroscopy of planetary nebulae (PNe) provides the means to investigate
s-process enrichments of neutron(n)-capture elements that cannot be detected in
asymptotic giant branch (AGB) stars. However, accurate abundance determinations
of these elements present a challenge. Corrections for unobserved ions can be
large and uncertain, since in many PNe only one ion of a given n-capture
element has been detected. Furthermore, the atomic data governing the
ionization balance of these species are not well-determined, inhibiting the
derivation of accurate ionization corrections. We present initial results of a
program that addresses these challenges. Deep high resolution optical
spectroscopy of ~20 PNe has been performed to detect emission lines from
trans-iron species including Se, Br, Kr, Rb, and Xe. The optical spectral
region provides access to multiple ions of these elements, which reduces the
magnitude and importance of uncertainties in the ionization corrections. In
addition, experimental and theoretical efforts are providing determinations of
the photoionization cross-sections and recombination rate coefficients of Se,
Kr, and Xe ions. These new atomic data will make it possible to derive robust
ionization corrections for these elements. Together, our observational and
atomic data results will enable n-capture element abundances to be determined
with unprecedented accuracy in ionized nebulae.Comment: 6 pages, 6 figures, to appear in "The Origin of the Elements Heavier
than Fe", Sep 25-27, 2008, Turin, Italy, PASA, eds. John C. Lattanzio and M.
Lugar
SCAMP:standardised, concentrated, additional macronutrients, parenteral nutrition in very preterm infants: a phase IV randomised, controlled exploratory study of macronutrient intake, growth and other aspects of neonatal care
<p>Abstract</p> <p>Background</p> <p>Infants born <29 weeks gestation are at high risk of neurocognitive disability. Early postnatal growth failure, particularly head growth, is an important and potentially reversible risk factor for impaired neurodevelopmental outcome. Inadequate nutrition is a major factor in this postnatal growth failure, optimal protein and calorie (macronutrient) intakes are rarely achieved, especially in the first week. Infants <29 weeks are dependent on parenteral nutrition for the bulk of their nutrient needs for the first 2-3 weeks of life to allow gut adaptation to milk digestion. The prescription, formulation and administration of neonatal parenteral nutrition is critical to achieving optimal protein and calorie intake but has received little scientific evaluation. Current neonatal parenteral nutrition regimens often rely on individualised prescription to manage the labile, unpredictable biochemical and metabolic control characteristic of the early neonatal period. Individualised prescription frequently fails to translate into optimal macronutrient delivery. We have previously shown that a standardised, concentrated neonatal parenteral nutrition regimen can optimise macronutrient intake.</p> <p>Methods</p> <p>We propose a single centre, randomised controlled exploratory trial of two standardised, concentrated neonatal parenteral nutrition regimens comparing a standard macronutrient content (maximum protein 2.8 g/kg/day; lipid 2.8 g/kg/day, dextrose 10%) with a higher macronutrient content (maximum protein 3.8 g/kg/day; lipid 3.8 g/kg/day, dextrose 12%) over the first 28 days of life. 150 infants 24-28 completed weeks gestation and birthweight <1200 g will be recruited. The primary outcome will be head growth velocity in the first 28 days of life. Secondary outcomes will include a) auxological data between birth and 36 weeks corrected gestational age b) actual macronutrient intake in first 28 days c) biomarkers of biochemical and metabolic tolerance d) infection biomarkers and other intravascular line complications e) incidence of major complications of prematurity including mortality f) neurodevelopmental outcome at 2 years corrected gestational age</p> <p>Trial registration</p> <p>Current controlled trials: <a href="http://www.controlled-trials.com/ISRCTN76597892">ISRCTN76597892</a>; EudraCT Number: 2008-008899-14</p
Improved Neutron-Capture Element Abundances in Planetary Nebulae
Spectroscopy of planetary nebulae (PNe) provides the means to investigate
s-process enrichments of neutron(n)-capture elements that cannot be detected in
asymptotic giant branch (AGB) stars. However, accurate abundance determinations
of these elements present a challenge. Corrections for unobserved ions can be
large and uncertain, since in many PNe only one ion of a given n-capture
element has been detected. Furthermore, the atomic data governing the
ionization balance of these species are not well-determined, inhibiting the
derivation of accurate ionization corrections. We present initial results of a
program that addresses these challenges. Deep high resolution optical
spectroscopy of ~20 PNe has been performed to detect emission lines from
trans-iron species including Se, Br, Kr, Rb, and Xe. The optical spectral
region provides access to multiple ions of these elements, which reduces the
magnitude and importance of uncertainties in the ionization corrections. In
addition, experimental and theoretical efforts are providing determinations of
the photoionization cross-sections and recombination rate coefficients of Se,
Kr, and Xe ions. These new atomic data will make it possible to derive robust
ionization corrections for these elements. Together, our observational and
atomic data results will enable n-capture element abundances to be determined
with unprecedented accuracy in ionized nebulae.Comment: 6 pages, 6 figures, to appear in "The Origin of the Elements Heavier
than Fe", Sep 25-27, 2008, Turin, Italy, PASA, eds. John C. Lattanzio and M.
Lugar
JWST observations of the Ring Nebula (NGC 6720): I. Imaging of the rings, globules, and arcs
We present JWST images of the well-known planetary nebula NGC 6720 (the Ring
Nebula), covering wavelengths from 1.6m to 25 m. The bright shell is
strongly fragmented with some 20 000 dense globules, bright in H, with a
characteristic diameter of 0.2 arcsec and density -
cm. The shell contains a thin ring of polycyclic aromatic hydrocarbon
(PAH) emission. H is found throughout the shell and in the halo. H in
the halo may be located on the swept-up walls of a biconal polar flow. The
central cavity is shown to be filled with high ionization gas and shows two
linear structures. The central star is located 2 arcsec from the emission
centroid of the cavity and shell. Linear features (`spikes') extend outward
from the ring, pointing away from the central star. Hydrodynamical simulations
are shown which reproduce the clumping and possibly the spikes. Around ten
low-contrast, regularly spaced concentric arc-like features are present; they
suggest orbital modulation by a low-mass companion with a period of about 280
yr. A previously known much wider companion is located at a projected
separation of about 15 000 au; we show that it is an M2-M4 dwarf. The system is
therefore a triple star. These features, including the multiplicity, are
similar to those seen in the Southern Ring Nebula (NGC 3132) and may be a
common aspect of such nebulae.Comment: 25 pages, 23 figures. Submitted to Monthly Notices of the Royal
Astronomical Society. Corrected typos in metadat
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