419 research outputs found
Hyperfine quenching of the metastable states in divalent atoms
Hyperfine quenching rates of the lowest-energy metastable and
states of Mg, Ca, Sr, and Yb atoms are computed. The calculations are carried
out using ab initio relativistic many-body methods. The computed lifetimes may
be useful for designing novel ultra-precise optical clocks and trapping
experiments with the fermionic isotopes. The resulting natural widths of
the clock transition are 0.44 mHz for Mg, 2.2 mHz for
Ca, 7.6 mHz for Sr, 43.5 mHz for Yb, and 38.5 mHz for
Yb. Compared to the bosonic isotopes, the lifetime of the states
in fermionic isotopes is noticeably shortened by the hyperfine quenching but
still remains long enough for trapping experiments.Comment: 10 pages, 1 figure, submitted to Phys. Rev.
On the feasibility of cooling and trapping metastable alkaline-earth atoms
Metastability and long-range interactions of Mg, Ca, and Sr in the
lowest-energy metastable state are investigated. The calculated
lifetimes are 38 minutes for Mg*, 118 minutes for Ca*, and 17 minutes for Sr*,
supporting feasibility of cooling and trapping experiments. The
quadrupole-quadrupole long-range interactions of two metastable atoms are
evaluated for various molecular symmetries. Hund's case (c) 4_g potential
possesses a large 100-1000 K potential barrier. Therefore magnetic trap losses
can possibly be reduced using cold metastable atoms in a stretched M=2 state.
Calculations were performed in the framework of ab initio relativistic
configuration interaction method coupled with the random-phase approximation.Comment: 8 pages, 2 figures; to appear in PR
Laser gas-discharge absorption measurements of the ratio of two transition rates in argon
The ratio of two line strengths at 922.7 nm and 978.7 nm of argon is measured
in an argon pulsed discharge with the use of a single-mode Ti:Sapphire laser.
The result 3.29(0.13) is in agreement with our theoretical prediction 3.23 and
with a less accurate ratio 2.89(0.43) from the NIST database.Comment: 5 pages, 2 figures, 1 tabl
Study of the photon flux from the night sky at La Palma and Namibia, in the wavelength region relevant for imaging atmospheric Cherenkov telescopes
The level of the night sky background light at La Palma and Namibia was
determined, with emphasis on the wavelength region and solid angle coverage
relevant for the operation of imaging atmospheric Cherenkov telescopes. The
dependence of the night sky background light both on celestial coordinates
(alt, az) and on galactic coordinates (b, l) was measured, with an angular
resolution of about 1 degr. Average light levels near the zenith are similar in
both locations -- 2.2 x 10^12 to 2.6 x 10^12 photons per sr s m^2 for 300 nm <
lambda < 650 nm. With increasing zenith angle the level of background light
increases at La Palma, whereas a constant level is measured in Namibia. Near
the center of the Milky Way, background light levels are increased by a factor
up to 4 and more. Also the level of light backscattered from the ground has
been studied.Comment: 21 Pages, 12 figures, Latex, Nucl. Instr. Meth., in pres
[Fe IV] emission in ionized nebulae
This paper presents an analysis of [Fe IV] emission based on new
identifications and previous measurements of [Fe IV] lines in 30 Doradus, IC
4846, M42, SMC N88A, and SBS 0335-052. The Fe abundances obtained by adding the
abundances of the relevant Fe ions (mainly Fe^{++} and Fe^{3+}) are found to be
lower, by factors in the range 2.6-5.9, than the Fe abundances implied by [Fe
III] emission and an ionization correction factor derived from ionization
models. The most likely explanation of this discrepancy is that either the
collision strengths for [Fe IV] or the Fe ionization fractions predicted by
models are unreliable. The available data neither allow one to distinguish
between these two possibilities nor to exclude another possible explanation:
that the discrepancy implies the presence of a gradient in the Fe abundance
within the ionized gas. Further measurements of [Fe IV] lines and checks on the
Fe^{3+} atomic data and ionization models are needed to reach a definitive
conclusion. The discrepancy introduces an uncertainty in the determination of
Fe abundances in ionized nebulae. This uncertainty has implications for our
understanding of both the evolution of dust in ionized nebulae and the chemical
history of low metallicity galaxies.Comment: 23 pages, 3 figures, accepted for publication in ApJ, a new table and
several comments adde
Ionization Structure and Spectra of Iron in Gaseous Nebulae}
The emission spectra and the ionization structure of the low ionization
stages of iron, Fe I--IV, in gaseous nebulae are studied. This work includes:
(i) new atomic data: photoionization cross sections, total e-ion recombination
rates, excitation collision strengths, and transition probabilities; (ii)
detailed study of excitation mechanisms for the [Fe II], [Fe III], and [Fe IV]
emission, and spectroscopic analysis of the observed IR, optical, and UV
spectra; (iii) study of the physical structure and kinematics of the nebulae
and their ionization fronts. Spectral analysis of the well observed Orion
nebula is carried out as a test case, using extensive collisional-radiative and
photoionization models. It is shown that the [Fe II] emission from the Orion
nebula is predominantly excited via electron collisions in high density
partially ionized zones; radiative fluorescence is relatively less effective.
Further evidence for high density zones is derived from the [O I] and [Ni II]
spectral lines, as well as from the kinematic measurements of ionic species in
the nebula. The ionization structure of iron in Orion is modeled using the
newly calculated atomic data, showing some significant differences from
previous models. The new model suggests a fully ionized H II region at
densities on the order of cm, and a dynamic partially ionized H
II/H I region at densities of \cm3. Photoionization models also
indicate that the optical [O I] and [Fe II] emission originates in high density
partially ionized regions within ionization fronts. The gas phase iron
abundance in Orion is estimated from observed spectra.Comment: AAS LaTex, 60 pages 18 figures. Astrophysical Journal. in pres
Properties of metastable alkaline-earth-metal atoms calculated using an accurate effective core potential
The first three electronically excited states in the alkaline-earth-metal
atoms magnesium, calcium, and strontium comprise the (nsnp) triplet P^o_J
(J=0,1,2) fine-structure manifold. All three states are metastable and are of
interest for optical atomic clocks as well as for cold-collision physics. An
efficient technique--based on a physically motivated potential that models the
presence of the ionic core--is employed to solve the Schroedinger equation for
the two-electron valence shell. In this way, radiative lifetimes, laser-induced
clock shifts, and long-range interaction parameters are calculated for
metastable Mg, Ca, and Sr.Comment: 13 pages, 9 table
Late Spectral Evolution of SN 1987A: I. Temperature and Ionization
The temperature and ionization of SN 1987A is modeled between 200 and 2000
days in its nebular phase, using a time-dependent model. We include all
important elements, as well as the primary composition zones in the supernova.
The energy input is provided by radioactive decay of Co-56, Co-57, and Ti-44.
The thermalization of the resulting gamma-rays and positrons is calculated by
solving the Spencer-Fano equation. Both the ionization and the individual level
populations are calculated time-dependently. Adiabatic cooling is included in
the energy equation. Charge transfer is important for determining the
ionization and is included with available and estimated rates. Full, multilevel
atoms are used for the observationally important ions. As input models to the
calculations we use explosion models for SN 1987A calculated by Woosley et al
and Nomoto et al. The most important result in this paper refers to the
evolution of the temperature and ionization of the various abundance zones. The
metal-rich core undergoes a thermal instability, often referred to as the
IR-catastrophe, at 600 - 1000 days. The hydrogen-rich zones evolve
adiabatically after 500 - 800 days, while in the helium region both adiabatic
cooling and line cooling are of equal importance after ~1000 days. Freeze-out
of the recombination is important in the hydrogen and helium zones. Concomitant
with the IR-catastrophe, the bulk of the emission shifts from optical and
near-IR lines to the mid- and far-IR. After the IR-catastrophe, the cooling is
mainly due to far-IR lines and adiabatic expansion. Dust cooling is likely to
be important in the zones where dust forms. We find that the dust condensation
temperatures occur later than ~500 days in the oxygen-rich zones, and the most
favorable zone for dust condensation is the iron core.Comment: 53 pages, including 10 figures; ApJ (Main Journal); scheduled for
April 1, 1998, Vol. 49
Evidence for binarity in the bipolar planetary nebulae A79, He2-428 and M1-91
We present low and high resolution long-slit spectra of three bipolar
planetary nebulae (PNe) with bright central cores: A79, He2-428 and M1-91.
He2-428 and M1-91 have high density (from 10^3.3 to 10^6.5 cm^-3) unresolved
nebular cores that indicate that strong mass loss/exchange phenomena are
occurring close to their central stars. An F0 star is found at the centre of
symmetry of A79; its reddening and distance are consistent with the association
of the star with the nebula. The spectrum of the core of He2-428 shows
indications of the presence of a hot star with red excess emission, probably
arising in a late-type companion. A79 is one of the richest PNe in N and He,
the abundances of M1-91 are at the lower end of the range spanned by bipolar
PNe, and He2-428 shows very low abundances, similar to those measured for halo
PNe. The extended nebulae of A79 and He2-428 have inclined equatorial rings
expanding at a velocity of approx. 15 km/s, with kinematical ages 10^4 yr. The
association of these aged, extended nebulae with a dense nebular core (He2-428)
or a relatively late type star (A79) is interpreted as evidence for the
binarity of their nuclei.Comment: 13 pages including 8 tables. A&A accepted; also available at
http://www.iac.es/publicaciones/preprints.htm
Recurrent sudden unexpected death in infancy : a case series of sibling deaths
Objectives: To determine the rate of sudden unexpected death in infancy (SUDI) for infants born after a previous SUDI in the same family, and to establish the causes of death and the frequency of child protection concerns in families with recurrent SUDI.
Design: Observational study using clinical case records.
Setting: The UKâs Care of Next Infant (CONI) programme, which provides additional care to families who have experienced SUDI with their subsequent children.
Patients: Infants registered on CONI between January 2000 and December 2015.
Main outcome measures: Cause of death, presence of modifiable risk factors for SUDI and child protection concerns.
Results: There were 6608 live-born infants registered in CONI with 29 deaths. 26 families had 2 deaths, and 3 families had 3 deaths. The SUDI rate for infants born after one SUDI is 3.93 (95% CI 2.7 to 5.8) per 1000 live births. Cause of death was unexplained for 19 first and 15 CONI deaths. Accidental asphyxia accounted for 2 first and 6 CONI deaths; medical causes for 3 first and 4 CONI deaths; and homicide for 2 first and 4 CONI deaths. 10 families had child protection concerns.
Conclusions: The SUDI rate for siblings is 10 times higher than the current UK SUDI rate. Homicide presenting as recurrent SUDI is very rare. Many parents continued to smoke and exposed infants to hazardous co-sleeping situations, with these directly leading to or contributing to the death of six siblings. SUDI parents need support to improve parenting skills and reduce risk to subsequent infants
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