281 research outputs found
Comments on ``The first detections of the Extragalactic Background Light at 3000, 5500, and 8000 A'' by Bernstein, Freedman and Madore
A critical discussion is presented of the data analysis applied by Bernstein,
Freedman and Madore (2002 ApJ, 571, 56; and ApJ 571, 85) in their measurement
of the Extragalactic Background Light. There are questionable assumptions in
the analysis of the ground-based observations of the Zodiacal Light. The
modeling of the Diffuse Galactic Light is based on an underestimated value of
the dust column density along the line of sight. Comparison with the previously
presented results from the same observations reveals a puzzling situation: in
spite of a large difference in the atmospheric scattered light corrections the
derived Extragalactic Background Light values are exactly the same. The claim
of the paper of a ``detection of the Extragalactic Background Light'' appears
premature.Comment: 6 pages, accepted for Ap
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
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
Multiconfiguration Dirac-Hartree-Fock energy levels and transition probabilities for 3d^5 in Fe IV
Multiconfiguration Dirac-Hartree-Fock electric quadrupole (E2) and magnetic
dipole (M1) transition probabilities are reported for transitions between
levels of 3d^5 in [Fe IV]. The accuracy of the ab initio energy levels and the
agreement in the length and velocity forms of the line strength for the E2
transitions are used as indicators of accuracy. The present E2 and M1
transition probabilities are compared with earlier Breit-Pauli results and
other theories. An extensive set of transition probabilites with indicators of
accuracy are reported in Appendices A and B. Recommended values of A(E2) +
A(M1) are listed in Appendix C.Comment: 16 pages, three appendices containing accuracy indicators and
recommended values for E2 and M1 transition rate
The FERRUM project: Transition probabilities for forbidden lines in [FeII] and experimental metastable lifetimes
Accurate transition probabilities for forbidden lines are important
diagnostic parameters for low-density astrophysical plasmas. In this paper we
present experimental atomic data for forbidden [FeII] transitions that are
observed as strong features in astrophysical spectra.
Aims: To measure lifetimes for the 3d^6(^3G)4s a ^4G_{11/2} and 3d^6(^3D)4s b
^4D_{1/2} metastable levels in FeII and experimental transition probabilities
for the forbidden transitions 3d^7 a ^4F_{7/2,9/2}- 3d^6(^3G)4s a ^4G_{11/2}.
Methods: The lifetimes were measured at the ion storage ring facility CRYRING
using a laser probing technique. Astrophysical branching fractions were
obtained from spectra of Eta Carinae, obtained with the Space Telescope Imaging
Spectrograph onboard the Hubble Space Telescope. The lifetimes and branching
fractions were combined to yield absolute transition probabilities.
Results: The lifetimes of the a ^4G_{11/2} and the b ^4D_{1/2} levels have
been measured and have the following values, 0.75(10) s and 0.54(3) s
respectively. Furthermore, we have determined the transition probabilities for
two forbidden transitions of a ^4F_{7/2,9/2}- a ^4G_{11/2} at 4243.97 and
4346.85 A. Both the lifetimes and the transition probabilities are compared to
calculated values in the literature.Comment: 5 pages, accepted for publication in A&
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
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
The ground state of the Lithium atom in strong magnetic fields
The ground and some excited states of the Li atom in external uniform
magnetic fields are calculated by means of our 2D mesh Hartree-Fock method for
field strengths ranging from zero up to 2.35 10^8 T. With increasing field
strength the ground state undergoes two transitions involving three different
electronic configurations: for weak fields the ground state configuration
arises from the field-free 1s^22s configuration, for intermediate fields from
the 1s^22p_{-1} configuration and in high fields the 1s2p_{-1}3d_{-2}
electronic configuration is responsible for the properties of the atom. The
transition field strengths are determined. Calculations on the ground state of
the Li+ ion allow us to describe the field-dependent ionization energy of the
Li atom. Some general arguments on the ground states of multi-electron atoms in
strong magnetic fields are provided.Comment: 11 pages, 6 figures, submitted to Physical Review
[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
The ground state of the carbon atom in strong magnetic fields
The ground and a few excited states of the carbon atom in external uniform
magnetic fields are calculated by means of our 2D mesh Hartree-Fock method for
field strengths ranging from zero up to 2.35 10^9 T. With increasing field
strength the ground state undergoes six transitions involving seven different
electronic configurations which belong to three groups with different spin
projections S_z=-1,-2,-3. For weak fields the ground state configuration arises
from the field-free 1s^2 2s^2 2p_0 2p_{-1}, S_z=-1 configuration. With
increasing field strength the ground state involves the four S_z=-2
configurations 1s^22s2p_0 2p_{-1}2p_{+1}, 1s^22s2p_0 2p_{-1}3d_{-2}, 1s^22p_0
2p_{-1}3d_{-2}4f_{-3} and 1s^22p_{-1}3d_{-2}4f_{-3}5g_{-4}, followed by the two
fully spin polarized S_z=-3 configurations 1s2p_02p_{-1}3d_{-2}4f_{-3}5g_{-4}
and 1s2p_{-1}3d_{-2}4f_{-3}5g_{-4}6h_{-5}. The last configuration forms the
ground state of the carbon atom in the high field regime \gamma>18.664. The
above series of ground state configurations is extracted from the results of
numerical calculations for more than twenty electronic configurations selected
due to some general energetical arguments.Comment: 6 figures,acc. Phys.Rev.
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