383 research outputs found
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
Stationary components of HeI in strong magnetic fields - a tool to identify magnetic DB white dwarfs
In only three of the 61 known magnetic white dwarfs helium has been
identified unambiguously while about 20% of all non-magnetic stars of this
class are known to contain HeI or HeII. Until recently, data for HeI data were
available only for magnetic fields below 20MG. This changed with the
publication of extensive data by the group in Heidelberg. The corresponding
calculations have now been completed for the energetically lowest five states
of singlet and triplet symmetry for the subspaces with |m| <= 3; selected
calculations have been performed for even higher excitations. In strongly
magnetized white dwarfs only line components are visible whose wavelengths vary
slowly with respect to the magnetic field, particularly stationary components
which have a wavelength minimum or maximum in the range of the magnetic fields
strengths on the stellar surface. In view of the many ongoing surveys finding
white dwarfs we want to provide the astronomical community with a tool to
identify helium in white dwarfs for fields up to 5.3GG. To this end we present
all calculated helium line components whose wavelengths in the UV, optical, and
near IR vary slowly enough with respect to the field strength to produce
visible absorption features. We also list all stationary line components in
this spectral range. Finally, we find series of minima and maxima which occur
as a result of series of extremal transitions to increasingly higher
excitations. We estimated the limits for 8 series which can possibly give rise
to additional absorption in white dwarf spectra; one strong absorption feature
in GD229 which is yet unexplained by stationary components is very close to two
estimated series limits.Comment: 8 pages, 2 figures, accepted for publication by Astronomy and
Astrophysic
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
Irreducible tensor-form of the relativistic corrections to the M1 transition operator
The relativistic corrections to the magnetic dipole moment operator in the
Pauli approximation were derived originally by Drake (Phys. Rev. A 3(1971)908).
In the present paper, we derive their irreducible tensor-operator form to be
used in atomic structure codes adopting the Fano-Racah-Wigner algebra for
calculating its matrix elements.Comment: 26 page
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.
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
Regulatory de novo mutations underlying intellectual disability
The genetic aetiology of a major fraction of patients with intellectual disability (ID) remains unknown. De novo mutations (DNMs) in protein-coding genes explain up to 40% of cases, but the potential role of regulatory DNMs is still poorly understood. We sequenced 63 whole genomes from 21 ID probands and their unaffected parents. In addition, we analysed 30 previously sequenced genomes from exome-negative ID probands. We found that regulatory DNMs were selectively enriched in fetal brain-specific enhancers as compared with adult brain enhancers. DNM-containing enhancers were associated with genes that show preferential expression in the prefrontal cortex. Furthermore, we identified recurrently mutated enhancer clusters that regulate genes involved in nervous system development (CSMD1, OLFM1, and POU3F3). Most of the DNMs from ID probands showed allele-specific enhancer activity when tested using luciferase assay. Using CRISPR-mediated mutation and editing of epigenomic marks, we show that DNMs at regulatory elements affect the expression of putative target genes. Our results, therefore, provide new evidence to indicate that DNMs in fetal brain-specific enhancers play an essential role in the aetiology of ID
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