760 research outputs found
Absolute Dimensions and Apsidal Motion of the Young Detached System LT Canis Majoris
New high resolution spectra of the short period (P~1.76 days) young detached
binary LT CMa are reported for the first time. By combining the results from
the analysis of new radial velocity curves and published light curves, we
determine values for the masses, radii and temperatures as follows: M_1= 5.59
(0.20) M_o, R_1=3.56 (0.07) R_o and T_eff1= 17000 (500) K for the primary and
M_2=3.36 (0.14) M_o, R_2= 2.04 (0.05) R_o and T_eff2= 13140 (800) K for the
secondary. Static absorbtion features apart from those coming from the close
binary components are detected in the several spectral regions. If these
absorbtion features are from a third star, as the light curve solutions
support, its radial velocity is measured to be RV_3=70(8) km s^-1. The orbit of
the binary system is proved to be eccentric (e=0.059) and thus the apsidal
motion exists. The estimated linear advance in longitude of periastron
corresponds to an apsidal motion of U=694+/-5 yr for the system. The average
internal structure constant log k_2,obs=-2.53 of LT CMa is found smaller than
its theoretical value of log k_2,theo=-2.22 suggesting the stars would have
more central concentration in mass. The photometric distance of LT CMa
(d=535+/-45 pc) is found to be much smaller than the distance of CMa OB1
association (1150 pc) which rules out membership. A comparison with current
stellar evolution models for solar metallicity indicates that LT CMa (35 Myr)
is much older than the CMa OB1 association (3 Myr), confirming that LT CMa is
not a member of CMa OB1. The kinematical and dynamical analysis indicate LT CMa
is orbiting the Galaxy in a circular orbit and belongs to the young thin-disk
population.Comment: 19 pages, 6 figures and 6 tables, accepted for publication in
Publication of the Astronomical Society of Japa
Study of Eclipsing Binary and Multiple Systems in OB Associations: I. Ori OB1a - IM Mon
All available photometric and spectroscopic observations were collected and
used as the basis of a detailed analysis of the close binary IM Mon. The
orbital period of the binary was refined to 1.19024249(0.00000014) days. The
Roche equipotentials, fractional luminosities (in (B, V) and H_p bands) and
fractional radii for the component stars in addition to mass ratio q,
inclination i of the orbit and the effective temperature T_eff of the secondary
cooler less massive component were obtained by the analysis of light curves. IM
Mon is classified to be a detached binary system in contrast to the contact
configuration estimations in the literature. The absolute parameters of IM Mon
were derived by the simultaneous solutions of light and radial velocity curves
as M_1,2=5.50(0.24)M_o and 3.32(0.16)M_o, R1,2=3.15(0.04)R_o and 2.36(0.03)R_o,
T_eff1,2=17500(350) K and 14500(550) K implying spectral types of B4 and B6.5
ZAMS stars for the primary and secondary components respectively. The modelling
of the high resolution spectrum revealed the rotational velocities of the
component stars as V_rot1=147(15) km/s and V_rot2=90(25) km/s. The photometric
distance of 353(59) pc was found more precise and reliable than Hipparcos
distance of 341(85) pc. An evolutionary age of 11.5(1.5) Myr was obtained for
IM Mon. Kinematical and dynamical analysis support the membership of the young
thin-disk population system IM Mon to the Ori OB1a association dynamically.
Finally, we derived the distance, age and metallicity information of Ori OB1a
sub-group using the information of IM Mon parameters.Comment: 26 pages, 5 figures and 6 tables, accepted for publication in
Publication of the Astronomical Society of Japa
Molecular Regulation of Paused Pluripotency in Early Mammalian Embryos and Stem Cells
The energetically costly mammalian investment in gestation and lactation requires plentiful nutritional sources and thus links the environmental conditions to reproductive success. Flexibility in adjusting developmental timing enhances chances of survival in adverse conditions. Over 130 mammalian species can reversibly pause early embryonic development by switching to a near dormant state that can be sustained for months, a phenomenon called embryonic diapause. Lineage-specific cells are retained during diapause, and they proliferate and differentiate upon activation. Studying diapause thus reveals principles of pluripotency and dormancy and is not only relevant for development, but also for regeneration and cancer. In this review, we focus on the molecular regulation of diapause in early mammalian embryos and relate it to maintenance of potency in stem cells in vitro. Diapause is established and maintained by active rewiring of the embryonic metabolome, epigenome, and gene expression in communication with maternal tissues. Herein, we particularly discuss factors required at distinct stages of diapause to induce, maintain, and terminate dormancy
d_{x^2-y^2}-wave superconductivity and the Hubbard model
The numerical studies of d_{x^2-y^2}-wave pairing in the two-dimensional (2D)
and the 2-leg Hubbard models are reviewed. For this purpose, the results
obtained from the determinantal Quantum Monte Carlo and the density-matrix
renormalization-group calculations are presented. These are calculations which
were motivated by the discovery of the high-T_c cuprates. In this review, the
emphasis is placed on the microscopic many-body processes which are responsible
for the d_{x^2-y^2}-wave pairing correlations observed in the 2D and the 2-leg
Hubbard models. In order to gain insight into these processes, the results on
the effective pairing interaction as well as the magnetic, density and the
single-particle excitations will be reviewed. In addition, comparisons will be
made with the other numerical approaches to the Hubbard model and the numerical
results on the t-J model. The results reviewed here indicate that an effective
pairing interaction which is repulsive at (pi,pi) momentum transfer and
enhanced single-particle spectral weight near the (pi,0) and (0,pi) points of
the Brillouin zone create optimum conditions for d_{x^2-y^2}-wave pairing.
These are two effects which act to enhance the d_{x^2-y^2}-wave pairing
correlations in the Hubbard model. Finding additional ways is an active
research problem.Comment: 85 pages, 63 figures, to appear in Advances in Physics, vol. 51, no.
6 (2002
Hole-hole superconducting pairing in the t-J model induced by spin-wave exchange
We study numerically the hole pairing induced by spin-wave exchange. The
contact hole-hole interaction is taken into account as well. It is assumed that
antiferromagnetic order is preserved at all scales relevant to pairing. The
strongest pairing is obtained for the d-wave symmetry of the gap. Dependence of
the value of the gap on hole concentration and temperature is presented. For
the critical temperature we obtain Tc about 100 K at the hole concentration
delta = 0.2-0.3.Comment: replaced with a revised version to appear in PRB, 6 pages, REVTeX
3.0, figures not change
Superconductivity and spin triplet collective mode in the t-J model close to antiferromagnetic instability
To investigate relations between long-range antiferromagnetic (AF) order,
superconductivity and two particle triplet collective excitations we consider a
modified two dimensional t-J model at doping close to half filling. The model
includes additional hopping t'' and nearest sites Coulomb repulsion V. The
additional parameters allow us to control closeness of the system to the AF
instability. We demonstrate the possibility of co-existence of long-range AF
order and d-g-wave superconductivity. In the phase with long-range AF order we
find, analytically, superconducting gaps and spin wave renormalization. We
demonstrate that at approaching the point of the AF instability the spin
triplet collective excitation arises with energy below the superconducting gap.Comment: 9 page
HOT or not: examining the basis of high-occupancy target regions
High-occupancy target (HOT) regions are segments of the genome with unusually high number of transcription factor binding sites. These regions are observed in multiple species and thought to have biological importance due to high transcription factor occupancy. Furthermore, they coincide with house-keeping gene promoters and consequently associated genes are stably expressed across multiple cell types. Despite these features, HOT regions are solemnly defined using ChIP-seq experiments and shown to lack canonical motifs for transcription factors that are thought to be bound there. Although, ChIP-seq experiments are the golden standard for finding genome-wide binding sites of a protein, they are not noise free. Here, we show that HOT regions are likely to be ChIP-seq artifacts and they are similar to previously proposed 'hyper-ChIPable' regions. Using ChIP-seq data sets for knocked-out transcription factors, we demonstrate presence of false positive signals on HOT regions. We observe sequence characteristics and genomic features that are discriminatory of HOT regions, such as GC/CpG-rich k-mers, enrichment of RNA-DNA hybrids (R-loops) and DNA tertiary structures (G-quadruplex DNA). The artificial ChIP-seq enrichment on HOT regions could be associated to these discriminatory features. Furthermore, we propose strategies to deal with such artifacts for the future ChIP-seq studies
Magnetic Impurity in the two-dimensional Heisenberg Antiferromagnet
We analyze the ground state properties of the two-dimensional quantum
antiferromagnet with a S=1/2 Kondo impurity. Perturbation theory around the
strong Kondo coupling limit is developed and the results compared with studies,
based on exact diagonalization of small clusters. We find that at intermediate
coupling the impurity is partially screened and the magnetization locally
suppressed. A local singlet between the impurity and the host spin is formed
asymptotically.Comment: 12 REVTex pages, 4 Postscript figure
Single-hole properties in the - and strong-coupling models
We report numerical results for the single-hole properties in the -
model and the strong-coupling approximation to the Hubbard model in two
dimensions. Using the hopping basis with over states we discuss (for an
infinite system) the bandwidth, the leading Fourier coefficients in the
dispersion, the band masses, and the spin-spin correlations near the hole. We
compare our results with those obtained by other methods. The band minimum is
found to be at () for the - model for , and for the strong-coupling model for . The bandwidth
in both models is approximately at large , in rough agreement with
loop-expansion results but in disagreement with other results. The
strong-coupling bandwidth for t/J\agt6 can be obtained from the - model
by treating the three-site terms in first-order perturbation theory. The
dispersion along the magnetic zone face is flat, giving a large
parallel/perpendicular band mass ratio.Comment: 1 RevTeX file with epsf directives to include 8 .eps figures 8 figure
files encoded using uufile
Kink Structure in the Quasiparticle Band of Doped Hubbard Systems
By making use of the self-consistent projection operator method with
high-momentum and high-energy resolutions, we find a kink structure in the
quasiparticle excitation spectrum of the two-dimensional Hubbard model in the
underdoped regime. The kink is caused by a mixing between the quasiparticle
state and excitations with short-range antiferromagnetic order. We suggest that
this might be the origin of the strong concentration dependence of the 'kink'
found in La_{2-x}Sr_{x}CuO_{4} (x=0.03-0.07).Comment: 3 pages, 4 figures. to be published in J. Phys. Soc. Jpn., Vol. 74,
No. 9, September 15, 200
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