623 research outputs found
Modeling the spectrum of V4334 Sgr (Sakurai's Object)
Theoretical spectral energy distributions were computed for a grid of
hydrogen-deficient and carbon-rich model atmospheres of T(eff) in the range of
5000-6250 K and log g = 1.0 - 0.0 by the technique of opacity sampling, taking
into account continuous, molecular band and atomic line absorption. These
energy distributions were compared with the spectrum of V4334 Sgr (Sakurai's
object) of April, 1997 in the wavelength interval 300-1000 nm. We show that (1)
the shape of the theoretical spectra depends strongly on T(eff) but only very
weakly on the hydrogen abundance; (2) the comparison of the observed and
computed spectra permits to estimate T(eff) approximately 5500 K for V4334 Sgr
in April, 1997, and its interstellar reddening (plus a possible circumstellar
contribution) E(B-V) approximately 0.70.Comment: 7 pages, 8 figures, LaTeX, accepted by Astronomy and Astrophysic
Boron in Very Metal-Poor Stars
We have observed the B I 2497 A line to derive the boron abundances of two
very metal-poor stars selected to help in tracing the origin and evolution of
this element in the early Galaxy: BD +23 3130 and HD 84937. The observations
were conducted using the Goddard High Resolution Spectrograph on board the
Hubble Space Telescope. A very detailed abundance analysis via spectral
synthesis has been carried out for these two stars, as well as for two other
metal-poor objects with published spectra, using both Kurucz and OSMARCS model
photospheres, and taking into account consistently the NLTE effects on the line
formation. We have also re-assessed all published boron abundances of old disk
and halo unevolved stars. Our analysis shows that the combination of high
effective temperature (Teff > 6000 K, for which boron is mainly ionized) and
low metallicity ([Fe/H]<-1) makes it difficult to obtain accurate estimates of
boron abundances from the B I 2497 A line. This is the case of HD 84937 and
three other published objects (including two stars with [Fe/H] ~ -3), for which
only upper limits can be established. BD +23 3130, with [Fe/H] ~ -2.9 and
logN(B)_NLTE=0.05+/-0.30, appears then as the most metal-poor star for which a
firm measurement of the boron abundance presently exists. The evolution of the
boron abundance with metallicity that emerges from the seven remaining stars
with Teff < 6000 K and [Fe/H]<-1, for which beryllium abundances were derived
using the same stellar parameters, shows a linear increase with a slope ~ 1.
Furthermore, the B/Be ratio found is constant at a value ~ 20 for stars in the
range -3<[Fe/H]<-1. These results point to spallation reactions of ambient
protons and alpha particles with energetic particles enriched in CNO as the
origin of boron and beryllium in halo stars.Comment: 38 pages, 11 Encapsulated Postscript figures (included), uses
aaspp4.sty. Accepted for publication in The Astrophysical Journal. The
preprint is also available at: http://www.iac.es/publicaciones/preprints.htm
Optical studies of the X-ray transient XTE J2123-058 -I. Photometry
We present optical photometry of the X-ray transient XTE J2123-058, obtained
in July-October 1998. The light curves are strongly modulated on the 5.95hrs
orbital period, and exhibit dramatic changes in amplitude and form during the
decline. We used synthetic models which include the effect of partial eclipses
and X-ray heating effects, to estimate the system parameters, and we constrain
the binary inclination to be i=73+-4 degrees. The model is successful in
reproducing the light curves at different stages of the decay by requiring the
accretion disc to become smaller and thinner by 30% as the system fades by 1.7
mags in the optical. From Aug 26 the system reaches quiescence with a mean
magnitude of R=21.7+-0.1 and our data are consistent with the optical
variability being dominated by the companion's ellipsoidal modulation.Comment: 6 pages, 6 figure
Masses, Oxygen and Carbon abundances in CHEPS dwarf stars
Reproduced with permission from Astronomy & Astrophysics. © 2019 ESOContext. We report the results from the determination of stellar masses, carbon, and oxygen abundances in the atmospheres of 107 stars from the Calan-Hertfordshire Extrasolar Planet Search (CHEPS) programme. Our stars are drawn from a population with a significantly super-solar metallicity. At least 10 of these stars are known to host orbiting planets. Aims. In this work, we set out to understand the behaviour of carbon and oxygen abundance in stars with different spectral classes, metallicities, and V sin i within the metal-rich stellar population. Methods. Masses of these stars were determined using data from Gaia DR2. Oxygen and carbon abundances were determined by fitting the absorption lines. We determined oxygen abundances with fits to the 6300.304 Å O I line, and we used 3 lines of the C I atom and 12 lines of the C 2 molecule for the determination of carbon abundances. Results. We determine masses and abundances of 107 CHEPS stars. There is no evidence that the [C/O] ratio depends on V sin i or the mass of the star within our constrained range of masses, i.e. 0.82 5 km s -1) are massive stars.Peer reviewedFinal Published versio
Electronic charge and orbital reconstruction at cuprate-titanate interfaces
In complex transition metal oxide heterostructures of physically dissimilar
perovskite compounds, interface phenomena can lead to novel physical properties
not observed in either of their constituents. This remarkable feature opens new
prospects for technological applications in oxide electronic devices based on
nm-thin oxide films. Here we report on a significant electronic charge and
orbital reconstruction at interfaces between YBa2Cu3O6 and SrTiO3 studied using
local spin density approximation (LSDA) with intra-atomic Coulomb repulsion
(LSDA+U). We show that the interface polarity results in the metallicity of
cuprate-titanate superlattices with the hole carriers concentrated
predominantly in the CuO2 and BaO layers and in the first interface TiO2 and
SrO planes. We also find that the interface structural relaxation causes a
strong change of orbital occupation of Cu 3d orbitals in the CuO2 layers. The
concomitant change of Cu valency from +2 to +3 is related to the partial
occupation of the Cu orbitals at the interface with SrO planes
terminating SrTiO3. Interface-induced predoping and orbital reconstruction in
CuO2 layers are key mechanisms which control the superconducting properties of
field-effect devices developed on the basis of cuprate-titanate
heterostructures.Comment: 11 pages, 8 figures, to appear in the "Proceedings of Third Joint
HLRB and KONWIHR Result and Reviewing Workshop", Springer 200
Josephson Vortex Lattice Melting in Bi-2212
In a recent study The experiment was conducted on overdoped Bi-2212 layered structures (mesas) (see We measured the oscillating Josephson vortex flow resistance of the mesa as a function of the parallel magnetic field at several constant temperatures, with a small temperature step (see The experimentally determined Josephson vortex lattice melting diagram is consistent with the theoretical model. The maximum temperature T 0 at which the BKT phase exists corresponds to zero-field BKT transition. The critical field B * estimated for our samples with γ = 500 [11] is 0.5 T, which is close to the measured 0.6-0.7 T. The upper boundary of the triangularlattice state is also in qualitative agreement with the theoretically predicted melting line B ( T ): the field B increases with decreasing T . The continuous decrease to zero in oscillation amplitude with increasing temperature or magnetic field suggests that the boundary corresponds to a second-order phase transition. This points to a melting transition to a BKT state, because the melting transition from Josephson-lattice to BKT state must be a second-order transition at γ > 9 and B > B * In contrast to the upper boundary, the lower boundary is characterized by rapid decrease in oscillation amplitude with applied field. Currently, the mechanism underlying the lower boundary is not well understood. The oscillations occur at B > 0.5 T and are almost independent of temperature. This corresponds to the minimum of 5-7 triangular-lattice periods required for the commensuration. We believe that the lower boundary of triangular lattice states is associated with formation of a dense lattice. ACKNOWLEDGMENTS We thank A.M. Nikitina (Institute of Radio Engineering and Electronics, Russian Academy of Sciences) for providing us with single-crystal Bi-2212 whiskers. This work was supported under programs for collaboration between Russian Academy of Sciences and KOSEF (Republic of Korea) and between CRTBT (France) and IREE RAS (RFBR project no. 03-02-22001-NTsNI_a), as well as by Division of Physical Sciences of the RAS under the program "Strongly correlated electron systems and quantum critical phenomena."
Two-dimensional electron liquid state at LaAlO3-SrTiO3 interfaces
Using tunneling spectroscopy we have measured the spectral density of states
of the mobile, two-dimensional electron system generated at the LaAlO3-SrTiO3
interface. As shown by the density of states the interface electron system
differs qualitatively, first, from the electron systems of the materials
defining the interface and, second, from the two-dimensional electron gases
formed at interfaces between conventional semiconductors
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