40,277 research outputs found
Chiral Hierarchies, Compositeness and the Renormalization Group
A wide class of models involve the fine--tuning of significant hierarchies
between a strong--coupling ``compositeness'' scale, and a low energy dynamical
symmetry breaking scale. We examine the issue of whether such hierarchies are
generally endangered by Coleman--Weinberg instabilities. A careful study using
perturbative two--loop renormalization group methods finds that consistent
large hierarchies are not generally disallowed.Comment: 22 pp + 5 figs (uuencoded and submitted separately),
SSCL-Preprint-490; FERMI-PUB-93/035-
XMM-Newton and INTEGRAL analysis of the Supergiant Fast X-ray Transient IGR J17354-3255
We present the results of combined INTEGRAL and XMM-Newton observations of
the supergiant fast X-ray transient (SFXT) IGR J173543255. Three XMM-Newton
observations of lengths 33.4 ks, 32.5 ks and 21.9 ks were undertaken, the first
an initial pointing to identify the correct source in the field of view and the
latter two performed around periastron. Simultaneous INTEGRAL observations
across of the orbital cycle were analysed but the source was neither
detected by IBIS/ISGRI nor by JEM-X. The XMM-Newton light curves display a
range of moderately bright X-ray activity but there are no particularly strong
flares or outbursts in any of the three observations. We show that the spectral
shape measured by XMM-Newton can be fitted by a consistent model throughout the
observation, suggesting that the observed flux variations are driven by
obscuration from a wind of varying density rather than changes in accretion
mode. The simultaneous INTEGRAL data rule out simple extrapolation of the
simple powerlaw model beyond the XMM-Newton energy range.Comment: 13 pages, 9 figures, This article has been accepted for publication
in Monthly Notices of the Royal Astronomical Society Published by Oxford
University Pres
Li-rich RGB stars in the Galactic Bulge
We present Lithium abundance determination for a sample of K giant stars in
the galactic bulge. The stars presented here are the only 13 stars with
detectable Lithium line (6767.18 A) among ~400 stars for which we have spectra
in this wavelength range, half of them in Baade's Window (b=-4) and half in a
field at b=-6. The stars were observed with the GIRAFFE spectrograph of
FLAMES@VLT, with a spectral resolution of R~20,000. Abundances were derived via
spectral synthesis and the results are compared with those for stars with
similar parameters, but no detectable Li line. We find 13 stars with a
detectable Li line, among which 2 have abundances A(Li)>2.7. No clear
correlations were found between the Li abundance and those of other elements.
With the exception of the two most Li rich stars, the others follow a fairly
tight A(Li)-T_eff correlation. It would seems that there must be a Li
production phase during the red giant branch (RGB), acting either on a very
short timescale, or selectively only in some stars. The proposed Li production
phase associated with the RGB bump cannot be excluded, although our targets are
significantly brighter than the predicted RGB bump magnitude for a population
at 8 kpcComment: 8 pages, 9 figures, accepted for publication in A&
New insights on accretion in Supergiant Fast X-ray Transients from XMM-Newton and INTEGRAL observations of IGR J175442619
XMM-Newton observations of the supergiant fast X-ray transient
IGRJ175442619 are reported and placed in the context of an analysis of
archival INTEGRAL/IBIS data that provides a refined estimate of the orbital
period at 4.92720.0004 days. A complete outburst history across the
INTEGRAL mission is reported. Although the new XMM-Newton observations (each
lasting 15 ks) targeted the peak flux in the phase-folded hard X-ray
light curve of IGRJ175442619, no bright outbursts were observed, the
source spending the majority of the exposure at intermediate luminosities of
the order of several 10ergs (0.510keV) and
displaying only low level flickering activity. For the final portion of the
exposure, the luminosity of IGRJ175442619 dropped to
410ergs (0.5 - 10 keV), comparable with the
lowest luminosities ever detected from this source, despite the observations
being taken near to periastron. We consider the possible orbital geometry of
IGRJ175442619 and the implications for the nature of the mass transfer
and accretion mechanisms for both IGRJ175442619 and the SFXT population.
We conclude that accretion under the `quasi-spherical accretion' model provides
a good description of the behaviour of IGRJ175442619, and suggest an
additional mechanism for generating outbursts based upon the mass accumulation
rate in the hot shell (atmosphere) that forms around the NS under the
quasi-spherical formulation. Hence we hope to aid in explaining the varied
outburst behaviours observed across the SFXT population with a consistent
underlying physical model.Comment: 12 pages, 5 figures, accepted for publication in MNRA
Zinc abundances in Galactic bulge field red giants: implications for DLA systems
Zinc in stars is an important reference element because it is a proxy to Fe
in studies of damped Lyman-alpha systems, permitting a comparison of chemical
evolution histories of bulge stellar populations and DLAs. In terms of
nucleosynthesis, it behaves as an alpha element because it is enhanced in
metal-poor stars. The aim of this work is to derive the iron-peak element Zn
abundances in 56 bulge giants from high resolution spectra. These results are
compared with data from other bulge samples, as well as from disk and halo
stars, and damped Lyman-alpha systems, in order to better understand the
chemical evolution in these environments. High-resolution spectra were obtained
using FLAMES+UVES on the Very Large Telescope. We find [Zn/Fe]=+0.24+-0.02 in
the range -1.3 < [Fe/H] < -0.5 and [Zn/Fe]=+0.06+-0.02 in the range -0.5 <
[Fe/H] -0.1, it shows a spread of -0.60 < [Zn/Fe]
< +0.15, with most of these stars having low [Zn/Fe]<0.0. These low zinc
abundances at the high metallicity end of the bulge define a decreasing trend
in [Zn/Fe] with increasing metallicities. A comparison with Zn abundances in
DLA systems is presented, where a dust-depletion correction was applied for
both Zn and Fe. Finally, we present a chemical evolution model of Zn enrichment
in massive spheroids, representing a typical classical bulge.Comment: Accepted in Astronomy & Astrophysics, in press Date of acceptance:
13/05/2015. 19 pages, 14 Figs in Astronomy & Astrophysics, 201
Validity of the second law in nonextensive quantum thermodynamics
The second law of thermodynamics in nonextensive statistical mechanics is
discussed in the quantum regime. Making use of the convexity property of the
generalized relative entropy associated with the Tsallis entropy indexed by q,
Clausius' inequality is shown to hold in the range of q between zero and two.
This restriction on the range of the entropic index, q, is purely quantum
mechanical and there exists no upper bound of q for validity of the second law
in classical theory.Comment: 12 pages, no figure
Non-LTE abundances of Mg and K in extremely metal-poor stars and the evolution of [O/Mg], [Na/Mg], [Al/Mg] and [K/Mg] in the Milky Way
LTE abundances of light elements in extremely metal-poor (EMP) stars have
been previously derived from high quality spectra. New derivations, free from
the NLTE effects, will better constrain the models of the Galactic chemical
evolution and the yields of the very first supernovae. The NLTE profiles of the
magnesium and potassium lines have been computed in a sample of 53 extremely
metal-poor stars with a modified version of the program MULTI and adjusted to
the observed lines in order to derive the abundances of these elements. The
NLTE corrections for magnesium and potassium are in good agreement with the
works found in the literature. The abundances are slightly changed, reaching a
better precision: the scatter around the mean of the abundance ratios has
decreased. Magnesium may be used with confidence as reference element. Together
with previously determined NLTE abundances of sodium and aluminum, the new
ratios are displayed, for comparison, along the theoretical trends proposed by
some models of the chemical evolution of the Galaxy, using different models of
supernovae
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