1,219 research outputs found
Local integrals of motion detection of localization-protected topological order
Many-body-localized (MBL) phases can be topologically distinct, but
distinguishing these phases using order parameters can be challenging. Here we
show how topologically distinct local integrals of motion, variationally
parametrized by quantum circuits, can be used to numerically demonstrate the
topological inequivalence of MBL phases. We illustrate our approach on a
fermionic chain where both topologically distinct MBL phases and benchmark
comparisons to order parameters are possible. We also use our approach,
augmented by the DMRG-X algorithm, to extract high-energy topological doublets.
We describe applying our methods to higher dimensions to identify MBL
topological order and topological multiplets hidden by the dense many-body
spectrum.Comment: 8 pages, 7 figures; v2: accepted manuscript; v3: typo fixe
Germination at extreme temperatures : implications for alpine shrub encroachment
Worldwide, shrub cover is increasing across alpine and tundra landscapes in response to warming ambient temperatures and declines in snowpack. With a changing climate, shrub encroachment may rely on recruitment from seed occurring outside of the optimum temperature range. We used a temperature gradient plate in order to determine the germination niche of 14 alpine shrub species. We then related the range in laboratory germination temperatures of each species to long-term average temperature conditions at: (1) the location of the seed accession site and (2) across each species geographic distribution. Seven of the species failed to germinate sufficiently to be included in the analyses. For the other species, the germination niche was broad, spanning a range in temperatures of up to 17 ◦C, despite very low germination rates in some species. Temperatures associated with the highest germination percentages were all above the range of temperatures present at each specific seed accession site. Optimum germination temperatures were consistently within or higher than the range of maximum temperatures modelled across the species’ geographic distribution. Our results indicate that while some shrub species germinate well at high temperatures, others are apparently constrained by an inherent seed dormancy. Shrub encroachment in alpine areas will likely depend on conditions that affect seed germination at the microsite-scale, despite overall conditions becoming more suitable for shrubs at high elevations
VLT/FLAMES spectroscopy of red giant branch stars in the Fornax dwarf spheroidal galaxy
Fornax is one of the most massive dwarf spheroidal galaxies in the Local
Group. The Fornax field star population is dominated by intermediate age stars
but star formation was going on over almost its entire history. It has been
proposed that Fornax experienced a minor merger event. Despite recent progress,
only the high metallicity end of Fornax field stars ([Fe/H]>-1.2 dex) has been
sampled in larger number via high resolution spectroscopy. We want to better
understand the full chemical evolution of this galaxy by better sampling the
whole metallicity range, including more metal poor stars. We use the VLT-FLAMES
multi-fibre spectrograph in high-resolution mode to determine the abundances of
several alpha, iron-peak and neutron-capture elements in a sample of 47
individual Red Giant Branch stars in the Fornax dwarf spheroidal galaxy. We
combine these abundances with accurate age estimates derived from the age
probability distribution from the colour-magnitude diagram of Fornax. Similar
to other dwarf spheroidal galaxies, the old, metal-poor stars of Fornax are
typically alpha-rich while the young metal-rich stars are alpha-poor. In the
classical scenario of the time delay between SNe II and SNe Ia, we confirm that
SNe Ia started to contribute to the chemical enrichment at [Fe/H] between -2.0
and -1.8 dex. We find that the onset of SNe Ia took place between 12-10 Gyrs
ago. The high values of [Ba/Fe], [La/Fe] reflect the influence of SNe Ia and
AGB stars in the abundance pattern of the younger stellar population of Fornax.
Our findings of low [alpha/Fe] and enhanced [Eu/Mg] are compatible with an
initial mass function that lacks the most massive stars and with star formation
that kept going on throughout the whole history of Fornax. We find that massive
stars kept enriching the interstellar medium in alpha-elements, although they
were not the main contributor to the iron enrichment.Comment: Resubmitted to A&A (18/09/2014) after Referee's comment
Dynamics of nonequilibrium quasiparticles in a double superconducting tunnel junction detector
We study a class of superconductive radiation detectors in which the
absorption of energy occurs in a long superconductive strip while the redout
stage is provided by superconductive tunnel junctions positioned at the two
ends of the strip. Such a device is capable both of imaging and energy
resolution. In the established current scheme, well studied from the
theoretical and experimental point of view, a fundamental ingredient is
considered the presence of traps, or regions adjacent to the junctions made of
a superconducting material of lower gap. We reconsider the problem by
investigating the dynamics of the radiation induced excess quasiparticles in a
simpler device, i.e. one without traps. The nonequilibrium excess
quasiparticles can be seen to obey a diffusion equation whose coefficients are
discontinuous functions of the position. Based on the analytical solution to
this equation, we follow the dynamics of the quasiparticles in the device,
predict the signal formation of the detector and discuss the potentiality
offered by this configuration.Comment: 16 pages, 5 figures Submitted to Superconducting Science and
Technolog
Could the Ultra Metal-poor Stars be Chemically Peculiar and Not Related to the First Stars?
Chemically peculiar stars define a class of stars that show unusual elemental
abundances due to stellar photospheric effects and not due to natal variations.
In this paper, we compare the elemental abundance patterns of the ultra
metal-poor stars with metallicities [Fe/H] to those of a subclass of
chemically peculiar stars. These include post-AGB stars, RV Tauri variable
stars, and the Lambda Bootis stars, which range in mass, age, binarity, and
evolutionary status, yet can have iron abundance determinations as low as
[Fe/H] . These chemical peculiarities are interpreted as due to the
separation of gas and dust beyond the stellar surface, followed by the
accretion of dust depleted-gas. Contrary to this, the elemental abundances in
the ultra metal-poor stars are thought to represent yields of the most
metal-poor supernova and, therefore, observationally constrain the earliest
stages of chemical evolution in the Universe. The abundance of the elements in
the photospheres of the ultra metal-poor stars appear to be related to the
condensation temperature of that element; if so, then their CNO abundances
suggest true metallicities of [X/H]~ -2 to -4, rather than their present
metallicities of [Fe/H] < -5.Comment: Accepted for ApJ. 17 pages, 10 figure
Accurate Fundamental Parameters or A, F, and G-type Supergiants in the Solar Neighbourhood
The following parameters are determined for 63 Galactic supergiants in the
solar neighbourhood: effective temperature Teff, surface gravity log g, iron
abundance log e(Fe), microturbulent parameter Vt, mass M/Msun, age t and
distance d. A significant improvement in the accuracy of the determination of
log g and, all parameters dependent on it, is obtained through application of
van Leeuwens (2007) re-reduction of the Hipparcos parallaxes. The typical error
in the log g values is now +-0.06 dex for supergiants with distances d < 300 pc
and +-0.12 dex for supergiants with d between 300 and 700 pc; the mean error in
Teff for these stars is +-120 K. For supergiants with d > 700 pc parallaxes are
uncertain or unmeasurable, so typical errors in their log g values are 0.2-0.3
dex.
A new Teff scale for A5-G5 stars of luminosity classes Ib-II is presented.
Spectral subtypes and luminosity classes of several stars are corrected.
Combining the Teff and log g with evolutionary tracks, stellar masses and ages
are determined; a majority of the sample has masses between 4 Msun and 15 Msun
and, hence, their progenitors were early to middle B-type main sequence stars.
Using Fe ii lines, which are insensitive to departures from LTE, the
microturbulent parameter Vt and the iron abundance log e(Fe) are determined
from high-resolution spectra. The parameter Vt is correlated with gravity: Vt
increases with decreasing log g. The mean iron abundance for the 48 supergiants
with distances d < 700 pc is log e(Fe)=7.48+-0.09, a value close to the solar
value of 7.45+-0.05, and thus the local supergiants and the Sun have the same
metallicity.Comment: 12 pages, 9 figures. Will be published at MNRA
Chemical Composition of Extremely Metal-Poor Stars in the Sextans Dwarf Spheroidal Galaxy
Chemical abundances of six extremely metal-poor ([Fe/H]<-2.5) stars in the
Sextans dwarf spheroidal galaxy are determined based on high resolution
spectroscopy (R=40,000) with the Subaru Telescope High Dispersion Spectrograph.
(1) The Fe abundances derived from the high resolution spectra are in good
agreement with the metallicity estimated from the Ca triplet lines in low
resolution spectra. The lack of stars with [Fe/H]=<-3 in Sextans, found by
previous estimates from the Ca triplet, is confirmed by our measurements,
although we note that high resolution spectroscopy for a larger sample of stars
will be necessary to estimate the true fraction of stars with such low
metallicity. (2) While one object shows an overabundance of Mg (similar to
Galactic halo stars), the Mg/Fe ratios of the remaining five stars are similar
to the solar value. This is the first time that low Mg/Fe ratios at such low
metallicities have been found in a dwarf spheroidal galaxy. No evidence for
over-abundances of Ca and Ti are found in these five stars, though the
measurements for these elements are less certain. Possible mechanisms to
produce low Mg/Fe ratios, with respect to that of Galactic halo stars, are
discussed. (3) Ba is under-abundant in four objects, while the remaining two
stars exhibit large and moderate excesses of this element. The abundance
distribution of Ba in this galaxy is similar to that in the Galactic halo,
indicating that the enrichment of heavy elements, probably by the r-process,
started at metallicities [Fe/H] < -2.5, as found in the Galactic halo.Comment: 15 pages, 6 figures, 6 tables, A&A, in pres
The Highly Unusual Chemical Composition of the Hercules Dwarf Spheroidal Galaxy
We report on the abundance analysis of two red giants in the faint Hercules
dwarf spheroidal (dSph) galaxy. These stars show a remarkable deficiency in the
neutron-capture elements, while the hydrostatic alpha-elements (O, Mg) are
strongly enhanced. Our data indicate [Ba/Fe] and [Mg/Fe] abundance ratios of
<-2 dex and ~+0.8 dex, respectively, with essentially no detection of other
n-capture elements. In contrast to the only other dSph star with similar
abundance patterns, Dra 119, which has a very low metallicity at [Fe/H]=-2.95
dex, our objects, at [Fe/H]~-2.0 dex, are only moderately metal poor. The
measured ratio of hydrostatic/explosive alpha-elements indicates that high-mass
(~35 M_sun) Type II supernovae progenitors are the main, if not only,
contributors to the enrichment of this galaxy. This suggests that star
formation and chemical enrichment in the ultrafaint dSphs proceeds
stochastically and inhomogeneously on small scales, or that the IMF was
strongly skewed to high mass stars. The neutron capture deficiencies and the
[Co/Fe] and [Cr/Fe] abundance ratios in our stars are similar to those in the
extremely low metallicity Galactic halo. This suggests that either our stars
are composed mainly of the ejecta from the first, massive, population III stars
(but at moderately high [Fe/H]), or that SN ejecta in the Hercules galaxy were
diluted with ~30 times less hydrogen than typical for extreme metal-poor stars.Comment: 5 pages, 3 figures, accepted by Astrophysical Journal Letter
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