132 research outputs found
Periodically kicked turbulence
Periodically kicked turbulence is theoretically analyzed within a mean field
theory. For large enough kicking strength A and kicking frequency f the
Reynolds number grows exponentially and then runs into some saturation. The
saturation level can be calculated analytically; different regimes can be
observed. For large enough Re we find the saturation level to be proportional
to A*f, but intermittency can modify this scaling law. We suggest an
experimental realization of periodically kicked turbulence to study the
different regimes we theoretically predict and thus to better understand the
effect of forcing on fully developed turbulence.Comment: 4 pages, 3 figures. Phys. Rev. E., in pres
Discovery of long-period variable stars in the very-metal-poor globular cluster M15
We present a search for long-period variable (LPV) stars among giant branch
stars in M15 which, at [Fe/H] ~ -2.3, is one of the most metal-poor Galactic
globular clusters. We use multi-colour optical photometry from the 0.6-m Keele
Thornton and 2-m Liverpool Telescopes. Variability of delta-V ~ 0.15 mag is
detected in K757 and K825 over unusually-long timescales of nearly a year,
making them the most metal-poor LPVs found in a Galactic globular cluster. K825
is placed on the long secondary period sequence, identified for metal-rich
LPVs, though no primary period is detectable. We discuss this variability in
the context of dust production and stellar evolution at low metallicity, using
additional spectra from the 6.5-m Magellan (Las Campanas) telescope. A lack of
dust production, despite the presence of gaseous mass loss raises questions
about the production of dust and the intra-cluster medium of this cluster.Comment: 13 pages, 9 figures, accepted by MNRA
An Over-Massive Black Hole in the Compact Lenticular Galaxy NGC1277
All massive galaxies likely have supermassive black holes at their centers,
and the masses of the black holes are known to correlate with properties of the
host galaxy bulge component. Several explanations have been proposed for the
existence of these locally-established empirical relationships; they include
the non-causal, statistical process of galaxy-galaxy merging, direct feedback
between the black hole and its host galaxy, or galaxy-galaxy merging and the
subsequent violent relaxation and dissipation. The empirical scaling relations
are thus important for distinguishing between various theoretical models of
galaxy evolution, and they further form the basis for all black hole mass
measurements at large distances. In particular, observations have shown that
the mass of the black hole is typically 0.1% of the stellar bulge mass of the
galaxy. The small galaxy NGC4486B currently has the largest published fraction
of its mass in a black hole at 11%. Here we report observations of the stellar
kinematics of NGC 1277, which is a compact, disky galaxy with a mass of 1.2 x
10^11 Msun. From the data, we determine that the mass of the central black hole
is 1.7 x 10^10 Msun, or 59% its bulge mass. Five other compact galaxies have
properties similar to NGC 1277 and therefore may also contain over-sized black
holes. It is not yet known if these galaxies represent a tail of a
distribution, or if disk-dominated galaxies fail to follow the normal black
hole mass scaling relations.Comment: 7 pages. 6 figures. Nature. Animation at
http://www.mpia.de/~bosch/blackholes.htm
Dust, pulsation, chromospheres and their role in driving mass loss from red giants in Galactic globular clusters
Context: Mass loss from red giants in old globular clusters affects the
horizontal branch (HB) morphology and post-HB stellar evolution including the
production of ultraviolet-bright stars, dredge up of nucleosynthesis products
and replenishment of the intra-cluster medium. Studies of mass loss in globular
clusters also allows one to investigate the metallicity dependence of the mass
loss from cool, low-mass stars down to very low metallicities.
Aims: We present an analysis of new VLT/UVES spectra of 47 red giants in the
Galactic globular clusters 47 Tuc (NGC 104), NGC 362, omega Cen (NGC 5139), NGC
6388, M54 (NGC 6715) and M15 (NGC 7078). The spectra cover the wavelength
region 6100-9900A at a resolving power of R = 110,000. Some of these stars are
known to exhibit mid-infrared excess emission indicative of circumstellar dust.
Our aim is to detect signatures of mass loss, identify the mechanism(s)
responsible for such outflows, and measure the mass-loss rates.
Methods: We determine for each star its effective temperature, luminosity,
radius and escape velocity. We analyse the H-alpha and near-infrared calcium
triplet lines for evidence of outflows, pulsation and chromospheric activity,
and present a simple model for estimating mass-loss rates from the H-alpha line
profile. We compare our results with a variety of other, independent methods.
Results: We argue that a chromosphere persists in Galactic globular cluster
giants and controls the mass-loss rate to late-K/early-M spectral types, where
pulsation becomes strong enough to drive shock waves at luminosities above the
RGB tip. This transition may be metallicity-dependent. We find mass-loss rates
of ~10^-7 to 10^-5 solar masses per year, largely independent of metallicity.Comment: 23 pages, 17 figures, accepted for publication in Astronomy and
Astrophysic
A spectral atlas of post-main-sequence stars in omega Centauri: kinematics, evolution, enrichment and interstellar medium
We present a spectral atlas of the post-main-sequence population of the most
massive Galactic globular cluster, omega Centauri. Spectra were obtained of
more than 1500 stars selected as uniformly as possible from across the (B, B-V)
colour-magnitude diagram of the proper motion cluster member candidates of van
Leeuwen et al. (2000). The spectra were obtained with the 2dF multi-fibre
spectrograph at the Anglo Australian Telescope, and cover the approximate range
lambda~3840-4940 Angstroem. We measure the radial velocities, effective
temperatures, metallicities and surface gravities by fitting ATLAS9 stellar
atmosphere models. We analyse the cluster membership and stellar kinematics,
interstellar absorption in the Ca II K line at 3933 Angstroem, the RR Lyrae
instability strip and the extreme horizontal branch, the metallicity spread and
bimodal CN abundance distribution of red giants, nitrogen and s-process
enrichment, carbon stars, pulsation-induced Balmer line emission on the
asymptotic giant branch (AGB), and the nature of the post-AGB and UV-bright
stars. Membership is confirmed for the vast majority of stars, and the radial
velocities clearly show the rotation of the cluster core. We identify
long-period RR Lyrae-type variables with low gravity, and low-amplitude
variables coinciding with warm RR Lyrae stars. A barium enhancement in the
coolest red giants indicates that 3rd dredge-up operates in AGB stars in omega
Cen. This is distinguished from the pre-enrichment by more massive AGB stars,
which is also seen in our data. The properties of the AGB, post-AGB and
UV-bright stars suggest that RGB mass loss may be less efficient at very low
metallicity, [Fe/H]<<-1, increasing the importance of mass loss on the AGB. The
catalogue and spectra are made available via CDS.Comment: accepted for publication in MNRA
Transport properties of strongly correlated metals:a dynamical mean-field approach
The temperature dependence of the transport properties of the metallic phase
of a frustrated Hubbard model on the hypercubic lattice at half-filling are
calculated. Dynamical mean-field theory, which maps the Hubbard model onto a
single impurity Anderson model that is solved self-consistently, and becomes
exact in the limit of large dimensionality, is used. As the temperature
increases there is a smooth crossover from coherent Fermi liquid excitations at
low temperatures to incoherent excitations at high temperatures. This crossover
leads to a non-monotonic temperature dependence for the resistance,
thermopower, and Hall coefficient, unlike in conventional metals. The
resistance smoothly increases from a quadratic temperature dependence at low
temperatures to large values which can exceed the Mott-Ioffe-Regel value, hbar
a/e^2 (where "a" is a lattice constant) associated with mean-free paths less
than a lattice constant. Further signatures of the thermal destruction of
quasiparticle excitations are a peak in the thermopower and the absence of a
Drude peak in the optical conductivity. The results presented here are relevant
to a wide range of strongly correlated metals, including transition metal
oxides, strontium ruthenates, and organic metals.Comment: 19 pages, 9 eps figure
Formation of Supermassive Black Holes
Evidence shows that massive black holes reside in most local galaxies.
Studies have also established a number of relations between the MBH mass and
properties of the host galaxy such as bulge mass and velocity dispersion. These
results suggest that central MBHs, while much less massive than the host (~
0.1%), are linked to the evolution of galactic structure. In hierarchical
cosmologies, a single big galaxy today can be traced back to the stage when it
was split up in hundreds of smaller components. Did MBH seeds form with the
same efficiency in small proto-galaxies, or did their formation had to await
the buildup of substantial galaxies with deeper potential wells? I briefly
review here some of the physical processes that are conducive to the evolution
of the massive black hole population. I will discuss black hole formation
processes for `seed' black holes that are likely to place at early cosmic
epochs, and possible observational tests of these scenarios.Comment: To appear in The Astronomy and Astrophysics Review. The final
publication is available at http://www.springerlink.co
A novel approach to locate Phytophthora infestans resistance genes on the potato genetic map
Mapping resistance genes is usually accomplished by phenotyping a segregating population for the resistance trait and genotyping it using a large number of markers. Most resistance genes are of the NBS-LRR type, of which an increasing number is sequenced. These genes and their analogs (RGAs) are often organized in clusters. Clusters tend to be rather homogenous, viz. containing genes that show high sequence similarity with each other. From many of these clusters the map position is known. In this study we present and test a novel method to quickly identify to which cluster a new resistance gene belongs and to produce markers that can be used for introgression breeding. We used NBS profiling to identify markers in bulked DNA samples prepared from resistant and susceptible genotypes of small segregating populations. Markers co-segregating with resistance can be tested on individual plants and directly used for breeding. To identify the resistance gene cluster a gene belongs to, the fragments were sequenced and the sequences analyzed using bioinformatics tools. Putative map positions arising from this analysis were validated using markers mapped in the segregating population. The versatility of the approach is demonstrated with a number of populations derived from wild Solanum species segregating for P. infestans resistance. Newly identified P. infestans resistance genes originating from S. verrucosum, S. schenckii, and S. capsicibaccatum could be mapped to potato chromosomes 6, 4, and 11, respectively
The Formation of the First Massive Black Holes
Supermassive black holes (SMBHs) are common in local galactic nuclei, and
SMBHs as massive as several billion solar masses already exist at redshift z=6.
These earliest SMBHs may grow by the combination of radiation-pressure-limited
accretion and mergers of stellar-mass seed BHs, left behind by the first
generation of metal-free stars, or may be formed by more rapid direct collapse
of gas in rare special environments where dense gas can accumulate without
first fragmenting into stars. This chapter offers a review of these two
competing scenarios, as well as some more exotic alternative ideas. It also
briefly discusses how the different models may be distinguished in the future
by observations with JWST, (e)LISA and other instruments.Comment: 47 pages with 306 references; this review is a chapter in "The First
Galaxies - Theoretical Predictions and Observational Clues", Springer
Astrophysics and Space Science Library, Eds. T. Wiklind, V. Bromm & B.
Mobasher, in pres
An internal ribosome entry site in the 5′ untranslated region of epidermal growth factor receptor allows hypoxic expression
The expression of epidermal growth factor receptor (EGFR/ERBB1/HER1) is implicated in the progress of numerous cancers, a feature that has been exploited in the development of EGFR antibodies and EGFR tyrosine kinase inhibitors as anti-cancer drugs. However, EGFR also has important normal cellular functions, leading to serious side effects when EGFR is inhibited. One damaging characteristic of many oncogenes is the ability to be expressed in the hypoxic conditions associated with the tumour interior. It has previously been demonstrated that expression of EGFR is maintained in hypoxic conditions via an unknown mechanism of translational control, despite global translation rates generally being attenuated under hypoxic conditions. In this report, we demonstrate that the human EGFR 5′ untranslated region (UTR) sequence can initiate the expression of a downstream open reading frame via an internal ribosome entry site (IRES). We show that this effect is not due to either cryptic promoter activity or splicing events. We have investigated the requirement of the EGFR IRES for eukaryotic initiation factor 4A (eIF4A), which is an RNA helicase responsible for processing RNA secondary structure as part of translation initiation. Treatment with hippuristanol (a potent inhibitor of eIF4A) caused a decrease in EGFR 5′ UTR-driven reporter activity and also a reduction in EGFR protein level. Importantly, we show that expression of a reporter gene under the control of the EGFR IRES is maintained under hypoxic conditions despite a fall in global translation rates
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