5,736 research outputs found
Feasibility and conceptual design study - Vibration generator transient waveform control system Final report, Jul. 1968 - Jun. 1969
Design and characteristics of on-line transient waveform control of electromagnetic and hydraulic vibrator
Modulation of calmodulin lobes by different targets: an allosteric model with hemiconcerted conformational transitions
Calmodulin, the ubiquitous calcium-activated second messenger in eukaryotes,
is an extremely versatile molecule involved in many biological processes:
muscular contraction, synaptic plasticity, circadian rhythm, and cell cycle,
among others. The protein is structurally organised into two globular lobes,
joined by a flexible linker. Calcium modulates calmodulin activity by favoring
a conformational transition of each lobe from a closed conformation to an open
conformation. Most targets have a strong preference for one conformation over
the other, and depending on the free calcium concentration in a cell,
particular sets of targets will preferentially interact with calmodulin. In
turn, targets can increase or decrease the calcium affinity of the calmodulin
molecules to which they bind. Interestingly, experiments with the tryptic
fragments showed that most targets have a much lower affinity for the N-lobe
than for the C-lobe. Hence, the latter predominates in the formation of most
calmodulin-target complexes. We showed that a relatively simple allosteric
mechanism, based the classic MWC model, can capture the observed modulation of
both the isolated C-lobe, and intact calmodulin, by individual targets.
Moreover, our model can be naturally extended to study how the calcium affinity
of a single pool of calmodulin is modulated by a mixture of competing targets
in vivo
Damped Lyman Alpha Systems at z<1.65: The Expanded SDSS HST Sample
We present results of our HST Cycle 11 Survey for low-redshift (z<1.65) DLAs
in the UV spectra of quasars selected from the SDSS Early Data Release. These
quasars have strong intervening MgII-FeII systems which are known signatures of
high column density neutral gas. In total, UV observations of Ly-alpha
absorption in 197 MgII systems with z<1.65 and rest equivalent width (REW)
W2796 \ge 0.3A have now been obtained. The main results are: (1) 36(+/- 6)% of
systems with W2796 \ge 0.5 A and FeII W2600 \ge 0.5 A are DLAs. This increases
to 42(+/- 7)% for systems with W2796/W2600 0.1 A. (2) The
mean N(HI) of MgII systems with 0.3 A \le W2796 < 0.6 A is a factor of ~36
lower than that of systems with W2796 \ge 0.6 A. (3) The DLA incidence per unit
redshift is consistent with no evolution for z <~ 2 (Omega_L=0.7, Omega_M =
0.3), but exhibits significant evolution for z >~ 2. (4) Omega_{DLA} is
constant for 0.5<z<5.0 to within the uncertainties. This is larger than
Omega_{gas}(z=0) by a factor of ~2. (5) The slope of the N(HI) distribution
does not change significantly with redshift. However, the low redshift
distribution is marginally flatter due to the higher fraction of high N(HI)
systems in our sample. (6) Finally, using the precision of MgII survey
statistics, we find that there may be evidence of a decreasing Omega_{DLA} from
z=0.5 to z=0. We reiterate the conclusion of Hopkins, Rao, & Turnshek that very
high columns of neutral gas might be missed by DLA surveys because of their
very small cross sections, and therefore, that Omega_{DLA} might not include
the bulk of the neutral gas mass in the Universe. (Abridged)Comment: Accepted for publication in ApJ. 22 pages, 22 figure
High Metallicity Mg II Absorbers in the z < 1 Lyman alpha Forest of PKS 0454+039: Giant LSB Galaxies?
We report the discovery of two iron-group enhanced high-metallicity Mg II
absorbers in a search through 28 Lyman Alpha forest clouds along the PKS
0454+039 sight line. Based upon our survey and the measured redshift number
densities of W_r(MgII) <= 0.3 A absorbers and Lyman Alpha absorbers at z ~ 1,
we suggest that roughly 5% of Lyman Alpha absorbers at z < 1 will exhibit
"weak" Mg II absorption to a 5-sigma W_r(2796) detection limit of 0.02 A. The
two discovered absorbers, at redshifts z = 0.6248 and z = 0.9315, have W_r(Lya)
= 0.33 and 0.15 A, respectively. Based upon photoionization modeling, the H I
column densities are inferred to be in the range 15.8 <= log N(HI) <= 16.8
cm^-2. For the z = 0.6428 absorber, if the abundance pattern is solar, then the
cloud has [Fe/H] > -1; if its gas-phase abundance follows that of depleted
clouds in our Galaxy, then [Fe/H] > 0 is inferred. For the z = 0.9315 absorber,
the metallicity is [Fe/H] > 0, whether the abundance pattern is solar or
suffers depletion. Imaging and spectroscopic studies of the PKS 0454+039 field
reveal no candidate luminous objects at these redshifts. We discuss the
possibility that these Mg II absorbers may arise in the class of "giant" low
surface brightness galaxies, which have [Fe/H] >= -1, and even [Fe/H] >= 0, in
their extended disks. We tentatively suggest that a substantial fraction of
these "weak" Mg II absorbers may select low surface brightness galaxies out to
z ~ 1.Comment: Accepted The Astrophysical Journal; 25 pages; 6 encapsulated figure
Barkhausen Noise Analysis by Surrounding Coil
The Barkhausen effect (or Barkhausen noise)[1], discovered in 1919, was initially identified as the revealing of irreversible and discontinuous changes of magnetisation induced by an external magnetic field[2]. The interest of Barkhausen noise as a non destructive testing technique of magnetic materials is due to the interaction between the magnetic microstructure (magnetic domains, Bloch wall motion dynamic), microstructural state and stress state of the material [3,4,5,6]
Constraints on the Lyman continuum radiation from galaxies: first results with FUSE on Mrk 54
We present Far Ultraviolet Spectroscopic Explorer observations of the
star-forming galaxy Mrk 54 at z = 0.0448. The Lyman continuum radiation is not
detected above the HI absorption edge in our Galaxy. An upper limit is
evaluated by comparison with the background measured in regions of the detector
adjacent to the observed spectrum. A spectral window of 16 A, reasonably free
of additional HI Lyman series line absorption is used. No correction is needed
for molecular hydrogen absorption in our Galaxy but a foreground extinction of
0.29 mag is accounted for. An upper limit of 6.15 10^{-16} erg/cm^2/s/A is
obtained for the flux at ~ 900 A in the rest frame of Mrk 54. By comparison
with the number of ionizing photons derived from the H-alpha flux, this limit
translates into an upper limit of f_esc < 0.062 for the fraction of Lyman
continuum photons that escape the galaxy without being absorbed by interstellar
material. This limit compares with the limits obtained in three other nearby
galaxies and is compatible with the escape fractions predicted by models.
The upper limits obtained in nearby galaxies contrasts with the detection of
Lyman continuum flux in the composite spectrum of Lyman-break galaxies at z ~
3.4. The difficulties and implications of a comparison are discussed.Comment: 9 pages, 3 figures, accepted for publication in A&A include aa.cls
v5.0
The scatter and evolution of the global hot gas properties of simulated galaxy cluster populations
We use the cosmo-OWLS suite of cosmological hydrodynamical simulations to
investigate the scatter and evolution of the global hot gas properties of large
simulated populations of galaxy groups and clusters. Our aim is to compare the
predictions of different physical models and to explore the extent to which
commonly-adopted assumptions in observational analyses (e.g. self-similar
evolution) are violated. We examine the relations between (true) halo mass and
the X-ray temperature, X-ray luminosity, gas mass, Sunyaev-Zel'dovich (SZ)
flux, the X-ray analogue of the SZ flux () and the hydrostatic mass. For
the most realistic models, which include AGN feedback, the slopes of the
various mass-observable relations deviate substantially from the self-similar
ones, particularly at late times and for low-mass clusters. The amplitude of
the mass-temperature relation shows negative evolution with respect to the
self-similar prediction (i.e. slower than the prediction) for all models,
driven by an increase in non-thermal pressure support at higher redshifts. The
AGN models predict strong positive evolution of the gas mass fractions at low
halo masses. The SZ flux and show positive evolution with respect to
self-similarity at low mass but negative evolution at high mass. The scatter
about the relations is well approximated by log-normal distributions, with
widths that depend mildly on halo mass. The scatter decreases significantly
with increasing redshift. The exception is the hydrostatic mass-halo mass
relation, for which the scatter increases with redshift. Finally, we discuss
the relative merits of various hot gas-based mass proxies.Comment: 31 pages (21 before appendices), 19 figures, 12 tables, accepted by
MNRAS after minor revisio
Assessment of Irradiation Damage on Stainless Steel by Acoustic Miroscopy
The plan to increase the life cycle of nuclear power stations opens up a new field of investigation for methods of characterizing materials. One of the main problems encountered by the operator is knowing how to evaluate the remaining useful life of components in its generating unit to prevent critical parts from suddenly breaking in service [1]. The aim is to end up with indicators of the degree of damage suffered by metal structures using nondestructive measurement tests whose effectiveness will have been proved in an industrial environment
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