4,284 research outputs found
Wide-field multi-color photometry of the Galactic globular cluster NGC 1261
(Abriged)This work studies in more detail the stellar population, including
its photometric properties and characteristics, in the rarely studied southern
Galactic globular cluster NGC 1261. We focus on the brighter sequences of the
cluster's color-magnitude diagram (CMD). Like in our previous works, we rely
upon photometry in several passbands to achieve more reliable results and
conclusions. We carried out and analyzed new multi-color photometry of NGC 1261
in UBVI reaching below the turnoff point in all passbands in a fairly extended
cluster field, about 14'x14'. We found several signs of the inhomogeneity
("multiplicity") in the stellar population. The most prominent of them are: (1)
the dependence of the radial distribution of sub-giant branch (SGB) stars in
the cluster on their U magnitude, with brighter stars less centrally
concentrated at the 99.9 \% level than their fainter counterparts; (2) the
dependence of the location of red giant branch (RGB) stars in the U-(U-B) CMD
on their radial distance from the cluster center, with the portion of stars
bluer in the (U-B) color increasing towards the cluster outskirts.
Additionally, the radial variation of the RGB luminosity function in the bump
region is suspected. We assume that both the SGB stars brighter in the U and
the RGB stars bluer in the (U-B) color are probably associated with blue
horizontal branch stars, because of a similarity in their radial distribution
in the cluster. We estimated the metalicity of NGC 1261 from the slope of the
RGB in U-based CMDs and the location of the RGB bump on the branch. These
metallicity indicators give [Fe/H]zw = -1.34 +/- 0.16 dex and [Fe/H]zw = -1.41
+/- 0.10 dex, respectively. We isolated 18 probable blue straggler candidates.
They are more centrally concentrated than the lower red giants of comparable
brightness at the 97.9 \% level.Comment: 8 pages, 9 figures, accepted for publication in A&
The 1723 AD violent Strombolian and phreatomagatic eruption at Irazu volcano (Costa Rica)
The largest of the recorded historic eruptions at IrazĂș volcano began on February 16, 1723 and lasted
until at least December 11. We here critically examine deposits of this eruption exposed on the summit of IrazĂș. Our
reconstruction of the eruption is based on the unique chronicle of the Spanish governor Diego de la Haya. The eruption
began with a < 10 cm thick surge deposit of phreatic origin showing block sag structures. The deposit is overlain by
6 m-thick coarse-grained basaltic andesitic non-graded juvenile fallout tephra consisting of highly vesicular (22-59
vol.%) bombs and lapilli with minor hydrothermally altered lapilli (1-7 vol.%) and rare light colored andesitic vesicular
lapilli (< 1%). These fallout deposits are interpreted as strombolian, possible generated during a short-lived scoria cone
at the end of February 1723, dominate volumetrically in the proximal facies. Overlying <1.2 m thick phreatomagmatic
deposits of fi nely laminated lapilli-bearing gray ash (fallout and surge deposits) some with contorted bedding and sag
structures, are in turn overlain by a 1.2 m thick bed of ash matrix-rich bomb/block deposit. The 1723 eruption was
accompanied by shallow volcano-tectonic earthquakes (Modifi ed Mercalli scale Intensity MMI VI-VII, magnitude ML
~5.5) that possibly facilitated magma/water interaction. Phenocrysts in the basaltic andesite (~53-55 wt.% SiO2) bombs
comprise plagioclase (6.1-21.6 vol.%, An52-35), clinopyroxene (2.5-10 vol.%), orthopyroxene (0.7-2 vol.%), olivine
(0.1-2.2 vol.%; Fo76-88) and Fe/Ti-oxides (0.1-1%), in a groundmass (66.5-90.3 vol. %), dominated by plagioclase
(An69-54), clinopyroxene and opaques in brown and black glass with the same range of chemical composition (SiO2=
57-64 wt.%). Rare white pumiceous lapilli in the scoria deposits are high-K, hornblende andesite (SiO2: 58-60 wt.%),
geochemically unrelated to the scoria deposits. Thus, two different magmas co-existing in the magma chamber were
mingled shortly before, and during, the eruption, suggesting that the eruption was triggered by magma mingling between
hornblende andesite and basaltic andesite magma
Interferometric detection of spin-polarized transport in the depletion layer of a metal-GaAs Schottky barrier
It is shown that the Kerr rotation of spin-polarized electrons is modulated
by the distance of the electrons from the sample surface. Time-resolved Kerr
rotation of optically-excited spin-polarized electrons in the depletion layer
of n-doped GaAs displays fast oscillations that originate from an interference
between the light reflected from the semiconductor surface and from the front
of the electron distribution moving into the semiconductor. Using this effect,
the dynamics of the photogenerated charge carriers in the depletion layer of
the biased Schottky barrier is measured.Comment: 10 pages, 4 figure
Signatures of dynamically polarized nuclear spins in all-electrical lateral spin transport devices
The effect of nuclear spins in Fe/GaAs all-electrical spin-injection devices
is investigated. At temperatures below 50 K, strong modifications of the
non-local spin signal are found that are characteristic for hyperfine coupling
between conduction electrons and dynamically polarized nuclear spins. The
perpendicular component of the nuclear Overhauser field depolarizes electron
spins near zero in-plane external magnetic field, and can suppress such
dephasing when antialigned with the external field, leading to satellite peaks
in a Hanle measurement. The features observed agree well with a Monte Carlo
simulation of the spin diffusion equation including hyperfine interaction, and
are used to study the nuclear spin dynamics and relate it to the spin
polarization of injected electrons.Comment: 6 pages, 4 figure
Active biopolymer networks generate scale-free but euclidean clusters
We report analytical and numerical modelling of active elastic networks,
motivated by experiments on crosslinked actin networks contracted by myosin
motors. Within a broad range of parameters, the motor-driven collapse of active
elastic networks leads to a critical state. We show that this state is
qualitatively different from that of the random percolation model.
Intriguingly, it possesses both euclidean and scale-free structure with Fisher
exponent smaller than . Remarkably, an indistinguishable Fisher exponent and
the same euclidean structure is obtained at the critical point of the random
percolation model after absorbing all enclaves into their surrounding clusters.
We propose that in the experiment the enclaves are absorbed due to steric
interactions of network elements. We model the network collapse, taking into
account the steric interactions. The model shows how the system robustly drives
itself towards the critical point of the random percolation model with absorbed
enclaves, in agreement with the experiment.Comment: 6 pages, 7 figure
Temperature dependence of the nonlocal voltage in an Fe/GaAs electrical spin injection device
The nonlocal spin resistance is measured as a function of temperature in a
Fe/GaAs spin-injection device. For nonannealed samples that show minority-spin
injection, the spin resistance is observed up to room temperature and decays
exponentially with temperature at a rate of 0.018\,K. Post-growth
annealing at 440\,K increases the spin signal at low temperatures, but the
decay rate also increases to 0.030\,K. From measurements of the
diffusion constant and the spin lifetime in the GaAs channel, we conclude that
sample annealing modifies the temperature dependence of the spin transfer
efficiency at injection and detection contacts. Surprisingly, the spin transfer
efficiency increases in samples that exhibit minority-spin injection.Comment: 10 pages, 4 figure
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