12,360 research outputs found
Initial in situ measurements of perennial meltwater storage in the Greenland firn aquifer
pre-printA perennial storage of water in a firn aquifer was discovered in southeast Greenland in 2011. We present the first in situ measurements of the aquifer, including densities and temperatures. Water was present at depths between ~12 and 37m and amounted to 18.7 ± 0.9 kg in the extracted core. The water filled the firn to capacity at ~35m. Measurements show the aquifer temperature remained at the melting point, representing a large heat reservoir within the firn. Using model results of liquid water extent and aquifer surface depth from radar measurements, we extend our in situ measurements to the Greenland ice sheet. The estimated water volume is 140 ± 20 Gt, representing ~0.4mm of sea level rise (SLR). It is unknown if the aquifer temporary buffers SLR or contributes to SLR through drainage and/or ice dynamics
Cascade of magnetic field induced Lifshitz transitions in the ferromagnetic Kondo lattice material YbNi4P2
A ferromagnetic quantum critical point is thought not to exist in two and
three-dimensional metallic systems yet is realized in the Kondo lattice
compound YbNi4(P,As)2, possibly due to its one-dimensionality. It is crucial to
investigate the dimensionality of the Fermi surface of YbNi4P2 experimentally
but common probes such as ARPES and quantum oscillation measurements are
lacking. Here, we studied the magnetic field dependence of transport and
thermodynamic properties of YbNi4P2. The Kondo effect is continuously
suppressed and additionally we identify nine Lifshitz transitions between 0.4
and 18 T. We analyze the transport coefficients in detail and identify the type
of Lifshitz transitions as neck or void type to gain information on the Fermi
surface of YbNi4P2. The large number of Lifshitz transitions observed within
this small energy window is unprecedented and results from the particular flat
renormalized band structure with strong 4f-electron character shaped by the
Kondo lattice effect.Comment: 6 pages, 4 figure
Constraining the Nature of the Galactic Center X-ray Source Population
We searched for infrared counterparts to the cluster of X-ray point sources
discovered by Chandra in the Galactic Center Region (GCR). While the sources
could be white dwarfs, neutron stars, or black holes accreting from stellar
companions, their X-ray properties are consistent with magnetic Cataclysmic
Variables, or High Mass X-ray Binaries (HMXB) at low accretion-rates. A direct
way to decide between these possibilities and hence between alternative
formation scenarios is to measure or constrain the luminosity distribution of
the companions. Using infrared (J, H, K, Br-gamma) imaging, we searched for
counterparts corresponding to typical HMXB secondaries: spectral type B0V with
K<15 at the GCR. We found no significant excess of bright stars in Chandra
error circles, indicating that HMXBs are not the dominant X-ray source
population, and account for fewer than 10% of the hardest X-ray sources.Comment: 4 pages, 3 figures, 1 table, accepted in ApJ Letters for publicatio
Initial in Situ Measurements of Perennial Meltwater Storage in the Greenland Firn Aquifer
A perennial storage of water in a firn aquifer was discovered in southeast Greenland in 2011. We present the first in situ measurements of the aquifer, including densities and temperatures. Water was present at depths between approx. 12 and 37m and amounted to 18.7 +/- 0.9 kg in the extracted core. The water filled the firn to capacity at approx. 35m. Measurements show the aquifer temperature remained at the melting point, representing a large heat reservoir within the firn. Using model results of liquid water extent and aquifer surface depth from radar measurements, we extend our in situ measurements to the Greenland ice sheet. The estimated water volume is 140 +/- 20 Gt, representing approx. 0.4mm of sea level rise (SLR). It is unknown if the aquifer temporary buffers SLR or contributes to SLR through drainage and/or ice dynamics
On Estimation of Fully Entangled Fraction
We study the fully entangled fraction (FEF) of arbitrary mixed states. New
upper bounds of FEF are derived. These upper bounds make complements on the
estimation of the value of FEF. For weakly mixed quantum states, an upper bound
is shown to be very tight to the exact value of FEF.Comment: 8 pages, 2 figure
ATLASGAL-selected massive clumps in the inner Galaxy: VI. Kinetic temperature and spatial density measured with formaldehyde
We aim to directly determine the kinetic temperature and spatial density with
formaldehyde for the 100 brightest ATLASGAL-selected clumps at 870 m
representing various evolutionary stages of high-mass star formation. Ten
transitions ( = 3-2 and 4-3) of ortho- and para-HCO near 211, 218, 225,
and 291 GHz were observed with the APEX 12 m telescope. Using non-LTE models
with RADEX, we derive the gas kinetic temperature and spatial density using the
measured p-HCO 3-2/3-2,
4-3/4-3, and 4-3/3-2
ratios. The gas kinetic temperatures derived from the p-HCO
3-2/3-2 and 4-3/4-3
line ratios are high, ranging from 43 to 300 K with an unweighted average of
91 4 K. Deduced values from the = 3-2 and 4-3
transitions are similar. Spatial densities of the gas derived from the
p-HCO 4-3/3-2 line ratios yield 0.6-8.3
10 cm with an unweighted average of 1.5 (0.1)
10 cm. A comparison of kinetic temperatures derived from p-HCO,
NH, and the dust emission indicates that p-HCO traces a distinctly
higher temperature than the NH (2,2)/(1,1) transitions and the dust,
tracing heated gas more directly associated with the star formation process.
The HCO linewidths are found to be correlated with bolometric luminosity
and increase with the evolutionary stage of the clumps, which suggests that
higher luminosities tend to be associated with a more turbulent molecular
medium. It seems that the spatial densities measured with HCO do not vary
significantly with the evolutionary stage of the clumps. However, averaged gas
kinetic temperatures derived from HCO increase with time through the
evolution of the clumps.Comment: Accepted for publication in A&
Photoassociation of cold atoms with chirped laser pulses: time-dependent calculations and analysis of the adiabatic transfer within a two-state model
This theoretical paper presents numerical calculations for photoassociation
of ultracold cesium atoms with a chirped laser pulse and detailed analysis of
the results. In contrast with earlier work, the initial state is represented by
a stationary continuum wavefunction. In the chosen example, it is shown that an
important population transfer is achieved to vibrational levels in
the vicinity of the v=98 bound level in the external well of the
potential. Such levels lie in the energy range swept by
the instantaneous frequency of the pulse, thus defining a ``photoassociation
window''. Levels outside this window may be significantly excited during the
pulse, but no population remains there after the pulse. Finally, the population
transfer to the last vibrational levels of the ground (6s + 6s)
is significant, making stable molecules. The results are interpreted in the
framework of a two state model as an adiabatic inversion mechanism, efficient
only within the photoassociation window. The large value found for the
photoassociation rate suggests promising applications. The present chirp has
been designed in view of creating a vibrational wavepacket in the excited state
which is focussing at the barrier of the double well potential.Comment: 49 pages, 9 figures, submitted to Phys. Rev.
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