20,585 research outputs found
Protoplanetary gas disks in the far infrared
The physical and chemical conditions in young protoplanetary disks set the
boundary conditions for planet formation. Although the dust in disks is
relatively easily detected as a far-IR photometric ``excess'' over the expected
photospheric emission, much less is known about the gas phase. It seems clear
that an abrupt transition from massive optically thick disks (gas-rich
structures where only ~1% of the total mass is in the form of dust) to tenuous
debris disks almost devoid of gas occurs at ~10^7 years, by which time the
majority of at least the giant planets must have formed. Indeed, these planets
are largely gaseous and thus they must assemble before the gas disk dissipates.
Spectroscopic studies of the disk gas content at different evolutive stages are
thus critical. Far-IR water vapor lines and atomic fine structure lines from
abundant gas reservoirs (e.g., [OI]63um, [SI]56um, [SiII]34um) are robust
tracers of the gas in disks. Spectrometers on board Herschel will detect some
of these lines toward the closest, youngest and more massive protoplanetary
disks. However, according to models, Herschel will not reach the required
sensitivity to (1) detect the gas residual in more evolved and tenuous
transational disks that are potentially forming planets and (2) detect the gas
emission from less massive protoplanetary disks around the most numerous stars
in the Galaxy (M-type and cooler dwarfs). Both are unique goals for
SPICA/SAFARI. Besides, SAFARI will be able to detect the far-IR modes of water
ice at ~44 and ~62um, and thus allow water ice to be observed in many
protoplanetary systems and fully explore its impact on planetary formation and
evolution.Comment: To appear in Proc. Workshop "The Space Infrared Telescope for
Cosmology & Astrophysics: Revealing the Origins of Planets and Galaxies".
Eds. A.M. Heras, B. Swinyard, K. Isaak, and J.R. Goicoeche
Strong Secrecy on a Class of Degraded Broadcast Channels Using Polar Codes
Different polar coding schemes are proposed for the memoryless degraded
broadcast channel under different reliability and secrecy requirements: layered
decoding and/or layered secrecy. In this setting, the transmitter wishes to
send multiple messages to a set of legitimate receivers keeping them masked
from a set of eavesdroppers. The layered decoding structure requires receivers
with better channel quality to reliably decode more messages, while the layered
secrecy structure requires eavesdroppers with worse channel quality to be kept
ignorant of more messages. The implementation of the proposed polar coding
schemes is discussed and their performance is evaluated by simulations for the
symmetric degraded broadcast channel.Comment: 35 pages. Published in "MDPI Entropy". A short version of this paper
had been accepted to the 3rd Workshop on Physical-Layer Methods for Wireless
Security, IEEE CNS 201
The single-electron transport in a three-ion magnetic molecule modulated by a transverse field
We study single-electron transport in a three-ion molecule with strong
uniaxial anisotropy and in the presence of a transverse magnetic field. Two
magnetic ions are connected to each other through a third, nonmagnetic ion. The
magnetic ions are coupled to ideal metallic leads and a back gate voltage is
applied to the molecule, forming a field-effect transistor. The microscopic
Hamiltonian describing this system includes inter-ion hopping, on-site
repulsions, and magnetic anisotropies. For a range of values of the parameters
of the Hamiltonian, we obtain an energy spectrum similar to that of
single-molecule magnets in the giant-spin approximation where the two states
with maximum spin projection along the uniaxial anisotropy axis are well
separated from other states. In addition, upon applying an external in-plane
magnetic field, the energy gap between the ground and first excited states of
the molecule oscillates, going to zero at certain special values of the field,
in analogy to the diabolical points resulting from Berry phase interference in
the giant spin model. Thus, our microscopic model provides the same
phenomenological behavior expected from the giant spin model of a
single-molecule magnet but with direct access to the internal structure of the
molecule, thus making it more appropriate for realistic electronic transport
studies. To illustrate this point, the nonlinear electronic transport in the
sequential tunneling regime is evaluated for values of the field near these
degeneracy points. We show that the existence of these points has a clear
signature in the I-V characteristics of the molecule, most notably the
modulation of excitation lines in the differential conductance.Comment: 10 pages, 13 figure
Stellar Feedback in the ISM Revealed by Wide-Field Far-Infrared Spectral-Imaging
The radiative and mechanical interaction of stars with their environment
drives the evolution of the ISM and of galaxies as a whole. The far-IR emission
(lambda ~30 to 350 microns) from atoms and molecules dominates the cooling of
the warm gas in the neutral ISM, the material that ultimately forms stars.
Far-IR lines are thus the most sensitive probes of stellar feedback processes,
and allow us to quantify the deposition and cycling of energy in the ISM. While
ALMA (in the (sub)mm) and JWST (in the IR) provide astonishing sub-arcsecond
resolution images of point sources and their immediate environment, they cannot
access the main interstellar gas coolants, nor are they designed to image
entire star-forming regions (SFRs). Herschel far-IR photometric images of the
interstellar dust thermal emission revealed the ubiquitous large-scale
filamentary structure of SFRs, their mass content, and the location of
thousands of prestellar cores and protostars. These images, however, provide a
static view of the ISM: not only they dont constrain the cloud dynamics,
moreover they cannot reveal the chemical composition and energy transfer within
the cloud, thus giving little insight into the regulation process of star
formation by stellar feedback. In this white paper we emphasize the need of a
space telescope with wide-field spectral-imaging capabilities in the critical
far-IR domain.Comment: White Paper submitted to the Astro 2020 Decadal Survey on Astronomy
and Astrophysics (National Academies of Science, Engineering, and Medicine
Recommended from our members
Tropidophis celiae
Number of pages: 8Geological SciencesIntegrative Biolog
Modeling Space-Charge Limited Currents in Organic Semiconductors: Extracting Trap Density and Mobility
We have developed and applied a mobility edge model that takes into account
drift and diffusion currents to characterize the space charge limited current
in organic semiconductors. The numerical solution of the drift-diffusion
equation allows the utilization of asymmetric contacts to describe the built-in
potential within the device. The model has been applied to extract information
of the distribution of traps from experimental current-voltage measurements of
a rubrene single crystal from Krellner et al. [Phys. Rev. B, 75(24), 245115]
showing excellent agreement across several orders of magnitude of current.
Although the two contacts are made of the same metal, an energy offset of 580
meV between them, ascribed to differences in the deposition techniques
(lamination vs. evaporation) was essential to correctly interpret the shape of
the current-voltage characteristics at low voltage. A band mobility 0.13 cm2/Vs
for holes was estimated, which is consistent with transport along the long axis
of the orthorhombic unit cell. The total density of traps deeper than 0.1 eV
was 2.2\times1016 cm-3. The sensitivity analysis and error estimation in the
obtained parameters shows that it is not possible to accurately resolve the
shape of the trap distribution for energies deeper than 0.3 eV or shallower
than 0.1 eV above the valence band edge. The total number of traps deeper than
0.3 eV however can be estimated. Contact asymmetry and the diffusion component
of the current play an important role in the description of the device at low
bias, and are required to obtain reliable information about the distribution of
deep traps
On the robustness of least-squares Monte Carlo (LSM) for pricing American derivatives
This paper analyses the robustness of Least-Squares Monte Carlo, a technique recently proposed by Longstaff and Schwartz (2001) for pricing American options. This method is based on least-squares regressions in which the explanatory variables are certain polynomial functions. We analyze the impact of different basis functions on option prices. Numerical results for American put options provide evidence that a) this approach is very robust to the choice of different alternative polynomials and b) few basis functions are required. However, these conclusions are not reached when analyzing more complex derivatives.Least-Squares Monte Carlo, option pricing, American options
- …