70,685 research outputs found
Radiation absorption noise for molecular information transfer
Molecular signaling is ubiquitous across scales in nature and finds useful applications in precision medicine and heavy industry. Characterizing noise in communication systems is essential to understanding its information capacity. To date, research in molecular nano communication (MNC) primarily considers the molecular dynamics within the medium, where various forms of stochastic effects generate noise. However, in many real-world scenarios, external effects can also influence molecular dynamics and cause noise. Here, the noise due to the temperature fluctuations from incident electromagnetic (EM) radiation is considered, with applications ranging from cell signaling to chemical engineering. EM radiation and subsequent molecular absorption cause temperature fluctuations which affect molecular dynamics and can be considered as an exogenous noise source for MNC. In this paper, the probability density function of the radiation absorption noise (RAN) is analyzed and to demonstrate applicability, we include characteristics of different tissues of the human body. Furthermore, the closed-form expression of error probability (EP) for MNC under the radiation noise is derived. Numerical analysis is demonstrated on different tissues of the human body: skin, brain, and blood, as well as the polarization factor of incident EM radiation is demonstrated. The coupling relationship between the radiation frequency and the intrinsic impedance of the human body on the PDF of radiation absorption noise is presented. This is useful for understanding how mutual information changes with external radiation source
Hot HCN around young massive stars at 0.1" resolution
Massive stars form deeply embedded in dense molecular gas, which they stir
and heat up and ionize. During an early phase, the ionization is confined to
hypercompact HII regions, and the stellar radiation is entirely absorbed by
dust, giving rise to a hot molecular core. To investigate the innermost
structure of such high-mass star-forming regions, we observed vibrationally
excited HCN (via the direct -type transition of v2=1, =0, J=13,
which lies 1400 K above ground) toward the massive hot molecular cores
G10.47+0.03, SgrB2-N, and SgrB2-M with the Very Large Array (VLA) at 7 mm,
reaching a resolution of about 1000 AU (0.1"). We detect the line both in
emission and in absorption against HII regions. The latter allows to derive
lower limits on the column densities of hot HCN, which are several times
cm. We see indication of expansion motions in G10.47+0.03 and
detect velocity components in SgrB2-M at 50, 60, and 70 km/s relative to the
Local Standard of Rest. The emission originates in regions of less than 0.1 pc
diameter around the hypercompact HII regions G10.47+0.03 B1 and SgrB2-N K2, and
reaches brightness temperatures of more than 200 K. Using the three-dimensional
radiative transfer code RADMC-3D, we model the sources as dense dust cores
heated by stars in the HII regions, and derive masses of hot (>300 K) molecular
gas of more than 100 solar masses (for an HCN fractional abundance of
10), challenging current simulations of massive star formation. Heating
only by the stars in the HII regions is sufficient to produce such large
quantities of hot molecular gas, provided that dust is optically thick to its
own radiation, leading to high temperatures through diffusion of radiation.Comment: 12 pages, 14 figures, accepted for publication in A&
LIME - a flexible, non-LTE line excitation and radiation transfer method for millimeter and far-infrared wavelengths
We present a new code for solving the molecular and atomic excitation and
radiation transfer problem in a molecular gas and predicting emergent spectra.
This code works in arbitrary three dimensional geometry using unstructured
Delaunay latices for the transport of photons. Various physical models can be
used as input, ranging from analytical descriptions over tabulated models to
SPH simulations. To generate the Delaunay grid we sample the input model
randomly, but weigh the sample probability with the molecular density and other
parameters, and thereby we obtain an average grid point separation that scales
with the local opacity. Our code does photon very efficiently so that the slow
convergence of opaque models becomes traceable. When convergence between the
level populations, the radiation field, and the point separation has been
obtained, the grid is ray-traced to produced images that can readily be
compared to observations. Because of the high dynamic range in scales that can
be resolved using this type of grid, our code is particularly well suited for
modeling of ALMA data. Our code can furthermore deal with overlapping lines of
multiple molecular and atomic species.Comment: 13 pages, 12 figures, Accepted by A&A on 06/08/201
LIME - a flexible, non-LTE line excitation and radiation transfer method for millimeter and far-infrared wavelengths
We present a new code for solving the molecular and atomic excitation and
radiation transfer problem in a molecular gas and predicting emergent spectra.
This code works in arbitrary three dimensional geometry using unstructured
Delaunay latices for the transport of photons. Various physical models can be
used as input, ranging from analytical descriptions over tabulated models to
SPH simulations. To generate the Delaunay grid we sample the input model
randomly, but weigh the sample probability with the molecular density and other
parameters, and thereby we obtain an average grid point separation that scales
with the local opacity. Our code does photon very efficiently so that the slow
convergence of opaque models becomes traceable. When convergence between the
level populations, the radiation field, and the point separation has been
obtained, the grid is ray-traced to produced images that can readily be
compared to observations. Because of the high dynamic range in scales that can
be resolved using this type of grid, our code is particularly well suited for
modeling of ALMA data. Our code can furthermore deal with overlapping lines of
multiple molecular and atomic species.Comment: 13 pages, 12 figures, Accepted by A&A on 06/08/201
A novel satellite mission concept for upper air water vapour, aerosol and cloud observations using integrated path differential absorption LiDAR limb sounding
We propose a new satellite mission to deliver high quality measurements of upper air water vapour. The concept centres around a LiDAR in limb sounding by occultation geometry, designed to operate as a very long path system for differential absorption measurements. We present a preliminary performance analysis with a system sized to send 75 mJ pulses at 25 Hz at four wavelengths close to 935 nm, to up to 5 microsatellites in a counter-rotating orbit, carrying retroreflectors characterized by a reflected beam divergence of roughly twice the emitted laser beam divergence of 15 µrad. This provides water vapour profiles with a vertical sampling of 110 m; preliminary calculations suggest that the system could detect concentrations of less than 5 ppm. A secondary payload of a fairly conventional medium resolution multispectral radiometer allows wide-swath cloud and aerosol imaging. The total weight and power of the system are estimated at 3 tons and 2,700 W respectively. This novel concept presents significant challenges, including the performance of the lasers in space, the tracking between the main spacecraft and the retroreflectors, the refractive effects of turbulence, and the design of the telescopes to achieve a high signal-to-noise ratio for the high precision measurements. The mission concept was conceived at the Alpbach Summer School 2010
Determination of the Boltzmann constant by laser spectroscopy as a basis for future measurements of the thermodynamic temperature
In this paper, we present the latest results on the measurement of the
Boltzmann constant kB, by laser spectroscopy of ammonia at 10 ?m. The Doppler
absorption profile of a ro-vibrational line of an NH3 gas sample at thermal and
pressure equilibrium is measured as accurately as possible. The absorption cell
is placed inside a large 1m3 thermostat filled with an ice-water mixture, which
sets the temperature very close to 273.15 K. Analysing this profile, which is
related to the Maxwell-Boltzmann molecular speed distribution, leads to a
determination of the Boltzmann constant via a measurement of the Doppler width
(proportional tosqrt(kBT)). A spectroscopic determination of the Boltzmann
constant with an uncertainty as low as 37 ppm is obtained. Recent improvements
with a new passive thermostat lead to a temperature accuracy, stability and
homogeneity of the absorption cell better than 1 ppm over a day
Analytical modelling of the effect of noise on the terahertz in-vivo communication channel for body-centric nano-networks
The paper presents an analytical model of the terahertz (THz) communication channel (0.1 - 10 THz) for in-vivo nano-networks by considering the effect of noise on link quality and information rate. The molecular absorption noise model for in-vivo nano-networks is developed based on the physical mechanisms of the noise present in the medium, which takes into account both the radiation of the medium and the molecular absorption from the transmitted signal. The signal-to-noise ratio (SNR) of the communication channel is investigated for different power allocation schemes and the maximum achievable information rate is studied to explore the potential of THz communication inside the human body. The obtained results show that the information rate is inversely proportional to the transmission distance. Based on the studies on channel performance, it can be concluded that the achievable transmission distance of in-vivo THz nano-networks should be restrained to approximately 2 mm maximum, while the operation band of in-vivo THz nano-networks should be limited to the lower band of the THz band. This motivates the utilisation of hierarchical/cooperative networking concepts and hybrid communication techniques using molecular and electromagnetic methods for future body-centric nano-networks
Molecular Line Emission from Massive Protostellar Disks: Predictions for ALMA and the EVLA
We compute the molecular line emission of massive protostellar disks by
solving the equation of radiative transfer through the cores and disks produced
by the recent radiation-hydrodynamic simulations of Krumholz, Klein, & McKee.
We find that in several representative lines the disks show brightness
temperatures of hundreds of Kelvin over velocity channels ~10 km s^-1 wide,
extending over regions hundreds of AU in size. We process the computed
intensities to model the performance of next-generation radio and submillimeter
telescopes. Our calculations show that observations using facilities such as
the EVLA and ALMA should be able to detect massive protostellar disks and
measure their rotation curves, at least in the nearest massive star-forming
regions. They should also detect significant sub-structure and non-axisymmetry
in the disks, and in some cases may be able to detect star-disk velocity
offsets of a few km s^-1, both of which are the result of strong gravitational
instability in massive disks. We use our simulations to explore the strengths
and weaknesses of different observational techniques, and we also discuss how
observations of massive protostellar disks may be used to distinguish between
alternative models of massive star formation.Comment: 15 pages, 9 figures, emulateapj format, accepted for publication in
ApJ. Resolution of figures severely degraded to fit within size limits.
Download the full paper from
http://www.astro.princeton.edu/~krumholz/recent.htm
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