77,664 research outputs found
The inner wind of IRC+10216 revisited: New exotic chemistry and diagnostic for dust condensation in carbon stars
Aims. We model the chemistry of the inner wind of the carbon star IRC+10216
and consider the effect of periodic shocks induced by the stellar pulsation on
the gas to follow the non-equilibrium chemistry in the shocked gas layers. We
consider a very complete set of chemical families, including hydrocarbons and
aromatics, hydrides, halogens and phosphorous-bearing species. Derived
abundances are compared to the latest observational data from large surveys and
Herschel. Results. The shocks induce a non-equilibrium chemistry in the dust
formation zone of IRC+10216 where the collision destruction of CO in the
post-shock gas triggers the formation of O-bearing species (H2O, SiO). Most of
the modelled abundances agree very well with the latest values derived from
Herschel data on IRC+10216. Hydrides form a family of abundant species that are
expelled into the intermediate envelope. In particular, HF traps all the atomic
fluorine in the dust formation zone. Halogens are also abundant and their
chemistry is independent of the C/O ratio of the star. Therefore, HCl and other
Cl-bearing species should also be present in the inner wind of O-rich AGB or
supergiant stars. We identify a specific region ranging from 2.5 R* to 4 R*,
where polycyclic aromatic hydrocarbons form and grow. The estimated carbon
dust-to-gas mass ratio derived from the mass of aromatics ranges from 1.2 x
10^(-3) to 5.8 x 10^{-3} and agrees well with existing observational values.
The aromatic formation region is located outside hot layers where SiC2 is
produced as a bi-product of silicon carbide dust synthesis. Finally, we predict
that some molecular lines will show flux variation with pulsation phase and
time (e.g., H2O) while other species will not (e.g., CO). These variations
merely reflect the non-equilibrium chemistry that destroys and reforms
molecules over a pulsation period in the shocked gas of the dust formation
zone.Comment: 15 pages, 10 figures. Accepted for publication in Astronomy &
Astrophysic
Generic Feasibility of Perfect Reconstruction with Short FIR Filters in Multi-channel Systems
We study the feasibility of short finite impulse response (FIR) synthesis for
perfect reconstruction (PR) in generic FIR filter banks. Among all PR synthesis
banks, we focus on the one with the minimum filter length. For filter banks
with oversampling factors of at least two, we provide prescriptions for the
shortest filter length of the synthesis bank that would guarantee PR almost
surely. The prescribed length is as short or shorter than the analysis filters
and has an approximate inverse relationship with the oversampling factor. Our
results are in form of necessary and sufficient statements that hold
generically, hence only fail for elaborately-designed nongeneric examples. We
provide extensive numerical verification of the theoretical results and
demonstrate that the gap between the derived filter length prescriptions and
the true minimum is small. The results have potential applications in synthesis
FB design problems, where the analysis bank is given, and for analysis of
fundamental limitations in blind signals reconstruction from data collected by
unknown subsampled multi-channel systems.Comment: Manuscript submitted to IEEE Transactions on Signal Processin
A simple model for the evolution of molecular codes driven by the interplay of accuracy, diversity and cost
Molecular codes translate information written in one type of molecules into
another molecular language. We introduce a simple model that treats molecular
codes as noisy information channels. An optimal code is a channel that conveys
information accurately and efficiently while keeping down the impact of errors.
The equipoise of the three conflicting needs, for minimal error-load, minimal
cost of resources and maximal diversity of vocabulary, defines the fitness of
the code. The model suggests a mechanism for the emergence of a code when
evolution varies the parameters that control this equipoise and the mapping
between the two molecular languages becomes non-random. This mechanism is
demonstrated by a simple toy model that is formally equivalent to a mean-field
Ising magnet.Comment: Keywords: molecular codes, rate-distortion theory, biological
information channels, stochastic maps, genetic code, genetic network
- âŠ