15,177 research outputs found
Lacunary generating functions of Hermite polynomials and symbolic methods
We employ an umbral formalism to reformulate the theory of Hermite polynomials and the derivation of the associated lacunary generating functions
Low EUV Luminosities Impinging on Protoplanetary Disks
The amount of high-energy stellar radiation reaching the surface of
protoplanetary disks is essential to determine their chemistry and physical
evolution. Here, we use millimetric and centimetric radio data to constrain the
EUV luminosity impinging on 14 disks around young (~2-10Myr) sun-like stars.
For each object we identify the long-wavelength emission in excess to the dust
thermal emission, attribute that to free-free disk emission, and thereby
compute an upper limit to the EUV reaching the disk. We find upper limits lower
than 10 photons/s for all sources without jets and lower than photons/s for the three older sources in our sample. These latter
values are low for EUV-driven photoevaporation alone to clear out
protoplanetary material in the timescale inferred by observations. In addition,
our EUV upper limits are too low to reproduce the [NeII] 12.81 micron
luminosities from three disks with slow [NeII]-detected winds. This indicates
that the [NeII] line in these sources primarily traces a mostly neutral wind
where Ne is ionized by 1 keV X-ray photons, implying higher photoevaporative
mass loss rates than those predicted by EUV-driven models alone. In summary,
our results suggest that high-energy stellar photons other than EUV may
dominate the dispersal of protoplanetary disks around sun-like stars.Comment: Accepted for publication to The Astrophysical Journa
A blockchain based approach for the definition of auditable Access Control systems
This work proposes to exploit blockchain technology to define Access Control systems that guarantee the auditability of access control policies evaluation. The key idea of our proposal is to codify attribute-based Access Control policies as smart contracts and deploy them on a blockchain, hence transforming the policy evaluation process into a completely distributed smart contract execution. Not only the policies, but also the attributes required for their evaluation are managed by smart contracts deployed on the blockchain. The auditability property derives from the immutability and transparency properties of blockchain technology. This paper not only presents the proposed Access Control system in general, but also its application to the innovative reference scenario where the resources to be protected are themselves smart contracts. To prove the feasibility of our approach, we present a reference implementation exploiting XACML policies and Solidity written smart contracts deployed on the Ethereum blockchain. Finally, we evaluate the system performances through a set of experimental results, and we discuss the advantages and drawbacks of our proposal
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A blockchain based approach for the definition of auditable Access Control systems
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