arXiv subjects

David Rodriguez

Publications and source records attributed to David Rodriguez.

47 records · Page 3Linked to original sources

arXiv2012

The kinematics of very low mass dwarfs: splinter session summary

Kinematic investigations are being increasingly deployed in studies of the lowest mass stars and brown dwarfs to investigate their origins, characterize their atmospheres, and examine the evolution of their physical parameters. This article summarizes the contributions made at the Kinematics of Very Low Mass Dwarfs Splinter Session. Results discussed include analysis of kinematic distributions of M, L and T dwarfs; theoretical tools for interpreting these distributions; identifications of very low mass halo dwarfs and wide companions to nearby stars; radial velocity variability among young and very cool brown dwarfs; and the search and identification of M dwarfs in young moving groups. A summary of discussion points at the conclusion of the Splinter is also presented.

astro-ph.SR↗

arXiv2012

Resolved Debris Discs Around A Stars in the Herschel DEBRIS Survey

The majority of debris discs discovered so far have only been detected through infrared excess emission above stellar photospheres. While disc properties can be inferred from unresolved photometry alone under various assumptions for the physical properties of dust grains, there is a degeneracy between disc radius and dust temperature that depends on the grain size distribution and optical properties. By resolving the disc we can measure the actual location of the dust. The launch of Herschel, with an angular resolution superior to previous far-infrared telescopes, allows us to spatially resolve more discs and locate the dust directly. Here we present the nine resolved discs around A stars between 20 and 40 pc observed by the DEBRIS survey. We use these data to investigate the disc radii by fitting narrow ring models to images at 70, 100 and 160 μm and by fitting blackbodies to full spectral energy distributions. We do this with the aim of finding an improved way of estimating disc radii for unresolved systems. The ratio between the resolved and blackbody radii varies between 1 and 2.5. This ratio is inversely correlated with luminosity and any remaining discrepancies are most likely explained by differences to the minimum size of grain in the size distribution or differences in composition. We find that three of the systems are well fit by a narrow ring, two systems are borderline cases and the other four likely require wider or multiple rings to fully explain the observations, reflecting the diversity of planetary systems.

astro-ph.EP↗

arXiv2011

Bell tests with photon-entanglement: LHV models and critical efficiencies at the light of Wigner-PDC optics

Within the Wigner-PDC picture of photon entanglement, detection "errors" are not independent (though they may look, on average), nor can they be controlled by means of a technological improvement on the detectors. Those two elements make possible the interpretation of experimental evidence without the need to exclude local realism: for that reason, we propose the abandonment of the usual (photon, particle-based) description of (PDC-generated) light states, in favour of an also quantum, but field-theoretical description (QED), a description that finds a one-to-one equivalent in the Wigner-PDC approach we have advocated in recent posts.

quant-ph↗

arXiv2011

Revisiting factorability and indeterminism

Perhaps it is not completely superfluous to remind that Clauser-Horne factorability, introduced in [1], is only necessary when λ, the hidden variable (HV), is sufficiently deterministic: for {M_i} a set of possible measurements (isolated or not by space-like intervals) on a given system, the most general sufficient condition for factorability on λ is obtained by finding a set of expressions M_i=M_i(λ,ξ_i), with {ξ_i} a set of HV's, all independent from one another and from λ. Otherwise, factorability can be recovered on γ= λ \oplus\ μ, with μ another additional HV, so that a description M_i=M_i(γ,ξ_i) is again found: conceptually, this is always possible; experimentally, it may not: μ may be unaccessible or even its existence unknown (and so, too, from the point of view of a phenomenological theory). Results here may help clarify our recent post in [6].

quant-ph↗

arXiv2011

Communication loophole in a Bell-EPR-Bohm experiment: standard no-signaling may not always be enough to exclude local realism

Assuming perfect detection efficiency, we present an (indeterministic) model for an EPR-Bohm experiment which reproduces the singlet correlations, without contradicting Bell's original locality condition. In this model we allow the probability distribution $ρ_λ$ of the state $λ$ at the source to depend parametrically on the orientation $ξ$ of one of the measuring devices: $ρ_λ(λ,ξ)$. In a Bell experiment, no-signaling between the source and each one of the devices would seem clearly sufficient to rule such an influence; however, not even schemes where the choice of observables takes place during the on-flight time of the particles can prevent, in some situations, a model of this type from violating the local bounds. In particular, a random shift $ρ_λ(λ,ξ_1) \rightarrow ρ_λ(λ,ξ_2) \rightarrow...\rightarrow ρ_λ(λ,ξ_n)$ allows the model to perform a "subensemble selection" for each of the terms involved in the inequality (analogous to what goes on with the efficiency loophole), whenever some correlation of those $ρ_λ$-shifts with the sequence of measurement choices is allowed. That correlation does not necessarily imply signaling during the photon on-flight time.

quant-ph↗

arXiv2010

Chandra X-ray Detection of the Enigmatic Field Star BP Psc

BP Psc is a remarkable emission-line field star that is orbited by a dusty disk and drives a parsec-scale system of jets. We report the detection by the Chandra X-ray Observatory of a weak X-ray point source coincident with the centroids of optical/IR and submillimeter continuum emission at BP Psc. As the star's photosphere is obscured throughout the visible and near-infrared, the Chandra X-ray source likely represents the first detection of BP Psc itself. The X-rays most likely originate with magnetic activity at BP Psc and hence can be attributed either to a stellar corona or to star-disk interactions. The log of the ratio of X-ray to bolometric luminosity (log(L_X/L_{bol}) lies in the range -5.8 to -4.2. This is smaller than log(L_X/L_{bol}) ratios typical of low-mass, pre-main sequence stars, but is well within the log(L_X/L_{bol}) range observed for rapidly-rotating (FK Com-type) G giant stars. Hence, the Chandra results favor an exotic model wherein the disk/jet system of BP Psc is the result of its very recently engulfing a companion star or giant planet, as the primary star ascended the giant branch.

astro-ph.SR↗

arXiv2010

Resolving debris discs in the far-infrared: early highlights from the DEBRIS survey

We present results from the earliest observations of DEBRIS, a Herschel Key Programme to conduct a volume- and flux-limited survey for debris discs in A-type through M-type stars. PACS images (from chop/nod or scan-mode observations) at 100 and 160 micron are presented toward two A-type stars and one F-type star: beta Leo, beta UMa and eta Corvi. All three stars are known disc hosts. Herschel spatially resolves the dust emission around all three stars (marginally, in the case of beta UMa), providing new information about discs as close as 11 pc with sizes comparable to that of the Solar System. We have combined these data with existing flux density measurements of the discs to refine the SEDs and derive estimates of the fractional luminosities, temperatures and radii of the discs.

astro-ph.SR↗

arXiv2010

Security properties in an open peer-to-peer network

This paper proposes to address new requirements of confidentiality, integrity and availability properties fitting to peer-to-peer domains of resources. The enforcement of security properties in an open peer-topeer network remains an open problem as the literature have mainly proposed contribution on availability of resources and anonymity of users. That paper proposes a novel architecture that eases the administration of a peer-to-peer network. It considers a network of safe peer-to-peer clients in the sense that it is a commune client software that is shared by all the participants to cope with the sharing of various resources associated with different security requirements. However, our proposal deals with possible malicious peers that attempt to compromise the requested security properties. Despite the safety of an open peer-to-peer network cannot be formally guaranteed, since a end user has privileges on the target host, our solution provides several advanced security enforcement. First, it enables to formally define the requested security properties of the various shared resources. Second, it evaluates the trust and the reputation of the requesting peer by sending challenges that test the fairness of its peer-to-peer security policy. Moreover, it proposes an advanced Mandatory Access Control that enforces the required peer-to-peer security properties through an automatic projection of the requested properties onto SELinux policies. Thus, the SELinux system of the requesting peer is automatically configured with respect to the required peer-to-peer security properties.

cs.CR↗

arXiv2009

Minimum detection efficiency for a loophole-free violation of the Braunstein-Caves chained Bell inequalities

The chained Bell inequalities of Braunstein and Caves involving N settings per observer have some interesting applications. Here we obtain the minimum detection efficiency required for a loophole-free violation of the Braunstein-Caves inequalities for any N > 2. We discuss both the case in which both particles are detected with the same efficiency and the case in which the particles are detected with different efficiencies.

quant-ph↗

arXiv2008

Necessary and sufficient detection efficiency for the Mermin inequalities

We prove that the threshold detection efficiency for a loophole-free Bell experiment using an $n$-qubit Greenberger-Horne-Zeilinger state and the correlations appearing in the $n$-partite Mermin inequality is $n/(2n-2)$. If the detection efficiency is equal to or lower than this value, there are local hidden variable models that can simulate all the quantum predictions. If the detection efficiency is above this value, there is no local hidden variable model that can simulate all the quantum predictions.

quant-ph↗

arXiv2008

Mermin inequalities for perfect correlations

Any n-qubit state with n independent perfect correlations is equivalent to a graph state. We present the optimal Bell inequalities for perfect correlations and maximal violation for all classes of graph states with n < 7 qubits. Twelve of them were previously unknown and four give the same violation as the Greenberger-Horne-Zeilinger state, although the corresponding states are more resistant to decoherence.

quant-ph↗