9,109 research outputs found
Effects of Extreme Obliquity Variations on the Habitability of Exoplanets
We explore the impact of obliquity variations on planetary habitability in
hypothetical systems with high mutual inclination. We show that large
amplitude, high frequency obliquity oscillations on Earth-like exoplanets can
suppress the ice-albedo feedback, increasing the outer edge of the habitable
zone. We restrict our exploration to hypothetical systems consisting of a
solar-mass star, an Earth-mass planet at 1 AU, and 1 or 2 larger planets. We
verify that these systems are stable for years with N-body simulations,
and calculate the obliquity variations induced by the orbital evolution of the
Earth-mass planet and a torque from the host star. We run a simplified energy
balance model on the terrestrial planet to assess surface temperature and ice
coverage on the planet's surface, and we calculate differences in the outer
edge of the habitable zone for planets with rapid obliquity variations. For
each hypothetical system, we calculate the outer edge of habitability for two
conditions: 1) the full evolution of the planetary spin and orbit, and 2) the
eccentricity and obliquity fixed at their average values. We recover previous
results that higher values of fixed obliquity and eccentricity expand the
habitable zone, but also find that obliquity oscillations further expand
habitable orbits in all cases. Terrestrial planets near the outer edge of the
habitable zone may be more likely to support life in systems that induce rapid
obliquity oscillations as opposed to fixed-spin planets. Such planets may be
the easiest to directly characterize with space-borne telescopes.Comment: 46 pages, 12 Figures, 5 Table
A Machine-Checked Formalization of the Generic Model and the Random Oracle Model
Most approaches to the formal analyses of cryptographic protocols make the perfect cryptography assumption, i.e. the hypothese that there is no way to obtain knowledge about the plaintext pertaining to a ciphertext without knowing the key. Ideally, one would prefer to rely on a weaker hypothesis on the computational cost of gaining information about the plaintext pertaining to a ciphertext without knowing the key. Such a view is permitted by the Generic Model and the Random Oracle Model which provide non-standard computational models in which one may reason about the computational cost of breaking a cryptographic scheme. Using the proof assistant Coq, we provide a machine-checked account of the Generic Model and the Random Oracle Mode
Automating Security Analysis: Symbolic Equivalence of Constraint Systems
We consider security properties of cryptographic protocols, that are either trace properties (such as confidentiality or authenticity) or equivalence properties (such as anonymity or strong secrecy). Infinite sets of possible traces are symbolically represented using deducibility constraints. We give a new algorithm that decides the trace equivalence for the traces that are represented using such constraints, in the case of signatures, symmetric and asymmetric encryptions. Our algorithm is implemented and performs well on typical benchmarks. This is the first implemented algorithm, deciding symbolic trace equivalence
HAZMAT VI: The Evolution of Extreme Ultraviolet Radiation Emitted from Early M Star
Quantifying the evolution of stellar extreme ultraviolet (EUV, 100 -- 1000
) emission is critical for assessing the evolution of
planetary atmospheres and the habitability of M dwarf systems. Previous studies
from the HAbitable Zones and M dwarf Activity across Time (HAZMAT) program
showed the far- and near-UV (FUV, NUV) emission from M stars at various stages
of a stellar lifetime through photometric measurements from the Galaxy
Evolution Explorer (GALEX). The results revealed increased levels of
short-wavelength emission that remain elevated for hundreds of millions of
years. The trend for EUV flux as a function of age could not be determined
empirically because absorption by the interstellar medium prevents access to
the EUV wavelengths for the vast majority of stars. In this paper, we model the
evolution of EUV flux from early M stars to address this observational gap. We
present synthetic spectra spanning EUV to infrared wavelengths of 0.4
0.05 M stars at five distinct ages between 10 and 5000 Myr, computed
with the PHOENIX atmosphere code and guided by the GALEX photometry. We model a
range of EUV fluxes spanning two orders of magnitude, consistent with the
observed spread in X-ray, FUV, and NUV flux at each epoch. Our results show
that the stellar EUV emission from young M stars is 100 times stronger than
field age M stars, and decreases as t after remaining constant for a few
hundred million years. This decline stems from changes in the chromospheric
temperature structure, which steadily shifts outward with time. Our models
reconstruct the full spectrally and temporally resolved history of an M star's
UV radiation, including the unobservable EUV radiation, which drives planetary
atmospheric escape, directly impacting a planet's potential for habitability.Comment: 23 pages, 15 figures, accepted to Ap
Highly Sensitive Dual-Core Photonic Metal Fiber
In this study, we propose an all-solid cladding dual-core metal fiber (DC-MF)
filled with toluene and ethanol for temperature sensing applications. Instead
of using air holes in the cladding region, we employ fluorine doped silica
glass to form an all-solid cladding. By selectively filling toluene and ethanol
into three air holes near the core region, we investigate the temperature
sensing characteristics numerically. Simulation results demonstrate that the
average sensitivity of the temperature sensing can reach -11.64 and -7.41 nm/C
within the temperature ranges of 0 to 70 C and -80 to 0 C, respectively, even
with a short DC-MF length of 1.6 mm. The maximum sensitivity in the considered
temperature ranges can reach up to -15 and -9 nm/C, respectively. Furthermore,
the proposed temperature sensor exhibits insensitivity to hydrostatic pressure
Proof Theory, Transformations, and Logic Programming for Debugging Security Protocols
We define a sequent calculus to formally specify, simulate, debug and verify security protocols. In our sequents we distinguish between the current knowledge of principals and the current global state of the session. Hereby, we can describe the operational semantics of principals and of an intruder in a simple and modular way. Furthermore, using proof theoretic tools like the analysis of permutability of rules, we are able to find efficient proof strategies that we prove complete for special classes of security protocols including Needham-Schroeder. Based on the results of this preliminary analysis, we have implemented a Prolog meta-interpreter which allows for rapid prototyping and for checking safety properties of security protocols, and we have applied it for finding error traces and proving correctness of practical examples
VPLanet: The Virtual Planet Simulator
We describe a software package called VPLanet that simulates fundamental
aspects of planetary system evolution over Gyr timescales, with a focus on
investigating habitable worlds. In this initial release, eleven physics modules
are included that model internal, atmospheric, rotational, orbital, stellar,
and galactic processes. Many of these modules can be coupled simultaneously to
simulate the evolution of terrestrial planets, gaseous planets, and stars. The
code is validated by reproducing a selection of observations and past results.
VPLanet is written in C and designed so that the user can choose the physics
modules to apply to an individual object at runtime without recompiling, i.e.,
a single executable can simulate the diverse phenomena that are relevant to a
wide range of planetary and stellar systems. This feature is enabled by
matrices and vectors of function pointers that are dynamically allocated and
populated based on user input. The speed and modularity of VPLanet enables
large parameter sweeps and the versatility to add/remove physical phenomena to
assess their importance. VPLanet is publicly available from a repository that
contains extensive documentation, numerous examples, Python scripts for
plotting and data management, and infrastructure for community input and future
development.Comment: 75 pages, 34 figures, 10 tables, accepted to the Proceedings of the
Astronomical Society of the Pacific. Source code, documentation, and examples
available at https://github.com/VirtualPlanetaryLaboratory/vplane
The nuclear spectrum of the radio galaxy NGC 5128 (Centaurus A)
We present near-infrared spectra of the nuclear disk in the nearby radio
galaxy NGC 5128 (Centaurus A). On the basis of the observed strengths of the [S
III] 0.9532um and [Fe II] 1.2567um lines, we classify NGC 5128 as a LINER.
Modeling of the strengths of these and additional lines suggests that the
nuclear region is powered by shocks rather than photoionization.Comment: 12 pages including 2 figures, accepted by ApJ Letter
Intraabdominal Intravascular Papillary Endothelial Hyperplasia (Masson's Tumor): A Rare and Novel Cause of Gastrointestinal Bleeding
Intravascular papillary endothelial hyperplasia (IPEH), or Masson's tumor, a rare benign vascular lesion, occurs mainly in the head, neck, and hands in the human population. Aberrant tumor locations have been rarely reported. We present a case of a patient with chronic abdominal pain and melena of variable severity due to a Masson's tumor, with no apparent Masson's tumor-associated comorbidities, along with a comprehensive review of the literature. Using PubMed, a search engine provided by the U.S. National Library of Medicine and the National Institutes of Health, we searched for all reports of Masson's tumor limited within the abdominal cavity. Furthermore, keywords such as âintravascular papillary endothelial hyperplasiaâ, ârenalâ, âgastrointestinalâ, âhepaticâ and âintraabdominalâ were used to facilitate the search. We thus found fourteen cases of intraabdominal Masson's tumors published. Six (42.9%) of these were located in the renal vein, 4 (28.6%) were reported in the gastrointestinal tract, 1 (7.1%) in the adrenal gland, 1 (7.1%) in the liver, and 1 (7.1%) instance with multiple lesion sites including the renal hilum and retroperitoneum. Among these patients, 9 (64.3%) were female and 5 (35.7%) male, with a mean age of 38.9 years (7â69). IPEH is a reactive process, having three subtypes, all involving the proliferation of epithelial cells around a thrombus in the setting of venous stasis. In its pure form, the organized thrombus is solely localized within the vascular lumen. Mixed-form IPEH is formed in preexisting vascular lesions (such as arteriovenous malformation, hemangioma, pyogenic granuloma, etc.). The rarest form is the extravascular variety, which arises in hematomas often from recent trauma to the area. In its pure form, IPEH has a zero recurrence rate when an R0 resection is performed; all mixed and extravascular forms show the highest recurrence rates. The exact histogenesis of these epithelial cells remains a mystery and multiple theories have been offered. Although difficult to distinguish from malignant angiosarcomas solely on presentation and radiologic work-up, Masson's tumors occur more frequently in women, demonstrate no local invasion, do not metastasize, are commonly located intravascularly, and are associated with a significantly more favorable prognosis than angiosarcomas. Only four Masson's tumors have been reported in the gastrointestinal tract, two of these cases were related to microvascular thrombosis secondary to paroxysmal nocturnal hemoglobinuria and two were formed secondary to arteriovenous malformations. Our case lacked solitary evidence of either of these comorbidities. An incidental finding of an enlarged ovary, which was removed during our exploratory laparoscopy, plus strong demographic statistics that suggest women have an increased prevalence of this lesion may help support a hormonal theory of pathogenesis
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