454 research outputs found
Catastrophic Evaporation of Rocky Planets
Short-period exoplanets can have dayside surface temperatures surpassing 2000
K, hot enough to vaporize rock and drive a thermal wind. Small enough planets
evaporate completely. We construct a radiative-hydrodynamic model of
atmospheric escape from strongly irradiated, low-mass rocky planets, accounting
for dust-gas energy exchange in the wind. Rocky planets with masses < 0.1
M_Earth (less than twice the mass of Mercury) and surface temperatures > 2000 K
are found to disintegrate entirely in < 10 Gyr. When our model is applied to
Kepler planet candidate KIC 12557548b --- which is believed to be a rocky body
evaporating at a rate of dM/dt > 0.1 M_Earth/Gyr --- our model yields a
present-day planet mass of < 0.02 M_Earth or less than about twice the mass of
the Moon. Mass loss rates depend so strongly on planet mass that bodies can
reside on close-in orbits for Gyrs with initial masses comparable to or less
than that of Mercury, before entering a final short-lived phase of catastrophic
mass loss (which KIC 12557548b has entered). Because this catastrophic stage
lasts only up to a few percent of the planet's life, we estimate that for every
object like KIC 12557548b, there should be 10--100 close-in quiescent
progenitors with sub-day periods whose hard-surface transits may be detectable
by Kepler --- if the progenitors are as large as their maximal, Mercury-like
sizes (alternatively, the progenitors could be smaller and more numerous).
According to our calculations, KIC 12557548b may have lost ~70% of its
formation mass; today we may be observing its naked iron core.Comment: Accepted to MNRAS with minor edits compared to version
Economic and political effects on currency clustering
This is an Accepted Manuscript of an article published by Taylor & Francis in Quantitative Finance vol. 19, no. 5, available online: https://www.tandfonline.com/doi/abs/10.1080/14697688.2018.1532101.We propose a new measure named the symbolic performance to better understand the structure of foreign exchange markets. Instead of considering currency pairs, we isolate a quantity that describes each currency’s position in the market, independent of a base currency. We apply the k-means++ clustering algorithm to analyze how the roles of currencies change over time, from reference status or minimal apprecia- tions and depreciations with respect to other currencies to large appreciations and depreciations. We show how different central bank interventions and economic and political developments, such as the cap on the Swiss franc to the euro enforced by the Swiss National Bank or the Brexit vote, affect the position of a currency in the global foreign exchange market.Accepted manuscrip
A Plan for a Comprehensive and Integrated Information Systems Curriculum
Curriculum guidelines have placed a strong emphasis on integration of theory and practice in information systems (IS). A parallel concern, as yet unaddressed, is the integration throughout the IS curriculum of key theoretical concepts from individual courses. This paper suggests an architectural plan that incorporates formal methods, technological team issues, and organizational theory considerations in the development of a fully integrated information systems curriculum
Community analysis of global financial markets
We analyze the daily returns of stock market indices and currencies of 56 countries over the period of 2002–2012. We build a network model consisting of two layers, one being the stock market indices and the other the foreign exchange markets. Synchronous and lagged correlations are used as measures of connectivity and causality among different parts of the global economic system for two different time intervals: non-crisis (2002–2006) and crisis (2007–2012) periods. We study community formations within the network to understand the influences and vulnerabilities of specific countries or groups of countries. We observe different behavior of the cross correlations and communities for crisis vs. non-crisis periods. For example, the overall correlation of stock markets increases during crisis while the overall correlation in the foreign exchange market and the correlation between stock and foreign exchange markets decrease, which leads to different community structures. We observe that the euro, while being central during the relatively calm period, loses its dominant role during crisis. Furthermore we discover that the troubled Eurozone countries, Portugal, Italy, Greece and Spain, form their own cluster during the crisis period.Published versio
Homogeneous nucleation: Comparison between two theories
The classical nucleation theory of Becker, D\"{o}ring and Zeldovich is
compared with the Langer coarse-grained field approach to the nucleation
phenomenon. Both formalisms have been applied to the condensation from a
supersaturated vapor. It is shown that the nucleation rate derived in the
classical theory can be expressed in a form equivalent to that of the field
nucleation theory. This equivalence serves as an explanation of the puzzling
fact that the numerical predictions of both theories for condensation of Xe and
CO are almost identical though the standard analytical expressions for the
nucleation rates are different. The results obtained can help to link the
theories of nucleation and their approximations.Comment: 12 pages, LaTeX, no figure
Surface Layer Accretion in Conventional and Transitional Disks Driven by Far-Ultraviolet Ionization
Whether protoplanetary disks accrete at observationally significant rates by
the magnetorotational instability (MRI) depends on how well ionized they are.
Disk surface layers ionized by stellar X-rays are susceptible to charge
neutralization by small condensates, ranging from ~0.01-micron-sized grains to
angstrom-sized polycyclic aromatic hydrocarbons (PAHs). Ion densities in
X-ray-irradiated surfaces are so low that ambipolar diffusion weakens the MRI.
Here we show that ionization by stellar far-ultraviolet (FUV) radiation enables
full-blown MRI turbulence in disk surface layers. Far-UV ionization of atomic
carbon and sulfur produces a plasma so dense that it is immune to ion
recombination on grains and PAHs. The FUV-ionized layer, of thickness 0.01--0.1
g/cm^2, behaves in the ideal magnetohydrodynamic limit and can accrete at
observationally significant rates at radii > 1--10 AU. Surface layer accretion
driven by FUV ionization can reproduce the trend of increasing accretion rate
with increasing hole size seen in transitional disks. At radii < 1--10 AU,
FUV-ionized surface layers cannot sustain the accretion rates generated at
larger distance, and unless turbulent mixing of plasma can thicken the
MRI-active layer, an additional means of transport is needed. In the case of
transitional disks, it could be provided by planets.Comment: Final proofed version. Corrects X-ray-driven accretion rates in the
high PAH case for Figures 8 and
Forming Planetesimals by Gravitational Instability: II. How Dust Settles to its Marginally Stable State
Dust at the midplane of a circumstellar disk can become gravitationally
unstable and fragment into planetesimals if the local dust-to-gas density ratio
mu is sufficiently high. We simulate how dust settles in passive disks and ask
how high mu can become. We settle the dust using a 1D code and test for
dynamical stability using a 3D shearing box code. This scheme allows us to
explore the behavior of small particles having short but non-zero stopping
times in gas: 0 < t_stop << the orbital period. The streaming instability is
thereby filtered out. Dust settles until shearing instabilities in the edges of
the dust layer threaten to overturn the entire layer. In this state of marginal
stability, mu=2.9 for a disk whose bulk (height-integrated) metallicity is
solar. For a disk whose bulk metallicity is 4x solar, mu reaches 26.4. These
maximum values of mu, which depend on the background radial pressure gradient,
are so large that gravitational instability of small particles is viable in
disks whose bulk metallicities are just a few (<4) times solar. Earlier studies
assumed that dust settles until the Richardson number Ri is spatially constant.
Our simulations are free of this assumption but provide support for it within
the dust layer's edges, with the proviso that Ri increases with bulk
metallicity in the same way that we found in Paper I. Only modest enhancements
in bulk metallicity are needed to spawn planetesimals directly from small
particles.Comment: Accepted to Ap
COUPLED NETWORK APPROACH TO PREDICTABILITY OF FINANCIAL MARKET RETURNS AND NEWS SENTIMENTS
High-energy sub-nanosecond optical pulse generation with a semiconductor laser diode for pulsed TOF laser ranging utilizing the single photon detection approach
Bulk and quantum well laser diodes with a large equivalent spot size of da/Γa ≈ 3 µm and stripe width/cavity length of 30 µm/3 mm were realized and tested. They achieved a pulse energy and pulse length of the order of ~1 nJ and ~100 ps, respectively, with a peak pulse current of 6–8 A and a current pulse width of 1 ns. The 2D characteristics of the optical output power versus wavelength and time were also analyzed with a monochromator/streak camera set-up. The far-field characteristics were studied with respect to the time-homogeneity and energy distribution. The feasibility of a laser diode with a large equivalent spot size in single photon detection based laser ranging was demonstrated to a non-cooperative target at a distance of a few tens of meters
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