7,634 research outputs found
Core-Accretion Model Predicts Few Jovian-Mass Planets Orbiting Red Dwarfs
The favored theoretical explanation for giant planet formation -- in both our
solar system and others -- is the core accretion model (although it still has
some serious difficulties). In this scenario, planetesimals accumulate to build
up planetary cores, which then accrete nebular gas. With current opacity
estimates for protoplanetary envelopes, this model predicts the formation of
Jupiter-mass planets in 2--3 Myr at 5 AU around solar-mass stars, provided that
the surface density of solids is enhanced over that of the minimum-mass solar
nebula (by a factor of a few). Working within the core-accretion paradigm, this
paper presents theoretical calculations which show that the formation of
Jupiter-mass planets orbiting M dwarf stars is seriously inhibited at all
radial locations (in sharp contrast to solar-type stars). Planet detection
programs sensitive to companions of M dwarfs will test this prediction in the
near future.Comment: 10 pages including 2 figures; accepted to ApJ Letter
Doppler Probe of Accretion onto a T Tauri star
The YY Ori stars are T Tauri stars with prominent time-variable redshifted
absorption components that flank certain emission lines. One of the brightest
in this class is S CrA, a visual double star. We have obtained a series of
high-resolution spectra of the two components during four nights with the UVES
spectrograph at the Very Large Telescope. We followed the spectral changes
occurring in S CrA to derive the physical structure of the accreting gas.
We found that both stars are very similar with regard to surface temperature,
radius, and mass. Variable redshifted absorption components are particularly
prominent in the SE component. During one night, this star developed a spectrum
unique among the T Tauri stars: extremely strong and broad redshifted
absorption components appeared in many lines of neutral and ionized metals, in
addition to those of hydrogen and helium. The absorption depths of cooler, low
ionization lines peak at low velocities - while more highly ionized lines have
peak absorption depths at high velocities. The different line profiles indicate
that the temperature and density of the accretion stream increase as material
approaches the star. We derive the physical conditions of the flow at several
points along the accretion funnel directly from the spectrum of the infalling
gas. We estimated mass accretion rates of about 10^(-7) solar masses per year,
which is similar to that derived from the relation based on the strength of H
alpha emission line.
This is the first time the density and temperature distributions in accretion
flows around a T Tauri star have been inferred from observations. Compared with
predictions from standard models of accretion in T Tauri stars, which assume a
dipole stellar magnetic field, we obtained higher densities and a steeper
temperature rise toward the star.Comment: Replaces 1408.1846 4 pages, 4 figures. Appears in Astronomy and
Astrophysics, 201
Multiple Transition States and Roaming in Ion-Molecule Reactions: a Phase Space Perspective
We provide a dynamical interpretation of the recently identified `roaming'
mechanism for molecular dissociation reactions in terms of geometrical
structures in phase space. These are NHIMs (Normally Hyperbolic Invariant
Manifolds) and their stable/unstable manifolds that define transition states
for ion-molecule association or dissociation reactions. The associated dividing
surfaces rigorously define a roaming region of phase space, in which both
reactive and nonreactive trajectories can be trapped for arbitrarily long
times.Comment: 20 pages, 6 figure
A New Method for Laminar Boundary Layer Transition Visualization in Flight: Color Changes in Liquid Crystal Coatings
The visualization of laminar to turbulent boundary layer transition plays an important role in flight and wind tunnel aerodynamic testing of aircraft wing and body surfaces. Visualization can help provide a more complete understanding of both transition location as well as transition modes; without visualization, the transition process can be very difficult to understand. In the past, the most valuable transition visualization methods for fight applications included sublimating chemicals and oil flows. Each method has advantages and limitations. In particular, sublimating chemicals are impractical to use in subsonic applications much above 20,000 feet because of the greatly reduced rates of sublimation at lower temperatures (less than -4 degrees Fahrenheit). Both oil flow and sublimating chemicals have the disadvantage of providing only one good data point per flight. Thus, for many important flight conditions, transition visualization has not been readily available. This paper discusses a new method for visualizing transition in fight by the use of liquid crystals. The new method overcomes the limitations of past techniques, and provides transition visualization capability throughout almost the entire altitude and speed ranges of virtually all subsonic aircraft flight envelopes. The method also has wide applicability for supersonic transition visualization in flight and for general use in wind tunnel research over wide subsonic and supersonic speed ranges
On the Radii of Extrasolar Giant Planets
We have computed evolutionary models for extrasolar planets which range in
mass from 0.1 to 3.0 Jovian Masses, and which range in equilibrium temperature
from 113 K to 2000 K. We present four sequences of models, designed to show the
structural effects of a solid core and of internal heating due to the
conversion of kinetic to thermal energy at pressures of tens of bars. The model
planetary radii are intended for comparisons with radii derived from
observations of transiting extrasolar planets. To provide such comparisons, we
expect that of order 10 transiting planets with orbital periods less than 200
days can be detected around bright (V<10) main-sequence stars for which
accurate, well-sampled radial velocity measurements can be readily accumulated.
Through these observations, structural properties of the planets will be
derivable, particularly for low-mass, high-temperature planets. Implications
regarding the transiting companion to OGLE-TR-56 recently announced by Konacki
et al. are discussed.
With regard to the confirmed transiting planet, HD 209458b, we find, in
accordance with other recent calculations, that models without internal heating
predict a radius that is 35 percent smaller than the observed radius. We
explore the possibility that HD 209458b owes its large size to dissipation of
energy arising from ongoing tidal circularization of the orbit. We show that
residual scatter in the current radial velocity data set for HD 209458b is
consistent with the presence of an as-of-yet undetected second companion, and
that further radial velocity monitoring of the star is indicated.Comment: 23 pages, 3 figures, accepted by Astrophysical Journa
Changes in the gut microbiota of mice orally exposed to methylimidazolium ionic liquids
Ionic liquids are salts used in a variety of industrial processes, and being relatively non-volatile, are proposed as environmentally-friendly replacements for existing volatile liquids. Methylimidazolium ionic liquids resist complete degradation in the environment, likely because the imidazolium moiety does not exist naturally in biological systems. However, there is limited data available regarding their mammalian effects in vivo. This study aimed to examine the effects of exposing mice separately to 2 different methylimidazolium ionic liquids (BMI and M8OI) through their addition to drinking water. Potential effects on key target organs-the liver and kidney-were examined, as well as the gut microbiome. Adult male mice were exposed to drinking water containing ionic liquids at a concentration of 440 mg/L for 18 weeks prior to examination of tissues, serum, urine and the gut microbiome. Histopathology was performed on tissues and clinical chemistry on serum for biomarkers of hepatic and renal injury. Bacterial DNA was isolated from the gut contents and subjected to targeted 16S rRNA sequencing. Mild hepatic and renal effects were limited to glycogen depletion and mild degenerative changes respectively. No hepatic or renal adverse effects were observed. In contrast, ionic liquid exposure altered gut microbial composition but not overall alpha diversity. Proportional abundance of Lachnospiraceae, Clostridia and Coriobacteriaceae spp. were significantly greater in ionic liquid-exposed mice, as were predicted KEGG functional pathways associated with xenobiotic and amino acid metabolism. Exposure to ionic liquids via drinking water therefore resulted in marked changes in the gut microbiome in mice prior to any overt pathological effects in target organs. Ionic liquids may be an emerging risk to health through their potential effects on the gut microbiome, which is implicated in the causes and/or severity of an array of chronic disease in humans
Radio Field Strength Propagation Data and Pathloss calculation Methods in UMTS Network
The design of future-generation mobile communication systems depends critically on the pathloss prediction methods and their suitability to various signal propagation regions. An accurate estimation of radio pathloss is useful for predicting coverage areas of base stations, frequency assignments, determination of electric field strength, interference analysis, handover optimization, and power level adjustment. The radio path loss will also affect other elements such as the required receiver sensitivity, the form of transmission used and several other factors. As a result, it is necessary to understand the reasons for radio path loss, and to be able to determine the levels of the signal loss for a given radio path. In this paper, we investigated the radio signal path attenuation behavior, by conducting an experimental measurement survey in a UMTS network transmitting at 2100MHz band in Government Reservation Area (GRA), Benin City. The measured field strength data collected at various distances from the base stations was used to estimate the pathloss. Firstly, the effect of different parameters, such as distance from base stations was studied and it is observed that path loss increases with distance from the signal source due to a corresponding decrease in field strength. Secondly, the calculated pathloss data have been compared with data from other existing pathloss prediction methods. We find that the Okumura-Hata model pathloss values were closest of all the propagation models considered classifying the environment into consideration. Thus, the performance of Okumura-Hata model shows its suitability for path attenuation loss prediction in UMTS networks in GRA
Parasitism of \u3ci\u3eCidia\u3c/i\u3e Spp. (Lepidoptera: Tortricidae) on \u3ci\u3eSophora Chrysophylla\u3c/i\u3e (Fabaceae) Along an Elevation Gradient of Dry Subalpine Forest on Mauna Kea, Hawaii
The biology and ecological importance of Hawaiian endemic Cydia spp. (Lepidoptera: Tortricidae) are poorly known. Cydia larvae arc an important food to palila, an endangered Hawaiian bird that inhabits Sophora woodlands on Mauna Kea, Hawaii. We quantified Cydia larval abundance in seeds of Sophora chrysophylla Salisbury (Fabaceae) and larval mortality caused by parasitism. Four new host plant associations are reported: C. crassicornis [Walsingham], C. fulsifalcella [Walsingham], C. obliqlla [Walsingham], and C. storeella [Walsingham]. Four parasitoid wasp species were consistently reared from larval Cydia: Calliephialtes grapholithae [Cresson], Diadegma blackburni [Cameron], Pristomerus hawaiiensis Perkins (Hymenoptera: Ichneumonidae), and Euderus metallicus [Ashmead] (Hymenoptera: Eulophidae). The three Ichneumonidae appear to be accidental introductions, while E. metallicus is likely to be native to Hawaii. Parasitism rates by all four wasps combined decreased with elevation from 94% at 1800 m to 20% at 2700 m
Parasitism of \u3ci\u3eCidia\u3c/i\u3e Spp. (Lepidoptera: Tortricidae) on \u3ci\u3eSophora Chrysophylla\u3c/i\u3e (Fabaceae) Along an Elevation Gradient of Dry Subalpine Forest on Mauna Kea, Hawaii
The biology and ecological importance of Hawaiian endemic Cydia spp. (Lepidoptera: Tortricidae) are poorly known. Cydia larvae arc an important food to palila, an endangered Hawaiian bird that inhabits Sophora woodlands on Mauna Kea, Hawaii. We quantified Cydia larval abundance in seeds of Sophora chrysophylla Salisbury (Fabaceae) and larval mortality caused by parasitism. Four new host plant associations are reported: C. crassicornis [Walsingham], C. fulsifalcella [Walsingham], C. obliqlla [Walsingham], and C. storeella [Walsingham]. Four parasitoid wasp species were consistently reared from larval Cydia: Calliephialtes grapholithae [Cresson], Diadegma blackburni [Cameron], Pristomerus hawaiiensis Perkins (Hymenoptera: Ichneumonidae), and Euderus metallicus [Ashmead] (Hymenoptera: Eulophidae). The three Ichneumonidae appear to be accidental introductions, while E. metallicus is likely to be native to Hawaii. Parasitism rates by all four wasps combined decreased with elevation from 94% at 1800 m to 20% at 2700 m
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