1,912 research outputs found
Parts and materials application review for space systems
Parts and materials application review for project management of space systems engineerin
Flows, Fragmentation, and Star Formation. I. Low-mass Stars in Taurus
The remarkably filamentary spatial distribution of young stars in the Taurus
molecular cloud has significant implications for understanding low-mass star
formation in relatively quiescent conditions. The large scale and regular
spacing of the filaments suggests that small-scale turbulence is of limited
importance, which could be consistent with driving on large scales by flows
which produced the cloud. The small spatial dispersion of stars from gaseous
filaments indicates that the low-mass stars are generally born with small
velocity dispersions relative to their natal gas, of order the sound speed or
less. The spatial distribution of the stars exhibits a mean separation of about
0.25 pc, comparable to the estimated Jeans length in the densest gaseous
filaments, and is consistent with roughly uniform density along the filaments.
The efficiency of star formation in filaments is much higher than elsewhere,
with an associated higher frequency of protostars and accreting T Tauri stars.
The protostellar cores generally are aligned with the filaments, suggesting
that they are produced by gravitational fragmentation, resulting in initially
quasi-prolate cores. Given the absence of massive stars which could strongly
dominate cloud dynamics, Taurus provides important tests of theories of
dispersed low-mass star formation and numerical simulations of molecular cloud
structure and evolution.Comment: 32 pages, 9 figures: to appear in Ap
Acute Malnutrition and Under-5 Mortality, Northeastern Part of India.
We assessed the prevalence of childhood acute malnutrition and under-five mortality rate (U5MR) in Darbhanga district, India, using a two-stage 49-cluster household survey. A total of 1379 households comprising 8473 people were interviewed. During a 90-day recall period, U5MR was 0.5 [95% confidence interval (CI), 0.2-1.4] per 10 000 per day. The prevalence of global acute malnutrition among 1405 children aged 6-59 months was 15.4% (NCHS) and 19.4% (2006 WHO references). This survey suggests that in Darbhanga district, the population is in a borderline food crisis with few food resources. Appropriate strategies should be developed to improve the overall nutritional and health status of children
The Thermal Regulation of Gravitational Instabilities in Protoplanetary Disks II. Extended Simulations with Varied Cooling Rates
In order to investigate mass transport and planet formation by gravitational
instabilities (GIs), we have extended our 3-D hydrodynamic simulations of
protoplanetary disks from a previous paper. Our goal is to determine the
asymptotic behavior of GIs and how it is affected by different constant cooling
times. Initially, Rdisk = 40 AU, Mdisk = 0.07 Mo, M* = 0.5 Mo, and Qmin = 1.8.
Sustained cooling, with tcool = 2 orps (outer rotation periods, 1 orp ~ 250
yrs), drives the disk to instability in ~ 4 orps. This calculation is followed
for 23.5 orps. After 12 orps, the disk settles into a quasi-steady state with
sustained nonlinear instabilities, an average Q = 1.44 over the outer disk, a
well-defined power-law Sigma(r), and a roughly steady Mdot ~ 5(-7) Mo/yr. The
transport is driven by global low-order spiral modes. We restart the
calculation at 11.2 orps with tcool = 1 and 1/4 orp. The latter case is also
run at high azimuthal resolution. We find that shorter cooling times lead to
increased Mdots, denser and thinner spiral structures, and more violent dynamic
behavior. The asymptotic total internal energy and the azimuthally averaged
Q(r) are insensitive to tcool. Fragmentation occurs only in the high-resolution
tcool = 1/4 orp case; however, none of the fragments survive for even a quarter
of an orbit. Ring-like density enhancements appear and grow near the boundary
between GI active and inactive regions. We discuss the possible implications of
these rings for gas giant planet formation.Comment: Due to document size restrictions, the complete manuscript could not
be posted on astroph. Please go to http://westworld.astro.indiana.edu to
download the full document including figure
Observing biogeochemical cycles at global scales with profiling floats and gliders: prospects for a global array
Chemical and biological sensor technologies have advanced rapidly in the past five years. Sensors that require low power and operate for multiple years are now available for oxygen, nitrate, and a variety of bio-optical properties that serve as proxies for important components of the carbon cycle (e.g., particulate organic carbon). These sensors have all been deployed successfully for long periods, in some cases more than three years, on platforms such as profiling floats or gliders. Technologies for pH, pCO2, and particulate inorganic carbon are maturing rapidly as well. These sensors could serve as the enabling technology for a global biogeochemical observing system that might operate on a scale comparable to the current Argo array. Here, we review the scientific motivation and the prospects for a global observing system for ocean biogeochemistry
Multi--Pressure Polytropes as Models for the Structure and Stability of Molecular Clouds. I. Theory
Molecular clouds are supported by thermal pressure, magnetic pressure, and
turbulent pressure. Each of these can be modeled with a polytropic equation of
state, so that overall the total pressure is the sum of the individual
components. We model the turbulent pressure as being due to a superposition of
Alfven waves. The theory of polytropes is generalized to allow for the flow of
entropy in response to a perturbation, as expected for the entropy associated
with wave pressure. The equation of state of molecular clouds is "soft", so
that the properties of the clouds are generally governed by the conditions at
the surface. In general, the polytropes are not isentropic, and this permits
large density and pressure drops to occur between the center and the edge of
the polytropes, as is observed.Comment: Submitted to ApJ with 10 figure
Determination of the absorption coefficient of chromophoric dissolved organic matter from underway spectrophotometry
Measurements of the absorption coefficient of chromophoric dissolved organic matter (ay) are needed to validate existing ocean-color algorithms. In the surface open ocean, these measurements are challenging because of low ay values. Yet, existing global datasets demonstrate that ay could contribute between 30% to 50% of the total absorption budget in the 400–450 nm spectral range, thus making accurate measurement of ay essential to constrain these uncertainties. In this study, we present a simple way of determining ay using a commercially-available in-situ spectrophotometer operated in underway mode. The obtained ay values were validated using independent collocated measurements. The method is simple to implement, can provide measurements with very high spatio-temporal resolution, and has an accuracy of about 0.0004 m−1 and a precision of about 0.0025 m−1 when compared to independent data (at 440 nm). The only limitation for using this method at sea is that it relies on the availability of relatively large volumes of ultrapure water. Despite this limitation, the method can deliver the ay data needed for validating and assessing uncertainties in ocean-colour algorithms
Two Jovian-Mass Planets in Earthlike Orbits
We report the discovery of two new planets: a 1.94 M_Jup planet in a 1.8-year
orbit of HD 5319, and a 2.51 M_Jup planet in a 1.1-year orbit of HD 75898. The
measured eccentricities are 0.12 for HD 5319 b and 0.10 for HD 75898 b, and
Markov Chain Monte Carlo simulations based on derived orbital parameters
indicate that the radial velocities of both stars are consistent with circular
planet orbits. With low eccentricity and 1 < a < 2 AU, our new planets have
orbits similar to terrestrial planets in the solar system. The radial velocity
residuals of both stars have significant trends, likely arising from substellar
or low-mass stellar companions.Comment: 32 pages, including 11 figures and 5 tables. Accepted by Ap
Low Mass Companions for Five Solar-Type Stars from the Magellan Planet Search Program
We report low mass companions orbiting five Solar-type stars that have
emerged from the Magellan precision Doppler velocity survey, with minimum
(Msini) masses ranging from 1.2 to 25 Mjup. These nearby target stars range
from mildly metal-poor to metal-rich, and appear to have low chromospheric
activity. The companions to the brightest two of these stars have previously
been reported from the CORALIE survey. Four of these companions (HD 48265-b, HD
143361-b, HD 28185-b, HD 111232-b) are low-mass Jupiter-like planets in
eccentric intermediate and long-period orbits. On the other hand, the companion
to HD 43848 appears to be a long period brown dwarf in a very eccentric orbit.Comment: Accepted for publication on ApJ, 26 pages, 10 figures, 7 table
The Chromospheric Activity and Ages of M Dwarf Stars in Wide Binary Systems
We investigate the relationship between age and chromospheric activity for
139 M dwarf stars in wide binary systems with white dwarf companions. The age
of each system is determined from the cooling age of its white dwarf component.
The current limit for activity-age relations found for M dwarfs in open
clusters is 4 Gyr. Our unique approach to finding ages for M stars allows for
the exploration of this relationship at ages older than 4 Gyr. The general
trend of stars remaining active for a longer time at later spectral type is
confirmed. However, our larger sample and greater age range reveals additional
complexity in assigning age based on activity alone. We find that M dwarfs in
wide binaries older than 4 Gyr depart from the log-linear relation for clusters
and are found to have activity at magnitudes, colors and masses which are
brighter, bluer and more massive than predicted by the cluster relation. In
addition to our activity-age results, we present the measured radial velocities
and complete space motions for 161 white dwarf stars in wide binaries.Comment: 22 pages including 9 figures and 5 tables. Accepted for publication
in The Astronomical Journa
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