3,714 research outputs found
On the lithium abundance dispersion in late-type Pleiades stars
I present the results of a programme to monitor the strengths of the Li I
6708A, K I 7699A and chromospheric Halpha lines in a group of cool Pleiades
stars. Consistent instrumentation and analysis techniques are used to show that
there is no Li I variability on timescales of 1 year that could possibly
account for the apparent spread in Li abundances seen in Pleiades stars between
effective temperatures of 4800-5200K. Comparison with published data reveals
tentative evidence for variability on 10 year timescales, but at a very low
level. The lack of chromospheric activity variability above levels of 20 to 30
percent makes it difficult however, to rule out evenly distributed magnetic
activity regions causing a scatter in the Li I line strengths at a given
abundance. The similar star to star scatter of K I line strengths in these and
published data reinforces the conclusion that it is still unsafe to attribute
the Li I line strength dispersion to a large variation in Li depletion at a
given mass.Comment: Latex 7 pages, 3 postscript figures to appear in MNRA
Using rotation, magnetic activity and lithium to estimate the ages of low mass stars
The rotation rate, level of magnetic activity and surface lithium abundance
are age-dependent quantities in stars of about a solar mass and below. The
physical reasons for the evolution of these phenomena are qualitatively
understood, but accurate quantitative models remain dependent on empirical
calibration using the Sun and stars of known age, chiefly in clusters. In this
work I review the status of these "empirical age indicators", outlining the
astrophysics of their time dependence, describing the measurements, assessing
the precision (and accuracy) of age estimates when applied to individual stars,
and identifying their principle limitations in terms of the mass and age ranges
over which they are useful. Finally, I discuss the "lithium depletion boundary"
technique which, in contrast to the empirical methods, appears to provide
robust, almost model-independent ages that are both precise and accurate, but
which is only applicable to coeval groups of stars.Comment: 38 pages; contribution to the proceedings of the 23rd Evry Schatzman
School on Stellar Astrophysics, "The Ages of Stars", Roscoff 2013, EAS
Publications Series, eds. C. Charbonnel et al. This version has minor
corrections/additions to the bibliograph
Ages and Age Spreads in Young Stellar Clusters
I review progress towards understanding the time-scales of star and cluster
formation and of the absolute ages of young stars. I focus in particular on the
areas in which Francesco Palla made highly significant contributions -
interpretation of the Hertzsprung-Russell diagrams of young clusters and the
role of photospheric lithium as an age diagnostic.Comment: To appear in "Francesco's Legacy: Star Formation in Space and Time",
Memorie della SAIt, in press. Eds. R. Cesaroni, E. Corbelli and D. Galli. 5p
Analysis of GaAs and Si solar cell arrays for earth orbital and orbit transfer missions
Silicon and gallium arsenide arrays were studied and compared for low earth orbit (LE), geosynchronous orbit (GEO), and LEO to GEO electric propulsion orbit transfer missions. The sensitivities of total cost to parameters such as mission duration, array cost, cover glass thickness, and concentration ratio were determined along with cost tradeoffs between silicon and gallium arsenide arrays for selected mission classes. Results indicate that development of the technology for low cost, light weight concentrators should be increased and that cost reduction efforts for gallium arsenide cells be pursued
A lithium depletion boundary age of 21 Myr for the Beta Pictoris moving group
Optical spectroscopy is used to confirm membership for 8 low-mass candidates
in the young Beta Pic moving group (BPMG) via their radial velocities,
chromospheric activity and kinematic parallaxes. We searched for the presence
of the Li I 6708A resonance feature and combined the results with literature
measurements of other BPMG members to find the age-dependent lithium depletion
boundary (LDB) -- the luminosity at which Li remains unburned in a coeval
group. The LDB age of the BPMG is 21 +/- 4 Myr and insensitive to the choice of
low-mass evolutionary models. This age is more precise, likely to be more
accurate, and much older than that commonly assumed for the BPMG. As a result,
substellar and planetary companions of BPMG members will be more massive than
previously thought.Comment: Accepted for MNRAS Letter
Parametric analysis of hollow conductor parallel and coaxial transmission lines for high frequency space power distribution
A parametric analysis was performed of transmission cables for transmitting electrical power at high voltage (up to 1000 V) and high frequency (10 to 30 kHz) for high power (100 kW or more) space missions. Large diameter (5 to 30 mm) hollow conductors were considered in closely spaced coaxial configurations and in parallel lines. Formulas were derived to calculate inductance and resistance for these conductors. Curves of cable conductance, mass, inductance, capacitance, resistance, power loss, and temperature were plotted for various conductor diameters, conductor thickness, and alternating current frequencies. An example 5 mm diameter coaxial cable with 0.5 mm conductor thickness was calculated to transmit 100 kW at 1000 Vac, 50 m with a power loss of 1900 W, an inductance of 1.45 micron and a capacitance of 0.07 micron-F. The computer programs written for this analysis are listed in the appendix
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