4,799 research outputs found
Economic Comparison of the Farm, Nutrition, and Bioenergy Act (H.R. 2419) to Fairness in Farm and Food Policy Amendment to H.R. 2419
The House Agriculture Committee unanimously passed their farm bill, âFarm, Nutrition and Bioenergy Act of 2007â (H.R. 2419). An amendment, âFairness in Farm and Food Policy,â H.R. 2419 was announced by Ron Kind (D-WI). The purpose of this Briefing Report is to compare the likely economic impacts of H.R. 2419 and the FFFP Amendment on representative crop, dairy, and cattle farms.Agricultural and Food Policy,
The Rotation of Young Low-Mass Stars and Brown Dwarfs
We review the current state of our knowledge concerning the rotation and
angular momentum evolution of young stellar objects and brown dwarfs from a
primarily observational view point. Periods are typically accurate to 1% and
available for about 1700 stars and 30 brown dwarfs in young clusters.
Discussion of angular momentum evolution also requires knowledge of stellar
radii, which are poorly known for pre-main sequence stars. It is clear that
rotation rates at a given age depend strongly on mass; higher mass stars
(0.4-1.2 M) have longer periods than lower mass stars and brown dwarfs.
On the other hand, specific angular momentum is approximately independent of
mass for low mass pre-main sequence stars and young brown dwarfs. A spread of
about a factor of 30 is seen at any given mass and age. The evolution of
rotation of solar-like stars during the first 100 Myr is discussed. A broad,
bimodal distribution exists at the earliest observable phases (1 Myr) for
stars more massive than 0.4 M. The rapid rotators (50-60% of the
sample) evolve to the ZAMS with little or no angular momentum loss. The slow
rotators continue to lose substantial amounts of angular momentum for up to 5
Myr, creating the even broader bimodal distribution characteristic of 30-120
Myr old clusters. Accretion disk signatures are more prevalent among slowly
rotating PMS stars, indicating a connection between accretion and rotation.
Disks appear to influence rotation for, at most, 5 Myr, and considerably
less than that for the majority of stars. If the dense clusters studied so far
are an accurate guide, then the typical solar-like star may have only 1
Myr for this task. It appears that both disk interactions and stellar winds are
less efficient at braking these objects.Comment: Review chapter for Protostars and Planets V. 15 page and 8 figure
Indirect observation of unobservable interstellar molecules
It is suggested that the abundances of neutral non-polar interstellar molecules unobservable by radio astronomy can be systematically determined by radio observation of the protonated ions. As an example, observed N2H(+) column densities are analyzed to infer molecular nitrogen abundances in dense interstellar clouds. The chemistries and expected densities of the protonated ions of O2, C2, CO2, C2H2 and CH4 are then discussed. Microwave transition frequencies fo HCO2(+) and C2H3(+) are estimated, and a preliminary astronomical search for HCO2(+) is described
Rotation in the Orion Nebula Cluster
Eighteen fields in the Orion Nebula Cluster (ONC) have been monitored for one
or more observing seasons from 1990-99 with a 0.6-m telescope at Wesleyan
University. Photometric data were obtained in Cousins I on 25-40 nights per
season. Results from the first 3 years of monitoring were analyzed by Choi &
Herbst (1996; CH). Here we provide an update based on 6 more years of
observation and the extensive optical and IR study of the ONC by Hillenbrand
(1997) and Hillenbrand et al. (1998). Rotation periods are now available for
134 ONC members. Of these, 67 were detected at multiple epochs with identical
periods by us and 15 more were confirmed by Stassun et al. (1999) in their
study of Ori OBIc/d. The bimodal period distribution for the ONC is confirmed,
but we also find a clear dependence of rotation period on mass. This can be
understood as an effect of deuterium burning, which temporarily slows the
contraction and thus spin-up of stars with M <0.25 solar masses and ages of ~1
My. Stars with M <0.25 solar masses have not had time to bridge the gap in the
period distribution at ~4 days. Excess H-K and I-K emission, as well as CaII
infrared triplet equivalent widths (Hillenbrand et al. 1998), show weak but
significant correlations with rotation period among stars with M >0.25 solar
masses. Our results provide new observational support for the importance of
disks in the early rotational evolution of low mass stars. [abridged]Comment: 18 pages of text, 17 figures, and 4 tables; accepted for publication
in The Astronomical Journa
The Mass Dependence of Stellar Rotation in the Orion Nebula Cluster
We have determined new rotation periods for 404 stars in the Orion Nebula
Cluster using the Wide Field Imager attached to the MPG/ESO 2.2 m telescope on
La Silla, Chile. Mass estimates are available for 335 of these and most have M
< 0.3 M_sun. We confirm the existence of a bimodal period distribution for the
higher mass stars in our sample and show that the median rotation rate
decreases with increasing mass for stars in the range 0.1 < M <0.4 M_sun. While
the spread in angular momentum (J) at any given mass is more than a factor of
10, the majority of lower mass stars in the ONC rotate at rates approaching 30%
of their critical break-up velocity, as opposed to 5-10% for solar-like stars.
This is a consequence of both a small increase in observed specific angular
momentum (j=J/M) and a larger decrease in the critical value of j with
decreasing mass. Perhaps the most striking fact, however, is that j varies by
so little - less than a factor of two - over the interval 0.1-1.0 M_sun. The
distribution of rotation rates with mass in the ONC (age ~ 1 My) is similar in
nature to what is found in the Pleiades (age ~ 100 My). These observations
provide a significant new guide and test for models of stellar angular momentum
evolution during the proto-stellar and pre-main sequence phases.Comment: 11 pages, 3 figure
Relativistic Coulomb Problem: Analytic Upper Bounds on Energy Levels
The spinless relativistic Coulomb problem is the bound-state problem for the
spinless Salpeter equation (a standard approximation to the Bethe--Salpeter
formalism as well as the most simple generalization of the nonrelativistic
Schr\"odinger formalism towards incorporation of relativistic effects) with the
Coulomb interaction potential (the static limit of the exchange of some
massless bosons, as present in unbroken gauge theories). The nonlocal nature of
the Hamiltonian encountered here, however, renders extremely difficult to
obtain rigorous analytic statements on the corresponding solutions. In view of
this rather unsatisfactory state of affairs, we derive (sets of) analytic upper
bounds on the involved energy eigenvalues.Comment: 12 pages, LaTe
A Variational Approach to the Spinless Relativistic Coulomb Problem
By application of a straightforward variational procedure we derive a simple,
analytic upper bound on the ground-state energy eigenvalue of a
semirelativistic Hamiltonian for (one or two) spinless particles which
experience some Coulomb-type interaction.Comment: 7 pages, HEPHY-PUB 606/9
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