6,246 research outputs found
Advances in Atomic Data for Neutron-Capture Elements
Neutron(n)-capture elements (atomic number Z>30), which can be produced in
planetary nebula (PN) progenitor stars via s-process nucleosynthesis, have been
detected in nearly 100 PNe. This demonstrates that nebular spectroscopy is a
potentially powerful tool for studying the production and chemical evolution of
trans-iron elements. However, significant challenges must be addressed before
this goal can be achieved. One of the most substantial hurdles is the lack of
atomic data for n-capture elements, particularly that needed to solve for their
ionization equilibrium (and hence to convert ionic abundances to elemental
abundances). To address this need, we have computed photoionization cross
sections and radiative and dielectronic recombination rate coefficients for the
first six ions of Se and Kr. The calculations were benchmarked against
experimental photoionization cross section measurements. In addition, we
computed charge transfer (CT) rate coefficients for ions of six n-capture
elements. These efforts will enable the accurate determination of nebular Se
and Kr abundances, allowing robust investigations of s-process enrichments in
PNe.Comment: To be published in IAU Symp. 283: Planetary Nebulae, an Eye to the
Future; 2 page
Physical properties of solar polar jets: A statistical study with Hinode XRT data
The target of this work is to investigate the physical nature of polar jets
in the solar corona and their possible contribution to coronal heating and
solar wind flow based on the analysis of X-ray images acquired by the Hinode
XRT telescope. We estimate the different forms of energy associated with many
of these small-scale eruptions, in particular the kinetic energy and enthalpy.
Two Hinode XRT campaign datasets focusing on the two polar coronal holes were
selected to analyze the physical properties of coronal jets; the analyzed data
were acquired using a series of three XRT filters. Typical kinematical
properties (e.g., length, thickness, lifetime, ejection rate, and velocity) of
18 jets are evaluated from the observed sequences, thus providing information
on their possible contribution to the fast solar wind flux escaping from
coronal holes. Electron temperatures and densities of polar-jet plasmas are
also estimated using ratios of the intensities observed in different filters.
We find that the largest amount of energy eventually provided to the corona
is thermal. The energy due to waves may also be significant, but its value is
comparatively uncertain. The kinetic energy is lower than thermal energy, while
other forms of energy are comparatively low. Lesser and fainter events seem to
be hotter, thus the total contribution by polar jets to the coronal heating
could have been underestimated so far. The kinetic energy flux is usually
around three times smaller than the enthalpy counterpart, implying that this
energy is converted into plasma heating more than in plasma acceleration. This
result suggests that the majority of polar jets are most likely not escaping
from the Sun and that only cooler ejections could possibly have enough kinetic
energy to contribute to the total solar wind flow.Comment: 21 pages, 10 figures, Submitted and accepted for publishing in
Astronomy and Astrophysics journa
Single parameter testing
Frequency response testing of ac and dc amplifiers, and single parameter testing of X-Y PLOTTE
Helping Provisionally Licensed Middle School Science Teachers
The New Science Teachers\u27 Support Network is a National Science Foundation-funded project that provides a multifaceted support system to provisionally licensed middle and high school science teachers.The teachers in this project were all hired to teach science, and had science degrees, but had little or no education coursework or background. Research is being conducted on the effectiveness of the support system we employed for these teachers, particularly on the factors that characterize the practice of new teachers, and on factors that lead to teacher success and teacher retention. In this paper, we describe the design of the study and the results from the one-year pilot study. We focus upon our observations and experiences with the middle school teachers in the group of participants; and, we close with preliminary recommendations for supporting provisionally licensed science teachers so they have the best chance of being successful and staying in the teaching profession
Science Leadership: Impact of the New Science Coordinators Academy
This article discusses the impact of the New Science Coordinators Academy (NSCA) on two cohorts of participants. The NSCA is one of four components of the Virginia Initiative for Science Teaching and Achievement (VISTA), a United States Department of Education (USED) science education reform grant. The NSCA is designed to support new school district science coordinators (with less than five years of experience) and to continue building the state science education infrastructure. Research in education leadership traditionally focuses on teacher leaders, principals, and district office personnel. Interestingly, research on district office personnel rarely distinguishes between the different roles of district personnel. This article seeks to inform the field by sharing the impact of an academy designed for new science coordinators on their learning, and to begin to understand their role and impact in their district. The five-day Academy engaged participants in a variety of experiences designed to facilitate the following: 1) build leadership skills; 2) build a common understanding and vision for hands-on science, inquiry, problem-based learning, and nature of science in the science classroom; 3) investigate data to improve student learning goals; 4) and, develop a science strategic plan. The data indicate that the NSCA was successful at meeting its goals to support the participants and to build a common language among these new coordinators. Initial data also support the variety of responsibilities of these participants and the positive impact of the Academy on their district work
Why do axons differ in caliber?
CNS axons differ in diameter (d) by nearly 100-fold (∼0.1-10 μm); therefore, they differ in cross-sectional area (d(2)) and volume by nearly 10,000-fold. If, as found for optic nerve, mitochondrial volume fraction is constant with axon diameter, energy capacity would rise with axon volume, also as d(2). We asked, given constraints on space and energy, what functional requirements set an axon's diameter? Surveying 16 fiber groups spanning nearly the full range of diameters in five species (guinea pig, rat, monkey, locust, octopus), we found the following: (1) thin axons are most numerous; (2) mean firing frequencies, estimated for nine of the identified axon classes, are low for thin fibers and high for thick ones, ranging from ∼1 to >100 Hz; (3) a tract's distribution of fiber diameters, whether narrow or broad, and whether symmetric or skewed, reflects heterogeneity of information rates conveyed by its individual fibers; and (4) mitochondrial volume/axon length rises ≥d(2). To explain the pressure toward thin diameters, we note an established law of diminishing returns: an axon, to double its information rate, must more than double its firing rate. Since diameter is apparently linear with firing rate, doubling information rate would more than quadruple an axon's volume and energy use. Thicker axons may be needed to encode features that cannot be efficiently decoded if their information is spread over several low-rate channels. Thus, information rate may be the main variable that sets axon caliber, with axons constrained to deliver information at the lowest acceptable rate
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