7 research outputs found
Radiative Transfer Effects in He I Emission Lines
We consider the effect of optical depth of the 2 ^{3}S level on the nebular
recombination spectrum of He I for a spherically symmetric nebula with no
systematic velocity gradients. These calculations, using many improvements in
atomic data, can be used in place of the earlier calculations of Robbins. We
give representative Case B line fluxes for UV, optical, and IR emission lines
over a range of physical conditions: T=5000-20000 K, n_{e}=1-10^{8} cm^{-3},
and tau_{3889}=0-100. A FORTRAN program for calculating emissivities for all
lines arising from quantum levels with n < 11 is also available from the
authors.
We present a special set of fitting formulae for the physical conditions
relevant to low metallicity extragalactic H II regions: T=12,000-20,000 K,
n_{e}=1-300 cm^{-3}, and tau_{3889} < 2.0. For this range of physical
conditions, the Case B line fluxes of the bright optical lines 4471 A, 5876 A,
and 6678 A, are changed less than 1%, in agreement with previous studies.
However, the 7065 A corrections are much smaller than those calculated by
Izotov & Thuan based on the earlier calculations by Robbins. This means that
the 7065 A line is a better density diagnostic than previously thought. Two
corrections to the fitting functions calculated in our previous work are also
given.Comment: To be published in 10 April 2002 ApJ; relevant code available at
ftp://wisp.physics.wisc.edu/pub/benjamin/Heliu
Point Source C-Band Mueller Matrices for the Green Bank Telescope
C-Band Mueller matrices for the Green Bank Telescope are presented here which
enable on-sky Stokes parameters for point sources at the beam center to be
determined. Standard calibrators, 3C138 and 3C286, were observed using the
Spider program to steer the telescope across a broad range of Right Ascensions
on both sides of the zenith transit. For this analysis, only the observations
at the peak of the Spider pattern were used rather than the full sweep of the
runs. Therefore, the results presented here only apply to point sources at the
beam center. The Mueller matrices are shown to vary with frequency and with use
of the Hi-Cal or Lo-Cal noise diodes, due to the relative calibration gain
between the X and Y components of the feed.
However, the relative calibration gain can be determined from observations of
a source with known polarization. Correcting the data for the relative
calibration gain prior to data analysis allows for use of a frequency
independent Mueller matrix. This generic Mueller matrix is shown to provide
reliable C-Band polarization measurements.Comment: Acceptance for publication in The Astronomical Journa
Improving Predictions for Helium Emission Lines
We have combined the detailed He I recombination model of Smits with the
collisional transitions of Sawey & Berrington in order to produce new accurate
helium emissivities that include the effects of collisional excitation from
both the 2 (3)S and 2 (1) S levels. We present a grid of emissivities for a
range of temperature and densities along with analytical fits and error
estimates.
Fits accurate to within 1% are given for the emissivities of the brightest
lines over a restricted range for estimates of primordial helium abundance. We
characterize the analysis uncertainties associated with uncertainties in
temperature, density, fitting functions, and input atomic data. We estimate
that atomic data uncertainties alone may limit abundance estimates to an
accuracy of 1.5%; systematic errors may be greater than this. This analysis
uncertainty must be incorporated when attempting to make high accuracy
estimates of the helium abundance. For example, in recent determinations of the
primordial helium abundance, uncertainties in the input atomic data have been
neglected.Comment: ApJ, accepte
Point Source C-band Mueller Matrices for the Green Bank Telescope
C -band Mueller matrices for the Green Bank Telescope are presented here which enable on-sky Stokes parameters for point sources at the beam center to be determined. Standard calibrators, 3C 138 and 3C 286, were observed using the Spider program to steer the telescope across a broad range of R.A. on both sides of the zenith transit. For this analysis, only the observations at the peak of the Spider pattern were used rather than the full sweep of the runs. Therefore, the results presented here only apply to point sources at the beam center. The Mueller matrices are shown to vary with frequency and with use of the Hi-Cal or Lo-Cal noise diodes, due to the relative calibration gain between the X and Y components of the feed. However, the relative calibration gain can be determined from observations of a source with known polarization. Correcting the data for the relative calibration gain prior to data analysis allows for use of a frequency-independent Mueller matrix. This generic Mueller matrix is shown to provide reliable C -band polarization measurements
Effectiveness of Collaborative Ranking Tasks on Student Understanding of Key Astronomy Concepts
This research concerns the development and assessment of a program of introductory astronomy conceptual exercises called ranking tasks. These exercises were designed based on results from science education research, learning theory, and classroom pilot studies. The investigation involved a single-group repeated measures experiment across eight key introductory astronomy topics with 253 students at the University of Arizona. Student understanding of these astronomy topics was assessed before and after traditional instruction in an introductory astronomy course. Collaborative ranking tasks were introduced after traditional instruction on each topic, and student understanding was evaluated again. Results showed that average scores on multiple-choice tests across the eight astronomy topics increased from 32% before instruction, to 61% after traditional instruction, to 77% after the ranking-task exercises. A Likert scale attitude survey found that 83% of the students participating in the 16-week study thought that the ranking-task exercises helped their understanding of core astronomy concepts. Based on these results, we assert that supplementing traditional lecture-based instruction with collaborative ranking-task exercises can significantly improve student understanding of core astronomy topics