61 research outputs found
Mode Conversion Processes in Magnetized Plasmas
Mode conversion processes in plasmas allow wave energy to transferred between two or more different wave modes, and are often invoked in theories of space and astrophysical phenomena. For instance, electrostatic Langmuir waves which are trapped in the solar wind plasma can be converted to electromagnetic radiation and produce radio bursts, which can propagate through the plasma and thus be observed remotely. In environments where mode conversion has been invoked there is often a significant ambient magnetic field. This modifies the dispersion relations of the wave modes and can result in additional wave modes. However, magnetization effects have been neglected in the analyses of certain mode conversion processes. This thesis presents a number of investigations into mode conversion processes as they occur in magnetized plasmas, focusing on the magnetization of the Langmuir mode
Deep Photometry of the Globular Cluster M5: Distance Estimates from White Dwarf and Main-Sequence Stars
We present deep VI photometry of stars in the globular cluster M5 (NGC 5904) based on images taken with the Hubble Space Telescope. The resulting color-magnitude diagram reaches below V ≈27 mag, revealing the upper 2–3 mag of the white dwarf cooling sequence and main-sequence stars 8 mag and more below the turnoff. We fit the main sequence to subdwarfs of known parallax to obtain a true distance modulus of (m ̶ M )0 = 14:45 ± 0:11 mag. A second distance estimate based on fitting the cluster white dwarf sequence to field white dwarfs with known parallax yielded (m _ M)0 = 14:67 ± 0:18 mag. We discuss the nature of the difference between the two distance estimates and suggest approaches for reducing the uncertainty in white dwarf fitting estimates for future studies. We couple our distance estimates with extensive photometry of the cluster’s RR Lyrae variables to provide a calibration of the RR Lyrae absolute magnitude yielding MV (RR) = 0:42 ± 0:10 mag at [Fe/H] = ̶ 1:11 dex. We provide another luminosity calibration in the form of reddening-free Wasenheit functions. Comparison of our calibrations with predictions based on recent models combining stellar evolution and pulsation theories shows encouraging agreement, and the existing differences may provide useful feedback to the models
HST Photometry for the Halo Stars in the Leo Elliptical NGC 3377
We have used the ACS camera on HST to obtain (V,I) photometry for 57,000
red-giant stars in the halo of the Leo elliptical NGC 3377. We use this sample
of stars to derive the metallicity distribution function (MDF) for its halo
field stars, and comment on its chemical evolution history compared with both
larger and smaller E galaxies. Our ACS/WFC field spans a radial range extending
from 4 to 18 kpc projected distance from the center of NGC 3377 and thus covers
a significant portion of this galaxy's halo. We find that the MDF is broad,
reaching a peak at [m/H] ~ -0.6. It may, in addition, have relatively few
stars more metal-rich than [m/H] = -0.3$, although interpretation of the
high-metallicity end of the MDF is limited by photometric completeness that
affects the detection of the reddest, most metal-rich stars. NGC 3377 appears
to have an enrichment history intermediate between those of normal dwarf
ellipticals and the much larger giants. As yet, we find no clear evidence that
the halo of NGC 3377 contains a significant population of ``young'' (< 3 Gy)
stars.Comment: 40 pages, 17 figure
The Determination Of Reddening From Intrinsic VR Colors Of RR Lyrae Stars
New R-band observations of 21 local field RR Lyrae variable stars are used to
explore the reliability of minimum light (V-R) colors as a tool for measuring
interstellar reddening. For each star, R-band intensity mean magnitudes and
light amplitudes are presented. Corresponding V-band light curves from the
literature are supplemented with the new photometry, and (V-R) colors at
minimum light are determined for a subset of these stars as well as for other
stars in the literature. Two different definitions of minimum light color are
examined, one which uses a Fourier decomposition to the V and R light curves to
find (V-R) at minimum V-band light, (V-R)_{min}^F, and the other which uses the
average color between the phase interval 0.5-0.8, (V-R)_{min}^{\phi(0.5-0.8)}.
From 31 stars with a wide range of metallicities and pulsation periods, the
mean dereddened RR Lyrae color at minimum light is (V-R)_{min,0}^F = 0.28 pm
0.02 mag and (V-R)_{min,0}^{\phi(0.5-0.8)} = 0.27 pm 0.02 mag. As was found by
Guldenschuh et al. (2005) using (V-I) colors, any dependence of the star's
minimum light color on metallicity or pulsation amplitude is too weak to be
formally detected. We find that the intrinsic (V-R) of Galactic bulge RR Lyrae
stars are similar to those found by their local counterparts and hence that
Bulge RR0 Lyrae stars do not have anomalous colors as compared to the local RR
Lyrae stars.Comment: accepted by A
The Variable Stars and Blue Horizontal Branch of the Metal-Rich Globular Cluster NGC 6441
We present time-series VI photometry of the metal-rich ([Fe/H] = -0.53)
globular cluster NGC 6441. Our color-magnitude diagram shows that the extended
blue horizontal branch seen in Hubble Space Telescope data exists in the
outermost reaches of the cluster. The red clump slopes nearly parallel to the
reddening vector. A component of this slope is due to differential reddening,
but part is intrinsic. The blue horizontal branch stars are more centrally
concentrated than the red clump stars. We have discovered about 50 new variable
stars near NGC 6441, among them eight or more RR Lyrae stars which are very
probably cluster members. Comprehensive period searches over the range 0.2-1.0
days yielded unusually long periods (0.5-0.9 days) for the fundamental
pulsators compared with field RR Lyrae of the same metallicity. Three similar
long-period RR Lyrae are known in other metal-rich globulars. With over ten
examples in hand, it seems that a distinct sub-class of RR Lyrae is emerging.
The observed properties of the horizontal branch stars are in reasonable
agreement with recent models which invoke deep mixing to enhance the
atmospheric helium abundance, while they conflict with models which assume high
initial helium abundance. The light curves of the c-type RR Lyrae seem to have
unusually long rise times and sharp minima. Reproducing these light curves in
stellar pulsation models may provide another means of constraining the physical
variables responsible for the anomalous blue horizontal branch extension and
sloped red clump observed in NGC 6441.Comment: 30 pages plus 6 EPS and 6 JPEG figures; uses AAS TeX. Accepted by the
Astronomical Journal. Minor changes include computing He abundance,
modifications to Figs 1 and 8, and expansion on idea that blue HB stars may
be produced in binarie
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