1,675 research outputs found
Effects of Rotationally-Induced Mixing in Compact Binary Systems with Low-Mass Secondaries and in Single Solar-Type Stars
Many population synthesis and stellar evolution studies have addressed the
evolution of close binary systems in which the primary is a compact remnant and
the secondary is filling its Roche lobe, thus triggering mass transfer.
Although tidal locking is expected in such systems, most studies have neglected
the rotationally-induced mixing that may occur. Here we study the possible
effects of mixing in the mass-losing stars for a range in secondary star masses
and metallicities. We find that tidal locking can induce rotational mixing
prior to contact and thus affect the evolution of the secondary star if the
effects of the Spruit-Tayler dynamo are included both for angular momentum and
chemical transport. Once contact is made, the effect of mass transfer tends to
be more rapid than the evolutionary time scale, so the effects of mixing are no
longer directly important, but the mass transfer strips matter to inner layers
that may have been affected by the mixing. These effects are enhanced for
secondaries of 1-1.2 Msun and for lower metallicities. We discuss the possible
implications for the paucity of carbon in the secondaries of the cataclysmic
variable SS Cyg and the black hole candidate XTE J1118+480 and for the
progenitor evolution of Type Ia supernovae. We also address the issue of the
origin of blue straggler stars in globular and open clusters. We find that for
models that include rotation consistent with that observed for some blue
straggler stars, evolution is chemically homogeneous. This leads to tracks in
the HR diagram that are brighter and bluer than the non-rotating main-sequence
turn-off point. Rotational mixing could thus be one of the factors that
contribute to the formation of blue stragglers.Comment: 46 pages, 18 figure
The Fight is on in Texas: A History of African American Churches of Christ in the Lone Star State, 1865-2000
This groundbreaking work draws upon congregational histories and other primary sources to chronicle for the first time the story of African American Churches of Christ in Texas. Emerging out of the nineteenth-century Stone-Campbell Restoration Movement, the African American churches inherited from their white mentors both a Biblicist theology and a feisty spirit. Their “fight” was against religious error and in support of the true church as they understood it. Out of that “fight” emerged a growing network of congregations that by the mid-twentieth century reached throughout Texas. This book lifts out of obscurity the African American Christians who joined Ramsey’s “fight …out West” and who made black Churches of Christ in Texas what they are today.https://digitalcommons.acu.edu/acu_library_books/1024/thumbnail.jp
Long-Term Optical Observations Of Two Lmxbs: Uw Crb (=Ms 1603+260) And V1408 Aql (=4U 1957+115)
We present new optical photometry of two low-mass X-ray binary stars, UW CrB (MS 1603+260) and V1408 Aql (4U 1957+115). UW CrB is an eclipsing binary and we refine its eclipse ephemeris and measure an upper limit to the rate of change of its orbital period, vertical bar P vertical bar < 4.2 x 10(-11) (unitless). The light curve of UW CrB shows optical counterparts of type I X-ray bursts. We tabulate the times, orbital phases, and fluences of 33 bursts and show that the optical flux in the bursts comes primarily from the accretion disk, not from the secondary star. The new observations are consistent with a model in which the accretion disk in UW CrB is asymmetric and precesses in the prograde direction with a period of similar to 5.5 days. The light curve of V1408 Aql has a low-amplitude modulation at its 9.33 hr orbital period. The modulation remained a nearly pure sine curve in the new data as it was in 1984 and 2008, but its mean amplitude was lower, 18% against 23% in the earlier data. A model in which the orbital modulation is caused by the varying aspect of the heated face of the secondary star continues to give an excellent fit to the light curve. We derive a much improved orbital ephemeris for the system.NSF 0958783Astronom
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Time Resolved Spectroscopy Of Cataclysmic Variables - SS Cygni
NSF AST76-23882, AST 79-06340McDonald Observator
Ion bombardment experiments suggesting an origin for organic particles in pre-cometary and cometary ices
During the Giotto and Vega encounters with Comet Halley both organic particles called CHON and energetic ions were detected. The acceleration of ions to hundreds of keV in the vicinity of the bow shock and near the nucleus may be a demonstration of a situation occurring in the early solar system (perhaps during the T Tauri stage) that led to the formation of organic particles only now released. Utilizing a Van de Graaff accelerator and a target chamber having cryogenic and mass spectrometer capabilities, frozen gases were bombarded at 10 K with 175 keV protons with the result that fluffy solid material remains after sublimation of the ice. Initial experiments were carried out with a gas mixture in parts of 170 carbon monoxide, 170 argon, 25 water, 20 nitrogen, and 15 methane formulated to reflect an interstellar composition in experiments involving the freezing out of the products of a plasma. The plasma experiments resulted in a varnish-like film residue that exhibited luminescence when excited with ultraviolet radiation, while the ion bombardment created particulate material that was not luminescent
Earthshine as an Illumination Source at the Moon
Earthshine is the dominant source of natural illumination on the surface of
the Moon during lunar night, and at locations within permanently shadowed
regions that never receive direct sunlight. As such, earthshine may enable the
exploration of areas of the Moon that are hidden from solar illumination. The
heat flux from earthshine may also influence the transport and cold trapping of
volatiles present in the very coldest areas. In this study, Earth's spectral
radiance at the Moon is examined using a suite of Earth spectral models created
using the Virtual Planetary Laboratory (VPL) three dimensional modeling
capability. At the Moon, the broadband, hemispherical irradiance from Earth
near 0 phase is approximately 0.15 watts per square meter, with comparable
contributions from solar reflectance and thermal emission. Over the simulation
timeframe, spanning two lunations, Earth's thermal irradiance changes less than
a few mW per square meter as a result of cloud variability and the
south-to-north motion of sub-observer position. In solar band, Earth's
diurnally averaged light curve at phase angles < 60 degrees is well fit using a
Henyey Greenstein integral phase function. At wavelengths > 0.7 microns, near
the well known vegetation "red edge", Earth's reflected solar radiance shows
significant diurnal modulation as a result of the longitudinal asymmetry in
projected landmass, as well as from the distribution of clouds. A simple
formulation with adjustable coefficients is presented for estimating Earth's
hemispherical irradiance at the Moon as a function of wavelength, phase angle
and sub-observer coordinates. It is demonstrated that earthshine is
sufficiently bright to serve as a natural illumination source for optical
measurements from the lunar surface.Comment: 27 pages, 15 figures, 1 tabl
Differential Effects of Lipid-lowering Drugs in Modulating Morphology of Cholesterol Particles.
Treatment of dyslipidemia patients with lipid-lowering drugs leads to a significant reduction in low-density lipoproteins (LDL) level and a low to moderate level of increase in high-density lipoprotein (HDL) cholesterol in plasma. However, a possible role of these drugs in altering morphology and distribution of cholesterol particles is poorly understood. Here, we describe the in vitro evaluation of lipid-lowering drug effects in modulating morphological features of cholesterol particles using the plaque array method in combination with imaging flow cytometry. Image analyses of the cholesterol particles indicated that lovastatin, simvastatin, ezetimibe, and atorvastatin induce the formation of both globular and linear strand-shaped particles, whereas niacin, fibrates, fluvastatin, and rosuvastatin induce the formation of only globular-shaped particles. Next, purified very low-density lipoprotein (VLDL) and LDL particles incubated with these drugs showed changes in the morphology and image texture of cholesterol particles subpopulations. Furthermore, screening of 50 serum samples revealed the presence of a higher level of linear shaped HDL cholesterol particles in subjects with dyslipidemia (mean of 18.3%) compared to the age-matched normal (mean of 11.1%) samples. We also observed considerable variations in lipid-lowering drug effects on reducing linear shaped LDL and HDL cholesterol particles formation in serum samples. These findings indicate that lipid-lowering drugs, in addition to their cell-mediated hypolipidemic effects, may directly modulate morphology of cholesterol particles by a non-enzymatic mechanism of action. The outcomes of these results have potential to inform diagnosis of atherosclerosis and predict optimal lipid-lowering therapy
Large-Scale Outflows in Edge-on Seyfert Galaxies. III. Kiloparsec-Scale Soft X-ray Emission
We present ROSAT PSPC and HRI images of eight galaxies selected from a
distance-limited sample of 22 edge-on Seyfert galaxies. Kiloparsec-scale soft
X-ray nebulae extend along the galaxy minor axes in three galaxies (NGC 2992,
NGC 4388 and NGC 5506). The extended X-ray emission has 0.2-2.4 keV X-ray
luminosities of . The X-ray nebulae are
roughly co-spatial with the large-scale radio emission, suggesting that both
are produced by large-scale galactic outflows. Assuming pressure balance
between the radio and X-ray plasmas, the X-ray filling factor is \gapprox
10^4 times larger than the radio plasma filling factor, suggesting that
large-scale outflows in Seyfert galaxies are predominantly winds of thermal
X-ray emitting gas. We favor an interpretation in which large-scale outflows
originate as AGN-driven jets that entrain and heat gas on kpc scales as they
make their way out of the galaxy. AGN- and starburst-driven winds are also
possible explanations in cases where the winds are oriented along the rotation
axis of the galaxy disk.Comment: 24 pages, 7 ps figures, AASTEX 4.0, accepted for ApJ April 1, 199
Extranuclear X-ray Emission in the Edge-on Seyfert Galaxy NGC 2992
We found several extranuclear (r >~ 3") X-ray nebulae within 40" (6.3 kpc at
32.5 Mpc) of the nucleus of the Seyfert galaxy NGC 2992. The net X-ray
luminosity from the extranuclear sources is ~2-3 E39 erg/s (0.3-8.0 keV). The
X-ray core itself (r <~ 1") is positioned at 9:45:41.95 -14:19:34.8 (J2000) and
has a remarkably simple power-law spectrum with photon index Gamma=1.86 and
Nh=7E21 /cm2. The near-nuclear (3" <~ r <~ 18") Chandra spectrum is best
modelled by three components: (1) a direct AGN component with Gamma fixed at
1.86, (2) cold Compton reflection of the AGN component, and (3) a 0.5 keV
low-abundance (Z < 0.03 Zsolar) "thermal plasma," with ~10% of the flux of
either of the first two components. The X-ray luminosity of the 3rd component
(the "soft excess") is ~1.4E40 erg/s, or ~5X that of all of the detected
extranuclear X-ray sources. We suggest that most (~75-80%) of the soft excess
emission originates from 1" < r < 3", which is not imaged in our observation
due to severe CCD pile-up. We also require the cold reflector to be positioned
at least 1" (158 pc) from the nucleus, since there is no reflection component
in the X-ray core spectrum. Much of the extranuclear X-ray emission is
coincident with radio structures (nuclear radio bubbles and large-scale radio
features), and its soft X-ray luminosity is generally consistent with
luminosities expected from a starburst-driven wind (with the starburst scaled
from L_FIR). However, the AGN in NGC 2992 seems equally likely to power the
galactic wind in that object. Furthermore, AGN photoionization and
photoexcitation processes could dominate the soft excess, especially the
\~75-80% which is not imaged by our observations.Comment: 34 pages AASTEX, 9 (low-res) PS figures, ApJ, in press. For
full-resolution postscript file, visit
http://www.pha.jhu.edu/~colbert/n2992_chandra.ps.g
Submillimeter Observations of the Ultraluminous BAL Quasar APM 08279+5255
With an inferred bolometric luminosity of 5\times10^{15}{\rm \lsun}, the
recently identified z=3.87, broad absorption line quasar APM 08279+5255 is
apparently the most luminous object currently known. As half of its prodigious
emission occurs in the infrared, APM 08279+5255 also represents the most
extreme example of an Ultraluminous Infrared Galaxy. Here, we present new
submillimeter observations of this phenomenal object; while indicating that a
vast quantity of dust is present, these data prove to be incompatible with
current models of emission mechanisms and reprocessing in ultraluminous
systems. The influence of gravitational lensing upon these models is considered
and we find that while the emission from the central continuum emitting region
may be significantly enhanced, lensing induced magnification cannot easily
reconcile the models with observations. We conclude that further modeling,
including the effects of any differential magnification is required to explain
the observed emission from APM 08279+5255.Comment: 12 Pages with Two figures. Accepted for publication in the
Astrophysical Journal Letter
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