11,268 research outputs found
ULTRACAM photometry of the eclipsing cataclysmic variable OU Vir
We present high-speed, three-colour photometry of the faint eclipsing
cataclysmic variable OU Vir. For the first time in OU Vir, separate eclipses of
the white dwarf and bright spot have been observed. We use timings of these
eclipses to derive a purely photometric model of the system, obtaining a mass
ratio of q = 0.175 +/- 0.025, an inclination of i = 79.2 +/- 0.7 degrees and a
disc radius of Rd/a = 0.2315 +/- 0.0150. We separate the white dwarf eclipse
from the lightcurve and, by fitting a blackbody spectrum to its flux in each
passband, obtain a white dwarf temperature of T = 21700 +/- 1200 K and a
distance of D = 650 +/- 210 pc. Assuming that the primary obeys the Nauenberg
(1972) mass-radius relation for white dwarfs and allowing for temperature
effects, we also find a primary mass Mw/Msun = 0.90 +/- 0.19, primary radius
Rw/Rsun = 0.0097 +/- 0.0031 and orbital separation a/Rsun = 0.75 +/- 0.05.Comment: 8 pages LaTeX, 6 figures. Accepted by MNRAS; erratum added at end.
Mon.Not.Roy.Astron.Soc. 347 (2004) 1173, erratum in pres
Academic self-concept, gender and single-sex schooling
This paper assesses gender differences in academic self-concept for a cohort of children born in 1958 (the National Child Development Study). We address the question of whether attending single-sex or co-educational schools affected studentsâ perceptions of their own academic abilities (academic self-concept). Academic selfconcept was found to be highly gendered, even controlling for prior test scores. Boys had higher self-concepts in maths and science, and girls in English. Single-sex schooling reduced the gender gap in self-concept, while selective schooling was linked to lower academic self-concept overall
Dissipative Particle Dynamics with energy conservation
Dissipative particle dynamics (DPD) does not conserve energy and this
precludes its use in the study of thermal processes in complex fluids. We
present here a generalization of DPD that incorporates an internal energy and a
temperature variable for each particle. The dissipation induced by the
dissipative forces between particles is invested in raising the internal energy
of the particles. Thermal conduction occurs by means of (inverse) temperature
differences. The model can be viewed as a simplified solver of the fluctuating
hydrodynamic equations and opens up the possibility of studying thermal
processes in complex fluids with a mesoscopic simulation technique.Comment: 5 page
Constraints on the Lifetimes of Disks Resulting from Tidally Destroyed Rocky Planetary Bodies
Spitzer IRAC observations of 15 metal-polluted white dwarfs reveal infrared
excesses in the spectral energy distributions of HE 0110-5630, GD 61, and HE
1349-2305. All three of these stars have helium-dominated atmospheres, and
their infrared emissions are consistent with warm dust produced by the tidal
destruction of (minor) planetary bodies. This study brings the number of
metal-polluted, helium and hydrogen atmosphere white dwarfs surveyed with IRAC
to 53 and 38 respectively. It also nearly doubles the number of metal-polluted
helium-rich white dwarfs found to have closely orbiting dust by Spitzer. From
the increased statistics for both atmospheric types with circumstellar dust, we
derive a typical disk lifetime of log[t_{disk} (yr)] = 5.6+-1.1 (ranging from
3*10^4 - 5*10^6 yr). This assumes a relatively constant rate of accretion over
the timescale where dust persists, which is uncertain. We find that the
fraction of highly metal-polluted helium-rich white dwarfs that have an
infrared excess detected by Spitzer is only 23 per cent, compared to 48 per
cent for metal-polluted hydrogen-rich white dwarfs, and we conclude from this
difference that the typical lifetime of dusty disks is somewhat shorter than
the diffusion time scales of helium-rich white dwarf. We also find evidence for
higher time-averaged accretion rates onto helium-rich stars compared to the
instantaneous accretion rates onto hydrogen-rich stars; this is an indication
that our picture of evolved star-planetary system interactions is incomplete.
We discuss some speculative scenarios that can explain the observations.Comment: 20 pages, 9 figures, accepted to be published in Ap
Choosing the lesser of two evils, the better of two goods: Specifying the roles of ventromedial prefrontal cortex and dorsal anterior cingulate in object choice
The ventromedial prefrontal cortex (vmPFC) and dorsal anterior cingulate cortices (ACd) are considered important for reward-based decision making. However, work distinguishing their individual functional contributions has only begun. One aspect of decision making that has received little attention is that making the right choice often translates to making the better choice. Thus, response choice often occurs in situations where both options are desirable (e.g., choosing between mousse au chocolat or crème caramel cheesecake from a menu) or, alternatively, in situations where both options are undesirable. Moreover, response choice is easier when the reinforcements associated with the objects are far apart, rather than close together, in value. We used functional magnetic resonance imaging to delineate the functional roles of the vmPFC and ACd by investigating these two aspects of decision making: (1) decision form (i.e., choosing between two objects to gain the greater reward or the lesser punishment), and (2) between-object reinforcement distance (i.e., the difference in reinforcements associated with the two objects). Blood oxygen level-dependent (BOLD) responses within the ACd and vmPFC were both related to decision form but differentially. Whereas ACd showed greater responses when deciding between objects to gain the lesser punishment, vmPFC showed greater responses when deciding between objects to gain the greater reward. Moreover, vmPFC was sensitive to reinforcement expectations associated with both the chosen and the forgone choice. In contrast, BOLD responses within ACd, but not vmPFC, related to between-object reinforcement distance, increasing as the distance between the reinforcements of the two objects decreased. These data are interpreted with reference to models of ACd and vmPFC functioning
Accommodation of lattice mismatch in Ge_(x)Si_(1âx)/Si superlattices
We present evidence that the critical thickness for the appearance of misfit defects in a given material and heteroepitaxial structure is not simply a function of lattice mismatch. We report substantial differences in the relaxation of mismatch stress in Ge_(0.5)Si_(0.5)/Si superlattices grown at different temperatures on (100) Si substrates. Samples have been analyzed by xâray diffraction, channeled Rutherford backscattering, and transmission electron microscopy. While a superlattice grown at 365â°C demonstrates a high degree of elastic strain, with a dislocation density <10^5 cm^(â2) , structures grown at higher temperatures show increasing numbers of structural defects, with densities reaching 2Ă10^(10) cm^(â2) at a growth temperature of 530â°C. Our results suggest that it is possible to freeze a latticeâmismatched structure in a highly strained metastable state. Thus it is not surprising that experimentally observed critical thicknesses are rarely in agreement with those predicted by equilibrium theories
Dynamic sea surface topography, gravity and improved orbit accuracies from the direct evaluation of SEASAT altimeter data
A method for the simultaneous solution of dynamic ocean topography, gravity and orbits using satellite altimeter data is described. A GEM-T1 based gravitational model called PGS-3337 that incorporates Seasat altimetry, surface gravimetry and satellite tracking data has been determined complete to degree and order 50. The altimeter data is utilized as a dynamic observation of the satellite's height above the sea surface with a degree 10 model of dynamic topography being recovered simultaneously with the orbit parameters, gravity and tidal terms in this model. PGS-3337 has a geoid uncertainty of 60 cm root-mean-square (RMS) globally, with the uncertainty over the altimeter tracked ocean being in the 25 cm range. Doppler determined orbits for Seasat, show large improvements, with the sub-30 cm radial accuracies being achieved. When altimeter data is used in orbit determination, radial orbital accuracies of 20 cm are achieved. The RMS of fit to the altimeter data directly gives 30 cm fits for Seasat when using PGS-3337 and its geoid and dynamic topography model. This performance level is two to three times better than that achieved with earlier Goddard earth models (GEM) using the dynamic topography from long-term oceanographic averages. The recovered dynamic topography reveals the global long wavelength circulation of the oceans with a resolution of 1500 km. The power in the dynamic topography recovery is now found to be closer to that of oceanographic studies than for previous satellite solutions. This is attributed primarily to the improved modeling of the geoid which has occurred. Study of the altimeter residuals reveals regions where tidal models are poor and sea state effects are major limitations
Bostonia: The Boston University Alumni Magazine. Volume 29
Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs
Inactivation of Kell blood group antigens by 2-aminoethylisothiouronium bromide
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75007/1/j.1365-2141.1982.tb07295.x.pd
- âŚ