20,381 research outputs found
Data user's note: Apollo 15 lunar photography
Brief descriptions are given of the Apollo 15 mission objectives, photographic equipment, and photographic coverage and quality. The lunar photographic tasks were: (1) ultraviolet photography of the earth and moon; (2) photography of the gegenschein from lunar orbit; (3) service module orbital photographic tasks; and (4) command module photographic tasks
The instability of stellar structures intermediate between white dwarfs and neutron stars
Instability of stellar structures intermediate between dwarfs and neutron star
An investigation of pulsar searching techniques with the Fast Folding Algorithm
Here we present an in-depth study of the behaviour of the Fast Folding
Algorithm, an alternative pulsar searching technique to the Fast Fourier
Transform. Weaknesses in the Fast Fourier Transform, including a susceptibility
to red noise, leave it insensitive to pulsars with long rotational periods (P >
1 s). This sensitivity gap has the potential to bias our understanding of the
period distribution of the pulsar population. The Fast Folding Algorithm, a
time-domain based pulsar searching technique, has the potential to overcome
some of these biases. Modern distributed-computing frameworks now allow for the
application of this algorithm to all-sky blind pulsar surveys for the first
time. However, many aspects of the behaviour of this search technique remain
poorly understood, including its responsiveness to variations in pulse shape
and the presence of red noise. Using a custom CPU-based implementation of the
Fast Folding Algorithm, ffancy, we have conducted an in-depth study into the
behaviour of the Fast Folding Algorithm in both an ideal, white noise regime as
well as a trial on observational data from the HTRU-S Low Latitude pulsar
survey, including a comparison to the behaviour of the Fast Fourier Transform.
We are able to both confirm and expand upon earlier studies that demonstrate
the ability of the Fast Folding Algorithm to outperform the Fast Fourier
Transform under ideal white noise conditions, and demonstrate a significant
improvement in sensitivity to long-period pulsars in real observational data
through the use of the Fast Folding Algorithm.Comment: 19 pages, 15 figures, 3 table
The Role of Subsurface Flows in Solar Surface Convection: Modeling the Spectrum of Supergranular and Larger Scale Flows
We model the solar horizontal velocity power spectrum at scales larger than
granulation using a two-component approximation to the mass continuity
equation. The model takes four times the density scale height as the integral
(driving) scale of the vertical motions at each depth. Scales larger than this
decay with height from the deeper layers. Those smaller are assumed to follow a
Kolomogorov turbulent cascade, with the total power in the vertical convective
motions matching that required to transport the solar luminosity in a mixing
length formulation. These model components are validated using large scale
radiative hydrodynamic simulations. We reach two primary conclusions: 1. The
model predicts significantly more power at low wavenumbers than is observed in
the solar photospheric horizontal velocity spectrum. 2. Ionization plays a
minor role in shaping the observed solar velocity spectrum by reducing
convective amplitudes in the regions of partial helium ionization. The excess
low wavenumber power is also seen in the fully nonlinear three-dimensional
radiative hydrodynamic simulations employing a realistic equation of state.
This adds to other recent evidence suggesting that the amplitudes of large
scale convective motions in the Sun are significantly lower than expected.
Employing the same feature tracking algorithm used with observational data on
the simulation output, we show that the observed low wavenumber power can be
reproduced in hydrodynamic models if the amplitudes of large scale modes in the
deep layers are artificially reduced. Since the large scale modes have reduced
amplitudes, modes on the scale of supergranulation and smaller remain important
to convective heat flux even in the deep layers, suggesting that small scale
convective correlations are maintained through the bulk of the solar convection
zone.Comment: 36 pages, 6 figure
Dates of birth and seasonal changes in well-being among 4904 subjects completing the seasonal pattern assessment questionnaire
Background: Abnormal distributions of birthdates, suggesting intrauterine aetiological factors, have been found in several psychiatric disorders, including one study of out-patients with Seasonal Affective Disorder (S.A.D.). We investigated birthdate distribution in relation to seasonal changes in well-being among a cohort who had completed the Seasonal Pattern Assessment Questionnaire (SPAQ). Method: A sample of 4904 subjects, aged 16 to 64, completed the SPAQ. 476 were cases of S.A.D. on the SPAQ and 580 were cases of sub-syndromal S.A.D. (S-S.A.D.). 92 were interview confirmed cases of S.A.D. Months and dates of birth were compared between S.A.D. cases and all others, between S.A.D. and S-S.A.D. cases combined and all others, and between interview confirmed cases and all others. Seasonality, as measured through seasonal fluctuations in well-being on the Global Seasonality Scores (GSS) of the SPAQ, was compared for all subjects by month and season of birth. Results: There was no evidence of an atypical pattern of birthdates for subjects fulfilling criteria for S.A.D., for the combined S.A.D. / S-S.A.D. group or for interview confirmed cases. There was also no relationship between seasonality on the GSS and month or season of birth. Limitations: Diagnoses of S.A.D. made by SPAQ criteria are likely to be overinclusive. Conclusion: Our findings differ from studies of patients with more severe mood disorders, including psychiatric out-patients with S.A.D. The lack of association between seasonality and birthdates in our study adds credence to the view that the aetiology of S.A.D. relates to separable factors predisposing to affective disorders and to seasonality
Formation of Kuiper Belt Binaries
The discovery that a substantial fraction of Kuiper Belt objects (KBOs)
exists in binaries with wide separations and roughly equal masses, has
motivated a variety of new theories explaining their formation. Goldreich et
al. (2002) proposed two formation scenarios: In the first, a transient binary
is formed, which becomes bound with the aid of dynamical friction from the sea
of small bodies (L^2s mechanism); in the second, a binary is formed by three
body gravitational deflection (L^3 mechanism). Here, we accurately calculate
the L^2s and L^3 formation rates for sub-Hill velocities. While the L^2s
formation rate is close to previous order of magnitude estimates, the L^3
formation rate is about a factor of 4 smaller. For sub-Hill KBO velocities (v
<< v_H) the ratio of the L^3 to the L^2s formation rate is 0.05 (v/v_H)
independent of the small bodies' velocity dispersion, their surface density or
their mutual collisions. For Super-Hill velocities (v >> v_H) the L^3 mechanism
dominates over the L^2s mechanism. Binary formation via the L^3 mechanism
competes with binary destruction by passing bodies. Given sufficient time, a
statistical equilibrium abundance of binaries forms. We show that the frequency
of long-lived transient binaries drops exponentially with the system's lifetime
and that such transient binaries are not important for binary formation via the
L^3 mechanism, contrary to Lee et al. (2007). For the L^2s mechanism we find
that the typical time, transient binaries must last, to form Kuiper Belt
binaries (KBBs) for a given strength of dynamical friction, D, increases only
logarithmically with D. Longevity of transient binaries only becomes important
for very weak dynamical friction (i.e. D \lesssim 0.002) and is most likely not
crucial for KBB formation.Comment: 20 pages, 3 figures, Accepted for publication in ApJ, correction of
minor typo
On the Snow Line in Dusty Protoplanetary Disks
The snow line, in Hayashi's (1981) model, is where the temperature of a black
body that absorbed direct sunlight and re-radiated as much as it absorbed,
would be 170~K. It is usually assumed that the cores of the giant planets,
e.g., Jupiter, form beyond the snow line. Since Hayashi, there have been a
series of more detailed models of the absorption by dust of the stellar
radiation, and of accretional heating, which alter the location of the snow
line. We have attempted a "self-consistent" model of a T Tauri disk in the
sense that we used dust properties and calculated surface temperatures that
matched observed disks. We then calculated the midplane temperature for those
disks, with no accretional heating or with small (<10^-8) accretion rates. Our
models bring the snow line in to the neighbourhood of 1 AU; not far enough to
explain the close planetary companions to other stars, but much closer than in
recent starting lines for orbit migration scenarios.Comment: 9 pages, 1 figure, to appear in ApJ,528,200
A Census Of Highly Symmetric Combinatorial Designs
As a consequence of the classification of the finite simple groups, it has
been possible in recent years to characterize Steiner t-designs, that is
t-(v,k,1) designs, mainly for t = 2, admitting groups of automorphisms with
sufficiently strong symmetry properties. However, despite the finite simple
group classification, for Steiner t-designs with t > 2 most of these
characterizations have remained longstanding challenging problems. Especially,
the determination of all flag-transitive Steiner t-designs with 2 < t < 7 is of
particular interest and has been open for about 40 years (cf. [11, p. 147] and
[12, p. 273], but presumably dating back to 1965). The present paper continues
the author's work [20, 21, 22] of classifying all flag-transitive Steiner
3-designs and 4-designs. We give a complete classification of all
flag-transitive Steiner 5-designs and prove furthermore that there are no
non-trivial flag-transitive Steiner 6-designs. Both results rely on the
classification of the finite 3-homogeneous permutation groups. Moreover, we
survey some of the most general results on highly symmetric Steiner t-designs.Comment: 26 pages; to appear in: "Journal of Algebraic Combinatorics
Modification of surface energy in nuclear multifragmentation
Within the statistical multifragmentation model we study modifications of the
surface and symmetry energy of primary fragments in the freeze-out volume. The
ALADIN experimental data on multifragmentation obtained in reactions induced by
high-energy projectiles with different neutron richness are analyzed. We have
extracted the isospin dependence of the surface energy coefficient at different
degrees of fragmentation. We conclude that the surface energy of hot fragments
produced in multifragmentation reactions differs from the values extracted for
isolated nuclei at low excitation. At high fragment multiplicity, it becomes
nearly independent of the neutron content of the fragments.Comment: 11 pages with 13 figure
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