1,692 research outputs found
Evaluation of LACIE phase 3 yield models, detailed data
There are no author-identified significant results in this report
Lunar highland rock types: Their implications for impact induced fractionation
The first step in a petrologic study must be a classification based on observed textures and mineralogy. Lunar rocks, may be classified into three major groups: (1) coarse-grained igneous rocks, (2) fine-grained igneous rocks and (3) breccias. Group 1 is interpreted as primitive lunar crustal rocks that display various degrees of crushing and/or annealing. Group 2 is interpreted as volcanic rocks. Group 3 is interpreted as resulting from impacts on the lunar surface and is subdivided on the basis of matrix textures into fragmental breccias, crystalline breccias that have been annealed, and crystalline breccias with igneous matrices. A synthesis of the relevant data concerning lunar highlands polymict breccias from the fields of petrography, chemistry, photogeology, and impact studies compels the prediction that the breccias should have homogeneous matrices from rock to rock within regions of the highlands of limited size where impact mixing has been efficient and extensive
Birth kicks as the origin of pulsar rotation
Radio pulsars are thought to born with spin periods of 0.02–0.5 s
and space velocities of 100–1,000 kms^(-1), and they are inferred to
have initial dipole magnetic fields of 10^(11)–10^(13) G. The
average space velocity of their progenitor stars is less than 15 kms^(-1),
which means that pulsars must receive a substantial ‘kick’ at birth.
Here we propose that the birth characteristics of pulsars have a
simple physical connection with each other. Magnetic fields
maintained by differential rotation between the core and envelope
of the progenitor would keep the whole star in a state of
approximately uniform rotation until 10 years before the explosion.
Such a slowly rotating core has 1,000 times less angular
momentum than required to explain the rotation of pulsars. The
specific physical process that ‘kicks’ the neutron star at birth has
not been identified, but unless its force is exerted exactly head-on
it will also cause the neutron star to rotate. We identify this
process as the origin of the spin of pulsars. Such kicks may cause a
correlation between the velocity and spin vectors of pulsars. We
predict that many neutron stars are born with periods longer than
2 s, and never become radio pulsars
Radio wave scattering by circumgalactic cool gas clumps
We consider the effects of radio wave scattering by cool ionized clumps (T ∼ 10^4 K) in circumgalactic media (CGMs). The existence of such clumps is inferred from intervening quasar absorption systems, but has long been something of a theoretical mystery. We consider the implications for compact radio sources of the ‘fog-like’ two-phase model of the CGM recently proposed by McCourt et al. In this model, the CGM consists of a diffuse coronal gas (T ≳ 10^6 K) in pressure equilibrium with numerous ≲1 pc scale cool clumps or ‘cloudlets’ formed by shattering in a cooling instability. The areal filling factor of the cloudlets is expected to exceed unity in ≳10^(11.5) M⊙ haloes, and the ensuing radio wave scattering is akin to that caused by turbulence in the Galactic warm ionized medium. If 30 per cent of cosmic baryons are in the CGM, we show that for a cool-gas volume fraction of fv ∼ 10^(−3), sources at z_s ∼ 1 suffer angular broadening by ∼15μ as and temporal broadening by ∼1 ms at λ = 30 cm, due to scattering by the clumps in intervening CGM. The former prediction will be difficult to test (the angular broadening will suppress Galactic scintillation only for <10μ Jy compact synchrotron sources). However the latter prediction, of temporal broadening of localized fast radio bursts, can constrain the size and mass fraction of cool ionized gas clumps as a function of halo mass and redshift, and thus provides a test of the model proposed by McCourt et al
Introduction to the Apollo collections: Part 2: Lunar breccias
Basic petrographic, chemical and age data for a representative suite of lunar breccias are presented for students and potential lunar sample investigators. Emphasis is on sample description and data presentation. Samples are listed, together with a classification scheme based on matrix texture and mineralogy and the nature and abundance of glass present both in the matrix and as clasts. A calculus of the classification scheme, describes the characteristic features of each of the breccia groups. The cratering process which describes the sequence of events immediately following an impact event is discussed, especially the thermal and material transport processes affecting the two major components of lunar breccias (clastic debris and fused material)
Radio wave scattering by circumgalactic cool gas clumps
We consider the effects of radio-wave scattering by cool ionized clumps
(K) in circumgalactic media (CGM). The existence of such clumps
are inferred from intervening quasar absorption systems, but have long been
something of a theoretical mystery. We consider the implications for compact
radio sources of the `fog-like' two-phase model of the circumgalactic medium
recently proposed by McCourt et al.(2018). In this model, the CGM consists of a
diffuse coronal gas (K) in pressure equilibrium with numerous
pc scale cool clumps or `cloudlets' formed by shattering in a
cooling instability. The areal filling factor of the cloudlets is expected to
exceed unity in haloes, and the ensuing radio-wave
scattering is akin to that caused by turbulence in the Galactic warm ionized
medium (WIM). If per-cent of cosmic baryons are in the CGM, we show that
for a cool-gas volume fraction of , sources at suffer angular broadening by as and temporal broadening
by ms at cm, due to scattering by the clumps in
intervening CGM. The former prediction will be difficult to test (the angular
broadening will suppress Galactic scintillation only for Jy compact
synchrotron sources). However the latter prediction, of temporal broadening of
localized fast radio bursts, can constrain the size and mass fraction of cool
ionized gas clumps as function of halo mass and redshift, and thus provides a
test of the model proposed by McCourt et al.(2018).Comment: In press MNRA
Preliminary data on boulders at station 6, Apollo 17 landing site
A cluster of boulders at Station 6 (Apollo 17 landing site) consists of breccias derived from the North Massif. Three preliminary lithologic units were established, on the basis of photogeologic interpretations; all lithologies identified photogeologically were sampled. Breccia clasts and matrices studied petrographically and chemically fall into two groups by modal mineralogy: (1) low-K Fra Mauro or high basalt composition, consisting of 50-60% modal feldspar, approximately 45% orthopyroxene and 1-7% Fe-Ti oxide; (2) clasts consisting of highland basalt composition, consisting of 70% feldspar, 30% orthopyroxene and olivine and a trace of Fe-Ti oxide
Relativistic Winds from Compact Gamma-ray Sources: I. Radiative Acceleration in the Klein-Nishina Regime
We consider the radiative acceleration to relativistic bulk velocities of a
cold, optically thin plasma which is exposed to an external source of
gamma-rays. The flow is driven by radiative momentum input to the gas, the
accelerating force being due to Compton scattering in the relativistic
Klein-Nishina limit. The bulk Lorentz factor of the plasma, Gamma, derived as a
function of distance from the radiating source, is compared with the
corresponding result in the Thomson limit. Depending on the geometry and
spectrum of the radiation field, we find that particles are accelerated to the
asymptotic Lorentz factor at infinity much more rapidly in the relativistic
regime; and the radiation drag is reduced as blueshifted, aberrated photons
experience a decreased relativistic cross section and scatter preferentially in
the forward direction. The random energy imparted to the plasma by gamma-rays
can be converted into bulk motion if the hot particles execute many Larmor
orbits before cooling. This `Compton afterburn' may be a supplementary source
of momentum if energetic leptons are injected by pair creation, but can be
neglected in the case of pure Klein-Nishina scattering. Compton drag by
side-scattered radiation is shown to be more important in limiting the bulk
Lorentz factor than the finite inertia of the accelerating medium. The
processes discussed here may be relevant to a variety of astrophysical
situations where luminous compact sources of hard X- and gamma-ray photons are
observed, including active galactic nuclei, galactic black hole candidates, and
gamma-ray bursts.Comment: LateX, 20 pages, 5 figures, revised version accepted for publication
in the Ap
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