2,530 research outputs found
Limitation of the Press-Schechter Formalism
The Press-Schechter(PS) formalism for the mass function of the collapsed
objects are reanalyzed. The factor of two in the Press-Schechter formalism is
argued to be correct in the sharp -space filter even when we use the another
approach proposed by Jedamzik(1994) in the cloud-in-cloud problem, which is
different from the previous approach by Peacock & Heavens(1990) and Bond et
al.(1991). The spatial correlation of the density fluctuations, however, had
been neglected in the cloud-in-cloud problem. The effects of this spatial
correlation is analyzed by using the Jedamzik formalism and it is found that
this effect alter the PS mass function especially on larger mass scales.
Furthermore the exact formula of deriving mass function is shown. We also find
that the probability of the overlap of the collapsed objects can be neglected
on very small mass scales while it might not be neglected on other mass scales.Comment: 23 pages, uuencoded compressed Postscrip
Statistical Study of the Reconnection Rate in Solar Flares Observed with YOHKOH/SXT
We report a statistical study of flares observed with the Soft X-ray
Telescope (SXT) onboard Yohkoh in the year of 2000. We measure physical
parameters of 77 flares, such as the temporal scale, the size, and the magnetic
flux density and find that the sizes of flares tend to be distributed more
broadly as the GOES class becomes weaker and that there is a lower limit of
magnetic flux density that depends on the GOES class. We also examine the
relationship between these parameters and find weak correlation between
temporal and spatial scales of flares. We estimate reconnection inflow
velocity, coronal Alfven velocity, and reconnection rate using above observed
values. The inflow velocities are distributed from a few km/s to several tens
km/s and the Alfven velocities in the corona are in the range from 10^3 to 10^4
km/s. Hence the reconnection rate is 10^-3 - 10^-2. We find that the
reconnection rate in a flare tends to decrease as the GOES class of the flare
increases. This value is within one order of magnitude from the theoretical
maximum value predicted by the Petschek model, although the dependence of the
reconnection rate on the magnetic Reynolds number tends to be stronger than
that in the Petschek model.Comment: 21 pages, 8 figures, accepted for publication in Ap
Baryonically Closed Galaxy Groups
Elliptical galaxies and their groups having the largest L_x/L_B lie close to
the locus in the L_x,L_B diagram expected for closed systems with baryon
fractions equal to the cosmic mean value, f_b = 0.16. The estimated baryon
fractions for several of these galaxies/groups are also close to 0.16 when the
gas density is extrapolated to the virial radius. Evidently they are the least
massive baryonically closed systems. Gas retention in these groups implies that
non-gravitational heating cannot exceed about 1 keV per particle, consistent
with the heating required to produce the deviation of groups from the L_x - T
correlation for more massive clusters. Isolated galaxies/groups with X-ray
luminosities significantly lower than baryonically closed groups may have
undermassive dark halos, overactive central AGNs, or higher star formation
efficiencies. The virial mass and hot gas temperatures of nearly or completely
closed groups correlate with the group X-ray luminosities and the optical
luminosities of the group-centered elliptical galaxy, an expected consequence
of their merging history. The ratio of halo mass to the mass of the central
galaxy for X-ray luminous galaxy/groups is about 80.Comment: 7 pages; Accepted by ApJ Letter
Pickoff and spin-conversion quenchings of ortho-positronium in oxygen
The quenching processes of the thermalized ortho-positronium(o-Ps) on an
oxygen molecule have been studied by the positron annihilation age-momentum
correlation techinique(AMOC). The Doppler broadening spectrum of the 511 keV
gamma-rays from the 2gamma annihilation of o-Ps in O_2 has been measured as a
function of the o-Ps age. The rate of the quenching, consisting of the pickoff
and the spin-conversion, is estimated from the positron lifetime spectrum. The
ratio of the pickoff quenching rate to the spin-conversion rate is deduced from
the Doppler broadening of the 511 keV gamma-rays from the annihilation of the
o-Ps. The pickoff parameter ^1Z_eff, the effective number of the electrons per
molecule which contribute to the pickoff quenching, for O_2 is determined to be
0.6 +- 0.4. The cross-section for the elastic spin-conversion quenching is
determined to be (1.16 +- 0.01) * 10^{-19} cm^2.Comment: 4 pages with 5 eps figures, LaTeX2e(revtex4
Discovery of new mineral addibischoffite, Ca_2Al_6Al_6O_(20) ,IN A Ca-Al-Rich refractory inclusion from the Acfer 214 CH3 meteorite
Introduction: During a mineralogy investigation of the Acfer 214 CH3 carbonaceous chondrite, a new calcium aluminate mineral, named “addibischoffite”, Ca_2Al_6Al_6O_(20) with the P-1 aenigmatite structure, was identified in a Ca-Al-rich inclusion (CAI). Field-emission scanning electron microscope, electron back-scatter diffraction, electron microprobe and ion microprobe were used to characterize its chemical and oxygen-isotope compositions, structure, and associated phases. Synthetic CaAl_6O_(10) was reported but not fully characterized [e.g., 1]. Presented here is its first natural occurrence as a new refractory mineral in a primitive meteorite. The mineral has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2015-006). The name is in honor of Addi Bischoff, a cosmochemist at Münster University, Germany, for his many contributions to research on CAIs in carbonaceous chondrites, including CH chondrites
Forsterite-Bearing Type B CAI with a Relict Eringaite-Bearing Ultra-Refractory CAI
Forsterite-bearing Type B (FoB) Ca,Al-rich
inclusions (CAIs) are a rare type of coarse-grained igneous CAIs
found almost exclusively in CV3 chondrites [1–5]. Here we
describe the mineralogy, petrography, and oxygen-isotope
compositions of a FoB CAI Al-2 from Allende containing a relict
eringaite-bearing ultra-refractory (UR) inclusion. Eringaite is a
Sc-rich garnet [Ca_3(Sc,Y,Ti)_2Si_3O_(12)] that has been recently
identified in a cluster of UR inclusion fragments within an
amoeboid olivine aggregate in Vigarano [6]
A perspective from extinct radionuclides on a Young Stellar Object: The Sun and its accretion disk
Meteorites, which are remnants of solar system formation, provide a direct
glimpse into the dynamics and evolution of a young stellar object (YSO), namely
our Sun. Much of our knowledge about the astrophysical context of the birth of
the Sun, the chronology of planetary growth from micrometer-sized dust to
terrestrial planets, and the activity of the young Sun comes from the study of
extinct radionuclides such as 26Al (t1/2 = 0.717 Myr). Here we review how the
signatures of extinct radionuclides (short-lived isotopes that were present
when the solar system formed and that have now decayed below detection level)
in planetary materials influence the current paradigm of solar system
formation. Particular attention is given to tying meteorite measurements to
remote astronomical observations of YSOs and modeling efforts. Some extinct
radionuclides were inherited from the long-term chemical evolution of the
Galaxy, others were injected into the solar system by a nearby supernova, and
some were produced by particle irradiation from the T-Tauri Sun. The chronology
inferred from extinct radionuclides reveals that dust agglomeration to form
centimeter-sized particles in the inner part of the disk was very rapid (<50
kyr), planetesimal formation started early and spanned several million years,
planetary embryos (possibly like Mars) were formed in a few million years, and
terrestrial planets (like Earth) completed their growths several tens of
million years after the birth of the Sun.Comment: 49 pages, 9 figures, 1 table. Uncorrected preprin
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