2,621 research outputs found
CO and C_2 Absorption Toward W40 IRS 1a
The H II region W40 harbors a small group of young, hot stars behind roughly
9 magnitudes of visual extinction. We have detected gaseous carbon monoxide
(CO) and diatomic carbon (C_2) in absorption toward the star W40 IRS 1a. The
2-0 R0, R1, and R2 lines of 12CO at 2.3 micron were measured using the CSHELL
on the NASA IR Telescope Facility (with upper limits placed on R3, R4, and R5)
yielding an N_CO of (1.1 +/- 0.2) x 10^18 cm^-2. Excitation analysis indicates
T_kin > 7 K. The Phillips system of C_2 transitions near 8775 Ang. was measured
using the Kitt Peak 4-m telescope and echelle spectrometer. Radiative pumping
models indicate a total C_2 column density of (7.0 +/- 0.4) x 10^14 cm^-2, two
excitation temperatures (39 and 126 K), and a total gas density of n ~ 250
cm^-3. The CO ice band at 4.7 micron was not detected, placing an upper limit
on the CO depletion of delta < 1 %. We postulate that the sightline has
multiple translucent components and is associated with the W40 molecular cloud.
Our data for W40 IRS 1a, coupled with other sightlines, shows that the ratio of
CO/C_2 increases from diffuse through translucent environs. Finally, we show
that the hydrogen to dust ratio seems to remain constant from diffuse to dense
environments, while the CO to dust ratio apparently does not.Comment: To appear in The Astrophysical Journal 17 pages total, 5 figures Also
available at http://casa.colorado.edu/~shuping/research/w40/w40.htm
Galaxy Formation by Galactic Magnetic Fields
Galaxies exhibit a sequence of various morphological types, i.e., the Hubble
sequence, and they are basically composed of spheroidal components (elliptical
galaxies and bulges in spiral galaxies) and disks. It is known that spheroidal
components are found only in relatively massive galaxies with M=10^{10-12}
M_sun, and all stellar populations in them are very old, but there is no clear
explanation for these facts. Here we present a speculative scenario for the
origin of the Hubble sequence, in which magnetic fields ubiquitously seen in
galaxies have played a crucial role. We first start from a strange
observational fact that magnetic field strengths observed in spiral galaxies
sharply concentrate at a few microgauss, for a wide range of galaxy luminosity
and types. We then argue that this fact and the observed correlation between
star formation activity and magnetic field strength in spiral galaxies suggest
that spheroidal galaxies have formed by starbursts induced by strong magnetic
fields. Then we show that this idea naturally leads to the formation of
spheroidal systems only in massive and high-redshift objects in hierarchically
clustering universe, giving a simple explanation for various observations.Comment: 7 pages including 2 figures. Accepted by ApJ Letter
Diffusive propagation of UHECR and the propagation theorem
We present a detailed analytical study of the propagation of ultra high
energy (UHE) particles in extragalactic magnetic fields. The crucial parameter
which affects the diffuse spectrum is the separation between sources. In the
case of a uniform distribution of sources with a separation between them much
smaller than all characteristic propagation lengths, the diffuse spectrum of
UHE particles has a {\em universal} form, independent of the mode of
propagation. This statement has a status of theorem. The proof is obtained
using the particle number conservation during propagation, and also using the
kinetic equation for the propagation of UHE particles. This theorem can be also
proved with the help of the diffusion equation. In particular, it is shown
numerically, how the diffuse fluxes converge to this universal spectrum, when
the separation between sources diminishes. We study also the analytic solution
of the diffusion equation in weak and strong magnetic fields with energy losses
taken into account. In the case of strong magnetic fields and for a separation
between sources large enough, the GZK cutoff can practically disappear, as it
has been found early in numerical simulations. In practice, however, the source
luminosities required are too large for this possibility.Comment: 16 pages, 13 eps figures, discussion of the absence of the GZK
cut-off in strong magnetic field added, a misprint in figure 6 corrected,
version accepted for publication in Ap
Coherent acoustic vibration of metal nanoshells
Using time-resolved pump-probe spectroscopy we have performed the first
investigation of the vibrational modes of gold nanoshells. The fundamental
isotropic mode launched by a femtosecond pump pulse manifests itself in a
pronounced time-domain modulation of the differential transmission probed at
the frequency of nanoshell surface plasmon resonance. The modulation amplitude
is significantly stronger and the period is longer than in a gold nanoparticle
of the same overall size, in agreement with theoretical calculations. This
distinct acoustical signature of nanoshells provides a new and efficient method
for identifying these versatile nanostructures and for studying their
mechanical and structural properties.Comment: 5 pages, 3 figure
New Hampshire and Vicinity Continental Shelf: Sand and Gravel Resources
The continental shelf off New Hampshire (NH) has extensive marine-modified glacial deposits and associated shoals. These features are potential targets for sand and gravel resources for beach nourishment and other efforts to build coastal resiliency. The distribution of sand and gravel deposits was evaluated based on the synthesis of relatively recent high-resolution bathymetry, new surficial sediment and geoform maps, and an extensive data archive that includes over ~1280 km of seismic profiles, ~750 grain size analyses, and 23 vibracores. This work heavily utilizes the results of previous research on mineral resources on the NH shelf by Birch (1984) and others. Unfortunately, much of the archived data was collected before the Global Navigation Satellite System (GNSS) was used routinely for navigation on research vessels. Consequently, much of the critical data from the archives has a large uncertainty associated with the positioning. Furthermore, the seismics are of variable quality. Nevertheless, the data archives coupled with recent high-resolution bathymetry and surficial sediment mapping, provided the basis to develop an initial or first order evaluation of the sand and gravel resources and identify areas where follow-up field campaigns are warranted. This report focuses on four sites where sand and fine gravel deposits may be suitable for extraction for beach nourishment. The most promising sites are referred to as the Northern Sand Body (NSB) and the Southern Sand Deposits (SSD). Estimates of the volume of sand and fine gravel potentially available in the NSB and the SSD are on the order of 17.3 million m3 and 16.4 million m3, respectively. However, these values represent the total volume defined by subbottom seismics and include very fine sand and mud. Therefore, the volume of material that may be available for beach nourishment is likely considerably less. Both of these areas, as well as other potential sites identified, need high-resolution seismic surveys and vibracores to fully evaluate the potential sand and fine gravel resources
A Magnetized Local Supercluster and the Origin of the Highest Energy Cosmic Rays
A sufficiently magnetized Local Supercluster can explain the spectrum and
angular distribution of ultra-high energy cosmic rays. We show that the
spectrum of extragalactic cosmic rays with energies below eV may
be due to the diffusive propagation in the Local Supercluster with fields of
Gauss. Above eV, cosmic rays propagate
in an almost rectilinear way which is evidenced by the change in shape of the
spectrum at the highest energies. The fit to the spectrum requires that at
least one source be located relatively nearby at Mpc away from the
Milky Way. We discuss the origin of magnetic fields in the Local Supercluster
and the observable predictions of this model.Comment: 11 pages, 2 figures, submitted to PR
Mutations in DYNC2LI1 disrupt cilia function and cause short rib polydactyly syndrome.
The short rib polydactyly syndromes (SRPSs) are a heterogeneous group of autosomal recessive, perinatal lethal skeletal disorders characterized primarily by short, horizontal ribs, short limbs and polydactyly. Mutations in several genes affecting intraflagellar transport (IFT) cause SRPS but they do not account for all cases. Here we identify an additional SRPS gene and further unravel the functional basis for IFT. We perform whole-exome sequencing and identify mutations in a new disease-producing gene, cytoplasmic dynein-2 light intermediate chain 1, DYNC2LI1, segregating with disease in three families. Using primary fibroblasts, we show that DYNC2LI1 is essential for dynein-2 complex stability and that mutations in DYNC2LI1 result in variable length, including hyperelongated, cilia, Hedgehog pathway impairment and ciliary IFT accumulations. The findings in this study expand our understanding of SRPS locus heterogeneity and demonstrate the importance of DYNC2LI1 in dynein-2 complex stability, cilium function, Hedgehog regulation and skeletogenesis
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