2,647 research outputs found
Cosmological Constraints on Theories with Large Extra Dimensions
In theories with large extra dimensions, constraints from cosmology lead to
non-trivial lower bounds on the fundamental scale M_F, corresponding to upper
bounds on the radii of the compact extra dimensions. These constraints are
especially relevant to the case of two extra dimensions, since only if M_F is
10 TeV or less do deviations from the standard gravitational force law become
evident at distances accessible to planned sub-mm gravity experiments. By
examining the graviton decay contribution to the cosmic diffuse gamma
radiation, we derive, for the case of two extra dimensions, a conservative
bound M_F > 110 TeV, corresponding to r_2 < 5.1 times 10^-5 mm, well beyond the
reach of these experiments. We also consider the constraint coming from
graviton overclosure of the universe and derive an independent bound M_F > 6.5
h^(-1/2) TeV, or r_2 < .015 h mm.Comment: 10 pages, references adde
Study of Fe K-alpha lines in Non-magnetic Cataclysmic Variables using Chandra HEG data
Results from a study of Fe K-alpha emission lines for a sample of six
non-magnetic Cataclysmic Variables (CVs) using the high resolution X-ray data
from the Chandra High Energy Transmission Grating (HETG) are presented. Two of
the sources, SS Cyg and U Gem are observed in both quiescent and outburst
states whereas V603 Aql, V426 Oph, WX Hyi and SU UMa are observed only in
quiescence. The fluorescent Fe line is prominent in V603 Aql, V426 Oph and SS
Cyg during quiescence indicating the presence of a conspicuous reflection
component in these sources. The observed equivalent width of the fluorescent Fe
line is consistent with reflection from a white dwarf surface that subtends 2pi
solid angle at the X-ray source. During the outburst in SS Cyg, the fluorescent
line is red-shifted by about 2300 km/s. The Fe XXV triplet at 6.7 keV is found
to be dominant in all sources. The value of the G-ratio derived from the Fe XXV
triplet indicates that the plasma is in collisional ionization equilibrium
during the quiescent state. The Fe XXV line is significantly broadened in U Gem
and SS Cyg during the outbursts compared to quiescence, indicating the presence
of a high velocity material near the white dwarf during the outburst. The ratio
of Fe XXVI/XXV indicates a higher ionization temperature during quiescence than
in outburst in U Gem and SS Cyg.Comment: 24 pages, including 4 figures and 3 tables, To appear in The
Astrophysical Journal, 10 April 2006 issue, vol. 64
Structural basis for ligase-specific conjugation of linear ubiquitin chains by HOIP
Linear ubiquitin chains are important regulators of cellular signaling pathways that control innate immunity and inflammation through NF-κB activation and protection against TNFα-induced apoptosis(1-5). They are synthesized by HOIP, which belongs to the RBR (RING-between-RING) family of E3 ligases and is the catalytic component of LUBAC (linear ubiquitin chain assembly complex), a multi-subunit E3 ligase(6). RBR family members act as RING/HECT hybrids, employing RING1 to recognize ubiquitin-loaded E2 while a conserved cysteine in RING2 subsequently forms a thioester intermediate with the transferred or “donor” ubiquitin(7). Here we report the crystal structure of the catalytic core of HOIP in its apo form and in complex with ubiquitin. The C-terminal portion of HOIP adopts a novel fold that, together with a zinc finger, forms an ubiquitin-binding platform which orients the acceptor ubiquitin and positions its α-amino group for nucleophilic attack on the E3~ubiquitin thioester. The carboxy-terminal tail of a second ubiquitin molecule is located in close proximity to the catalytic cysteine providing a unique snapshot of the ubiquitin transfer complex containing both donor and acceptor ubiquitin. These interactions are required for activation of the NF-kB pathway in vivo and explain the determinants of linear ubiquitin chain specificity by LUBAC
Chemical evolution of some selected complex organic molecules in low-mass star-forming regions
The destiny of complex organic molecules (COMs) in star-forming regions is
interlinked with various evolutionary phases. Therefore, identifying these
species in diversified environments of identical star-forming regions would
help to comprehend their physical and chemical heritage. We identified multiple
COMs utilizing the Large Program `Astrochemical Surveys At IRAM' (ASAI) data,
dedicated to chemical surveys in Sun-like star-forming regions with the IRAM 30
m telescope. It was an unbiased survey in the millimetre regime, covering the
prestellar core, protostar, outflow region, and protoplanetary disk phase.
Here, we have reported some transitions of seven COMs, namely, methanol
(CH3OH), acetaldehyde (CH3CHO), methyl formate (CH3OCHO), ethanol (C2H5OH),
propynal (HCCCHO), dimethyl ether (CH3OCH3), and methyl cyanide (CH3CN) in some
sources L1544, B1-b, IRAS4A, and SVS13A. We found a trend among these species
from the derived abundances using the rotational diagram method and MCMC fit.
We have found that the abundances of all of the COMs, except for HCCCHO,
increase from the L1544 (prestellar core) and peaks at IRAS16293-2422 (class 0
phase). It is noticed that the abundance of these molecules correlate with the
luminosity of the sources. The obtained trend is also visible from the previous
interferometric observations and considering the beam dilution effect.Comment: 44 pages, 25 figures, and 12 tables. Accepted for the publication in
the Astrophysical Journa
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Characterization of Niobium Oxide Films Deposited by High Target Utilization Sputter Sources
High quality, refractory metal, oxide coatings are required in a variety of applications such as laser optics, micro-electronic insulating layers, nano-device structures, electro-optic multilayers, sensors and corrosion barriers. A common oxide deposition technique is reactive sputtering because the kinetic mechanism vaporizes almost any solid material in vacuum. Also, the sputtered molecules have higher energies than those generated from thermal evaporation, and so the condensates are smoother and denser than those from thermally-evaporated films. In the typical sputtering system, target erosion is a factor that drives machine availability. In some situations such as nano-layered capacitors, where the device's performance characteristics depends on thick layers, target life becomes a limiting factor on the maximizing device functionality. The keen interest to increase target utilization in sputtering has been addressed in a variety of ways such as target geometry, rotating magnets, and/or shaped magnet arrays. Also, a recent sputtering system has been developed that generates a high density plasma, directs the plasma beam towards the target in a uniform fashion, and erodes the target in a uniform fashion. The purpose of this paper is to characterize and compare niobia films deposited by two types of high target utilization sputtering sources, a rotating magnetron and a high density plasma source. The oxide of interest in this study is niobia because of its high refractive index. The quality of the niobia films were characterized spectroscopically in optical transmission, ellipsometrically, and chemical stoichiometry with X-ray photo-electron spectroscopy. The refractive index, extinction coefficients, Cauchy constants were derived from the ellipsometric modeling. The mechanical properties of coating density and stress are also determined
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