1,057 research outputs found
Ultra-luminous X-ray sources and remnants of massive metal-poor stars
Massive metal-poor stars might form massive stellar black holes (BHs), with
mass 25<=mBH/Msun<=80, via direct collapse. We derive the number of massive BHs
(NBH) that are expected to form per galaxy through this mechanism. Such massive
BHs might power most of the observed ultra-luminous X-ray sources (ULXs). We
select a sample of 64 galaxies with X-ray coverage, measurements of the star
formation rate (SFR) and of the metallicity. We find that NBH correlates with
the number of observed ULXs per galaxy (NULX) in this sample. We discuss the
dependence of our model on the SFR and on the metallicity. The SFR is found to
be crucial, consistently with previous studies. The metallicity plays a role in
our model, since a lower metallicity enhances the formation of massive BHs.
Consistently with our model, the data indicate that there might be an
anticorrelation between NULX, normalized to the SFR, and the metallicity. A
larger and more homogeneous sample of metallicity measurements is required, in
order to confirm our results.Comment: 21 pages, 8 figures, accepted for publication in MNRA
Effects of dark matter annihilation on the first stars
We study the evolution of the first stars in the universe (Population III)
from the early pre-Main Sequence until the end of helium burning in the
presence of WIMP dark matter annihilation inside the stellar structure. The two
different mechanisms that can provide this energy source are the contemporary
contraction of baryons and dark matter, and the capture of WIMPs by scattering
off the gas with subsequent accumulation inside the star. We find that the
first mechanism can generate an equilibrium phase, previously known as a "dark
star", which is transient and present in the very early stages of pre-MS
evolution. The mechanism of scattering and capture acts later, and can support
the star virtually forever, depending on environmental characteristic of the
dark matter halo and on the specific WIMP model.Comment: Proceedings of the IAU Symposium 255, "Low-Metallicity Star
Formation: From the First Stars to Dwarf Galaxies"; L.K. Hunt, S. Madden and
R. Schneider ed
Growth and morphogenetic factors in bone induction: role of osteogenin and related bone morphogenetic proteins in craniofacial and periodontal bone repair.
Bone has considerable potential for repair as illustrated by the phenomenon of fracture healing. Repair and regeneration of bone recapitulate the sequential stages of development. It is well known that demineralized bone matrix has the potential to induce new bone formation locally at a heterotopic site of implantation. The sequential development of bone is reminiscent of endochondral bone differentiation during bone development. The collagenous matrix-induced bone formation is a prototype model for matrix-cell interactions in vivo. The developmental cascade includes migration of progenitor cells by chemotaxis, attachment of cells through fibronectin, proliferation of mesenchymal cells, and differentiation of bone. The bone inductive protein, osteogenin, was isolated by heparin affinity chromatography. Osteogenin initiates new bone formation and is promoted by other growth factors. Recently, the genes for osteogenin and related bone morphogenetic proteins were cloned and expressed. Recombinant osteogenin is osteogenic in vivo. The future prospects for bone induction are bright, and this is an exciting frontier with applications in oral and orthopaedic surgery.TS2016
Josephson effect in MgB_2 break junctions
We present the first observation of the DC and AC Josephson effect in MgB_2
break junctions. The junctions, obtained at 4.2 K in high-quality, high-density
polycrystalline metallic MgB_2 samples, show a non-hysteretic DC Josephson
effect. By irradiating the junctions with microwaves we observe clear Shapiro
steps spaced by the ideal value. The temperature dependence of the
DC Josephson current and the dependence of the height of the steps on the
microwave power are obtained. These results are a direct prove for the
existence of pairs with charge 2e in this new metallic superconductor and give
evidence of the superconductor-normal metal-superconductor weak link character
of these junctions.Comment: 4 RevTEX pages, 4 eps figure
Intergalactic medium heating by dark matter
We derive the evolution of the energy deposition in the intergalactic medium
(IGM) by dark matter (DM) decays/annihilations for both sterile neutrinos and
light dark matter (LDM) particles. At z > 200 sterile neutrinos transfer a
fraction f_abs~0.5 of their rest mass energy into the IGM; at lower redshifts
this fraction becomes <~ 0.3 depending on the particle mass. The LDM particles
can decay or annihilate. In both cases f_abs~0.4-0.9 at high (> 300) redshift,
dropping to ~0.1 below z=100. These results indicate that the impact of DM
decays/annihilations on the IGM thermal and ionization history is less
important than previously thought. We find that sterile neutrinos (LDM) decays
are able to increase the IGM temperature by z=5 at most up to 4K (100K), about
50-200 times less than predicted by estimates based on the assumption of
complete energy transfer to the gas.Comment: 13 pages, 7 figures; MNRAS, in press; matches the published versio
Performance optimization of detector electronics for millimeter laser ranging
The front-end electronic circuitry plays a fundamental role in determining the performance actually obtained from ultrafast and highly sensitive photodetectors. We deal here with electronic problems met working with microchannel plate photomultipliers (MCP-PMTs) and single photon avalanche diodes (SPADs) for detecting single optical photons and measuring their arrival time with picosecond resolution. The performance of available fast circuits is critically analyzed. Criteria for selecting the most suitable electronics are derived and solutions for exploiting the detector performance are presented and discussed
On a semiclassical formula for non-diagonal matrix elements
Let be a Schr\"odinger operator on the real
line, be a bounded observable depending only on the coordinate and
be a fixed integer. Suppose that an energy level intersects the potential
in exactly two turning points and lies below
. We consider the semiclassical limit
, and where is the th
eigen-energy of . An asymptotic formula for , the
non-diagonal matrix elements of in the eigenbasis of , has
been known in the theoretical physics for a long time. Here it is proved in a
mathematically rigorous manner.Comment: LaTeX2
Ultra-luminous X-ray sources and remnants of massive metal-poor stars
Massive metal-poor stars might form massive stellar black holes (BHs), with mass 25 ≤mBH/M⊙≤ 80, via direct collapse. We derive the number of massive BHs (NBH) that are expected to form per galaxy through this mechanism. Such massive BHs might power most of the observed ultra-luminous X-ray sources (ULXs). We select a sample of 64 galaxies with X-ray coverage, measurements of the star formation rate (SFR) and of the metallicity. We find that NBH correlates with the number of observed ULXs per galaxy (NULX) in this sample. We discuss the dependence of our model on the SFR and on the metallicity. The SFR is found to be crucial, consistently with previous studies. The metallicity plays a role in our model, since a lower metallicity enhances the formation of massive BHs. Consistently with our model, the data indicate that there might be an anticorrelation between NULX, normalized to the SFR, and the metallicity. A larger and more homogeneous sample of metallicity measurements is required, in order to confirm our result
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