3,212 research outputs found
Simulating a White Dwarf-dominated Galactic Halo
Observational evidence has suggested the possibility of a Galactic halo which
is dominated by white dwarfs (WDs). While debate continues concerning the
interpretation of this evidence, it is clear that an initial mass function
(IMF) biased heavily toward WD precursors (1 < m/Msol < 8), at least in the
early Universe, would be necessary in generating such a halo. Within the
framework of homogeneous, closed-box models of Galaxy formation, such biased
IMFs lead to an unavoidable overproduction of carbon and nitrogen relative to
oxygen (as measured against the abundance patterns in the oldest stars of the
Milky Way). Using a three-dimensional Tree N-body smoothed particle
hydrodynamics code, we study the dynamics and chemical evolution of a galaxy
with different IMFs. Both invariant and metallicity-dependent IMFs are
considered. Our variable IMF model invokes a WD-precursor-dominated IMF for
metallicities less than 5% solar (primarily the Galactic halo), and the
canonical Salpeter IMF otherwise (primarily the disk). Halo WD density
distributions and C,N/O abundance patterns are presented. While Galactic haloes
comprised of ~5% (by mass) of WDs are not supported by our simulations, mass
fractions of ~1-2% cannot be ruled out. This conclusion is consistent with the
present-day observational constraints.Comment: accepted for publication in MNRA
MACHOs, White Dwarfs, and the Age of the Universe
(Abridged Abstract) A favored interpretation of recent microlensing
measurements towards the Large Magellanic Cloud implies that a large fraction
(i.e. 10--50%) of the mass of the galactic halo is composed of white dwarfs. We
compare model white dwarf luminosity functions to the data from the
observational surveys in order to determine a lower bound on the age of any
substantial white dwarf halo population (and hence possibly on the age of the
Universe). We compare various theoretical white dwarf luminosity functions, in
which we vary hese three parameters, with the abovementioned survey results.
From this comparison, we conclude that if white dwarfs do indeed constitute
more than 10% of the local halo mass density, then the Universe must be at
least 10 Gyr old for our most extreme allowed values of the parameters. When we
use cooling curves that account for chemical fractionation and more likely
values of the IMF and the bolometric correction, we find tighter limits: a
white dwarf MACHO fraction of 10% (30%) requires a minimum age of 14 Gyr (15.5
Gyr). Our analysis also indicates that the halo white dwarfs almost certainly
have helium-dominated atmospheres.Comment: Final version accepted for publication, straight TeX formate, 6 figs,
22 page
Chemical Abundance Constraints on White Dwarfs as Halo Dark Matter
We examine the chemical abundance constraints on a population of white dwarfs
in the Halo of our Galaxy. We are motivated by microlensing evidence for
massive compact halo objects (Machos) in the Galactic Halo, but our work
constrains white dwarfs in the Halo regardless of what the Machos are. We focus
on the composition of the material that would be ejected as the white dwarfs
are formed; abundance patterns in the ejecta strongly constrain white dwarf
production scenarios. Using both analytical and numerical chemical evolution
models, we confirm that very strong constraints come from Galactic Pop II and
extragalactic carbon abundances. We also point out that depending on the
stellar model, significant nitrogen is produced rather than carbon. The
combined constraints from C and N give from
comparison with the low C and N abundances in the Ly forest. We note,
however, that these results are subject to uncertainties regarding the
nucleosynthesis of low-metallicity stars. We thus investigate additional
constraints from D and He, finding that these light elements can be kept
within observational limits only for \Omega_{WD} \la 0.003 and for a white
dwarf progenitor initial mass function sharply peaked at low mass (2).
Finally, we consider a Galactic wind, which is required to remove the ejecta
accompanying white dwarf production from the galaxy. We show that such a wind
can be driven by Type Ia supernovae arising from the white dwarfs themselves,
but these supernovae also lead to unacceptably large abundances of iron. We
conclude that abundance constraints exclude white dwarfs as Machos. (abridged)Comment: Written in AASTeX, 26 pages plus 4 ps figure
Laser cooling of a nanomechanical resonator mode to its quantum ground state
We show that it is possible to cool a nanomechanical resonator mode to its
ground state. The proposed technique is based on resonant laser excitation of a
phonon sideband of an embedded quantum dot. The strength of the sideband
coupling is determined directly by the difference between the electron-phonon
couplings of the initial and final states of the quantum dot optical
transition. Possible applications of the technique we describe include
generation of non-classical states of mechanical motion.Comment: 5 pages, 3 figures, revtex
Intellectual Property Management in Health and Agricultural Innovation: Executive Guide
Prepared by and for policy-makers, leaders of public sector research establishments, technology transfer professionals, licensing executives, and scientists, this online resource offers up-to-date information and strategies for utilizing the power of both intellectual property and the public domain. Emphasis is placed on advancing innovation in health and agriculture, though many of the principles outlined here are broadly applicable across technology fields. Eschewing ideological debates and general proclamations, the authors always keep their eye on the practical side of IP management. The site is based on a comprehensive Handbook and Executive Guide that provide substantive discussions and analysis of the opportunities awaiting anyone in the field who wants to put intellectual property to work. This multi-volume work contains 153 chapters on a full range of IP topics and over 50 case studies, composed by over 200 authors from North, South, East, and West. If you are a policymaker, a senior administrator, a technology transfer manager, or a scientist, we invite you to use the companion site guide available at http://www.iphandbook.org/index.html The site guide distills the key points of each IP topic covered by the Handbook into simple language and places it in the context of evolving best practices specific to your professional role within the overall picture of IP management
Intellectual Property Management in Health and Agricultural Innovation: A Handbook of Best Practices, Vol. 1
Prepared by and for policy-makers, leaders of public sector research establishments, technology transfer professionals, licensing executives, and scientists, this online resource offers up-to-date information and strategies for utilizing the power of both intellectual property and the public domain. Emphasis is placed on advancing innovation in health and agriculture, though many of the principles outlined here are broadly applicable across technology fields. Eschewing ideological debates and general proclamations, the authors always keep their eye on the practical side of IP management. The site is based on a comprehensive Handbook and Executive Guide that provide substantive discussions and analysis of the opportunities awaiting anyone in the field who wants to put intellectual property to work. This multi-volume work contains 153 chapters on a full range of IP topics and over 50 case studies, composed by over 200 authors from North, South, East, and West. If you are a policymaker, a senior administrator, a technology transfer manager, or a scientist, we invite you to use the companion site guide available at http://www.iphandbook.org/index.html The site guide distills the key points of each IP topic covered by the Handbook into simple language and places it in the context of evolving best practices specific to your professional role within the overall picture of IP management
Mirror Dark Matter and Core Density of Galaxies
We present a particle physics realization of a recent suggestion by Spergel
and Steinhardt that collisional but dissipationless dark matter may resolve the
core density problem in dark matter-dominated galaxies such as the dwarf
galaxies. The realization is the asymmetric mirror universe model introduced to
explain the neutrino puzzles and the microlensing anomaly. The mirror baryons
are the dark matter particles with the desired properties. The time scales are
right for resolution of the core density problem and formation of mirror stars
(MACHOs observed in microlensing experiments). The mass of the region
homogenized by Silk damping is between a dwarf and a large galaxy.Comment: 9 pages, LaTex. The present version shows that atomic scattering
inherent in the mirror model can solve the core density problem without the
need for an extra U(1) discussed in the original version; all conclusions are
unchanged. This version is accepted for publication in Phys. Rev.
Transmission X-ray Diffraction (XRD) Patterns Relevant to the MSL Chemin Amorphous Component: Sulfates And Silicates
The Mars Science Laboratory (MSL) CheMin instrument on the Curiosity rover is a transmission X-ray diffractometer (Co-Kalpha radiation source and a approx.5deg to approx.52deg 2theta range) where the analyzed powder samples are constrained to have discrete particle diameters <150 microns by a sieve. To date, diffraction patterns have been obtained for one basaltic soil (Rocknest (RN)) and four drill fines of coherent rock (John Klein (JK), Cumberland (CB), Windjana (WJ), and Confidence Hills (CH)). The CheMin instrument has detected and quantified the abundance of both primary igneous (e.g., feldspar, olivine, and pyroxene) and secondary (e.g., Ca-sulfates, hematite, akaganeite, and Fe-saponite) minerals. The diffraction patterns of all CheMin samples are also characterized by a broad diffraction band centered near 30deg 2theta and by increasing diffraction intensity (scattering continuum) from approx.15deg to approx.5deg, the 2theta minimum. Both the broad band and the scattering continuum are attributed to the presence of an XRD amorphous component. Estimates of amorphous component abundance, based on the XRD data itself and on mass-balance calculations using APXS data crystalline component chemistry derived from XRD data, martian meteorites, and/or stoichiometry [e.g., 6-9], range from approx.20 wt.% to approx.50 wt.% of bulk sample. The APXSbased calculations show that the amorphous component is rich in volatile elements (esp. SO3) and is not simply primary basaltic glass, which was used as a surrogate to model the broad band in the RN CheMin pattern. For RN, the entire volatile inventory (except minor anhydrite) is assigned to the amorphous component because no volatile-bearing crystalline phases were reported within detection limits [2]. For JK and CB, Fesaponite, basanite, and akaganeite are volatile-bearing crystalline components. Here we report transmission XRD patterns for sulfate and silicate phases relevant to interpretation of MSL-CheMin XRD amorphous components
Black Hole Binary Formation in the Expanding Universe --- Three Body Problem Approximation ---
We study black hole MACHO binary formation through three-body interactions in
the early universe at s. The probability distribution functions
of the eccentricity and the semimajor axis of binaries as well as of the
coalescence time are obtained assuming that the black holes are randomly formed
in space. We confirm that the previous order-of-magnitude estimate for the
binary parameters is valid within error. We find that the
coalescence rate of the black hole MACHO binaries is events/year/galaxy taking into consideration several possible
factors which may affect this estimate. This suggests that the event rate of
coalescing binary black holes will be at least several events per year within
15 Mpc. The first LIGO/VIRGO interferometers in 2001 will be able to verify
whether the MACHOs are black holes or not.Comment: Revtex, 25 pages, 10 figures, to appear in PR
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