Death of Stellar Baryonic Dark Matter

The nature of the dark matter in the haloes of galaxies is one of the outstanding questions in astrophysics. All stellar candidates, until recently thought to be likely baryonic contributions to the Halo of our Galaxy, are shown to be ruled out. Faint stars and brown dwarfs are found to constitute only a few percent of the mass of the Galaxy. Stellar remnants, including white dwarfs and neutron stars, are shown to be very constrained as well. High energy gamma-rays observed in HEGRA data place the strongest constraints, $\Omega_{WD} < 3 \times 10^{-3} h^{-1}$, where $h$ is the Hubble constant in units of 100 km s$^{-1}$ Mpc$^{-1}$. Hence one is left with several unanswered questions: 1) What are MACHOs seen in microlensing surveys? 2) What is the dark matter in our Galaxy? Indeed a nonbaryonic component in the Halo seems to be required.Comment: 6 pages ps fil

Massive Compact Halo Objects Viewed from a Cosmological Perspective: Contribution to the Baryonic Mass Density of the Universe

[Abridged] We estimate the contribution of Massive Compact Halo Objects (Machos) and their stellar progenitors to the mass density of the Universe. If the Machos that have been detected reside in the Halo of our Galaxy, then a simple extrapolation of the Galactic population (out to 50 kpc) of Machos to cosmic scales gives a cosmic density \rho_{Macho} = (1-5) \times 10^9 h \msun \Mpc^{-3}, which in terms of the critical density corresponds to $\Omega_{Macho}=(0.0036-0.017) h^{-1}$. Such a mass density is comparable to the baryon density implied by Big Bang Nucleosynthesis. If we take the central values of the estimates, then Machos dominate the baryonic content of the Universe today, with $\Omega_{Macho}/\Omega_{Baryon} \sim 0.7 h$. However, the cumulative uncertainties in the density determinations only require that $\Omega_{Macho}/\Omega_{Baryon} \geq 1/6 h f_{gal}$, where the fraction of galaxies that contain Machos $f_{gal} > 0.17$, and $h$ is the Hubble constant in units of 100 km s$^{-1}$ Mpc$^{-1}$. Our best estimate for $\Omega_{Macho}$ is hard to reconcile with the current best estimates of the baryonic content of the intergalactic medium indicated by measurements of the Lyman-$\alpha$ forest. We explore the addition constraints that arise if the Machos are white dwarfs as suggested by the present microlensing data. We discuss the challenges this scenario presents at both the local and cosmic scales, emphasizing in particular the constraints on the required mass budget and nucleosynthesis products (particularly carbon).Comment: 18 pages, LaTeX, uses AASTeX macros. In press, New Astronomy (submitted Jan. 20, 1998

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 $\Omega_{WD} h < 2 \times 10^{-4}$ from comparison with the low C and N abundances in the Ly$\alpha$ 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 $^4$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$M_\odot$). 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

A Theoretical Model for the Mbh−σM_{\rm bh}-\sigma Relation for Supermassive Black Holes in Galaxies

We construct a model for the formation of black holes within galactic bulges. The initial state is a slowly rotating isothermal sphere, characterized by effective transport speed \aeff and rotation rate $\Omega$. The black hole mass is determined when the centrifugal radius of the collapse flow exceeds the capture radius of the central black hole. This model reproduces the observed correlation between black hole masses and galactic velocity dispersions, \mbh \approx 10^8 M_\odot (\sigma/200 \kms)^4, where \sigma = \sqrt{2} \aeff. This model also predicts the ratio \mrat of black hole mass to host mass: \mrat $\approx$ 0.004 (\sigma/200 \kms).Comment: 9 pages, 2 figures, submitted to Astrophysical Journal Letter

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

On the evaluation and improvement of arabic wordnet coverage and usability

The final publication is available at Springer via http://dx.doi.org/10.1007/s10579-013-9237-0[EN] Built on the basis of the methods developed for Princeton WordNet and EuroWordNet, Arabic WordNet (AWN) has been an interesting project which combines WordNet structure compliance with Arabic particularities. In this paper, some AWN shortcomings related to coverage and usability are addressed. The use of AWN in question/answering (Q/A) helped us to deeply evaluate the resource from an experience-based perspective. Accordingly, an enrichment of AWN was built by semi-automatically extending its content. Indeed, existing approaches and/or resources developed for other languages were adapted and used for AWN. The experiments conducted in Arabic Q/A have shown an improvement of both AWN coverage as well as usability. Concerning coverage, a great amount of named entities extracted from YAGO were connected with corresponding AWN synsets. Also, a significant number of new verbs and nouns (including Broken Plural forms) were added. In terms of usability, thanks to the use of AWN, the performance for the AWN-based Q/A application registered an overall improvement with respect to the following three measures: accuracy (+9.27 % improvement), mean reciprocal rank (+3.6 improvement) and number of answered questions (+12.79 % improvement).The work presented in Sect. 2.2 was done in the framework of the bilateral Spain-Morocco AECID-PCI C/026728/09 research project. The research of the two first authors is done in the framework of the PROGRAMME D'URGENCE project (grant no. 03/2010). The research of the third author is done in the framework of WIQEI IRSES project (grant no. 269180) within the FP 7 Marie Curie People, DIANA-APPLICATIONS-Finding Hidden Knowledge in Texts: Applications (TIN2012-38603-C02-01) research project and VLC/CAMPUS Microcluster on Multimodal Interaction in Intelligent Systems. We would like to thank Manuel Montes-y-Gomez (INAOE-Puebla, Mexico) and Sandra Garcia-Blasco (Bitsnbrain, Spain) for their feedback on the work presented in Sect. 2.4. We would like finally to thank Violetta Cavalli-Sforza (Al Akhawayn University in Ifrane, Morocco) for having reviewed the linguistic level of the entire document.Abouenour, L.; Bouzoubaa, K.; Rosso, P. (2013). On the evaluation and improvement of arabic wordnet coverage and usability. Language Resources and Evaluation. 47(3):891-917. https://doi.org/10.1007/s10579-013-9237-0S891917473Abbès, R., Dichy, J., & Hassoun, M. (2004). The architecture of a standard Arabic lexical database: Some figures, ratios and categories from the DIINAR.1 source program. In Workshop on computational approaches to Arabic script-based languages, Coling 2004. Geneva, Switzerland.Abouenour, L., Bouzoubaa, K., & Rosso, P. (2009a). Structure-based evaluation of an Arabic semantic query expansion using the JIRS passage retrieval system. 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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.

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 $t\sim 10^{-5}$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 $\sim 50$ error. We find that the coalescence rate of the black hole MACHO binaries is $\sim 5 \times 10^{-2} \times 2^{\pm 1}$ 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

Elastic Wave Transmission at an Abrupt Junction in a Thin Plate, with Application to Heat Transport and Vibrations in Mesoscopic Systems

The transmission coefficient for vibrational waves crossing an abrupt junction between two thin elastic plates of different widths is calculated. These calculations are relevant to ballistic phonon thermal transport at low temperatures in mesoscopic systems and the Q for vibrations in mesoscopic oscillators. Complete results are calculated in a simple scalar model of the elastic waves, and results for long wavelength modes are calculated using the full elasticity theory calculation. We suggest that thin plate elasticty theory provide a useful and tractable approximation to the full three dimensional geometry.Comment: 35 pages, including 12 figure