HST Star Counts at High Galactic Latitudes

We use star counts from 13 deep HST fields imaged with the {\it Wide Field Camera - 2} in order to constrain the amount of dark matter in the Galaxy that can be in the form of low-mass main sequence stars or white-dwarfs. Based on the number of red stars found in our fields, we exclude the possibility that more than 25\% of the massive dark halo is made up of M dwarfs or subdwarfs; fairly massive ($M \sim 0.2 M_\odot$) and yet extremely faint ($M_I$ \gtsima $14.5$) stellar candidates would have to be invoked in order to make the observed number of stars compatible with that predicted by a stellar dark halo. White dwarfs must also be intrinsically very faint ($M_I$ \gtsima $14$) in order to be consistent with the observed number of faint stars in the HST fields. We also rule out an increasing or flat stellar luminosity function beyond $M_V \sim 13$. The inferred slope of the disk luminosity function is intermediary between local, volume-limited surveys and ground-based photometric ones. Finally, the magnitude counts are well fitted by existing models for the structure of the Galaxy, with only small changes in the fiducial values of the model parameters. The colour distribution, however, is not well reproduced by the models. It is unclear at present if this reflects inadequacies of the available models or uncertainties in the colour-magnitude diagrams for low metallicity stars and in the photometric calibration.Comment: 18 pages plus 6 figure

Phase diagram of Landau-Zener phenomena in coupled one-dimensional Bose quantum fluids

We study stationary and dynamical properties of the many-body Landau-Zener dynamics of a Bose quantum fluid confined in two coupled one-dimensional chains, using a many-body generalization recently reported [Y.-A. Chen et al.], within the decoupling approximation and the one-level band scheme. The energy spectrum evidences the structure of the avoided level crossings as a function of the on-site inter particle interaction strength. On the dynamical side, a phase diagram of the transfer efficiency across ground-state and inverse sweeps is presented. A totally different scenario with respect to the original single-particle Landau-Zener scheme is found for ground-state sweeps, in which a breakdown of the adiabatic region emerges as the sweep rate decreases. On the contrary, the transfer efficiency across inverse sweeps reveals consistent results with the single-particle Landau-Zener predictions. In the strong coupling regime, we find that there is a critical value of the on-site interaction for which the transfer of particles starts to vanish independently of the sweep rate. Our results are in qualitative agreement with those of the experimental counterpart.Comment: 15 pages, submitted to Phys. Rev. A (new version

HST Observations of the Field Star Population in the Large Magellanic Cloud

We present $V$ and $I$ photometry, obtained with the Hubble Space Telescope, for $\sim 15,800$ stars in a field in the inner disk of the Large Magellanic Cloud. We confirm previous results indicating that an intense star formation event, probably corresponding to the formation of the LMC disk, occurred a few times $10^9$ years ago. We find a small but real difference between our field and one further out in the disk observed by Gallagher et al (1996): either star formation in the inner disk commenced slightly earlier, or the stars are slightly more metal rich. We also find evidence for a later burst, around 1 Gyr ago, which may correspond to the formation of the LMC bar. About 5% of the stars in our field are substantially older than either burst, and are probably members of an old disk or halo population with age $\sim 12$ Gyr.Comment: 18 pages, 7 figures: only 3 available ellectronically - complete copies by request from [email protected]

Effective nucleon-nucleon interactions and nuclear matter equation of state

Nuclear matter equations of state based on Skyrme, Myers-Swiatecki and Tondeur interactions are written as polynomials of the cubic root of density, with coefficients that are functions of the relative neutron excess $\delta$. In the extrapolation toward states far away from the standard one, it is shown that the asymmetry dependence of the critical point ($\rho_c, \delta_c$) depends on the model used. However, when the equations of state are fitted to the same standard state, the value of $\delta_c$ is almost the same in Skyrme and in Myers-Swiatecki interactions, while is much lower in Tondeur interaction. Furthermore, $\delta_c$ does not depend sensitively on the choice of the parameter $\gamma$ in Skyrme interaction.Comment: 15 pages, 9 figure

Nuclear matter properties and relativistic mean-field theory

Nuclear matter properties are calculated in the relativistic mean field theory by using a number of different parameter sets. The result shows that the volume energy $a_1$ and the symmetry energy $J$ are around the acceptable values 16MeV and 30MeV respectively; the incompressibility $K_0$ is unacceptably high in the linear model, but assumes reasonable value if nonlinear terms are included; the density symmetry $L$ is around $100MeV$ for most parameter sets, and the symmetry incompressibility $K_s$ has positive sign which is opposite to expectations based on the nonrelativistic model. In almost all parameter sets there exists a critical point $(\rho_c, \delta_c)$, where the minimum and the maximum of the equation of state are coincident and the incompressibility equals zero, falling into ranges 0.014fm$^{-3}<\rho_c<0.039$fm$^{-3}$ and $0.74<\delta_c\le0.95$; for a few parameter sets there is no critical point and the pure neutron matter is predicted to be bound. The maximum mass $M_{NS}$ of neutron stars is predicted in the range 2.45M$_\odot\leq M_{NS}\leq 3.26$M$_\odot$, the corresponding neutron star radius $R_{NS}$ is in the range 12.2km$\leq R_{NS}\leq 15.1$km.Comment: 10 pages, 5 figure

Magnetic Instability in Strongly Correlated Superconductors

Recently a new phenomenological Hamiltonian has been proposed to describe the superconducting cuprates. This so-called Gossamer Hamiltonian is an apt model for a superconductor with strong on-site Coulomb repulsion betweenthe electrons. It is shown that as one approaches half-filling the Gossamer superconductor, and hence the superconducting state, with strong repulsion is unstable toward an antiferromagnetic insulator an can undergo a quantum phase transition to such an insulator if one increases the on-site Coulomb repulsion

Globular Cluster Streams as Galactic High-Precision Scales - The Poster Child Palomar 5

Using the example of the tidal stream of the Milky Way globular cluster Palomar 5 (Pal 5), we demonstrate how observational data on streams can be efficiently reduced in dimensionality and modeled in a Bayesian framework. Our approach combines detection of stream overdensities by a Difference-of-Gaussians process with fast streakline models, a continuous likelihood function built from these models, and inference with MCMC. By generating $\approx10^7$ model streams, we show that the geometry of the Pal 5 debris yields powerful constraints on the solar position and motion, the Milky Way and Pal 5 itself. All 10 model parameters were allowed to vary over large ranges without additional prior information. Using only SDSS data and a few radial velocities from the literature, we find that the distance of the Sun from the Galactic Center is $8.30\pm0.25$ kpc, and the transverse velocity is $253\pm16$ km/s. Both estimates are in excellent agreement with independent measurements of these quantities. Assuming a standard disk and bulge model, we determine the Galactic mass within Pal 5's apogalactic radius of 19 kpc to be $(2.1\pm0.4)\times10^{11}$ M$_\odot$. Moreover, we find the potential of the dark halo with a flattening of $q_z = 0.95^{+0.16}_{-0.12}$ to be essentially spherical within the radial range that is effectively probed by Pal 5. We also determine Pal 5's mass, distance and proper motion independently from other methods, which enables us to perform vital cross-checks. We conclude that with more observational data and by using additional prior information, the precision of this method can be significantly increased.Comment: 28 pages, 14 figures, submitted to ApJ (revised version), comments welcom