Bose Einstein Condensation of incommensurate solid 4He

It is pointed out that simulation computation of energy performed so far cannot be used to decide if the ground state of solid 4He has the number of lattice sites equal to the number of atoms (commensurate state) or if it is different (incommensurate state). The best variational wave function, a shadow wave function, gives an incommensurate state but the equilibrium concentration of vacancies remains to be determined. In order to investigate the presence of a supersolid phase we have computed the one--body density matrix in solid 4He for the incommensurate state by means of the exact Shadow Path Integral Ground State projector method. We find a vacancy induced Bose Einstein condensation of about 0.23 atoms per vacancy at a pressure of 54 bar. This means that bulk solid 4He is supersolid at low enough temperature if the exact ground state is incommensurate.Comment: 5 pages, 2 figure

Up to date on the use of triptans for child and adolescent migraine: “the state of the art”

The introduction of triptans, in the early 1990s, has improved the therapy for acute migraine attack, offering a new quality of life for those patients who suffer from this disabling neurological disorder. Epidemiological data point out that about 10% of school–age children suffer from migraine, with a progressive increase in incidence and prevalence up to the threshold of adulthood. The increase in extent and prevalence of migraine from the years of growth stresses the importance of the application and adjustment of ad hoc therapeutic (either pharmacological or not) and diagnostic measures. Indeed, the peculiar neurobiological and psychological aspects which are typical of an “evolving” organism preclude the use, by simple “transposition” or “proportion”, of the knowledge acquired from adult–targeted studies. That requires the implementation of studies to analyze the specific responses of children and adolescents to the triptans. To date, the studies on such issues are absolutely insufficient to draw definitive conclusions and indications for the use of triptans for child and adolescent migraineurs

Secondary Star Formation in a Population III Object

We explore the possibility of subsequent star formation after a first star forms in a Pop III object, by focusing on the radiation hydrodynamic (RHD) feedback brought by ionizing photons as well as H2 dissociating photons. For the purpose, we perform three-dimensional RHD simulations, where the radiative transfer of ionizing photons and H2 dissociating photons from a first star is self-consistently coupled with hydrodynamics based on a smoothed particle hydrodynamics method. As a result, it is shown that density peaks above a threshold density can keep collapsing owing to the shielding of H2 dissociating radiation by an H2 shell formed ahead of a D-type ionization front. But, below the threshold density, an M-type ionization front accompanied by a shock propagates, and density peaks are radiation hydrodynamically evaporated by the shock. The threshold density is dependent on the distance from a source star, which is $\approx 10^2 cm^{-3}$ for the source distance of 30pc. Taking into consideration that the extent of a Pop III object is $\approx 100$pc and density peaks within it have the density of $10^{2-4}$cm$^{-3}$, it is concluded that the secondary star formation is allowed in the broad regions in a Pop III object.Comment: 4pages, 2 figures, submitted to Ap

Imaginary Time Correlations and the phaseless Auxiliary Field Quantum Monte Carlo

The phaseless Auxiliary Field Quantum Monte Carlo method provides a well established approximation scheme for accurate calculations of ground state energies of many-fermions systems. Here we apply the method to the calculation of imaginary time correlation functions. We give a detailed description of the technique and we test the quality of the results for static and dynamic properties against exact values for small systems.Comment: 13 pages, 6 figures; submitted to J. Chem. Phy

Gravitational collapse of magnetized clouds II. The role of Ohmic dissipation

We formulate the problem of magnetic field dissipation during the accretion phase of low-mass star formation, and we carry out the first step of an iterative solution procedure by assuming that the gas is in free-fall along radial field lines. This so-called ``kinematic approximation'' ignores the back reaction of the Lorentz force on the accretion flow. In quasi steady-state, and assuming the resistivity coefficient to be spatially uniform, the problem is analytically soluble in terms of Legendre's polynomials and confluent hypergeometric functions. The dissipation of the magnetic field occurs inside a region of radius inversely proportional to the mass of the central star (the ``Ohm radius''), where the magnetic field becomes asymptotically straight and uniform. In our solution, the magnetic flux problem of star formation is avoided because the magnetic flux dragged in the accreting protostar is always zero. Our results imply that the effective resistivity of the infalling gas must be higher by several orders of magnitude than the microscopic electric resistivity, to avoid conflict with measurements of paleomagnetism in meteorites and with the observed luminosity of regions of low-mass star formation.Comment: 20 pages, 4 figures, The Astrophysical Journal, in pres

Population III star formation in a Lambda CDM universe, II: Effects of a photodissociating background

We examine aspects of primordial star formation in the presence of a molecular hydrogen-dissociating ultraviolet background. We compare a set of AMR hydrodynamic cosmological simulations using a single cosmological realization but with a range of ultraviolet background strengths in the Lyman-Werner band. This allows us to study the effects of Lyman-Werner radiation on suppressing H2 cooling at low densities as well as the high-density evolution of the collapsing core in a self-consistent cosmological framework. We find that the addition of a photodissociating background results in a delay of the collapse of high density gas at the center of the most massive halo in the simulation and, as a result, an increase in the virial mass of this halo at the onset of baryon collapse. We find that, contrary to previous results, Population III star formation is not suppressed for J$_{21} \geq 0.1$, but occurs even with backgrounds as high as J$_{21} = 1$. We find that H2 cooling leads to collapse despite the depressed core molecular hydrogen fractions due to the elevated H2 cooling rates at $T=2-5 \times 10^3$ K. We observe a relationship between the strength of the photodissociating background and the rate of accretion onto the evolving protostellar cloud core, with higher LW background fluxes resulting in higher accretion rates. Finally, we find that the collapsing halo cores in our simulations do not fragment at densities below $n \sim 10^{10}$ cm$^{-3}$ regardless of the strength of the LW background, suggesting that Population III stars forming in halos with T$_{vir} \sim 10^4$ K may still form in isolation.Comment: 46 pages, 14 figures (9 color). Accepted by the Astrophysical Journal, some minor revision

Exact ground state Monte Carlo method for Bosons without importance sampling

Generally ``exact'' Quantum Monte Carlo computations for the ground state of many Bosons make use of importance sampling. The importance sampling is based, either on a guiding function or on an initial variational wave function. Here we investigate the need of importance sampling in the case of Path Integral Ground State (PIGS) Monte Carlo. PIGS is based on a discrete imaginary time evolution of an initial wave function with a non zero overlap with the ground state, that gives rise to a discrete path which is sampled via a Metropolis like algorithm. In principle the exact ground state is reached in the limit of an infinite imaginary time evolution, but actual computations are based on finite time evolutions and the question is whether such computations give unbiased exact results. We have studied bulk liquid and solid 4He with PIGS by considering as initial wave function a constant, i.e. the ground state of an ideal Bose gas. This implies that the evolution toward the ground state is driven only by the imaginary time propagator, i.e. there is no importance sampling. For both the phases we obtain results converging to those obtained by considering the best available variational wave function (the Shadow wave function) as initial wave function. Moreover we obtain the same results even by considering wave functions with the wrong correlations, for instance a wave function of a strongly localized Einstein crystal for the liquid phase. This convergence is true not only for diagonal properties such as the energy, the radial distribution function and the static structure factor, but also for off-diagonal ones, such as the one--body density matrix. From this analysis we conclude that zero temperature PIGS calculations can be as unbiased as those of finite temperature Path Integral Monte Carlo.Comment: 11 pages, 10 figure

Sub-structure formation in starless cores

Motivated by recent observational searches of sub-structure in starless molecular cloud cores, we investigate the evolution of density perturbations on scales smaller than the Jeans length embedded in contracting isothermal clouds, adopting the same formalism developed for the expanding Universe and the solar wind. We find that initially small amplitude, Jeans-stable perturbations (propagating as sound waves in the absence of a magnetic field), are amplified adiabatically during the contraction, approximately conserving the wave action density, until they either become nonlinear and steepen into shocks at a time $t_{\rm nl}$, or become gravitationally unstable when the Jeans length decreases below the scale of the perturbations at a time $t_{\rm gr}$. We evaluate analytically the time $t_{\rm nl}$ at which the perturbations enter the non-linear stage using a Burgers' equation approach, and we verify numerically that this time marks the beginning of the phase of rapid dissipation of the kinetic energy of the perturbations. We then show that for typical values of the rms Mach number in molecular cloud cores, $t_{\rm nl}$ is smaller than $t_{\rm gr}$, and therefore density perturbations likely dissipate before becoming gravitational unstable. Solenoidal modes grow at a faster rate than compressible modes, and may eventually promote fragmentation through the formation of vortical structures.Comment: 8 pages, 4 figure