Poemas

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The Kinematics of HH 34 from HST Images with a Nine-year Time Baseline

We study archival HST [S II] 6716+30 and Hα images of the HH 34 outflow, taken in 1998.71 and in 2007.83. The ~9 yr time baseline and the high angular resolution of these observations allow us to carry out a detailed proper-motion study. We determine the proper motions of the substructure of the HH 34S bow shock (from the [S II] and Hα frames) and of the aligned knots within ~30'' from the outflow source (only from the [S II] frames). We find that the present-day motions of the knots along the HH 34 jet are approximately ballistic, and that these motions directly imply the formation of a major mass concentration in ~900 yr, at a position similar to the one of the present-day HH 34S bow shock. In other words, we find that the knots along the HH 34 jet will merge to form a more massive structure, possibly resembling HH 34S

Galactic cold dark matter as a Bose-Einstein condensate of WISPs

We propose here the dark matter content of galaxies as a cold bosonic fluid composed of Weakly Interacting Slim Particles (WISPs), represented by spin-0 axion-like particles and spin-1 hidden bosons, thermalized in the Bose-Einstein condensation state and bounded by their self-gravitational potential. We analyze two zero-momentum configurations: the polar phases in which spin alignment of two neighbouring particles is anti-parallel and the ferromagnetic phases in which every particle spin is aligned in the same direction. Using the mean field approximation we derive the Gross-Pitaevskii equations for both cases, and, supposing the dark matter to be a polytropic fluid, we describe the particles density profile as Thomas-Fermi distributions characterized by the halo radii and in terms of the scattering lengths and mass of each particle. By comparing this model with data obtained from 42 spiral galaxies and 19 Low Surface Brightness (LSB) galaxies, we constrain the dark matter particle mass to the range $10^{-6}-10^{-4} eV$ and we find the lower bound for the scattering length to be of the order $10^{-14} fm$.Comment: 13 pages; 6 figures; references added; v.3: typo corrected in the abstract, published in JCA

Role of oxygen and fluorine in passivation of the GaSb(111) surface depending on its termination

The mechanism of the chemical bonding of oxygen and fluorine on the GaSb(111) surface depending on its termination is studied by the projector augmented-waves method within density functional theory. It is shown that on an unreconstructed (111) surface with a cation termination, the adsorption of fluorine leads to the removal of surface states from the band gap. The binding energy of fluorine on the cation-terminated surface in the most preferable Ga-T position is lower by ~0.4 eV than that of oxygen, but it is significantly lower (by ~0.8 eV) on the anion-terminated surface. We demonstrate that the mechanism of chemical bonding of electronegative adsorbates with the surface has an ionic–covalent character. The covalence of the O–Sb bond is higher than the F–Sb one, and it is higher than both O–Ga and F–Ga bonds. Trends in the change in the electronic structure of the GaSb(111) surface upon adsorption of fluorine and oxygen are discussed. It is found that an increase in the oxygen concentration on the Sb-terminated GaSb(111) surface promotes a decrease in the density of surface states in the band gap

Bounds on the mass and abundance of dark compact objects and black holes in dwarf spheroidal galaxy halos

We establish new dynamical constraints on the mass and abundance of compact objects in the halo of dwarf spheroidal galaxies. In order to preserve kinematically cold the second peak of the Ursa Minor dwarf spheroidal (UMi dSph) against gravitational scattering, we place upper limits on the density of compact objects as a function of their assumed mass. The mass of the dark matter constituents cannot be larger than 1000 solar masses at a halo density in UMi's core of 0.35 solar masses/pc^3. This constraint rules out a scenario in which dark halo cores are formed by two-body relaxation processes. Our bounds on the fraction of dark matter in compact objects with masses >3000 solar masses improve those based on dynamical arguments in the Galactic halo. In particular, objects with masses $\sim 10^{5}$ solar masses can comprise no more than a halo mass fraction $\sim 0.01$. Better determinations of the velocity dispersion of old overdense regions in dSphs may result in more stringent constraints on the mass of halo objects. For illustration, if the preliminary value of 0.5 km/s for the secondary peak of UMi is confirmed, compact objects with masses above $\sim 100$ solar masses could be excluded from comprising all its dark matter halo.Comment: 6 pages, 2 figures, accepted for publication in ApJ Letter