Hydromagnetic and gravitomagnetic crust-core coupling in a precessing neutron star

We consider two types of mechanical coupling between the crust and the core of a precessing neutron star. First, we find that a hydromagnetic (MHD) coupling between the crust and the core strongly modifies the star's precessional modes when $t_a\le\sim (T_s\times T_p)^{1/2}$; here $t_a$ is the Alfven crossing timescale, and $T_s$ and $T_p$ are the star's spin and precession periods, respectively. We argue that in a precessing pulsar PSR B1828-11 the restoring MHD stress prevents a free wobble of the crust relative to the non-precessing core. Instead, the crust and the proton-electron plasma in the core must precess in unison, and their combined ellipticity determines the period of precession. Link has recently shown that the neutron superfluid vortices in the core of PSR B1828-11 cannot be pinned to the plasma; he has also argued that this lack of pinning is expected if the proton Fermi liquid in the core is type-I superconductor. In this case, the neutron superfluid is dynamically decoupled from the precessing motion. The pulsar's precession decays due to the mutual friction between the neutron superfluid and the plasma in the core. The decay is expected to occur over tens to hundreds of precession periods and may be measurable over a human lifetime. Such a measurement would provide information about the strong n-p interaction in the neutron-star core. Second, we consider the effect of gravitomagnetic coupling between the neutron superfluid in the core and the rest of the star and show that this coupling changes the rate of precession by about 10%. The general formalism developed in this paper may be useful for other applications.Comment: 6 page

Long-Term evolution of Discs around Magnetic Stars

We investigate the evolution of a thin viscous disc surrounding magnetic star, including the spindown of the star by the magnetic torques it exerts on the disc. The transition from an accreting to a non-accreting state, and the change of the magnetic torque across the corotation radius are included in a generic way, the widths of the transition taken in the range suggested by numerical simulations. In addition to the standard accreting state, two more are found. An accreting state can develop into a 'dead' disc state, with inner edge well outside corotation. More often, a 'trapped' state develops, in which the inner disc edge stays close to corotation even at very low accretion rates. The long-term evolution of these two states is different. In the dead state the star spins down incompletely, retaining much of its initial spin. In the trapped state the star asymptotically can spin down to arbitarily low rates, its angular momentum transferred to the disc. We identify these outcomes with respectively the rapidly rotating and the very slowly rotating classes of Ap stars and magnetic white dwarfs.Comment: 14 pages, 8 figures. Accepted by MNRA

Protoplanets with core masses below the critical mass fill in their Roche lobe

We study the evolution of a protoplanet of a few earth masses embedded in a protoplanetary disc. If we assume that the atmosphere of the protoplanet, i.e. the volume of gas in hydrostatic equilibrium bound to the core, has a surface radius smaller than the Roche lobe radius, we show that it expands as it accretes both planetesimals and gas at a fixed rate from the nebula until it fills in the Roche lobe. The evolution occurs on a timescale shorter than the formation or migration timescales. Therefore, we conclude that protoplanets of a few earth masses have an atmosphere that extends to the Roche lobe surface, where it matches to the nebula. This is true even when the Bondi radius is smaller than the Roche lobe radius. This is in contrast to the commonly used models in which the static atmosphere extends up to the Bondi radius and is surrounded by a cold accretion flow. As a result, any calculation of the tidal torque exerted by the disc onto the protoplanet should exclude the material present in the Roche lobe, since it is bound to the protoplanet.Comment: 17 pages, 1 figure, accepted by MNRA

Adeus ao Binário: o direito à diversidade sexual como pauta nas relações de trabalho

Em 10 de dezembro de 1948, a Declaração Universal dos Direitos Humanos foi adotada pela Organização das Nações Unidas (ONU), delineando a inter-relação de direitos humanos básicos. A partir dessa concepção, compreendeu-se que todos os seres humanos nascem livres e iguais em dignidade e direitos. Entretanto, hoje, em diferentes partes do mundo, sobretudo no Brasil, os princípios da igualdade e não discriminação tendem a ser violados em razão das diferentes manifestações de identidade sexual e de gênero, impossibilitando, inclusive, a efetiva inserção de pessoas LGBTI+ no mercado de trabalho. Em virtude disso, o presente estudo configura-se como uma análise pormenorizada, à luz das garantias constitucionais, sobre o reconhecimento da diversidade sexual e de gênero como direito fundamental, visando provocar um olhar crítico acerca do tema, bem como, a consolidação de uma ordem jurídica mais inclusiva que avance na luta contra a discriminação da população LGBTI+ na sociedade e no âmbito das relações de trabalho

Modelling human choices: MADeM and decision‑making

Research supported by FAPESP 2015/50122-0 and DFG-GRTK 1740/2. RP and AR are also part of the Research, Innovation and Dissemination Center for Neuromathematics FAPESP grant (2013/07699-0). RP is supported by a FAPESP scholarship (2013/25667-8). ACR is partially supported by a CNPq fellowship (grant 306251/2014-0)

Dead discs, unstable discs and the stars they surround

Strong stellar magnetic fields significantly alter the behaviour of surrounding accretion discs. Recent work has demonstrated that at low accretion rates a large amount of mass can remain confined in the disc, contrary to the standard assumption that the magnetic field will expel the disc in an outflow (the “propeller regime”). These “dead discs” often become unstable, causing cycles of accretion onto the central star. Here I present the main predictions of this model, and argue that it provides a good explanation for the peculiar behaviour seen in several accreting sources with strong magnetic fields. I will focus in particular on three accreting millisecond X-ray pulsars: SAX J1808.4-3658, NGC 6440 X-2 and IGR J00291+5934. These sources all show low-frequency quasi-periodic oscillations consistent with a variable accretion rate, as well as unusual outburst patterns that suggest gas is confined in the inner disc regions during quiescence

Adeus ao Binário: o direito à diversidade sexual como pauta nas relações de trabalho

Em 10 de dezembro de 1948, a Declaração Universal dos Direitos Humanos foi adotada pela Organização das Nações Unidas (ONU), delineando a inter-relação de direitos humanos básicos. A partir dessa concepção, compreendeu-se que todos os seres humanos nascem livres e iguais em dignidade e direitos. Entretanto, hoje, em diferentes partes do mundo, sobretudo no Brasil, os princípios da igualdade e não discriminação tendem a ser violados em razão das diferentes manifestações de identidade sexual e de gênero, impossibilitando, inclusive, a efetiva inserção de pessoas LGBTI+ no mercado de trabalho. Em virtude disso, o presente estudo configura-se como uma análise pormenorizada, à luz das garantias constitucionais, sobre o reconhecimento da diversidade sexual e de gênero como direito fundamental, visando provocar um olhar crítico acerca do tema, bem como, a consolidação de uma ordem jurídica mais inclusiva que avance na luta contra a discriminação da população LGBTI+ na sociedade e no âmbito das relações de trabalho