370 research outputs found
Convective envelopes in rotating OB stars
We study the effects of rotation on the outer convective zones of massive
stars. We examine the effects of rotation on the thermal gradient and on the
Solberg--Hoiland term by analytical developments and by numerical models.
Writing the criterion for convection in rotating envelopes, we show that the
effects of rotation on the thermal gradient are much larger and of opposite
sign to the effect of the Solberg-Hoiland criterion. On the whole, rotation
favors convection in stellar envelopes at the equator and to a smaller extent
at the poles. In a rotating 20 Msun star at 94% of the critical angular
velocity, there are two convective envelopes, with the bigger one having a
thickness of 13.2% of the equatorial radius. In the non-rotating model, the
corresponding convective zone has a thickness of only 4.6% of the radius. The
occurrence of outer convection in massive stars has many consequences.Comment: 4 pages, 3 figures, accepted by Astronomy and Astrophysic
O potencial da plataforma educativa Esybee para uma educação de qualidade : The potential of the educational platform Esybee for quality education
Neste artigo apresenta-se um estudo exploratório, com recurso a um estudo de caso selecionado através de uma amostragem por conveniência realizado numa turma de jovens com deficiência e/ou Necessidades Educativas Especiais (NEE) no colégio de St. Piers, Young Epilepsy (YE), Inglaterra durante o ano letivo 2020-2021 (ano da pandemia COVID-19). Pretendeu-se compreender a importância da plataforma de gestão educativa Earwig Academic no quotidiano destes estudantes, recorrendo-se à monotorização do seu desempenho em quatro sessões no colégio. Das opções formativas oferecidas em St. Piers que se encontram organizadas em quatro áreas: a) Competências funcionais; b) Estudos vocacionais; c) Atividades de enriquecimento e d) A vida depois do Colégio, selecionou-se as sessões de Competências Funcionais, Culinária, Desporto e Fazenda para também se compreender a importância da diferenciação educativa no desenvolvimento dos estudantes. A recolha de dados foi realizada através da observação não participante e participante, conversas formais e informais, com recurso a evidências fotográficas, folhas de exercícios e notas de campo que ajudaram na monotorização do estudante na Earwig Academic. Esta análise qualitativa permitiu compreender: a) as fragilidades dos ambientes online utilizados; b) a importância da diferenciação educativa para colmatar as dificuldades identificadas e c) a relevância de uma equipa educativa capacitada para o sucesso educativo. A importância do recurso a uma plataforma de aprendizagem, de base construtivista como a Esybee é clara. Este sistema de suporte multicamadas (Multi-Tiered System of Support), com recurso à resposta à intervenção (response to intervention - RTI), representa uma medida importante na promoção da educação de qualidade
Direct Dark Matter Detection with Velocity Distribution in the Eddington approach
Exotic dark matter together with the vacuum energy (associated with the
cosmological constant) seem to dominate the Universe. Thus its direct detection
is central to particle physics and cosmology. Supersymmetry provides a natural
dark matter candidate, the lightest supersymmetric particle (LSP). One
essential ingredient in obtaining the direct detection rates is the density and
the velocity distribution of the LSP in our vicinity. In the present paper we
study simultaneously density profiles and velocity distributions in the context
of the Eddington approach. In such an approach, unlike the commonly assumed
Maxwell-Boltzmann (M-B) distribution, the upper bound of the velocity arises
naturally from the potential.Comment: 21 LaTex pages, 27 figure
Turbulent Mixing in the Surface Layers of Accreting Neutron Stars
During accretion a neutron star (NS) is spun up as angular momentum is
transported through its surface layers. We study the resulting differentially
rotating profile, focusing on the impact this has for type I X-ray bursts. The
predominant viscosity is likely provided by the Tayler-Spruit dynamo. The
radial and azimuthal magnetic field components have strengths of ~10^5 G and
~10^10 G, respectively. This leads to nearly uniform rotation at the depths of
interest for X-ray bursts. A remaining small shear transmits the accreted
angular momentum inward to the NS interior. Though this shear gives little
viscous heating, it can trigger turbulent mixing. Detailed simulations will be
required to fully understand the consequences of mixing, but our models
illustrate some general features. Mixing has the greatest impact when the
buoyancy at the compositional discontinuity between accreted matter and ashes
is overcome. This occurs at high accretion rates, at low spin frequencies, or
may depend on the ashes from the previous burst. We then find two new regimes
of burning. The first is ignition in a layer containing a mixture of heavier
elements from the ashes. If ignition occurs at the base of the mixed layer,
recurrence times as short as ~5-30 minutes are possible. This may explain the
short recurrence time of some bursts, but incomplete burning is still needed to
explain these bursts' energetics. When mixing is sufficiently strong, a second
regime is found where accreted helium mixes deep enough to burn stably,
quenching X-ray bursts. We speculate that the observed change in X-ray burst
properties near one-tenth the Eddington accretion rate is from this mechanism.
The carbon-rich material produced by stable helium burning would be important
for triggering and fueling superbursts. (abridged)Comment: Accepted for publication in The Astrophysical Journal, 16 pages, 15
figure
Visualization and spectral synthesis of rotationally distorted stars
Simple spherical, non-rotating stellar models are inadequate when describing
real stars in the limit of very fast rotation: Both the observable spectrum and
the geometrical shape of the star deviate strongly from simple models. We
attempt to approach the problem of modeling geometrically distorted, rapidly
rotating stars from a new angle: By constructing distorted geometrical models
and integrating standard stellar models with varying temperature, gravity, and
abundances, over the entire surface, we attempt a semi-empirical approach to
modeling. Here we present our methodology, and present simple examples of
applications.Comment: 7 pages, 5 figures. Contribution to GREAT-ESF Workshop on "Stellar
Atmospheres in the Gaia Era", Brussels 23-24 June 201
The effects of inclination, gravity darkening and differential rotation on absorption profiles of fast rotators
Mechanisms influencing absorption line profiles of fast rotating stars can be
sorted into two groups; (i) intrinsic variations sensitive to temperature and
pressure, and (ii) global effects common to all spectral lines. I present a
detailed study on the latter effects focusing on gravity darkening and
inclination for various rotational velocities and spectral types. It is shown
that the line shapes of rapidly and rigidly rotating stars mainly depend on the
equatorial velocity , not on the projected rotational velocity which determines the lines width. The influence of gravity darkening
and spectral type on the line profiles is shown. The results demonstrate the
possibility of determining the inclination angle of single fast rotators,
and they show that constraints on gravity darkening can be drawn for stellar
samples. While significant line profile deformation occurs in stars rotating as
fast as v_{\rm e} \ga 200 km s, for slower rotators profile distortion
are marginal. In these cases spectral signatures induced by, e.g., differential
rotation are not affected by gravity darkening and the methods applicable to
slow rotators can be applied to these faster rotators, too.Comment: 7 pages, accepted by A&
Asymmetric transit curves as indication of orbital obliquity: clues from the late-type dwarf companion in KOI-13
KOI-13.01, a planet-sized companion in an optical double star was announced
as one of the 1235 Kepler planet candidates in February 2011. The transit
curves show significant distortion that was stable over the ~130 days time-span
of the data. Here we investigate the phenomenon via detailed analyses of the
two components of the double star and a re-reduction of the Kepler data with
pixel-level photometry. Our results indicate that KOI-13 is a common proper
motion binary, with two rapidly rotating components (v sin i ~ 65--70 km/s). We
identify the host star of KOI-13.01 and conclude that the transit curve
asymmetry is consistent with a companion orbiting a rapidly rotating, possibly
elongated star on an oblique orbit. After correcting the Kepler light curve to
the second light of the optical companion star, we derive a radius of 2.2 R_J
for the transiter, implying an irradiated late-type dwarf, probably a hot brown
dwarf rather than a planet. KOI-13 is the first example for detecting orbital
obliquity for a substellar companion without measuring the Rossiter-McLaughlin
effect from spectroscopy.Comment: Accepted by ApJ Letters. A new part added about KOI-13.01 (Sect. 3.4
pars 2-4
Superconvergent Perturbation Method in Quantum Mechanics
An analogue of Kolmogorov's superconvergent perturbation theory in classical
mechanics is constructed for self adjoint operators. It is different from the
usual Rayleigh--Schr\"odinger perturbation theory and yields expansions for
eigenvalues and eigenvectors in terms of functions of the perturbation
parameter.Comment: 11 pages, LaTe
An Extremely Lithium-Rich Bright Red Giant in the Globular Cluster M3
We have serendipitously discovered an extremely lithium-rich star on the red
giant branch of the globular cluster M3 (NGC 5272). An echelle spectrum
obtained with the Keck I HIRES reveals a Li I 6707 Angstrom resonance doublet
of 520 milli-Angstrom equivalent width, and our analysis places the star among
the most Li-rich giants known: log[epsilon(Li)] ~= +3.0. We determine the
elemental abundances of this star, IV-101, and three other cluster members of
similar luminosity and color, and conclude that IV-101 has abundance ratios
typical of giants in M3 and M13 that have undergone significant mixing. We
discuss mechanisms by which a low-mass star may be so enriched in Li, focusing
on the mixing of material processed by the hydrogen-burning shell just below
the convective envelope. While such enrichment could conceivably only happen
rarely, it may in fact regularly occur during giant-branch evolution but be
rarely detected because of rapid subsequent Li depletion.Comment: 7-page LaTeX file, including 2 encapsulated ps figures + 1 table;
accepted for publication in the Astrophysical Journal Letter
Ionization structure in the winds of B[e] supergiants II. Influence of rotation on the formation of equatorial hydrogen neutral zones
Context: B[e] supergiants are known to have non-spherical winds, and the
existence of disks that are neutral in hydrogen close to their stellar surface
has been postulated. A suitable mechanism to produce non-spherical winds seems
to be rapid rotation, and at least for three B[e] supergiants in the Magellanic
Clouds rotation velocities at a substantial fraction of their critical velocity
have been found. Aims: We want to find suitable recombination distances in the
equatorial plane of rapidly rotating stars that explain the observed huge
amounts of neutral material in the vicinity of B[e] supergiants. Methods: We
perform ionization structure calculations in the equatorial plane around
rapidly rotating luminous supergiants. The restriction to the equatorial plane
allows us to treat the ionization balance equations 1-dimensionally, while the
stellar radiation field is calculated 2-dimensionally, taking into account the
latitudinal variation of the stellar surface parameters. The stellar parameters
used correspond to those known for B[e] supergiants. The assumptions made in
the computations all have in common that the total number of available ionizing
photons at any location within the equatorial plane is overestimated, resulting
in upper limits for the recombination distances. Results: We find that despite
the drop in equatorial surface density of rapidly rotating stars (neglecting
effects like bi-stability and/or wind compression), hydrogen and helium
recombine at or close to the stellar surface, for mass loss rates Mdot > 5d-5
M_sun/yr and rotation speeds in excess of v(rot,eq)/v(crit)=0.8.Comment: 10 pages, 5 figures, accepted for publication in A&A (03/05/2006),
Eq. (3) corrected, language improve
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