1o semestre de 2012

Data

Apresentador

Título

06/Mar

Bruno Dias

Quebra de Paradigma sobre Aglomerados Estelares

13/Mar

Marcio Avellar

Entropy, complexity and disequilibrium in compact stars

20/Mar

Mairan Teodoro

SN II causadas por YSG: desafio para os modelos de evolução de estrelas de alta massa?

27/Mar

Viviane Alves

Uma estrela extremamente primitiva no halo galáctico

03/Abr

SEMANA SANTA

10/Abr

CANCELADO (RELATÓRIO DE DEPTO)

17/Abr

Phillip Galli

Métodos de Detecção de Planetas Extra-Solares

24/Abr

Tala Monroe

Dusty Planetary Debris Disks

01/Mai

DIA DO TRABALHADOR

08/Mai

Rodrigo Vieira

Filamentos em nuvens: grandes berçários vistos pelo Herschel

15/Mai

Patricia Spinelli

The anti-bullet cluster

22/Mai

Bruno Quint

Afinal, o que é o BTFI?

29/Mai

Daryel

Magnetized strange quark matter. Astrophysical implications

05/Jun

Raul Puebla

The Transition Mass-loss Rate: Calibrating the Role of Line-driven Winds in Massive Star Evolution

29/Mai

Danilo Teixeira

No evidence for Bardeen-Peterson alignment in conservative GRMHD simulations of a relatively thin, tilted accretion disk.

19/Jun

Valentin Perret

Last results of the "Mass Assembly Survey with SINFONI in VVDS (MASSIV)

26/Jun

Vinicius Placco

The s-process in low-metallicity stars - III. Individual analysis of CEMP-s and CEMP-s/r with asymptotic giant branch models


Quebra de Paradigma sobre Aglomerados Estelares
Bruno Dias
Resumo
Nos cursos de Astrofísica Estelar aprendemos que um aglomerado estelar é um conjunto de estrelas formadas a partir da mesma nuvem de gás e que, portanto, têm a mesma idade e a mesma composição química. Com os avanços na instrumentação de alta resolução nas últimas décadas, em fotometria e espectroscopia, foram detectados vários aglomerados da Via Láctea apresentando estrelas com diferentes idades (turnoffs múltiplos no Diagrama HR) e diferentes metalicidades (histograma de metalicidades largo ou com 2 picos). Para entender o motivo dessas constatações sistemáticas, é preciso entender bem qual a origem de um aglomerado estelar. Focarei, em particular, em modelos recentes de formação de aglomerados que explicam a dispersão de metalicidade dos objetos mais massivos, com M~10^6 M_sun ou maiores.
Referências
- Bekki 2012, MNRAS, accepted (arXiv:1112.3710)
- Bekki & Freeman 2003, MNRAS, 346, 11
- Bruens & Kroupa 2011, ApJ, 729, 69


Entropy, complexity and disequilibrium in compact stars
Marcio Avellar
Resumo
We used the statistical measurements of information entropy, disequilibrium and complexity to infer a hierarchy of equations of state for two types of compact stars from the broad class of neutron stars, namely, with hadronic composition and with strange quark composition. Our results show that, since order costs energy, Nature would favour the exotic strange stars even though the question of how to form the strange stars cannot be answered within this approach.
Referências
- Avellar et al., 2012


SN II causadas por YSG: desafio para os modelos de evolução de estrelas de alta massa?
Mairan Teodoro
Resumo
Os modelos atuais de evolução de estrelas de alta massa preveêm que estrelas com massa no intervalo de 10 a 20 Msun devam atingir a fase de supergigante vermelha (RSG) logo antes de explodirem como SN II. Entretanto, existem evidências de que algumas SNe II não foram geradas por RSGs. De fato, alguns progenitores de SNe II foram classificados como tendo 3.6 < log(Teff) < 3.8 - caracterizando-as como supergigantes amarelas (YSG) -, enquanto as RSG localizam-se em uma faixa de temperatura muito bem definida entre 3.54 < log(Teff) < 3.58. Neste seminário, apresentarei as alternativas para explicar tais observações em desacordo com os modelos atuais de evolução estelar.
Referências
- Georgy, C., 2012. A&A, 538, p.L8.
- Arcavi, I. et al., 2011. ApJL, 742(2), p.L18.
- Maund, J.R. et al., 2011. ApJL, 739(2), p.L37.
- Murphy, J.W. et al., 2011. ApJL, 742(1), p.L4.
- Van Dyk, S.D. et al., 2011. ApJL, 741(2), p.L28.


Uma estrela extremamente primitiva no halo galáctico
Viviane Alves
Resumo
O trabalho descreve a composição química de uma estrela do halo galáctico com uma metalicidade muito baixa (Z < 6.9 x 10-7 ) e com um padrão químico típico de uma estrela extremamente pobre em metais clássica, isto é, sem enriquecimento de C, N ou O. Por razões teóricas e por não se observar estrelas com Z < 1.5 x 10-5 , tem sido sugerido que estrelas de baixa massa não puderam ser formar a partir do meio interestelar primitivo até que ele fosse enriquecido acima do valor crítico de Z (estimado no intervalo de 1.5 x 10-8 a 1.5 x 10-6 ), no entanto a descoberta deste estrela sugere o contrário.
Referências
- Caffau, E. et al. 2011, Nature, 477, 67


Métodos de Detecção de Planetas Extra-Solares
Phillip Galli
Resumo
A descoberta de planetas que orbitam outras estrelas que não o Sol, é uma das maiores conquistas científicas dos últimos tempos. O interesse chama atenção não só dos cientistas, mas também do público em geral pela possibilidade de encontrar outras formas de vida no Universo. A procura por planetas com métodos diretos é difícil e extremamente desafiadora para a instrumentação atual. Métodos indiretos que medem o efeito do planeta na estrela hospedeira são mais utilizados e conduziram à descoberta da maioria dos planetas conhecidos até hoje. No próximo encontro apresentarei os principais métodos utilizados para detectar planetas relembrando descobertas importantes dos últimos anos. Em particular, será discutido as vantagens e perspectivas da detecção de planetas por astrometria com o PRIMA/VLTI.
Referências
- Lagrange et al., 2010, Science, 329, 57.
- Deeg et al., 2010, Nature, 464, 384.
- Setiawan et al., 2008, Nature, 451, 38.
- Launhardt, 2009, New Astronomy Reviews, 53, 294.
- Quirrenbach, 2011, EPJ Web of Conferences 16, 07005


Dusty Planetary Debris Disks
Tala Monroe
Resumo
Debris disks are the dusty observational consequences of planetesimal and planet formation. The dust observed in these disks is thought to have been generated during collisions of planetesimals that have been gravitationally stirred, either by massive planets or the most massive planetesimals that have formed. In this discussion I will highlight some key results from studies of debris disks obtained with Spitzer Space Telescope observations, recent results from the Herschel Space Observatory, and early results of ALMA observations, particularly in relation to the presence of planets. I will also compare these dusty extrasolar disks with the disk of dust in our own Solar system.
Referências
- Moro-Martin, A. (2012), arXiv:1203.0005.
- Wyatt (2008), ARA&A 46 339W.
- Boley et al. (2012), arXiv:1204.0007.
- Eiroa et al. (2011) A&A 536L 4E.


Filamentos em nuvens: grandes berçários vistos pelo Herschel
Rodrigo Vieira
Resumo
No nosso próximo encontro apresentarei alguns resultados recentes obtidos com o satélite Herschel. As observações indicam que o processo de formação estelar ocorre preferencialmente na junção das estruturas filamentares, presentes nos complexos de nuvens moleculares.
Referências
- Hill et al. (2011)
- Schneider et al. (2012)


The anti-bullet cluster
Patrícia Spinelli
Resumo
Neste seminário, vou discutir o recente artigo de Jee et al. 2012 (arXiv:1202.6368) que foi tema do comunicado de imprensa do dia 2 de Março de 2012 do telescópio espacial Hubble (http://hubblesite.org/newscenter/archive/releases/2012/10/full/). O artigo entitulado como "A Study of the Dark Core in A520 with Hubble Space Telescope: The Mystery Deepens" traz um estudo do aglomerado de galáxias Abell 520, que parece ser um contra-exemplo do famoso "Bullet Cluster". O estudo mostra que neste aglomerado, o pico da distribuição de matéria coincide com o pico em raios-X mas não com a distribuição de galáxias o que não pode ser explicado com as teorias de Cold Dark Matter (CDM), deixando caminho livre para explicações alternativas.
Referências
- Clowe, D. et al. 2006, ApJ, 648L, 109 (revisão);
- Jee, M. J. et al. 2012, ApJ, 747, 96;
- Moni Bidin, C. et al. 2012, ApJ, 751, 30


Afinal, o que é o BTFI?
Bruno Quint
Resumo
No mês de julho deste ano iremos comemorar dois anos desde a primeira luz do instrumento Brazilian Tunable Filter Imager ou, como é mais conhecido, BTFI. Mas o que é exatamente o BTFI? O Journal Club deste dia será destinado à apresentar este instrumento aos pós-graduandos que, provavelmente, já ouviram falar deste instrumento mas que ainda não sabem exatamente o que é e como funciona. O BTFI, instalado no telescópio SOAR, no Chile, é um espectrógrafo tri-dimensional que utiliza duas novas tecnologias para produzir cubos de dados espectrais. Aqui, apresentarei quais são estas novas tecnologias, como a informação das fontes astrofísicas é transformada em cubos de dados e quais as operações necessárias para podermos extrair dados relevantes destes cubos.
Referências
- Blais-Oeuellette et al. (2006)
- Shopbell et al. (1992)
- Quint et al. (2010)
- Taylor et al. (2010)


Magnetized strange quark matter. Astrophysical implications
Daryel
Resumo
I will review the stability of magnetized strange quark matter (MSQM) within the phenomenological MIT bag model, taking into account the variation of the relevant input parameters, namely, the strange quark mass, baryon density, magnetic field and bag parameter. A comparison with magnetized asymmetric quark matter in $\beta$-equilibrium as well as with strange quark matter (SQM) is presented. We obtain that the energy per baryon for MSQM decreases as the magnetic field increases, and its minimum value at vanishing pressure is lower than the value found for SQM, which implies that MSQM is more stable than non-magnetized SQM. The mass-radius relation for magnetized strange quark stars is also obtained in this framework.
Referências
- D. M. Paret and A. P. Martinez, arXiv:1010.0909 [astro-ph.HE]
- A. P. Martinez, R. G. Felipe and D. M. Paret, 2010
- R. G. Felipe and A. P. Martinez, 2009


The Transition Mass-loss Rate: Calibrating the Role of Line-driven Winds in Massive Star Evolution
Raul Puebla
Resumo
A debate has arisen regarding the importance of stationary versus eruptive mass loss for massive star evolution. The reason is that stellar winds have been found to be clumped, which results in the reduction of unclumped empirical mass-loss rates. Most stellar evolution models employ theoretical mass-loss rates which are already reduced by a moderate factor of sime2-3 compared to non-corrected empirical rates. A key question is whether these reduced rates are of the correct order of magnitude, or if they should be reduced even further, which would mean that the alternative of eruptive mass loss becomes necessary. Here we introduce the transition mass-loss rate \dot{M}_trans between O and Wolf-Rayet stars. Its novelty is that it is model independent. All that is required is postulating the spectroscopic transition point in a given data set, and determining the stellar luminosity, which is far less model dependent than the mass-loss rate. The transition mass-loss rate is subsequently used to calibrate stellar wind strength by its application to the Of/WNh stars in the Arches cluster. Good agreement is found with two alternative modeling/theoretical results, suggesting that the rates provided by current theoretical models are of the right order of magnitude in the ~50 M sun mass range. Our results do not confirm the specific need for eruptive mass loss as luminous blue variables, and current stellar evolution modeling for Galactic massive stars seems sound. Mass loss through alternative mechanisms might still become necessary at lower masses, and/or metallicities, and the quantification of alternative mass loss is desirable.
Referências
- Vink & Grafener 2012, ApJ, 751, 34
- Vink et al. 2011, A&A, 531A, 132
- Vink et al. 2000, A&A, 362, 295


No evidence for Bardeen-Peterson alignment in conservative GRMHD simulations of a relatively thin, tilted accretion disk.
Danilo Teixeira
Resumo
We present 3D GRMHD simulations of a tilted accretion disk with a disk scale-height of 0.08, where our simulations have been done with the astrophysical code Cosmos++. The objective of this work is to study the evolution of the tilt angle, the angle between the angular momentum of the accretion disk and the angular momentum of the black hole and check if this angle will reach some steady state behavior and if the inner parts will align with the equatorial plane of the black hole. We have run four different simulations, two low resolutions and two high resolutions where initially we started with an untilted torus and allowed it to evolve until t=2060GM/c3 . After this time, we have kept this simulation running and started a new one where we tilted the black hole in 10 degrees and keep both simulations running until t=20000GM/c3 . We observe that the tilt angle of our simulation reached a steady-state where the tilt angle has not gone to zero in the inner regions as expected and our simulations also showed that we are in the transition between diffusive and wave-like regimes for the propagation of the warps but another important result is that we have not found warps in our simulations and also that our disk precess as a rigid body according to the profile of our precession angle. Another important fact is that we were able to measure the azimuthal and vertical component of the stress and see if the values that have been used in SPH simulations to study tilted accretion disk are reasonable. Our results are compatible with the new relativistic accretion disk model from Zhuravlev & Ivanov (MNRAS, 2011, 415, 2012) where for our value of the azimuthal component of the stress, scale-height and spin, we are in a region in their model where the tilt angle should present a small increase in xthe inner regions.
Referências
- Teixeira & Fragile, 2012, in prep


Last results of the "Mass Assembly Survey with SINFONI in VVDS (MASSIV)
Valentin Perret
Resumo
Understanding how galaxies evolve and assemble their mass across cosmic time is still a fundamental unsolved issue. Processes driving mass assembly are expected to evolve on different timescales along cosmic time. A transition might happen around z=1 as the cosmic star formation rate starts its decrease. To get insight into the various processes of galaxy mass assembly, the Mass Assembly Survey with SINFONI in VVDS (MASSIV) aims at probing the kinematical and chemical properties of a significant and representative sample of high-redshift (0.9< z< 1.8) star-forming galaxies. This sample contains 84 star-forming galaxies, selected from the VIMOS VLT Deep Survey (VVDS) and observed with the SINFONI integral-field spectrograph at the VLT. The MASSIV selection function, based on star formation criteria provides a good representation of ''normal'' star-forming galaxies to be compared to massive galaxies at the same age. I will present the last results of this survey. I will focus on my work which is related to the analysis of the kinematics of these galaxies.
Referências
- Epinat, B. et al. 2012, A&A, 539, 92


The s-process in low-metallicity stars - III. Individual analysis of CEMP-s and CEMP-s/r with asymptotic giant branch models
Vinicius Placco
Resumo
This paper provides an individual analysis of 94 carbon-enhanced metal-poor stars showing an s-process enrichment (CEMP-s) collected from the literature. The s-process enhancement observed in these stars is ascribed to mass transfer by stellar winds in a binary system from a more massive companion evolving faster towards the asymptotic giant branch (AGB) phase. The theoretical AGB nucleosynthesis models have been presented in Bisterzo et al. (Paper I of this series). Several CEMP-s show an enhancement in both s- and r-process elements (CEMP-s/r). In order to explain the peculiar abundances observed in CEMP-s/r, the authors assume that the molecular cloud from which CEMP-s formed was previously enriched in r-elements by supernova pollution. A general discussion and the method adopted in order to interpret the observations have been provided in Bisterzo et al. (Paper II of this series). This paper presents a detailed study of spectroscopic observations of individual stars, considering all elements from carbon to bismuth, with particular attention to the three s-process peaks, ls (Y, Zr), hs (La, Nd, Sm) and Pb, and their ratios [hs/ls] and [Pb/hs]. The presence of an initial r-process contribution may be typically evaluated by [La/Eu]. The authors found possible agreements between theoretical predictions and spectroscopic data. In general, the observed [Na/Fe] (and [Mg/Fe]) provides information on the AGB initial mass, while [hs/ls] and [Pb/hs] are mainly indicators of the s-process efficiency. A range of 13C-pocket strengths are required to interpret the observations. However, major discrepancies between models and observations exist. The authors also highlight star by star the agreements and the main problems encountered and, when possible, suggest potential indications for further studies. These discrepancies provide starting points of debate for unsolved problems in which spectroscopic and theoretical studies may intervene.
Referências
- Bisterzo, S. et al. 2012, MNRAS, 422, 849