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Anãs brancas de massa extremamente baixa (ELM WDs) são entendidas como o resultado da evolução binária na qual uma estrela doadora de baixa massa — massa da ordem da do Sol — é despida por sua companheira, deixando para trás uma anã branca com núcleo de hélio de massa de 0,2–0,3 massas solares e gravidade superficial de dez a mil vezes menor do que a de uma anã branca canônica. O objetivo geral deste trabalho é obter modelos de ELM compatíveis com a distribuição observada de ELMs e pré-ELMs. Antes da interação, (i.e., da transferência de massa), acredita-se que a evolução binária é dominada pelo mecanismo do freamento magnético, o qual é a maior fonte de incertezas na modelagem desse tipo de sistema. Nos modelos de formação de ELM WDs orbitando pulsares de milissegundos em órbitas muito compactas (2–9 horas) publicados, um severo ajuste fino no período orbital inicial foi necessário, sugerindo que a formulação do freamento magnético utilizado precisava ser revisada. Uma modificação ao tratamento do freamento magnético tradicional foi proposta por Van & Ivanova (2019), nomeada Convection And Rotation Boosted (CARB). Essa receita para a lei do freamento magnético inclui duas novas melhorias no entendimento dos campos e ventos magnéticos, introduzindo modificações na evolução de sistemas binários de raios-X de baixa massa (LMXB) devido à força do campo magnético e à rotação. Computando sequências de modelos usando o código de evolução estelar MESA, nós aplicamos a formulação CARB em modelos de formação de ELM WDs em sistemas binários compactos e encontramos que o ajuste fino severo no período orbital inicial não é mais necessário. Em particular, nós encontramos que para períodos orbitais no intervalo de 4–250 dias os produtos finais da evolução são ELMs e anãs brancas de baixa massa com massas no intervalo 0,15–0,40 massas solares. Também, o período de bifurcação — o período orbital inicial que separa os sistemas em que, ao final da evolução, terão seus componentes aproximados ou afastados — é deslocado para valores maiores (de 2,75–2,8 para 20–25 dias) quando o freamento magnético CARB é levado em conta. Como resultado, obtivemos modelos de ELM WDs com massas tão baixas quando 0,26 massas solares em sistemas convergentes mesmo com períodos orbitais iniciais tão longos quanto 20 dias. Nós expandimos a nossa grade de modelos para estudar os efeitos de diferentes parâmetros iniciais. Os seguinte cenários foram considerados: o aumento da massa da estrela doadora de 1,0 para 1,2 vezes a massa do Sol a fim de diminuir o tempo na sequência principal; diferentes metalicidades (Z = 0.02, 0.01 and 0.001) pois há progenitores em diferentes partes da Galáxia; e estudar alta (70%) e baixa (20%) eficiência na transferência de massa, já que é um parâmetro bastante incerto. Nós então repetimos as mesmas configurações descritas acima mas considerando como acretora uma anã branca massiva (0,8 massa solar), compatível com sistemas de variáveis cataclísmicas (CV). As nossas conclusões indicam que a prescrição CARB é adequada para reproduzir as propriedades de LMXBs e CVs observadas, mesmo no regime de períodos orbitais iniciais curtos. Comparando nossos modelos com dados observacionais de He-WDs em sistemas binários com pulsares de milissegundos, o uso do freamento magnético CARB se mostra compatível com a formação de ELM WDs em LMXBs. Em adição aos sistemas LMXB e CV, o freamento CARB torna possível a formação de binários de raios-X ultra-compactos (UCXB) e pulsares de milissegundos de órbita ampla, bem como ELM WDs separadas em companhia de estrelas de nêutrons e de anãs brancas massivas. Todavia, dados de ELM WDs em sistemas com anãs brancas apresentam grande dispersão em massa e no período orbital do sistema binário, sugerindo que outros canais de formação também estão presentes. Nota-se que não simulamos modelos de Envelope Comum (CE). Além disso, nós encontramos que as propriedades observacionais como temperatura efetiva e gravidade superficial desses novos modelos são compatíveis com a maioria das ELM WDs confirmadas ou candidatas. Os resultados combinados dos mais de trezentos modelos que calculamos deixam claro que a massa final das ELM WDs é uma quantidade extremamente degenerada no espaço de parâmetros estudado, já que existem inúmeras interdependências entre metalicidade, ocorrência de flashes de hidrogênio, eficiência na transferência de massa, a massa inicial das estrelas, o período orbital inicial, e a reação do sistema binário quanto à transferência de massa. Extremely low-mass white dwarfs (ELM WDs) are understood to be the result of binary evolution in which a low-mass donor star — mass of the order of that of the Sun — is stripped by its companion, leaving behind a helium-core white dwarf with a mass below 0.2–0.3 solar masses and with surface gravity ten to a thousand times smaller than a canonical white dwarf. The overall objective of this work is to obtain ELM models compatible with the observed distribution of ELMs and pre-ELMs. Prior to the interaction (i.e., the mass transfer), the binary evolution is thought to be dominated by the magnetic braking mechanism, which is the main uncertainty in modelling such systems. In published models of the formation of the ELM WDs orbiting millisecond pulsars in very compact orbits (2–9 hours), a severe fine-tuning of the initial orbital period was necessary, suggesting that standard magnetic braking formulation needs to be revised. A modification to the traditional treatment of magnetic braking has been proposed by Van & Ivanova (2019), named Convection And Rotation Boosted (CARB). This recipe for the magnetic braking law includes two recent improvements in understanding stellar magnetic fields and magnetised winds, introducing changes in the evolution of low-mass X-ray binary (LMXB) systems due to the magnetic field strength and the rotation. Computing model sequences using the stellar evolutionary code MESA, we apply the CARB formulation to the formation of ELM WDs in LMXB systems and find that fine-tuning of the initial periods is not required. In particular, we find that for initial orbital periods in the range of 4–250 days, the final product of the evolution are ELM and low mass WDs with masses in the range 0.15–0.40 solar masses. Also, the bifurcation period — the initial orbital period that separates the systems which, at the end of evolution, will have their components close together or far apart — is shifted to longer ones (from 2.75–2.8 to 20–25 days) when the CARB magnetic braking is considered. As a result, we obtain ELM white dwarf models with masses as low as 0.26 solar mass in converging binary systems even with initial orbital periods as long as 20 days. We expand the model grid to study the effects of different initial parameters. The following scenarios were considered: increase of the donor star mass from 1.0 to 1.2 times the solar mass to decrease the main sequence lifetime; study different metallicities (Z = 0.02, 0.01 and 0.001) because there are progenitors from different parts of the Galaxy; study high (70%) and low (20%) efficiencies in mass transfer since this is a very uncertain parameter. We then repeat the same setups described above but considering a massive white dwarf (0.8 solar mass) accretor, compatible with cataclysmic variable (CV) systems. Our conclusion indicates that the CARB prescription is adequate to reproduce the observed LMXBs and CVs properties, even in the regime of short initial orbital periods. Comparing our models with observational data from He-WDs in binary systems with millisecond pulsars, the use of CARB magnetic braking is shown to be compatible with the formation of ELM WDs in LMXBs. In addition to the LMXB and CV systems, the use of CARB magnetic braking makes it possible to form ultra-compact X-ray binaries (UCXB) systems and wide-orbit binary millisecond pulsars, as well as detached ELM WDs companions to neutron stars and massive white dwarfs. Nevertheless, data from ELM WDs in systems with white dwarf companions present great dispersion in mass and orbital period of the binary system, suggesting that other formation channels are also present. Note that we do not simulate Common Envelope models. In addition, we found that the observational properties such as effective temperature and surface gravity of these new models are compatible with the majority of confirmed or candidate ELM WDs. The combined result of the more than three hundred models we calculated makes it clear that the final mass of ELM WDs is a highly degenerate quantity in the studied parameter space, as there are numerous interdependencies between metallicity, hydrogen shell flash occurrence, efficiency of mass transfer, the initial mass of the stars, initial orbital period, and the reaction of the orbital separation regarding the mass transfer.

We present the discovery of 255 binary and 6 multiple system candidates with wide (> 5 arcsec) separation composed by ultracool dwarfs (UCDs) companions to stars, plus nine double ultracool dwarf systems. These systems were selected based on common distance criteria. About 90 per cent of the total sample has proper motions available and 73 per cent of the systems also satisfy a common proper motion criterion. The sample of ultracool candidates was taken from the Dark Energy Survey (DES) and the candidate stellar primaries are from Gaia DR2 and DES data. We compute chance alignment probabilities in order to assess the physical nature of each pair. We find that 174 candidate pairs with Gaia DR2 primaries and 81 pairs with a DES star as a primary have chance alignment probabilities

Submitted by Angela Maria de Oliveira (amolivei@uepg.br) on 2020-05-08T00:38:58Z No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Derisnei Mendes Reis.pdf: 18312671 bytes, checksum: c055c3faa0acbe5819c0fd2671f0a799 (MD5) Made available in DSpace on 2020-05-08T00:38:58Z (GMT). No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Derisnei Mendes Reis.pdf: 18312671 bytes, checksum: c055c3faa0acbe5819c0fd2671f0a799 (MD5) Previous issue date: 2019-10-30 Neste trabalho apresentamos um estudo de classificação de estrelas binárias das curvas de luz cromáticas observadas com o satélite CoRoT. O experimento teve como missão fazer fotometria de um grande campo com alta precisão. O satélite observou o centro e o anti-centro da galáxia por longos períodos de tempo. O satélite possuía quatro CCD’s: duas dedicadas ao estudo de sismologia estelar e as outras duas dedicadas à descoberta de exoplanetas. Durante os seis anos e quatro meses que esteve operando, a missão observou um total de 163.665 estrelas em 26 campos estelares, produzindo cerca de 177.454 curvas de luz. Neste trabalho estudamos 286 curvas de luz. Após corrigirmos as curvas de luz cromáticas, classificamos os sistemas de estrelas binárias e identificamos binárias eclipsantes, sendo, 95 binárias do tipo Algol e que são sistemas de semi-contato; 11 binárias de contato do tipo Beta Lyrae; 61 binárias de supercontato do tipo W Ursae Majoris; 90 binárias destacadas de não-contato e 4 binárias que pertencem a outros tipos. Após a correção das curvas de luz, realizamos uma modelagem de alta precisão para oito sistemas, modelagem que nos permitiu medir parâmetros físicos importantes em um sistema binário. A análise ainda revelou 12 estrelas variáveis intrínsecas. Para a classe de variáveis intrínsecas, foram encontradas duas possíveis RR Lyrae classificadas incorretamente na literatura. 29 sistemas estelares binários e seus períodos foram determinados de forma inédita neste trabalho. In this work we present a study of binary stars classification using chromatic lightcurves observed by the CoRoT satellite . The experiment had as mission to make photometry of a large field with high precision. The satellite observed the center and anti-center of the galaxy for long periods of time. The satellite had four CCDs: two dedicated to the study of stellar seismology and the other two dedicated to the discovery of exoplanets. During the six years and four months it was operating, the mission observed a total of 163,665 stars in 26 star fields, producing about 177,454 light curves. In this work we study 286 light curves. After correcting the chromatic light-curves, we classify the binary star systems and identify eclipsing binaries, as 95 Algol-type binaries, which are semi-contact systems; 11 contact binaries of the Beta Lyrae type; 61 binary supercontact type W Ursae Majoris; 90 binary non-contact and 4 binary belonging to other types. After correcting the light curves, we performed a high precision modeling for eight systems, modeling that allowed us to measure important physical parameters in a binary star system. The analysis also revealed 12 intrinsic variable stars. For the class of intrinsic variables, two possible Lyrae RRs were incorrectly classified in the literature. 29 binary star systems and their periods have been determined for the first time in this work.

Context. The ESA Gaia mission provides a unique time-domain survey for more than 1.6 billion sources with G less than or similar to 21 mag., Aims. We showcase stellar variability in the Galactic colour-absolute magnitude diagram (CaMD). We focus on pulsating, eruptive, and cataclysmic variables, as well as on stars that exhibit variability that is due to rotation and eclipses., Methods. We describe the locations of variable star classes, variable object fractions, and typical variability amplitudes throughout the CaMD and show how variability-related changes in colour and brightness induce "motions". To do this, we use 22 months of calibrated photometric, spectro-photometric, and astrometric Gaia data of stars with a significant parallax. To ensure that a large variety of variable star classes populate the CaMD, we crossmatched Gaia sources with known variable stars. We also used the statistics and variability detection modules of the Gaia variability pipeline. Corrections for interstellar extinction are not implemented in this article., Results. Gaia enables the first investigation of Galactic variable star populations in the CaMD on a similar, if not larger, scale as was previously done in the Magellanic Clouds. Although the observed colours are not corrected for reddening, distinct regions are visible in which variable stars occur. We determine variable star fractions to within the current detection thresholds of Gaia. Finally, we report the most complete description of variability-induced motion within the CaMD to date., Conclusions. Gaia enables novel insights into variability phenomena for an unprecedented number of stars, which will benefit the understanding of stellar astrophysics. The CaMD of Galactic variable stars provides crucial information on physical origins of variability in a way that has previously only been accessible for Galactic star clusters or external galaxies. Future Gaia data releases will enable significant improvements over this preview by providing longer time series, more accurate astrometry, and additional data types (time series BP and RP spectra, RVS spectra, and radial velocities), all for much larger samples of stars.

We have examined ultraviolet (UV) and optical UB spectra of 20 UV-emitting Asymptotic Giant Branch (AGB) stars of various variability classes to study the intensity of the continuum and emission lines as a function of stellar visual magnitude to shed light on the origin of their UV emission. A significant fraction (60 per cent) of these stars show Fe I and Fe II emission lines and ∼1/4 show Balmer lines in emission. The emission in the GALEX [FUV] and [NUV] bands is dominated by continuum emission, with a limited (≤36 per cent) contribution from emission lines. The UV spectra of sources with multiple GALEX or IUE observations reveal short-term (of a few days or less) variability, which does not follow the pulsation cycle. The intensity of the Mg II λ2800 doublet, a classical diagnostic of chromospheric activity, is anticorrelated with the spectral slope in the near-UV that could be partially attributed to temperature variations in a stellar chromosphere. We observed that the intensity of Mg II λ2800 in o Cet has a sharp maximum at the phase φ 0.35 after the light curve maximum. Other LPV stars (T Cet and R Com) show strong UV Fe II emission lines near this same phase and, like the Mg II doublet, their excitation can be driven by pulsation. Our results suggest that far-UV emission from AGB stars might be external (hot companion, accretion disc), but contemporary photometric and spectral UV observations covering the whole pulsation period are required to ascertain its true origin.© 2018 The Author(s)., M.A.G. acknowledges support of the grant AYA 2014-57280-P, co-funded with FEDER funds.

The physical nature of the sdA stars – cool hydrogen-rich objects with spectroscopic surface gravities intermediate between main-sequence and canonical-mass white dwarfs – has been elusive since they were found in Sloan Digital Sky Survey Data Release 12 spectra. The population is likely dominated by metal-poor A/F stars in the halo with overestimated surface gravities, with a small contribution of extremely low-mass white dwarfs and their precursors, i.e. ELMs and pre-ELMs. In this work, we seek to identify (pre-)ELMs with radii smaller than what is possible for main-sequence stars, allowing even for very low metallicity. We analyse 3891 sdAs previously identified in the Sloan Digital Sky Survey using Gaia DR2 data. Our Monte Carlo analysis supports that 90 of these are inconsistent with the main sequence. 37 lie close to or within the canonical white dwarf cooling sequence, while the remaining 53 lie between the canonical white dwarfs and main sequence, which we interpret as likely (pre-)ELMs given their spectral class. Of these, 30 pass more conservative criteria that allow for higher systematic uncertainties on the parallax, as well as an approximate treatment of extinction. Our identifications increase the number of known (pre-)ELMs by up to 50 per cent, demonstrating how Gaia astrometry can reveal members of the compact (pre-)ELM subpopulation of the sdA spectral class.

The tidal interaction of a Be star with a binary companion forms two spiral arms that cause orbital modulation of the Be disc structure. The aim of this work is to identify observables in which this modulation is apparent. The structure of a Be disc in a coplanar circular binary system is computed with a smoothed-particle hydrodynamics code, and a radiation transfer code calculates the spectral energy distribution. Line depolarisation was confirmed, with polarisation profiles nearly reverse to emission-line profiles. The continuum flux maximizes for pole-on discs, but photometric variability maximizes for edge-on discs. The linear polarisation exhibits one or two maxima per orbital cycle. While polarisation variability in visible passbands is important only at low inclinations, infrared bands may demonstrate high orbital variability even at large inclinations. More evident is the modulation in the polarisation angle (PA) for low inclinations. The latter can be used to track azimuthal asymmetries for pole-on discs, where the spectroscopic variability in the violet-to-red (V/R) emission-component ratio disappears. PA reversals coincide with phases where V/R=1, tracking lines of sight directed towards regions where the approaching and receding arms overlap. Continuum flux and polarisation are mostly in phase for neighbouring wavelength regions. It is suggested that studies of non-symmetric discs distorted by tidal forces from a secondary star may be used to study disc variabilities of other origins., Comment: 20 pages, 19 figures; accepted for publication in MNRAS

Estrelas anãs brancas são o último estágio evolutivo observável de mais de 95% das estrelas e também um resultado comum na evolução de estrelas binárias. O estudo de anãs brancas é, portanto, uma ferramenta poderosa na compreensão da evolução de estrelas simples e binárias, da função de massa inicial local, e da perda de massa após a sequência principal, levando-nos a uma melhor compreensão do histórico de formação e evolução estelar de diferentes populações. Para que esses estudos sejam possíveis, é necessária uma amostra grande e preferencialmente completa de anãs brancas, cobrindo todo o intervalo de parâmetros físicos. A maneira mais simples de obter isso é utilizando dados de grandes projetos de mapeamento. O Sloan Digital Sky Survey já permitiu o aumento do número de anãs brancas conhecidas em cinco vezes até o data release 10. Neste trabalho, estendemos a busca por anãs brancas aos novos objetos no data release 12, descobrindo 3 157 novas anãs brancas e 1 349 novas subanãs. Pela primeira vez, estendemos essa busca para log g < 6.5, correspondente a M < 0.3 M⊙. Anãs brancas abaixo desse limite de massa não podem formar-se em um tempo de Hubble; contudo, se a estrela é parte de um sistema de binárias próximas, a perda de massa pode ser tão intensa que a anã branca resultante tem massa inferior ao limite por evolução simples. Esses objetos são chamados anãs brancas de massa extremamente-baixa (ELMs, do inglês extremely-low mass white dwarfs) Elas têm Teff < 20 000 K e 5.0 . log g . 6.5 e espectros muito similares a estrelas A de sequência principal. Menos de cem são conhecidas, e a maioria dos objetos foi descoberta tendo em conta um critério de seleção tendencioso, que excluiu ELMs frias (Teff < 9 000 K) e com mais baixa massa (M . 0.15 M⊙), tornando difícil verificar modelos e compreender as propriedades das ELMs como classe. Nós identificamos milhares de objetos cujas propriedades físicas, temperatura efetiva e log g, estão no intervalo de produtos de evolução binária, como as ELMs. Nós os chamamos de sdAs, referindo-nos ao seu log g que as coloca abaixo da sequência principal e seu espectro dominado por hidrogênio. As sdAs parecem conter populações estelares sobrepostas, e encontramos que 7% são mais compatíveis com (pre-)ELMs do que com objetos de sequência principal. Obtivemos espectroscopia resolvida temporalmente para 26 objetos e pudemos confirmar que 15 estão em binárias. Um objeto faz parte de um sistema eclipsante, enquanto outro é pulsante — o oitavo membro da classe de ELMs pulsantes. Outros seis objetos também apresentaram pulsações em nossas observações, cinco desses estão na vizinhança da faixa de instabilidade das ELMs. Com estes resultados, aumentamos a população de ELMs por um fator de 20%, elevando a fração de ELMs de 4 para 20%, um valor que é consistente com as previsões de modelos evolucionários. White dwarf stars are the final observable evolutionary state of over 95% of stars and also a common outcome in binary evolution. Therefore, studying white dwarfs is a powerful tool to understand both single and binary stellar evolution, local initial mass function, and post-main sequence mass loss, leading us to a better comprehension of the history of stellar formation and evolution of different stellar populations. In order to make this type of studies possible, a large and preferably complete sample of white dwarf stars, covering the whole range of physical parameters, is required. The simplest way to achieve that is to take advantage of data provided by large surveys. The Sloan Digital Sky Survey has already allowed the increase of the number of known white dwarf stars fivefold up to its data release 10. In this work, we extended the search for white dwarfs to the new objects in the data release 12, discovering 3 157 new white dwarfs and 1 349 new subdwarfs. For the first time, we have extended this search to log g < 6.5, corresponding to M < 0.3 M⊙. White dwarfs below this mass limit cannot be formed through single evolution within a Hubble time; however, if the star is part of a close binary system, the mass loss of the system may be so intense that the resulting white dwarf has mass below the single evolution limit. These objects are known as extremely-low mass white dwarfs (ELMs) They show Teff < 20 000 K and 5.0 . log g . 6.5 and spectra very similar to main sequence A stars. Less than a hundred of them are known, and most objects were discovered relying on biased selection criteria, that excluded cool (Teff < 9 000 K), lowermass (M . 0.15 M⊙) ELMs, making it difficult to validate the models and comprehend the properties of the ELMs as a class. We have identified thousands of objects whose physical properties, effective temperature and surface gravity, place them in the range of by-products of binary interaction such as the ELMs. We have called them sdAs, referring to their sub-main sequence log g and hydrogen dominated spectra. They seem to be composed of overlapping stellar populations, and we found that at least 7% are more likely ELMs or their precursors, the pre-ELMs, than main sequence stars. Obtaining time-resolved spectroscopy for 26 objects, we could confirm 15 to be in close binaries. One of them is also an eclipsing system, while another is a pulsator — the eighth member of the pulsating ELM class. Other six new pulsators were found as part of our follow-up, five of them in the vicinity of the ELM instability strip. With these results, we increase the population of ELMs by 20%, raising the fraction of cool ELMs from 4 to 20%, which is consistent with the predictions from the evolutionary models.

The spectral classification ‘subdwarfA’ (sdA) is given to stars showingH-rich spectra and submain- sequence surface gravities, but effective temperature lower than the zero-age horizontal branch. Their evolutionary origin is an enigma. In this work, we discuss the results of followup observations of selected sdAs. We obtained time-resolved spectroscopy for 24 objects and time-series photometry for another 19 objects. For two targets, we report both spectroscopy and photometry observations. We confirm seven objects to be new extremely low-mass white dwarfs (ELMs), one of which is a known eclipsing star.We also find the eighth member of the pulsating ELM class.

The first results of radiative transfer calculations on decretion discs of binary Be stars are presented. A smoothed particle hydrodynamics code computes the structure of Be discs in coplanar circular binary systems for a range of orbital and disc parameters. The resulting disc configuration consists of two spiral arms, and can be given as input into a Monte Carlo code, which calculates the radiative transfer along the line of sight for various observational coordinates. Making use of the property of steady disc structure in coplanar circular binaries, observables are computed as functions of the orbital phase. Orbital-phase series of line profiles are given for selected parameter sets under various viewing angles, to allow comparison with observations. Flat-topped profiles with and without superimposed multiple structures are reproduced, showing, for example, that triple-peaked profiles do not have to be necessarily associated with warped discs and misaligned binaries. It is demonstrated that binary tidal effects give rise to phase-locked variability of the violet-to-red (V/R) ratio of hydrogen emission lines. The V/R ratio exhibits two maxima per cycle; in certain cases those maxima are equal, leading to a clear new V/R cycle every half orbital period. This study opens a way in identifying binaries and in constraining the parameters of binary systems that exhibit phase-locked variations induced by tidal interaction with a companion star., 12 pages, 16 figures; accepted for publication by MNRAS

The central parsec of the Milky Way is a very special region of our Galaxy; it contains the supermassive black hole associated with Sgr A* as well as a significant number of early-type stars and a complex structure of streamers of neutral and ionized gas, within two parsecs from the centre, representing a unique laboratory. We report the identification of a high velocity compact nebular filament 2.2 arcsec south of Sgr A*. The structure extends over ~1 arcsec and presents a strong velocity gradient of ~200 km s^{-1} arcsec^{-1}. The peak of maximum emission, seen in [Fe III] and He I lines, is located at d{\alpha} = +0.20 +/- 0.06 arcsec and d{\delta} = -2.20 +/- 0.06 arcsec with respect to Sgr A*. This position is near the star IRS 33N. The velocity at the emission peak is Vr = -267 km s^{-1}. The filament has a position angle of PA = 115{\degr} +/- 10{\degr}, similar to that of the Bar and of the Eastern Arm at that position. The peak position is located 0.7 arcsec north of the binary X-ray and radio transient CXOGX J174540.0-290031, a low-mass X-ray binary with an orbital period of 7.9 hr. The [Fe III] line emission is strong in the filament and its vicinity. These lines are probably produced by shock heating but we cannot exclude some X-ray photoionization from the low-mass X-ray binary. Although we cannot rule out the idea of a compact nebular jet, we interpret this filament as a possible shock between the Northern and the Eastern Arm or between the Northern Arm and the mini-spiral "Bar"., Comment: 7 pages, 4 figures, published online in MNRAS

Planet searches using the radial velocity technique show a paucity of companions to solar-type stars within ~5 AU in the mass range of ~10 - 80 M$_{\text{Jup}}$. This deficit, known as the brown dwarf desert, currently has no conclusive explanation. New substellar companions in this region help asses the reality of the desert and provide insight to the formation and evolution of these objects. Here we present 10 new brown dwarf and two low-mass stellar companion candidates around solar-type stars from the Multi-object APO Radial-Velocity Exoplanet Large-Area Survey (MARVELS) of the Sloan Digital Sky Survey III (SDSS-III). These companions were selected from processed MARVELS data using the latest University of Florida Two Dimensional (UF2D) pipeline, which shows significant improvement and reduction of systematic errors over previous pipelines. The 10 brown dwarf companions range in mass from ~13 to 76 M$_{\text{Jup}}$ and have orbital radii of less than 1 AU. The two stellar companions have minimum masses of ~98 and 100 M$_{\text{Jup}}$. The host stars of the MARVELS brown dwarf sample have a mean metallicity of [Fe/H] = 0.03 $\pm$ 0.08 dex. Given our stellar sample we estimate the brown dwarf occurrence rate around solar-type stars with periods less than ~300 days to be ~0.56%., Comment: 20 pages, 22 figures, 3 tables

Although the majority of known binary Asymptotic Giant Branch (AGB) stars are symbiotic systems (i.e. with a WD as a secondary star), main-sequence companions of AGB stars can be more numerous, even though they are more difficult to find because the primary high luminosity hampers the detection of the companion at visual wavelengths. However, in the ultraviolet the flux emitted by a secondary with T eff > 5500 ~ 6000 K may prevail over that of the primary, and then it can be used to search for candidates to binary AGB stars. In this work, theoretical atmosphere models are used to calculate the UV excess in the GALEX near- and far-UV bands due to a main-sequence companion. After analysing a sample of confirmed binary AGB stars, we propose as a criterium for binarity: (1) the detection of the AGB star in the GALEX far-UV band and/or (2) a GALEX near-UV observed-to-predicted flux ratio > 20. These criteria have been applied to a volume-limited sample of AGB stars within 500 pc of the Sun; 34 out of the sample of 58 AGB stars ( ~ 60%) fulfill them, implying to have a MS companion of spectral type earlier than K0. The excess in the GALEX near- and far-UV bands cannot be attributed to a single temperature companion star, thus suggesting that the UV emission of the secondary might be absorbed by the extended atmosphere and circumstellar envelope of the primary or that UV emission is produced in accretion flows., Accepted for publication in MNRAS, 12 pages, 4 figures

Neste trabalho descrevo aspectos enriquecedores para a compreensão e fixação de importantes conceitos de física através de temas astronômicos, em específico com o estudo de uma classe especial de estrelas, as estrelas binárias. Em particular, as conhecidas estrelas binárias espectroscópicas são exploradas para fins didáticos, mostrando como a aplicação do efeito Doppler-Fizeau pode ser utilizada para determinação de grandezas como as velocidades das estrelas. São apresentados ainda os conceitos de espectro de radiação eletromagnética e espectros estelares, temas estes que podem ser discutidos em uma abordagem didática utilizando as estrelas binárias para o aprendizado contextualizado de física. Como aspecto suplementar, são também discutidas as principais características das estrelas binárias no contexto astrofísico e sua importância para diversos estudos em subáreas da física e astronomia In this work I describe some aspects of physics teaching based on astronomical subjects using a special class of stars, the binary or double stars. The known spectroscopic binary stars are studied for teaching purposes. In special, it is shown how that the application of the Doppler-Fizeau effect can be used to determine physical data such as the velocity of stars. It is also presented the basics of electromagnetic radiation spectrum, stellar spectra, and how these topics can be further discussed using the binary stars as a motivation to learning physics. As a supplementary aspect, it is also discussed the main features of the binary stars in the astrophysical approach and the importance of binary stars to several studies in physics and astronomy

Made available in DSpace on 2014-12-02T11:16:54Z (GMT). No. of bitstreams: 0 Previous issue date: 2014-03-20Bitstream added on 2014-12-02T11:20:56Z : No. of bitstreams: 1 000791753.pdf: 2751318 bytes, checksum: 273e7cbadcaa9ed81c675b41ad4a9fb7 (MD5) Neste trabalho investigamos a evolução orbital de partículas de poeira que escapam das superfícies de ambos satélites pequenos de Plutão: Nix e Hidra, produzidas através do impacto de micrometeoroides nas superfícies dos mesmos, sob influencia da pressão de radiação solar e dos efeitos gravitacionais de Plutão, Caronte e dos próprios satélites fontes (Nix e Hidra). A taxa de produção de massa dos grãos de poeira micrométricos foi obtida e simulações numéricas foram realizadas para obter o tempo de vida destes grãos no sistema plutoniano. Os grãos ejetados de Nix e Hidra formam um anel de largura de aproximadamente 16.000 km, o que corresponde a distância radial aproximada entre as órbitas de Nix e Hidra. Através das integrações numéricas verificamos que colisões de grãos com os corpos massivos do sistema e escape em _orbitas hiperbólicas constituem os principais mecanismos de perda de material e são determinadas pelo efeito da pressão de radiação solar. Este importante mecanismo de desestabilização de grãos de poeira em Plutão, remove 30% do conjunto inicial de partículas com raios de 1 µ em apenas 1 ano. As demais partículas que permanecem no sistema formam um anel muito tênue com uma profundidade óptica máxima de 4x10¹¹. Exploramos também a possibilidade dos progenitores dos satélites pequenos de Plutão terem sido capturados pelo binário Plutão-Caronte do disco primordial massivo heliocêntrico, no qual Plutão estaria inserido. Encontramos que, para objetos com baixas velocidades de aproximação com o binário, capturas poderiam ocorrer com probabilidade não é negligenciável devido a natureza do próprio encontro: planetesimal e par de objetos massivosNo entanto, os objetos capturados permaneceriam em orbitas bastante excêntricas em relação ao baricentro do binário, assim o tempo tópico de ... (Resumo completo, clicar acesso eletrônico abaixo) In this work, we investigate the orbital evolution of the escaping dust from both small satellites of Pluto: Nix and Hydra, produced via impacts of micrometeoroids on the surfaces of the small satellites, under the influence of the solar radiation pressure combined with the gravitational ejects of Pluto, Charon, and the parent-bodies (Nix and Hydra). The mass production rate of micron-sized dust particles is obtained and numerical simulations are performed to derive the lifetime of the ejecta. The ejected particles form a wide ring of about 16,000 km, which corresponds to the radial distance between the orbits of Nix and Hydra. Through the numerical simulations we verified that collisions with the massive bodies within the system and escape into hyperbolic orbits are the main mechanisms of loss of material, which are mainly determined by the solar radiation pressure acting on the grains. This important loss mechanism removes 30% of the initial set of 1 µ sized particles in 1 year. The surviving particles form a ring too faint to be detectable with a derived maximum optical depth of 4x10¹¹. We also explore the possibility that the progenitors of the small satellites of Pluto have been captured by the Pluto-Charon binary from the massive heliocentric planetesimal disk in which Pluto was originally embedded into. Wend that debris with small approximation velocities to the binary can be captured temporarily by Pluto-Charon with non-negligible probability, due to the dynamical perturbations exerted by the binary nature of the Pluto-Charon pair... (Complete abstract click eletronic access below)

Context. NGC6357 is an active star-forming region located in the Sagittarius arm that displays several star clusters, which makes it a very interesting target to investigate star formation and early cluster evolution. Aims. We explore NGC6357 with the VISTA Variables in the Vía Láctea (VVV) photometry of seven embedded clusters (ECs), and one open cluster projected in the outskirts of the complex. Methods. Photometric and structural properties (age, reddening, distance, core, and total radii) of the star clusters were derived. VVV saturated stars were replaced by their 2MASS counterparts. Field-decontaminated VVV photometry was used to analyse colour– magnitude diagrams (CMDs) and stellar radial density profiles and determine astrophysical parameters. Results. We report the discovery of four ECs and one intermediate-age cluster in the complex area. We derive a revised distance estimate for NGC6357 of 1.78 ± 0.1 kpc based on the cluster CMD morphologies. Of the ECs, one contains the binary star WR93, while the remaining ones are dominated by pre-main sequence stars, young stellar objects, and/or and have a developed main sequence. These features reflect a significant age spread among the clusters. Evidence is found that the relatively populous cluster Pismis 24 hosts two subclusters.

Fast magnetic reconnection events can be a very powerful mechanism operating in the core region of microquasars and AGNs. In earlier work, it has been suggested that the power released by fast reconnection events between the magnetic field lines lifting from the inner accretion disk region and the lines anchored into the central black hole could accelerate relativistic particles and produce the observed radio emission from microquasars and low luminosity AGNs (LLAGNs). Moreover, it has been proposed that the observed correlation between the radio emission and the mass of these sources, spanning $10^{10}$ orders of magnitude in mass, might be related to this process. In the present work, we revisit this model comparing two different fast magnetic reconnection mechanisms, namely, fast reconnection driven by anomalous resistivity (AR) and by turbulence (as described in Lazarian and Vishiniac 1999). We apply the scenario above to a much larger sample of sources (including also blazars, and gamma-ray bursts - GRBs), and find that LLAGNs and microquasars do confirm the trend above. Furthermore, when driven by turbulence, not only their radio but also their gamma-ray emission can be due to magnetic power released by fast reconnection, which may accelerate particles to relativistic velocities in the core region of these sources. Thus the turbulent-driven fast reconnection model is able to reproduce better the observed emission than the AR model. On the other hand, the emission from blazars and GRBs does not follow the same trend as that of the LLAGNs and microquasars, suggesting that the radio and gamma-ray emission in these cases is produced further out along the jet, by another population of relativistic particles, as expected., Comment: 22 pages, 8 figures, accepted to be published in the ApJ

We report the discovery of a highly eccentric, double-lined spectroscopic binary star system (TYC 3010-1494-1), comprising two solar-type stars that we had initially identified as a single star with a brown dwarf companion. At the moderate resolving power of the MARVELS spectrograph and the spectrographs used for subsequent radial-velocity (RV) measurements (R ~, 16 pages, 11 figures, 6 tables

We describe the discovery of a likely brown dwarf (BD) companion with a minimum mass of 31.7 +/- 2.0 M_Jup to GSC 03546-01452 from the MARVELS radial velocity survey, which we designate as MARVELS-6b. For reasonable priors, our analysis gives a probability of 72% that MARVELS-6b has a mass below the hydrogen-burning limit of 0.072 M_Sun, and thus it is a high-confidence BD companion. It has a moderately long orbital period of 47.8929 +0.0063/-0.0062 days with a low eccentricty of 0.1442 +0.0078/-0.0073, and a semi-amplitude of 1644 +12/-13 m/s. Moderate resolution spectroscopy of the host star has determined the following parameters: T_eff = 5598 +/- 63, log g = 4.44 +/- 0.17, and [Fe/H] = +0.40 +/- 0.09. Based upon these measurements, GSC 03546-01452 has a probable mass and radius of M_star = 1.11 +/- 0.11 M_Sun and R_star = 1.06 +/- 0.23 R_Sun with an age consistent with less than ~6 Gyr at a distance of 219 +/- 21 pc from the Sun. Although MARVELS-6b is not observed to transit, we cannot definitively rule out a transiting configuration based on our observations. There is a visual companion detected with Lucky Imaging at 7.7 arcsec from the host star, but our analysis shows that it is not bound to this system. The minimum mass of MARVELS-6b exists at the minimum of the mass functions for both stars and planets, making this a rare object even compared to other BDs., Comment: 15 pages, 15 figures, 5 tables. Accepted for publication in The Astronomical Journal

We report the discovery of a candidate brown dwarf (BD) or a very low mass stellar companion (MARVELS-5b) to the star HIP 67526 from the Multi-object Apache point observatory Radial Velocity Exoplanet Large-area Survey (MARVELS). The radial velocity curve for this object contains 31 epochs spread over 2.5 yr.OurKeplerian fit, using a Markov Chain Monte Carlo approach, reveals that the companion has an orbital period of 90.2695+0.0188 −0.0187 days, an eccentricity of 0.4375±0.0040, and a semi-amplitude of 2948.14+16.65 −16.55 m s−1. Using additional high-resolution spectroscopy, we find the host star has an effective temperature Teff = 6004 ± 34 K, a surface gravity log g (cgs) = 4.55 ± 0.17, and a metallicity [Fe/H] = +0.04 ± 0.06. The stellar mass and radius determined through the empirical relationship of Torres et al. yields 1.10±0.09M and 0.92±0.19R . The minimummass ofMARVELS- 5b is 65.0±2.9MJup, indicating that it is likely to be either a BD or a very low mass star, thus occupying a relatively sparsely populated region of the mass function of companions to solar-type stars. The distance to this system is 101 ± 10 pc from the astrometric measurements of Hipparcos. No stellar tertiary is detected in the high-contrast images taken by either FastCam lucky imaging or Keck adaptive optics imaging, ruling out any star with mass greater than 0.2 M at a separation larger than 40 AU.

It is still unclear how common the Sun is when compared to other similar stars in regards to some of its physical properties, such as rotation. Considering that gyrochronology relations are widely used today to estimate ages of stars in the main sequence, and that the Sun is used to calibrate it, it is crucial to assess if these procedures are acceptable. We analyze the rotational velocities -- limited by the unknown rotation axis inclination angle -- of an unprecedented large sample of solar twins in order to study the rotational evolution of Sun-like stars, and assess if the Sun is a typical rotator. We use high-resolution ($R = 115000$) spectra obtained with the HARPS spectrograph and ESO's 3.6 m telescope at La Silla Observatory. The projected rotational velocities for 82 solar twins are estimated by line profile fitting with synthetic spectra. Macroturbulence velocities are inferred from a prescription that accurately reflects their dependence with effective temperature and luminosity of the stars. Our sample of solar twins include some spectroscopic binaries with enhanced rotational velocities, and we do not find any non-spectroscopic binaries with unusually high rotation velocities. We verified that the Sun does not have a peculiar rotation, but the solar twins exhibit rotational velocities that depart from the Skumanich relation. The Sun is a regular rotator when compared to solar twins with a similar age. Additionally, we obtain a rotational braking law that better describes the stars in our sample ($v \propto t^{-0.6}$) in contrast to previous, often-used scalings., Comment: 10 pages, 5 figures, accepted for publication in A&A

Aims. Our goal is to study the circumstellar environment associated with each component of the wide intermediate-mass pre-main sequence binary system PDS 144 using broadband polarimetry. Methods. We present near-infrared (NIR) linear polarimetric observations of PDS 144 gathered with the IAGPOL imaging polarimeter along with the CamIV infrared camera at the Observatorio do Pico dos Dias (OPD). In addition, we re-analyzed OPD archive optical polarization to separate the binary and estimate the interstellar polarization using foreground stars. Results. After discounting the interstellar component, we found that both stars of the binary system are intrinsically polarized. The polarization vectors at optical and NIR bands of both components are aligned with the local magnetic field and the jet axis. These findings indicate an interplay between the interstellar magnetic field and the formation of the binary system. We also found that the PDS 144N is less polarized than its southern companion in the optical. However, in the NIR PDS 144N is more polarized. Our polarization data can only be explained by high inclinations (i 80 ◦ ) for the disks of both members. In particular, comparisons of our NIR data with young stellar objects disk models suggest predominantly small grains in the circumstellar environment of PDS 144N. In spite of the different grain types in each component, the infrared spectral indexes indicate a coeval system. We also found evidence of coplanarity between the disks.

TYC 4110-01037-1 has a low-mass stellar companion, whose small mass ratio and short orbital period are atypical among binary systems with solar-like (Teff 6000 K) primary stars. Our analysis of TYC 4110-01037-1 reveals it to be a moderately aged ( 5 Gyr) solar-like star having a mass of 1.07 ± 0.08M and radius of 0.99 ± 0.18 R . We analyze 32 radial velocity (RV) measurements from the SDSS-III MARVELS survey as well as 6 supporting RV measurements from the SARG spectrograph on the 3.6 m Telescopio Nazionale Galileo telescope obtained over a period of ∼2 years. The best Keplerian orbital fit parameters were found to have a period of 78.994 ± 0.012 days, an eccentricity of 0.1095 ± 0.0023, and a semi-amplitude of 4199 ± 11 m s−1. We determine the minimum companion mass (if sin i = 1) to be 97.7 ± 5.8 MJup. The system’s companion to host star mass ratio, 0.087 ± 0.003, places it at the lowest end of observed values for short period stellar companions to solar-like (Teff 6000 K) stars. One possible way to create such a system would be if a triple-component stellar multiple broke up into a short period, low q binary during the cluster dispersal phase of its lifetime. A candidate tertiary body has been identified in the system via single-epoch, high contrast imagery. If this object is confirmed to be comoving, we estimate it would be a dM4 star. We present these results in the context of our larger-scale effort to constrain the statistics of low-mass stellar and brown dwarf companions to FGK-type stars via the MARVELS survey.

Context: CoRoT is a pioneering space mission whose primary goals are stellar seismology and extrasolar planets search. Its surveys of large stellar fields generate numerous planetary candidates whose lightcurves have transit-like features. An extensive analytical and observational follow-up effort is undertaken to classify these candidates. Aims: The list of planetary transit candidates from the CoRoT LRa01 star field in the Monoceros constellation towards the Galactic anti-center is presented. The CoRoT observations of LRa01 lasted from 24 October 2007 to 3 March 2008. Methods: 7470 chromatic and 3938 monochromatic lightcurves were acquired and analysed. Instrumental noise and stellar variability were treated with several filtering tools by different teams from the CoRoT community. Different transit search algorithms were applied to the lightcurves. Results: Fifty-one stars were classified as planetary transit candidates in LRa01. Thirty-seven (i.e., 73 % of all candidates) are "good" planetary candidates based on photometric analysis only. Thirty-two (i.e., 87 % of the "good" candidates) have been followed-up. At the time of this writing twenty-two cases have been solved and five planets have been discovered: three transiting hot-Jupiters (CoRoT-5b, CoRoT-12b, and CoRoT-21b), the first terrestrial transiting planet (CoRoT-7b), and another planet in the same system (CoRoT-7c, detected by radial velocity survey only). Evidences of another non-transiting planet in the CoRoT-7 system, namely CoRoT-7d, have been recently found., Comment: 30 pages, 15 figures, accepted for publication in A&A, Section 14 "Catalogs and Data"

Context. Spectroscopic binaries are important in studies of binary star-formation, since original information linked to the forming stars, such as the mass ratio, can be kept in the system and observationally retrieved. Aims. We aim to derive the more probable mass ratio distributions, to test a possible Initial Mass Functions acting on the formation of spectroscopic binaries, and to derive the value of q0, the minimal mass ratio for which both binary components are simultaneously in the main sequence. Methods. The sample has 249 systems, selected from the Ninth Catalogue of Spectroscopic Binaries, with selection criteria that include only those systems with periods longer than four days and magnitudes brighter than mV = 7.0, and main-sequence primaries, to avoid mass exchange. Monte Carlo simulations of observational parameters were performed.We performed simulations by comparing an observational quantity, Yobs, equal to f (m)/m1, where f (m) is the mass function, to theoretical Yth, which is a function of the system inclination (assumed to be random), and of the mass ratio q = m2/m1. We considered a minimal mass ratio q0, which is the limiting condition, to ensure that both components are simultaneously on main sequence, avoiding evolutionary effects leading to mass exchange and loss of information on the original masses. Calculations were done using several q0 values. The form of the distribution of mass ratios was studied by simulations of Y, involving several functions, with slopes which are increasing, decreasing, constant, or bimodal. Other simulations were done by the generation of stars following several initial mass function laws. We combined pairs of stars generated from different IMFs, assuming that binaries arise from the pairing of stars simultaneously formed. In this case, several values of q0 were also used in the input equations. Results. Our results indicate that decreasing q distributions match better with observations and also that a unique IMF gives better fits to observations than a composite IMF, with a slope around 1.4, rather than the widely used 2.35 slope. Values of q0 tend to be lower than what is suggested by previous studies, which point to q0 around 0.2; all simulations suggest that q0 values as low as 0.08 produce the best fits to observations.

In the present work, we have studied the nature of the physical processes of the coronal heating, considering as basis significant samples of single and binary evolved stars, that have been achieved with the ROSAT satellite. In a total of 191 simple stars were studied, classified in the literature as giants with spectral type F, G and K. The results were compared with those obtained from 106 evolved stars of spectral type F, G and K, which belong to the spectroscopic binary systems. Accurate measurements on rotation and information about binarity were obtained from De Medeiros s catalog. We have analysed the behavior of the coronal activity in function of diverse stellar parameters. With the purpose to better clarify the profile of the stars evolution, the HR diagram was built for the two samples of stars, the single and the binary ones. The evolved traces added in the diagram were obtained from the Toulouse-Geneve code, Nascimento et al. (2000). The stars were segregated in this diagram not only in range of rotational speed but also in range of X-ray flux. Our analysis shows clearly that the single stars and the binary ones have coronal activity controlled by physical process independent on the rotation. Non magnetic processes seem to be strongly influencing the coronal heating. For the binary stars, we have also studied the behavior of the coronal emission as a function of orbital parameters, such as period and eccentricity, in which it was revealed the existence of a discontinuity in the emission of X-rays around an orbital period of 100 days. The study helped to conclude that circular orbits of the binary stars are presented as a necessary property for the existence of a higher level ofX-rays emission, suggesting that the effect of the gravitational tide has an important role in the coronal activity level. When applied the Kolmogorov-Smirnov test (KS test ) for the Vsini and FX parameters to the samples of single and binary stars, we could evidence very relevant aspects for the understanding of the mechanisms inherent to the coronal activity. For the Vsini parameter, the differences between the single stars and the binary ones for rotation over 6.3 km/s were really remarkable. We believe, therefore, that the existence of gravitational tide is, at least, one of the factors that most contribute for this behavior. About the X-rays flux, the KS test showed that the behavior of the single and the binary stars, regarding the coronal activity, comes from the same origin No presente trabalho, estudamos a natureza dos processos físicos do aquecimento coronal, tomando como base amostras significativas de estrelas evoluídas simples e binárias, obtidas com o satélite ROSAT. No total, foram estudadas 191 estrelas simples, classificadas na literatura como gigantes com tipo espectral F, G e K. Os resultados foram comparados com os obtidos das 106 estrelas evoluídas de tipo espectral F, G e K pertencentes a sistemas binários espectroscópicos. Medidas precisas sobre rotação e informações sobre binaridade foram obtidas do catálogo de De Medeiros. Analisamos o comportamento da atividade coronal em função de diversos parâmetros estelares. Para uma maior clareza do perfil de evolução das estrelas foi construído o diagrama HR para as duas amostras de estrelas, as simples e as binárias. Os traçados evolutivos adicionados nos diagramas foram obtidos a partir do código de Toulouse-Geneve, Nascimento et al. (2000). As estrelas foram segregadas nesse diagrama tanto por intervalo de velocidade rotacional como por intervalo de fluxo de raio-X. Nossa análise mostra claramente que as estrelas simples e as binárias possuem atividade coronal dominada por processo físico independente da rotação. Processos não magnéticos parecem estar influenciando fortemente o aquecimento coronal. Para as estrelas binárias, estudamos ainda o comportamento da emissão coronal em função de parâmetros orbitais, como período e excentricidade, tendo sido revelada a existência de uma descontinuidade na emissão de raio-X em torno de um período orbital de 100 dias. O estudo permitiu concluir que a circularização das órbitas das estrelas bináriasapresenta-se como uma propriedade necessária para a existência de um nível de emissão de raio-X mais elevado, sugerindo que efeito de maré gravitacional tem importante papel no nível de atividade coronal. Ao aplicarmos o teste Kolmogorov-Smirnov (teste KS) para os parâmetros Vsini e FX às amostras de estrelas simples e binárias, evidenciamos aspectos bastante relevantes para a compreensão dos mecanismos inerentes à atividade coronal. Para a grandeza Vsini, ficaram marcantes as diferenças existentes entre as estrelas simples e as binárias para rotações acima de 6,3 km/s. Acreditamos, pois, ser a existência de maré gravitacional, pelo menos, um dos fatores que contribui para esse comportamento. Quanto ao fluxo de raio-X, o teste KS demonstrou que o comportamento das estrelas simples e binárias, no que diz respeito à atividade coronal, provém de uma mesma origem

New and existing X-ray, UBVJHKsW(1 - 4), and spectroscopic observations were analyzed to constrain fundamental parameters for M 25, NGC 7790, and dust along their sight-lines. The star clusters are of particular importance because they host the classical Cepheids U Sgr, CF Cas, and the visual binary Cepheids CEa and CEb Cas. Precise results from the multiband analysis, in tandem with a comprehensive determination of the Cepheids' period evolution (dP/dt) from ~140 years of observations, helped to resolve concerns raised regarding the clusters and their key Cepheid constituents. Specifically, the distances derived for members of M 25 and NGC 7790 are 630 ± 25 pc and 3.40 ± 0.15 kpc, respectively.

The periodic spectroscopic events in η Carinae are now well established and occur near the periastron passage of two massive stars in a very eccentric orbit. Several mechanisms have been proposed to explain the variations of different spectral features, such as an eclipse by the wind-wind collision (WWC) boundary, a shell ejection from the primary star or accretion of its wind onto the secondary. All of them have problems explaining all the observed phenomena. To better understand the nature of the cyclic events, we performed a dense monitoring of η Carinae with five Southern telescopes during the 2009 low-excitation event, resulting in a set of data of unprecedented quality and sampling. The intrinsic luminosity of the He II λ4686 emission line (L 310 L) just before periastron reveals the presence of a very luminous transient source of extreme UV radiation emitted in the WWC region. Clumps in the primary's wind probably explain the flare-like behavior of both the X-ray and He II λ4686 light curves. After a short-lived minimum, He II λ4686 emission rises again to a new maximum, when X-rays are still absent or very weak. We interpret this as a collapse of the WWC onto the "surface" of the secondary star, switching off the hard X-ray source and diminishing the WWC shock cone. The recovery from this state is controlled by the momentum balance between the secondary's wind and the clumps in the primary's wind., Facultad de Ciencias Astronómicas y Geofísicas

We report the discovery, in the course of a survey for high-latitude, ultraviolet-excess objects in the southern hemisphere, of two new hybrid DAB stars. Both objects exhibit a weak He I λ4471 line superposed onto the usual Balmer line spectrum. Analyses of the Balmer series with both homogeneous and stratified models show these stars to have spectroscopic effective temperatures around 27,000 K. However, low­ dispersion ultraviolet observations with the lU E reveal energy distributions considerably flatter than those of normal DA stars near that effective temperature. These energy distributions are inconsistent as well as those of stratified models calculated at the temperature and hydrogen-layer mass determined from the optical spectrum. We demonstrate instead that consistent fits to the optical and ultraviolet spectrum, as well as to the energy distribution, can be achieved if these objects are unresolved, composite systems consisting of a DA white dwarf together with a DB or DBA star. The DAB spectral class thus appears inhomogeneous, and may contain both composite systems and stratified stars.

Recent interest in the variable (DAV) white dwarf G29-38 has been stirred by a tentative report of a radial velocity variation that may be due to an unseen companion. Earlier evidence for a brown dwarf in the system has come from an observed infrared excess in the star's spectrum. For asteroseismological reasons, we have accumulated more than five seasons of high-speed photometric data on the star. By measuring the phase of an isolated, stable frequency in the power spectrum, we show the measured variation is not due (at least in its entirety) to an orbital companion. Because any orbital radial velocity variation must result in a systematic phase variation, the data we present can be used to place stringent limits on the types of companions the system may contain.

Using an extensive set of high-speed photometric observations obtained with the Whole Earth Telescope network, we show that the complex light curve of the ZZ Ceti (DAV) star G29-38 is dominated by a single, constant amplitude period of 615 s during the time span of our observations. The pulse arrival times for this period exhibit a systematic variation in phase readily explained by light-travel time effects produced by reflex orbital motion about an unseen companion. Our best-fit model to the observations indicates a highly eccen­ tric orbit, a period of 109 ± 13 days and a minimum mass of 0.5 Mʘ for the companion. Radial velocity variations predicted by this model are not observed, however, nor are these phase variations seen in another independent pulsation, so the origin of the phase variation remains a mystery. Any model involving intrinsic pulsation mechanisms must explain the large (~200 s) phase change with no corresponding change in pulsation amplitude, and it shape, which mimics quite exactly the effects of binary orbital motion.