22 research outputs found

    Estudo dos envoltórios de estrelas OH/IR

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    O objetivo dessa monografia é o estudo dos envoltórios de estrelas OH/IR. Tais objetos apresentam emissão maser da molécula OH localizadas nas regiões mais externas dos envoltórios circunstelares. A linha em 1612 MHz é a mais proeminente e apresenta um perfil de duplo pico (delta v ~ 20-50 Km/s). A duração dessa emissão é de aproximadamente 1700 anos e, ela deve permanecer detectável em envoltórios de poeira ricos em oxigênio até a fase pós-AGB. Em um artigo de 2002, Lewis discute que o fenômeno de desaparecimento do maser OH é um fenômeno comum e apresenta 4 estrelas que "perderam" o maser OH e uma que está perdendo. O autor sugere que estas estrelas ainda estão sobre o AGB e que a emissão do maser OH seja periódica e esteja ligada a fase de pulsos térmicos. Para isso apresenta também 3 estrelas com detecção recente do maser OH, as OH/IR recém-nascidas. Nesse trabalho modelamos as estrelas da amostra de Lewis (2002) e também apresentamos uma nova amostra de estrelas que estão sofrendo variações significativas em suas emissões maser OH. Além disso, estudamos também, em uma amostra diferente, a existência de silicatos cristalinos juntamente com silicatos amorfos nos envoltórios circunstelares de poeira. Calculamos a razão entre as duas espécies e verificamos que essa razão é mais alta que a esperada para objetos que estão sobre o AGB. Os modelos utilizados nesse trabalho tratam o problema da transferência radiativa em envoltórios ricos em poeira via simulação numérica de tipo Monte Carlo

    High-resolution observations of gas and dust around Mira using ALMA and SPHERE/ZIMPOL

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    The outflows of oxygen-rich asymptotic giant branch (AGB) stars are thought to be driven by radiation pressure due to the scattering of photons on relatively large grains, with sizes of tenths of microns. The details of the formation of dust in the extended atmospheres of these stars and, therefore, the mass-loss process, is still not well understood. Aims. We aim to constrain the distribution of the gas and the composition and properties of the dust grains that form in the inner circumstellar environment of the archetypal Mira variable o Cet. Methods. We obtained quasi-simultaneous observations using ALMA and SPHERE/ZIMPOL on the Very Large Telescope (VLT) to probe the distribution of gas and large dust grains, respectively. Results. The polarized light images show dust grains around Mira A, but also around the companion, Mira B, and a dust trail that connects the two sources. The ALMA observations show that dust around Mira A is contained in a high-gas-density region with a significant fraction of the grains that produce the polarized light located at the edge of this region. Hydrodynamical and wind-driving models show that dust grains form efficiently behind shock fronts caused by stellar pulsation or convective motions. The distance at which we observe the density decline (a few tens of au) is, however, significantly larger than expected for stellar-pulsation-induced shocks. Other possibilities for creating the high-gas-density region are a recent change in the mass-loss rate of Mira A or interactions with Mira B. We are not able to determine which of these scenarios is correct. We constrained the gas density, temperature, and velocity within a few stellar radii from the star by modelling the CO v = 1, J = 3-2 line. We find a mass (~3.8 \ub1 1.3) 7 104 M to be contained between the stellar millimetre photosphere, R338 GHz, and 4 R338 GHz. Our best-fit models with lower masses also reproduce the 13CO v = 0, J = 3-2 line emission from this region well. We find TiO2 and AlO abundances corresponding to 4.5% and <0.1% of the total titanium and aluminium expected for a gas with solar composition. The low abundance of AlO allows for a scenario in which Al depletion into dust happens already very close to the star, as expected from thermal dust emission observations and theoretical calculations of Mira variables. The relatively large abundance of aluminium for a gas with solar composition allows us to constrain the presence of aluminium oxide grains based on the scattered light observations and on the gas densities we obtain. These models imply that aluminium oxide grains could account for a significant fraction of the total aluminium atoms in this region only if the grains have sizes 0.02 μm. This is an order of magnitude smaller than the maximum sizes predicted by dust-formation and wind-driving models. Conclusions. The study we present highlights the importance of coordinated observations using different instruments to advance our understanding of dust nucleation, dust growth, and wind driving in AGB stars

    Modelling the carbon AGB star R Sculptoris: Constraining the dust properties in the detached shell based on far-infrared and sub-millimeter observations

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    Context. On the asymptotic giant branch (AGB), Sun-like stars lose a large portion of their mass in an intensive wind and enrich the surrounding interstellar medium with nuclear processed stellar material in the form of molecular gas and dust. For a number of carbon-rich AGB stars, thin detached shells of gas and dust have been observed. These shells are formed during brief periods of increased mass loss and expansion velocity during a thermal pulse, and open up the possibility to study the mass-loss history of thermally pulsing AGB stars. Aims. We study the properties of dust grains in the detached shell around the carbon AGB star R Scl and aim to quantify the influence of the dust grain properties on the shape of the spectral energy distribution (SED) and the derived dust shell mass. Methods. We modelled the SED of the circumstellar dust emission and compared the models to observations, including new observations of Herschel/PACS and SPIRE (infrared) and APEX/LABOCA (sub-millimeter). We derived present-day mass-loss rates and detached shell masses for a variation of dust grain properties (opacities, chemical composition, grain size, and grain geometry) to quantify the influence of changing dust properties to the derived shell mass. Results. The best-fitting mass-loss parameters are a present-day dust mass-loss rate of 2 7 10-10M⊙yr-1and a detached shell dust mass of (2.9 \ub1 0.3) 7 10-5M⊙. Compared to similar studies, the uncertainty on the dust mass is reduced by a factor of 4. We find that the size of the grains dominates the shape of the SED, while the estimated dust shell mass is most strongly affected by the geometry of the dust grains. Additionally, we find a significant sub-millimeter excess that cannot be reproduced by any of the models, but is most likely not of thermal origin

    CO envelope of the symbiotic star R Aquarii seen by ALMA

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    The symbiotic star R Aqr is part of a small sample of binary AGB stars observed with the Atacama Large Millimeter/submillimeter Array (ALMA). The sample stars are: R Aqr, Mira, W Aql, and π1Gru. The sample covers a range in binary separation and wind properties, where R Aqr is the source with the smallest separation. The R Aqr binary pair consists of an M-type AGB star and a white dwarf at a separation of 45 mas, equivalent to about 10 AU at 218 pc. The aim of the ALMA study is to investigate the dependence of the wind shaping on the binary separation and to provide constraints for hydrodynamical binary interaction models. R Aqr is particularly interesting as the source with the smallest separation and a complex circumstellar environment that is strongly affected by the interaction between the two stars and by the high-energy radiation resulting from this interaction and from the hot white dwarf companion. The CO(J = 3 →2) line emission has been observed with ALMA at ∼0.5′′ spatial resolution. The CO envelope around the binary pair is marginally resolved, showing what appears to be a rather complex distribution. The outer radius of the CO emitting region is estimated from the data and found to be about a factor of 10 larger than previously thought. This implies an average mass-loss rate during the past ∼100 yr of 2 710-7M∗yr-1, a factor of 45 less than previous estimates. The channel maps are presented and the molecular gas distribution is discussed and set into the context of what was previously known about the system from multiwavelength observations. Additional molecular line emission detected within the bandwidth covered by the ALMA observations is also presented. Because of the limited extent of the emission, firm conclusions about the dynamical evolution of the system will have to wait for higher spatial resolution observations. However, the data presented here support the assumption that the mass-loss rate from the Mira star strongly varies and is focused on the orbital plane

    Constraints on the H2O formation mechanism in the wind of carbon-rich AGB stars

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    Context. The recent detection of warm H2_2O vapor emission from the outflows of carbon-rich asymptotic giant branch (AGB) stars challenges the current understanding of circumstellar chemistry. Two mechanisms have been invoked to explain warm H2_2O vapor formation. In the first, periodic shocks passing through the medium immediately above the stellar surface lead to H2_2O formation. In the second, penetration of ultraviolet interstellar radiation through a clumpy circumstellar medium leads to the formation of H2_2O molecules in the intermediate wind. Aims. We aim to determine the properties of H2_2O emission for a sample of 18 carbon-rich AGB stars and subsequently constrain which of the above mechanisms provides the most likely warm H2_2O formation pathway. Methods, Results, and Conclusions. See paper

    The shock-heated atmosphere of an asymptotic giant branch star resolved by ALMA

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    Our current understanding of the chemistry and mass-loss processes in solar-like stars at the end of their evolution depends critically on the description of convection, pulsations and shocks in the extended stellar atmosphere. Three-dimensional hydrodynamical stellar atmosphere models provide observational predictions, but so far the resolution to constrain the complex temperature and velocity structures seen in the models has been lacking. Here we present submillimeter continuum and line observations that resolve the atmosphere of the asymptotic giant branch star W Hya. We show that hot gas with chromospheric characteristics exists around the star. Its filling factor is shown to be small. The existence of such gas requires shocks with a cooling time larger than commonly assumed. A shocked hot layer will be an important ingredient in the models of stellar convection, pulsation and chemistry that underlie our current understanding of the late stages of stellar evolution.Comment: 30 pages, 9 figures, including Supplementary information. Author manuscript version before editorial/copyediting by Nature Astronomy. Journal version available via http://rdcu.be/xUW

    Chemical content of the circumstellar envelope of the oxygen-rich AGB star R Doradus: Non-LTE abundance analysis of CO, SiO, and HCN

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    Context. The stellar outflows of low- to intermediate-mass stars are characterised by a rich chemistry. Condensation of molecular gas species into dust grains is a key component in a chain of physical processes that leads to the onset of a stellar wind. In order to improve our understanding of the coupling between the micro-scale chemistry and macro-scale dynamics, we need to retrieve the abundance of molecules throughout the outflow. Aims. Our aim is to determine the radial abundance profile of SiO and HCN throughout the stellar outflow of R Dor, an oxygen-rich AGB star with a low mass-loss rate. SiO is thought to play an essential role in the dust-formation process of oxygen-rich AGB stars. The presence of HCN in an oxygen-rich environment is thought to be due to non-equilibrium chemistry in the inner wind. Methods. We analysed molecular transitions of CO, SiO, and HCN measured with the APEX telescope and all three instruments on the Herschel Space Observatory, together with data available in the literature. Photometric data and the infrared spectrum measured by ISO-SWS were used to constrain the dust component of the outflow. Using both continuum and line radiative transfer methods, a physical envelope model of both gas and dust was established. We performed an analysis of the SiO and HCN molecular transitions in order to calculate their abundances. Results. We have obtained an envelope model that describes the dust and the gas in the outflow, and determined the abundance of SiO and HCN throughout the region of the stellar outflow probed by our molecular data. For SiO, we find that the initial abundance lies between 5.5 7 10 -5 and 6.0 7 10 -5 with respect to H 2 . The abundance profile is constant up to 60 \ub1 10 R, after which it declines following a Gaussian profile with an e-folding radius of 3.5 \ub1 0.5 7 10 13 cm or 1.4 \ub1 0.2 R. For HCN, we find an initial abundance of 5.0 7 10 -7 with respect to H 2 . The Gaussian profile that describes the decline starts at the stellar surface and has an e-folding radius r e of 1.85 \ub1 0.05 7 10 15 cm or 74 \ub1 2 R. Conclusions. We cannot unambiguously identify the mechanism by which SiO is destroyed at 60 \ub1 10 R. The initial abundances found are higher than previously determined (except for one previous study on SiO), which might be due to the inclusion of higher-J transitions. The difference in abundance for SiO and HCN compared to high mass-loss rate Mira star IK Tau might be due to different pulsation characteristics of the central star and/or a difference in dust condensation physics

    Maser emission from the CO envelope of the asymptotic giant branch star W Hydrae

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    Context. Observation of CO emission around asymptotic giant branch (AGB) stars is the primary method to determine gas mass-loss rates. While radiative transfer models have shown that molecular levels of CO can become mildly inverted, causing maser emission, CO maser emission has yet to be confirmed observationally. Aims. High-resolution observations of the CO emission around AGB stars now have the brightness temperature sensitivity to detect possible weak CO maser emission. Methods. We used high angular resolution observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the small-scale structure of CO J = 3-2 emission around the oxygen-rich AGB star W Hya. Results. We find CO maser emission amplifying the stellar continuum with an optical depth tau approximate to-0.55. The maser predominantly amplifies the limb of the star because CO J = 3-2 absorption from the extended stellar atmosphere is strongest towards the centre of the star. Conclusions. The CO maser velocity corresponds to a previously observed variable component of high-frequency H2O masers and with the OH maser that was identified as the amplified stellar image. This implies that the maser originates beyond the acceleration region and constrains the velocity profile since we find the population inversion primarily in the inner circumstellar envelope. We find that inversion can be explained by the radiation field at 4.6 mu m and that the existence of CO maser emission is consistent with the estimated mass-loss rates for W Hya. However, the pumping mechanism requires a complex interplay between absorption and emission lines in the extended atmosphere. Excess from dust in the circumstellar envelope of W Hya is not sufficient to contribute significantly to the required radiation field at 4.6 mu m. The interplay between molecular lines that cause the pumping can be constrained by future multi-level CO observations

    Imaging the dust and the gas around Mira using ALMA and SPHERE/ZIMPOL

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    The mass-loss mechanism of asymptotic giant branch stars has long been thought to rely on two processes: Stellar pulsations and dust formation. The details of the mass-loss mechanism have remained elusive, however, because of the overall complexity of the dust formation process in the very dynamical pulsation-enhanced atmosphere. Recently, our understanding of AGB stars and the associated mass loss has evolved significantly, thanks both to new instruments which allow sensitive and high-angular-resolution observations and the development of models for the convective AGB envelopes and the dust formation process. ALMA and SPHERE/ZIMPOL on the VLT have been very important instruments in driving this advance in the last few years by providing high-angular resolution images in the sub-mm and visible wavelengths, respectively. I will present observations obtained using these instruments at the same epoch (2.5 weeks apart) of the AGB star Mira that resolve even the stellar disk. The ALMA data reveals the distribution and dynamics of the gas around the star, while the polarised light imaged using SPHERE shows the distribution of the dust grains expected to drive the outflows. Moreover, the observations show a central source surrounded by asymmetric distributions of gas and dust, with complementary structures seen in the two components. We model the observed CO v = 1, J = 3-2 line to determine the density, temperature and velocity of gas close to the star. This model is then used to estimate the abundance of AlO. Our results show that only a very small fraction of aluminium (‰20.1%) is locked in AlO molecules. We also calculate models to fit the observed polarised light based on the gas densities we find. The low level of visible-light polarisation detected using ZIMPOL implies that, at the time of the observations, aluminium atoms are either not efficiently depleted into dust or the aluminium-oxide grains are relatively small (‰20.02μm)