Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userShells of dust around AGB stars: effects on the integrated spectrum of Single Stellar Populations
66
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
In this paper we present models for SSPs of intermediate and old ages where dust enshrouded AGB stars are introduced. To this aim, particular care is devoted to follow the evolution of the AGB stars throughout the quiet and thermally pulsing regimes, to evaluate the effect of self contamination in the outermost layers by the third dredge-up mechanism, to follow the transition from oxygen-rich to carbon-rich objects, and finally to estimate the efficiency of mass-loss by stellar winds. In addition to this, accurate physical models of the dusty shells are presented in which the re-processing of radiation from the central stars is calculated. The resulting spectral energy distribution (SED) is examined to show how important features evolve with time. The SEDs are then convolved with the IRAS filters to obtain the flux in various pass-bands for individual AGB stars of different, mass, chemical composition, and age. The comparison is made by means of SSPs along which AGB stars of the same age but different initial masses are located. The theoretical results are compared to the observational data for selected groups of stars. The same is made for the J,H,K,L pass-bands of the Johnson system. Finally, from the integrated SEDs of the SSPs, we derive the integrated Johnson J,H,K,L magnitudes and colors to be compared to infrared data for star clusters of the Magellanic Clouds.
rate research
Read More
The evolution of AGB stars is notoriously complex. The confrontation of AGB population models with observed stellar populations is a useful alternative to the detailed study of individual stars in efforts to converge towards a reliable evolution theory. I review here the impact of studies of star clusters on AGB models and AGB population synthesis, deliberately leaving out any more complex stellar populations. Over the last 10 years, despite much effort, the absolute uncertainties in the predictions of the light emitted by intermediate age populations have not been reduced to a satisfactory level. Observational sample definitions, as well as the combination of the natural variance in AGB properties with small number statistics, are largely responsible for this situation. There is hope that the constraints may soon become strong enough, thanks to large unbiased surveys of star clusters, resolved colour-magnitude diagrams, and new analysis methods that can account for the stochastic nature of AGB populations in clusters.
Detached shells are believed to be created during a thermal pulse, and constrain the time scales and physical properties of one of the main drivers of late stellar evolution. We aim at determining the morphology of the detached dust shells around the carbon AGB stars R Scl and V644 Sco, and compare this to observations of the detached gas shells. We observe the polarised, dust-scattered stellar light around these stars using the PolCor instrument mounted on the ESO 3.6m telescope. Observations were done with a coronographic mask to block out the direct stellar light. The polarised images clearly show the detached shells. Using a dust radiative transfer code to model the dust-scattered polarised light, we constrain the radii and widths of the shells to 19.5 arcsec and 9.4 arcsec for the detached dust shells around R Scl and V644 Sco, respectively. Both shells have an overall spherical symmetry and widths of approx. 2 arcsec. For R Scl we can compare the observed dust emission directly with high spatial-resolution maps of CO(3-2) emission from the shell observed with ALMA. We find that the dust and gas coincide almost exactly, indicating a common evolution. The data presented here for R Scl are the most detailed observations of the entire dusty detached shell to date. For V644 Sco these are the first direct measurements of the detached shell. Also here we find that the dust most likely coincides with the gas shell. The observations are consistent with a scenario where the detached shells are created during a thermal pulse. The determined radii and widths will constrain hydrodynamical models describing the pre-pulse mass loss, the thermal pulse, and post-pulse evolution of the star.
Dust is formed in the expanding atmosphere during late stages of stellar evolution. Dust influences the dynamics and thermodynamics of the stellar atmosphere by its opacity. The dust opacity depends both on the optical properties of the grain material as well as on the amount of dust present. A rich source of information on some mineral phases of dust in AGB stars comes from the study of presolar grains from meteorites. This paper presents a short overview of presolar grains studies and describes how the optical properties of dust grains are obtained in the laboratory.
Far-infrared Herschel/PACS images at 70 and 160 micron of a sample of 78 Galactic evolved stars are used to study the (dust) emission structures, originating from stellar wind-ISM interaction. In addition, two-fluid hydrodynamical simulations of the coupled gas and dust in wind-ISM interactions are used to compare with the observations. Four distinct classes of wind-ISM interaction (i.e. fermata, eyes, irregular, and rings) are identified and basic parameters affecting the morphology are discussed. We detect bow shocks for ~40% of the sample and detached rings for ~20%. De-projected stand-off distances (R_0) -- defined as the distance between the central star and the nearest point of the interaction region -- of the detected bow shocks (fermata and eyes) are derived from the PACS images and compared to previous results, model predictions and the simulations. All observed bow shocks have stand-off distances smaller than 1 pc. Observed and theoretical stand-off distances are used together to independently derive the local ISM density. Both theoretical (analytical) models and hydrodynamical simulations give stand-off distances for adopted stellar properties that are in good agreement with the measured de-projected stand-off distance of wind-ISM bow shocks. The possible detection of a bow shock -- for the distance limited sample -- appears to be governed by its physical size as set roughly by the stand-off distance. In particular the stars peculiar space velocity and the density of the ISM appear decisive in detecting emission from bow shocks or detached rings. Tentatively, the eyes class objects are associated to (visual) binaries, while the rings generally appear not to occur for M-type stars, only for C or S-type objects that have experienced a thermal pulse.
We present first results from an on-going survey of the stellar populations of the bulges and inner disks of spirals at various points along the Hubble sequence. In particular, we are investigating the hypotheses that bulges of early-type spirals are akin to (and may in fact originally have been) intermediate-luminosity ellipticals while bulges of late-type spirals are formed from dynamical instabilities in their disks. Absorption-line spectroscopy of the central regions of Sa--Sd spirals is combined with stellar population models to determine integrated mean ages and metallicities. These ages and metallicities are used to investigate stellar population differences both between the bulges and inner disks of these spirals and between bulges and ellipticals in an attempt to place observational constraints on the formation mechanisms of spiral bulges.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
