In the last decade, we have been able to observe the envelopes of Asymptotic Giant Branch and Red Supergiant stars by several different means. Many aspects of the evolution of these cool objects have seen several breakthroughs, thanks also to the high angular resolution observations able to reach down to the stellar surface. However the spatially resolved observations were for many years only limited to very few points across the stellar discs. Asymmetric structures have been detected, but their interpretation has been difficult and highly non-unique. With the advancement in the instrumentations and with the increasing amount of apertures available, the new generation of interferometers allows today to reconstruct the brightness distribution of the surface of these stars with incredible details. Convective patterns and asymmetric ejections of the size of the astronomical unit can be finally characterised. 3D models are slowly catching up, and are now ready to be challenged by these new observations. In this presentation I will review the results achieved by using high angular resolution techniques in the infrared. I will discuss the recently obtained images of the stellar surface of AGBs and RSGs, and I will conclude highlighting how our field of research will benefit from the advent of the second generation of VLTI instruments. There is enough to keep us busy for the next ten years.
3D models of AGB stars have revealed complex structures, with self-excited pulsations. To fully understand the implications of different stellar parameters a grid of models of AGB stars, based on 3D star-in-a-box simulations using CO5BOLD, has been calculated. This grid contains 8 models with effective temperatures ranging from 2500K to 2800K and luminosities from 5000 to 10000 solar luminosities. The mass of all models is kept at one solar mass. With several models of different stellar parameters it is possible to extract pulsation properties, such as period and radius, and investigate the resulting trends. This is used to compare with observations and corresponding properties of 1D models, where good consistency is found. These results indicate that the 3D models give a satisfactory description of the stellar interior, and could be used to investigate the interplay between self-excited pulsation, shocks and dust creation.
We present near-infrared $H$ band interferometric imaging observations of the carbon AGB star R Scl with the goals of (1) constraining and testing available dynamic atmosphere and wind models by near-infrared interferometry, (2) revealing the detailed morphology of the stellar atmosphere and innermost mass-loss region, and (3) constraining fundamental stellar properties of R Scl, which may further constrain the binary companion model. Our images reveal one dominant surface spot and a spiral-like surface structure, which may be caused by large-scale convection cells and/or localized mass loss.
Time-dependent wind models have successfully been able to reproduce both dynamical and photometric observations of C-type and M-type AGB stars in the Milky Way (e.g. Eriksson et al. 2014, Bladh et al. 2015). In the dynamical wind models for C-type AGB stars the outflows are predominately driven by photon absorption on amorphous carbon grains and in the models for M- type AGB stars the stellar winds are triggered by photon-scattering on Fe-free silicates. What happens to the wind properties of AGB stars in a low metallicity environment such as the LCM or SMC? C-type AGB stars produce their own carbon during the AGB phase and the mass-loss rates should therefore not be significantly affected by metallicity. The outflows in M-type AGB stars, however, are driven by dust material consisting of elements that cannot be produced by the stars themselves. A low metallicity environment could for that reason have a strong impact on the mass loss of these stars. In order to investigate the properties of M-type AGB stars in a low metallicity environment we here report the first tentative results from a set of wind models for M-type AGB stars with metallicity similar to that of the LMC. The grid is set-up much like the previous grid of wind models with solar metallicity (Bladh et al. 2015). The dynamical properties of this set verify the assumption that a low metallicity environment affects the mass loss of M-type AGB stars: the models produce stellar winds only for very luminous AGB stars, the resulting wind velocities are very low and the mass-loss rates span a narrow range. Current measurements of the mass-loss rates of M-type AGB stars in LMC, although uncertain, indicate lower mass-loss rates and higher wind velocities (Gullieuszik et al. 2012).
Radio and submillimetre interferometry has enabled the study of Asymptotic Giant Branch (AGB) and Red Supergiant (RSG) stars from the stellar photosphere to the outer wind, at up to sub-milliarcsecond resolution. In this talk, I will focus on VLBI and ALMA observations of the gas dynamics of the stellar extended atmospheres / inner circumstellar envelopes (CSEs), where shocks are believed to play an important role in levitating gas to larger radii, enabling dust formation. I will also show submm continuum and maser observations that provide indications of asymmetry, clumpiness and inhomogeneity in the inner CSE, with mass loss occurring in localised directions. Finally, I will discuss evidence for dynamically important magnetic fields permeating the CSE, and the questions that this generates.
Spatially resolved cm/mm/sub-mm continuum observations can be a powerful means by which to study the partially ionized extended atmospheres around red supergiants as they allow us to probe the essential region where mass loss is initiated. To date, only Betelguese has been spatially resolved at cm and mm wavelengths with the data showing that the atmosphere is highly extended and that the mean gas temperature beyond 2 stellar radii is cool (Tgas < 3000K). Here, I present the initial results of our ALMA long baseline and e-MERLIN continuum observations of Betelgeuse's extended atmosphere which probe it at unprecedented angular resolution and sensitivity.
Red supergiant stars (RSG) can be considered as the massive counterparts of AGBs, sharing properties such as extended atmospheres, prodigious mass loss, and a chemically complex circumstellar envelopes. They are also known to present few giant convective cells at the photospheric level. In 2010, a weak magnetic field has been detected at the surface of Betelgeuse. Until recently this was the only M-type supergiant with a direct detection of a surface field. With the spectropolarimeter Narval (TBL Pic du Midi, France) we have initiated in spring 2015 a 2-year campaign dedicated to a sample of cool evolved stars, including several red supergiant stars (RSG). We present detection of surface magnetic field in RSGs obtained from circular polarisation data and multiline analysis technics. Because of their slow rotation (and thus high Rossby number, up to 100!) convection in these stars is not expected to efficiently generate a global magnetic field. Our detections may thus rather point toward local transitory fields, which may play a role in the mass loss mechanism. I will also introduce the diagnosis induced by linear polarisation in these stars, focusing on the Betelgeuse twin Erakis.
Dynamic models for the atmospheres of C-rich Asymptotic Giant Branch stars are quite advanced and have been overall successful in reproducing spectroscopic and photometric observations. Interferometry provides independent information and is thus an important technique tostudy the atmospheric stratification and to further constrain the dynamic models. We observed a sample of six C-rich AGBs with the mid infrared interferometer VLTI/MIDI. These observations, combined with photometric and spectroscopic data from the literature, are compared with synthetic observables derived from dynamic model atmospheres (DMA, Eriksson et al. 2014). The SEDs can be reasonably well modelled and the interferometry supports the extended and multi-component structure of the atmospheres, but some differences remain. We discuss the possible reasons for these differences and we compare the stellar parameters derived from this comparison with stellar evolution models. Finally, we point out the high potential of MATISSE, the second generation VLTI instrument allowing interferometric imaging in the L, M, and N bands, for further progress in this field.
In the recent years, new high angular resolution instrument such as ALMA, SPHERE/VLT and VLTI/PIONIER started a small revolution in the study of circumstellar environment around evolved stars. We do not talk about sub-arcsecond observations any more but about milliarcsecs. This is revolutionising our view of the close environement of these stars, the way they interact with binary companions and the formation of discs. I will review the results obtained using these observational techniques and present new insights on the mass-loss process from evolved stars of low, intermediate and high masses.
High spatial resolution observations have provided us with an amazing view on the winds of evolved cool giants and supergiants. The longstanding assumption of smooth spherically symmetric winds seems often not validated. Large scale structures in the form of spirals, circumstellar disks, bipolar outflows, bowshocks etc. are detected and smaller scale clumpiness seems omnipresent. These novel data challenge our understanding of the wind launching process. The observations serve as critical benchmark for 3D hydrodynamical models. Even more, these data push the theoretical models to include a higher form of complexity, in particular to incorporate a (more) consistent approach of chemistry, dynamics, and radiative transfer. In this talk, I will give a review of the recent observational results obtained with ALMA, SPHERE and Herschel. I will show how these data yield detailed information on the wind structure of evolved stars and elucidate which chemical and physical phenomena should be captured in theoretical wind models. I will summarize ongoing efforts to improve these theoretical models, both in terms of numerical modeling and based on novel laboratory experiments. I will show how these theoretical models serve as a guideline to further improve the observing strategies.
We present our recent investigation aimed at constraining the typical size and optical properties of carbon dust grains in Circumstellar envelopes (CSEs) of C-stars in the Small Magellanic Cloud. To achieve this goal, we apply our recent dust growth model, coupled with a radiative transfer code, to the CSEs of C-stars evolving along the TP-AGB, for which we compute spectra and colors. We then compare our modeled colors in the NIR and MIR bands with the observed ones, testing different assumptions in our dust model and employing several optical constants data sets for carbon dust available in the literature. Different assumptions adopted in our dust model change the typical size of the carbon grains produced. We finally constrain carbon dust properties by selecting the combination of typical grain size and optical constants which best reproduce several colors in the NIR and MIR at the same time. The approach is new and has never been adopted so far. We conclude that the complete set of selected NIR and MIR colors are best reproduced by small grains, with sizes between 0.06 and 0.1 microns, rather than by large grains of 0.2-0.4 microns. Remarkably, the inability of large grains to reproduce NIR and MIR colors seems independent of the adopted optical data set. We also find a possible trend of the typical grain size with the dust reddening in the CSEs of these stars. We finally emphasize that this work is preparatory to follow-up studies aimed at calibrating the TP-AGB phase through resolved stellar populations in star clusters and galaxies which include dusty, mass-losing evolved stars.
Using observations from the Herschel Inventory of The Agents of Galaxy Evolution (HERITAGE) survey of the Magellanic Clouds, we have found 32 evolved stars that are bright in the far-infrared. These sources span a wide range in luminosity and hence initial mass. We found 13 low- to intermediate-mass evolved stars, including asymptotic giant branch (AGB) stars, post-AGB stars, planetary nebulae, and a symbiotic star. We also identify 10 high mass stars, including 3 extreme red supergiants that are highly enshrouded by dust and detect 9 probable evolved objects which were previously undescribed in the literature. These sources are likely to be among the dustiest evolved objects in the Magellanic Clouds. The Herschel emission may either be due to dust produced by the evolved star or it may arise from swept-up interstellar medium material.
Cool, evolved stars, e.g., asymptotic giant branch stars and red (super)giants, lose copious amounts of mass and momentum through powerful, dense stellar winds. The interaction of these outflows with their surroundings results in highly structured and complex circumstellar environments, often featuring knots, arcs, shells and spirals. Recent improvements in computational power and techniques have led to the development of detailed, multi-dimensional simulations that have given new insight into the origin of these structures, and better understanding of the physical mechanisms driving their formation. In this talk, I review one of the main mechanisms that shapes the outflows of evolved stars: interaction with a companion. I will discuss both wind-wind interactions where the companion also ejects a stellar outflow, and mass-transfer interactions where the companion has a weak or insignicant outflow. I will also highlight the broader implications of these stellar wind interactions for other phenomena, e.g, for planetary nebulae, symbiotic and X-ray binaries, novae and supernovae.
Asymptotic Giant Branch (AGB) stars are a very important contributor to the total dust mass injected into the ISM in galaxies. Good estimations of the dust mass injection by AGB stars in the Magellanic Clouds have been achieved using Spitzer data (Riebel et al. 2012; Srinivasan et al, 2016). However, the last estimate of the dust injection rate in the Milky Way was done in the late '80s for a non-all sky sample (Jura & Kleinmann 1989). In this work we revisit the total dust mass-loss rate from AGB stars in the Solar neighborhood. It is especially hard to evaluate distances to dusty AGB stars in the Milky Way, as the highest mass-loss rate objects are not in the Hipparcos catalogue, due to circumstellar extinction. Using present-day all-sky infrared facilities (WISE, 2MASS, and others), we have constructed spectral energy distributions for all AGB stars within 1 kpc from the Sun. We use the GRAMS model grid (Sargent, Srinivasan & Meixner 2011; Srinivasan, Sargent & Meixner 2011) to estimate the dust production for this sample of AGB stars. Preliminary results show an increase in the number of known dusty objects within 1 kpc. An integrated dust production rate of ~ 10^-5 Msun/year or an average of ~ 10^-7 Msun/year per object is obtained. The result for the Solar Neighborhood will be extrapolated for the entire Milky Way, using a suitable stellar distribution function. In addition, we compare our results to those of the Magellanic Clouds and other Local Group galaxies, for which the distance determination problems do not exist. This work is a step towards more reliable determinations of the mass loss of AGB stars, and it aims to provide new insight on the discrepancy between the dust mass produced by AGB stars and that estimated to be present in the ISM.
Our research concerns the detailed dust composition surrounding Mira variables. These regular pulsators are easily observed in the optical and infrared due to their changes in brightness. Data on 25 Miras were obtained with the Spitzer IRS instrument in 2008-09 under a GO program led by Creech-Eakman. The stars were observed once per month to track changes in their brightness and spectral features. This dataset is unique for both the number of observations of each star and the high SNR due to quick exposure times to avoid saturation of the detectors. The reduced spectra reveal a wonderful “forest” of features that provide insight into the stellar atmospheres and circumstellar dust composition. The sample of stars in this study span the range of oxygen- to carbon-rich, with each type exhibiting certain known solid state components (i.e dust). Preliminary examination of the oxygen-rich Miras shows many of the rimary known features, such as broad silicate emission and aluminum oxide (Al2O3). Using the radiative transfer modeling code, DUSTY, we are attempting to identify several broad, and some sharp, dust features by implementing recently derived laboratory spectral indeces for dust opacities. Prominent features seen in oxygen-rich stars include: water ice emission, as well as amorphous and crystalline silicates, and potentially corundum. We will show preliminary results for a sub- sample of the set and discuss plans for publishing the entire sample.
Red supergiant (RSG) stars are the later stage of evolution of massive stars. The strong increase in size that massive stars experience when they leave the main sequence (at a mostly constant mass) causes their surface gravity to be very weak. This in principle favors the triggering of mass loss from their surface, consistently with their important observed mass loss rate. However, we do not know exactly which are the physical processes that effectively trigger the outflow. Nearby red supergiants are easy targets for optical/infrared interferometers, as they exhibit very large angular diameters. They significantly resolve the features present on their photosphere, in particular convective cells. I will present our recent observational and modeling results on the photosphere of the two nearby RSGs: Betelgeuse and Antares. I will also briefly present our ongoing project aiming at getting a better understanding of the convective activity of RSG.