ORCID Profile
0000-0002-3443-3416
Current Organisations
KU Leuven
,
University of Gothenburg
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Publisher: EDP Sciences
Date: 03-2023
DOI: 10.1051/0004-6361/202245398
Abstract: Context . Low- and intermediate-mass asymptotic giant stars and massive red supergiant stars are important contributors to the chemical enrichment of the Universe. They are among the most efficient dust factories of the Galaxy, harboring chemically rich circumstellar environments. Yet, the processes that lead to dust formation or the large-scale shaping of the mass loss still escape attempts at modeling. Aims . Through the A TOMIUM project, we aim to present a consistent view of a s le of 17 nearby cool evolved stars. Our goals are to unveil the dust-nucleation sites and morphologies of the circumstellar envelope of such stars and to probe ambient environments with various conditions. This will further enhance our understanding of the roles of stellar convection and pulsations, and that of companions in shaping the dusty circumstellar medium. Methods . Here we present and analyze VLT/SPHERE-ZIMPOL polarimetric maps obtained in the visible (645–820 nm) of 14 out of the 17 A TOMIUM sources. They were obtained contemporaneously with the ALMA high spatial resolution data. To help interpret the polarized signal, we produced synthetic maps of light scattering by dust, through 3D radiative transfer simulations with the RADMC3D code. Results . The degree of linear polarization (DoLP) observed by ZIMPOL spreads across several optical filters. We infer that it primarily probes dust located just outside of the point spread function of the central source, and in or near the plane of the sky. The polarized signal is mainly produced by structures with a total optical depth close to unity in the line of sight, and it represents only a fraction of the total circumstellar dust. The maximum DoLP ranges from 0.03–0.38 depending on the source, fractions that can be reproduced by our 3D pilot models for grains composed of olivine, melilite, corundum, enstatite, or forsterite. The spatial structure of the DoLP shows a erse set of shapes, including clumps, arcs, and full envelopes. Only for three sources do we note a correlation between the ALMA CO υ = 0, J = 2−1 and SiO υ = 0, J = 5−4 lines, which trace the gas density, and the DoLP, which traces the dust. Conclusions . The clumpiness of the DoLP and the lack of a consistent correlation between the gas and the dust location show that, in the inner environment, dust formation occurs at very specific sites. This has potential consequences for the derived mass-loss rates and dust-to-gas ratio in the inner region of the circumstellar environment. Except for π 1 Gru and perhaps GY Aql, we do not detect interactions between the circumstellar wind and the hypothesized companions that shape the wind at larger scales. This suggests that the orbits of any other companions are tilted out of the plane of the sky.
Publisher: American Association for the Advancement of Science (AAAS)
Date: 18-09-2020
Abstract: Stars less than eight times the mass of the Sun end their lives as planetary nebulae, structures of ionized gas thrown off by the star and heated by the exposed stellar core. Planetary nebulae are often bipolar in shape or contain complex morphological features such as rings or spirals. Decin et al. observed the stellar winds of 14 stars during their asymptotic giant branch (AGB) phase of stellar evolution, which immediately precedes the planetary nebula phase. They found morphologies in the AGB winds similar to planetary nebulae and demonstrated that they are produced by the influence of a binary companion on the AGB wind. Science , this issue p. 1497
Publisher: EDP Sciences
Date: 04-2022
DOI: 10.1051/0004-6361/202140431
Abstract: This overview paper presents ATOMIUM , a Large Programme in Cycle 6 with the Atacama Large Millimeter/submillimeter Array (ALMA). The goal of ATOMIUM is to understand the dynamics and the gas phase and dust formation chemistry in the winds of evolved asymptotic giant branch (AGB) and red supergiant (RSG) stars. A more general aim is to identify chemical processes applicable to other astrophysical environments. Seventeen oxygen-rich AGB and RSG stars spanning a range in (circum)stellar parameters and evolutionary phases were observed in a homogeneous observing strategy allowing for an unambiguous comparison. Data were obtained between 213.83 and 269.71 GHz at high (∼0″.025–0″.050), medium (∼0″.13–0″.24), and low (∼1″) angular resolution. The sensitivity per ∼1.3 km s −1 channel was 1.5–5 mJy beam −1 , and the line-free channels were used to image the millimetre wave continuum. Our primary molecules for studying the gas dynamics and dust formation are CO, SiO, AlO, AlOH, TiO, TiO 2 , and HCN secondary molecules include SO, SO 2 , SiS, CS, H 2 O, and NaCl. The scientific motivation, survey design, s le properties, data reduction, and an overview of the data products are described. In addition, we highlight one scientific result – the wind kinematics of the ATOMIUM sources. Our analysis suggests that the ATOMIUM sources often have a slow wind acceleration, and a fraction of the gas reaches a velocity which can be up to a factor of two times larger than previously reported terminal velocities assuming isotropic expansion. Moreover, the wind kinematic profiles establish that the radial velocity described by the momentum equation for a spherical wind structure cannot capture the complexity of the velocity field. In fifteen sources, some molecular transitions other than 12 CO v = 0 J = 2 − 1 reach a higher outflow velocity, with a spatial emission zone that is often greater than 30 stellar radii, but much less than the extent of CO. We propose that a binary interaction with a (sub)stellar companion may (partly) explain the non-monotonic behaviour of the projected velocity field. The ATOMIUM data hence provide a crucial benchmark for the wind dynamics of evolved stars in single and binary star models.
Publisher: American Astronomical Society
Date: 13-03-2018
Publisher: EDP Sciences
Date: 06-2023
DOI: 10.1051/0004-6361/202245193
Abstract: Context. Water (H 2 O) and the hydroxyl radical (OH) are major constituents of the envelope of O-rich late-type stars. Transitions involving energy levels that are rotationally or vibrationally highly excited (energies ≳4000 K) have been observed in both H 2 O and OH. These and more recently discovered transitions can now be observed at a high sensitivity and angular resolution in the inner wind close to the stellar photosphere with the Atacama Large Millimeter/submillimeter Array (ALMA). Aims. Our goals are: (1) to identify and map the emission and absorption of H 2 O in several vibrational states, and of OH in Λ-doubling transitions with similar excitation energies and (2) to determine the physical conditions and kinematics in gas layers close to the extended atmosphere in a s le of asymptotic giant branch stars (AGBs) and red supergiants (RSGs). Methods. Spectra and maps of H 2 O and OH lines observed in a 27 GHz aggregated bandwidth and with an angular resolution of ~0."02−1."0 were obtained at two epochs with the main ALMA array. Additional observations with the Atacama Compact Array (ACA) were used to check for time variability of water transitions. Radiative transfer models of H 2 O were revisited to characterize masing conditions. Up-to-date chemical models were used for comparison with the observed OH/H 2 O abundance ratio. Results. Ten rotational transitions of H 2 O with excitation energies ~4000–9000 K were observed in vibrational states up to (υ 1 ,υ 2 ,υ 3 ) = (0,1,1). All but one are new detections in space, and from these we have derived accurate rest frequencies. Hyperfine split Λ-doubling transitions in υ = 0, J = 27/2 and 29/2 levels of the 2 Π 3/2 state, as well as J = 33/2 and 35/2 of the 2 Π 1/2 state of OH with excitation energies of ~4780–8900 K were also observed. Four of these transitions are new detections in space. Combining our measurements with earlier observations of OH, the υ = 0 and υ = 1 Λ-doubling frequencies have been improved. Our H 2 O maps show compact emission toward the central star and extensions up to twelve stellar radii or more. The 268.149 GHz emission line of water in the υ 2 = 2 state is time variable, tends to be masing with dominant radiative pumping, and is widely excited in AGBs and RSGs. The widespread but weaker 262.898 GHz water line in the υ 2 = 1 state also shows signs of maser emission. The OH emission is weak and quasithermally excited. Emission and absorption features of H 2 O and OH reveal an infall of matter and complex kinematics influenced by binarity. From the OH and H 2 O column densities derived with nonmasing transitions in a few sources, we obtain OH/H 2 O abundance ratios of ~(0.7–2.8) × 10 −2 .
Location: Italy
No related grants have been discovered for David Gobrecht.