Context.
Despite being the best studied red supergiant star in our Galaxy, the physics behind the photometric variability and mass loss of Betelgeuse is poorly understood. Moreover, recently the star ...has experienced an unusual fading with its visual magnitude reaching a historical minimum. The nature of this event was investigated by several studies where mechanisms, such as episodic mass loss and the presence of dark spots in the photosphere, were invoked.
Aims.
We aim to relate the atmospheric dynamics of Betelgeuse to its photometric variability, with the main focus on the dimming event.
Methods.
We used the tomographic method which allowed us to probe different depths in the stellar atmosphere and to recover the corresponding disk-averaged velocity field. The method was applied to a series of high-resolution HERMES observations of Betelgeuse. Variations in the velocity field were then compared with photometric and spectroscopic variations.
Results.
The tomographic method reveals that the succession of two shocks along our line-of-sight (in February 2018 and January 2019), the second one amplifying the effect of the first one, combined with underlying convection and/or outward motion present at this phase of the 400 d pulsation cycle, produced a rapid expansion of a portion of the atmosphere of Betelgeuse and an outflow between October 2019 and February 2020. This resulted in a sudden increase in molecular opacity in the cooler upper atmosphere of Betelgeuse and, thus, in the observed unusual decrease of the star’s brightness.
Aims. We assess the ability to image the photosphere of red supergiants and, in particular Betelgeuse, through the modelling of the observed linear polarization in atomic spectral lines. We also aim ...to analyse the resulting images over time, to measure the size and dynamics of the convective structures in these stars. Methods. Rayleigh scattering polarizes the continuum and spectral lines depolarize it. This depolarization is seen as a linear polarization signal parallel to the radial direction on the stellar disk. Integrated over the disk, it would result in a null signal, except if brightness asymmetries/inhomogeneities are present. This is the basic concept behind our imaging technique. Through several tests and comparisons, we have tried to assess and extend its validity, and to determine what can be learnt unambiguously through it. Results. The several tests and comparisons performed prove that our technique reliably retrieves the salient brightness structures in the photosphere of Betelgeuse, and should be relevant to other red supergiants. For Betelgeuse, we demonstrate that these structures we infer are convective cells, with a characteristic size of more than 60% of the stellar radius. We also derive the characteristic upflow and downflow speeds, 22 and 10 km s−1, respectively. We find weak magnetic fields concentrated in the downflow lanes in between granules, similar to the quiet sun magnetism. We follow those convective structures in time. Changes happen on timescales of 1 week, but individual structures can be tracked over 4 yr of observations. Conclusions. The measured characteristics of the convection in Betelgeuse confirm the predictions of numerical simulations in both the strong, supersonic upflows and the size of the convective cells. They also concur in the presence of weak magnetic fields that are completely dominated by the convective flows and constrained to the dark lanes of down-flowing plasma.
Context. Betelgeuse is an M-type supergiant that presents a circularly polarized (Stokes V) signal in its line profiles, interpreted in terms of a surface magnetic field. Aims. The weak circular ...polarization signal has been monitored over 7.5 years in order to follow its evolution on different timescales, and eventually to determine its physical origin. Linear polarization measurements have also been obtained regularly in the last few years. Methods. We used both the ESPaDOnS and Narval spectropolarimeters to obtain high signal-to-noise ratio spectra, which were processed by means of the least-squares deconvolution method. In order to ensure the reality of the very weak circular polarization, special care has been taken to limit instrumental effects. In addition, several tests were performed on the Stokes V signal to establish its stellar and Zeeman origin. Results. We confirm the magnetic nature of the circular polarization, pointing to a surface magnetic field of the order of 1 G. The Stokes V profiles present variations over different timescales, the most prominent one being close to the long secondary period (LSP; around 2000 d for Betelgeuse) often invoked in red evolved stars. This long period is also dominant for all the other Stokes parameters. The circular polarization is tentatively modeled by means of magnetic field concentrations mimicking spots, showing in particular that the velocity associated with each “spot” also follows the long timescale, and that this signal is nearly always slightly redshifted. Conclusions. From the coupled variations of both linear and circular polarization signatures in amplitude, velocity and timescale, we favour giant convection cells as the main engine at the origin of polarization signatures and variations in all the Stokes parameters. This strengthens support for the hypothesis that large convective cells are at the origin of the LSP.
Context.
Red supergiant stars are one of the latest stages in the evolution of massive stars. Their photospheric convection may play an important role in the launching mechanism of their mass loss; ...however, its characteristics and dynamics are still poorly constrained.
Aims.
By observing red supergiant stars with near infrared interferometry at different epochs, we expect to reveal the evolution of bright convective features on their stellar surface.
Methods.
We observed the M2Iab-Ib red supergiant star CE Tau with the VLTI/PIONIER instrument in the
H
band at two different epochs separated by one month.
Results.
We derive the angular diameter of the star and basic stellar parameters, and reconstruct two reliable images of its
H
-band photosphere. The contrast of the convective pattern of the reconstructed images is 5 ± 1% and 6 ± 1% for our two epochs of observation.
Conclusions.
The stellar photosphere shows few changes between the two epochs. The contrast of the convective pattern is below the average contrast variations obtained on 30 randomly chosen snapshots of the best matching 3D radiative hydrodynamics simulation: 23 ± 1% for the original simulation images and 16 ± 1% for the maps degraded to the reconstruction resolution. We offer two hypotheses to explain this observation. CE Tau may be experiencing a quiet convective activity episode or it could be a consequence of its warmer effective temperature (hence its smaller radius) compared to the simulation.
Context. Betelgeuse is the closest red supergiant (RSG); therefore, it is well suited for studying the complex processes in its atmosphere that lead to the chemical enrichment of the interstellar ...medium. Aims. We intend to investigate the shape and composition of the close molecular layer (also known as the MOLsphere) that surrounds the star. This analysis is part of a wider program that aims at understanding the dynamics of the circumstellar envelope of Betelgeuse. Methods. On January and February 2011, Betelgeuse was observed using the Astronomical Multi-BEam combineR (AMBER) instrument of the Very Large Telescope Interferometer (VLTI) in the H and K bands. Using the medium spectral resolution of the instrument (R ~ 1500), we were able to investigate the carbon monoxide band heads and the water-vapor bands. We used two different approaches to analyse our data: a model fit in both the continuum and absorption lines and then a fit with a radiative hydrodynamics (RHD) simulation. Results. Using the continuum data, we derive a uniform disk diameter of 41.01 ± 0.41 mas, a power law type limb-darkened disk diameter of 42.28 ± 0.43 mas and a limb-darkening exponent of 0.155 ± 0.009. Within the absorption lines, using a single layer model, we obtain parameters of the MOLsphere. Using a RHD simulation, we unveil the convection pattern in the visibilities. Conclusions. We derived a new value of the angular diameter of Betelgeuse in the K band continuum. Our observations in the absorption lines are well reproduced by a molecular layer at 1.2 stellar radii containing both CO and H2O. The visibilities at higher spatial frequencies are matching a convection pattern in a RHD simulation.
Context.
The ‘Great Dimming’ of the prototypical red supergiant Betelgeuse, which occurred between December 2019 and April 2020, gives us unprecedented insight into the processes occurring on the ...stellar surface and in the inner wind of this type of star. In particular it may bring further understanding of their dust nucleation and mass-loss processes.
Aims.
Here, we present and analyse VLTI/MATISSE observations in the N band (8–13 µm) taken near the brightness minimum in order to assess the status of the dusty circumstellar environment.
Methods.
We explored the compatibility of a dust clump obscuring the star with our mid-infrared interferometric observations using continuum 3D radiative transfer modelling, and probed the effect of adding multiple clumps close to the star on the observables. We also tested the viability of a large cool spot on the stellar surface without dust present in the ambient medium.
Results.
Using the visibility data, we derived a uniform disk diameter of 59.02 ± 0.64 mas in the spectral range 8–8.75 µm. We find that both the dust clump and the cool spot models are compatible with the data. Further to this, we note that the extinction and emission of our localised dust clump in the line of sight of the star directly compensate for each other, making the clump undetectable in the spectral energy distribution and visibilities. The lack of infrared brightening during the Great Dimming therefore does not exclude extinction due to a dust clump as one of the possible mechanisms. The visibilities can be reproduced by a spherical wind with dust condensing at 13 stellar radii and a dust mass-loss rate of (2.1–4.9) × 10
−10
M
⊙
yr
−1
; however, in order to reproduce the complexity of the observed closure phases, additional surface features or dust clumps would be needed.
ATOMIUM Baudry, A.; Wong, K. T.; Etoka, S. ...
Astronomy and astrophysics (Berlin),
2023, Volume:
674
Journal Article
Peer reviewed
Open access
Context: Water (H2O) 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 & GSIM;4000 K) have been observed in both H2O 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 H2O in several vibrational states, and of OH in Lambda-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 sample of asymptotic giant branch stars (AGBs) and red supergiants (RSGs).
Methods: Spectra and maps of H2O and OH lines observed in a 27 GHz aggregated bandwidth and with an angular resolution of similar to 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 H2O were revisited to characterize masing conditions. Up-to-date chemical models were used for comparison with the observed OH/H2O abundance ratio.
Results: Ten rotational transitions of H2O with excitation energies similar to 4000-9000 K were observed in vibrational states up to (& upsilon;(1),& upsilon;(2),& upsilon;(3)) = (0,1,1). All but one are new detections in space, and from these we have derived accurate rest frequencies. Hyperfine split Lambda-doubling transitions in & upsilon; = 0, J = 27/2 and 29/2 levels of the (2)& pi;(3/2) state, as well as J = 33/2 and 35/2 of the (2)& pi;(1/2) state of OH with excitation energies of similar to 4780-8900 K were also observed. Four of these transitions are new detections in space. Combining our measurements with earlier observations of OH, the & upsilon; = 0 and & upsilon; = 1 Lambda-doubling frequencies have been improved. Our H2O 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 & upsilon;(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 & upsilon;(2) = 1 state also shows signs of maser emission. The OH emission is weak and quasithermally excited. Emission and absorption features of H2O and OH reveal an infall of matter and complex kinematics influenced by binarity. From the OH and H2O column densities derived with nonmasing transitions in a few sources, we obtain OH/H2O abundance ratios of similar to(0.7-2.8) x 10(-2).
The mass-loss mechanism of cool massive evolved stars is poorly understood. The proximity of Betelgeuse makes it an appealing target to study its atmosphere, map the shape of its envelope, and follow ...the structure of its wind from the photosphere out to the interstellar medium. A link is suspected between the powerful convective motions in Betelgeuse and its mass loss. We aim to constrain the spatial structure and temporal evolution of the convective pattern on the photosphere and to search for evidence of this link. We report new interferometric observations in the infrared H-band using the VLTI/PIONIER instrument. We monitored the photosphere of Betelgeuse between 2012 January and 2014 November to look for evolutions that may trigger the outflow. We bring new evidence for the presence of a convective pattern in the photosphere of red supergiants. We propose that the large observed feature is modifying the signature of the convective pattern at the surface of the star in a way that simulations cannot reproduce.
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