91 research outputs found
The nearby AGB star L2 Puppis: the birth of a planetary nebula ?
Adaptive optics observations in the infrared (VLT/NACO, Kervella et al. 2014)
and visible (VLT/SPHERE, Kervella et al. 2015) domains revealed that the nearby
AGB star L2 Pup (d=64 pc) is surrounded by a dust disk seen almost edge-on.
Thermal emission from a large dust "loop" is detected at 4 microns up to more
than 10 AU from the star. We also detect a secondary source at a separation of
32 mas, whose nature is uncertain. L2 Pup is currently a relatively "young" AGB
star, so we may witness the formation of a planetary nebula. The mechanism that
breaks the spherical symmetry of mass loss is currently uncertain, but we
propose that the dust disk and companion are key elements in the shaping of the
bipolar structure. L2 Pup emerges as an important system to test this
hypothesis.Comment: 6 pages, 3 figures, Proceedings of the Physics of Evolved Stars
conference, 8-12 June 2015, Nice, Franc
Asymmetries on red giant branch surfaces from CHARA/MIRC optical interferometry
Context. Red giant branch (RGB) stars are very bright objects in galaxies and
are often used as standard candles. Interferometry is the ideal tool to
characterize the dynamics and morphology of their atmospheres. Aims. We aim at
precisely characterising the surface dynamics of a sample of RGB stars.
Methods. We obtained interferometric observations for three RGB stars with the
MIRC instrument mounted at the CHARA interfer- ometer. We looked for
asymmetries on the stellar surfaces using limb-darkening models. Results. We
measured the apparent diameters of HD197989 (Epsilon Cyg) = 4.61+-0.02 mas,
HD189276 (HR7633) = 2.95+-0.01 mas, and HD161096 (Beta Oph) = 4.43+-0.01 mas.
We detected departures from the centrosymmetric case for all three stars with
the tendency of a greater effect for lower logg of the sample. We explored the
causes of this signal and conclude that a possible explanation to the
interferometric signal is the convection-related and/or the magnetic-related
surface activity. However, it is necessary to monitor these stars with new
observations, possibly coupled with spectroscopy, in order to firmly establish
the cause.Comment: Accepted for publication as a Letter in Astronomy and Astrophysics,
section 1. Letters to the Editor. The official date of acceptance is
06/03/201
The SPHERE data center: a reference for high contrast imaging processing
The objective of the SPHERE Data Center is to optimize the scientific return
of SPHERE at the VLT, by providing optimized reduction procedures, services to
users and publicly available reduced data. This paper describes our motivation,
the implementation of the service (partners, infrastructure and developments),
services, description of the on-line data, and future developments. The SPHERE
Data Center is operational and has already provided reduced data with a good
reactivity to many observers. The first public reduced data have been made
available in 2017. The SPHERE Data Center is gathering a strong expertise on
SPHERE data and is in a very good position to propose new reduced data in the
future, as well as improved reduction procedures.Comment: SF2A proceeding
NOEMA maps the CO environment of the red supergiant Cep
Red supergiant stars are surrounded by a gaseous and dusty circumstellar
environment created by their mass loss which spreads heavy elements into the
interstellar medium. The structure and the dynamics of this envelope are
crucial to understand the processes driving the red supergiant mass loss and
the shaping of the pre-supernova ejecta. We have observed the emission from the
CO line from the red supergiant star ~Cep with the NOEMA
interferometer. In the line the synthesized beam was ~arcsec
(~au at 641~pc). The continuum map shows only the unresolved
contribution of the free-free emission of the star chromosphere. The
continuum-subtracted channel maps reveal a very inhomogeneous and clumpy
circumstellar environment. In particular, we detected a bright CO clump, as
bright as the central source in the line, at 1.80~arcsec south-west from the
star, in the blue channel maps. After a deprojection of the radial velocity
assuming two different constant wind velocities, the observations were modelled
using the 3D radiative transfer code \textsc{lime} to derive the
characteristics of the different structures. We determine that the gaseous
clumps observed around ~Cep are responsible for a mass loss rate of , in addition to a
spatially unresolved wind component with an estimated mass-loss rate of . Therefore, the clumps have a
significant role in ~Cep's mass loss (). We cannot exclude that
the unresolved central outflow may be made of smaller unresolved clumps.Comment: 15 pages, 4 tables, 9 figures. 2nd version : one co-author removed
and acknowledgement updated (consistent with erratum
https://doi.org/10.1093/mnras/stz1006
The extended atmosphere and circumstellar environment of the cool evolved star VX Sagittarii as seen by MATISSE
Context. VX Sgr is a cool, evolved, and luminous red star whose stellar parameters are difficult to determine, which affects its classification. Aims: We aim to spatially resolve the photospheric extent as well as the circumstellar environment. Methods: We used interferometric observations obtained with the MATISSE instrument in the L (3-4 μm), M (4.5-5 μm), and N (8-13 μm) bands. We reconstructed monochromatic images using the MIRA software. We used 3D radiation-hydrodynamics simulations carried out with CO5BOLD and a uniform disc model to estimate the apparent diameter and interpret the stellar surface structures. Moreover, we employed the radiative transfer codes OPTIM3D and RADMC3D to compute the spectral energy distribution for the L, M, and N bands, respectively. Results: MATISSE observations unveil, for the first time, the morphology of VX Sgr across the L, M, and N bands. The reconstructed images show a complex morphology with brighter areas whose characteristics depend on the wavelength probed. We measured the angular diameter as a function of the wavelength and showed that the photospheric extent in the L and M bands depends on the opacity through the atmosphere. In addition to this, we also concluded that the observed photospheric inhomogeneities can be interpreted as convection-related surface structures. The comparison in the N band yielded a qualitative agreement between the N-band spectrum and simple dust radiative transfer simulations. However, it is not possible to firmly conclude on the interpretation of the current data because of the difficulty in constraing the model parameters using the limited accuracy of our absolute flux calibration. Conclusions: MATISSE observations and the derived reconstructed images unveil the appearance of VX Sgr's stellar surface and circumstellar environment across a very large spectral domain for the first time. Based on the observations made with VLTI-ESO Paranal, Chile under the programme IDs 0103.D-0153(D, E, G). The data are available at oidb.jmmc.f
Spatially Resolved Ultraviolet Spectroscopy of the Great Dimming of Betelgeuse
The bright supergiant, Betelgeuse (Alpha Orionis, HD 39801) experienced a
visual dimming during 2019 December and the first quarter of 2020 reaching an
historic minimum 2020 February 713. During 2019 September-November, prior to
the optical dimming event, the photosphere was expanding. At the same time,
spatially resolved ultraviolet spectra using the Hubble Space Telescope/Space
Telescope Imaging Spectrograph revealed a substantial increase in the
ultraviolet spectrum and Mg II line emission from the chromosphere over the
southern hemisphere of the star. Moreover, the temperature and electron density
inferred from the spectrum and C II diagnostics also increased in this
hemisphere. These changes happened prior to the Great Dimming Event. Variations
in the Mg II k-line profiles suggest material moved outwards in response to the
passage of a pulse or acoustic shock from 2019 September through 2019 November.
It appears that this extraordinary outflow of material from the star, likely
initiated by convective photospheric elements, was enhanced by the coincidence
with the outward motions in this phase of the 400 day pulsation cycle.
These ultraviolet observations appear to provide the connecting link between
the known large convective cells in the photosphere and the mass ejection event
that cooled to form the dust cloud in the southern hemisphere imaged in 2019
December, and led to the exceptional optical dimming of Betelgeuse in 2020
February.Comment: 11 pages, 8 figures, Astrophysical Journal, accepte
The VLT/SPHERE view of the ATOMIUM cool evolved star sample. I. Overview:Sample characterization through polarization analysis
Aims. Through the ATOMIUM project, based on an ALMA large program, we aim to
present a consistent view of a sample of 17 nearby cool evolved stars
(Aymptotic Giant Branch and red supergiant stars).
Methods. Here we present VLT/SPHERE-ZIMPOL polarimetric maps obtained in the
visible of 14 out of the 17 ATOMIUM 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, and in or near the plane of the sky,
with a total optical depth close to unity in the line of sight, representing
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 common dust species. The spatial structure
of the DoLP shows a diverse set of shapes. Only for three sources do we note a
correlation between the ALMA CO and SiO lines, which trace the gas density, and
the DoLP, which traces the dust.
Conclusion. The clumpiness of the DoLP and the lack of a consistent
correlation between the gas and the dust location show that, in the inner
circumstellar environment (CSE), 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 CSE. Except for ~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.Comment: Accepted for publication in Astronomy & Astrophysics. 22 pages, 15
figures, 5 table
The VLT/SPHERE view of the ATOMIUM cool evolved star sample. I. Overview: Sample characterization through polarization analysis
Aims. Through the ATOMIUM project, based on an ALMA large program, we aim to
present a consistent view of a sample of 17 nearby cool evolved stars
(Aymptotic Giant Branch and red supergiant stars).
Methods. Here we present VLT/SPHERE-ZIMPOL polarimetric maps obtained in the
visible of 14 out of the 17 ATOMIUM 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, and in or near the plane of the sky,
with a total optical depth close to unity in the line of sight, representing
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 common dust species. The spatial structure
of the DoLP shows a diverse set of shapes. Only for three sources do we note a
correlation between the ALMA CO and SiO lines, which trace the gas density, and
the DoLP, which traces the dust.
Conclusion. The clumpiness of the DoLP and the lack of a consistent
correlation between the gas and the dust location show that, in the inner
circumstellar environment (CSE), 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 CSE. Except for ~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.Comment: Accepted for publication in Astronomy & Astrophysics. 22 pages, 15
figures, 5 table
(Sub)stellar companions shape the winds of evolved stars
Binary interactions dominate the evolution of massive stars, but their role is less clear for low- and intermediate-mass stars. The evolution of a spherical wind from an asymptotic giant branch (AGB) star into a nonspherical planetary nebula (PN) could be due to binary interactions. We observed a sample of AGB stars with the Atacama Large Millimeter/submillimeter Array (ALMA) and found that their winds exhibit distinct nonspherical geometries with morphological similarities to planetary nebulae (PNe). We infer that the same physics shapes both AGB winds and PNe; additionally, the morphology and AGB mass-loss rate are correlated. These characteristics can be explained by binary interaction. We propose an evolutionary scenario for AGB morphologies that is consistent with observed phenomena in AGB stars and PNe
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