J/A+A/626/A81 4 AGB ALMA images (Vlemmings+, 2019)
Resolving the extended stellar atmospheres of asymptotic giant branch stars at
(sub)millimetre wavelengths.
Vlemmings W.H.T., Khouri T., Olofsson H.
<Astron. Astrophys. 626, A81 (2019)>
=2019A&A...626A..81V 2019A&A...626A..81V (SIMBAD/NED BibCode)
ADC_Keywords: Stars, giant ; Millimetric/submm sources
Keywords: stars: AGB and post-AGB - stars: atmospheres - stars: activity -
stars: imaging - submillimetre: stars
Abstract:
The initial conditions for mass loss during the asymptotic giant
branch (AGB) phase are set in their extended atmospheres, where, among
others, convection and pulsation driven shocks determine the physical
conditions.
High resolution observations of AGB stars at (sub)millimetre
wavelengths can now directly determine the morphology, activity,
density, and temperature close to the stellar photosphere.
We used Atacama Large Millimeter/submillimeter Array (ALMA) high
angular resolution observations to resolve the extended atmospheres of
four of the nearest AGB stars: W Hya, Mira A, R Dor, and R Leo. We
interpreted the observations using a parameterised atmosphere model.
We resolve all four AGB stars and determine the brightness temperature
structure between 1 and 2 stellar radii. For W Hya and R Dor we
confirm the existence of hotspots with brightness temperatures >3000
to 10000K. All four stars show deviations from spherical symmetry. We
find variations on a timescale of days to weeks, and for R Leo we
directly measure an outward motion of the millimetre wavelength
surface with a velocity of at least 10.6±1.4km/s. For all objects
but W Hya we find that the temperature-radius and size-frequency
relations require the existence of a (likely inhomogeneous) layer of
enhanced opacity.
The ALMA observations provide a unique probe of the structure of the
extended AGB atmosphere.We find highly variable structures of hotspots
and likely convective cells. In the future, these observations can be
directly compared to multi-dimensional chromosphere and atmosphere
models that determine the temperature, density, velocity, and
ionisation structure between the stellar photosphere and the dust
formation region. However, our results show that for the best
interpretation, both very accurate flux calibration and
near-simultaneous observations are essential.
Description:
We observed four nearby AGB stars with ALMA at high angular resolution
to describe their extended atmosphere at (sub)millimetre wavelengths.
The observations were taken between 21 September and 25 November 2017
in some of the longest baseline ALMA configurations with maximum
baselines between 10 and 16.2km.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
stars.dat 31 4 List of studied stars
list.dat 158 23 List of fits images
fits/* . 23 Individual fits images
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Byte-by-byte Description of file: stars.dat
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Bytes Format Units Label Explanations
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1- 6 A6 --- Name Star name
9- 10 I2 h RAh Simbad right ascension (J2000)
12- 13 I2 min RAm Simbad right ascension (J2000)
15- 19 F5.2 s RAs Simbad right ascension (J2000)
21 A1 --- DE- Declination sign (J2000)
22- 23 I2 deg DEd Declination (J2000)
25- 26 I2 arcmin DEm Declination (J2000)
28- 31 F4.1 arcsec DEs Declination (J2000)
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 23 I4 --- Nx Number of pixels along X-axis
25- 28 I4 --- Ny Number of pixels along Y-axis
30- 55 A26 "datime" Obs.date Observation date
57- 63 F7.3 GHz Freq Observed frequency
65- 68 I4 Kibyte size Size of FITS file
70- 81 A12 --- FileName Name of FITS file, in subdirectory fits
83-158 A76 --- Title Title of the FITS file
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Acknowledgements:
Wouter Vlemmings, wouter.vlemmings(at)chalmers.se
(End) Patricia Vannier [CDS] 09-May-2019