J/A+A/626/A81ivo://CDS.VizieR/J/A+A/626/A81doi:10.26093/cds/vizier.36260081CDSVlemmings W.H.T.Khouri T.Olofsson H.2019-07-02T06:33:39Z2019-06-19T09:09:31ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frgiant-starsmillimeter-astronomysubmillimeter-astronomyThe 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.2019A&A...626A..81Vhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/626/A81CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/626/A81https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/626/A81http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/626/A81https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/626/A81https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/626/A81http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/626/A81GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/626/A81/stars (List of studied stars)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/626/A81/stars?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/626/A81/stars?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/626/A81/stars?GETtext/xml+votable180.050000true34.836625-2.97763890.005555555555555556Cone search capability for table J/A+A/626/A81/list (List of fits images)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/626/A81/list?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/626/A81/list?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/626/A81/list?GETtext/xml+votable180.050000true34.83668-2.978810.0055555555555555566/4355 17716 33233 43890https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/626/A81?format=asciiMillimeterdefaultJ/A+A/626/A81/starsList of studied starsrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintNameStar namemeta.id;meta.maincharRAJ2000Simbad right ascension (J2000)pos.eq.ra;meta.mainDEJ2000Declination (J2000)pos.eq.dec;meta.mainSimbadSimbad column added by the CDSmeta.ref.urlcharJ/A+A/626/A81/listList of fits imagesrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintRAJ2000Right Ascension of center (J2000)degpos.eq.ra;meta.mainDEJ2000Declination of center (J2000)degpos.eq.dec;meta.mainNxNumber of pixels along X-axismeta.numberintNyNumber of pixels along Y-axismeta.numberintObs.dateObservation datestime.epoch;obsnullableFreqObserved frequencyGHzem.freqfloatsizeSize of FITS fileKibytephys.size;meta.fitsintFileNameName of FITS file, in subdirectory fitsmeta.id;meta.fitscharTitleTitle of the FITS filemeta.title;meta.fitschar