Diffraction‐limited Near‐IR Imaging at Keck Reveals Asymmetric, Time‐variable Nebula around Carbon Star CIT 6

Monnier, John D.; Tuthill, P. G.; Danchi, W. C.

doi:10.1086/317857

Cited by 37 publications

(40 citation statements)

References 23 publications

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“…Alternatively, it is possible that the CO gas motion detected by our AMBER observations may represent local mass ejections. High-resolution studies of the dusty RSG VY CMa and less massive evolved stars, such as IRC+10216 and CIT6, suggest that the mass loss is accompanied by episodic mass ejections in clumps or arcs (Humphreys et al 2007;Smith et al 2009;Weigelt et al 2002;Monnier et al 2000). While the circumstellar envelope around Betelgeuse is spherical on a large scale unlike VY CMa, inhomogeneities such as clumps and plumes have been detected (Plez & Lambert 2002;Smith et al 2009).…”

Section: Discussionmentioning

confidence: 99%

Spatially resolving the inhomogeneous structure of the dynamical atmosphere of Betelgeuse with VLTI/AMBER

Ohnaka¹,

Hofmann²,

Benisty

et al. 2009

A&A

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Aims. We present spatially resolved, high-spectral resolution K-band observations of the red supergiant Betelgeuse (α Ori) using AMBER at the Very Large Telescope Interferometer (VLTI). Our aim is to probe inhomogeneous structures in the dynamical atmosphere of Betelgeuse. Methods. Betelgeuse was observed in the wavelength range between 2.28 and 2.31 μm with VLTI/AMBER using baselines of 16, 32, and 48 m. The spectral resolutions of 4800−12 000 allow us to study inhomogeneities seen in the individual CO first overtone lines. Results. Spectrally dispersed interferograms have been successfully obtained in the second, third, and fifth lobes, which represents the highest spatial resolution (9 mas) achieved for Betelgeuse. This corresponds to five resolution elements over its stellar disk. The AMBER visibilities and closure phases in the K-band continuum can be reasonably fitted by a uniform disk with a diameter of 43.19 ± 0.03 mas or a limb-darkening disk with 43.56 ± 0.06 mas and a limb-darkening parameter of (1.2 ± 0.07) × 10 −1 . These AMBER data and the previous K-band interferometric data taken at various epochs suggest that Betelgeuse seen in the K-band continuum shows much smaller deviations from the above uniform disk or limb-darkened disk than predicted by recent 3-D convection simulations for red supergiants. On the other hand, our AMBER data in the CO lines reveal salient inhomogeneous structures. The visibilities and phases (closure phases, as well as differential phases representing asymmetry in lines with respect to the continuum) measured within the CO lines show that the blue and red wings originate in spatially distinct regions over the stellar disk, indicating an inhomogeneous velocity field that makes the star appear different in the blue and red wings. Our AMBER data in the CO lines can be roughly explained by a simple model, in which a patch of CO gas is moving outward or inward with velocities of 10−15 km s −1 , while the CO gas in the remaining region in the atmosphere is moving in the opposite direction at the same velocities. Also, the AMBER data are consistent with the presence of warm molecular layers (so-called MOLsphere) extending to ∼1.4−1.5 R with a CO column density of ∼1 × 10 20 cm −2 . Conclusions. Our AMBER observations of Betelgeuse are the first spatially resolved study of the so-called macroturbulence in a stellar atmosphere (photosphere and possibly MOLsphere as well) other than the Sun. The spatially resolved CO gas motion is likely to be related to convective motion in the upper atmosphere or intermittent mass ejections in clumps or arcs.

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Section: Discussionmentioning

confidence: 99%

Spatially resolving the inhomogeneous structure of the dynamical atmosphere of Betelgeuse with VLTI/AMBER

Ohnaka¹,

Hofmann²,

Benisty

et al. 2009

A&A

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“…Clumpy dust clouds are detected in other nearby AGB stars as well. For example, clumpy dust clouds have been imaged in the innermost circumstellar envelope of the well-studied carbonrich AGB stars IRC+10216 (Weigelt et al 1998;Haniff & Buscher 1998;Stewart et al 2016 and references therein) and CIT6 (Monnier et al 2000). Therefore, the formation of clumpy structures might be intrinsic to the mass loss phenomenon in AGB stars.…”

Section: Sphere-zimpol Polarimetric Imagesmentioning

confidence: 99%

Clumpy dust clouds and extended atmosphere of the AGB star W Hydrae revealed with VLT/SPHERE-ZIMPOL and VLTI/AMBER

Ohnaka

Weigelt

Hofmann

2016

A&A

126

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Context. Dust formation is thought to play an important role in the mass loss from stars at the asymptotic giant branch (AGB); however, where and how dust forms is still open to debate. Aims. We present visible polarimetric imaging observations of the well-studied AGB star W Hya taken with VLT/SPHERE-ZIMPOL as well as high spectral resolution long-baseline interferometric observations taken with the AMBER instrument at the Very Large Telescope Interferometer (VLTI). Our goal is to spatially resolve the dust and molecule formation region within a few stellar radii. Methods. We observed W Hya with VLT/SPHERE-ZIMPOL at three wavelengths in the continuum (645, 748, and 820 nm), in the Hα line at 656.3 nm, and in the TiO band at 717 nm. The VLTI/AMBER observations were carried out in the wavelength region of the CO first overtone lines near 2.3 µm with a spectral resolution of 12000. Results. Taking advantage of the polarimetric imaging capability of SPHERE-ZIMPOL combined with the superb adaptive optics performance, we succeeded in spatially resolving three clumpy dust clouds located at ∼50 mas (∼2 R ) from the central star, revealing dust formation very close to the star. The AMBER data in the individual CO lines suggest a molecular outer atmosphere extending to ∼3 R . Furthermore, the SPHERE-ZIMPOL image taken over the Hα line shows emission with a radius of up to ∼160 mas (∼7 R ). We found that dust, molecular gas, and Hα-emitting hot gas coexist within 2-3 R . Our modeling suggests that the observed polarized intensity maps can reasonably be explained by large (0.4-0.5 µm) grains of Al 2 O 3 , Mg 2 SiO 4 , or MgSiO 3 in an optically thin shell (τ 550nm = 0.1 ± 0.02) with an inner and outer boundary radius of 1.9-2.0 R and 3 ± 0.5 R , respectively. The observed clumpy structure can be reproduced by a density enhancement of a factor of 4 ± 1. Conclusions. The grain size derived from our modeling of the SPHERE-ZIMPOL polarimetric images is consistent with the prediction of the hydrodynamical models for the mass loss driven by the scattering due to micron-sized grains. The detection of the clumpy dust clouds close to the star lends support to the dust formation induced by pulsation and large convective cells as predicted by the 3D simulations for AGB stars.

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“…), while very few extreme objects depart from spherical morphology in the inner dust shell (e.g. IRC+10216 and CIT6;McCarthy et al 1980;Weigelt et al 1998;Monnier et al 2000;Tuthill et al 2000;Chandler et al 2007). To our knowledge this is the first time that MIDI has detected a signature of deviation from central symmetry for a carbon Mira variable through the differential phase.…”

Section: Observations and Data Reductionmentioning

confidence: 99%

Detection of an asymmetry in the envelope of the carbon Mira R Fornacis using VLTI/MIDI

Paladini

Sacuto

Klotz

et al. 2012

A&A

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Aims. We present a study of the envelope morphology of the carbon Mira R For with VLTI/MIDI. This object is one of the few asymptotic giant branch (AGB) stars that underwent a dust-obscuration event. The cause of such events is still a matter of discussion. Several symmetric and asymmetric scenarios have been suggested in the literature. Methods. Mid-infrared interferometric observations were obtained separated by two years. The observations probe different depths of the atmosphere and cover different pulsation phases. The visibilities and the differential phases were interpreted using GEM-FIND, a tool for fitting spectrally dispersed interferometric observations with the help of wavelength-dependent geometric models. Results. We report the detection of an asymmetric structure revealed through the MIDI differential phase. This asymmetry is observed at the same baseline and position angle two years later. The observations are best simulated with a model that includes a uniform-disc plus a Gaussian envelope plus a point-source. The geometric model can reproduce both the visibilities and the differential phase signatures. Conclusions. Our MIDI data favour explanations of the R For obscuration event that are based on an asymmetric geometry. We clearly detect a photocentre shift between the star and the strongly resolved dust component. This might be caused by a dust clump or a substellar companion. However, the available observations do not allow us to distinguish between the two options. The finding has strong implications for future studies of the geometry of the envelope of AGB stars: if this is a binary, are all AGB stars that show an obscuration event binaries as well? Or are we looking at asymmetric mass-loss processes (i.e. dusty clumps) in the inner part of a carbon-rich Mira?

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Diffraction‐limited Near‐IR Imaging at Keck Reveals Asymmetric, Time‐variable Nebula around Carbon Star CIT 6

Cited by 37 publications

References 23 publications

Spatially resolving the inhomogeneous structure of the dynamical atmosphere of Betelgeuse with VLTI/AMBER

Spatially resolving the inhomogeneous structure of the dynamical atmosphere of Betelgeuse with VLTI/AMBER

Clumpy dust clouds and extended atmosphere of the AGB star W Hydrae revealed with VLT/SPHERE-ZIMPOL and VLTI/AMBER

Detection of an asymmetry in the envelope of the carbon Mira R Fornacis using VLTI/MIDI

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