Study of the inner dust envelope and stellar photosphere of the AGB star R Doradus using SPHERE/ZIMPOL

Abstract

Context. On the asymptotic giant branch (AGB) low-and intermediate-mass stars eject a large fraction of their envelope, but the mechanism driving these outflows is still poorly understood. For oxygen-rich AGB stars, the wind is thought to be driven by radiation pressure caused by scattering of radiation off dust grains. Aims. We study the photosphere, the warm molecular layer, and the inner wind of the close-by oxygen-rich AGB star R Doradus. We focus on investigating the spatial distribution of the dust grains that scatter light and whether these grains can be responsible for driving the outflow of this star. Methods. We use high-angular-resolution images obtained with SPHERE/ZIMPOL to study R Dor and its inner envelope in a novel way. We present observations in filters V, cntHα, and cnt820 and investigate the surface brightness distribution of the star and of the polarised light produced in the inner envelope. Thanks to second-epoch observations in cntHα, we are able to see variability on the stellar photosphere. We study the polarised-light data using a continuum-radiative-transfer code that accounts for direction-dependent scattering of photons off dust grains. Results. We find that in the first epoch the surface brightness of R Dor is asymmetric in V and cntHα, the filters where molecular opacity is stronger, while in cnt820 the surface brightness is closer to being axisymmetric. The second-epoch observations in cntHα show that the morphology of R Dor has changed completely in a timespan of 48 days to a more axisymmetric and compact configuration. This variable morphology is probably linked to changes in the opacity provided by TiO molecules in the extended atmosphere. The observations show polarised light coming from a region around the central star. The inner radius of the region from where polarised light is seen varies only by a small amount with azimuth. The value of the polarised intensity, however, varies by between a factor of 2.3 and 3.7 with azimuth for the different images. We fit the radial profile of the polarised intensity using a spherically symmetric model and a parametric description of the dust density profile, ρ(r) = ρ • r −n . On average, we find exponents of −4.5 ± 0.5 that correspond to a much steeper density profile than that of a wind expanding at constant velocity. The dust densities we derive imply an upper limit for the dust-to-gas ratio of ∼2 × 10 −4 at 5.0 R . Considering all the uncertainties in observations and models, this value is consistent with the minimum values required by wind-driving models for the onset of a wind, of ∼3.3 × 10 −4 . However, if the steep density profile we find extends to larger distances from the star, the dust-to-gas ratio will quickly become too small for the wind of R Dor to be driven by the grains that produce the scattered light.

Study of the aluminium content in AGB winds using ALMA

“…5). The SPHERE/ZIMPOL data of R Dor and W Hya (Khouri et al 2016;Ohnaka et al 2016) confirm the earlier result by Norris et al (2012) that a halo of large transparent grains is present at ∼1.5-3 R . Estimated grain sizes are around 0.3-0.5 µm and Fe-free silicates or Al 2 O 3 are put forward as the possible dust species that produce the scattered light.…”

“…From an analysis of Herschel and sub-millimeter data, Khouri (2014) and Van de Sande et al (2017) have derived that some 90% of atomic silicon is locked up in gaseous SiO and the remainder in silicates. Data from VLTI/NACO and SPHERE/ZIMPOL support the presence of a close halo of large transparent grains (∼0.3 µm) at ∼1.5−2 R (Norris et al 2012;Khouri et al 2016).…”

“…Recent polarized light data obtained with the VLT/SPHERE instrument offer us another way to look at the inner wind chemical and morphological structure. Again, these data prove the presence of grains close to the star, albeit it is not evident to put stringent constraints on the grain size (Khouri et al 2016;Ohnaka et al 2016). However, neither the NACO or the SPHERE data can pinpoint the exact chemical constitution of these micron-sized transparent grains.…”

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

“…Therefore, it is more likely that it represents a bright spot in the atmosphere of the star. Similar inhomogeneities and their time variations within 48 days have recently been detected in another AGB star, R Dor, by Khouri et al (2016). They interpret the inhomogeneities and their time variations as due to the fluctuations in the density and/or molecular abundance and/or excitation.…”

“…Given that the morphology of the images of R Dor changed within 48 days (Khouri et al 2016), SPHERE-ZIMPOL observations of W Hya as frequent as every few weeks are useful for clarifying the physical process behind the time variations. Milliarcsecond-resolution velocity-resolved aperture-synthesis imaging of the extended atmosphere with VLTI/AMBER or VLTI/GRAVITY (Eisenhauer et al 2008) is crucial for examining whether or not the gas at the locations of the dust clouds shows systematic outward motions.…”