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 .
We use high-angular-resolution images obtained with SPHERE/ZIMPOL to study the photosphere , the warm molecular layer , and the inner wind of the close-by oxygen-rich AGB star R Doradus and its inner envelope .
We present observations in filters V , cntH \alpha , 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 cntH \alpha , 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 .
We find that in the first epoch the surface brightness of R Dor is asymmetric in V and cntH \alpha , the filters where molecular opacity is stronger , while in cnt820 the surface brightness is closer to being axisymmetric .
The second-epoch observations in cntH \alpha 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 , \rho ( r ) = \rho _ { \circ } r ^ { - n } .
On average , we find exponents of -4.5 \pm 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 \sim 2 \times 10 ^ { -4 } at 5.0 R _ { \star } .
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 \sim 3.3 \times 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 .