Exploring the innermost dust formation region of the oxygen-rich AGB star IK~Tau with VLT/SPHERE-ZIMPOL and VLTI/AMBER

Abstract

Low- and intermediate-mass stars at the asymptotic giant branch (AGB) are known to be prevalent dust providers to galaxies. However, the mechanisms responsible for the formation and acceleration of dust in the cool extended atmospheres of AGB stars are still open to debate. We present visible polarimetric imaging observations of the oxygen-rich AGB star IK Tau obtained with SPHERE-ZIMPOL (phase 0.27) as well as interferometric observations with AMBER. IK Tau was observed with ZIMPOL at three wavelengths in the pseudo-continuum (645, 748, and 820 nm), in the H$\alpha$ line at 656.3 nm, and in the TiO band at 717 nm. The AMBER observations were carried out in the wavelength region of the CO first overtone lines near 2.3 $\mu$m (R=12 000). The polarimetric imaging capabilities of SPHERE-ZIMPOL have allowed us to spatially resolve clumpy dust clouds at 20--50 mas from the central star, which corresponds to 2--5 $R_{\star }$ when combined with a central star's angular diameter of 20.7$\pm$1.53 mas measured with AMBER. The diffuse, asymmetric dust emission extends out to $\sim$73 $R_{\star }$. We find that the TiO emission extends to 150 mas (15 $R_{\star }$). The AMBER data in the CO lines also suggest a molecular outer atmosphere extending to $\sim$1.5 $R_{\star }$. The results of our 2-D Monte Carlo radiative transfer modelling of dust clumps suggest that the polarized intensity and degree of linear polarization can be reasonably explained by small-sized (0.1 $\mu$m) grains of Al${}_2$O${}_3$, MgSiO${}_3$, or Mg${}_2$SiO${}_4$ in an optically thin shell (${\tau }_{550\mathrm{nm}}$=0.5$\pm$0.1) with an inner boundary radius of 3.5 $R_{\star }$. IK Tau's mass-loss rate is 20 to 50 times higher than the previously studied AGB stars W Hya, R Dor, and $o$ Cet. Nevertheless, our observations of IK Tau revealed that clumpy dust formation occurs close to the star as seen in those low mass-rate AGB stars.

arXiv comment14 pages, 12 figures, 4 tables

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