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

Abstract

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?

The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI

“…It is clear from this figure that the 128 m baseline differential phases show a significant deviation from zero in the 8−9 μm band 3 , greater than the expected upper-limit uncertainty of 10 • . A similar differential phase signature was found by Paladini et al (2012) in the close environment of the C-type star R For. 3 Differential phase measurements for wavelengths longer than 12.0 μm are more uncertain owing to the increasing effect of the atmospheric water vapor and edge band noise.…”

Tomography of silicate dust around M-type AGB stars

“…In a very simple (1D) picture, the close environment of the star, where the temperature stratifications of the three cases presented here are the same, is probed at higher angular resolution. However, it should be kept in mind that asymmetric structures play a role at higher angular resolution observations (i.e., visibilities below 0.1), as documented in the literature (Deroo et al 2007;Ohnaka et al 2008;Paladini et al 2012;Sacuto et al 2013;Paladini et al 2017). In any investigation including 1D models, such low visibilities should be regarded with care.…”

New insights into the dust formation of oxygen-rich AGB stars

“…Asymmetries can be investigated either by comparing visibility data obtained at the same projected baseline length, but different position angles, or by analyzing the differential phases that are computed as a part of the EWS data reduction. The latter method was previously used by Ohnaka et al (2008); Paladini et al (2012); Sacuto et al (2013) to detect asymmetric intensity distributions of the environments of AGB stars.…”

“…Various infrared interferometric observations revealed deviations from spherical symmetry already in the AGB phase (Lopez et al 1997;Monnier et al 2004;Ragland et al 2006;Wittkowski et al 2011;Paladini et al 2012). Non-spherical structures in the molecular and dust shells are theoretically predicted by three dimensional radiation hydrodynamic simulations of the convective interior and the stellar atmosphere of AGB stars (Freytag & Höfner 2008), as well as by pulsation-and shock-induced weak chaotic motion in the extended atmosphere (Icke et al 1992;Ireland et al 2008;Wittkowski et al 2011).…”