Observations of increasingly higher spatial resolution reveal the existence of asymmetries in the circumstellar envelopes of a small fraction of asymptotic giant branch (AGB) stars. Although there is no general consensus for their origin, a binary companion star may be responsible. Within this framework, we investigate the gravitational effects associated with a sufficiently wide binary system, where Roche lobe overflow is unimportant, on the outflowing envelopes of AGB stars using three dimensional hydrodynamic simulations. The effects due to individual binary components are separately studied, enabling investigation of the stellar and circumstellar characteristics in detail. The reflex motion of the AGB star alters the wind velocity distribution, thereby, determining the overall shape of the outflowing envelope. On the other hand, the interaction of the companion with the envelope produces a gravitational wake, which exhibits a vertically thinner shape. The two patterns overlap and form clumpy structures. To illustrate the diversity of shapes, we present the numerical results as a function of inclination angle. Not only is spiral structure produced by the binary interaction, but arc patterns are also found that represent the former structure when viewed at different inclinations. The arcs reveal a systematic shift of their centers of curvature for cases when the orbital speed of the AGB star is comparable to its wind speed. They take on the shape of a peanut for inclinations nearly edge-on. In the limit of slow orbital motion of the AGB star relative to the wind speed, the arc pattern becomes nearly spherically symmetric. We find that the aspect ratio of the overall oblate shape of the pattern is an important diagnostic probe of the binary as it can be used to constrain the orbital velocity of the AGB star, and moreover the binary mass ratio.

With the advent of high-resolution high-sensitivity observations, spiral patterns have been revealed around several asymptotic giant branch (AGB) stars. Such patterns can provide possible evidence for the existence of central binary stars embedded in outflowing circumstellar envelopes. Here, we suggest the viability of explaining the previously observed incomplete ring-like patterns with the spiral-shell structure due to the motion of (unknown) binary components viewed at an inclination with respect to the orbital plane. We describe a method of extracting such spiral-shells from an incomplete ring-like pattern to place constraints on the characteristics of the central binary stars. The use of gas kinematics is essential in facilitating a detailed modeling for the three-dimensional structure of the circumstellar pattern. We show that a hydrodynamic radiative transfer model can reproduce the structure of the HC 3 N molecular line emission of the extreme carbon star, CIT 6. This method can be applied to other sources in the AGB phase and to the outer ring-like patterns of pre-planetary nebulae for probing the existence of embedded binary stars, which are highly anticipated with future observations using the Atacama Large Millimeter/submillimeter Array.

Preplanetary nebulae (pPNe) and planetary nebulae (PNe) are evolved, mass-losing stellar objects that show a wide variety of morphologies. Many of these nebulae consist of outer structures that are nearly spherical (spiral/shell/arc/halo) and inner structures that are highly asymmetric (bipolar/multipolar) [1,2]. The coexistence of such geometrically distinct structures is enigmatic because it hints at the simultaneous presence of both wide and close binary interactions, a phenomenon that has been attributed to stellar binary systems with eccentric orbits [3]. Here we report new high-resolution molecular-line observations of the circumstellar spiral-shell pattern of AFGL 3068, an asymptotic giant branch (AGB) star transitioning to the pPN phase. The observations clearly reveal that the dynamics of the mass loss is influenced by the presence of an eccentric-orbit binary. This quintessential object opens a new window on the nature of deeply embedded binary stars through the circumstellar spiral-shell patterns that reside at distances of several thousand Astronomical Units (AU) from the stars.AFGL 3068, an extreme carbon star at the tip of the AGB evolutionary phase, is a remarkable source with the best-characterized, complete spiral pattern in its circumstellar envelope (CSE). This unambiguous spiral pattern was the first ever revealed surrounding an evolved star in a dust-scattered light image in the optical band (at 0.6 µm) of the Hubble Space Telescope (HST) [4,5]. The striking discovery of the presence of this very well-defined pattern has prompted new research on how binarity can affect mass outflows during late stages of stellar evolution (AGB, pPN, and PN). In particular, recent theoretical investigations have shown that such patterns can naturally be explained by the orbital motion of a mass-losing star in a binary system [6][7][8][9][10]. In the case of AFGL 3068, there are indeed two point-like sources in its central region detected with Keck adaptive optics near-infrared imaging, revealing a projected binary separation of 109 AU [4]. Constraints on its binary parameters have been derived on the basis of these HST and Keck images, assuming a circular orbit [9]. This further indicated that the degeneracy imposed by the two-dimensional image of the three-dimensional structure can be lifted by high-resolution molecular line observations.Our new observations of AFGL 3068 taken with the Atacama Large Millimeter/submillimeter Array (ALMA; see Methods for details on observations and data calibrations) unveil exceptionally detailed features in its CSE (Fig. 1; individual molecular lines are presented in Supplementary Figs 1-3). A

The binary characteristics of asymptotic giant branch (AGB) stars are imprinted in the asymmetric patterns of their circumstellar envelopes (CSEs). We develop a simple method for constraining the orbital parameters of such binary stars from the characteristics of a spiral-like pattern observed at large distances from the central stars. We place constraints on the properties of AFGL 3068 (i.e., the masses of binary components, the viewing inclination of the orbital plane, as well as the orbital period, velocity, and separation). In particular, the mass of the companion star of AFGL 3068 is estimated to be greater than 2.6M ⊙ . This method is applicable to other AGB stars, providing a step toward understanding the role binary stars can play in explaining the diverse patterns in observed CSEs.

We present near-infrared coronagraphic imaging polarimetry of RY Tau. The scattered light in the circumstellar environment was imaged at the H band at a high resolution (∼0. 05) for the first time, using Subaru/HiCIAO. The observed polarized intensity (PI) distribution shows a butterfly-like distribution of bright emission with an angular scale similar to the disk observed at millimeter wavelengths. This distribution is offset toward the blueshifted jet, indicating the presence of a geometrically thick disk or a remnant envelope, and therefore the earliest stage of the Class II evolutionary phase. We perform comparisons between the observed PI distribution and disk models with (1) full radiative transfer code, using the spectral energy distribution (SED) to constrain the disk parameters; and (2) monochromatic simulations of scattered light which explore a wide range of parameters space to constrain the disk and dust parameters. We show that these models cannot consistently explain the observed PI distribution, SED, and the viewing angle inferred by millimeter interferometry. We suggest that the scattered light in the near-infrared is associated with an optically thin and geometrically thick layer above the disk surface, with the surface responsible for the infrared SED. Half of the scattered light and thermal radiation in this layer illuminates the disk surface, and this process may significantly affect the thermal structure of the disk.