The large-scale nebular pattern of a superwind binary in an eccentric orbit

Abstract

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

Reduction of the maximum mass-loss rate of OH/IR stars due to unnoticed binary interaction

“…Assuming a circular orbit, the orbital period can be estimated from the slope of the observed emission in the angle-radius plot 40 , which for OH 26.5+0.6 yields ∼430 years. Another method exploits the spiralarm spacing 39,41 .…”

Evidence for orbital motion of CW Leonis from ground-based astrometry

“…However, our results would tend to indicate a longer orbital period (by a factor of two or so) than that tentatively reported by D15. The TOPP observations cannot directly constrain the orbital eccentricity, that could very well be significant as a consequence of the mass loss and mass exchange between the binary components (e.g., Bonačić Marinović et al 2008;Kim et al 2017).…”

Mass loss of stars on the asymptotic giant branch

“…As a consequence, spirals can often be seen instead as concentric arcs. Ways to derive properties of the binary system based on the CSE morphology are discussed in Huggins et al (2009), Kim and Taam (2012c), and Kim et al (2013), and interpretations of observed morphologies through hydrodynamical simulations can be found in, e.g., Maercker et al (2012) and Kim et al (2017), Fig. 21, right panel.…”

“…The number of examples is steadily increasing, e.g., CW Leo Decin et al 2015), RW LMi (Dinh-V-Trung 2009;Claussen et al 2011;Kim et al 2015), R Scl , R Aqr and W Aql (Mayer et al 2013), and π 1 Gru . A prominent example is that of the dust and gas spiral towards AFGL3068 (Mauron and Huggins 2006;Kim et al 2017), Fig. 21, left panel.…”