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

Abstract

Recent ALMA observations indicate that CW Leo, the closest carbon-rich AGB star to the Sun, might have a low-mass stellar companion. We present archival groundbased astrometric measurements of CW Leo obtained within the context of the Torino Parallax Program and with > 6 yr (1995 − 2001) of time baseline. The residuals to a single-star solution show significant curvature, and they are strongly correlated with the well-known I-band photometric variations due to stellar pulsations. We describe successfully the astrometry of CW Leo with a variability-induced motion (VIM) + acceleration model. We obtain proper motion and parallax of the center-of-mass of the binary, the former in fair agreement with recent estimates, the latter at the near end of the range of inferred distances based on indirect methods. The VIM + acceleration model results allow us to derive a companion mass in agreement with that inferred by ALMA, they point towards a somewhat longer period than implied by ALMA, but are not compatible with much longer period estimates. These data will constitute a fundamental contribution towards the full understanding of the orbital architecture of the system when combined with Gaia astrometry, providing a ∼ 25 yr time baseline.

The Evolving Radio Photospheres of Long-period Variable Stars

“…These values agree with the determination of M12 to within uncertainties. Our µ x determination also agrees with Sozzetti et al (2017), but we find significant disagreement in the y component of motion, where the latter authors report µ y =30.22±2.02 mas yr −1 based on a single-star fit to the data and µ y =25.43±1.69 mas yr −1 based on a "VIM+acceleration" model, intended to account from perturbations from a suspected binary companion. A possible explanation for this discrepancy in the y component of motion is that (1969.274, 1987.419, 1993.07, and 2006.145) taken from Becklin et al (1969), Menten et al (2006), and M12.…”

“…The time elapsed between our recent measurement and the previous high-precision radio measurement of M12 (∼8 yr) is slightly larger than the time span of the measurements presented by Sozzetti et al (2017). However, we find no evidence for excursions of the motion of the star compared with an extrapolation of the data between 1969-2006.…”

“…More recently, Sozzetti et al (2017) reported astrometric measurements of IRC+10216 based on archival (1995-2001) I-band (∼800 nm) data. They found an east-west component of the proper motion, µ x , comparable to within uncertainties with that derived by M12, but reported a significantly different north-south component, µ y , and postulated that this difference may be due to the effects of a binary companion and its so-called variability-induced motion (VIM) on the apparent motion of the star.…”

Templates of Binary-induced Spiral-shell Patterns around Mass-losing Post-main-sequence Stars

“…From various line surveys for IRC+10216, which include molecular lines emanating only in the very central region, the spatial profile of the wind velocity has been explored and the increase of the velocity with radius has been interpreted on the basis of several dust acceleration models (e.g., Patel et al 2011;Decin et al 2015). However, if IRC+10216 is a binary system as proposed by some authors (e.g., Decin et al 2015;Cernicharo et al 2015;Kim et al 2015;Sozzetti et al 2017), the interpretation of its velocity profile requires an additional consideration. As seen in the rightmost column of Figure 5 in the central 100 AU region, the fluid velocity can be accelerated by the gravitational potential of the companion star.…”