Epsilon Aurigae: An Improved Spectroscopic Orbital Solution

Abstract

A rare eclipse of the mysterious object Aurigae will occur in 2009-2011. We report an updated single-lined spectroscopic solution for the orbit of the primary star based on 20 years of monitoring at the CfA, combined with historical velocity observations dating back to 1897. There are 518 new CfA observations obtained between 1989 and 2009. Two solutions are presented. One uses the velocities outside the eclipse phases together with mid-times of previous eclipses, from photometry dating back to 1842, which provide the strongest constraint on the ephemeris. This yields a period of 9896.0 ± 1.6 days (27.0938 ± 0.0044 years) with a velocity semiamplitude of 13.84 ± 0.23 km s −1 and an eccentricity of 0.227 ± 0.011. The middle of the current ongoing eclipse predicted by this combined fit is JD 2,455,413.8 ± 4.8, corresponding to 2010 August 5. If we use only the radial velocities, we find that the predicted middle of the current eclipse is nine months earlier. This would imply that the gravitating companion is not the same as the eclipsing object. Alternatively, the purely spectroscopic solution may be biased by perturbations in the velocities due to the short-period oscillations of the supergiant.

A high angular and spectral resolution view into the hidden companion ofεAurigae

“…Moreover, this angular momentum vector is parallel to the observed orbital motion of the disk around the system center of mass (Kloppenborg et al 2010). Thus, we can assume that the F star rotation axis is parallel to these, and consistent with its own orbital motion (Chadima et al 2011;Stefanik et al 2010). …”

“…14 in Chadima et al 2011), which is comparable to the full amplitude of the orbital motion of the F primary (28-29 km s −1 ; Stefanik et al 2010;Chadima et al 2010). This means that even the latest published sets of orbital elements, despite being derived carefully (Stefanik et al 2010;Chadima et al 2010), must be accepted with some caution. Both studies are inevitably based on RV measurements of a number of investigators.…”

“…ε Aur (7 Aur, HD 31964, HR 1605) is a bright (V 3. m 0), singleline eclipsing binary with a very long orbital period of 27.1 years (9890. d 3) (Ludendorff 1903;Stefanik et al 2010;Chadima et al 2010) and a nearly two years long primary eclipse. During the last eclipse, which ended in the summer of 2011, the near-IR interferometric observations carried out by Kloppenborg et al (2010, K10 hereafter) demonstrated beyond any doubt that the eclipsing body is indeed a huge, dark and flattened disk as seen in the 1.6 μm continuum.…”

“…The spectral variability affects different spectral lines differently (Struve 1956), and can cause apparent cyclic radial-velocity (RV hereafter) variations with a full amplitude of some 30 km s −1 (see, e.g., Fig. 14 in Chadima et al 2011), which is comparable to the full amplitude of the orbital motion of the F primary (28-29 km s −1 ; Stefanik et al 2010;Chadima et al 2010). This means that even the latest published sets of orbital elements, despite being derived carefully (Stefanik et al 2010;Chadima et al 2010), must be accepted with some caution.…”

“…Since no secondary eclipse has ever been observed, one must be aware that the geometry of the orbit need not be exactly the one derived from the orbital solution. The difference to the true orbit can actually be quite large, as is documented by the fact that the orbital solution by Stefanik et al (2010), based on RVs only, predicts the recent mid-eclipse for the end of October 2009, while it actually occurred somewhere on the boundary of July and August 2010. Both Stefanik et al (2010) and Chadima et al (2010) had to use combined solutions based on RVs and photometry to obtain a good prediction of the latest mid-eclipse.…”