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Old Dog, Old Tricks, Old Data — A Mass for Proxima Cen c

Published onJun 01, 2020
Old Dog, Old Tricks, Old Data — A Mass for Proxima Cen c

An investigation using astrometry secured with HST Fine Guidance Sensor 3 (Benedict et al. 1999) established a Proxima Centauri companion mass detection upper limit (Mp < 0.8MJup) with an upper period limit, P < 1000 days. In response to the discovery with precision radial velocity measurements (Damasso et al. 2020) of the long-period Proxima Cen c companion (P ∼ 1900 days) we re-analyzed Proxima astrometry, now informed by that orbit. Our goals included a refined parallax for Proxima Cen and a mass for the perturbing companion, Proxima Cen c. We have described the combined radial velocity and astrometry analysis used to produce system parallax, proper motion, and component mass in detail (Benedict et al. 2017). The astrometric reference frame for Proxima consists of eight stars. At each of 65 epochs we measured a subset of reference stars (typically six, depending on spacecraft roll) each 1–3 times, and Proxima Centauri 1-4 times for a total of 105 measurements of Proxima, spanning 2,031 days. Departing significantly from our previous analyses, we adopt Gaia DR2 (Gaia Collaboration et al. 2018) parallax and proper motion priors for the reference stars. We also include the Damasso et al. (2020) orbital elements; period, P, time of inferior conjunction, Tconj, and RV amplitude, Kc as priors. Damasso et al. (2020) assumed an eccentricity, εc = 0. We include all priors as observations with associated errors, arbitrarily assuming an error on the eccentricity of 0.1. We constrained the astrometry with the Damasso et al. (2020) RV results (c.f. Pourbaix & Jorissen 2000). Adopting M = 0.11 Msun (Benedict et al. 1999), we find M = 18 ± 5 M. With a combination of the Hipparcos catalog and Gaia DR2, Kervella et al. (2019) detected a deviation from a purely linear tangential proper motion in the Proxima Cen tangential velocity, significant at 1.8σ. More recently their reanalysis (Kervella et al. 2020) yields a more precise mass estimate. Our preliminary mass determination for Proxima Cen c is compatible with both their initial prediction of a planet with true mass 10 to 20 M⊕ with a semi major axis of 1 to 2 AU and their more recent measurement, M = 12(+12-5) M. Assuming a coplanar system, Proxima Cen b would have Mb = 3 ± 0.3 M. Gaia will eventually provide better companion masses.

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