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A Mass for Proxima Centauri C

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A Mass for Proxima Centauri C Old Dogs, Old Tricks, Old (and NEW!!) Data Masses for Proxima Centauri and the Exoplanet Companion Proxima Centauri c G. Fritz Benedict & Barbara E. McArthur McDonald Observatory University of Texas A Definition of Astrometry The object of the game is to figure out how the Universe works by watching tiny lights move across the sky George Johnson NYT Books 5 June 2005 Adventures in Astrometry - GFB Astrometry, a simple example 5 "plates" different scales Star Positions different orientations to Astrometry * * * * * * * * * * * * Modeling, universal * * * scale, rotation, and * * * 1 2 3 offset * * xi = ax + by + c * * 4 * eta = -bx + ay + f * * * * * * * The resulting six 5 (xi, eta) pairs are a catalog with Result of Overlap Solution to * * associated errors. Plate #1 * Also get the * * Precision = standard deviation of the distribution of residuals ( ) from the residuals to the fit. * model-derived positions (*) I 0.002 arcsec HST lets us Measure angles with millisecond of arc precision, a quarter 1500 miles away. 1992-1999 HST Fine Guidance Sensor (FGS) has millisecond of arc astrometric precision. We found nothing, BUT only looked for companions with periods shorter than 1000 days February 2020 Period = 1907 days. Real planet? HST 30th - GFB Past investigations yielding exoplanet masses from a combination of RV and HST/FGS astrometry Bean, J. L., McArthur, B. E., Benedict, G. F., et al. 2007, AJ, 134, 749 Benedict, G. F., McArthur, B. E., Bean, J. L., et al. 2010, AJ, 139, 1844 Benedict, G. F., McArthur, B. E., Forveille, T., et al. 2002a, ApJL, 581, L115 Benedict, G. F., McArthur, B. E., Gatewood, G., et al. 2006, AJ, 132, 2206 Martioli, E., McArthur, B. E., Benedict, G. F., et al. 2010, ApJ, 708, 625 McArthur, B. E., Endl, M., Cochran, W. D., et al. 2004, ApJL, 614, L81 McArthur, B. E., Benedict, G. F., Barnes, R., et al. 2010, ApJ, 715, 1203 McArthur, B. E., Benedict, G. F., Henry, G. W., et al. 2014, ApJ, 795, 41 Benedict, G. F. & Harrison, T. E. 2017, AJ, 153, 228 Benedict, G. F. , Harrison, T. E. , Endl, M. & Torres, G. 2018, RNAAS, 2, 7 March - April 2020 An old dog (me), doing old tricks (FGS Astrometry + RV priors) with old data (FGS data from 1992-1997) Estimated the mass of Proxima Cen c at 18±5 M ⊕ , about the mass of Uranus. Real Planet. Agrees with Kervella+ 2020, arXiv 2003.13106, deduced from Hipparcos and Gaia proper motion changes Later in April 2020 With great effort Gratton et al. tease out 9 separations and position angles covering 3.3 yr of a 5.5 yr orbit. This is the planet! Really? 1.5 N E Proxima Cen c 1.0 orbit from Gratton et al. 0.5 2020 measures. 0.0 Dec ["] Looks pretty good. -0.5 -1.0 -1.5 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 R.A. ["] The Gratton et al. (2020) position angles and separations let us treat the Proxima Cen system as a binary star system. We apply the Benedict et al (2016, AJ, 152 141) modeling. Our combined modeling *priors from the radial velocity orbit of Proxima Cen A *HST/FGS astrometry of Proxima Cen A *Sphere astrometry of Proxima Cen c yields a perturbation orbit and an exoplanet orbit, hence a mass fraction, and masses for both components. 1.5 N E 2 1.0 1 0.5 0 orbit y [mas] -1 0.0 -2 Dec ["] 2 1 -0.5 0 -1.0 orbit x [mas] -1 -2 -1.5 48500 49000 49500 50000 50500 51000 JD-2400000 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 R.A. ["] Results For Proxima Cen c P=5.54±0.03 y For Proxima Cen A P=2023±11 d p=767.00±0.1 mas a=1.105±0.027” a=1.44±0.01 AU a= 0.229±0.048 mas inc=131.6°±1.0° ecc=0.00±0.02 M=0.097±0.007 M☉ M=7±2 M ⊕ Conclusions • HST/FGS astrometry + RV yields Mc= 18±5 M⊕ • HST/FGS astrometry + RV + SPHERE yields Mc= 7±2 M ⊕ • Does the tension between these two mass estimates indicate a possible issue with the SPHERE measurements? • Need more SPHERE observations of Proxima Cen c • Gaia will significantly improve Proxima Cen A perturbation. • Gaia results by 2025. .
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