Old Dogs, Old Tricks, Old (and NEW!!) Data

Masses for Proxima Centauri and the Exoplanet Companion Proxima Centauri c

G. Fritz Benedict & Barbara E. McArthurMcDonald 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 JohnsonNYT Books5 June 2005

Adventures in Astrometry - GFB

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Astrometry, a simple example5 "plates"different scalesdifferent orientations

Result of OverlapSolution toPlate #1

Precision = standard deviation of thedistribution of residuals ( ) from themodel-derived positions (*)

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I 0.002 arcsec

Star Positions to AstrometryModeling, universal scale, rotation, and offset

xi = ax + by + ceta = -bx + ay + f

The resulting six (xi, eta) pairs are a catalog with associated errors. Also get the residuals to the fit.

HST lets us Measure angles with millisecond of arc precision, a quarter 1500 miles away.

We found nothing, BUT only looked for companions with periods shorter than 1000 days

1992-1999

HST Fine Guidance Sensor (FGS) has millisecond of arc astrometric precision.

HST 30th - GFBPeriod = 1907 days. Real planet?

February 2020

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, 2206Martioli, 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, 41Benedict, G. F. & Harrison, T. E. 2017, AJ, 153, 228Benedict, G. F. , Harrison, T. E. , Endl, M. & Torres, G. 2018, RNAAS, 2, 7

Past investigations yielding exoplanet masses from a combination of RV

and HST/FGS astrometry

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 at18±5 M , about themass of Uranus. Real Planet.

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Agrees with Kervella+ 2020, arXiv 2003.13106, deduced from Hipparcos and Gaiaproper 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?

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Proxima Cen corbit from Gratton et al. 2020 measures.

Looks pretty good.

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.

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ResultsFor Proxima Cen cP=5.54±0.03 yP=2023±11 da=1.105±0.027”a=1.44±0.01 AUinc=131.6°±1.0°ecc=0.00±0.02

M=7±2 M⊕☉

For Proxima Cen Ap=767.00±0.1 mas

a= 0.229±0.048 mas

M=0.097±0.007 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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