An Astrometrically Measured Mass for Extrasolar Planet Gliese 876b

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An Astrometrically Measured Mass for Extrasolar
Planet Gliese 876b

• By Michael McElwain
• Presented March 14, 2003

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Paper details

• A Mass For the Extrasolar Planet Gliese 876b
  Determined from Hubble Space Telescope Fine
  Guidance Sensor 3 Astrometry and High-Precision
  Radial Velocities
• Authors G.F. Benedict, B.E. McArthur, T.
  Forveille, X. Delfosse, E. Nelan, R.P. Butler, W.
  Spiesman, G. Marcy, B. Goldman, C. Perrier, W.H.
  Jefferys, M. Mayor
• 2002ApJ...581L.115B, published 2002 November 25

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Detecting Extrasolar Planets

• Radial Velocity Measurements
• Astrometry
• Photometry
• Interferometry
• Gravitational Microlensing

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Extrasolar Planet Update

• 91 planetary systems
• 105 planets
• 12 multiple planets systems

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Precisely Known Extrasolar Masses

• 1. HD 209458 - 0.69 0.05 MJ (transit)
• 2. Gl 876b 1.89 0.34 MJ (astrometry)
• 3. ?

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Mass Calculation from Transits

• r3GMP2/4p2
• Vpl2GM/r
• MplMV/Vpl -- observe KVsini
• Transits, sini 1, so you know Mpl
• Kepler mission 10-5 changes in flux

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Mass Calculation from Astrometry

• asini/pabs PK1(1-e2)1/2/2p4.7405
• Pourbaix Jorissen 2000
• HST FGS3, SIM

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GL 876 Profile

• Sp. Type M4V
• ? (mas) 214.6 or d (pc) 4.7
• m_v (mag) 10.16
• B-V (mag) 1.597
• T_eff (K) 3200
• 2 Extrasolar planets Gl 876b, Gl 876c
• Trilling searched for a circumstellar disk around
  this object, not found.

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Observations

• Radial Velocities provided ephemeris for HST
  observations and orbital parameters P, K1, e.
  Taken from Keck, Lick, Coralie, Elodie.
  periastron, apastron
• HST Fine Guidance Sensor 3 (FGS 3)

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HST Fine Guidance Sensors

• 3 Sensors
• 2 used for positioning HST, and the 3rd can be
  used for astrometry
• operational modes POS, TRANS
• advantage from narrow psf

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Results

• 27 data sets were taken over 2 years.
• Measured 5 reference stars, one turned out to be
  binary.
• Residuals were on the order of 2 mas.

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Residuals

• Measure the distance between periastron and
  apastron, which is 2a.
• Really measuring the photocenter. It could be
  influenced by the companion, but is not the case
  as shown in simulations. The companion must have
  ?m 5.

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Orbital Fit

• a 0.25 0.06 mas
• i 84o 6o
• pabs 214.6 0.2 mas

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Future Prospects

• Use the FGS3 to measure masses of other
  extrasolar planets resolution problems
• SIM much higher resolution (µ)
• Planet formation theory
• What type of planets form around what type of
  stars?

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