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Precise Astrometric Mass Derivation of an Unresolved BD Companion Using AO Techniques

This research aims to accurately determine the mass of an unresolved Brown Dwarf companion to an M-Dwarf using astrometric observations aided by adaptive optics (AO). The study involves combining radial velocity data with astrometric measurements to derive the true mass and orbital parameters of the companion. Observations of the candidate system GJ 1046, along with challenges such as parallax movement and data reduction techniques, are discussed. Astrometric corrections, data reduction methods, and the current status of the research are detailed. The importance of additional techniques beyond RV astrometry, the need for high precision, and monitoring plate-scale variability are highlighted. The study emphasizes the careful monitoring of plate-scale changes and the challenges of reference star selection due to the limitations of current AO systems' field of view.

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Precise Astrometric Mass Derivation of an Unresolved BD Companion Using AO Techniques

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  1. Deriving the true mass of an unresolved BD companion by AO aided astrometry Eva Meyer MPIA, Heidelberg Supervisor: Martin Kürster New Technologies for Probing the Diversity of Brown Dwarfs and Exoplanets 19-24 July 2009

  2. What do I do ? • Astrometric follow-up observation of an RV discovered Brown Dwarf companion to an M-Dwarf from ground with AO (Kürster et al. 2008) • That means: measuring the movement of the host star in the plane perpendicular to the line of sight • Combining RV data with astrometric data to derive the true mass • Orbital parameters from RV: P, e, a, ω, T0 • RV only gives minimum mass m sini

  3. Astrometric follow up observations

  4. The Fit Parameters • Need to fit 7 parameters simultaneously • 2 coordinate zero-points, 0, 0 • 2 proper motions, ,  • 1 parallax,  • 1 inclination, i • 1 longitude of the ascending node,  4 observations minimum

  5. The candidate - GJ 1046 • Brown Dwarf orbiting an M2.5V-star (Kürster et al. 2008) • K = 7.03 mag • Distance ~ 14 pc • Minimum companion mass: m sini = 27 MJup • P = 169 d, a = 0.42 AU, e = 0.26 • Brown Dwarf desert candidate • Expected minimum peak-to-peak signal: 3.7 mas • Aimed precision: 0.5 mas • Reference star at separation of ~ 30’’ (by chance)

  6. Observations • Observations started last summer with NACO @ VLT, S27 camera • 8 observations in roughly 3 week intervals K = 7.03 mag K = 13.52 mag DSS

  7. i=45 =60 i=30 =150 no companion Difficulties • Parallax movement • to faint for a good HIPPARCOS parallax • Maximum mass: 112 MJup • Probability of stellar companion: 2.9 %

  8. Data Reduction • Flatfield, dark correction • Images stacked with Jitter routine (eclipse) • PSF fitted with Moffat-function • Positional error estimation from fit with Bootstrap resampling method • 0.009 mas (0.012 mas) bright star • 0.286 mas (0.579 mas) faint reference star

  9. Astrometric Corrections • Differential Aberration: • Relativistic effect due to movement of earth • need to know exact position of stars • Error due to abs. pos. error ~1μas or less • Atmospheric Refraction • Negligible due to narrow band filter • Plate-scale changes • Less than 1% (Köhler et al. 2008)

  10. Reference field in 47 Tuc • Immediately before target • Derive change of plate-scale over observations • Check rotation of field

  11. Current Status • Working on orbit-fit • Derive plate-scale changes and see how big this effect is • Observing time in P83, last chance to observe target with NACO, + 3 datapoints • Derive proper motion independently, long baseline

  12. Summary: • One needs additional techniques to derive mass of a planet besides RV astrometry (transit) • 7 parameters to fit • Very high precision ~0.5 mas or better • But plate-scale variability needs to be monitored carefully • More than one reference star is preferable but difficult with the small FoV of today’s AO systems

  13. Thank you!

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