Deriving the true mass of an unresolved BD companion by AO aided astrometry Eva Meyer MPIA,...
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Transcript of Deriving the true mass of an unresolved BD companion by AO aided astrometry Eva Meyer MPIA,...
Deriving the true mass of an unresolved BD companion by
AO aided astrometry
Eva MeyerMPIA, Heidelberg
Supervisor: Martin Kürster
New Technologies for Probing the Diversity of Brown Dwarfs and Exoplanets
19-24 July 2009
Eva Meyer
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
Eva Meyer
Astrometric follow up observations
Eva Meyer
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
Eva Meyer
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)
Eva Meyer
Observations
• Observations started last summer with NACO @ VLT, S27 camera
• 8 observations in roughly 3 week intervals
K = 7.03 mag
K = 13.52 magDSS
Eva Meyer
Difficulties
• Parallax movement– to faint for a good HIPPARCOS parallax
• Maximum mass: 112 MJup
• Probability of stellar companion: 2.9 %
i=45 =60i=30 =150no companion
Eva Meyer
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
Eva Meyer
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)
Eva Meyer
Reference field in 47 Tuc
• Immediately before target
• Derive change of plate-scale over observations
• Check rotation of field
Eva Meyer
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
Eva Meyer
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
Eva Meyer
Thank you!