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!