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Deriving the true mass of an unresolved BD companion by AO aided astrometry Eva Meyer MPIA, Heidelberg Supervisor: Martin Kürster New Technologies for.

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Presentation on theme: "Deriving the true mass of an unresolved BD companion by AO aided astrometry Eva Meyer MPIA, Heidelberg Supervisor: Martin Kürster New Technologies for."— Presentation transcript:

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 July 2009

2 Eva Meyer Outline Introduction, or what do I do Candidate Data Reduction Problems and Difficulties Current Status Summary

3 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, T 0 RV only gives minimum mass m sini

4 Eva Meyer Astrometric follow-up observations Reminder: Orbital parameters RV -> planet -> parameters: P, e, a, ω But one only gets a lower mass limit: msini Period P Semi-mayor axis a Eccentricity e Inclination i Argument of perihelion ω Longitude of the acsending node Ω

5 Eva Meyer Astrometric follow up observations

6 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

7 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 M Jup 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)

8 Eva Meyer Observations Observations started last summer with VLT, S27 camera 8 observations in roughly 3 week intervals K = 7.03 mag K = mag DSS

9 Eva Meyer Difficulties Parallax movement –to faint for a good HIPPARCOS parallax Maximum mass: 112 M Jup Probability of stellar companion: 2.9 % i=45  =60 i=30  =150 no companion

10 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

11 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 ~1 μ as or less Atmospheric Refraction –Negligible due to narrow band filter Plate-scale changes –Less than 1% (Köhler et al. 2008)

12 Eva Meyer Reference field in 47 Tuc Immediately before target Derive change of plate-scale over observations Check rotation of field

13 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

14 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

15 Eva Meyer Thank you!


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