Multiple systems as a source of false positives in exo-planet detections

Transcription

1 Multiple systems as a source of false positives in exo-planet detections Andrei Tokovinin ( (NOAO/CTIO) Jean-Louis Halbwachs (Observatoire de Strasbourg) Astrometric companion to Zeta Aqr-A imaged at SOAR May 2013, Marseille 1

2 Outline Review of multiplicity statistics History Sample of F,G dwarfs within 67pc Companion detection limits Distribution of periods and mass ratios Statistics of hierarchies Focus on binary secondaries (false positives?) Only ~ 10% of solar-type stars have 2 companions, but about 2% may be false-positives May 2013, Marseille 2

3 Multiplicity statistics in the solar neighborhood 1924 Öpik, Multiple stars seem to represent the general rule in our stellar universe 1976 Abt & Levy Bright stars, sgle:dble:triple:4uple = 42:46:9:2 % 1991 Duquennoy & Mayor 22pc, 164**, 57 % of G- dwarf primaries have a companion with q > Tokovinin MSC catalogue; 612 multiple systems 2010 Raghavan et al. 454** 25pc, sgle:dble:triple:4uple = 56:33:8:3 % 2013? Tokovinin et al. New sample 5000** 67pc Multiplicity statistics is still a challenge! May 2013, Marseille 3

4 Three ways to study statistics Sample, get them all (Raghavan et al. 2010) Sample, corrected for incompleteness (FG-67pc sample, 2013) Use catalogs (uncertain selection!) May 2013, Marseille 4

5 More on the FG-67 sample 4846 systems from Hipparcos: 0.5 < V-I <0.8 Plx >15 mas +/-10% Triple: ~8.5% Quadruple etc: ~2.5% Complete! Un-evolved May 2013, Marseille 5

6 Raw numbers in the FG-67pc Single, binary, triple, quadr = 3083, 1422, 285, 51 Hierarchical level Number The raw numbers must be corrected for selection May 2013, Marseille 6

7 Companion detection: Speckle & AdaptiveOptic AO (8-m): ΔK<7, ρ>0.05 (only a small subset) Speckle (4-m): ΔI<5.3, ρ>0.03 (y), (I) K-band AO 2MASS May 2013, Marseille 7

8 Speckle detects binary secondaries New triple HIP How many secondary companions are actually close pairs? May 2013, Marseille 8

9 Robo-AO survey at Palomar Laser Guide Star system at 1.5-m, robotic In progress, ~400 targets covered in 2012B Try resolving (faint) secondary companions May 2013, Marseille 9

10 Binaries from astrometry resolved with Gemini-S NearInfraredCoronImager: NaturalGuideStar AO system, 2 λ simult. 104 targets surveyed, ~33% resolved (AJ, 2013, submitted) Unexpected binaries May 2013, Marseille 10

11 Coverage of the parameter space ~78% detection May 2013, Marseille 11

12 Distribution of all companions Unknown periods 257 SB without orbit 142 SB & astrometric 244 astrometric 50% detection May 2013, Marseille 12

13 Period distribution (all pairs) FG-67 Raghavan et al. (2010) May 2013, Marseille 13

14 /! Fitted models /! Frequency of binaries in space : /!\ Preliminary results 1 + β f(p,q)= ε q β exp[-(x-x 0 ) 2 /(2σ 2 )] σ (2π ) X = log 10 (P/1d) ε = fract of binaries Selection effects applied to simulated binaries parameters adjusted by ML Case ε x 0 σ β Level 1 (f M ) All pairs (f C ) f M =fract of n-sngl syst f C = N companion /N prim target When the selection effects are neglected: Fraction of non-single systems, f M = ( ) / 4841 = Average number of companions per primary target, f C = 2145/4841 = May 2013, Marseille

15 Fitted models /! /!\ Preliminary results 1 + β f(p,q)= ε q β exp[-(x-x 0 ) 2 /(2σ 2 )] σ (2π ) X = log 10 (P/1d) ε = fract of syst. Selection effects applied to simulated binaries parameters adjusted by ML Case f M ε x 0 σ β Level Level f M =fract of systems with respect to the upper level May 2013, Marseille

16 Statistical model of hierarchies Select inner and outer systems from the generating distribution, keep only stable combinations log(p L /P S) > depending on eccentricity May 2013, Marseille 16

17 Period distribution, level 11 For dynamical stability, P out /P in > 4.7 (1 e L ) -1.8 (1+e L ) 0.6 (1+q out )0.1 Inner binaries can t have periods as long as the outer ones May 2013, Marseille 17

18 Fitted models /! /!\ Preliminary results 1 + β f(p,q)= ε q β exp[-(x-x 0 ) 2 /(2σ 2 )] σ (2π ) X = log 10 (P/1d) ε = fract of syst. Introduction of a cut-off in period after generation of the components at level 11 and 12 Case ε x 0 σ β Level 11 (0.21) Generate Generate May 2013, Marseille

19 Analytical estimates of sub-system system frequency Double integral of fitted models over outer and inner periods, in desired P intervals Fraction of level 11 (all P): 0.10 Fraction of level 12 for outer period<10 5 d: 0.030, but when systems with level 11 are discarded This model can be used for first-order evaluation of false positives in exo-planet work. Include mass ratio, if necessary. May 2013, Marseille 19

20 HIP 11072: a potential false positive HIP = HR 695 = κ For at 20pc. P=25.8yr ΔI=3.7 m B is a binary! P~ 3.7 days Lines of Ba,Bb are detected Would be a problem if P~1yr Tokovinin 2013 AJ, 145, 76 May 2013, Marseille 20

21 Exo-planets or binary secondaries? HIP (Chauvin et al. 2011): 72-yr astrometric binary resolved. Planet around A, P=3.6yr. Or Ba,Bb? HD (T.Roell et al. 2010): F8V+M3V at 2. B is a binary (resolved), P 682 d, close to P planet =536 d. HIP 12184: New companion at 0.5 discovered with RoboAO. Is it binary, too, or is the 335-d planet real? CONCLUSION: Focus on binary secondaries May 2013, Marseille 21