A.K.A., a Simple Explanation of All Exoplanet Images to Date
Total Page:16
File Type:pdf, Size:1020Kb
TheThe RoleRole ofof StellarStellar MassMass inin High-ContrastHigh-Contrast ImagingImaging JustinJustin R.R. Crepp,Crepp, JohnJohn A.A. JohnsonJohnson (a.k.a.,(a.k.a., aa simplesimple explanationexplanation ofof CaliforniaCalifornia InstituteInstitute ofof TechnologyTechnology arXiv:1103:4910 (accepted to ApJ) allall exoplanetexoplanet imagesimages toto date)date) (1)(1) WhyWhy havehave thethe firstfirst handfulhandful ofof exoplanetexoplanet imagesimages beenbeen (5)(5) TableTable 11 andand ourour simulationsimulation resultsresults areare consistentconsistent withwith thethe aroundaround M,KM,K andand AA stars,stars, butbut nono spectralspectral typestypes inin between?between? (3)(3) TwoTwo additionaladditional observationalobservational resultsresults supportsupport presencepresence ofof twotwo differentdifferent planetplanet formationformation mechanisms:mechanisms: oneone operatingoperating ThisThis mustmust bebe tellingtelling usus somethingsomething aboutabout planetplanet formation.formation. interpretationinterpretation ofof BetaBeta PicPic bb andand thethe HRHR 87998799 planetsplanets asas closeclose toto starsstars thatthat resultsresults inin strongstrong correlationscorrelations betweenbetween starstar massmass formingforming byby ªcore-accretionº:ªcore-accretionº: andand planetplanet properties,properties, andand anotheranother operatingoperating furtherfurther fromfrom starsstars (or(or (i) Beta Pic b and HR 8799 bcde fall in the possiblypossibly atat allall separations)separations) wherewhere therethere existsexists littlelittle correspondencecorrespondence same sector of the mass / semi-major axis Beta Pic b plot as RV planets (Currie et al. 2011). betweenbetween starstar massmass andand planetplanet properties.properties. High metallicity! (ii) Spectra of HR 8799 b indicate a very HR 8799 bcde Some observational constraints on the high metal content (Barman et al. 2011). presence of ultra-wide companions: ~1% ~1% “Extrapolating the inferred companion Directly imaged planetary-mass (m<15M ) companions with stellar primaries (http://exoplanet.eu/). mass function to <13M , we find that Fig. 1: Directly imaged planetary-mass (m<15MJup) companions with stellar primaries (http://exoplanet.eu/). J Mode 1 Fig. 3 ± Mass ratio vs. semi-major axis for planetary mass companions from Currie et al. 2011. Given sufficient subdeuterium-burning “planetary” There currently exists an apparent dichotomy between the detection of planets around A-stars in the solar companions, if able to form through gravo- ~80 AU Mode 1 time baselines, Doppler RV teams will eventually discover companions with the same orbital period as Beta Pic turbulent fragmentation, exist in wide neighborhood and ultra-wide separation planets around M, K stars in clusters. time baselines, Doppler RV teams will eventually discover companions with the same orbital period as Beta Pic orbits around ~1% of Sun-like stars” b and HR 8799 e (indeed, many RV trends already exist in current data sets!). Given their co-rotation and orbit ~5% configuration, it is difficult to justify invoking separate formation mechanisms for HR 8799 e and HR 8799 b. Metchev & Hillenbrand 2010 ~20% Further, HR 8799 e has a high metal content, and our simulations of an extended (a<100 AU) RV population indicate that it is natural to image the first exoplanets around nearby A-type stars. “... naïve estimate of the frequency (of Mode 2 Mode 2 (2)(2) TheThe DopplerDoppler techniquetechnique hashas shownshown thatthat strongstrong ultra-wide planetary-mass companions) is approximately 4% ...” correlationscorrelations existexist betweenbetween starstar massmass andand planetplanet properties:properties: (4)(4) HowHow dodo wewe explainexplain thethe bottombottom portionportion ofof TableTable 1?1? Ireland et al. 2011 A-starA-star M-starM-star (1) the occurrence rate of gas giant planets correlates with star mass (Johnson et al. 2010) ObservationsObservations ofof aa groupgroup ofof starsstars havinghaving thethe samesame distancedistance andand (2) planets orbiting A-stars are more massive than those orbiting lower-mass stars (Bowler et al. 2010) Fig. 5 ± The simplest explanation for all exoplanet images to date: two modes of formation. Companions that form close to their star (3) planets orbiting A-stars have wider orbits than those orbiting lower-mass stars (Bowler et al. 2010) ageage representsrepresents aa specialspecial casecase inin high-contrasthigh-contrast imaging,imaging, exhibit very strong correlations between star mass and planet properties: planet occurrence rate, semimajor axis extent, and mass all rise with increasing star mass. Mode 1 (blue region) is likely ªcore-accretionº and is also responsible for creating the Doppler RV planet CanCan extrapolationextrapolation ofof thethe radialradial velocityvelocity (RV)(RV) planetplanet neutralizingneutralizing twotwo importantimportant trade-offstrade-offs betweenbetween low-masslow-mass starsstars population. Another mechanism, Mode 2 (orange region) has a lower occurrence rate at wide separations. Our simulations indicate that companions with ultra-wide orbits may exhibit very little correspondence between star mass and planet properties. These results populationpopulation toto separationsseparations accessibleaccessible toto high-contrasthigh-contrast andand high-masshigh-mass stars.stars. SimulationsSimulations ofof aa tighttight clustercluster showshow thatthat are consistent with the predictions of Boley et al. 2009. instrumentsinstruments explainexplain thethe toptop portionportion ofof TableTable 1?1? therethere isis aa situationsituation wherewhere low-masslow-mass starsstars cancan dominatedominate bothboth YES!YES! (6)(6) ThereThere isis anotheranother importantimportant effecteffect toto consider:consider: youngyoung stellarstellar clustersclusters thethe numbernumber ANDAND efficiencyefficiency ofof detections.detections. ThisThis occursoccurs whenwhen areare subjectsubject toto anan observationalobservational biasbias relatedrelated toto smallsmall numbernumber statistics.statistics. allall correlationscorrelations betweenbetween starstar massmass andand planetplanet propertiesproperties areare Low-massLow-mass starsstars produceproduce thethe firstfirst ultra-wideultra-wide separationseparation exoplanetexoplanet imagesimages removedremoved (the(the redred curvescurves below).below). irrespectiveirrespective ofof correlationscorrelations betweenbetween starstar massmass andand planetplanet properties.properties. Cluster N D (pc) Age (Myr) AB Dor 89 34+/-26 70 Argus 64 106+/-51 40 Beta pic 50 31+/-21 10 Tuc-hor 49 48+/-7 30 Columba 41 82+/-30 30 Eta Cha 24 108+/-9 6 Carina 23 85+/-35 30 TW hydra 22 48+/-13 8 Fig. 6 ± The number of detections in a (tightly bound) young stellar cluster always favors Octans 15 141+/-34 20 low-mass stars. Since the stellar IMF falls off faster than the planet occurrence rate grows, MK stars are the first spectral-types to produce planet images. This bias is exacerbated by the fact that: (i) half of star systems are binaries and high-contrast teams avoid binaries; (ii) the youngest stars are at a distance of ~140 pc, forcing the overall planet occurrence rate to take on its value at wide separations; (iii) the occurrence rate of (7)(7) Conclusions:Conclusions: ultra-wide separation planets appears to be only several percent! (i)(i) ExtrapolationExtrapolation ofof thethe DopplerDoppler RVRV planetplanet populationpopulation toto widewide (tens(tens ofof AU)AU) separationsseparations resultsresults inin anan excellentexcellent matchmatch betweenbetween simulationssimulations andand observationsobservations toto date,date, bothboth detectionsdetections andand non-detections.non-detections. Fig. 2 ± Planet detection numbers (top) and rates (bottom) as a function of host star mass for a 50 pc age-and- (ii)(ii) HRHR 87998799 bcdebcde andand BetaBeta PicPic bb likelylikely formedformed byby thethe samesame processprocess asas thosethose ofof thethe brightness selected ground-based imaging survey using contrast levels comparable to present-day instruments. DopplerDoppler RVRV population.population. Over-plotted are non-detections from a number of previous programs. Extrapolation of the Doppler RV population to wide semimajor axes yields an excellent agreement between simulations and observations to (iii)(iii) A-starsA-stars areare idealideal high-contrasthigh-contrast imagingimaging targetstargets forfor reasonsreasons inin additionaddition toto theirtheir date, in terms of both absolute and relative detection rates. Thus far, only 2 planetary systems have been Fig. 4 ± Planet detection numbers (top) and rates (bottom) as a function of host star mass for simulations imaged from the ground in the parameter space considered above (bright, young, nearby stars): HR 8799 and intrinsicintrinsic youthyouth andand abilityability toto serveserve asas aa brightbright beaconbeacon forfor ªextremeºªextremeº AOAO systems.systems. of a young stellar association synthesized to resemble Tucana-Horologium. When correlations between Beta Pic. Both are A-stars. Therefore, the slope of the detection rate curve as a function of star mass must be star mass and planet properties are removed, the number of detections essentially follows the IMF. (iv)(iv) ObservationsObservations ofof aa young,young, tightlytightly boundbound clustercluster representsrepresents aa specialspecial casecase inin positive. This occurs when correlations exist between star mass and planet properties. high-contrasthigh-contrast imagingimaging wherewhere MK-starsMK-stars dominatedominate thethe numbernumber ofof detections.detections..