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Radial Velocity Detection of Planets: II. Results • To date 701 planets have been detected with the RV method • ca 500 planets discovered with the RV method. The others are from transit searches • 94 are in Multiple Systems → exoplanets.org Telescope 1-m MJUO 1.2-m Euler Telescope 1.8-m BOAO 1.88-m Okayama Obs, 1.88-m OHP 2-m TLS 2.2m ESO/MPI La Silla 2.7m McDonald Obs. 3-m Lick Observatory 3.8-m TNG 3.9-m AAT 3.6-m ESO La Silla 8.2-m Subaru Telescope 8.2-m VLT 9-m Hobby-Eberly 10-m Keck Instrument Hercules CORALIE BOES HIDES SOPHIE Coude Echelle FEROS Tull Spectroraph Hamilton Echelle SARG UCLES HARPS HDS UVES HRS HiRes Wavelength Reference Th-Ar Th-Ar Iodine Cell Iodine Cell Th-Ar Iodine Cell Th-Ar Iodine Cell Iodine Cell Iodine Cell Iodine Cell Th-Ar Iodine Cell Iodine Cell Iodine Cell Iodine Cell Campbell & Walker: The Pioneers of RV Planet Searches 1988: 1980-1992 searched for planets around 26 solar-type stars. Even though they found evidence for planets, they were not 100% convinced. If they had looked at 100 stars they certainly would have found convincing evidence for exoplanets. Campbell, Walker, & Yang 1988 „Probable third body variation of 25 m s–1, 2.7 year period, superposed on a large velocity gradient“ e Eri was a „probable variable“ The first extrasolar planet around a normal star: HD 114762 with Msini = 11 MJ discovered by Latham et al. (1989) Filled circles are data taken at McDonald Observatory using the telluric lines at 6300 Ang as a wavelength reference Rate of Radial Velocity Planet Discoveries 51 Peg 51 Pegasi b: The Discovery that Shook up the Field Period = 4,3 Days Semi-major axis = 0,05 AU (10 Stellar Radii!) Mass ~ 0,45 MJupiter Discovered by Michel Mayor & Didier Queloz, 1995 Global Properties of Exoplanets: Mass Distribution i decreasing probability decreasing Because we only measure msini one could argue that all of these companions are not planets but low mass stars viewed near i = 0 degrees. Argument against stars #1 P(i < q) = 1– cos q Probability an orbit has an inclination less than q e.g. for m sin i = 0.5 MJup for this to have a true mass of 0.5 Msun sin i would have to be 0.01. This implies q = 0.6 deg or P =0.00005: highly unlikely! This argument was probably valid when you had 10 exoplanets, but with 700 it is highly unlikely that all of them are stellar companions viewed at a low inclination Argument against stars #2 We have detected approximately 200 transiting planets where we know the inclination. All of these have masses in the planetary regime. Global Properties of Exoplanets: Mass Distribution The Brown Dwarf Desert Planet: M < 13 MJup → no nuclear burning Brown Dwarf: 13 MJup < M < ~80 MJup → deuterium burning Star: M > ~80 MJup → Hydrogen burning Brown Dwarf Desert: Although there are ~100-200 Brown dwarfs as isolated objects, and several in long period orbits, there is a paucity of brown dwarfs (M= 13–50 MJup) in short (P < few years) as companion to stars An Oasis in the Brown Dwarf Desert: HD 137510 = HR 5740 Brown Dwarfs versus Planets Bump due to deuterium burning The distinction between brown dwarfs and planets is vague. Until now the boundary was taken as ~ 13 MJup where deuterium burning is possible. But this is arbitrary as deuterium burning has little influence on the evolution of the brown dwarf compared to the planet A better boundary is to use the different distributions between stars and planets: By this definition the boundary between planets and non-planets is 20 MJup A note on the naming convention: Name of the star: 16 Cyg If it is a binary star add capital letter B, C, D If it is a planet add small letter: b, c, d 55 CnC b : first planet to 55 CnC 55 CnC c: second planet to 55 CnC 16 Cyg B: fainter component to 16 Cyg binary system 16 Cyg Bb: Planet to 16 Cyg B The IAU has yet to agree on a rule for the naming of extrasolar planets Semi-Major Axis Distribution The lack of long period planets is a selection effect since these take a long time to detect The short period planets are also a selection effect: they are the easiest to find and now transiting surveys are geared to finding these. Eccentricity versus Orbital Distance Note that there are few highly eccentric orbits close into the star. This is due to tidal forces which circularizes the orbits quickly. Eccentricity distribution Fall off at high eccentricity may be partially due to an observing bias… e=0.4 e=0.6 e=0.8 w=0 w=90 w=180 …high eccentricity orbits are hard to detect! For very eccentric orbits the value of the eccentricity is is often defined by one data point. If you miss the peak you can get the wrong mass! At opposition with Earth would be 1/5 diameter of full moon, 12x brighter than Venus e Eri 16 Cyg Bb was one of the first highly eccentric planets discovered 2 ´´ Comparison of some eccentric orbit planets to our solar system Mass versus Orbital Distance There is a relative lack of massive close-in planets Classes of planets: 51 Peg Planets: Jupiter mass planets in short period orbits Classes of planets: 51 Peg Planets • ~40% of known extrasolar planets are 51 Peg planets with orbital periods of less than 20 d. This is a selection effect due to: 1. These are easier to find. 2. RV work has concentrated on transiting planets • 0.5–1% of solar type stars have giant planets in short period orbits • 5–10% of solar type stars have a giant planet (longer periods) Another short period giant planet Classes of planets: Hot Neptunes Santos et al. 2004 McArthur et al. 2004 Butler et al. 2004 Note that the scale on the yaxes is a factor of 100 smaller than the previous orbit showing a hot Jupiter Msini = 14-20 MEarth If there are „hot Jupiters“ and „hot Neptunes“ it makes sense that there are „hot Superearths“ CoRoT-7b Mass = 7.4 ME P = 0.85 d Hot Superearths were discovered by space-based transit searches Classes of Planets: The Massive Eccentrics • Masses between 7–20 MJupiter • Eccentricities, e > 0.3 • Prototype: HD 114762 discovered in 1989! m sini = 11 MJup Classes: The Massive Eccentrics As of 2011 there were no massive planets in circular orbits Classes: The Massive Eccentrics Now there is more, but still relatively few. Ignoring the blue points (close in planets) there are ~ 10 planets with masses > 10 MJup with e < 0.2 and ~20 with e > 0.2 Red: Planets with masses < 4 MJup Blue: Planets with masses > 4 MJup Planet-Planet Interactions Initially you have two giant planets in circular orbits These interact gravitationally. One is ejected and the remaining planet is in an eccentric orbit Lin & Ida, 1997, Astrophysical Journal, 477, 781L Classes: Planets in Binary Systems Why should we care about binary stars? • Most stars are found in binary systems • Does binary star formation prevent planet formation? • Do planets in binaries have different characteristics? • What role does the environment play? • Are there circumbinary planets? (see Kepler Lecture!) Some Planets in known Binary Systems: Star 16 Cyg B 55 CnC HD 46375 Boo And HD 222582 HD 195019 a (AU) 800 540 300 155 1540 4740 3300 For more examples see Mugrauer & Neuhäuser 2009, Astronomy & Astrophysics, vol 494, 373 and references therein There are very few planets in close binaries. The exception is g Cep. If you look hard enough, many exoplanet host stars in fact have stelar companions A new stellar companion to the planet hosting star HD 125612 Mugrauer & Neuhäuser 2009 Approximately 17% of the exoplanet hosting stars have stellar companions (Mugrauer & Neuhäuser 2009). Most of these are in wide systems. g Cep Ab: A planet that challenges formation theories The first extra-solar Planet may have been found by Walker et al. in 1992 in a binary system: Ca II is a measure of stellar activity (spots) g Cephei Planet Period Msini 2.47 Years 1.76 MJupiter e a K 0.2 2,13 AU 26.2 m/s Binary Period Msini 56.8 ± 5 Years ~ 0,4 ± 0,1 MSun e a 0,42 ± 0,04 18.5 AU K 1.98 ± 0,08 km/s g Cephei Primary star (A) Secondary Star (B) Planet (b) Neuhäuser et al. Derive an orbital inclination of AB of 119 degrees. If the binary and planet orbit are in the same plane then the true mass of the planet is 1.8 MJup. The planet around g Cep is difficult to form and on the borderline of being impossible. Standard planet formation theory: Giant planets form beyond the snowline where the solid core can form. Once the core is formed the protoplanet accretes gas. It then migrates inwards. In binary systems the companion truncates the disk. In the case of g Cep this disk is truncated just at the ice line. No ice line, no solid core, no giant planet to migrate inward. g Cep can just be formed, a giant planet in a shorter period orbit would be problems for planet formation theory. The interesting Case of 16 Cyg B Effective Temperature: A=5760 K, B=5760 K Surface gravity (log g): 4.28, 4.35 Log [Fe/H]: A= 0.06 ± 0.05, B=0.02 ± 0.04 16 Cyg B has 6 times less Lithium These stars are identical and are „solar twins“. 16 Cyg B has a giant planet with 1.7 MJup in a 800 d period Kozai Mechanism: One Explanation for the high eccentricty of 16 Cyg B Two stars are in long period orbits around each other. A planet is in a shorter period orbit around one star. If the orbit of the planet is inclined, the outer planet can „pump up“ the eccentricity of the planet. Planets can go from circular to eccentric orbits. This was first investigated by Kozai who showed that satellites in orbit around the Earth can have their orbital eccentricity changed by the gravitational influence of the Moon Kozai Mechanism: changes the inclination and eccentricity Planetary Systems: 94 Multiple Systems The first: Some Extrasolar Planetary Systems Star P (d) MJsini a (AU) e HD 82943 221 0.9 0.7 0.54 444 1.6 1.2 0.41 GL 876 47 UMa 30 61 1095 2594 0.6 2.0 2.4 0.8 HD 37124 153 0.9 550 1.0 55 CnC 2.8 0.04 14.6 0.8 44.3 0.2 260 0.14 5300 4.3 Ups And 4.6 0.7 241.2 2.1 1266 4.6 HD 108874 395.4 1.36 1605.8 1.02 HD 128311 448.6 2.18 919 3.21 HD 217107 7.1 1.37 3150 2.1 0.1 0.2 2.1 3.7 0.27 0.10 0.06 0.00 0.5 2.5 0.04 0.1 0.2 0.78 6.0 0.06 0.8 2.5 1.05 2.68 1.1 1.76 0.07 4.3 0.20 0.40 0.17 0.0 0.34 0.2 0.16 0.01 0.28 0.27 0.07 0.25 0.25 0.17 0.13 0.55 Star P (d) MJsini HD 74156 51.6 1.5 2300 7.5 HD 169830 229 2.9 2102 4.0 HD 160691 9.5 0.04 637 1.7 2986 3.1 HD 12661 263 1444 HD 168443 58 1770 HD 38529 14.31 2207 HD 190360 17.1 2891 HD 202206 255.9 1383.4 HD 11964 37.8 1940 2.3 1.6 7.6 17.0 0.8 12.8 0.06 1.5 17.4 2.4 0.11 0.7 a (AU) 0.3 3.5 0.8 3.6 0.09 1.5 0.09 e 0.65 0.40 0.31 0.33 0 0.31 0.80 0.8 2.6 0.3 2.9 0.1 3.7 0.13 3.92 0.83 2.55 0.23 3.17 0.35 0.20 0.53 0.20 0.28 0.33 0.01 0.36 0.44 0.27 0.15 0.3 The 5-planet System around 55 CnC 0.17MJ 5.77 MJ •0.11 M J Red lines: solar system plane orbits 0.82MJ • •0.03M J The Planetary System around GJ 581 16 ME 7.2 ME 5.5 ME Inner planet 1.9 ME Can we find 4 planets in the RV data for GL 581? Note: for Fourier analysis we deal with frequencies (1/P) and not periods n1 = 0.317 cycles/d n2 = 0.186 n3 = 0.077 n4 = 0.015 Almost: The Period04 solution: P1 = 5.38 d, K = 12.7 m/s Published solution: P1 = 5.37 d, K = 12.5 m/s P2 = 12.99 d, K = 3.2 m/s P2 = 12.93 d, K = 2.63 m/s P3 = 83.3 d, K = 2.7 m/s P3 = 66.8 d, K = 2.7 m/s P4 = 3.15, K = 1.05 m/s P4 = 3.15, K = 1.85 m/s s=1.17 m/s s=1.53 m/s Conclusions: 5.4 d and 12.9 d probably real, 66.8 d period is suspect, 3.15 d may be due to noise and needs confirmation. A better solution is obtained with 1.4 d instead of 3.15 d, but this is above the Nyquist sampling frequency Resonant Systems Systems Star P (d) MJsini a (AU) e HD 82943 221 0.9 0.7 0.54 444 1.6 1.2 0.41 → GL 876 30 61 55 CnC 14.6 44.3 2:1 0.6 2.0 0.1 0.2 0.27 0.10 → 2:1 0.8 0.2 0.1 0.2 0.0 0.34 → 3:1 HD 108874 395.4 1.36 1605.8 1.02 1.05 2.68 0.07 0.25 → 4:1 HD 128311 448.6 2.18 919 3.21 1.1 1.76 0.25 0.17 → 2:1 2:1 → Inner planet makes two orbits for every one of the outer planet Eccentricities • Period (days) Red points: Systems Blue points: single planets Mass versus Orbital Distance Eccentricities Red points: Systems Blue points: single planets Idea: If you divide the disk mass among several planets, they each have a smaller mass? The Dependence of Planet Formation on Stellar Mass Poor precision Too faint (8m class tel.). Ideal for 3m class tel. RV Error (m/s) Main Sequence Stars 2.9 2.0 1.6 1.2 1.05 0.9 0.8 0.7 0.5 Stellar Mass (solar masses) The shape of the previous histogram merely reflects the detection bias of the radial velocity method Exoplanets around low mass stars (Mstar < 0.4 Msun) Programs: • ESO UVES program (Kürster et al.): 40 stars • HET Program (Endl & Cochran) : 100 stars • Keck Program (Marcy et al.): 200 stars • HARPS Program (Mayor et al.):~200 stars Results: • ~15 planets around low mass (M = 0.15-0.4 Msun) • Giant planets (2) around GJ 876. Giant planets around low mass M dwarfs seem rare • Hot neptunes around several → low mass start tend to have low mass planets Currently too few planets around M dwarfs to make any real conclusions GL 876 System 1.9 MJ 0.6 MJ Inner planet 0.02 MJ Exoplanets around massive stars Difficult with the Doppler method because more massive stars have higher effective temperatures and thus few spectral lines. Plus they have high rotation rates. A way around this is to look for planets around giant stars. This will be covered in „Planets around evolved stars“ Result: Only a few planets around early-type, more massive stars, and these are mostly around F-type stars (~ 1.4 solar masses) Galland et al. 2005 HD 33564 M* = 1.25 msini = 9.1 MJupiter P = 388 days e = 0.34 F6 V star A Planet around an F star from the Tautenburg Program HD 8673 Mplanet = 14.6 MJup Period = 4.47 Years ecc = 0.72 An F4 V star from the Tautenburg Program P = 328 days Msini = 8.5 Mjupiter e = 0.24 Scargle Power M* = 1.4 Mסּ Frequency (c/d) Mstar ~ 1.4 Msun Mstar = 0.2-0.5 Msun Mstar ~ 1 Msun Preliminary conclusions: more massive stars have more massive planets with higher frequency. Less massive stars have less massive planets → planet formation is a sensitive function of the planet mass. Planets and the Properties of the Host Stars: The StarMetallicity Connection Astronomer‘s Metals More Metals ! Even more Metals !! The „Bracket“ [Fe/H] Take the abundance of heavy elements (Fe for instance) Ratio it to the solar value Take the logarithm e.g. [Fe/H] = –1 → 1/10 the iron abundance of the sun The Planet-Metallicity Connection? These are stars with metallicity [Fe/H] ~ +0.3 – +0.5 Valenti & Fischer There is believed to be a connection between metallicity and planet formation. Stars with higher metalicity tend to have a higher frequency of planets. This is often used as evidence in favor of the core accretion theory There are several problems with this hypothesis Endl et al. 2007: HD 155358 two planets and.. …[Fe/H] = –0.68. This certainly muddles the metallicity-planet connection The Hyades The Hyades • Hyades stars have [Fe/H] = 0.2 • According to V&F relationship 10% of the stars should have giant planets, • Paulson, Cochran & Hatzes surveyed 100 stars in the Hyades • According to V&H relationship we should have found 10 planets • We found zero planets! Something is funny about the Hyades. False Planets or How can you be sure that you have actually discovered a planet? HD 166435 In 1996 Michel Mayor announced at a conference in Victoria, Canada, the discovery of a new „51 Peg“ planet in a 3.97 d. One problem… HD 166435 shows the same period in in photometry, color, and activity indicators. This is not a planet! What can mimic a planet in Radial Velocity Variations? 1. Spots or stellar surface structure 2. Stellar Oscillations 3. Convection pattern on the surface of the star Radial Velocity (m/s) Starspots can produce Radial Velocity Variations Spectral Line distortions in an active star that is rotating rapidly 10 -10 0 0. 0. 0. 4 Phase 6 2 Rotation 0. 8 Tools for confirming planets: Photometry Starspots are much cooler than the photosphere Light Variations Color Variations Relatively easy to measure Tools for confirming planets: Ca II H&K Active star Inactive star Ca II H & K core emission is a measure of magnetic activity: HD 166435 Ca II emission measurements Tools for confirming planets: Bisectors Bisectors can measure the line shapes and tell you about the nature of the RV variations: Curvature Span What can change bisectors: • Spots • Pulsations • Convection pattern on star Spots produce an „anti-correlation“ of Bisector Span versus RV variations: Correlation of bisector span with radial velocity for HD 166435 Activity Effects: Convection Hot rising cell Cool sinking lane •The integrated line profile is distorted. •The ratio of dark lane to hot cell areas changes with the solar cycle This is a Jupiter! RV changes can be as large as 10 m/s with an 11 year period One has to worry even about the nature long period RV variations The Planet around TW Hya? Figueira et al. 2010, Astronomy and Astrophysics, 511, 55 A constant star In the IR the radial velocity variations have 1/3 the amplitude in the optical. This is what expected from spots that have a smaller contrast in the IR How do you know you have a planet? 1. Is the period of the radial velocity reasonable? Is it the expected rotation period? Can it arise from pulsations? • E.g. 51 Peg had an expected rotation period of ~30 days. Stellar pulsations at 4 d for a solar type star was never found 2. Do you have Ca II data? Look for correlations with RV period. 3. Get photometry of your object 4. Measure line bisectors 5. And to be double sure, measure the RV in the infrared! Radial Velocity Planets Period in years → 30 90 Red line: Current detection limits Green line detection limit for a precision of 1 m/s 1000 Summary Radial Velocity Method Pros: • Most successful detection method • Gives you a dynamical mass • Distance independent • Will provide the bulk (~1000) discoveries in the next 10+ years Summary Radial Velocity Method Cons: • Only effective for cool stars. • Most effective for short (< 10 – 20 yrs) periods • Only high mass planets (no Earths…yet!) • Only get projected mass (msin i) • Other phenomena (pulsations, spots, etc.) can mask as an RV signal. Must be careful in the interpretation Summary of Exoplanet Properties from RV Studies • ~5% of normal solar-type stars have giant planets • ~10% or more of stars with masses ~1.5 M סּhave giant planets that tend to be more massive (more on this later in the course) • < 1% of the M dwarfs stars (low mass) have giant planets, but may have a large population of neptune-mass planets → low mass stars have low mass planets, high mass stars have more planets of higher mass → planet formation may be a steep function of stellar mass • 0.5–1% of solar type stars have short period giant plants • Exoplanets have a wide range of orbital eccentricities (most are not in circular orbits) • Massive planets tend to be in eccentric orbits and have large orbital radii •Stars with higher metallicity tend to have a higher frequency of planets, but this needs confirmation