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Radial Velocity Detection of Planets: II. Results • To date 1783 exoplanets have been discovered • ca 558 planets discovered with the RV method. The others are from transit searches • 98 are in Multiple Systems (RV) → exoplanet.eu 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 Spectrograph 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“ ε Eri was a „probable variable“ The first extrasolar planet around a normal star: HD 114762 with Msini = 11 MJ P = 84 d 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 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 Rate of Radial Velocity Planet Discoveries 51 Peg 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 Mass Distribution at Low Masses 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 ω=0 ω=90 ω=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 ε 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 Another short period giant planet 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) 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 Earth-mass Planet: Kepler 78b Pepe et al. 2013, Howard et al. 2013 Mass = 1.31± 0.25 MEarth (Amplitude = 1.34 m/s) Period = 8.5 hours 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 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? Some Planets in known Binary Systems: 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 γ 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. γ Cep Ab: A planet that challenges formation theories The first extra-solar Planet may have been found by Walker et al. in 1988 in a binary system: Ca II is a measure of stellar activity (spots) γ 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 γ Cephei Primary star (A) Secondary Star (B) Planet (b) The planet around γ 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 γ Cep this disk is truncated just at the ice line. No ice line, no solid core, no giant planet to migrate inward. γ 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: ~ 100 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 HD 168443 HD 38529 HD 190360 HD 202206 HD 11964 263 1444 58 1770 14.31 2207 17.1 2891 255.9 1383.4 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 MJ 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 ν1 = 0.317 cycles/d ν2 = 0.186 ν3 = 0.077 ν4 = 0.015 Yes! The Period04 solution: Published solution: P1 = 5.37 d, K = 12.7 m/s P1 = 5.37 d, K = 12.5 m/s P2 = 12.92 d, K = 3.2 m/s P2 = 12.93 d, K = 2.63 m/s P3 = 66.7 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 σ=1.2 m/s σ=1.53 m/s 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? 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 Transiting surveys are finding more planets around M dwarfs 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 solar masses m sini = 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 P = 328 days Msini = 8.5 Mjupiter e = 0.24 Scargle Power An F4 main sequence star from the Tautenburg program M* = 1.4 M Frequency (c/d) ε Eri: A „complete“ System • Long period planet • Very young star • Has a dusty ring • Nearby (3.2 pcs) • Astrometry (1-2 mas) • Imaging (Δm =20-22 mag) • Other planets? Clumps in Ring can be modeled with a planet here (Liou & Zook 2000) Radial Velocity Measurements of ε Eri Hatzes et al. 2000 Large scatter is because this is an active star. It has been argued that this is not a planet at all, but rather the signal due to activity. Scargle Periodogram of ε Eri Radial velocity measurements False alarm probability ~ 10–8 Scargle Periodogram of Ca II measurements Figure 10 from The HARPS-TERRA Project. I. Description of the Algorithms, Performance, and New Measurements on a Few Remarkable Stars Observed by HARPS Guillem Anglada-Escudé and R. Paul Butler 2012 ApJS 200 15 doi:10.1088/0067-0049/200/2/15 Hatzes et al. 2000 Anglada-Escude & Butler 2011 Period: 2501 ± days Period: 2651 ± 36 days Eccentricity: 0.61 ± 04 Eccentricity: 0.40 ± 0.1 ω : 49 ± 4degrees ω : 141 ± 10 degrees K: 19.0 ± 1.7 m/s K: 11.8 ± 1.1 m/s msini : 0.86 MJupiter msini : 0.64 MJupiter Anglada-Escude & Butler argue that the variations are due to an activity cycle. 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! Fake Planets 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 4. Noise Radial Velocity (m/s) Starspots can produce Radial Velocity Variations Spectral Line distortions in an active star that is rotating rapidly 10 -10 0 0.2 0.4 0.6 Rotation Phase 0.8 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 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 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! The Non-Planet around TW Hya Figueira et al. 2010, Astronomy and Astrophysics, 511, 55 A constant star Points: IR measurements, Solid line is the orbital solution using optical radial velocity measurements, but with one-third the optical amplitude → No planet! Period = 3.24 d K = 0.5 m/s Msini = 1.13 MEarth FAP = 0.02% Is Alpha Cen Bb really there? Claimed detection: Dumusque et al. 2012 False alarm probability (FAP) ~ 0.02 % False alarm probability = 0.4 False alarm probability = 0.00010 Data Fake Planet Maybe not! 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 • Important for transit searches Summary Radial Velocity Method Cons: • Only effective for cool stars. • Most effective for short (< 10 – 20 yrs) periods • Only high mass planets, but getting closer to Earth mass planets • Only get projected mass (msin i) • Other phenomena (pulsations, spots, etc.) can mask as an RV signal. Must be careful in the interpretation The Radial Velocity Method is successful, but highly biased – we only know about planets around solartype stars! Summary of Exoplanet Properties from RV Studies • ~10 % of normal solar-type stars have giant planets • < 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