ASTRO 2233 Fall 2010 Planet Detec4on Issues Lecture 18 Thursday October 28, 2010
Astrometry: Advantages: Direct measurement of mass of the planet assumes we know star s mass from stellar type i.e. spectral class Sensi4ve to large planets a long way from the star Disadvantages: θ 1 / Distance to the star => nearby stars only [θ max for Sun Jupiter from 10 light years 1.6 milli arc sec] Velocity measurements: Advantages: Sensi4ve to large planets close to the star Not directly dependent on distance to the star just need sensi4vity Disadvantages: m p Sin i - lower limit on the mass Not sensi4ve to planets at large distances from the star
For discrete data: µ = mean = Gaussian or normal distribu4on Probability that: x > 1 σ = 0.32 x > 2 σ = 0.05 x > 3 σ = 0.003
Image: Nigel Sharp
R = 50,000 Angstroms = 0.1 nm R = 5,000 KECK HIRES spectrum at two resolu4ons from Echelle spectrograph Resolu4on of the spectrograph given by R = / where is the resolu4on in wavelengths. For R = 50,000 1 Angstrom (0.1 nm)
AO system and Echelle spectrometer R = 100,000 on the ESO VLT 8m telescopes in Chile
Broadening of an absorp4on line due to the star s rota4on - 1.5 1.5 Doppler shid (km/s) Doppler broadening = x v/c = 500 nm x 3 / 3 x 10 5 nm = 0.005 nm = 0.05 Angstroms 3 m/s corresponds to a Doppler shid of the wavelength of 0.00005 angstroms
How do you measure Doppler shids this small when the resolu4on of your spectrometer is only 1 Angstrom (0.1 nm) and each line in the star s spectrum is ~1,000 4mes wider than the required measurement accuracy? Need a reference spectrum with mul4ple lines whose frequencies are very accurately known! Use an Iodine absorp4on cell pass the light through Iodine gas Compare with emission spectra of ThoriumArgon gas (now gives best accuracy - see HARPS system, reaches ~1 m/s accuracy)
For calibra4on reference pass light through an Iodine cell many absorp4on lines of known wavelength between about 5,000 and 6,000 angstroms (0.5 and 0.6 um) Can also use Thorium Argon (ThAr) emission spectra - see HARPS system
Thorium Argon spectrum from ThAr Atlas - NOAO Lines from about 3,000 to 11,000 Angstroms 0.3 um to 11 um.
Sources of noise in the RV method that limits accuracy. Best system is currently HARPS on the 3.6 m European Southern Observatory telescope in Chile - can reach 1 m/s in 1 minute for bright stars magnitude 7.5. Two types: 1. Intrinsic to the measurement process 2. Intrinsic to the star
Intrinsic to the star (see Dumesque et al, arxiv 1010.2616v1, Oct 13, 2010): 1. Oscilla4ons of the star s surface due to pressure oscilla4ons (acous4c waves) with periods of 5 to 15 minutes (for the Sun) with veloci4es of 10 to 400 cm/s. 2. Granula4on convec4ve cell with sizes of ~1,000 km, life4mes of less than 25 min effects similar to acous4c waves. 3 Star spots and plages roughly cancel each other out not completely => asymmetric brightness distribu4on with period related to the star s rota4on period since star spots can persist for several rota4ons For Sun amplitude is about 40 cm/s For very high precision measurements s4ck with quiescent stars.
Granules Diameter: ~1,000 km Life4me: 8-20 min Sunspots - dark, low temperature areas Plages/faculae (bright spots that form in the canyons between solar granules, short- lived convec4on cells several thousand kilometers across that constantly form and dissipate over 4mescales of several minutes. Faculae are produced by concentra4ons of magne4c field lines.
Lovis et al, ASP 398, p455 to 459.
Observed 6 stars chosen at random from most stable stars in their data base for 150 days, total of ~50 measurements per star. Top right and center show possible periods of ~50 and ~100 days, center led show possible three low mass planets, botom right shows drid possibly indica4ve of a planet in a long period orbit. Lovis et al, ASP 398, p455 to 459.
CoRoT - French and ESA transit planet detec4on mission launched Dec 2006 Monitors 120,000 stars Feb, 2009 discovered a 1.7 Earth radius planet in a 0.85 day orbit period about a G9V Sun- like star 130 pc (400 ly) from Earth. A young, 1.2 billion years, star so very ac4ve making a 0.033% transit depth difficult to detect and radial velocity measurement even more difficult.
153 observa4ons averaged, Rouan et al, ASP 430, p 158-163 Radial velocity measurements: Showed two planets, 7b and 7c, in 0.85 and 3.7 day periods
Planet 7b: Density ~5.6 g/cm 3 same as Earth so probably silicate planet Distance from the star: 0.0172 AU - ~2.4 million km Tidally locked to the star same face to the star always Temperature of star facing side 1,800 to 2,600 K mel4ng rock Temperature of side facing away from star ~50K - no atmosphere probably May be temperate area between the two about 90 km wide but no water as it would condense on the dark side.
The measured maximum velocity is given by v max = 28.4 p - 1/3 {m p Sin i / M J } m - 2/3 m sec - 1 Where p is the orbit period in years, Sin i is the sine of the orbit inclina4on rela4ve to the line- of- sight from Earth, M J is the mass of Jupiter and m is the mass of the star in solar masses. CoRoT 7b: CoRoT 7c m ~1.0 ~1.0 solar masses P 0.85 days = 0.0023 years 3.7 days = 0.01 years Sin i 1? - probably near 1 (no transit detec4on) m p 4.8 Earth mass = 0.016 Mj 8.4 Earth mass = 0.028 Mj Velocity 3.44 m/sec < 3.7 m/sec Difficult detec4ons with an ac4ve star