1 / 45

Astrometric Detection of Exoplanets

Astrometric Detection of Exoplanets. Angles & Coordinates:. 1 full circle = 360 degrees 1 degree = 60 arcminutes 1 arcminute = 60 arcseconds ~ 1 inch @ 100 yards (2.908 cm at 100 meters) 1 milliarcsec (mas) = 0.001 arcsec 1 microarcsec ( µas) = 0.000001 arcsec

lorriegarcia
Download Presentation

Astrometric Detection of Exoplanets

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Astrometric Detection of Exoplanets

  2. Angles & Coordinates: • 1 full circle = 360 degrees • 1 degree = 60 arcminutes • 1 arcminute = 60 arcseconds ~ 1 inch @ 100 yards (2.908 cm at 100 meters) 1 milliarcsec (mas) = 0.001 arcsec 1 microarcsec (µas) = 0.000001 arcsec Astronomical coordinates on sky: E-W: Right Ascension (RA) in h:m:s (0-24h) N-S: Declination (DEC) in deg:arcm:arcs (-90 - +90)

  3. Stellar Motion There are 4 types of stellar „motion“ that astrometry can measure: 1. Parallax (distance): the motion of stars caused by viewing them from different parts of the Earth‘s orbit 2. Proper motion: the true motion of stars through space 3. Motion due to the presence of companion 4. „Fake“ motion due to other physical phenomena

  4. Our solar system from 32 light years (10 pcs) 1 milliarcsecond

  5. Galileo was the first to try measure distance to stars using a 2.5 cm telescope. He of course failed. Brief History • Astrometry - the branch of astronomy that deals with the measurement of the position and motion of celestial bodies • It is one of the oldest subfields of the astronomy dating back at least to Hipparchus (130 B.C.), who combined the arithmetical astronomy of the Babylonians with the geometrical approach of the Greeks to develop a model for solar and lunar motions. He also invented the brightness scale used to this day. • Hooke, Flamsteed, Picard, Cassini, Horrebrow, Halley also tried and failed

  6. Modern astrometry was founded by Friedrich Bessel with his Fundamenta astronomiae, which gave the mean position of 3222 stars. • 1838 first stellar parallax (distance) was measured independently by Bessel (heliometer), Struve (filar micrometer), and Henderson (meridian circle). • 1887-1889 Pritchard used photography for astrometric measurements

  7. Mitchell at McCormick Observatory (66 cm) telescope started systematic parallax work using photography • Astrometry is also fundamental for fields like celestial mechanics, stellar dynamics and galactic astronomy. Astrometric applications led to the development of spherical geometry. • Astrometry is also fundamental for cosmology. The cosmological distance scale is based on the measurements of nearby stars.

  8. Astrometry: Parallax Distant stars 1 AU projects to 1 arcsecond at a distance of 1 pc = 3.26 light years

  9. Astrometry: Proper motion Discovered by Halley who noticed that Sirius, Arcturus, and Aldebaran were over ½ degree away from the positions Hipparchus measured 1850 years earlier

  10. Astrometry: Proper motion Barnard is the star with the highest proper motion (~10 arcseconds per year) Barnard‘s star in 1950 Barnard‘s star in 1997

  11. D To convert to an angular displacement you have to divide by the distance, D Astrometry: Orbital Motion a1m1 = a2m2 a1 = a2m2 /m1 a2 × a1

  12. q = P2/3 m D M2/3 Astrometry: Orbital Motion The astrometric signal is given by: This is in radians. More useful units are arcseconds (1 radian = 206369 arcseconds) or milliarcseconds (0.001 arcseconds) = mas m a q = M D m = mass of planet M = mass of star a = orbital radius D = distance of star Note: astrometry is sensitive to companions of nearby stars with large orbital distances Radial velocity measurements are distance independent, but sensitive to companions with small orbital distances

  13. Astrometry: Orbital Motion With radial velocity measurements and astrometry one can solve for all orbital elements

  14. So we find our astrometric orbit But the parallax can disguise it And the proper motion can slinky it

  15. Astrometric Detections of Exoplanets The Challenge: for a star at a distance of 10 parsecs (=32.6 light years):

  16. The Observable Model Must take into account: • Location and motion of target • Instrumental motion and changes • Orbital parameters • Physical effects that modify the position of the stars

  17. The Importance of Reference stars Example Focal „plane“ Detector Perfect instrument Perfect instrument at a later time • Reference stars: • Define the „plate scale“ • Monitor changes in the plate scale (instrumental effects) • Give additional measures of your target Typical plate scale on a 4m telescope (Focal ratio = 13) = 3.82 arcsecs/mm = 0.05 arcsec/pixel (15 mm) = 57mas/pixel. The displacement of a star at 10 parsecs with a Jupiter-like planet would make a displacement of 1/100 of a pixel (0.00015 mm)

  18. They can have their own (and different) parallax 2. They can have their own (and different) proper motion Good Reference stars can be difficult to find: 3. They can have their own companions (stellar and planetary) 4. They can have starspots, pulsations, etc (as well as the target)

  19. Barnard´s star Astrometric detections: attempts and failures To date no extrasolar planet has been discovered with the astrometric method, although there have been several false detections

  20. New cell in lens installed Lens re-aligned Hershey 1973 Van de Kamp detection was most likely an instrumental effect

  21. Real Astrometric Detections with the Hubble Telescope Fine Guidance Sensors

  22. HST uses „Narrow Angle Interferometry“! G. Fritz Benedict (McD Obs.) HST is achieving astrometric precision of 0.1–1 mas !

  23. One of our planets is missing: sometimes you need the true mass! B HD 33636 b P = 2173 d Msini = 10.2 MJup Bean et al. 2007AJ....134..749B i = 4 deg → m = 142MJup = 0.142 Msun

  24. The mass of Gl876b • The more massive companion to Gl 876 (Gl 876b) has a mass Mb = 1.89 ± 0.34 MJup and an orbital inclination i = 84° ± 6°. • Assuming coplanarity, the inner companion (Gl 876c) has a mass Mc = 0.56 MJup

  25. 55Cnc d • Perturbation due to component d, • P = 4517 days • = 1.9 ± 0.4 mas i = 53° ± 7° Mdsin i = 3.9 ± 0.5MJ Md= 4.9 ± 1.1MJ Combining HST astrometry and ground-based RV • McArthur et al. 2004 ApJL, 614, L81

  26. The 55 Cnc (= r1 Cnc) planetary system, from outer- to inner-most ID r(AU) M (MJup) d 5.26 4.9 ± 1.1 c 0.24 0.27 ± 0.07 b 0.12 0.98 ±0.19 e 0.04 0.06 ± 0.02 Where we have invoked coplanarity for c, b, and e = (17.8 ± 5.6 Mearth) a Neptune!!

  27. The Planet around e Eridani Distance = 3.22 pcs = 10 light years Period = 6.9 yrs

  28. HST Astrometry of the extrasolar planet ofe Eridani e Eri p = 0.3107 arcsec (parallax) a = 2.2 mas (semi-major axis) i = 30° (inclination) X-displacement (arc-seconds) Y-displacement (arc-seconds) Mass (true) = 1.53 ± 0.29 MJupiter

  29. Orbital inclination of 30 degrees is consistent with inclination of dust ring

  30. One worrisome point: The latest radial velocities do not fit the orbit:

  31. The Planetary System of u And

  32. Note: the planets do not have the same inclination!

  33. Planets c and d are inclined by 30 degrees to each other!

  34. The Purported Planet around Vb10 Up until now astrometric measurements have only detected known exoplanets. Vb10 was purported to be the first astrometric detection of a planet. Prada and Shalkan 2009 claimed to have found a planet using the STEPS: A CCD camera mounted on the Palomar 5m. 9 years of data were obtained.

  35. The astrometric perturbation of Vb 10 Mass = 6.4 MJup

  36. The RV data does not support the astrometry. The only way is to have eccentric orbits which is ruled out by the astrometric measurements.

  37. Radial Velocity Astrometry 1. Measure a displacement of a spectral line on a detector 1. Measure a displacement of a stellar image on a detector 2. Thousands of spectral lines (decrease error by √Nlines) 2. One stellar image 3. Hundreds of reference lines (Th-Ar or Iodine) to define „plate solution“ (wavelength solution) 3. 1-10 reference stars to define plate solution 4. Reference lines are stable 4. Reference stars move! Comparison between Radial Velocity Measurements and Astrometry. Astrometry and radial velocity measurements are fundamentally the same: you are trying to measure a displacement on a detector

  38. Summary • Astrometry is the oldest branch of Astronomy • It is sensitive to planets at large orbital distances → complimentary to radial velocity • Gives you the true mass • Very useful for system architecture (e.g. ups And) • Least successful of all search techniques because the precision is about a factor of 1000 too large. • Will have to await space based missions to have a real impact

More Related