Measuring Radial Velocities of Stars Via the Doppler Shift (Effect)with light
Stars Discovered using Star Radial Velocity
"Marcel Mayor and Didier Queloz of the Geneva Observatory discovered the planet around 51 Peg and many other recent systems"
OVER 60 PLANETS DISCOVERED TO DATE
Including Some Planetary SYSTEMS—multiple planets
THE UPSILON ANDROMEDAE SYSTEM
Radial velocity is the motion, in kilometers per second, that a star has along your line of sight. It can be accurately measured from the shifts in the spectral lines from the light from the object.
The “result suggests that Jovian planets may be more common in short-period binary stars than in single-star systems,”Recently, though, astronomers realized that lensing can also be used to look at closer star systems within our own Milky Way galaxy. They call it
microlensing. Several research teams now patrol the skies looking for microlensing events. They hope these will help identify some of the so-called dark
matter that surrounds our galaxy. Dark matter makes up most of the galaxy’s mass, yet it is invisible to infrared, radio, optical, and X-ray telescopes.
Astronomers know that it’s there because of the strength with which the Milky Way’s gravity pulls on other nearby galaxies. If a dark matter object
moves in front of a background star, the star appears to brighten as microlensing focuses its light.
Planet research is a potential secondary payoff. If the object being looked at is a star system, any planets inside would influence the lensing with
their gravity. That’s why dark matter researchers alert planet hunters to suspicious microlensing events. This is a really good quote (suggests a better, new technique): Dr. Bennett says, as a detection technique, microlensing complements search by wobble, called the radial-velocity technique. “We are able to detect substantially lower-mass planets than the radial-velocity technique,” he says.
Also, microlensing is more sensitive in picking up planets orbiting at Jupiter-like distances from their star. Radial-velocity analysis is better with close-in planets.
Light from a star moving towards us has a shorter
wavelength (blue shift)
Light from a star moving away from us has a longer
wavelength (red shift)
51 PEG is comparable to the size of Mercury, 58 million KM from the sun (.4 AU)
Upsilon Andromedae: 3 Planets (4.6, 24.1, and 1267 days)
Masses 0.71, 2.11, and 4.61 MJ
Distances: 0.06, 0.83, and 2.50 AU.
HD 168443: 2 Planets (58days and 5.9 years.)
Gliese 876: 2 Planets (30 and 60 days)
HD 82943: 2 Planets (221 and 444 days)
HD 74156: 2 Planets (51 and 2300 days)
The article suggests that the planet discovered is that of a binary star
system, which is something that has been overlooked because scientists
didn't think it was probable for planets to form around binary
systems. However, the planet discovered is that of a binary star
system...something they have overlooked with star radial-velocity. Planet
hunters using the radial-velocity technique have avoided binary stars. If
binary star solar systems are as common as the MPS team suspects,
microlensing searches should swell the harvest of alien worlds.
http://zorba.as.utexas.edu/abstracts/esopap.html
Results by: A.P. Hatzes, M. Küurster, W.D. Cochran, K. Dennerl, & S. Döbereiner
We report on the progress of a program to search for extra-solar planets
using high precision radial velocity (RV) measurements taken at the
European Southern Observatory. Instrumental errors in the radial velocity
measure are minimized by placing a temperature stabilized molecular iodine
absorption cell in front of the entrance slit to the spectrograph. This
imposes an iodine absorption spectrum on top of the stellar spectrum. All
stellar radial velocity shifts are measured with respect to the iodine
absorption spectrum. The current program consists of a sample of 26 F, G,
and K dwarfs and 1 dMe star. Radial velocity measurements taken since
November 1992 indicate that a typical long-term radial velocity precision
for this program is about 18 m/s. One of our program stars, Tau Ceti, shows
constant radial velocity variability with a scatter of 6 m/s.
Representative radial velocity measurements are given for three stars
showing no significant radial velocity variations, three stars showing
variations indicative of low mass companions, and for the triple system
Alpha Centauri. The radial velocity measurements of Alpha Centauri A and B
are consistent with the astrometric orbital solution, although the data
points have a slightly steeper slope than the calculated orbit. Continued
radial velocity monitoring of this star should eventually result in a
better orbital solution.
This site tells how to measure radial velocity: "..placing a temperature
stabilized molecular iodine absorption cell in front of the entrance slit
to the spectrograph. This imposes an iodine absorption spectrum on the top
of the stellar spectrum. All stellar veclocity shifts are measured with
respect to the iodine absorption spectrum." A "typical" long-term radial
velocity precision is about 18 m/s for this method. "Continued radial
velocity monitoring of this star should eventually result in a better
orbital solution"
http://www.phys.canterbury.ac.nz/~phys159/Astr109/Lecture7.htm Back
http://seds.lpl.arizona.edu/~rme/espirit
http://iac.es/galaria/hdeeg/pubs2002/eslabpro14_final.htm
http://www.space.com/searchforlife/seti_doyle_020502.html
http://itss.raytheon.com/cafe/qadir/q930.html
Microlensing