Transcript Document

AST 443/PHY 517 : Observational Techniques
November 6, 2007
ASTROMETRY
By: Jackie Faherty
ASTROMETRY:
THE BASICS
• PARALLAX:
Distances to Stars
Which Star is closer, A or B?
QuickTime™ and a
H.264 decompressor
are needed to see this picture.
• PROPER MOTION
Motion Left after Parallax Has
been removed.
• RADIAL VELOCITY
The motion along the line of
site
http://upload.wikimedia.org/wikipedia/commons/6/6c/Barnard2005.gif
ASTROMETRY:
THE BASICS
• Space Motion:
When you combine
all three astrometric
measurements you
can look at groups
of objects moving
together and begin
to analyze formation models and statistics of nearby
stars.
http://www.astronomy.ohio-state.edu/~pogge/Ast162/Movies/umapm.mov
ASTROMETRY:
THE BASICS
Detailed look at Parallax
Which Star is Closer??? A or B??
Distance in Parsecs
Parallax in Arcseconds
D=1/p”
http://instruct1.cit.cornell.edu/courses/astro101/java/parallax/parallax.html
ASTROMETRY:
THE BASICS
Case Study a Parallax from start to finish
Step 1. You need to know your detector and what the limits
will be. Plate Scale and Field of View are VERY important.
ACS HRC Chip 0.027 arcsec/pixel
Field of View 27” x 27”
ACS WFC Chip 0.05 arcsec/pixel
Field of View 202” x 202”
ASTROMETRY:
THE BASICS
Case Study a Parallax from start to finish
What distances can be covered with those plate scales?
ACS HRC Chip 0.027 arcsec/pixel
ACS WFC Chip 0.05 arcsec/pixel
PSF fitting for HST does 0.01 pixels, or half a mili-arcsecond so the
minimal detections with HST go out to 2-3 kilaparsecs (WFC or HRC)
ASTROMETRY:
THE BASICS
HST is optimal. What can you do with SMARTS?
Plate Scale of Andicam? 0.137”/pixel
FOV : ~2.4 x 2.4 arcmin square
RULE of Thumb: When you centroid you can get
down to ~1/20th of a pixel.
So we can measure out to ~150 parsecs
ASTROMETRY:
THE BASICS
Step 2. Use photometric distance or proper motion to get an idea for the
Distance to your target:
For L Dwarfs at K band
MK=10.33+0.324(STL)
For T dwarfs at K band
MK=13.22-0.055(STT)+0.060(STT)^2
d=10[0.2(m-M)+1.0] at K band
Example: L0 Brown dwarf with K=13.0
Would be 34parsecs away and suitable to
Measure with ANDICAM
ASTROMETRY:
THE BASICS
Proper Motion as a distance Indicator
H_J [reduced proper motion at J band]
Reduced Proper Motion Diagram
J - K (mag)
http://www.cosmobrain.com/cosmobrain/res/nearstar.html
ASTROMETRY:
THE BASICS
Step 3. Make sure that the FOV is large enough so you have enough
background reference stars to compute a parallax
BAD: 2 Reference Stars At the Edge
GOOD: 8 Reference Stars
ASTROMETRY:
THE BASICS
Step 4: Narrow Targets by Appropriate RA and DEC
• You want to observer on either side of the parallactic ellipse and ideally
at the maximum parallactic factor.
• Measure often and multiple times!
ASTROMETRY:
THE BASICS
Step 5: Starting the Analysis. Solving for Astrometric Distortions
Optical Systems do not
have a constant plate
scale over the field!
There is a radial
distortion pattern which
is usually solved by a
third order polynomial
ASTROMETRY:
THE BASICS
Step 5: Starting the Analysis. Solving for Astrometric Distortions
Take a relatively crowded field and dither so the same star is moved
around the image enough so you can see the position across the chip
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
ASTROMETRY:
THE BASICS
Step 5. Continued: Then you can look at the residuals of a dither on the
same pointing and decide on the errors.
ASTROMETRY:
THE BASICS
Review of what you have at this point:
1) Target list with an idea of distances that do not exceed what is
possible with your detector
2) Observing strategy (if ground based) to obtain targets at max.
parallactic factor and many times over the course of the year
3) Distortion Solution on hand and a handle of the systematic errors
you will work with
ASTROMETRY:
THE BASICS
PIPELINE!
Step 1. Centroid or PSF fit to get X,Y coordinates for all stars in your
image
Step 2. Assume that the Parallax and Proper Motion of the reference
stars are zero
Step 3. Choose a “Standard Plate” (typically your first observation) and
transform all other images into it using the method of least-squares and
simultaneously solve for Parallax and proper motion
Xtrans = a*X+b*Y+c
Resx= Xtrans - µx*tc - pix
Ytrans = d*X+e*Y+f
Resy= Ytrans - µy*tc - piy
Resx,y are the residuals against Epoch1
a,b,c,d,e,f are Plate Constants
tc is the time between Epoch 1 and appropriate Epochs
pix and piy contain the parallactic factors
Step 4. Delete Outliers and repete if necessary
ASTROMETRY:
EXAMPLE GEMINGA
ASTROMETRY:
EXAMPLE GEMINGA
Observation Dates:
10-07-2003
03-18-2004
09-21-2004
03-22-2005
ASTROMETRY:
EXAMPLE GEMINGA
Epoch 2,4
Epoch 1
Epoch 3
π = 0.0039+/-0.0012 arcsec
µ = 172.0+/-1.0 mas/yr
Position angle 50.7 +/- 0.4 deg
ASTROMETRY:
Large Astrometry Projects
Hipparcos Astrometry Mission
European Space Agency (ESA)
Targeted 118,218 stars with high precision
Targeted 2,539,913 stars wil lesser precision
Launched 1989 Mission Completed March 1993
Errors on Average 1 mas
ASTROMETRY:
Large Astrometry Projects
The Yale Parallax Catalogue
• 41 Telescope/Observatory Combinations
• 8,112 stars with 15,994 parallaxes
• Definitive Ground Based Parallaxes
• Mostly completed with Small Telescopes
and lots of coverage (~81 years)
ASTROMETRY:
Large FUTURE Astrometry Projects
GAIA
Will target 1,000,000,000 stars or 1% of the
Galactic stellar population
Accuracy will be 20 micro arcseconds!
Measure Radial Velocity, Proper Motion and
Parallax (Full Space Motion)
Estimated Launch date is 2012
SIM
Will target fewer stars (more like a few
thousand) searching for planets
Accuracy will be 4 micro arcseconds!
Measure Radial Velocity, Proper Motion and
Parallax (Full Space Motion)
Estimated Launch date is ????
ASTROMETRY:
SCIENCE WITH ASTROMETRY
These are less then 100 pc and have known ages (8-50 Myr)
ASTROMETRY:
SCIENCE WITH ASTROMETRY
ASTROMETRY:
SCIENCE WITH ASTROMETRY
• U Velocity: The component of a star’s motion AWAY from
the galactic center. So a negative U velocity means it is
moving towards the GC. The Sun U velocity -9km/s
ASTROMETRY:
SCIENCE WITH ASTROMETRY
• V Velocity: The component of a star’s motion in the
direction of Galactic rotation as measured relative to a star in
a circular orbit. If it moves faster then if it were in a circular
orbit the V velocity is positive. The Sun’s V velocity 12km/s
ASTROMETRY:
SCIENCE WITH ASTROMETRY
• W Velocity: The component of a star’s motion perpendicular
to the Galactic plane. If a star is moving up its W velocity is
positive. The suns W velocity is 7km/s
ASTROMETRY:
SCIENCE WITH ASTROMETRY
Of the 192 stars present in this volume, the 5% fastest are highlighted as
light color dots. Among them, the asterisks identify those objects/groups
with velocity difference less than 42km/s.
ASTROMETRY:
SCIENCE WITH ASTROMETRY
http://video.google.com/videoplay?docid=9094050937621304915&q=galactic+ce
nter&total=138&start=0&num=10&so=0&type=search&plindex=5
ASTROMETRY:
That’s All from me!!!