ast 443/phy 517 : observational techniques november 6, 2007 astrometry by: jackie faherty

AST 443/PHY 517 : Observational Techniques

November 6, 2007

ASTROMETRY

By: Jackie Faherty

ASTROMETRY:THE BASICS

• PARALLAX:Distances to StarsWhich Star is closer, A or B?

• PROPER MOTIONMotion Left after Parallax Has been removed.

• RADIAL VELOCITYThe motion along the line of site

QuickTime™ and aH.264 decompressor

are needed to see this picture.

http://upload.wikimedia.org/wikipedia/commons/6/6c/Barnard2005.gif





• Space Motion: When you combineall three astrometricmeasurements youcan look at groupsof objects movingtogether and beginto analyze formation models and statistics of nearby stars.

http://www.astronomy.ohio-state.edu/~pogge/Ast162/Movies/umapm.mov






Detailed look at Parallax

Which Star is Closer??? A or B??

Distance in ParsecsParallax in Arcseconds

D=1/p”

http://instruct1.cit.cornell.edu/courses/astro101/java/parallax/parallax.html





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 WFC Chip 0.05 arcsec/pixel

ACS HRC Chip 0.027 arcsec/pixel

Field of View 27” x 27” Field of View 202” x 202”


Case Study a Parallax from start to finish

What distances can be covered with those plate scales?

ACS WFC Chip 0.05 arcsec/pixel

ACS HRC Chip 0.027 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)


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


Step 2. Use photometric distance or proper motion to get an idea for the Distance to your target:

For L Dwarfs at K bandMK=10.33+0.324(STL)

For T dwarfs at K bandMK=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=13.0Would be 34parsecs away and suitable toMeasure with ANDICAM


http://www.cosmobrain.com/cosmobrain/res/nearstar.html

Proper Motion as a distance Indicator

Reduced Proper Motion Diagram

J - K (mag)

H_J [reduced proper motion

at J band]







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 EdgeGOOD: 8 Reference Stars


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!


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


Step 5: Starting the Analysis. Solving for Astrometric DistortionsTake 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 aTIFF (Uncompressed) decompressor

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Step 5. Continued: Then you can look at the residuals of a dither on the same pointing and decide on the errors.


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 BASICSPIPELINE!

Step 1. Centroid or PSF fit to get X,Y coordinates for all stars in your imageStep 2. Assume that the Parallax and Proper Motion of the reference stars are zeroStep 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 motionXtrans = 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 Epoch1a,b,c,d,e,f are Plate Constantstc is the time between Epoch 1 and appropriate Epochspix and piy contain the parallactic factorsStep 4. Delete Outliers and repete if necessary

ASTROMETRY:EXAMPLE GEMINGA

Observation Dates:10-07-200303-18-200409-21-200403-22-2005



π = 0.0039+/-0.0012 arcsecµ = 172.0+/-1.0 mas/yrPosition angle 50.7 +/- 0.4 deg

Epoch 1

Epoch 2,4

Epoch 3

ASTROMETRY:Large Astrometry Projects

Hipparcos Astrometry MissionEuropean Space Agency (ESA)Targeted 118,218 stars with high precisionTargeted 2,539,913 stars wil lesser precisionLaunched 1989 Mission Completed March 1993Errors 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

ProjectsGAIAWill target 1,000,000,000 stars or 1% of the Galactic stellar populationAccuracy will be 20 micro arcseconds!Measure Radial Velocity, Proper Motion and Parallax (Full Space Motion)Estimated Launch date is 2012

SIMWill target fewer stars (more like a few thousand) searching for planetsAccuracy 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)


• 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


• 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


• 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


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.


http://video.google.com/videoplay?docid=9094050937621304915&q=galactic

+center&total=138&start=0&num=10&so=0&type=search&plindex=5

ASTROMETRY:That’s All from me!!!

ast 443/phy 517 : observational techniques november 6, 2007 astrometry by: jackie faherty

Documents

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