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? PROPER MOTION Motion Left after Parallax Has been removed. RADIAL VELOCITY The motion along the line of site

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.

ASTROMETRY: THE BASICS Detailed look at Parallax Which Star is Closer??? A or B?? Distance in Parsecs Parallax in Arcseconds D=1/p”

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 WFC Chip 0.05 arcsec/pixel ACS HRC Chip arcsec/pixel Field of View 27” x 27” 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 WFC Chip 0.05 arcsec/pixel ACS HRC Chip 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 M K = (ST L ) For T dwarfs at K band M K = (ST T )+0.060(ST T )^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 Reduced Proper Motion Diagram J - K (mag) H_J [reduced proper motion at J band]

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 EdgeGOOD: 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

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 X trans = a*X+b*Y+c Res x = X trans - µ x *tc - pi x Y trans = d*X+e*Y+f Res y = Y trans - µ y *tc - pi y Res x,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 pi x and pi y contain the parallactic factors Step 4. Delete Outliers and repete if necessary

ASTROMETRY: EXAMPLE GEMINGA

Observation Dates: ASTROMETRY: EXAMPLE GEMINGA

ASTROMETRY: EXAMPLE GEMINGA π = / arcsec µ = /-1.0 mas/yr Position angle /- 0.4 deg Epoch 1 Epoch 2,4 Epoch 3

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

ASTROMETRY: That’s All from me!!!