Comet Measurement Techniques. Karen Meech Institute for Astronomy Session 27 1/18/05

Transcription

1 Comet Measurement Techniques Karen Meech Institute for Astronomy Session 27 1/18/05 Image copyright, R. Wainscoat, IfA Image courtesy K. Meech

2 Techniques Summary Imaging & Detectors Photometry Deep Imaging Surface Brightness Dust Modeling Astrometry Infrared Techniques Applications Some examples Spectroscopy Excitation Mechanisms Physics Spectrographs Applications Radio Astronomy X-ray Astronomy Mass Spectrometry

3 CCD Detectors CCD is an array of detectors -- pixels Each has a different sensitivity We need to calibrate the instrumental effects

4 Image Processing Steps Bias Subtraction Removes the low level e- put in so none left behind Flat Field Correction Exposures on a bright uniform source (sky, dome) Cosmetic Corrections Fixing bad columns Fixing bad pixels Cosmic Ray cleaning

5 Cosmetic Changes Random bright spots High energy particles from space single pixel hits Only a problem if land on stars Can affect photometry Can Remove but usually not necessary

6 Image Combining Increases S/N on object Decreases noise Can remove background defects Can simulate non- sidereal tracking

7 Photometry Counting up how much light falls on the detector from star Aperture How big to use? Sky Background Where to measure? How? Background objects Images, Courtesy J. Bauer

8 Deep Imaging Composite Image Single exp, 400 sec

9 Deep Imaging Composite Image Single exp, 400 sec 12000s sum

10 Deep Imaging Composite Image Single exp, 400 sec 12000s sum (zoomed 80 )

11 Deep Imaging Composite Image Single exp, 400 sec 12000s sum (zoomed 80 ) Median combined

12 Deep Imaging Composite Image Single exp, 400 sec 12000s sum (zoomed 80 ) Median combined Shift & sum for KBO rate

13 Deep Imaging Composite Image Single exp, 400 sec 12000s sum (zoomed 80 ) Median combined Shift & sum for KBO rate Median star subtracted

14 Finson-Probstein Dust Models Observations: Surface brightness versus time. Model: Evaluate motion of dust after leaving comet Add up the scattered light from grains Fit to surface brightness of coma versus time Want observations spread so observing geometry changes a lot Plots below show predicted synchrones and syndynes β = F Rad / F Grav = 5.74 x 10-4 Q pr / ρ dust a Synchrone various β released at same time Syndyne same β released at different times Determine: v(β,t,t) f(β,t,t) N d (t) Velocity distribution Particle size distribution Dust production rate

15 Aug Oct Nov FP Dust Modelling Dec Predictions Syndynes (dashed) 0.001, 0.003, 0.01, 0.03, (few 100 µm to few) Synchrones (solid) 2000: by 25 dy 2004: to -600 by 200 dy 2005: -500 to -150 by 100 dy FP Models shown Jan Mar May Feb Apr May Jun

16 Astrometry 6.6 FOV (1/2 CCD) [above] 40 section [lower] Precise position meas Map 3-D sky to 2-D image Technique Measure centroids of many stars Fit for plate center Fit for scale, rotation Fit for warping, stretching Requirements Finding the objects Blinking Different color planes memorization Images, Courtesy K. Meech

17 Astrometry 6.6 FOV (1/2 CCD) [above] 40 section [lower] Precise position meas Map 3-D sky to 2-D image Technique Measure centroids of many stars Fit for plate center Fit for scale, rotation Fit for warping, stretching Requirements Finding the objects Blinking Different color planes memorization Images, Courtesy K. Meech

18 Infrared Observations Types of Observations Organics features in IR Re-radiated E: size, p v Nucleus radius & albedo Optical brightness: p v, size p v R 2 N = const r 2 Δ (msun-m) Thermal radiation: albedo, rotation, surface properties F(1-A)/r 2 = βσt 4

19 Nucleus Sizes 9P/Tempel 1 10 microns R band Direct in-situ meas Simultaneous IR/opt Need thermal model Distant Observations Assume albedo Assume no activity PSF modeling Assume symmetric Model coma in optical Scale and subtract at 10 µm From Fernandez, et al

20 Resonance Fluorescence Coma dominated by photolysis products Each molecule formed via many paths Same daughters from different parents Most electronic bands in UV (outside λ of HST) Converting intensity Q Excitation & emission proc Molecular spatial distn. Excitation Mechanisms Solar radiation Collision (small, usually) Amount of species i M i = Q i τ i Q i production rate of all sources Brightness of species i L i = M i g i g i fluorescence efficiency Atomic physics Solar flux Single scattering albedo

21 Photolytic Destruction of Water H 2 O + hν OH + H OH(A 2 Σ + ) + H H 2 + O( 1 D) H 2 + O( 1 S) H + H + O( 3 P) H 2 O + + e H + OH + + e H 2 + O + + e OH + H + + e OH + hν H 2 + hν O + hν H + hν O + H OH + + e H + H H e H + H + + e O + + e H + + e

22 Spectral Regions UV dominated by main H 2 O dissociation Optical dense; molecular bands Infrared cometary organics Radio/submm fluorescence pumping deviation in population levels from equilibrium

23 The Grating Equation Reflection off grooved surface Different path lengths interference Path diff, ΔS S in phase if mλ = d(sinβ + sinα) Spectrograph parts Slit focal pt of telescope Collimator parallel light Grating disperses, l come off at different angles Lens focuses dispersed light to CCD detector

24 1999 UG5 Unique Surface Photometry in 2000 Different colors Sep-Dec Period = hr Phase function G=-0.13 porous Spectra Hapke models Amorphous H2O H O ice Amorphous carbon Side 1: Triton tholin Side 2: Titan tholin, CH 3 OH (Bauer et al, 2002)

25 Radio Observations Radio is good to study parent gas species coming directly off the nucleus Isotopes large dust grains Radio Visibility poor Need bright comets : long- period. Very few short-period observations Instruments Imaging: bolometer sensitive to wide range of radio λ. Need a comet with lots of mm-size dust Spectroscopy: usually spectrometer is tuned to several specific λ of interest Most common mode with comets.

26 Radio Observations Image of the HCN emission in C/1995 O1 Hale-Bopp taken from BIMA Wright et al 1998, AJ SCUBA JCMT map of the large-grain dust coma from C/1995 O1 Hale Bopp Jewitt & Matthews 1999 AJ

27 Summary

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29 C/1996 B2 Hyakutake X rays

30 Photometry Types Copyright R. Wainscoat Absolute True brightness Photometric (Eggen-o-metric( Eggen-o-metric) Requires standards Differential Tolerates some cloud Corrects for extinction Airmass cloud Measure many stars as reference Noise & repeat measurements S/N Images, Courtesy K. Meech

31 Differential Light curve Images, Courtesy K. Meech

32 Surface Brightness Result: Q < 0.01 kg/s F = S o πa gr2 p v Qφ / 2r 2 Δ 2 v gr Constants: S o, π, r, Δ, φ Assume: a gr = 0.1 µm m (max lifted off) p v = 0.04, v gr = 0.1 km/s (CO)

33 Dust/Gas > 2 Q dust ~ AU > 20 µm m dust dominate coma (no Si feature) Density of old dust in orbit plane low wrt new dust Largest dust cm Dust Properties IRAS 1983 trail C. Lisse, et al.

34 9P/Tempel 1 Dust Trail Ishiguro et al. 81P/Wild 2, r = 2.9 AU, 3600 sec, R 22P/Kopff, r = 3.0 AU; 3900 sec, R Orbit plane crossings: May & Dec 1-2 5/28/00 2 images (600s) 12/2/02 4 images UH 2.2m 4800 sec, R r = 4.72 AU Dec 2003 Meech time Nov 29 & 30. Dec 2004 Meech,, YF, CL (UH, IRTF)