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Spectroscopic Mapping of Comets at Radio Frequencies Amy Lovell, Agnes Scott College
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Comet “Anatomy” nucleus (<30km) atmosphere (near sun)
Neutral (dust/gas) tails Magnetic fields direct ions Comet 1P/Halley -->
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Tempel 1 The largest dimension is shorter than the distance from Decatur to downtown Atlanta 7.6km x 4.8km
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67P/Churyumov-Gerasimenko Rosetta .
550 km distance 55m/pix – about 4km long ~4km
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Why Comets? Remnants of planet formation era
- some comets preserved/frozen early - others more processed by solar heating Coma gases sublimate from the nucleus ices -composition & physical conditions
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Interesting Chemistry & Physics
production/flow of sublimating gases off nucleus release of dust from nucleus* collisions between particles in inner coma UV dissociation of “parent” molecules into “daughters” chemical reactions interactions with the solar wind Orbital characteristics Coma composition (inferred nucleus comp) Gas-to-dust ratios Gas and dust production varies along orbit Nucleus size Physical properties, density, strength
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Radio Astronomy Complementary observing opportunities
Energetics appropriate for parent molecules Velocity details in high-resolution spectra 3rd dimension to images H2O, HDO, OH, H2O+, CO, CO2, CO+, HCO+, H2S, SO, SO2, OCS, CS, CH3OH, H2CO, HCOOH, HCN, CH3CN, HNC, HC3N, HNCO, CN, NH2, NH, CH4, C2H2, C2H6, C3, C2, Na, H13CN, HC15N, C34S
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Earth’s atmosphere
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Green Bank Telescope (100m GBT) Spatial resolution 7.4' Arecibo 305m
Mapping pattern, spectral & spatial resolution
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Radio spectra (and maps) can assess:
Gas production rates Gas outflow velocities Outgassing Asymmetries Coma excitation conditions/density Support optical/IR observations
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Parents (from ice) HCN, H2O UV Daughters (secondary products) CN, OH
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Lines at rest approaching Emitting molecules receding
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Line width shows expansion velocity
Width = outflow velocity Height relates to quantity of gas Shape = where gas is emitted Line width shows expansion velocity Line shape suggests a/symmetry, acceleration Line area relates to gas production
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OH Radio Bands OH is visible in the 18cm L-doublet
1667 and 1665 MHz primary lines, Pumped by solar UV amplifies or absorbs the cosmic background Despois et al. (A&A, 1981); Schleicher & A'Hearn (ApJ, 1988) Mapping pattern, spectral & spatial resolution
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Redshift to sun Blueshift to sun
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Emission lines Absorption lines
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Quenching Constraints
May 21, better quenching figure is needed…
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Mapping Layout OFF ON
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Monte Carlo OH coma simulation
H2O lifetime 82,000 s random time, direction OH lifetime 150,000s “kick” v=1.05 km/s Surviving OH forms synthetic spectrum binned by observed Doppler velocity
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Mapping simulation Fits minimize c2 data
Outflow velocity & quenching, possible asymmetry across the coma Arecibo rH = 2 AU 4’ 205,000 km
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B ½ size = more productive area
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Post-perihelion pre-perihelion ~ rh -2.5 C/Garradd Production Rate
Gas production rates log(Q) are between and in mid-June and between 28.2 and 28.6 a few weeks later. Late-July gas production rates are lower, between and Collisional quenching is present at predicted (Gérard et al. 1998) levels, with 11 July appearing above predictions.
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Gas outflow velocity Distance to sun (AU)
High variability near the sun – levels off at larger distances Distance to sun (AU) 25
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Summary Radio Observations reveal unique physics “big picture” view
supplement multiwavelength campaigns Velocity results challenge predictions Mapping spectroscopy is best constraint
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