OH Observations of Comets Ellen Howell (Arecibo Observatory) and Amy Lovell (Agnes Scott College)
-
Upload
belinda-watson -
Category
Documents
-
view
219 -
download
1
Transcript of OH Observations of Comets Ellen Howell (Arecibo Observatory) and Amy Lovell (Agnes Scott College)
Comets: where do they come from?
Icy bodies ejected during giant planet formation
Remnants of early solar system
Why Study Comets?
Solar System Origin: Remnants of Giant Planet formation
Kuiper Belt and Oort cloud are sources of comets
-“New” comets were preserved
-”Old” comets processed by solar heating
- Source of water on Earth? Maybe.
Coma gases released from nuclear ices
-composition of the solar nebula
-physical conditions present at formation
Anatomy of a comet• Nucleus is 1-30 km solid body of
ice/rock• Coma - can extend few 105 km
around nucleus– Gas mostly water, ammonia, methane,
HCN, other trace gases– Dust – silicates and organic solids
• Tail – can extend for degrees (few AU)– Ion tail is anti-sunward at solar wind
velocity (400 km/s)– Neutral gas and dust outflow at 0.5-2
km/s
Blueshift to sun
Redshift to sun
Extent depends on lifetime, outflow
Dense inner coma (thermal/collisional) maser quenched
OH Radio Bands
OH 18cm L-doublet
1667, 1665 MHz primary
1612, 1720 weaker
Pumped by solar UV
amplifies or
absorbs backgroundDespois et al. (A&A, 1981); Schleicher & A'Hearn (ApJ, 1988)
Observational Goals
Radio spectra (and maps) can assess:– Gas production rates – Gas outflow velocities – Day/Night Asymmetries– Coma density
Support optical/IR observations
Monte Carlo coma simulationFree parameters: outflow v, quenching rQ
Day/Night independent
Random production/destruction time/angle
Distribution binned to spectrum
c2 minimized for BEST FIT
water/OH lifetimes 8.2 x 104 s (~1 day) 1.5 x 105 s (1.7 days)
OH photodissociation “kick” 1.05 km/s
Outflow Velocity & Gas ProductionProduction rate Q
–# molecules/second to produce observed column density
Velocities (widths) 0.5–2.5 km/s Low velocity for low-Q or distant
Q<1029 mol s-1 r >1.0 km s-1
Large variations near sun, large Q
OH Collisional Quenching
At high density, OH is thermalized
-no pumping sustained
-”ons” suppressed
-”offs” enhanced
-Production rates
under-estimated
SummaryObserved “average” comets for “big picture”
Exploiting unique capabilities of radio astronomy:
Gas production monitoring
Outflow velocities vary widely
Mapping observations are essential
If you don’t have $1 billion…
Groundbased radar images can show surface features – like a fly-
by mission: