OH Observations of Comets
Ellen Howell (Arecibo Observatory) and
Amy Lovell (Agnes Scott College)
Hyakutake
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
Extent depends on
lifetime, outflow
Blueshift
to sun
Redshift
to sun
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 background
Despois et al. (A&A, 1981); Schleicher & A'Hearn (ApJ, 1988)
Emission
lines
Absorption
lines
Mapping Layout
OFF
ON
• Arecibo
• Spatial resolution 4'


Green Bank Telescope
(100m GBT)
Spatial resolution 7.4'
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 simulation
Free 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
Mapping
Arecibo
rH = 2 AU 4’
205,000 km
Outflow Velocity & Gas Production
Production 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
300 kg/s, would
fill swimming pool
in 4 minutes
Outflow Velocity
OH Collisional Quenching
At high density, OH is thermalized
-no pumping sustained
-”ons” suppressed
-”offs” enhanced
-Production rates
under-estimated
Quenching
Summary
Observed “average” comets for “big picture”
Exploiting unique capabilities of radio astronomy:
Gas production monitoring
Outflow velocities vary widely
Mapping observations are essential
Rosetta spacecraft views nucleus of
Comet 67P/Churyumov-Gerasimenko
Surface terrain
Landing Site
If you don’t have $1 billion…
Groundbased radar images can
show surface features – like a flyby mission:
Comet 209P/LINEAR
27 May 2014
•First comet nucleus
closer than 0.06 AU
since 1983
•Resolution of 15-m
• Rotation rate 11-hrs,
•Movie is in two parts
with pause between,
analysis ongoing