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

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)

Emission lines

Absorption lines

Mapping Layout

ON

OFF

• 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 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

Mapping

Arecibo rH = 2 AU 4’ 205,000 km

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

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

SummaryObserved “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 fly-

by 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