Transcript 2011-OSU-Spectroscopy.pptx

IS WATER ICE THE PRECURSOR
+
+
TO OH AND H2O IN ORION KL?
Paul Rimmer and Eric Herbst
Department of Physics, Chemistry, Astronomy
Ohio State University
Image from NASA, ESA, Robberto (STScI/ESA),
Orion Treasury Project Team
The Orion KL Region
• Orion KL near the Orion
Nebula.
• Thousands of young nearby
stars produce X-Rays and a
very high UV field (χ = 104 in
Draine units)
• Warm (100-300 K), dense
(> 104 cm-3) region.
• Promising source for
detecting transient
molecules.
H2O from ice?
Herschel Observations
• OH+ and H2O+ observed at
column densities of
1013 cm-2
• H3O+ was not observed (!)
• Upper limits for H3O+ are
around 1012 cm-2
• H2O at around 1017 cm-2
(Gupta et. al. 2010)
Standard PDR Results
The Reasons Why: OH+
• OH+ is formed in two different ways.
• At the cloud edge, H is ionized by cosmic rays,
and exchanges charge with an O atom
(Herbst 1973; Mitchell 1983; Stancil 1999).
Then (Smith 1978; Viggiano 1980):
O+ + H2 -> OH+ + H
• Deeper in the cloud, H2 is ionized by cosmic rays,
reacting with neutral H2 to form H3+ (Oka,
McCall). Then (Felsenfeld 1976; Milligan 2000):
H3+ + O -> OH+ + H2
The Reasons Why: Chemical Network
What do we do
to get more OH+
+
and H2O
without getting
more H3O+?
Does Ice Provide the Answer?
• Water ice forms on grains in cold
dense regions.
• When these regions heat up at the
advent of star formation, water
evaporates from the grains.
• The water is dissociated by
photons.
• The products, OH and H, are
ionized by high fluxes of cosmic
rays and X-rays.
• This greatly enhances OH+ and
H2O+ formation.
Initial Conditions for a Gas-Grain PDR
Model
• Initial Conditions:
–
–
–
–
–
–
TMC-1 chemical abundances
n = 104 cm-3
χ(t = 0) = 1 in Draine units
ζ(t = 0) = 10-17 s-1
Temperature from thermal balance
ζ and χ are time-dependent, and scaled to the star
formation rates from Hillenbrand (1997).
– ζ is also column-dependent, based on Rimmer 2011
(submitted).
– We incorporate X-rays into our ζ value, but using the
column-dependence from Maloney (1996)
Star Formation and Physical
Parameters in the Orion KL Region
Phase 2:
1:
Based on data from Hillenbrand (1997)
HST, False Color
χ = 104
ζ = 5 X10-15 s-1
χ = 700
T = 110 K
T = 400 K
Phase 1
Phase 2
Results: OH+, H2O+, H3O+, H2O
Results
integrated
to Av = 10
Results: H3+, HCO+, HC3N
Results
integrated
to Av = 10
Conclusion and Future Work
• Does ice solve the problem?
• Possibly. In part. But a high cosmic ray and X-ray
flux is necessary.
• This model can be tested against predicted
abundances of H3+, HCO+, HC3N and other
species.
• Future work will involve adding radiative transfer.
• The gas-grain PDR model can also be applied to
Sgr B2, to explore an even more complex scenario
involving OH+, H2O+, H3O+, and H3+.
Acknowledgements
• Harshal Gupta and John Pearson, for their
observations and analysis, and for helpful
discussions
• Nanase Harada for her help with incorporating
X-Rays
• John Black, for discussions about radiative
transfer