Searching for Molecular Oxygen in the Interstellar Medium Paul F Goldsmith Senior Research Scientist Jet Propulsion Laboratory California Institute of Technology A Prescient Paper Mentioning CO CS HCN H ID: 615120
Download Presentation The PPT/PDF document "Can You Breathe in Space?" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Slide1
Can You Breathe in Space?Searching for Molecular Oxygen in the Interstellar Medium
Paul F. Goldsmith
Senior Research Scientist
Jet Propulsion Laboratory, California Institute of TechnologySlide2
A Prescient Paper – Mentioning CO, CS, HCN, H2O,and NH3 as “of interest to radio astronomy”
1957 International Astronomical Union Symposium 4, 92
And now
the very last
sentence:
O
2
was measured in Earth’s atmosphere by microwave absorption in 1972 (or before). But what about interstellar space? Why should we care about this molecule in particular?Slide3
Gas Phase Chemistry for O, H2
O, O
2
and CO is Relatively Simple
All key reaction rates have been measured in laboratory, both at room temperature & at temperatures of dense interstellar clouds
Branching ratio measured by ASTRID and CRYRING experiments (Jensen et al. 2000; Neau et al. 2000) f(H
2
O):f(OH) = 0.25:0.75
OH + O −> O
2
is
an
exothermic
neutral-neutral reaction
Measurements (Carty et al. (2006) and full quantum calculations (Lique 2010) indicate ~ temp-indep. rate from 300 K to very low temperatures ≅ 4x10-11 cm3s-1Slide4
Standard Gas-Phase Chemistry Models
Also Predict Large Abundance
of O
2
The time dependent evolution of a gas phase chemistry model. The
physical
conditions are n(H
2
) = 10
4
cm
-3
, T = 10 K, and Av = 10 mag. The oxygen is initially entirely atomic (K. Willacy). Slide5
Searching for O2 in Molecular Clouds Goes Back a Long Time (1979)
O
2
could be important coolantO2
could be a good diagnostic with multiple transitions near 60 GHzSINTOX experiment proposed for Space Shuttle 3.4m dia. a
ntenna &500 K (DSB) noise temperature at 60 GHz
Sigfrid
Yngvesson
was a postdoc in Townes group, working on K-band masersSlide6
Ground-Based Search for Molecular Oxygen
O
2
in Milky Way cannot be observed from Earth’s surface or airplane, or even from a balloon due to absorption by lines in the atmosphere.
This could be considered a good thing!
Unlike homonuclear O
2, 16O18O has both even and odd values of rotational angular momentum. But 18
O/
16
O ~ 1/500 – makes it difficult!
The J = 2-1 transition is at ~ 234 GHzCooled Schottky mixer receiverNewly-installed teflon radome and realigned surface allowed operation at 1.3mmNo detections; upper limit was O2/CO < 0.5 which is not very constrainingNeed to go for 16O16O – and for that a space mission is requiredSlide7
Search for O2 with SWAS
Submillimeter Wave Astronomy Satellite
1
st
astronomy Small Explorer Mission (SMEX)
Launched Dec 199855 x 71 cm offset Cassegrain
antennaTwo Schottky diode second harmonic mixer receivers; designed by N. Erickson (UC Berkeley PhD with R. Chiao)Four spectral lines targeted:Species Transition Frequency
Eu
(K)
O
2 31 – 32 487.3 GHz 26C0 3P1 – 3P0 492.1 GHz 2413CO 5 – 4 550.9 GHz 79H2O 110– 101 556.9 GHz 27Slide8
X(O
2
) in IS Clouds from Odin & SWAS is M
ore Than
100X
Below that
Predicted by Gas
-Phase
Chemistry
Odin
Odin
Slide9
O2 Results from SWAS
No unambiguous detections; one tentative detection in Rho
Oph
was probably erroneous
In 20 clouds, [O2]/[H2
] < 10-6; more than 100x below prediction of chemical models
Low abundance of O2 confirmed by Swedish Odin satelliteWhat is going on?
The “missing piece” is the
interestellar
dust grains
In early phase of cloud evolution oxygen atoms collide with and stick to dust grainsResult is that gas-phase oxygen abundance is reducedAvailable O bound up as CO and little is left to make O2Slide10
Can we Ever Hope to Detect O2 in Space?
Where might we get oxygen off dust grain mantles and back into gas phase where O
2
can be made?
Two possibilities –
Interstellar shocks can “clean” grains and if shock velocity is modest, you get OH formed in postshock gas which then leads to O
2Radiation from embedded young stars heats dust grains to > 100 K, at which point the ice desorbs and water returned to gas phase; this is largely converted to O2
in ~1 million years
Look at
postshock gas and at massive, embedded young starsSlide11
Herschel Space Observatory
ESA cornerstone mission with major NASA participation
Launched 14 May, 2009
3.5m
telescope
cooled to~ 80 K3 instruments including photometers, med.-res. spectrometers and high resolution spectrometer covering 60 μm to 600 μm.3 prime O
2 lines covered by the HIFI instrumentHerschel Oxygen Project (“HOP”) targeted 10 most promising sources
Detections
only
in Rho
Oph and OrionSlide12
First Multi-Line Detection of O2: Herschel HIFI Observations of H2 Peak 1 in Orion
2 μm emission from shocked H
2Slide13
New Capability: the ALMA array in Chile
Huge collecting area
High angular resolution
Extremely low-noise receivers
Very good atmosphere
Based on Herschel intensities and modeling suggesting small size, we calculated that
16
O1
8
O would be detectable, submitted a proposal, were awarded time, and observed two ALMA fields (~8hr total)
H2 Pk 1 (Shock Peak)Hot Core (Hot Dust Peak)Slide14
Conclusions
O
2
generally has very low abundance in interstellar molecular clouds, typically 1/106
of H2The explanation is that oxygen is largely tied up in ice mantles on dust grain surfaces
In a few special circumstances, the predictions of gas-phase chemistry DO hold, and O2 becomes the second or third most abundant molecule in space….
Still more questions to be answered, but after 35+ years we are beginning to get a handle on the perplexing O2 molecule
But don’t hold your breath.
Or maybe you should.