Submillimeter Wave Astronomy Satellite and Star Formation

Di LiSmithsonian Astrophysical Observatory,

CfA

July, 2002

Oxygen Budget in Molecular Clouds

Oxygen is the third abundant element after H and He.

• Grains: oxygen locked up in silicates, olivine, and oxides.

• CO gas: limited by the carbon abundances

• How about O2, H2O, icy mantle, and OI?

Others27%

Grains38%

CO Gas35%

Blocked Vision Through Atmosphere

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450 470 490 510 530 550 570 590Frequency (GHz)

A Good Night on Mauna Kea (1mm PWV)

SWAS Science Objectives

• Where is all the oxygen in the dense interstellar medium?

• Are H2O, and to a lesser extent O2, important coolants for molecular gas as it collapses to form stars?

• What are the important physical processes in the UV-illuminated surfaces of molecular clouds?

• What is the large scale structure of cold CI clouds?

Submillimeter-Wave Astronomy Satellite (SWAS)

A NASA Small-Explorer mission

Principle Investigator Gary Melnick, SAO

CO Investigators Scientists from SAO,

Cornell U, John Hopkins U, U Mass, and NASA AMES

Center

SWAS: Observed Molecules and Transitions

Species Transition Energy Above Ground State

(E/k)

Frequency (GHz)

Wavelength (m)

Critical Density (cm-3)

O2 3,3 1,2 26 K 487.249 615.276 103

CI 3P1 3P0 24 K 492.161 609.134 104

13CO J = 5 4 79 K 550.926 544.161 3 105

H2O 110 101 27 K 556.936 538.289 109

H218O 110 101 26 K 547.676 547.390 109

Transitions listed in red are ground-state transitions.

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SWAS System Performance

Integration Time (hrs) Integration Time (hrs)

rms

nois

e (

K)

[CI] and O2 H2O and 13CO

M17 M17

Expectations from Gas Phase Chemistry

Quiescent Gas Chemistry Model

-Abundance predictions after 106 yrs

- [O2]/[H2] ~5x10-5

- [H2O]/[H2] ~5x10-7

H2O in Molecular Cloud Cores

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Ant

enna

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pera

ture

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VLSR (km/s)

Ceph-A NGC 2024 NGC 1333 S140

Oph-A NGC6334i W3 TMC-1

Analysis of H2O Emission in Star-Forming Regions

• H2O emission is very optically thick, but lines are close to the weak line limit:Molecular hydrogen densities typically 105 - 106 cm-3.ncrit ~ 2 108 cm-3.

Collisional de-excitations are unimportant and every photon created by collisional excitation eventually escapes.

Lines are optically thick but effectively thin!

In this limit:

)N(H

vT

h

4

)cCn(H

k2 = O)HX(o

2

kT

h

32

3

2d

e

H2O and H218O Absorption

Toward Sgr B2

1

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0 100 200-100VLSR (km/s)

Flu

x/C

ontin

uu

m F

lux

H2O 110 101

H218O 110 101

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H2O and H218O Absorption

Toward Sgr B2

SWAS H2O Abundances from Emission Lines

predictions of chemical theory

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2O A

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The Elusive O2 Molecule

O2 Abundance Upper Limits (3)

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predictions of chemical theory

What is Missing

1. Depletion onto dust grainsRate: Time scale:

2. Grain surface chemistryO+g=> H2O

C+g=>CH4

3. Dynamic processes associated with star formationShocks Outflows

yrsKTHnR

HAnT

depdep

10/)(

102)(

2

92

SHvAnnR gdep )( 2

Gas and Grain Chemistry

• Pros– Depletes

water– Process

Oxygen into surface ice

– Enhance some species, such as HC3N

• Cons– SO will

disappear– Atomic oxygen

may be too little

Cloud Structures: the Rho Oph Clouds

CI and CO Correlation

• The positive correlation is not spatial dependent, occurs in both clouds in Ophiuchi region

• The turnover only happens at extremely high extinction Av>20, where 13co may also become optically thick.

Tentative Detection of O2

• Both wings have offsets from the center.

• 4.5 Sigma detection at outflow velocities. How

significant is this?

Statistical Tests

• Stable performance of the system: RMS ~1/ t0.5 still holds after hundreds of hours

• Number of independent channels and rms noise of our sample of spectra conform to Gaussian statistics.

• Fraction of scans with positive intensity have expected statistics with respect to RMS.

4.5 => O2 Abundance ~ 10-5

)2

1exp()()

2

1(

)2

2(

1)( 2

22

322

1

22 s

Ns

NN

sfNN

Chemical Models with Shocks

Pseudo time dependent

chemical model with depletion and surface chemistry

Shocks release H2O and CO from

grain surfaces and enable neutral-

neutral reactions

Pure gas phase chemistry. Later,

depletion dominates

depending on density.

Water in Other Systems

• PlanetsSWAS has measured water vapor vertical profiles of the Mars atmosphere. Water has been detected from Jupiter and Saturn.

• Comets comet C/1999 H1 (Lee), giving

H2O production rate to be 8x1028 s-1

• Carbon-Rich StarsIRC10216 (CW Leonis)

Expanding evaporation zone of this AGB stars deposit water into the circumstellar outflows from Kuiper belt objects—possible probe to extrasolar comets.

Conclusions

• SWAS has detected water in a variety of star forming regions (GMCs, dark cloud complexes, region with outflows), and in diffuse ISM. The water abundance [H2O]/[H2] ranges from 10-9 to 10-5

• SWAS has set significant upper limits to the abundance of molecular oxygen to these regions. [O2]/[H2]< 10-6 (except for Ophiuchi A).

• SWAS is showing that gas-phase H2O and O2 are not dominant coolants or major carriers of elemental oxygen in the cold dense regions of the ISM.

• SWAS has made large scale CI maps of Orion, M17 and Ophiuchi. The striking correlation between CI structures and that of the CO isotopologues suggest a clumpy cloud structure with high contrast between the clump and interclump medium.

• A comprehensive picture to explain SWAS data must involve chemical models with grain surface processes and possible contributions from cloud dynamics (e.g. shocks, circulation), and cloud structure (clumps). These knowledge would shed great lights on the molecular clouds, which is the birth place of young stars.