Star Formation in our Galaxy Dr Andrew Walsh (James Cook University, Australia) Lecture 2 – Chemistry and Star Formation 1.Basic chemical interactions

Star Formation in our Galaxy

Dr Andrew Walsh (James Cook University, Australia)

Lecture 2 – Chemistry and Star Formation

1. Basic chemical interactions

2. Abundances

3. Depletion and enhancement

4. Line surveys and common lines

5. Column density

6. Virial equilibrium

7. Rotation diagrams

8. Chemical clocks

Basic chemical interactions• High dust column densities block optical and UV-light in dark cores: molecules can form and survive

• Formation of molecules is an energy problem Possibilities: - Simultaneous collision with 3rd atom carrying away energy unlikely at the given low densities

Basic chemical interactions

Chemical reactions on earth:

A + B AB* (excited state, unstable, lifetime 10-12 s)

followed by

AB* AB + C + ΔEkin

the collision with a third particle C within the lifetime of AB* is needed toremove excess energy, otherwise the reaction

AB* A + B

will occur. Due to momentum conservation, the excess energy cannot beconverted into kinetic energy.

Basic chemical interactions

Chemical reactions in space:

The density is so low that no particle C will come by withinthe lifetime of AB*, so only reactions of the type

A + B C + D

or

A + B AB + hν

are possible. The second reaction product obeys energy andmomentum conservation laws.

In space, temperatures are between 10 and 300 K, so most endothermic reactionscannot occur since not enough energy is available.

In space, we have a low-energy, two-body-in two-body-out chemistry.

Basic chemical interactions• High dust column densities block optical and UV-light in dark cores: molecules can form and survive

• Formation of molecules is an energy problem Possibilities: - Simultaneous collision with 3rd atom carrying away energy unlikely at the given low densities

- Ion-molecule or ion-atom reactions can solve energy problem

- Neutral-neutral reactions on dust grain surfaces (catalytic) important

Basic chemical interactions - Neutral-neutral reactions on dust grain surfaces (catalytic) important

Dustgrain

H

H

HH

Abundances

The Chemical Elements

Z Element Parts per million

1 Hydrogen 739,000 2 Helium 240,000 8 Oxygen 10,4006 Carbon 4,600 10 Neon 1,340 26 Iron 1,090 7 Nitrogen 960 14 Silicon 650 12 Magnesium 580 16 Sulfur 440

Abundances

Molecule/Ion/Radical Relative Abundances

Molecule/Ion/Radical Relative Abundance

Reference

H2 1

CO 2 × 10–5 Dickman & Clemens 1983

13CO 1 × 10–6 Irvine et al. 1987

C18O 1 × 10–7 Frerking et al. 1982

CH3OH 2 × 10–6 Bisschop et al. 2007

CH3CN 1 × 10–7 Bisschop et al. 2007

CS 4 × 10–8 Garay et al. 2010

HCO+ 4 × 10–8 Hogerheijde et al. 1998

HCCCN 5 × 10–8 Sorochenko et al. 1986

NH3 1 × 10–8 Johnstone et al. 2010

C34S 4 × 10–10 Wilson & Rood 1994

N2H+ 2 × 10–10 Walsh et al. 2007

SiO 5 × 10–11 Garay et al. 2010

Abundances

“CS abundance is 3 × 10-9 on average, ranging from (4-8) × 10-10 in the cold source GL 7009S to

(1-2) × 10-8 in the two hot-core-type sources.”

van der Tak et al. 2000

In the coldest and densest regions, species suffer “depletion” (decrease in abundance) whereby they freeze-out onto dust grains

Shocks can increase the abundance of some species

Depletion in B68

1.2 mm Dust Continuum C18O N2H+

Optical Near-Infrared

Depletion

Common depleting molecules:

• ALL of them

• Some suffer strong depletion (eg. O-bearing and S-bearing species like CO, HCO+ and CS)

• Some are relatively robust against depletion (eg. N-bearing species and H-only species like NH3, N2H+ and H2D+)

Shock Enhancement

Walsh et al. 2007

Red & Blue = HCO+ (1-0)

Greyscale = N2H+ (1-0)

+ = dust continuum cores

Shock Enhancement

Species affected: CO, HCO+, CS, CH3OH, HCN, HNC, SiO...

N2H+ and NH3 tend to “avoid” shocked regions

Due to reactions with CO and HCO+ that quickly react with N2H+ and NH3 to form CH3CN, CH3OH and similar byproducts

both N2H+ and NH3 are reliable tracers of quiescent gas

Line Surveys and Common Lines

Line Survey:

• Observe as large a range of frequencies as possible

• Usually done in the millimetre or sub-millimetre

• Show the range of species that are detectable

Line Surveys and Common Lines

The Mopra Radiotelescope

Recent Mopra Upgrades

• On-the-fly mapping to quickly scan the sky

• New 3mm receiver covers 77-116GHz

• New 12mm receiver covers 16-28GHz

• The new spectrometer (MOPS) has instantaneous 8GHz bandwidth with up to 32,000 channels (2 polarisations) 0.25MHz per channel in broadband mode

Mopra Radiotelescope

The new Mopra spectrometer (MOPS)

• Instantaneous 8GHz bandwidth split between 4 IFs of 2.2GHz width each

IF0IF1

IF2IF3

8.4GHz

2.2GHz

G327.3-0.6

Glimpse 3-colour mid-infrared image4.5, 5.8 and 8.0 microns

Line surveys of many sources

OrionG327.3-0.617233-3606G305.2+0.2

83

Frequency (GHz)

8785 8684 88 89 90 91 92

Frequency (GHz)

91 92 93 94 95 9796 98 10099

99 100 101 102

Frequency (GHz)

103 104 105 106 107 108

Frequency (GHz)

107 108 109 110 111 112 113 114 115 116

83

Frequency (GHz)

8785 8684 88 89 90 91 92

83

Frequency (GHz)

8785 8684 88 89 90 91 92

83

Frequency (GHz)

8785 8684 88 89 90 91 92

Orion

G327.3-0.6

17233-3606

G305.2+0.2

83

Frequency (GHz)

8785 8684 88 89 90 91 92

83

Frequency (GHz)

8785 8684 88 89 90 91 92

Orion

G327.3-0.6

17233-3606

G305.2+0.2

83

Frequency (GHz)

8785 8684 88 89 90 91 92

Orion

G327.3-0.6

17233-3606

G305.2+0.2

83

Frequency (GHz)

8785 8684 88 89 90 91 92

Orion

G327.3-0.6

17233-3606

G305.2+0.2

CH3OCH3

(El/k = 1059K)

CH3OH(El/k = 1443K)

Molecules in SpaceAlClAlFAlNCFeOHClHFKClMgCNMgNCNaClNaCNPNCP

SiCc-SiC2

SiC2

SiC3

SiC4

SiCNSiHSiH4

SiNSiNCSiOSiS

C2SC3SCH3SHCSH2CSH2SH2S+

HCS+

HNCSHSHS+

OCSS2

NSSOSO+

SO2

C3NC5NCH2CHCNCH2CNCH2NHCH3C3NCH3CH2CNCH3CNCH3NCCH3NH2

CNCN+

H2C3N+

H2CNHCNHNCHCCNHC3NHC4NHC5NHC7NHC9NHC11NHCCNCHCNH+

COCO+

CO2

CO2+

H2CCOH2COH2OH2O+

H3CO+

H3O+

HC2CHOHCOHCO+

HCOOCH3

HCOOHHOC+

HOCH2CH2OHHOCO+

OHOH+

C2

C2HC2H2

C2H4

C3

c-C3Hl-C3Hc-C3H2

C4HC5

C5HC6HC6H2

C6H6

C7HC8HCHCH+

CH2

CH3

CH3CCHCH3C4HCH3CH3

CH4

H2CCCH2CCCCHCCCCHHCCCCCCH

H2

H3+

HNCCCHNCOHNCO-

HNON2H+

N2+

N2ONHNH2

NH3

NH4+

NH2CNNH2CHONOc-C2H4OCH3CH2OHC2OC3H4OC3OCH2OHCHOCH3CH2CHOCH3CHOCH3COCH3

CH3COOHCH3OCH3

CH3OH

Molecules in SpaceAlClAlFAlNCFeOHClHFKClMgCNMgNCNaClNaCNPNCP

SiCc-SiC2

SiC2

SiC3

SiC4

SiCNSiHSiH4

SiNSiNCSiOSiS

C2SC3SCH3SHCSH2CSH2SH2S+

HCS+

HNCSHSHS+

OCSS2

NSSOSO+

SO2

C3NC5NCH2CHCNCH2CNCH2NHCH3C3NCH3CH2CNCH3CNCH3NCCH3NH2

CNCN+

H2C3N+

H2CNHCNHNCHCCNHC3NHC4NHC5NHC7NHC9NHC11NHCCNCHCNH+

COCO+

CO2

CO2+

H2CCOH2COH2OH2O+

H3CO+

H3O+

HC2CHOHCOHCO+

HCOOCH3

HCOOHHOC+

HOCH2CH2OHHOCO+

OHOH+

C2

C2HC2H2

C2H4

C3

c-C3Hl-C3Hc-C3H2

C4HC5

C5HC6HC6H2

C6H6

C7HC8HCHCH+

CH2

CH3

CH3CCHCH3C4HCH3CH3

CH4

H2CCCH2CCCCHCCCCHHCCCCCCH

H2

H3+

HNCCCHNCOHNCO-

HNON2H+

N2+

N2ONHNH2

NH3

NH4+

NH2CNNH2CHONOc-C2H4OCH3CH2OHC2OC3H4OC3OCH2OHCHOCH3CH2CHOCH3CHOCH3COCH3

CH3COOHCH3OCH3

CH3OH

Some of the more important lines

H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN



HI - atomic hydrogen

Frequency(GHz)1.420

Ubiquitous low density gas tracerCritical density ~ 101 cm-3

Strong enough to be easilydetected in other galaxies – traces outer edges



GASS (Galactic All Sky Survey)



OH - Hydroxyl Radical

Maser and thermal emission

Found towards star forming regions,Evolved stars (post-AGB), SNRs,Extragalactic sources

Frequency(GHz)1.6121.6651.6671.7204.7656.035



NH3 - Ammonia

Maser and thermal emission

Ubiquitous medium to high densityGas tracer > 103 cm-3

Closely traces density structure

Frequency(GHz)23.69423.72223.87024.13924.53225.056

etc



Optical Depth:

Tmain (1 - e-τ)

Tsat (1 - e-aτ)

a = 0.28 (inner)a = 0.22 (outer)

τ = 0.5

=

Main line

Inner satellite

Outer satellite

NH3 (1,1)spectrum



H2O - Water

Maser only

Most common maser known

Traces outflows in star forming regions

Also found in other astrophysical objects(eg. evolved stars, extragalactic megamasers)

Frequency(GHz)22.235



HCN - Hydrogen Cyanide

Frequency(GHz)88.632Ubiquitous high density gas tracer

Hyperfine structure

Bright enough to be seen in thecentres of other galaxies



CO - Carbon Monoxide

Frequency(GHz)

115.271110.201109.978112.358

13COC18OC17O

Ubiquitous low density gas tracerCritical density ~102 cm-3

Strongly influenced byoutflows in our Galaxy

Found in the cores of galaxies

Can be traced right across the universe



CO - Carbon Monoxide

(Dame, Hartmann & Thaddeus, 2000)

Second most abundant moleculeX ~ 10-4 H2

CO (1-0) is the brightest thermal line



HCO+ - Oxomethylium

Frequency(GHz)89.18886.75485.162

H13CO+

HC18O+

Occurs in similar regions to COHigher critical density~2 105 cm-3

Like CO enhanced in outflows andsuffers from freeze-out onto dust grainsin cold, dense regions



N2H+ - Diazenylium


Reliable high density gas tracer

Hyperfine structure gives optical depth

Critical density ~ 2 105 cm-3

Does not show up in outflows

Less prone to freeze-out/depletion



CH3OH - Methanol

Frequency(GHz)6.66912.17924.93344.06996.741

etc

Both thermal and maser

MANY spectral lines (asymmetricrotor)



Thermal Methanol

Lines in 12mm and 3mm bands → rotation diagram

12mm ladder:24.928 CH3OH (32,1-31,2) E Energy = 35K24.933 CH3OH (42,2-41,3) E Energy = 44K24.959 CH3OH (52,3-51,4) E Energy = 56K25.018 CH3OH (62,4-61,5) E Energy = 70K…27.472 CH3OH (132,11-131,12) E Energy = 232K



Methanol Masers

Class I masers collisionally excitedClass II masers radiatively excited

Class I usually found offset from star formation sites

Class II closely associated with sitesof high-mass star formation (and nothing else)



CH3CN – Methyl Cyanide


110.353

Useful rotational ladders(close together)

Velocity (km/s)

Rotation diagram using the J=(5-4) & J=(6-5) transitions.

CH3CN Spectrum

(Purcell et al. 2006, MNRAS, 367, 553)



SiO – Silicon Monoxide

Frequency(GHz)43.42386.24386.847

Both maser and thermal emission

Maser emission in vibrationallyExcited states only seen towards2 or 3 sources. But results veryproductive in Orion.




Frequency(GHz)43.42386.24386.847

Both maser and thermal emissionMaser emission in vibrationallyExcited states only seen towards2 or 3 sources. But results veryproductive in Orion.

Matthews et al. 2007




Frequency(GHz)43.42386.24386.847

Both maser and thermal emissionMaser emission in vibrationallyExcited states only seen towards2 or 3 sources. But results veryproductive in Orion.

Thermal SiO closely associated withOutflows in star forming regions




IRAS 20126+4104Cesaroni et al. 1999 IRAS 20126+4104



CS – Carbon Sulfide

Frequency(GHz)48.99197.981

Ubiquitous tracer of high density gas

Critical density ~ 2 106 cm-3

Suffers from freeze-out ontodust grains (depletion)



HCCCN - Cyanoacetylene

Frequency(GHz)18.19627.29436.39290.980

100.078

Hot core molecule(tracer of high mass star formation)



HCCCN - Cyanoacetylene

Frequency(GHz)18.19627.29436.39290.980

100.078

Hot core molecule(tracer of high mass star formation)

HOPS results

HCCCN

NH3

Calculating Column Densities


Nu = 8 k 2

Aul h c3 ∫-∞

∞Tb dv

1 - e-( )


Nu = 8 k 2

Aul h c3 ∫-∞

∞Tb dv

1 - e-( )

Nu = Column density in upper energy level


Nu = 8 k 2

Aul h c3 ∫-∞

∞Tb dv

1 - e-( )

k = Boltzmann’s constant = 1.38 10-23 m2 kg s-2 K-1


Nu = 8 k 2

Aul h c3 ∫-∞

∞Tb dv

1 - e-( )

= frequency of line transition (eg. 115.271 GHz for CO(1-0))


Nu = 8 k 2

Aul h c3 ∫-∞

∞Tb dv

1 - e-( )

Aul = Einstein A coefficient for transition = 1633

3ohc3

|2|


Nu = 8 k 2

Aul h c3 ∫-∞

∞Tb dv

1 - e-( )


o = permittivity of free space = 8.854 10-12 m-3 kg-1 s4 A2

3ohc3

|2|


Nu = 8 k 2

Aul h c3 ∫-∞

∞Tb dv

1 - e-( )


= magnetic dipole moment(eg, for N2H+ =

3ohc3

|2|


Nu = 8 k 2

Aul h c3 ∫-∞

∞Tb dv

1 - e-( )


= magnetic dipole moment(eg, for N2H+ = 3.4 Debye

3ohc3

|2|


Nu = 8 k 2

Aul h c3 ∫-∞

∞Tb dv

1 - e-( )


= magnetic dipole moment(eg, for N2H+ = 3.4 Debye = 1.13 10-29 C m)

3ohc3

|2|


Nu = 8 k 2

Aul h c3 ∫-∞

∞Tb dv

1 - e-( )

Integrated Intensity(area under the curve)


Nu = 8 k 2

Aul h c3 ∫-∞

∞Tb dv

1 - e-( )

= optical depth

Optical Depth

Optically thick

Optically thin

1 TB TB B

1 TB TB B

→ Temperature probe

→ Column density probe


Nu = 8 k 2

Aul h c3 ∫-∞

∞Tb dv

1 - e-( )N = Nu

gu

eEu/kT Q(Tex)


Nu = 8 k 2

Aul h c3 ∫-∞

∞Tb dv

1 - e-( )N = Nu

gu

eEu/kT Q(Tex)

gu = upper energy level degeneracy = 2J+1


Nu = 8 k 2

Aul h c3 ∫-∞

∞Tb dv

1 - e-( )N = Nu

gu

eEu/kT Q(Tex)

Eu = upper energy level (K)


Nu = 8 k 2

Aul h c3 ∫-∞

∞Tb dv

1 - e-( )N = Nu

gu

eEu/kT Q(Tex)

Q(Tex) = partition function (a sum over all energy states) at a given temperature, Tex


Values for , , Eu and Q(Tex) can be found at “CDMS”(http://www.astro.uni-koeln.de/site/vorhersagen/)

Note that CDMS quotes El, rather than Eu and unitsare in cm-1, rather than K. (1K = 100 hc/k cm-1)

http://www.astro.uni-koeln.de/site/vorhersagen/

Applying Column DensitiesWalsh et al. 2007, ApJ, 655, 958

Applying Column DensitiesGiven column density of N2H+ clump in NGC1333:

• Assume LTE

• Assume size of clump

• Assume relative abundance of N2H+ to H2

(~1.8 x 10-10)

• Assume mean molecular weight 2.3

Mass of clump

Applying Column Densities

Compare to Virial Mass:

MVIR = 210 v2 r

M⊙ km/s pc

Assumes uniform density profileIf density falls off as r-2,

210 changes to 126.



N = Nu

gu

eEu/kT Q(Tex)

Rotation Diagrams

Nu N Eu

gu Q(T) kTex( )ln = ln( )

• Plot ln (Nu/gu) vs. Eu/k

• Slope = 1/T

• Y-intercept = ln (N/Q(T))

Rotation DiagramsAmmonia in a high mass star forming region

(1,1)

(2,2)

(4,4)

(5,5)

(Longmore et al. 2007, MNRAS, 379, 535)

Use chemical rate equations, together with an initial model of the physical conditions

• Abundance

• Temperature

• Density

• Structure

Chemical Clocks

T = 100KNH

2 = 1.8 x 104 cm-3

T = 200KNH

2 = 1.8 x 104 cm-3

T = 100KNH

2 = 8 x 104 cm-3

T = 200KNH

2 = 8 x 104 cm-3

Summary

Lecture 2 – Chemistry and Star Formation

1. Basic chemical interactions

2. Abundances

3. Depletion and enhancement

4. Line surveys and common lines

5. Column density

6. Virial equilibrium

7. Rotation diagrams

8. Chemical clocks

Documents

Star Formation in our Galaxy Dr Andrew Walsh (James Cook University, Australia) Lecture 2 – Chemistry and Star Formation 1.Basic chemical interactions