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 + ΔE kin the collision with a third particle C within the lifetime of AB* is needed to remove excess energy, otherwise the reaction AB*  A + B will occur. Due to momentum conservation, the excess energy cannot be converted into kinetic energy.

Basic chemical interactions Chemical reactions in space: The density is so low that no particle C will come by within the 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 and momentum conservation laws. In space, temperatures are between 10 and 300 K, so most endothermic reactions cannot 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 Dust grain H H H H

Abundances The Chemical Elements Z ElementParts per million 1 Hydrogen739,000 2 Helium 240,000 8 Oxygen 10,400 6 Carbon 4,600 10 Neon 1,340 26 Iron 1,090 7 Nitrogen 960 14 Silicon 650 12 Magnesium 580 16Sulfur 440

Abundances Molecule/Ion/Radical Relative Abundances Molecule/Ion/RadicalRelative Abundance Reference H2H2 1 CO2 × 10 –5 Dickman & Clemens 1983 13 CO1 × 10 –6 Irvine et al. 1987 C 18 O1 × 10 –7 Frerking et al. 1982 CH 3 OH2 × 10 –6 Bisschop et al. 2007 CH 3 CN1 × 10 –7 Bisschop et al. 2007 CS4 × 10 –8 Garay et al. 2010 HCO + 4 × 10 –8 Hogerheijde et al. 1998 HCCCN5 × 10 –8 Sorochenko et al. 1986 NH 3 1 × 10 –8 Johnstone et al. 2010 C 34 S4 × 10 –10 Wilson & Rood 1994 N2H+N2H+ 2 × 10 –10 Walsh et al. 2007 SiO5 × 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 C 18 O N 2 H + 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 NH 3, N 2 H + and H 2 D + )

Shock Enhancement Walsh et al. 2007 Red & Blue = HCO + (1-0) Greyscale = N 2 H + (1-0) + = dust continuum cores

Shock Enhancement Species affected: CO, HCO +, CS, CH 3 OH, HCN, HNC, SiO... N 2 H + and NH 3 tend to “avoid” shocked regions Due to reactions with CO and HCO + that quickly react with N 2 H + and NH 3 to form CH 3 CN, CH 3 OH and similar byproducts  both N 2 H + and NH 3 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 IF0 IF1 IF2 IF3 8.4GHz 2.2GHz

G327.3-0.6 Glimpse 3-colour mid-infrared image 4.5, 5.8 and 8.0 microns

Line surveys of many sources

Orion G327.3-0.6 17233-3606 G305.2+0.2 83 Frequency (GHz) 878586848889909192 Frequency (GHz) 919293949597969810099 100101102 Frequency (GHz) 103104105106107108 Frequency (GHz) 107108109110111112113114115116

83 Frequency (GHz) 878586848889909192

83 Frequency (GHz) 878586848889909192 Orion G327.3-0.6 17233-3606 G305.2+0.2

83 Frequency (GHz) 878586848889909192

83 Frequency (GHz) 878586848889909192 Orion G327.3-0.6 17233-3606 G305.2+0.2

83 Frequency (GHz) 878586848889909192 Orion G327.3-0.6 17233-3606 G305.2+0.2 CH 3 OCH 3 (E l /k = 1059K) CH 3 OH (E l /k = 1443K)

Molecules in Space AlCl AlF AlNC FeO HCl HF KCl MgCN MgNC NaCl NaCN PN CP SiC c-SiC 2 SiC 2 SiC 3 SiC 4 SiCN SiH SiH 4 SiN SiNC SiO SiS C 2 S C 3 S CH 3 SH CS H 2 CS H 2 S H 2 S + HCS + HNCS HS HS + OCS S 2 NS SO SO + SO 2 C 3 N C 5 N CH 2 CHCN CH 2 CN CH 2 NH CH 3 C 3 N CH 3 CH 2 CN CH 3 CN CH 3 NC CH 3 NH 2 CN CN + H 2 C 3 N + H 2 CN HCN HNC HCCN HC 3 N HC 4 N HC 5 N HC 7 N HC 9 N HC 11 N HCCNC HCNH + CO CO + CO 2 CO 2 + H 2 CCO H 2 CO H 2 O H 2 O + H 3 CO + H 3 O + HC 2 CHO HCO HCO + HCOOCH 3 HCOOH HOC + HOCH 2 CH 2 OH HOCO + OH OH + C 2 C 2 H C 2 H 2 C 2 H 4 C 3 c-C 3 H l-C 3 H c-C 3 H 2 C 4 H C 5 C 5 H C 6 H C 6 H 2 C 6 H 6 C 7 H C 8 H CH CH + CH 2 CH 3 CH 3 CCH CH 3 C 4 H CH 3 CH 4 H 2 CCC H 2 CCCC HCCCCH HCCCCCCH H2H3+H2H3+ HNCCC HNCO HNCO - HNO N 2 H + N 2 + N 2 O NH NH 2 NH 3 NH 4 + NH 2 CN NH 2 CHO NO c-C 2 H 4 O CH 3 CH 2 OH C 2 O C 3 H 4 O C 3 O CH 2 OHCHO CH 3 CH 2 CHO CH 3 CHO CH 3 COCH 3 CH 3 COOH CH 3 OCH 3 CH 3 OH

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN HI - atomic hydrogen Frequency (GHz) 1.420 Ubiquitous low density gas tracer Critical density ~ 10 1 cm -3 Strong enough to be easily detected in other galaxies – traces outer edges

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN GASS (Galactic All Sky Survey)

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN OH - Hydroxyl Radical Maser and thermal emission Found towards star forming regions, Evolved stars (post-AGB), SNRs, Extragalactic sources Frequency (GHz) 1.612 1.665 1.667 1.720 4.765 6.035

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN NH 3 - Ammonia Maser and thermal emission Ubiquitous medium to high density Gas tracer > 10 3 cm -3 Closely traces density structure Frequency (GHz) 23.694 23.722 23.870 24.139 24.532 25.056 etc

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN Optical Depth: T main (1 - e -τ ) T sat (1 - e -aτ ) a = 0.28 (inner) a = 0.22 (outer)  τ = 0.5 = Main line Inner satellite Outer satellite NH 3 (1,1) spectrum

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN H 2 O - 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

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN HCN - Hydrogen Cyanide Frequency (GHz) 88.632 Ubiquitous high density gas tracer Hyperfine structure Bright enough to be seen in the centres of other galaxies

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN CO - Carbon Monoxide Frequency (GHz) 115.271 110.201 109.978 112.358 13 CO C 18 O C 17 O Ubiquitous low density gas tracer Critical density ~10 2 cm -3 Strongly influenced by outflows in our Galaxy Found in the cores of galaxies Can be traced right across the universe

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN CO - Carbon Monoxide (Dame, Hartmann & Thaddeus, 2000) Second most abundant molecule X ~ 10 -4  H 2 CO (1-0) is the brightest thermal line

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN HCO + - Oxomethylium Frequency (GHz) 89.188 86.754 85.162 H 13 CO + HC 18 O + Occurs in similar regions to CO Higher critical density ~2  10 5 cm -3 Like CO enhanced in outflows and suffers from freeze-out onto dust grains in cold, dense regions

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN N 2 H + - Diazenylium Frequency (GHz) 93.173 Reliable high density gas tracer Hyperfine structure gives optical depth Critical density ~ 2  10 5 cm -3 Does not show up in outflows Less prone to freeze-out/depletion

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN CH 3 OH - Methanol Frequency (GHz) 6.669 12.179 24.933 44.069 96.741 etc Both thermal and maser MANY spectral lines (asymmetric rotor)

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN Thermal Methanol Lines in 12mm and 3mm bands → rotation diagram 12mm ladder: 24.928 CH 3 OH (3 2,1 -3 1,2 ) E Energy = 35K 24.933 CH 3 OH (4 2,2 -4 1,3 ) E Energy = 44K 24.959 CH 3 OH (5 2,3 -5 1,4 ) E Energy = 56K 25.018 CH 3 OH (6 2,4 -6 1,5 ) E Energy = 70K … 27.472 CH 3 OH (13 2,11 -13 1,12 ) E Energy = 232K

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN Methanol Masers Class I masers collisionally excited Class II masers radiatively excited Class I usually found offset from star formation sites Class II closely associated with sites of high-mass star formation (and nothing else)

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN CH 3 CN – Methyl Cyanide Frequency (GHz) 91.987 110.353 Useful rotational ladders (close together) Velocity (km/s) Rotation diagram using the J=(5-4) & J=(6-5) transitions. CH 3 CN Spectrum (Purcell et al. 2006, MNRAS, 367, 553)

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN SiO – Silicon Monoxide Frequency (GHz) 43.423 86.243 86.847 Both maser and thermal emission Maser emission in vibrationally Excited states only seen towards 2 or 3 sources. But results very productive in Orion.

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN SiO – Silicon Monoxide Frequency (GHz) 43.423 86.243 86.847 Both maser and thermal emission Maser emission in vibrationally Excited states only seen towards 2 or 3 sources. But results very productive in Orion. Matthews et al. 2007

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN SiO – Silicon Monoxide Frequency (GHz) 43.423 86.243 86.847 Both maser and thermal emission Maser emission in vibrationally Excited states only seen towards 2 or 3 sources. But results very productive in Orion. Thermal SiO closely associated with Outflows in star forming regions

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN SiO – Silicon Monoxide IRAS 20126+4104 Cesaroni et al. 1999 IRAS 20126+4104

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN CS – Carbon Sulfide Frequency (GHz) 48.991 97.981 Ubiquitous tracer of high density gas Critical density ~ 2  10 6 cm -3 Suffers from freeze-out onto dust grains (depletion)

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN HCCCN - Cyanoacetylene Frequency (GHz) 18.196 27.294 36.392 90.980 100.078 Hot core molecule (tracer of high mass star formation)

Some of the more important lines H OH NH 3 H 2 O HCN CO HCO + N 2 H + CH 3 OH CH 3 CN SiO CS HCCCN HCCCN - Cyanoacetylene Frequency (GHz) 18.196 27.294 36.392 90.980 100.078 Hot core molecule (tracer of high mass star formation) HOPS results HCCCN NH 3

Calculating Column Densities

N u = 8 k  2 A ul h c 3 ∫ -∞ ∞ T b dv  1 - e -  ( )

Calculating Column Densities N u = 8 k  2 A ul h c 3 ∫ -∞ ∞ T b dv  1 - e -  ( ) N u = Column density in upper energy level

Calculating Column Densities N u = 8 k  2 A ul h c 3 ∫ -∞ ∞ T b dv  1 - e -  ( ) k = Boltzmann’s constant = 1.38  10 -23 m 2 kg s -2 K -1

Calculating Column Densities N u = 8 k  2 A ul h c 3 ∫ -∞ ∞ T b dv  1 - e -  ( ) = frequency of line transition (eg. 115.271 GHz for CO(1-0))

Calculating Column Densities N u = 8 k  2 A ul h c 3 ∫ -∞ ∞ T b dv  1 - e -  ( ) A ul = Einstein A coefficient for transition = 16  3 3 3  o hc 3 |2||2|

Calculating Column Densities N u = 8 k  2 A ul h c 3 ∫ -∞ ∞ T b dv  1 - e -  ( ) A ul = Einstein A coefficient for transition = 16  3 3  o = permittivity of free space = 8.854  10 -12 m -3 kg -1 s 4 A 2 3  o hc 3 |2||2|

Calculating Column Densities N u = 8 k  2 A ul h c 3 ∫ -∞ ∞ T b dv  1 - e -  ( ) A ul = Einstein A coefficient for transition = 16  3 3  = magnetic dipole moment (eg, for N 2 H + = 3  o hc 3 |2||2|

Calculating Column Densities N u = 8 k  2 A ul h c 3 ∫ -∞ ∞ T b dv  1 - e -  ( ) A ul = Einstein A coefficient for transition = 16  3 3  = magnetic dipole moment (eg, for N 2 H + = 3.4 Debye 3  o hc 3 |2||2|

Calculating Column Densities N u = 8 k  2 A ul h c 3 ∫ -∞ ∞ T b dv  1 - e -  ( ) A ul = Einstein A coefficient for transition = 16  3 3  = magnetic dipole moment (eg, for N 2 H + = 3.4 Debye = 1.13  10 -29 C m) 3  o hc 3 |2||2|

Calculating Column Densities N u = 8 k  2 A ul h c 3 ∫ -∞ ∞ T b dv  1 - e -  ( ) Integrated Intensity (area under the curve)

Calculating Column Densities N u = 8 k  2 A ul h c 3 ∫ -∞ ∞ T b dv  1 - e -  ( )  = optical depth

Optical Depth Optically thick Optically thin → Temperature probe → Column density probe

Calculating Column Densities N u = 8 k  2 A ul h c 3 ∫ -∞ ∞ T b dv  1 - e -  ( ) N = N u gugu e E u /kT Q(T ex )

Calculating Column Densities N u = 8 k  2 A ul h c 3 ∫ -∞ ∞ T b dv  1 - e -  ( ) N = N u gugu e E u /kT Q(T ex ) g u = upper energy level degeneracy = 2J+1

Calculating Column Densities N u = 8 k  2 A ul h c 3 ∫ -∞ ∞ T b dv  1 - e -  ( ) N = N u gugu e E u /kT Q(T ex ) E u = upper energy level (K)

Calculating Column Densities N u = 8 k  2 A ul h c 3 ∫ -∞ ∞ T b dv  1 - e -  ( ) N = N u gugu e E u /kT Q(T ex ) Q(T ex ) = partition function (a sum over all energy states) at a given temperature, T ex

Calculating Column Densities Values for ,, E u and Q(T ex ) can be found at “CDMS” (http://www.astro.uni-koeln.de/site/vorhersagen/)http://www.astro.uni-koeln.de/site/vorhersagen/ Note that CDMS quotes E l, rather than E u and units are in cm -1, rather than K. (1K = 100 hc/k cm -1 )

Applying Column Densities Walsh et al. 2007, ApJ, 655, 958

Applying Column Densities Given column density of N 2 H + clump in NGC1333: Assume LTE Assume size of clump Assume relative abundance of N 2 H + to H 2 (~1.8 x 10 -10 ) Assume mean molecular weight 2.3 Mass of clump

Applying Column Densities Compare to Virial Mass: M VIR = 210  v 2 r M ⊙ km/s pc Assumes uniform density profile If density falls off as r -2, 210 changes to 126.

Applying Column Densities

N = N u gugu e E u /kT Q(T ex )

Rotation Diagrams N u N E u g u Q(T) kT ex ( ) ln = ln ( ) Plot ln (N u /g u ) vs. E u /k Slope = 1/T Y-intercept = ln (N/Q(T))

Rotation Diagrams Ammonia 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 = 100K N H 2 = 1.8 x 10 4 cm -3 T = 200K N H 2 = 1.8 x 10 4 cm -3 T = 100K N H 2 = 8 x 10 4 cm -3 T = 200K N H 2 = 8 x 10 4 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

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

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

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Presentation on theme: "Star Formation in our Galaxy Dr Andrew Walsh (James Cook University, Australia) Lecture 2 – Chemistry and Star Formation 1.Basic chemical interactions."— Presentation transcript:

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