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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 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 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+BC+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 H H Dust grain H H Abundances The Chemical Elements Z Element Parts per million 1 2 8 6 10 26 7 14 12 16 Hydrogen Helium Oxygen Carbon Neon Iron Nitrogen Silicon Magnesium Sulfur 739,000 240,000 10,400 4,600 1,340 1,090 960 650 580 440 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 Optical 1.2 mm Dust Continuum Near-Infrared C18O N2H+ 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 Red & Blue = HCO+ (1-0) Greyscale = N2H+ (1-0) + = dust continuum cores Walsh et al. 2007 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 IF0 IF2 IF1 IF3 2.2GHz 8.4GHz G327.3-0.6 Glimpse 3-colour mid-infrared image 4.5, 5.8 and 8.0 microns Line surveys of many sources 83 84 85 86 87 88 Frequency (GHz) 89 90 91 92 91 92 93 94 95 96 Frequency (GHz) 97 98 99 100 99 100 101 102 103 104 Frequency (GHz) 105 106 107 108 107 108 109 111 112 Frequency (GHz) 113 114 115 116 Orion G327.3-0.6 17233-3606 G305.2+0.2 110 83 84 85 86 87 88 Frequency (GHz) 89 90 91 92 83 84 85 86 87 88 Frequency (GHz) 89 90 91 92 83 84 Orion G327.3-0.6 17233-3606 G305.2+0.2 85 86 87 88 Frequency (GHz) 89 90 91 92 83 84 85 86 87 88 Frequency (GHz) 89 90 91 92 83 84 Orion G327.3-0.6 17233-3606 G305.2+0.2 85 86 87 88 Frequency (GHz) 89 90 91 92 83 84 Orion G327.3-0.6 17233-3606 G305.2+0.2 85 86 87 88 Frequency (GHz) 89 90 91 92 83 84 85 86 87 88 Frequency (GHz) 89 Orion G327.3-0.6 CH3OH (El/k = 1443K) 17233-3606 G305.2+0.2 CH3OCH3 (El/k = 1059K) 90 91 92 Molecules in Space AlCl AlF AlNC FeO HCl HF KCl MgCN MgNC NaCl NaCN PN CP SiC c-SiC2 SiC2 SiC3 SiC4 SiCN SiH SiH4 SiN SiNC SiO SiS C2S C3S CH3SH CS H2CS H2S H2S+ HCS+ HNCS HS HS+ OCS S2 NS SO SO+ SO2 H2 H3+ C3N C5N CH2CHCN CH2CN CH2NH CH3C3N CH3CH2CN CH3CN CH3NC CH3NH2 CN CN+ H2C3N+ H2CN HCN HNC HCCN HC3N HC4N HC5N HC7N HC9N HC11N HCCNC HCNH+ HNCCC HNCO HNCOHNO N2H+ N2+ N2O NH NH2 NH3 NH4+ NH2CN NH2CHO NO c-C2H4O CH3CH2OH C2O C3H4O C3O CH2OHCHO CH3CH2CHO CH3CHO CH3COCH3 CH3COOH CH3OCH3 CH3OH CO CO+ CO2 CO2+ H2CCO H2CO H2O H2O+ H3CO+ H3O+ HC2CHO HCO HCO+ HCOOCH3 HCOOH HOC+ HOCH2CH2OH HOCO+ OH OH+ C2 C2H C2H2 C2H4 C3 c-C3H l-C3H c-C3H2 C4H C5 C5H C6H C6H2 C6H6 C7H C8H CH CH+ CH2 CH3 CH3CCH CH3C4H CH3CH3 CH4 H2CCC H2CCCC HCCCCH HCCCCCCH Molecules in Space AlCl AlF AlNC FeO HCl HF KCl MgCN MgNC NaCl NaCN PN CP SiC c-SiC2 SiC2 SiC3 SiC4 SiCN SiH SiH4 SiN SiNC SiO SiS C2S C3S CH3SH CS H2CS H2S H2S+ HCS+ HNCS HS HS+ OCS S2 NS SO SO+ SO2 H2 H3+ C3N C5N CH2CHCN CH2CN CH2NH CH3C3N CH3CH2CN CH3CN CH3NC CH3NH2 CN CN+ H2C3N+ H2CN HCN HNC HCCN HC3N HC4N HC5N HC7N HC9N HC11N HCCNC HCNH+ HNCCC HNCO HNCOHNO N2H+ N2+ N2O NH NH2 NH3 NH4+ NH2CN NH2CHO NO c-C2H4O CH3CH2OH C2O C3H4O C3O CH2OHCHO CH3CH2CHO CH3CHO CH3COCH3 CH3COOH CH3OCH3 CH3OH CO CO+ CO2 CO2+ H2CCO H2CO H2O H2O+ H3CO+ H3O+ HC2CHO HCO HCO+ HCOOCH3 HCOOH HOC+ HOCH2CH2OH HOCO+ OH OH+ C2 C2H C2H2 C2H4 C3 c-C3H l-C3H c-C3H2 C4H C5 C5H C6H C6H2 C6H6 C7H C8H CH CH+ CH2 CH3 CH3CCH CH3C4H CH3CH3 CH4 H2CCC H2CCCC HCCCCH HCCCCCCH Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN HI - atomic hydrogen Ubiquitous low density gas tracer Critical density ~ 101 cm-3 Strong enough to be easily detected in other galaxies – traces outer edges Frequency (GHz) 1.420 Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN GASS (Galactic All Sky Survey) Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN 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 NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN NH3 - Ammonia Maser and thermal emission Ubiquitous medium to high density Gas tracer > 103 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 NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN Optical Depth: Tmain Tsat = NH3 (1,1) spectrum Main line (1 - e-τ) (1 - e-aτ) a = 0.28 (inner) a = 0.22 (outer) τ = 0.5 Inner satellite Outer satellite Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN 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 Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN HCN - Hydrogen Cyanide Ubiquitous high density gas tracer Hyperfine structure Bright enough to be seen in the centres of other galaxies Frequency (GHz) 88.632 Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN CO - Carbon Monoxide Ubiquitous low density gas tracer Critical density ~102 cm-3 Strongly influenced by outflows in our Galaxy Found in the cores of galaxies Can be traced right across the universe 13CO C18O C17O Frequency (GHz) 115.271 110.201 109.978 112.358 Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN CO - Carbon Monoxide Second most abundant molecule X ~ 10-4 H2 CO (1-0) is the brightest thermal line (Dame, Hartmann & Thaddeus, 2000) Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN HCO+ - Oxomethylium Occurs in similar regions to CO Higher critical density ~2 105 cm-3 Like CO enhanced in outflows and suffers from freeze-out onto dust grains in cold, dense regions H13CO+ HC18O+ Frequency (GHz) 89.188 86.754 85.162 Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN 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 Frequency (GHz) 93.173 Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN CH3OH - Methanol Both thermal and maser MANY spectral lines (asymmetric rotor) Frequency (GHz) 6.669 12.179 24.933 44.069 96.741 etc Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN Thermal Methanol Lines in 12mm and 3mm bands → rotation diagram 12mm ladder: 24.928 CH3OH (32,1-31,2) E Energy = 35K 24.933 CH3OH (42,2-41,3) E Energy = 44K 24.959 CH3OH (52,3-51,4) E Energy = 56K 25.018 CH3OH (62,4-61,5) E Energy = 70K … 27.472 CH3OH (132,11-131,12) E Energy = 232K Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN 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 NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN CH3CN – Methyl Cyanide Useful rotational ladders (close together) Frequency (GHz) 91.987 110.353 Velocity (km/s) CH3CN Spectrum (Purcell et al. 2006, MNRAS, 367, 553) Rotation diagram using the J=(5-4) & J=(6-5) transitions. Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN SiO – Silicon Monoxide Both maser and thermal emission Maser emission in vibrationally Excited states only seen towards 2 or 3 sources. But results very productive in Orion. Frequency (GHz) 43.423 86.243 86.847 Some of the more important lines Matthews et al. 2007 H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN SiO – Silicon Monoxide Both maser and thermal emission Maser emission in vibrationally Excited states only seen towards 2 or 3 sources. But results very productive in Orion. Frequency (GHz) 43.423 86.243 86.847 Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN SiO – Silicon Monoxide 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 Frequency (GHz) 43.423 86.243 86.847 Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN SiO – Silicon Monoxide IRAS 20126+4104 Cesaroni et al. 1999 IRAS 20126+4104 Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN CS – Carbon Sulfide Ubiquitous tracer of high density gas Critical density ~ 2 106 cm-3 Suffers from freeze-out onto dust grains (depletion) Frequency (GHz) 48.991 97.981 Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN HCCCN - Cyanoacetylene Hot core molecule (tracer of high mass star formation) Frequency (GHz) 18.196 27.294 36.392 90.980 100.078 Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN HCCCN - Cyanoacetylene Hot core molecule (tracer of high mass star formation) HOPS results HCCCN NH3 Frequency (GHz) 18.196 27.294 36.392 90.980 100.078 Calculating Column Densities Calculating Column Densities Nu = 8 k 2 Aul h c3 ∫ ∞ -∞ Tb dv ( 1 - e- ) Calculating Column Densities Nu = 8 k 2 Aul h c3 ∫ ∞ -∞ Tb dv ( 1 - e- Nu = Column density in upper energy level ) Calculating Column Densities Nu = 8 k 2 Aul h c3 ∫ ∞ -∞ k = Boltzmann’s constant = 1.38 10-23 m2 kg s-2 K-1 Tb dv ( 1 - e- ) Calculating Column Densities Nu = 8 k 2 Aul h c3 ∫ ∞ Tb dv -∞ = frequency of line transition (eg. 115.271 GHz for CO(1-0)) ( 1 - e- ) Calculating Column Densities Nu = 8 k 2 Aul h c3 ∫ ∞ -∞ Tb dv ( Aul = Einstein A coefficient for transition = 1633 |2| 3ohc3 1 - e- ) Calculating Column Densities Nu = 8 k 2 Aul h c3 ∫ ∞ Tb dv -∞ ( Aul = Einstein A coefficient for transition = 1633 |2| 3ohc3 o = permittivity of free space = 8.854 10-12 m-3 kg-1 s4 A2 1 - e- ) Calculating Column Densities Nu = 8 k 2 Aul h c3 ∫ ∞ Tb dv -∞ ( Aul = Einstein A coefficient for transition = 1633 |2| 3ohc3 = magnetic dipole moment (eg, for N2H+ = 1 - e- ) Calculating Column Densities Nu = 8 k 2 Aul h c3 ∫ ∞ Tb dv -∞ ( Aul = Einstein A coefficient for transition = 1633 |2| 3ohc3 = magnetic dipole moment (eg, for N2H+ = 3.4 Debye 1 - e- ) Calculating Column Densities Nu = 8 k 2 Aul h c3 ∫ ∞ -∞ Tb dv ( 1 - e- Aul = Einstein A coefficient for transition = 1633 |2| 3ohc3 = magnetic dipole moment (eg, for N2H+ = 3.4 Debye = 1.13 10-29 C m) ) Calculating Column Densities Nu = 8 k 2 Aul h c3 ∫ ∞ -∞ Tb dv ( 1 - e- Integrated Intensity (area under the curve) ) Calculating Column Densities Nu = 8 k 2 Aul h c3 ∫ ∞ -∞ Tb dv ( 1 - e- ) = optical depth Optical Depth 1 TB TB B Optically thick → Temperature probe Optically thin 1 TB TB B → Column density probe Calculating Column Densities Nu = 8 k 2 Aul h c3 N = Nu gu ∫ ∞ Tb dv -∞ eEu/kT Q(Tex) ( 1 - e- ) Calculating Column Densities Nu = 8 k 2 Aul h c3 N = Nu ∫ ∞ Tb dv -∞ eEu/kT Q(Tex) gu gu = upper energy level degeneracy = 2J+1 ( 1 - e- ) Calculating Column Densities Nu = 8 k 2 Aul h c3 N = Nu ∫ ∞ Tb dv -∞ eEu/kT Q(Tex) gu Eu = upper energy level (K) ( 1 - e- ) Calculating Column Densities Nu = 8 k 2 Aul h c3 N = Nu ∫ ∞ Tb dv -∞ eEu/kT Q(Tex) gu Q(Tex) = partition function (a sum over all energy states) at a given temperature, Tex ( 1 - e- ) Calculating Column Densities 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 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 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 M⊙ km/s r pc Assumes uniform density profile If density falls off as r-2, 210 changes to 126. Applying Column Densities Applying Column Densities N = Nu gu eEu/kT Q(Tex) Rotation Diagrams ln ( ) ( ) Nu gu = ln N Q(T) Eu kTex • Plot ln (Nu/gu) vs. Eu/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) Chemical Clocks Use chemical rate equations, together with an initial model of the physical conditions • Abundance • Temperature • Density • Structure T = 100K NH = 1.8 x 104 cm-3 T = 100K NH = 8 x 104 cm-3 T = 200K NH = 1.8 x 104 cm-3 T = 200K NH = 8 x 104 cm-3 2 2 2 2 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