Star Formation in our Galaxy

Star Formation in our Galaxy Dr Andrew Walsh (James Cook University, Australia) • Lecture 2 – Chemistry and Star Formation • Basic chemical interactions • Abundances • Depletion and enhancement • Line surveys and common lines • Column density • Virial equilibrium • Rotation diagrams • 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* + C  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 + 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 H H H Dust grain H

Abundances • The Chemical Elements • Z Element Parts per million • 1 Hydrogen 739,000 • 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 • 16 Sulfur 440

Abundances • Molecule/Ion/Radical Relative Abundances

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

Optical Near-Infrared 1.2 mm Dust Continuum C18O N2H+ Depletion in B68

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

IF0 IF2 IF1 IF3 2.2GHz 8.4GHz Mopra Radiotelescope • The new Mopra spectrometer (MOPS) • Instantaneous 8GHz bandwidth split between 4 IFs of 2.2GHz width each

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

Line surveys of many sources

99 83 87 85 86 84 88 89 98 91 90 91 92 93 94 95 97 92 116 100 96 114 99 100 101 102 103 104 105 108 115 106 107 108 109 110 111 112 113 107 Frequency (GHz) Frequency (GHz) Frequency (GHz) Frequency (GHz) Orion G327.3-0.6 17233-3606 G305.2+0.2

83 87 85 86 84 88 89 90 91 92 Frequency (GHz)

83 87 85 86 84 88 89 90 91 92 Frequency (GHz) Orion G327.3-0.6 17233-3606 G305.2+0.2

83 87 85 86 84 88 89 90 91 92 Frequency (GHz)

83 87 85 86 84 88 89 90 91 92 Frequency (GHz) Orion G327.3-0.6 17233-3606 G305.2+0.2

83 87 85 86 84 88 89 90 91 92 Frequency (GHz) Orion G327.3-0.6 17233-3606 G305.2+0.2 CH3OH (El/k = 1443K) CH3OCH3 (El/k = 1059K)

Molecules in Space 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 HNCCC HNCO HNCO- HNO N2H+ N2+ N2O NH NH2 NH3 NH4+ NH2CN NH2CHO NO CO CO+ CO2 CO2+ H2CCO H2CO H2O H2O+ H3CO+ H3O+ HC2CHO HCOHCO+ HCOOCH3 HCOOH HOC+ HOCH2CH2OH HOCO+ OH OH+ AlCl AlF AlNC FeO HCl HF KCl MgCN MgNC NaCl NaCN PN CP C2S C3S CH3SH CS H2CS H2S H2S+ HCS+ HNCS HS HS+ OCS S2 NS SO SO+ SO2 C3N C5N CH2CHCN CH2CN CH2NH CH3C3N CH3CH2CN CH3CN CH3NC CH3NH2 CN CN+ H2C3N+ H2CN HCN HNCHCCN HC3N HC4N HC5N HC7N HC9N HC11N HCCNC HCNH+ SiC c-SiC2 SiC2 SiC3 SiC4 SiCN SiH SiH4 SiN SiNC SiO SiS c-C2H4O CH3CH2OH C2O C3H4O C3O CH2OHCHO CH3CH2CHO CH3CHO CH3COCH3 CH3COOH CH3OCH3 CH3OH H2 H3+

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 Frequency (GHz) 1.420 Ubiquitous low density gas tracer Critical density ~ 101 cm-3 Strong enough to be easily detected in other galaxies – traces outer edges

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 Frequency (GHz) 1.612 1.665 1.667 1.720 4.765 6.035 Maser and thermal emission Found towards star forming regions, Evolved stars (post-AGB), SNRs, Extragalactic sources

Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN NH3 - Ammonia Frequency (GHz) 23.694 23.722 23.870 24.139 24.532 25.056 etc Maser and thermal emission Ubiquitous medium to high density Gas tracer > 103 cm-3 Closely traces density structure

Some of the more important lines Optical Depth: Tmain (1 - e-τ) = Main line • Tsat (1 - e-aτ) • a = 0.28 (inner) • a = 0.22 (outer) • τ= 0.5 H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN NH3 (1,1) spectrum 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 Frequency (GHz) 22.235 Maser only Most common maser known Traces outflows in star forming regions Also found in other astrophysical objects (eg. evolved stars, extragalactic megamasers)

Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN 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 NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN CO - Carbon Monoxide Frequency (GHz) 115.271 110.201 109.978 112.358 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

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 Frequency (GHz) 89.188 86.754 85.162 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+

Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN N2H+ - Diazenylium Frequency (GHz) 93.173 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

Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN CH3OH - 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 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 Velocity (km/s) H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN SiO CS HCCCN CH3CN – Methyl Cyanide Frequency (GHz) 91.987 110.353 Useful rotational ladders (close together) CH3CN Spectrum Rotation diagram using the J=(5-4) & J=(6-5) transitions. (Purcell et al. 2006, MNRAS, 367, 553)

Some of the more important lines H OH NH3 H2O HCN CO HCO+ N2H+ CH3OH CH3CN 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.

Star Formation in our Galaxy