12/12/2011. Amino acid glycine CH 2 NH 2 COOH? Physics of the Interstellar and Intergalactic Medium. Some Interstellar Molecules (129 in 2005)

Size: px

Start display at page:

Download "12/12/2011. Amino acid glycine CH 2 NH 2 COOH? Physics of the Interstellar and Intergalactic Medium. Some Interstellar Molecules (129 in 2005)"

Naomi Lucas
6 years ago
Views:

1 PY4A4 Seior Sophister Physics of the Iterstellar ad Itergalactic Medium Lecture 5: Molecules Dr Graham M. Harper School of Physics, TCD Ope Office: Moday (for today) 14:3-15:3 Some Iterstellar Molecules (19 i 5) CH+ HCN HCO HC3N CH3OH HC5N HCOOCH3 HC7N CS HNC HCS HOCHO CH3CN CH3CCH CH3C3N HC9N CO HCO HCN CHNH CH3NC CH3NH CH3COOH HC11N CN OCS HNCO CHCO CH3SH CH3CHO CHOHCHO CH5CN C CH HNCS NHCN NHCHO CHCHCN HC6 CH3C4H CH CH C3H C4H C5H C6H CH3C5N CO+ C3 c-c3h c-c3h HC4 c-ch4o CH3OCH3 CF+ CO C3N HC3 HC3NH+ CHCHOH CH5OH CO C3O CHCN CH3COCH3 CS C3S HCCNC OHCHCHOH HCO+ CH3 HNCCC HOC+ CH CH4 HCS+ HOCO+ HCOH+ HCNH+ Amio acid glycie CH NH COOH? Claimed detectio i molecular cloud regios by Kua et al, 3, Ap, 593, 848 Refuted by Syder et al. 5, Ap, 619, 914 Cofirmed i Comet Wild- Stardust missio(9) 1

2 Amio acid glycie CH NH COOH? [1] Claimed detectio i molecular cloud regios by Kua et al, 3, Ap, 593, 848 [] Refuted by Syder et al. 5, Ap, 619, 914 [3] Cofirmed i Comet Wild- Stardust missio(9) ND 3 i Iterstellar Clouds Deuteratio: deuterium species slightly more stable tha hydroge form Submillimetre detectio of ND 3 by Lis et al., Astrophysical oural, 571, L55 () ND 3 /NH 3 = 8 1-4, compared with (D/H) 3 ~

3 DIB s Polycyclic Aromatic Hydrocarbos (PAHs) ~8 m Iterstellar Ices Mostly water ice Substatial compoets: - CO, CO, CH 3 OH Mior compoets: - HCOOH, CH 4, H CO 3

4 5. Molecules i the ISM Simplest molecules Liear, e.g. CO carbo mooxide [structured spectrum] symmetric top, e.g., NH 3 (ammoia) [more complex spectrum] asymmetric top, e.g., H (water vapour) [forest] Liear molecules rotatio, vibratio, electroic Symmetric top rotatioal Why are there ay molecules? hard to form easy to destroy Reactio etworks rate equatios timescales 1 Liear molecules I Molecules (rigid rotators, liear molecules) is rotatioal quatum umber v is vibratioal quatum umber B is rotatioal costat I is momet of iertia D is cetrifugal stretchig term ω is vibratioal costat h 8 ci B E() = (+1) B (+1) D + (v+1/)ħω [Figure] Liear molecules II For molecules with permaet electric dipole Selectio rule Δ= ±1, e.g., OH, SiO, CO For molecules with o permaet dipole momet Electric quadrupole trasitio Δ=,±, e.g., H Defie the eergies i temperature uits, Molecule Rotatioal Vibratioal CO B/k.8 K ħω /k 31 K H B/k 87 K ħω /k 63 K E T k 4

5 Spectra: Frak Shu Vol 1 Radiatio, Chap. 3 E() = B (+1) + (v+1/)hω frequecy (ν=δe/h ) for each rovibratioal lie with differet iitial ad fial vibratioal quatum umbers for example -1, ad v= v=1 i.e., ν=δe/h= B [(+1) (-1)] + ω = B + ω lie frequecy icreases with lies are approximately equally spaced i frequecy. [Figure from Mitchell, G. F., et al. 1989, Ap, 341, 1] Populatio of rotatioal levels Both H ad CO have low critical desities, ad the lower rotatioal levels are ofte i thermal equilibrium: we ca use this fact to measure the molecular clouds temperatures g exp g ' E 1exp[ 1 B kt] kt Ratio of ay two levels ca be foud directly, i.e., 1exp[ 1B kt] ' 1exp[ ' ' 1 B kt] Populatio distributio To fid the partitio fuctio we sum all ad whe T B/k we replace the discrete sum with itegratio over cotiuous values max 1exp[ 1 B kt] 5

6 Populatio distributio To fid the partitio fuctio we sum all ad whe T B/k we replace the discrete sum with itegratio over cotiuous values max 1exp[ 1 B kt] 1exp[ 1B kt] d kt B Populatio distributio To fid the partitio fuctio we sum all ad whe T B/k we replace the discrete sum with itegratio over cotiuous values max 1exp[ 1 B kt] 1exp[ 1B kt] d kt B Which gives is a expressio for the relative populatios 1 B exp 1 B kt kt LTE iteral eergy The mea cotributio to the rotatioal iteral eergy per molecule rot max kt 1B 1B exp[ 1 1 B B kt] d kt So whe T >> B/k there is equipartitio of kt/ for each of the two idepedet axes of rotatio. 6

7 Burgh et al. 7, Ap, 658, 446: Diffuse ISM CO T=4.1K Symmetric top Molecules (oliear molecules) is total agular mometum K is projectio o symmetry axis A,B, C are rotatioal costats I A,I B, I C are the momet of iertia, prolate (cigar) I A < I B =I C e.g., propye (CH 3C CH) oblate (disk) I A =I B < I C e.g,, bezee (C 6H 6) h 8 ci A A h 8 ci B B Prolate E() = (+1)B + (A-B) K Oblate E() = (+1)B + (C-B) K Selectios rule ΔK= series of equal frequecy separatio lies h 8 ci C C Symmetric top Ammoia (NH 3 ) Oblate symmetric top =>3 GHz (1.3 cm) iversio trasitios: - Thermometer of ISM dese molecular clouds [credit oh Bally] 7

8 Formatio the problem Collisios (A+B) No time to relax to eergetic stable state i.e., remove eergy durig collisio iteractio time is about 1-13 s the fastest relaxatio electroic rate is 1 8 s -1 So oly 1 i 1, collisios may remove eergy ad lead to a molecule = radiative associatio Alteratively a 3-body reactio may carry away eergy eed all 3 to collide at oce: A+B+C rate ~ 1-44 m 6 s -1 whe A+B are iteractig: ISM ~1 7 m -3 this might work o/ear i a star/circumstellar disk (~1 17 m -3 ) but ot i the space betwee stars = collisioal associatio Grai surface reactios Lagmuir-Hishelwood surface diffusio Eley-Rideal direct hit Destructio Electroic photo-dissociatio i the galactic ultraviolet Example from course book (Dyso & Williams) Cross-sectio 1-1 m over 1 m bad Number of UV photos 1 1 m - s -1 m -1 Lifetime of 1 1 s = 3 years 8

Destructio Electroic photo-dissociatio i the galactic ultraviolet Example from course book (Dyso & Williams) Cross-sectio 1-1 m over 1 m bad Number of UV photos 1 1 m - s -1 m -1 Lifetime of 1 1 s =

9 Destructio Electroic photo-dissociatio i the galactic ultraviolet Example from course book (Dyso & Williams) Cross-sectio 1-1 m over 1 m bad Number of UV photos 1 1 m - s -1 m -1 Lifetime of 1 1 s = 3 years Formatio: chemical reactio A + +B C+D + Reactios betwee ios ad molecules are rapid, e.g., O + + H OH + + H io-molecule O + H + O + + H (io-atom versio) = charge exchage Rate coefficiet of 1-15 m 3 s -1 per molecule Turs out to be a typical rate: let us see why as it applies to more tha that chemical cross-sectios Neutral exchage A B C D E Neutral exchage reactios (exothermic) Fial state is eergetically favourable, e.g, CH + O CO + H 7.6 ev Departig atom carries of excess eergy Rate coefficiet of 1-17 m 3 s -1 per molecule (1 weaker tha iomolecule) Rate depeds o short rage forces (va der Waals) V 1 R 6 activatio eergy a problem i the very cold ISM Credit Tom Millar (Quees Uiv.) 9

The Evolution of Molecular Tracers Jürgen Ott New World New Horizons, Santa Fe 8 March Molecular Gas Tracers in Galaxies. Juergen Ott (NRAO)

The Evolution of Molecular Tracers Jürgen Ott New World New Horizons, Santa Fe 8 March Molecular Gas Tracers in Galaxies. Juergen Ott (NRAO) Molecular Gas Tracers in Galaxies Juergen Ott (NRAO) Star Formation How do stars form? 1) Atomic/ionized gas converts to molecular clouds Star Formation How do stars form? 1) Atomic/ionized gas converts